Symposium EN10 : Emerging Light-Emitting Materials and Devices—Perovskite Emitters, Quantum Dots and Other Low-Dimensional Nanoscale Emitters

The unique optical properties of lead halide perovskites have drawn significant attention towards their application in light emitting devices (LEDs) in recent years. While quantum yield, emission wavelength and stability are already in the focus of many research groups, the orientation of the emissive transition dipoles has rarely been investigated. As known from other thin film applications such as organic LEDs, this quantity can severely affect the light outcoupling of the device and thereby limit the external quantum efficiency.
In this work, we investigate CsPbBr₃ nanoplatelets of variable thickness and determine the orientation of their transition dipole moments from thin film radiation pattern analysis. We then apply optical simulations to elucidate the performance limits of perovskite based blue LEDs in prototypical device architectures.
As was already shown, the transition dipole moment (TDM) of CsPbBr₃ nanocrystals can be tuned from preferentially vertical in nanocubes to horizontal in ultra-thin nanoplatelets [1, 2]. At the same time, quantum confinement renders the emission color from green to deep blue. Here, we use a systematic thickness variation from 2 to 6 monolayer thick nanoplatelets to study the tunability of the TDM orientation and the concomitant color shift.
We find that with increasingly beneficial horizontal orientation, the outcoupling efficiency increases to values close to 30 %. However, since the photoluminescence quantum efficiency degrades considerably for decreasing thickness [3], the overall maximum device efficiency does not significantly improve beyond 20%. Thus, for the currently available material sets we can conclude that while for nanocubes the non-ideal orientation limits device performance, devices with nanoplatelets are limited by non-optimal PLQYs.
Nevertheless, our results reveal very promising new efficiency limits for solution-processed light emitting diodes: Solution-processed, highly efficient deep-blue LEDs, with strong horizontal TDM orientation made of CsPbBr3 nanoplatelets are in reach.

[1] Jurow et al. "Tunable Anisotropic Photon Emission from Self-Organized CsPbBr3 Perovskite Nanocrystals." Nano Letters 17 (7), 4534-4540 (2017).
[2] Jurow et al. "Manipulating the Transition Dipole Moment of CsPbBr3 Perovskite Nanocrystals for Superior Optical Properties." Nano Letters 19 (4), 2489-2496 (2019).
[3] Bohn et al. “Boosting Tunable Blue Luminescence of Halide Perovskite Nanoplatelets through Postsynthetic Surface Trap Repair”, Nano Letters 18 (8), 5231-5238 (2018).

2D Ruddlesden-Popper perovskites (2D R-P), are an emerging group of materials that derive from the rapidly growing family of organic-inorganic perovskites. The 2D R-P are formed by the addition of an organic cation to the 3D perovskite, which acts as a spacer and forces the perovskite layers to separate, offering an additional knob to tune the material’s properties. In general, the 2D R-P shows significantly improved stability in comparison to the 3D perovskites and the colloidal nano-perovskites, making these materials appealing for various optoelectronic applications, such as photovoltaic devices and light emitting diodes. An interesting aspect arises from 2D R-P’s versatile structure is the ability to control their photoluminescence properties (PL). Depending on the exact structure and composition, the 2D R-P may show narrow excitonic PL, tunable by quantum confinement effects, low energy emission related to edge states and for some specific structures broadband ‘white’ emission. The latter case is specifically desired as it shows fairly high PL quantum yield and offers a simple path toward devices. Yet, the exact physical origin for the wide emission and the materials’ criteria for obtaining it are still not fully resolved. Experimentally, it was found that perovskite structures exhibiting complex connectivity and strong distortion are more prone to show broadband emission, but there is no clear model of why. The current models explaining the wide emission are focused on study the kinetics between the excitonic states (leading to narrow emission) and the low energy states (leading to broadband PL).
In order to better understand the underlying structural and physical mechanism for the broadband emission, a material that shows a clear transition between the narrow and broadband emission will be beneficial. We found that the simple single layer 2D Ruddlesden-Popper perovskite – (CH₃(CH₂)₃NH₃)₂PbI₄ – can show such transition from narrow PL at room temperature to broadband PL at low temperature. This transition relates to a phase transition from the slightly distorted room temperature structure to a significantly distorted phase below 200 K. At lower temperatures the distortion becomes larger, leading to stronger broadband emission, and at even lower temperatures it becomes weaker in correlation with the previously published kinetic model. By studying additional single layer 2D R-P samples, with longer organic cations as the spacer groups, we further change the distortion and formulate a more general model for the narrow to broadband PL transition. This study enables us to gain a better understanding of how the octahedra distortion in perovskites activate the low energy emissive states and lead to broadband emission and give some simple design rules to rationally approach the synthesis of such materials.

Recently, lead halide perovskite materials have introduced a new paradigm for semiconducting materials. The controlled synthesis, detailed structural analysis, optical, and electronic properties are of great fundamental interest. A high degree of ionic bonding character in perovskites gives rise to higher coordination environments, and significant structural disorder; the collective result is a family of “easy to make, easy to break” materials that are highly dynamic and readily reconfigurable. Particularly, highly ionic perovskite materials exhibit exceptional photophysical and optoelectronic properties even with significant electron-phonon coupling and structural disorder, motivating an investigation into the origins of these outstanding properties. This talk will focus on the research scope from simple ABX₃ inorganic halide perovskite towards other more complex halide perovskites and understand their structures and lattice dynamics. We develop the advanced synthetic methodology of perovskite nanostructures with desired size, composition, and optical properties, including colloidal, solution-phase and vapor-phase approaches. We anticipate rich phase transformation behaviors and rich electronic structures in such complex halide perovskites. Moreover, the phase transition and soft ionic lattice dynamics in such materials were demonstrated in detail using time- and spatially-resolved spectroscopy techniques.

Organic-inorganic metal halide hybrids, including ABX₃ metal halide perovskites, are an emerging class of functional materials with exceptional structural and property tunability. During the last decade, remarkable progress has been made in using solution processable metal halide perovskites as active materials for optoelectronic devices, including photovoltaic cells (PVs), light emitting diodes (LEDs), photodetectors, and lasers, for their excellent optical and electronic properties. By controlling the morphological dimensionality, low dimensional metal halide perovskites, including 2D nanoplatelets, 1D nanowires, and 0D quantum dots, have been developed to exhibit distinct properties from their bulk counterparts, due to quantum size effects. For instance, the emission of CsPbBr₃ nanocrystals can be tuned from green for nanocrystals with sizes of larger than the exciton bohr radius (~ 7 nm), to blue for quantum dots, nanowires, nanoplatelets, and hollow nanocrystals with strong quantum confinement. Besides ABX₃ perovskites, organic metal halide hybrids, containing the same building blocks of metal halide octahedra (BX₆), can be assembled with low dimensional structures at the molecular level, i.e. single crystalline bulk assemblies of metal halides with 2D, quasi-2D, corrugated-2D, 1D, corrugated-1D, and 0D structures. Due to the strong quantum confinement and site isolation, these low dimensional metal halide hybrids at the molecular level exhibit remarkable and unique properties that are significantly different from those of ABX₃ perovskites. For instance, broadband white emissions have been achieved in single crystalline bulk assemblies of 1D metal halide wires and tubes; and near-unity photoluminescence quantum efficiency has been realized for a number of 0D organic metal halide hybrids containing various metal halide species or clusters. In this talk, I will discuss our recent efforts on the development and study of highly luminescent low dimensional metal halide perovskites and hybrids, from synthetic control to device integration.

Hybrid organic-inorganic halide perovskites are a newly rediscovered class of solution-processable semiconductor materials with surprisingly promising optoelectronic performance. When fabricated in a nanostructured form – either as layered 2D quantum wells or colloidal nanocrystals – hybrid perovskite nanomaterials exhibit a combination of interesting properties revealing both quantum mechanical and classical composite effects. In this talk, I will discuss the thermal, electronic, and excitonic properties of hybrid perovskite nanomaterials as a function of composition, structure, and temperature – and what these experimental observations tell us about the interactions between the organic and inorganic subphases of this interesting class of materials.

I will discuss progress in the realization of electrically-driven optical sources (LEDs) based on reduced-dimensional materials. I will discuss perovskites, quantum dots, and hybrids of the two.

Cesium lead halide perovskite nanocrystals have recently emerged as a promising material system for a number of lighting and lasing applications due to their extensive color tunability, high photoluminescence quantum yield, and relatively narrow emission linewidths. Furthermore, these materials exhibit relatively low-threshold optical gain. If perovskite nanomaterials are to meet this potential in light emission applications, it is crucial to have a comprehensive understanding of the properties – not just under low-flux excitation – but under high-flux, device relevant conditions as well. We have thoroughly investigated the properties of the doubly excited state (“biexciton”) in CsPbBr₃ nanocrystals utilizing time- and spectrally-resolved ensemble photoluminescence, flux-dependent transient absorption spectroscopy, and single nanocrystal second- and third-order photon correlation measurements. Our results demonstrate that the biexciton state has a negligible and even slightly repulsive interaction energy. Furthermore, the dynamics of the biexciton state are much more rapid than what would be predicted by established statistical scaling models, providing insight into the processes which dominate multiexciton emission.

Organometal hybrid perovskites nanocrystals (PeNCs) with ABX₃ structure (A=small organic cations, B=Pb and X=halides) have been widely adopted as the emitters owing to their superior optical properties, quantum efficiency and the tunable photoluminescence wavelength with high color purity. However, due to the dynamic electrostatic interaction between ligands and the quantum dots and the existence of surface defects, such PeNCs normally suffer from poor stability issues. In this study, PeNCs are dispersed in the polymer matrix and the stability of the PeNC is enhanced due to the compatibility of the polymer matrix with the ligand. In addition, we realize that with the controlled diffusion of moisture through the polymer matrix into PeNCs, the PL intensity increases by 125% after immersing in water for 2 months, which is induced by the surface passivation of PeNCs the during the recrystallization process. Our work provides a fundamental approach to improve the stability of PeNCs against the environment with controlled diffusion of moisture and oxygen through the polymer matrix.
Reference
[1] Cho, H., Jeong, S. H., Park, M. H., Kim, Y. H., Wolf, C., Lee, C. L., Heo, J. H., Sadhanala, A., Myoung, N. S., Yoo, S., Im, S. H., Friend, R. H., and Lee, T. W. Science 2015, 350, 1222.
[2] Wei, Y., Cheng, Z., and Lin, J. Chem. Soc. Rev. 2019, 48, 310.

Colloidal semiconductor quantum dots (QDs) are attractive materials for realizing highly flexible, solution-processable optical gain media with readily tunable operational wavelengths [1,2]. However, QDs are difficult to use in lasing due to extremely short optical gain lifetimes limited by nonradiative Auger recombination [3,4]. This, in particular, is a serious obstacle for realizing cw optically and electrically pumped lasing devices. Recently, we have explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell QDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination [5,6]. Using these specially engineered QDs, we have been able to realize optical gain with direct-current electrical pumping [7], which has been a long-standing goal in the field of colloidal nanostructures. Further, we apply these dots to practically demonstrated the viability of a ‘zero-threshold-optical-gain’ concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption [8]. Such charged QDs are optical-gain-ready without excitation, which allows us to reduce the lasing threshold to record-low values that are well below a fundamental single-exciton-per-dot limit. Most recently, we have developed QD devices that operate as both an electroluminescent (EL) structure and a low-threshold, single-mode optically pumped laser. These devices feature a distributed feedback resonator integrated into a bottom electrode of a p-i-n multilayered stack. By carefully engineering a refractive-index profile across the device, we have been able to demonstrate excellent lasing behavior even with a very thin active region, which comprises only three monolayers of the QDs. These ultrathin devices can be pumped electrically and exhibit strong EL with good efficiency ‘roll-off’ characteristics. All of these recent developments demonstrate a considerable promise of colloidal QDs for implementing solution-processable optically and electrically pumped laser devices operating across a wide range of wavelengths.

[1] Klimov, V. I.et al. Optical gain and stimulated emission in nanocrystal quantum dots. Science 290, 314 (2000).
[2] Klimov, V. I.et al. Single-exciton optical gain in semiconductor nanocrystals. Nature 447, 441 (2007).
[3] Klimov, V. I., et al., Quantization of multiparticle Auger rates in semiconductor quantum dots. Science 287, 1011 (2000).
[4] Robel, I., et al., Universal Size-Dependent Trend in Auger Recombination in Direct-Gap and Indirect-Gap Semiconductor Nanocrystals. Phys. Rev. Lett. 102, 177404 (2009).
[5] Park, Y.-S., Lim , J., Makarov, N. S., Klimov, V. I. Effect of Interfacial Alloying versus “Volume Scaling” on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots. Nano Lett. 17, 5607 (2017).
[6] Park, Y.-S., Lim, J. & Klimov, V. I. Asymmetrically strained quantum dots with non-fluctuating single-dot emission spectra and subthermal room-temperature linewidths, Nat. Mater., 18, 249 (2019).
[7] Lim, J., Park, Y.-S. & Klimov, V. I. Optical Gain in Colloidal Quantum Dots Achieved by Direct-Current Charge Injection. Nat. Mater. 17, 42 (2018).
[8] Wu, K., Park, Y.-S., Lim, J. & Klimov, V. I. Towards zero-threshold optical gain using charged semiconductor quantum dots. Nat. Nanotechnol.12, 1140 (2017).

Organic-inorganic (hybrid) perovskite have recently emerged as a new semiconductor platform for next generation optoelectronics devices. These perovskite solids feature weak bonds between their organic and inorganic building blocks, which results in an intrinsic softness and dynamics disorder of the lattice and an acute sensitivity to external stimuli. For example, our recent work demonstrated that continuous sunlight illumination leads to a uniform expansion of the perovskite lattice, which impact the optoelectronic properties of the three-dimensional (3D) perovskites. In particular, this effect is beneficial as it helps cure electronics impurities and lowers energetic barriers near the surface/interface. Here, we present comprehensive in-situ studies of light and electrical field induced structural dynamics of in layered two-dimensional (2D) hybrid perovskites. We correlate the changes in the structure of the 2D perovskite to modification of both the physical properties and the figures of merit in solar cells and light emitting devices. We propose a new microscopic model to explain the evolution of the structure and optoelectronic properties under external stimuli. These results demonstrate that structural characterization of hybrid perovskite under external perturbation is key in unraveling the fundamental physics of these materials.

The remarkable solar performance of lead halide perovskites can be attributed to their excellent physical properties that present many mysteries, challenges, as well as opportunities. Better control over the crystal growth of these fascinating materials and fabrication of heterostructures using two-dimensional (2D) Ruddlesden−Popper layered metal halide perovskites with different bandgaps would open up opportunities for exploring new properties and device applications. I will first briefly summarize our results on the solution and vapor-phase growth of single crystal nanowires and nanoplates of methylammonium (MA), formamidinium (FA), and all-inorganic cesium (Cs) lead halides perovskites (APbX₃). Then I will focus my discussion on the growth of multi-layered multi-colored vertical heterostructures of 2D Ruddlesden–Popper (RP) layered lead iodide perovskites with defined n phases and atomically sharp interfaces, in which the long chain ammonium ligands serve as the barriers to prevent ion migration across the junctions. We have further developed the controlled growth of large area nanosheets of RP perovskite phases with varying n values and different anions and well defined vertical heterostructures of these phases. These heterostuctures were used to study the carrier transfer mechanisms between different RP phases and as building blocks for optoelectronic devices. High performance room temperature lasing with broad tunability of emission was also demonstrated with single-crystal APbX₃ perovskite nanowires. The excellent properties of these single-crystal perovskite nanostructures of diverse families of perovskite materials with different cations, anions, and dimensionality make them ideal for fundamental physical studies of carrier transport and decay mechanisms, and for enabling high performance lasers, LEDs, and other optoelectronic applications.

Metal halide perovskites (MHPs) has been considered as promising materials in the field of light-emitting diodes (LEDs), owing to their advantages on light emission such as extremely high color purity (full width at half maximum ≈ 20 nm at room temperature), high photoluminescence quantum efficiency > 90%, easily tunable bandgap and low material cost.¹ In addition, the efficiency of perovskite light-emitting diodes (PeLEDs) based on three-dimensional (3D) polycrystalline perovskites has been greatly increased despite the short period of research.^2–4 However, the short lifetime during the operation has been a key challenge to be overcome for their practical applications.⁵ The most critical factor is the ion migration in perovskites, which can seriously degrade the crystal structure and luminescent efficiency during device operation, especially for PeLEDs because of the required high bias voltage for device operation.
Here, we demonstrate proton-transfer-induced 3D/2D hybrid perovskite that was obtained by adding a small amount of neutral amine instead of alkyl ammonium halide salt. Proton transfer from organic ammonium precursor to neutral amine in additive/perovskite composition enabled crystallization of 2D perovskite on the surfaces of 3D perovskite grains without destroying the 3D phase. In this study, we almost removed overshoot of luminance over time and highly extended the lifetime of PeLEDs by more than 20 times by developing a 3D/2D hybrid perovskite emitter; the 2D perovskite passivates the trap sites of 3D perovskite and blocks possible pathways of ion migration. We also identified the mechanism of remarkably suppressed luminance overshooting and significant lifetime extension by blocking ion migration in PeLEDs. Our work provides promising guidelines to improve the stability of PeLEDs with the exact explanation on degradation mechanism assisted by the ion migration in perovskite.

Reference
1. Kim, Y.-H., Cho, H. & Lee, T.-W. Metal halide perovskite light emitters. Proc. Natl. Acad. Sci. U. S. A. 113, 11694–11702 (2016).
2. Cho, H. et al. Overcoming the electroluminescence efficiency limitations of perovskite light-emitting diodes. Science (80-. ). 350, 1222–1225 (2015).
3. Park, M.-H. et al. Boosting Efficiency in Polycrystalline Metal Halide Perovskite Light-Emitting Diodes. ACS Energy Lett. 4, 1134–1149 (2019).
4. Xu, W. et al. Rational molecular passivation for high-performance perovskite light-emitting diodes. Nat. Photonics 13, 418–424 (2019).
5. Quan, L. N., García de Arquer, F. P., Sabatini, R. P. & Sargent, E. H. Perovskites for Light Emission. Adv. Mater. 30, 1801996 (2018).

Currently, all inorganic CsPbX₃ perovskite nanocrystals are among the most investigated semiconductor nanostructures due to their set of unique optical properties, which includes high emission quantum efficiency, tolerance to surface defects, and broadband tunability while maintaining narrow emission spectra. Fast emission lifetime (up to one order of magnitude faster than CdSe based nanomaterials at room temperature) is yet another relevant property of this class of nanomaterials, which is due to the fact that, different from other nanostructures, the band edge is defined by a bright triplet state. This triplet exciton fine structure is still not well explored and further understanding is necessary to turn these unique properties of perovskite NCs useful for optoelectronic application. However, accessing the ultrafast temporal evolution and the coherent dynamics of those states is difficult due to the inhomogeneous broadening. Here, we will show that we can get around this inhomogeneous broadening by using multi-dimensional coherent spectroscopy (MDCS), and access relevant information regarding the triplet fine structure.
In our experiment, we investigate, at cryogenics temperatures, an ensemble of CsPbI₃ quantum dots with side lengths of 8.7 ± 2.6 nm. Using a sequence of three ultrafast laser pulses, with specified relative polarization, we resonantly excite different triplet coherences. For co-linearly polarized laser pulses, we observe a strong central emission peak, with a zero-phonon line of about 0.12 meV (FWHM), and two weaker ones positioned about 0.30 meV apart from the main peak, and nearly as broad as the center one. While the central peak is attributed to the absorption and emission from the ground level to same excited state Ψ_i, the side band are related to the absorption and emission involving the ground state and Ψ_x and Ψ_y, through coherent coupling. When we rotate the polarization of the second pulse to be perpendicular to the two others, the central peak disappears, while the side peaks at 0.30 meV becomes weaker and a broader peak, with linewidth of nearly 0.87 meV, and about 1.60 meV apart from the center, emerges becoming the most intense one. Those peaks are related to the absorption and emission involving the ground state and Ψ_z and Ψ_y. We do not observe, in any configuration coherences between Ψ_x and Ψ_z, which suggests that the dipole moment corresponding to Ψ_y is much stronger than for the other two. From the inter-triplet coherences we can infer that the coherence time for each one of the triplet levels is T₂^x = 5.68 ps, T₂^y = 5.32 ps, and T₂^z = 0.76 ps. In addition to these, our data indicates that the triplet state splitting is size dependent and, by temperature dependence studies, we show that the inter-triplet coherence is only weakly dependent on the temperature up to about 50K.

The development of practical optical quantum technologies requires the reproducible and scalable production of single quantum emitters with long optical coherence times that can be readily integrated with devices. We have recently demonstrated the potential for chemically-made, colloidal lead halide perovskite (CsPbX3, X=Cl, Br, I) quantum dots (PQDs), to be single emitters of quantum light. Using Photon-Correlation Fourier spectroscopy (PCFS) at low temperatures, we found that PQDs exhibit long optical coherence times and small exciton fine-structure splittings. The long coherence (80 ps) and short radiative emission lifetimes (210 ps) render the PQD emission linewidth near the Fourier transform limit. Moreover, we show that spectral diffusion is dramatically reduced compared to other colloidal quantum dots and that the majority (50-80%) of photons are emitted coherently. This fraction of coherent photons is already comparable to silicon vacancy centers in diamond, which are common emitters in quantum photonics. Our results suggest that, while all other colloidal quantum dot materials suffer from prohibitively incoherent emission, PQDs can be explored as sources of indistinguishable single photons that can be processed from solution onto virtually any substrate and coupled with hybrid nano-photonic components like waveguides or plasmonic gap cavities.

Single photon super-radiance is a way to move the radiative decay time into the sub-nanosecond time regime. Single photon super-radiance was introduced by Dicke,¹ who demonstrated that single photon spontaneous emission of N coherently excited atoms occurs N times faster than the spontaneous emission rate of a single atom, t₀ ; in other words the single photon superadiative decay time: t_SR=t₀/N. Single photon super-radiance has been observed recently in inorganic CsPbX₃ (X=Cl,Br,I) perovskite nanocrystals (NCs) at liquid hydrogen temperatures.² Sub-nanosecond radiative decay time of weakly bound excitons can be also understood in terms of the phenomenon known as giant oscillator strength (GOS).³ GOS was introduced initially to explain the excitation spectra of shallow impurities. Despite relatively small concentration, these impurities are observed as narrow sharp lines with intensities comparable to the intensity of the exciton line in the band edge absorption spectrum. It was predicted theoretically⁴ that exciton weakly confined in a spherical NC, which radius a is much larger than the exciton radius a_ex is characterized by GOS which strength, f _NC=f₀(a/a_ex)³ >> f₀ , where f₀ is the exciton oscillator strength. The single photon super-radiance connected with GOS f _NC of NCs observed generally at liquid helium temperatures shows unusual and counter-intuitive increase of the radiative decay time with temperature. We demonstrate that the unusual behavior of the radiative decay time observed in perovskite NCs is connected with the thermal population of upper optically forbidden exciton states and with phonon-induced admixture between these states and the ground exciton level, which reduces GOS of the ground exciton state.⁵ The latter mixing process is dominant at low temperatures and leads to formation of an exciton-phonon polaron.
¹ R. H. Dicke, Coherence in Spontaneous Radiation Processes, Phys. Rev. 93, 99-110 (1954).
² M. A. Becker, et al. “Bright triplet excitons in caesium lead halide perovskites,” Nature, 553, 189-193 (2018).
³ E. I Rashba and G. E. Gurgenishvili, Edge absorption theory in semiconductors. Sov. Phys. Solid State, 4, 759-760 (1962).
⁴ Al. L Efros and A. L. Efros, Interband absorption of light in a semiconductor sphere, Sov. Phys. Semicond, 16, 772-775 (1982).
⁵ G. Raino, P. Sercel, M. M. Glazov, F. Krieg, M. A. Becker, D. Guggisberg, J. Michopoulos, T. Stoferle, R. F. Mahrt, Al. L. Efros, and M. K. Kovalenko, Deciphering the role of temperature in single photon super-radiance of perovskite nanocrystals, to be published.

Ever since the physics of quantum dot (QD) was discovered, much research effort has been carried out for more than 30 years, and lots of applications adopting QDs have been proposed. Especially, wide color gamut displays using QDs as active light emitting materials have drawn much attention. And, the QD-based consumer displays such as LED TVs, tablets, and special monitors are now on the market. They provide best color gamut, reasonable power efficiency, and affordable price showing superior competitive edge to OLED technology. However, there used to be issues and argues using Cadmium containing materials in practical consumer devices. In spite of the European RoHS Exemptions, we needed to be aware the environmental risk of producing large quantity of Cd-containing materials and using them in the consumer electronics. Samsung has dedicated to develop more environmentally friendly InP based QDs that showed considerably high efficiency and saturated color spectrum compared to the Cd-containing materials. The structure of Cd-free QD was specially tailored for display applications and the synthetic process was optimized to produce reliable materials in commercial scales. In order to improve the efficiency and stability of the QDs in the devices operating under severe atmosphere, specific composite materials were designed and the fabrication process was optimized. From 2015, Samsung has released Cd-free QD adopted UHD TV for major product line-up which show the best color gamut among the current displays. Now we are trying to expand this established Cd-free QD technology as a material platform and make additional breakthroughs in wider optoelectronic applications. Also, we are trying to understand the fundamental properties of defect states of InP QDs via density functional simulation and spectroscopic analyses.

Quantum dots are semiconductor nanocrystals whose size can be tuned to control their optical absorption and emission properties. When excited electrically or optically, a spatially confined electron-hole pair (exciton) is generated. The quantum dot’s emission wavelength can be changed through the confinement effect by tuning the quantum dot to be the size of the exciton’s Bohr radius or smaller. Due to their bulk bandgap, CdSe quantum dots are tunable across the entire visible spectrum. Quantum dots can also simultaneously sustain multiple excitations, which can result in two bound excitons (biexcitons), three bound excitons (triexcitons), or more. Multiexcitons have lower quantum yields than single excitons due to the addition of nonradiative Auger recombination and are typically generated under higher flux conditions, due to the Poissonian absorption statistics of quantum dots. The dynamics and recombination pathways of triexcitons specifically are not well understood and therefore their efficiency is not well optimized in quantum dot devices. Previous theoretical studies show that triexcitons in CdSe quantum dots occupy two angular momentum states of the band-edge 1S-like state (filling the 1S-like state) and one spin state of the higher-energy, 1P-like state. Furthermore, previous work in our group predicted that triexciton emission is dominated by band-edge S-like recombination rather than the P-like recombination, but not definitively proven. We utilize time-correlated single photon counting (TCSPC) with a four spectrally-filtered single-photon counting detectors to detect 1S-like and 1P-like emission from the triexciton state. We demonstrate that almost all triexciton emission occurs from band-edge S-like recombination, indicating that multiexciton interactions drastically reduce the emission rate from the P-like state. These results provide a crucial step to improving the efficiency of nanocrystal materials in device relevant conditions.

Metal halide perovskite nanocrystals (NCs) have emerged as a new generation of light emitting materials with narrow emissions and high photoluminescence quantum efficiencies (PLQEs). Various types of perovskite NCs, including 2D nanoplatelets, 1D nanowires, and 0D nanocubes, have been discovered to exhibit tunable emissions across the whole visible spectrum. Despite remarkable advances in the field of metal halide perovskite NCs over the last few years, many nanostructures have yet to be realized in metal halide perovskites. Producing highly efficient blue emitting perovskite NCs has also remained challenging and of great interest. In this talk, I will present our efforts in developing highly efficient blue emitting cesium lead bromide (CsPbBr₃) perovskite NCs with hollow structures. By utilizing facile solution processing, in-situ formation of hollow CsPbBr3 NCs with controlled particle and pore sizes was realized for the first time. Our synthetic control of hollow nanostructures with quantum confinement effects has enabled precise color tuning of CsPbBr3 NCs from green (525 nm) to blue (459 nm) with high PLQEs of up to 81 %.

Generally, the fabrication method perovskite (PRK) LED paper reported are solution processed, which actually faced with several deficiencies, such as solubility of different ingredient (for instance, CsBr shows inferior solubility in DMF or DMSO, which are both regarded as frequently used solvent), small efficient device area, rigorous preparation conditions and variable device performance. Even though solution processed planar PRK LED can achieve EQE with more than 20%, the poor stability, short lifetime as well as incapable of large electroluminescence with good uniformity still limit the further application of PRK LED.Vacuum thermal evaporation is an excellent method to deposit perovskite films with good film morphology. What is more, the thickness of vacuum-evaporated films could be precisely controlled compared with that of solution-processed ones. Herein, we first demonstrated organic-inorganic hybridized 2D perovskite thin film as light emitting layer via evaporation method. Base on our characterization, we got 2D nanoflake structure perovskite crystals with small crystal size. The one-dimensional confined structure not only can significantly improve electron-hole injection, but also can enhance luminescence efficiency due to quantum confinement. The planar device EQE boost to 4.8% with luminance about 11000cd/m² after constructing the 2D structure. In addition, larger device size by 1.5cm*2cm were also demonstrated, which shows potential in large area display.

Metal halide perovskites (MHPs) have recently demonstrated exceptional promise as emissive materials for light-emitting applications. They exhibit remarkable colour purity and high photoluminescence quantum yield (PLQY), tuneable across the visible spectrum. As a result, prototype small-area light-emitting diodes (LEDs) with external quantum efficiencies (EQE) exceeding 20 % have been achieved.
A significant attraction of MHPs is their inherent compatibility with solution-processing. Considering also the low cost of the precursor materials, MHPs have the potential to facilitate cost-effective, high throughput roll-to-roll (R2R) LED fabrication on flexible substrates. Despite the potential, there have been very few reports of large-area perovskite LEDs realized through scalable deposition techniques; the vast majority still rely on spin-coating. Spin-coating places an inherent limitation on scalability; as each substrate must be coated separately it is unsuitable for desirable continuous processing. Slot-die coating provides an attractive alternative. It is a straight-forward continuous process, suitable for R2R deposition. Furthermore, the volume of excess solution required is fixed, thus material wastage for large-scale deposition processes would become almost negligible. This is in stark contrast with spin-coating, where most of the solution is ejected from the substrate.
The applicability of slot-die coating for light-emitting diode fabrication has been demonstrated extensively for organic light-emitting diodes, establishing the capability of the technique to reproducibly deposit films of around 40 nm. Recently, Prakasam et. al. achieved the first slot-die coated LEDs by directing N₂ gas flow onto wet films to induce fast gas-assisted crystallization of MAPbBr₃.¹ They obtained modest current efficiency (CE) of 2.6 cd A^-1 for 4.46 cm² active area devices, which were slot-die coated as a 230 cm² strip. Similar methods have been reported several times for slot-die coating of bulk perovskite films for solar cells, where crystallization control is also a challenging issue.^[14]
MHP nanocrystals (NCs) provide a simpler alternative. As the perovskite phase has already been crystallized prior to film deposition, the NC ink can be slot-die coated without additional provisions. Thus, the development and optimization of a slot-die coating process for MHP NCs would provide a versatile large-area deposition method, demonstrating a significant advancement of perovskite LED scalability.
Herein, we report the fabrication of slot-die coated LEDs from our FAPbBr₃ NCs. The slot-die coating parameters were optimised to obtain full surface coverage over an active area measuring 16 cm². Maximum external quantum efficiency (EQE) of 7.26 % (CE = 31.9 cd A^-1) and maximum luminance of 1428 cd m^-2 were achieved. To the best of our knowledge, both of these values are the highest reported for perovskite LEDs fabricated using a fully scalable deposition technique. These results represent a significant enhancement of the scalability of perovskite light-emitting diodes towards future commercial viability.

1. V. Prakasam, D. Tordera, F. Di Giacomo, R. Abbel, A. Langen, G. Gelinckad and H. J. Bolink, J. Mater. Chem. C, 2019, 7, 3795-3801.

Hybrid perovskite materials have expanded their scope of interest to a wide research community owing to their desirable photo-physical properties, which made them useful for opto-electronic applications.¹ Low-dimensional hybrid perovskites, in particular, have been the subject of intensive research as of late due to the ease of tuning of their excitonic properties.²
Here in this work, we present the case of the ethylene diammonium cation as an intercalating agent for forming a 1D layered lead halide perovskite framework. We show that the addition of mono-ammonium bromide salt (up to 20 mol %) during the crystal growth enables the manipulation of the crystal’s framework from 1D to 2D-stair-case like perovskite. The ethylene diammonium cation along with water molecules stabilize this stair-case like crystal framework. The ethylene diammonium cation acts simultaneously as an inter-layer cation as well as a cation filling the in between adjacent voids of octahedrons. This mono-ammonium bromide salt incorporation in the single crystals shows simultaneous broadband and narrow emission depending on the excitation source. With a combination of structural, spectroscopic, and Density functional theory (DFT) approaches we investigate the origin of dual emission from single crystals of this layered perovskite. Our findings suggest that self-trapped excitons and the presence of water molecules inside the crystal lattice control the dual emission from these crystal systems.

High photoluminescence quantum yield (PLQY) is required to reach optimal performance in solar cells, lasers and light-emitting diodes (LEDs). Typically, PLQY can be increased by improving the material quality to reduce the non-radiative recombination rate. It is in principle equally effective to improve the optical design by nanostructuring a material to increase light out-coupling efficiency and introduce quantum confinement, both of which can increase the radiative recombination rate. However, increased surface recombination typically minimizes nanostructure gains in PLQY. Here a template guided vapor phase growth of perovskite nanowire (NW) arrays with unprecedented control of NW diameter from the bulk (250 nm) to the quantum-confined regime (5.7 nm) is demonstrated, while simultaneously providing a low surface recombination velocity of 18 cm s-1. This enables an obvious PLQY enhancement from 0.33% up to 42.6% for MAPbI₃ quantum wires, and up to ~90% for MAPbBr₃ quantum wires, exclusively using nanophotonic design, which will be promising for LEDs applications with high external quantum efficiency (EQE). The simple extension of this technique to a wide variety of semiconductors and the ultra-high density of vertical QWs may also provide interesting opportunities in quantum transport, electronics and memory devices in the future.

Recently metal halide perovskite light emitting diodes (LEDs) have been reported with high efficiencies, most often made by in situ generation of nanocrystals in a film or the formation of quantum well hetero-structures. In both cases, bulky ammonium cations are used which greatly improve the optoelectronic properties of the perovskite by direct chemical passivation, increased dielectric confinement and carrier funnelling, all which encourage efficient radiative bimolecular recombination.
However, the operational stability of these devices is poor with even the state of the art only able to function for a few hours. More often than not, this stability is overlooked in favour of achieving high efficiency, but this now needs to be urgently addressed.
In this work we investigate the influence of the bulky ammonium cation on the stability of CsPbBr₃ based perovskite LEDs. Upon addition of these molecules into precursor solutions we observe an increase in external quantum efficiency of four orders of magnitude. Despite this excellent improvement in efficiency we also demonstrate that the optimised LEDs are far less stable than those made without the additive. Through optical and structural characterisation we elucidate the reason for this drop in stability and provide an insight into alternative strategies for obtaining both high efficiency and stable LEDs.

Elastic mechanoluminescent (ML) materials convert energy of reversible elastic mechanical deformation into optical emission. ZnS:Mn stands out among thousands of ML compounds due to one of the lowest reported thresholds for ML appearance in the range 0.6 - 1 MPa = 0.6 -1 pN/nm² [1]. Material which is capable of reversible mechanoluminescence at so low pressure range is perfect candidate for nanoscale stress sensor demanded in many technological and biomedical applications. However, further development of ML-based technologies is stalled due to the limited understanding of mechanisms underlying appearance of elastic ML.

We use a new approach to study microscopic characteristics of low force elastic mechanoluminescence in single ZnS:Mn microparticles. It employs the combination of a micropositioner for force application and the Perfect Focus System (PFS) of a Nikon Ti2 widefield microscope to track pressure and ML from single microparticles which are 0.5 - 5 µm in size. We demonstrate that ML in ZnS:Mn is fully endogenous, non-destructive and reversible physical process. Furthermore, analysis of time-resolved microscopic images of ML patterns reveals striation, which does not overlap with analogous features seen in the photoexcited emission of the same particle. It implies that microparticle’s volume is heterogeneous in terms of ML, and certain microscopic structures within the volume are more effective ML emitters. We use Transmission Electron Microscopy (TEM), Focused Ion Beam (FIB) and Kelvin Probe Force Microscopy (KPFM) techniques to show that ZnS:Mn microparticles contain high concentration of zinc blende (ZB)/wurtzite (WZ) stacking faults with electrically charged interphase boundaries.

We propose a new mechanism of elastic mechanoluminescence in ZnS:Mn which relates its structural and mechanoluminescent properties and explains very low threshold for ML appearance. This mechanism relies on the presence of stacking faults and concomitant built-in electric fields which produce a saw-tooth like potential profile inside the crystal. In the faulted crystal, the electron density is shifted towards the positively charged interfaces of the stacking faults in order to compensate for the built-in fields. It leaves the negatively charged interfaces unscreened. Hence, the electrons and holes trapped at the oppositely charged interfaces serve as a source for ML photons. Application of mechanical stress induces an onset of piezoelectric field which is locally enhanced at the electron-depleted interfaces. The carriers trapped at the interfaces gain an excess energy and recombine either immediately or at the moment of the stress release. The local enhancement of the piezoelectric field provides the explanation for extremely low pressures (as low as 240 kPa) which are sufficient for ML excitation in ZnS:Mn. Based on the new model, the ML structural unit comprises two segments of one crystal phase separated by the inclusion of the other phase. Based on our TEM data, it can be scaled down to less than 30 nm what, along with the low threshold for ML appearance, makes ZnS:Mn a perfect candidate for nanoscale force sensor.

[1] Wang, X. et al. Dynamic pressure mapping of personalized handwriting by a flexible sensor matrix based on the mechanoluminescence process. Adv. Mater. 27 , 2324-2331 (2015).

Currently, much research interest related to achieving white emission has refocused on quantumdots (QD)-LEDs based on fluorescent semiconducting nanoparticles consisting of 10-100 atoms. QD-LEDs have a good color rendering index (CRI), high quantum efficiency, high purity, and narrow emission spectra due to the quantum confinement effect and the size-dependent tunability of the bandgap. Nevertheless, the development of Cd-free non-hazardous as well as In-free cost-effective QDs has become essential and is encouraging intensive research for future optoelectronic devices. Oxide semiconductors are appropriate as eco-friendly materials to produce white light if the visible light emission caused by the electron transition between the defect energy levels of interstitials or vacancies existing in the bandgap is effectively exploited. II-VI ZnO is one among such strong candidates. In general, the PL spectrum of ZnO is characterized by two features, near band edge (NBE) emission in the UV region and defect-related blue, green, yellow, and orange-red emissions in the visible region. Even though the origin of defect-related visible emission have not been manifestively revealed, Defects depending on the excitation wavelength (l_ex) related to the emissions of green, yellow, and orange-red from ZnO QDs are completely inhibited by removing oxygen vacancies via the hybridization of Zn interstitials with anti-bonding O-states of graphene oxide (GO) QDs. This also leadsto only l_ex independent violet-purple-blue (V-P-B) emissive ZnO-GO QDs with a high photoluminescence quantum yield (PLQY) of 92%. White-light emission with CIE (0.32, 0.34) from PL QD-LEDs is realized using a mixture of ZnO and ZnO-GO QDs excited by a UV LEDs chip. ZnO-GO QD-based deep-blue LEDs with luminance of ca 2,000 cd/m², a luminous efficacy (LE) of more than 2.5 cd/A, and external quantum efficiency (EQE) of about 3% with CIE (0.16, 0.11) are also sucessfully achieved.

Quantum dots (QDs) have emerged as a promising material to be applied in the next generation of lasers. The possibility of controlling its emission color by size, added to its facile synthesis stimulated intense research in this field in the last years¹. However, there is still many challenges that need to be overcome to make this material competitive with current technologies. Among them, we can highlight the ones related to increase its output efficiency under long term optical or electrical pumping. An important strategy to reach this goal is to build heterostructures, such as core/shell or core/alloy/shell QDs². In this way, it is possible to increase both its absorption cross section and decrease losses due to, for example, non-radioactive Auger recombination. However, increased lattice mismatch in large shell structures drastically decreases their photoluminescence quantum yield (PLQY) due to the creation of interfacial defects. A novel class of heterostructured nanomaterial, called spherical quantum well (SQW), composed by and internal CdS seed, followed by an intermediary emissive layer of CdSe and an outer CdS shell can reduce the crystalline strain between the shell and the emissive layer, allowing for structures that combine high absorption cross section and near unity PL quantum yield³. Here, we investigate the performance of this new class of nanomaterial for lasing applications, by studying their multi-exciton dynamics and amplified spontaneous emission (ASE) threshold. From time resolved photoluminescence studies, we show that the multi-exciton Auger recombination rate is highly reduced in this nanostructure. Varying the shell and well sizes, we report Auger lifetimes from 0.3 to 1.4 ns across samples for positive trions, meanwhile for the negative ones, the lifetime varied from 1.5 to 8.0 ns. Inspired on these characteristics, the size dependence of the ASE properties on SQWs were studied. From that, we show that the ASE threshold decreases as the nanoparticle volume increases, because of the higher cross section, reaching 50 μJ/cm² for the biggest sample. However, considering the average of dots needed to generate ASE, this trend is inverse, going from 1.30 for the smallest particle to 2.35 for the biggest. To understand this phenomenon, the bi-exciton binding energy of the samples were investigated. From that, we show that the reason for this is a transition from an attractive bi-exciton interaction regime to a repulsive one, as the shell thickness increases. To conclude, the unique set of properties presented by SQW brings new insights to overcome current challenges on the application of QD in new laser technologies. Additionally, the ideas presented here can be expanded to other kinds of compositions, making this structure suitable to different kind of lightening emission applications, such as in LEDs and luminescent solar concentrators.

1. Klimov, V.; Mikhailovsky, A.; Xu, S.; Malko, A.; Hollingsworth, J.; Leatherdale, a. C.; Eisler, H.-J.; Bawendi, M., Optical gain and stimulated emission in nanocrystal quantum dots. Science 2000, 290 (5490), 314-317.
2. Park, Y.-S.; Bae, W. K.; Baker, T.; Lim, J.; Klimov, V. I., Effect of Auger Recombination on Lasing in Heterostructured Quantum Dots with Engineered Core/Shell Interfaces. Nano Letters 2015, 15 (11), 7319-7328.
3. Jeong, B. G.; Park, Y.-S.; Chang, J. H.; Cho, I.; Kim, J. K.; Kim, H.; Char, K.; Cho, J.; Klimov, V. I.; Park, P., Colloidal spherical quantum wells with near-unity photoluminescence quantum yield and suppressed blinking. ACS nano 2016, 10 (10), 9297-9305.

Recently, perovskite light emitting diodes (LEDs) has been attracting great attention as a next-generation LED that can replace existing OLED and QLED. However, methylammonium lead halide (MLH) perovskite is not applicable to LED devices due to its low moisture stability. Quasi-2D perovskites (PEA)₂(MA)_n-1Pb_nBr_3n+1 has been reported to enhance the photoluminescence (PL) and water-stability compared to conventional MLH 3D perovskite (MAPbBr₃). However, the development of conventional ruddlesden-popper phase perovskites has been only focused on red and green emission region. In this regard, herein we tried to tailor the emission range of Br-based quasi-2D perovskite from green to blue region by integrating organic cation spacer. As the candidates for spacer, selected aromatic (phenylethylammonium (PEA), benzyltrimethylammonium (BTA)) and aliphatic cations (isopropylammonium (IPA), n-propylammonium (nPA)) were employed by controlling the relative concentration ratio. As a result, the PL intensities and the emission wavelength of the new class of quasi-2D perovskites, BTA and nPA-based perovskites, showed deep-blue emissions at 475 nm, with the uniform and well-structured surface coverage. In addition, The PL intensity of blue perovskite thin film was on par with that of green one ((PEA)₂(MA)_n-1Pb_nBr_3n+1). By controlling the ratio of BTA and nPA, we could fine-tune the wavelength of the PL wavelength from 610 nm (green) to 475 nm (blue) systematically (with 7 nm intervals). Based on our dimensionality-controlled perovskites, the blue-emissive Q-2D perovskite LED showed low turn-on voltage (less than 4 V) with high-current density (42 mA/cm²). The protocol and strategy established in this study can be exploited to enhance high level electron quantum efficiency (EQE) and luminance.

Calcined oyster shells contain an abundant number of color centers arising from Ca and O vacancies. Emissions occur at both 20K and 300K with intensity much greater than reported data on carbon supported ZnO nanocrystals. Optical transitions proceed through three-body mechanisms where nonradiative processes involve transverse acoustical and optical phonons. Oxides obtained from suppliers show a low profile of emission, attributed to water absorption that causes charge transfer and weakens spin-orbital coupling.

Photon upconversion (PUC) via triplet-triplet annihilation (TTA) is an attractive means for solar energy concentration, bio-imaging, and photochemical reaction, such as photoinduced drug delivery. In particular, generation of blue light (< 500 nm) via upconversion is important, because blue light is required for most photochemical reactions^1,2, as well as photocatalytic water splitting to produce hydrogen, since most materials with the efficient quantum yield for photocatalytic water splitting can harvest only photons with energy above 500 nm (e.g., GaN:ZnO, BiVO₄ - SrTiO₃:Rh(,La))^3,4. On the other hand, utilizing near infrared (NIR) photons is crucial for applications such as bio-imaging and drug delivery, as the window for high transparency of biological tissue lies in the NIR region (700 - 900 nm).^1,4-6 Thus, PUC from NIR to blue would be beneficial to many emerging applications.
As the difference in energy between blue and NIR photons is large (e.g., 470 nm: 2.64 eV and 800 nm: 1.55 eV), a reduction of energy loss in PUC process, leading to larger anti-Stokes shift (: (PUC emission energy) - (excitation energy)), is also important. However, there has been no report, to the best of our knowledge, of a material with an anti-Stokes shift greater than 1.0 eV that also has a linear excitation intensity dependency (slope 1 for the PUC process).⁷ One of the keys to reduce loss in energy for PUC is to minimize the driving force of TTA process (difference in energy between 2T₁ and S₁ of TTA material). Reports of annihilators fluorophores which can accept low-energy triplets (< 1.55 eV) and generate blue light have so far mainly focused on 9,10-Bis(phenylethynyl)anthracene (BPEA)^8-10 and perylene derivatives.^1,11,12 However, these materials function with low efficiency of TTA (e.g., BPEA)^8-10 and/or require large driving force for TTA (e.g., perylene derivative: 2 T₁- S₁ ≈ 0.38 eV) ^1,11,12. Therefore, there is a clear need for PUC materials with a large anti-Stokes shift as well as efficient PUC from NIR to blue.
Here, we demonstrate efficient PUC converting from NIR energy to blue photons using the commercially available material 9,10-Bis[(triisopropylsilyl)ethynyl]anthracene (TIPS-Ac) as an efficient TTA material despite a low driving energy for the TTA process (< 0.32eV). When TIPS-Ac was sensitized with Pd(II) meso-Tetraphenyl Tetrabenzoporphine (PdTPBP), a generates triplets via intersystem crossing, the PUC photoluminescence quantum yield with excitation at 635 nm was 27 ± 0.5 % with a TTA efficiency of 77 ± 3 %. Combining TIPS-Ac with Pt(II) meso-Tetraphenyl Tetrabenzoporphine (PtTPBP), where we show directly generates triplet via NIR excitation at 785nm, the PUC achieved an anti-Stokes shift of 1.03 eV with a linear excitation intensity dependency.
Our results highlight the use of direct triplet generation via NIR excitation as a useful path to achieving large anti-Stokes shift and also show that very high TTA efficiencies can be achieved even in the absence of large driving energies for the TTA process.
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Lead-halide perovskites are a family of semiconductor materials with excellent optoelectronic properties ideally suited for various light-emitting applications beyond solar cells. Particularly, inorganic perovskites CsPbX₃ nanostructures are drawing increasing research interests because of their better stability and prior properties. Comparing to conventional semiconductors, these halide perovskites present a unique soft and dynamical ionic lattice. Owing to facile ion migration, anion exchange chemistry was demonstrated in CsPbX₃ nanostructures with high PLQY throughout the exchange reaction process. Via developing a novel localized anion exchange method, we demonstrate spatially resolved multi-color CsPbX₃ nanowire heterojunctions. These perovskite heterojunctions show tunable photoluminescence over the entire visible spectrum with high resolution down to about 500 nm, which can represent key building blocks for high-resolution displays. Moreover, the intrinsic solid-solid anion interdiffusion dynamics can be visualized in these perovskite heterojunction nanowires through photoluminescence techniques. Besides, there are rich structural phase transitions in the inorganic perovskites. The non-perovskite phase with a large bandgap and poor photoactivity can be thermally-driven transformed into a meta-stable perovskite phase with a decreasing bandgap and excellent optical properties. Using in situ nanoscale cathodoluminescence (CL) microscopy, we can directly visualize the transition dynamics from a non-perovskite to a perovskite phase in single-crystal CsPbX₃ nanowires and resolve nanoscale nucleation and growth in the transient two-phase junctions. The reverse transition from perovskite phase to the non-perovskite phase can be further realized with moisture which introduces halide vacancy in the crystal lattice and lowers the kinetic barrier. These fundamental understandings can offer insightful guidelines for engineering the perovskite nanomaterials with novel functional devices.

Additive manufacturing has become a well-established manufacturing style for a range of metals and polymers. An exciting new area is the extension of additive manufacturing to include nanoscale semiconductors, which exhibit interesting properties that can differ from their bulk counterparts. Plasmas have found considerable use in the synthesis of nanoparticles due to their wide operation range which effectively accommodates the tunable properties of nanoparticles. Atmospheric pressure plasmas allow for the miniaturization of reactors easing the integration of the reactors into additive manufacturing systems that rely on an aerosol jets for deposition.
Presented here is our work on the additive manufacturing of silicon nanoparticle layers synthesized at atmospheric pressure with a nonthermal plasma. We generated a plasma with a 13.56 MHz RF power supply inside a glass capillary tube between two external ring electrodes. A silane precursor gas together with an argon background gas were flown to the reactor for the synthesis of silicon nanoparticles, with tunable size depending on gas flowrates. The plasma reactor was mounted to a programmable and motorized gantry responsible for driving the reactor through predetermined patterns in all three spatial dimensions.
While we achieved sub-millimeter linewidths with aerodynamic focusing using a tapered capillary, we also seek to exploit the innate nanoparticle charging properties of the plasma synthesis method to modify the deposition. Charge accumulation on the nanoparticles from the plasma and a high voltage power supply were used to control the linewidth of the deposition through electrostatic forces. Through this process we achieved even narrower linewidths, with thicknesses up to and exceeding 100µm as dictated by the dwell time of the reactor.
Transmission electron microscopy (TEM) and x-ray diffraction (XRD) indicated that the particle crystallinity and size can be altered in-process via controlling the supplied power and gas flowrates, translating to a dynamic control over nanoparticle properties as we deposit the predetermined patterns. Fourier-transform infrared spectroscopy (FTIR) revealed a stable surface with oxygen-and nitrogen-containing compounds passivating the Si nanoparticle surfaces; this along with the amorphous region present at the edges of crystalline nanoparticles indicates the presence of an oxynitride shell. Photoluminescence of the nanoparticles was observed at a wavelength of 705 nm upon excitation with UV radiation. The ability to synthesize and deposit the nanoparticles with property tunability in-process points to the promise of this approach for additive manufacturing using plasmas.

A new model all-inorganic Ruddlesden-Popper phases perovskite (AIRPP) Cs_n+1B_nX_3n+1 (n = 1, 2, …) exhibits a more stable crystal structure in the ambient atmosphere and provides a unique quantum well structure separated by inorganic spacer (AX, A= Cs⁺ X= Cl^-, Br^- and I^-). However, a mixed halide AIRPP Cs₂PbI₂Cl₂ exhibited a large band gap and display strong UV-light response in photodetectors. Therefore, it is still very important topic developing single halide AIRPP with the tunable optical properties in visible light range. In this study, we demonstrated phase stable quasi-2D all-inorganic Ruddlesden-Popper phases perovskite (AIRPP) Cs_n+1Pb_nBr_3n+1 nanosheets (n=3, 4) with single halide stabilized by additional stronger bounding surface ligand octadecanedioic acid (ODA). The pure phase of quasi-2D AIRPP nanosheets are confirmed by STEM, HR-TEM images and XRD measurements. The different n thickness and layer structure are also identified by density functional theory (DFT). From the optical characterization, the bandgap of Cs_n+1Pb_nBr_3n+1 nanosheets (n=3) was found to be ~2.7eV with photoluminescence quantum yield (PLQY) exceeding 40%. The film of Cs_n+1Pb_nBr_3n+1 nanosheets (n=3) also displays strong visible light photoresponse at a wavelength of 460nm. Moreover, these series of single halide AIRPP exhibited long-term stability over three months in ambient condition.

Electro-chemiluminescence (ECL) displays using electrochemical reaction are attracting attention owing to their remarkable advantages, such as simple structure, ultra-thinning, and use of electrodes without work function limitations. However, despite these advantages of ECL displays, a significant limitation still exists, which is the simultaneous realization of red (R), green (G), and blue (B) emissions with excellent emission characteristics. Related studies reported so far have succeeded in improving the individual emission characteristics of R, G, and B ECL display, but have not implemented all colors simultaneously. In order to implement next-generation displays, it is necessary to improve the B emission characteristics and simultaneously realize emission of the three primary colors of R, G, and B.
In this study, ion gel-based flexible ECL full-color display has been successfully demonstrated by overcoming the limitations of simultaneous implementation of R, G, and B emissions. The ECL display was implemented using Ru(bpy)₃(PF₆)₂, [Ir(Fppy)₂(dmb)][PF₆], and FIrMepic corresponding to R, G, and B emissions, respectively. In particular, to achieve improved B ECL displays, the blended blue (BB) ECL display was designed using a mixed-metal chelate coreactant system with mixing of G and B luminophores. The BB ECL display shows significantly improved properties in terms of the operating voltage, luminance, and stability. Furthermore, even after dynamic bending tests of 5000 cycles at a radius of curvature of 10 mm, the flexible ECL display shows stable emission properties. The optical and electrochemical properties of ECL gels were measured using a PL measurement system and a cyclic voltammogram measurement. To confirm the emission of the ECL display, the AC voltage generated by the function/arbitrary waveform generator was used. The EL spectra, luminance, and color coordinates were observed using an EL measurement system and chroma meter.

Keywords: electro-chemiluminescence, ion gel, mixed-metal chelate system, multi-color implementation, flexible display.

Colloidal lead halide perovskite nanocrystals, which are solution-processable, color-pure, tunable and highly luminescent, are newly arising semiconductor nanomaterials for light- emitting applications. And among nanocrystals with various geometries, quantum- and dielectric-confined two-dimensional colloidal lead halide perovskite nanoplatelet is one of the most favorable candidates for the next-generation light-emitting applications.
In this work, we demonstrate a facile room-temperature synthesis of colloidal perovskite nanoplatelet (Chemical formula: L₂[ABX₃]_n-1BX₄, L: organic alkylammonium ligands, A: methylammonium or formamidinium, B: lead, X: bromide and iodide, n: number of [BX₆]^4- octahedral layers in the direction of nanoplatelet thickness) via ligand-assisted reprecipitation. We also show that the band gap of the nanoplatelets can be continuously tuned throughout the visible range by halide composition engineering. Then it is further demonstrated that various organic species can be incorporated as surface-capping ligands without any major modifications in the synthetic protocols, opening up the possibility of optimizing the surface chemistry for various applications. Then we focus on the stability of deep-blue luminescent (λ_max = 437 nm) nanoplatelet species, which is essential for achieving wide color gamut, and systematically investigate the factors that affect the photostability¹. Photobleaching is found to be coming from intrinsic instability of the perovskite nanoplatelet lattice against UV irradiation, while moisture triggers the transformation of nanoplatelets into thicker, less-confined structures. It is further shown that the substitution of methylammonium for formamidinium and the addition of excess alkylammonium bromide ligands can effectively enhance both the photo- and ambient stability of the nanoplatelets. And lastly, we demonstrate stable color-pure deep-blue luminescence from the dropcasted film of methylammonium lead bromide nanoplatelets.

1. Ha, S. K.; Mauck, C. M.; Tisdale, W. A., Toward Stable Deep-Blue Luminescent Colloidal Lead Halide Perovskite Nanoplatelets: Systematic Photostability Investigation. Chem. Mater. 2019, 31 (7), 2486-2496.

We represent comparative study on core/shell heterostructure nanocrystals based on III-V/II-VI semiconductor with InP/Zinc chalcogenide geometry that show high photoluminescence quantum yield (>80%) with narrow emission linewidth (~45 nm). We compared the optical properties of nanocrystals with different interfacial geometries before growth of zinc chalcogenide shell. The comparative studies based on spectroscopic analysis and elemental analysis by XPS and ICP-OES reveals that non-radiative traps in the InP nanocrystals are effectively suppressed by controlled interface and surface. Controlled interface of InP/zinc chalcogenide nanocrystal produce energetically favorable interface without net charge, which is called "neutral interface", to successfully confine the electron and hole wavefunction into the InP nanocrystals. Therefore, the III-V/II-VI heterovalent heterostructures show high PL QY of InP based core/shell heterostructures.

The ionic crystals of perovskites have been widely investigated for application in light-emitting devices (LEDs) due to their adjustable optical bandgap by changing the composition of the halide anions. However, small ion size of Cs⁺ in inorganic perovskite causes poor crystallinity and small exciton binding energy owing to the inherent defects present in the bulk perovskite hinders the luminescence efficiency of perovskite LEDs (PeLEDs). Moreover, a cesium lead iodide (CsPbI₃) which is studied as an emitter of red PeLED is not only required to be heat-treated at a high temperature of 200 °C for phase stabilization but also vulnerable to oxygen and moisture in the air. So, in order to improve the crystallinity and prevent the phase separation of perovskite, it is preferable to use mixture of Cs⁺ and methylammonium (MA) or formamidinium (FA) cation. Furthermore, by adding hydrophilic polymer into the perovskite precursor, the crystal size can be reduced. So, a flat and uniform perovskite film can be formed, leading to decrease of non-radiative defects and leakage current. The phase-stabilized mixed-cation perovskite with hydrophilic polymer not only improve the crystallinity of the perovskite structure but also prevent the phase separation of perovskite.
In this study, to improve the crystallinity and prevent the phase separation of perovskite, Cs_xMA_1-xPbI₃ films were fabricated as mixed-cation perovskite by adding methylammonium iodide (MAI) and poly(2-ethyl-2-oxazoline) (PEOXA) as a hydrophilic polymer into the CsPbI₃ precursor. The optimized Cs_xMA_1-xPbI₃-PEOXA film deposited on poly-TPD showed a uniform surface coverage with a crystal size and thickness of 28.7 nm and 114.2 nm, respectively, and exhibited a strong electroluminescence peak centered at 662 nm. Furthermore, Cs_xMA_1-xPbI₃-PEOXA-based red PeLED showed stable device performance at 24 °C under 20.3% relative humidity for 3 days. Therefore, it is believed that the PeLED with a Cs_xMA_1-xPbI₃-PEOXA film can be a good candidate for red light-emitting PeLEDs.

The design and exploration of high stable organic luminescent materials with high luminous efficiency has become an important research area in the frontier of chemistry and materials science. Iridium complexes are one of the best phosphorescent materials. This project aims to synthesize a new "3+2+1" dentate coordination type of luminescent iridium complex, allowing the introduction of a specific ligand in a controlled way, controlling the luminescence properties of iridium complex through appropriate selection of ligand. Finally, a green emissive iridium complex has been synthesized, and the photophysical studies reveals it display a structured emission bands at 474 and 500 nm. These complexes display structural stability at ambient temperatures and quantum yields greater than 30%.
In addition to the study of physical properties, we cast this green phosphorescent material as phosphor powders on a 460 nm blue chip to form light-emitting diodes. It was found that the LED has realized pure green emitting and the Commission Internationale Ed I’Eclairage coordinate of the fabricated green LEDs was (0.3,0.6), Moreover, the green LEDs displayed luminous efficiency of 5 lm/W under an injection current of 30 mA.

In conventional organic light emitting diode (OLED) as well as perovskite light emitting diode (PeLED) device, polyvinyl carbazole (PVK) and Poly-TPD have been widely used as a hole transporting layer (HTL) in the past. However, PVK and Poly-TPD have disadvantage in terms of soluble property limitation during stacked-soluble process device preparation. So, more research is needed in terms of materials to apply these materials to perovskite devices. Therefore, we synthesized new polymer HTL material, PBCZCZ for PeLED device. It has non-soluble property to general solvent after thermal treatment and curing process. For the surface analysis of PBCZCZ and PVK, contact angle and AFM analysis were carried out. Contact angle of PBCZCZ is about 5 ° C lower than that (76 °) of PVK, and AFM analysis shows similar surface roughness. Also, we found that PBCZCZ achieves much higher mobility than PVK. The structure of PBCZCZ is that two new functional groups are added to the carbazole group. The device configuration is ITO/PEDOT:PSS(40nm)/PBCZCZ(15nm)/CsPbBr₃/TPBi(30nm)/LiF(1nm)/Al(100nm). When new polymer HTL material were used, it showed high luminance efficiency of 34.81 cd/A and EQE of 4.01%.

Room temperature (RT) synthesis of perovskite nanocrystals (NCs) is typically achieved employing a ligand-assisted reprecipitation (LARP) method that is compared to the hot-injection (HI) approach cost effective and industrially friendly. However, the method has many drawbacks that can hinder large scale production of NCs. In this work, we report an amine and an oleic acid free synthesis of lead bromide perovskite NCs using a combination of trioctylphosphine oxide (TOPO) and diisooctylphosphinic acid (TMPPA) ligands. The alternative combination of ligands allows us to synthesize NCs at room temperature (RT) and in air using industrially friendly solvents. We show how our approach can be used to synthesize fully inorganic, CsPbBr₃ NCs and hybrid organic-inorganic FAPbBr₃ (FA = formamidinium) NCs with the PL emission (fwhm ≈20 nm) between 530 – 535 nm which is in line with the Rec. 2020 color standards. Moreover, using spectroscopy techniques we investigated ligand interactions with the NC surfaces and find that the ligands used in this study are bond via a Pb-O-P bond. In addition, we show that compared to a typically used ligand combination of carboxylic acid and amines, phosphine ligands can be easier removed from the NC surfaces.

Lead halide perovskite (CsPbX₃, X = Cl, Br, I) quantum dots (QDs) have great potential for light-emitting devices (LED) owing to their high color purity and ease of solution processability^[1-3]. The mix-halide perovskite QDs can be controlled their emission wavelength, CsPb(Cl/Br)₃ for blue region by adjustment halide composition.
Here, we demonstrate anion-exchange blue perovskite QDs CsPb(Cl/Br)₃ from pristine CsPbBr₃ using metal chloride. CsPbBr₃ was synthesized by the general hot-injection method. The metal chloride was added into CsPbBr₃ solution. The anion-exchange CsPb(Br/Cl)₃ exhibit a strong blue-shift in their of photoluminescence (PL) spectrum and UV-vis absorption spectra. The PL peaks of the anion-exchange blue perovskite exhibited a blue-shift from 508 nm (pristine) to 470 nm owing to the replacement of Br^– anions by Cl^– anions in the perovskite QDs. Furthermore, the anion-exchange blue perovskite in toluene dispersion exhibited high PLQYs of about 90%. To investigate the reason for increase of PLQY, we performed X-ray photoelectron spectroscopy (XPS). We found the presence of anion defects causing non-radiative recombination in the pristine CsPbBr_3. On the other hand, anion defects ware suppressed in the anion-exchange blue perovskite. Blue CsPb(Cl/Br)₃ LED with anion exchange showed external quantum efficiency (EQE) of over 1%.

References : [1] L. Protesescu et al., Nano Lett. 15, 3692 (2015). [2] G. Nedelcuet et al., Nano Lett. 15, 5635 (2015). [3] T. Chiba et al., Nat. Photon. 12, 681 (2018).

The discovery of a myriad of two-dimensional (2D) materials has triggered enormous efforts to integrate distinct monolayers into van der Waals heterostructure for integrated nano-photonics and optoelectronics devices. Mixed dimensional (0D-2D) heterostructures have been previously studied only in relevance to interface charge transport, energy transfer, and photosensitization. In this work, we leverage on mixed dimensional van der Waals heterostructure and fabricate a vertical field-effect device using a combination of 2D and 3D materials to demonstrate quantum-confined Stark effect (QCSE) in highly luminescent perovskite-based (Cesium lead bromide, CsPbBr3) colloidal quantum dots (QDs) with strong excitonic properties. Quantum-confined Stark effect (QCSE) is a basic feature of semiconductor nanostructures that characterizes the change in optical response on applying an electric field perpendicular to the structure.
In our device, few layers of h-BN (hexagonal boron nitride) and monolayer graphene serve as the dielectric and electrical contacts respectively. The QDs are deposited in the center of the heterostructure. We study the photoluminescence (PL) from the perovskite quantum dots embedded within the heterostructure at 4K. Application of a DC voltage in pre-determined step leads to a shift in the PL emission energy. This shift can be reversed by sweeping the voltage back to zero. The maximum observed energy shift is about 30 meV. The spectral shift is also accompanied by a change in intensity and linewidth of the QD which is the hallmark of the QCSE.
In conclusion, we have demonstrated QCSE in perovskite-based CsPbBr₃ QDs by embedding them in a vertically stacked van der Waals heterostructure. Using 2D materials for QCSE based modulators makes it highly flexible and easy to integrate with any existing device. In future, such device concept can be used to resonantly tune the transition of a QD for strong coupling with another local emitter or a nano-cavity mode. Further, active feedback from such devices can also reduce slow spectral diffusion in QDs.

Lead halide perovskites have been demonstrated as high performance materials in solar cells and light-emitting devices. These materials are characterized by coherent band transport expected from crystalline semiconductors, but dielectric responses and phonon dynamics typical of liquids. Here we explain the essential physics in this class of materials based on their dielectric functions and dynamic symmetry breaking on nano scales. We show that the dielectric function in the THz region may lead to dynamic and local ordering of polar nano domains by an extra electron or hole, resulting a quasiparticle which we call a ferroelectric large polaron, a concept similar to solvation in chemistry. The collective nature of polarization in a ferroelectric large polaron may give rise to order(s)-of-magnitude larger reduction in the Coulomb potential. We develop Fourier transform coherent phonon spectroscopy to directly probe the energetics and local phonon responses of ferroelectric polarons. The ferroelectric polaron may explain the defect tolerance and low recombination rates of charge carriers in lead halide perovskites, as well as providing a design principle of the “perfect” semiconductor for solar cells.

Chemically prepared colloidal semiconductor quantum dots have long been proposed as scalable and color-tunable single emitters in quantum optics, but they have typically suffered from prohibitively incoherent emission. We now demonstrate that individual colloidal lead halide perovskite quantum dots (PQDs) display highly efficient single photon emission with optical coherence times as long as 80 ps, an appreciable fraction of their 210 ps radiative lifetimes [1]. These measurements suggest that PQDs should be explored as building blocks in sources of indistinguishable single photons and entangled photon pairs. Our results present a starting point for the rational design of lead halide perovskite-based quantum emitters with fast emission, wide spectral-tunability, scalable production, and which benefit from the hybrid-integration with nano-photonic components that has been demonstrated for colloidal materials.

[1] Utzat et al. Science, 2019, 363 (6431), pp. 1068-1072.

The extraordinary optoelectronic properties of metal halide perovskite semiconductors allowed the development of highly-efficient light-harnessing and light-emitting diodes based on this new generation of functional semiconductor materials. The outstanding power-conversion efficiencies shown by perovskite solar cells (PSCs) and remarkable external quantum efficiency displayed by perovskite-based light-emitting devices (PeLEDs) are realized, respectively, due to low photovoltage deficits and relatively high photoluminescence quantum yields. Therefore, it has become very important to investigate and understand the causation of high photoluminescence quantum yields, which translate into low-voltage losses in PSCs, and high external quantum efficiencies of PeLEDs. In my presentation, I will discuss the fabrication of high-performance mixed-cation perovskite-based LEDs and solar cells with an active layer based on a new composition. The improved external quantum efficiencies and photovoltages are obtained by improving the optoelectronic quality of the active layer, optimizing the charge extraction layers, and tuning the interfacial properties which in turn boasts the photoluminescence quantum efficiency, reducing the non-radiative recombination loss and thus enabling balance charge injection. The causation of high internal and external quantum efficiencies and photovoltage was investigated using various structural (Synchrotron-based x-ray scattering), morphological and spectroscopic (photothermal deflection absorption spectroscopy, photoluminescence quantum yield measurements, time-integrated and time-resolved photoluminescence) characterization techniques. The detailed insights gained through these various techniques will be discussed.

References
1. N. Arora, M. Ibrahim. Dar, R. H. Friend. et al. Efficient Perovskite Light Emitting Devices: Insights from Synchrotron X-ray Scattering and detailed Optoelectronic Studies. (Manuscript under preparation).

2. N. Arora, M. Ibrahim Dar, A. Hinderhofer, N. Pellet, F. Schreiber, S. M. Zakeeruddin, M. Grätzel. Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies greater than 20%. Science 2017, 358, 768-771. DOI: 10.1126/science.aam5655

3. N. Arora, M. Ibrahim Dar, M. Abdi-Jalebi, F. Giordano, N. Pellet, G. Jacopin, R. H. Friend, S. M. Zakeeruddin, M. Grätzel. Intrinsic and Extrinsic Stability of Formamidinium Lead Bromide Perovskite Solar Cells Yielding High Photovoltage. Nano Letters 2016, 16, 7155–7162. DOI: 10.1021/acs.nanolett.6b03455

Metal halide perovskites have been attracting great attention during the past few years as emerging optoelectronic materials in solar cells and light emitting devices (LEDs). Perovskite LEDs include both narrowband single color light emission and broadband white light emission. Recently, broadband white-light emission has been observed in one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) organic-inorganic lead halide perovskites. In addition to the broadband feature, the photoluminescence (PL) spectra of these materials show a massive Stokes shift compared to the corresponding absorption onset. The bandwidths of PL spectra are related to the strength of electron-phonon coupling. While it is weak in narrow band emission, it is strong in broadband emission forming self-trapped excitons (STEs). Interestingly, it has been observed in only certain three-dimensional and low-dimensional metal halide perovskites. In this talk, we will show by density-functional theory calculation that multiple STE structures exist in each perovskite exhibiting broadband emission. However, only the STE with Jahn-Teller like octahedral distortion is mainly responsible for the observed broadband emission, though it may not be the lowest energy structure. Both narrowband and broadband emission can be very efficient, which is attributed to the unique defect properties in metal halide perovskites, i.e., the dominant defects only create shallow levels in the bandgap.

Tin-based perovskites have long remained a side topic in current perovskite opto-electronic research. With the recent efficiency improvement in thin film solar cells [1] and the observation of a long hot carrier cooling time in formamidinium tin iodide (FASnI3) [2], a thorough understanding of the material's photophysics becomes a pressing matter. Since pronounced background doping can easily obscure the actual material properties, it is of paramount importance to understand how different processing conditions affect the observed behavior. Using photoluminescence spectroscopy, we therefore investigated thin films of FASnI3 fabricated through different protocols. We show that hot carrier relaxation occurs much faster in highly p-doped films due to carrier-carrier scattering. From high quality thin films, we extract the longitudinal optical phonon energy and the electron-phonon coupling constant, which are fundamental to understand carrier cooling. Importantly, high quality films allow
for the observation of a previously unreported state of microsecond lifetime at lower energy in FASnI3, that has important consequences for the discussion of long-lived emission features in the field of metal halide perovskites.

[1] S. Shao, J. Liu, G. Portale, H.-H. Fang, G. R. Blake, G. H. ten Brink, L. J. A. Koster, M. A. Loi. Adv. Energy Mater. 8, 1702019, 2018; E. Jokar, C.-H. Chien, C.-M. Tsai, A. Fathi, and E. W.-G. Diau. Adv. Mater., 31, 1804385, 2019.

[2] H.-H. Fang, S. Adjokatse, S. Shao, J. Even, M. A. Loi. Nat. Commun. 9, 243, 2018.

Due to their characteristic elongated shape and reflective end-facets, semiconductor nanowires can be used as Fabry-Perot resonators to fabricate nanolasers. As a way of studying the key elements for lasing operation in these devices, we have developed and successfully employed a large scale optical technique to study interwire functional inhomogeneity. Taking advantage of this novel method we are able to locate and optically study a large number of nanolasers by -Photoluminescence (Figure 1). Through a statistical comparison, different parameters such as nanowire length, material inhomogeneity, lasing wavelength etc. are correlated to identify which mechanisms contribute to low-power lasing. We discuss two applications of this approach; nanowires with multi-quantum-well GaAs/AlGaAs active region [1], and p-doped GaAs nanowires [2].

Taking advantage of a large scale optical technique, we have identified key parameters which would improve optoelectronic efficiency and potentially decrease the spread in performance of semiconductor nanowires. Also, we were able to find the best performing nanowire for single device applications. This approach is of industrial value, as well as providing identification of best operating nanolasers for the fabrication of devices for fundamental study.

[1] J. A. Alanis et al. “Large-scale statistics for threshold optimization of optically pumped nanowire lasers" Nano Letters, no. 17, pp. 4860-4865, 2017.
[2] J. A. Alanis et al. “Optical Study of p-Doping in GaAs Nanowires for Low-Threshold and High-Yield Lasing“, Nano Letters, no. 19, pp. 362-368, 2019.

Excitons are the dominant photo-excited species in 2D layered perovskites and the early-time photoexcited species in 3D bulk metal-halide perovskites. Strong spin orbit coupling in these materials enables optical excitation of excitonic states with a defined total angular momentum and the generation of circularly-polarised emission from polarised spin-states. Here, we use circularly-polarised broadband transient absorption and luminescence spectroscopy to study spin-dependent exciton interactions and radiative recombination in the two-dimensional (2D) Ruddlesden-Popper perovskites (BA)₂(MA/FA/Cs)₁Pb₂I₇ and 3D bulk MAPbBr₃. We spectrally resolve an ultrafast dynamic circular dichroism in the optical response that we discuss to arise from a photoinduced polarisation in the electronic state total angular momentum quantum number. We show that the optically-active states are split in energy via spin-dependent exchange interactions between excitons following photoexcitation, giving rise to shifts in the luminescence spectrum. We report that the room temperature exciton spin lifetimes in 2D metal-halide perovskites increases at low carrier densities, which we discuss as a spin scattering mechanism through Elliott-Yafet processes. Detailed analysis of the initial depolarisation kinetics indicate that the rate of carrier spin relaxation is reduced following the formation of ground state exciton from which luminescence occurs. Our results provide a fundamental picture of the role of exciton-exciton interactions in determining room temperature spin lifetimes and luminescence polarisation, highlighting the potential of hybrid perovskites for applications in spin-polarised light-emitting devices.

Strong quantum confinement unique to colloidal semiconductor quantum dots (QDs) significantly enhances spin exchange interactions between band carriers and Mn dopants. In this case, a single exciton can exert an effective exchange field of a few tesla on the embedded magnetic dopants, which leads to the formation of zero-dimensional magnetic polarons.^1,2 The strong spin exchange also enables extremely fast energy transfer between the excited Mn ion and the QD exciton which allows one to effectively manipulate nonequilibrium ‘hot’ carriers prior to their relaxation into the band-edge levels.³ This process can also be utilized to generate hot electrons with weak cw optical excitation.⁴
Mn-doped CdSe QDs represent attractive structures for studies of the effect of resonant versus off-resonant exchange interactions on photoexcited electronic dynamics in a coupled host-semiconductor/magnetic-ion system. However, a colloidal synthesis of high quality Cd_1-xMn_xSe QDs is still challenging. Among existing synthetic methods, the diffusion doping yields simultaneously the highest Mn²⁺ content and good size uniformity.⁵ This methods exploits a gradient between Mn²⁺ chemical potentials in the solution and the QD phases for making incorporation of magnetic ions into QDs thermodynamically favorable. In the present study, we aim to improve the Mn diffusion doping method by controlling the chemical potentials of Cd²⁺ in solution. In particular, during the initial synthesis of CdSe QDs, we use secondary phosphines for increasing the nucleation rate and thereby improving the consumption of the Cd precursor. This helps form a pure MnSe shell on top of the CdSe cores upon introduction of a Mn source without producing a Cd_1-xMn_xSe alloyed layer. This leads to the improved monodispersity of the resulting doped particles as indicated by the reduction of the absorption peak widening to less than 7 meV instead of >20 meV observed for the previous syntheses.⁵ Further, the removal of excess Cd from the reaction mixture by post-treatment with oleic acid accelerates incorporation of Mn²⁺ into the CdSe lattice by facilitating creation of Cd vacancies due to diffusion of Cd ions towards the QD surface. As a result of fast Mn incorporation, we can induce a 100-meV shift of the absorption edge in just 5 min of the reaction, which is ~50 times faster than for the previously reported procedures.⁵ This new method allows us to tune the QD band gap by 600 meV, which is accompanied by the dramatic change in the strength of the sp-d exchange coupling between the magnetic dopants and the semiconductor host. By applying CdS shelling to the doped structures, we boost their room-temperature emission efficiencies to more than 60%. By varying the QD size and the level of Mn doping, we observe a strong effect of tunable exchange interactions on electronic dynamics on a wide range of time scales from hundreds of femtoseconds to hundreds of nanoseconds.

1. W. D. Rice, W. Liu, T. A. Baker, N. A. Sinitsy, V. I. Klimov, S. A. Crooker, Nat. Nanotech. 2015, 11, 137-142.
2. R. Beaulac, L. Schneider, P. I. Archer, G. Bacher, D. R. Gamelin, Science 2009, 325, 973-976.
3. R. Singh, W. Liu, J. Lim, I. Robel, V. I. Klimov, submitted, 2019.
4. Y. Dong, J. Choi, H. Jeong, D. H. Son, J. Am. Chem. Soc. 2015, 137, 5549-5554.
5. W. A. Vlaskin, C. J. Barrows, C. S. Erickson, D. R. Gamelin, J. Am. Chem. Soc. 2013, 135, 14380-14389.

Luminescence properties of novel light-emitting materials are determined by the architecture and function of the respective devices and directly relate to the physical and chemical nature and quality of these materials. The understanding of photophysical processes as well as structure-property relationship of the new classes of light emitting materials are important steps toward the optimization of their properties to emit specific light for practical applications in different devices.

Time-resolved fluorescence spectroscopy is a modern and most valuable tool to investigate excited state dynamics in molecules, complexes, or semiconductors, which affect the light emitting properties of materials. This type of measurement technique became more and more popular in many scientific fields, including chemistry, biology, physics, as well as in life, materials or environmental sciences and is the ideal method for measuring also weak luminescence.

The combination of time-resolved luminescence spectroscopy with microscopic techniques is without doubt a highly modern and powerful kind of material investigation. Advantages of both techniques are combined in a variable, precise and powerful kind to investigate the photoinduced photophysical processes as well as structural properties of novel light-emitting materials.

Here we will demonstrate the performance of a spectrometer-microscope assembly for characterization and analysis of different light-emitting materials (e.g., halide perovskite, quantum dots and organic-inorganic nanomaterials) in terms of time resolution, ability to measure long decays (e.g., phosphorescence, delayed fluorescence) and record time-gated spectra using laser drivers with burst capabilities. Further more we present different kind of hardware as well as handling optimization for a combination of a state of the art photoluminescence spectrometer with a confocal upright microscope for steady-state, anisotropy, and time-resolved measurements. With this we give prove, that this multi-dimensional approach can be easily and successfully used to exploited investigations of e.g. photoinduced charge carrier dynamics in these materials and its correlation with the localized inhomogeneities and defect sites. Furthermore, surface effects, energy transfer transfer processes as well as the influence of dopants, impurities and defect sites on the luminescence properties of emitters will be discussed.

Metal halide perovskites have received remarkable attention as perovskite photovoltaic (PV) devices. These have already achieved unprecedented higher power conversion efficiency (PCE of 24%) approaching silicon-based PV (PCE of 27%). However, these outstanding optical efficiencies can only be realized by lead-based perovskites and the devices are chemically unstable in air and moisture. Therefore, the key to the large-scale production of perovskite-based solar cell will come down to address their “toxicity” and instability problems. In our research, we are taking up the challenge to replace lead with nontoxic or less toxic element, Sn, and partially or even completely substitute Sn with Ge. For stability, mostly inorganic elements will be chosen. In addition, synthesis of quantum dot (QD) of these materials with water-resistant shell encapsulation to further stabilize the perovskite material will be investigated. The QDs will be synthesized with varying size and compositions to cover the whole UV-VIS spectrum.

We demonstrated bar-coating followed by a fast solvent evaporation process to yield uniform, fully-covered and thickness-controllable films of metal halide perovskite (MHP) nanoparticles (NPs). Based on optimized device structure with uniformly printed MHP NPs, highly efficient perovskite light-emitting diodes (PeLEDs) were achieved. Large-area and active-matrix PeLEDs were also demonstrated. We also analyze the ion-migration and non-radiative recombination pathways in NP films by measuring capacitance, transient electroluminescence and magnetic-field dependent characteristics of NP based PeLEDs and NP films. This work provides a promising way to toward development of MHP emitters in large-scale industrial displays and solid-state lighting, and suggests analysis methods that can be useful for other perovskite optoelectronic devices.

Endowing quantum dots (QDs) with robustness and durability has been one of the most important area of research in this field, since the major limitations of QDs in practical applications are their thermal and oxidative instabilities. In this work, we propose a facile and effective passivation method to enhance the photochemical stability of QDs by structurally designing polymeric double shell network of thiol-terminated poly(methyl methacrylate-b-glycidyl methacrylate) (P(MMA-b-GMA)-SH) block copolymers ligated to QDs. To achieve this, the cross-linking reaction of GMA epoxides in PGMA block was conducted using Lewis acid catalyst under an ambient environment in order not to affect the photophysical properties of pristine QDs, which provides QDs with robust double layers consisting of highly weatherable and transparent PMMA outer-shell and oxidation-protective crosslinked inner-shell. Consequently, the resulting QDs surrounded by crosslinked double shell layers exhibited exceptional tolerance to heat and oxidants when dispersed in organic solvents or QD-nanocomposite films, as evidently corroborated under various harsh conditions with respect to temperature and oxidant species. In addition, it was shown that the outer polymer brush can provide the miscibility of resulting QDs with host polymer matrix to fabricate a well-defined QD-nanocomposite films via cost-effective solution process. The present approach not only provides simple yet effective chemical means to enhance the thermochemical stability of QDs, but also offers a promising platform for the hybridization of QDs with polymeric materials for developing flexible light-emitting or light-harvesting devices.

Studies conducted on the stability of perovskite nanocrystals have begun to aid understanding of how to handle these emerging materials. Through extensive experimental and computational studies, a consensus has emerged in the community surrounding the crucial nature of the surface of the nanocrystals and the complex, dynamic ligand shell around them. Different ligands have been show to both improve photoluminescence quantum yield to near unity, and bolster stability to washing and colloidal lifetimes. The binding groups of the ligands and their introduction in the synthesis or as a post treatment have been highlighted time and again.

To date these impressive developments have mostly focused on cesium lead bromide, CsPbBr₃. The perovskite family extends its compositional range in both the cation and anion sites, and the translation of successes is not straightforward. In this work, we investigate the fabrication of formamidinium lead bromide, FAPbBr₃, via a phosphine oxide assisted synthesis and the challenges faced when applying lessons learnt from the all inorganic perovskite to this system. Through nuclear magnetic resonance spectroscopy coupled with optical characterisation we demonstrate the influence of the labile formamidinium protons on the surface chemistry of the nanocrystals and how this in turn changes their stability, structural integrity and effects the reaction pathway in the formation of these nanocrystals compared to their inorganic counter-part.

Colloidal semiconductor quantum dots (QDs) have great potential as optical gain media for realizing solution processable lasers with readily tunable emission wavelengths. A present challenge in this area is the demonstration of electrically pumped lasing devices. Recent advances towards this goal include the realization of optical gain with direct current (d.c.) electrical pumping [1], the demonstration of high brightness QD light emission diodes (LEDs) [2], and the development of effective strategies for tackling the efficiency ‘droop’ problem [3]. Next tasks include pushing current densities to ~100 A cm^-2 (or above) levels and incorporation of an optical resonator into a QD-LED device. Here, we address the first of these challenges by practically demonstrating pulsed QD-LEDs that achieve ultrahigh current densities exceeding 1,000 A cm^-2. This allows us to inject more than 10 excitons per dot and thereby realize population inversion of both the ground-state (1S) and the excited-state (1P) transitions.
This demonstration exploits continuously graded QDs (cg-QDs) embedded into a ‘current focusing’ p-i-n LED [1] actuated using short-pulse electrical bias. As we demonstrated previously, cg-QDs exhibit strong suppression of nonradiative Auger decay [1, 4], which is essential for achieving highly emissive multiexciton states. These dots are integrated into an inverted p-i-n LED with hybrid (inorganic/organic) charge transport layers. An interlayer of an insulating material with a narrow (~50 μm) gap is deposited on top of the QDs to limit the injection area and thereby increase current density (J). Using this approach, we achieve J of ~50 A cm^-2 with d.c. current excitation. We able to push current density to ~600 A cm^-2 using 5 μs voltage pulses, and increase it further to >1,000 A cm^-2 with 1 μs pulses. Using these ultrahigh current densities, we are able to realize an unusual electroluminescence (EL) regime when the intensity of the 1P band exceeds that of the 1S emission. Based on the quantitative analysis of the EL spectra and comparison with optically excited photoluminescence, we conclude that using electrical pumping we are able to excite at least 10 (and potentially >20) excitons per dots. Such excitation levels greatly exceed the thresholds for both 1S and 1P optical gain. Importantly, these extremely high current densities are realized using a fairly thick active medium comprising up to 4 QD monolayers. We further demonstrate that the layers of this thickness are capable of lasing if combined with optimized distributed feedback cavities. Not only these results prove the feasibility of electrically pumped lasing with colloidal QDs but they also suggest that the goal of practically realizing such devices is within close reach.
1. J. Lim, Y.-S. Park and V. I. Klimov, Optical gain in colloidal quantum dots achieved with direct-current electrical pumping, Nat. Mater. 17, 42-49 (2018)
2. H. Shen, Q. Gao, Y. Zhang, Y. Lin, Q. Lin, Z. Li, L. Chen, Z. Zeng, X. Li, Y. Jia, S. Wang, Z. Du, L. S. Li and Z. Zhang, Visible quantum dot light-emitting diodes with simultaneous high brightness and efficiency, Nat. photon. 13, 192-197 (2019)
3. J. Lim, Y.-S. Park, K. Wu, H. J. Yun, and V. I. Klimov, Droop-free colloidal quantum dot light emitting diodes, Nano Lett. 18, 6645–6653 (2018)
4. K. Wu, Y.-S. Park, J. Lim and V. I. Klimov, Toward zero-threshold optical gain using charged semiconductor quantum dots, Nat. Nanotechnol. 12, 1140-1147 (2017)

Metal halide perovskite nanocrystals (NCs) are promising materials for use in optoelectronics. However, further improvements in stability, reproducibility, and photoluminescence quantum yield (Φ_PL) are essential for enabling commercial applications. Inadequate surface passivation is a major cause of instability, irreproducibility, and low Φ_PL. We find that post-synthetic treatment with alkanethiols reproducibly yields stable NCs with near unity Φ_PL for a range of synthetic conditions and varying initial Φ_PL of the as-synthesized NCs. A mechanistic investigation shows that thiol addition leads to thioether formation via the thiol-ene reaction with octadecene, oleic acid, and oleylamine. Both thiolates and thioethers are suspected to bind to undercoordinated Pb atoms on the NC surfaces, and the surface binding and Φ_PL increases can be rapidly accelerated through exposure to blue or UV light. Furthermore, we show that metallic Pb nanoparticles appear in many batches of synthesized CsPbBr₃ NCs and that dodecanethiol addition eliminates these metallic Pb particles. Applying similar alkanethiol treatment to thin films processed with varying Cs:Pb or methylammonium:Pb stoichiometry we find that alkanethiols only bind to the film surfaces when a significant excess of PbX₂ is included in the processing solution.

The interfacial phenomenon in the case of MAPbBr₃based organic-inorganic hybrid light-emitting diode (LEDs) depends upon the existence of vacant sites that enable electric-field driven ion migration which in turn causes detrimental effect on the device efficiency. A clear deeper knowledge of the fundamental interfacial mechanism in hybrid LEDs that is dependent on the material properties must be essentially brought in focus so as to improve the optoelectronic properties.
In this present study, we have attempted to bring out this interfacial phenomenal occurrences using Admittance Spectroscopy and demonstrate the fundamental mechanism in such perovskite LED and further establish a method to tune the interfacial kinetics by appropriate passivation of perovskite layer by additive engineering. Defect passivation via additive engineering improved the LED device performance incredibly evidenced by the lowering of built-in potential (V_bi) indicating faster light turn-on and reduced interface potential revealing better interfacial charge injection behavior. Moreover, the current-voltage (I-V) hysteresis behavior speculated to be arising due to ion migration is suppressed. Carrier dynamics is altered by enhancing the carrier injection due to improved interfacial conduction and noticeable change from inductive behavior to capacitive behavior due to increased carrier density. The DC bias dependences of characteristic times and carrier distribution are analyzed to understand the complex kinetic behavior of hybrid LED as it is composed of both organic and inorganic components. This clear understanding of the interfacial kinetics and carrier dynamics will enable materials scientists to establish strategic design to overcome the existing disadvantages in near future and improve the optoelectronic functionalities and properties.

Cesium lead halide perovskite nanocrystals are a highly attractive class of materials for coherent light emission, with implications for lasing, light-emitting diodes, and quantum computing. Fine-tuning their properties for the above applications requires an exact understanding of their exciton fine structure, in particular, spacing and polarization of their triplet and singlet states. Experimental reports have been controversial, implicating that the Rashba effect may be inducing an inversion in the order of bright and dark states.
To aid in the resolution of this debate, we performed an investigation of the fine structure of the triplet emission properties in these materials. Using the wave functions generated via DFT calculations including spin-orbit coupling for cubic, orthorhombic and tetragonal cesium lead halide perovskite nanocrystals of ~3 nm in diameter, we further augmented them with Coulomb coupling between the exciton configurations, to resolve the absorption and emission fine structure in a configuration interaction method.
We anticipate our work will aid in the resolution of the debated emission fine structure of CsPbBr3 nanocrystals and thereafter allow for the development of bright materials for optoelectronics.

Confined versions of metal-halide perovskites have emerged over the past years as a versatile class of semiconductors for high-performance optoelectronic devices.^[1] Despite their unique properties, this family of perovskites has however been observed to display photo-instability, especially when emitting in the blue region of the electromagnetic spectrum. Moreover, the use of ligands to mitigate agglomeration and the large density of surface defects in the material hamper charge carrier injection and radiative recombination and thus their definitive integration in real-world devices.^[2,3]

In this talk, we will present a detailed photophysical characterisation of blue emitting CsPbBr₃ perovskite nanoplatelets (NPLs) in which a controlled Br- and light-based passivation strategy is employed to improve both their optical and electrical properties. In this regard, we will introduce a new powerful technique with which we can extract the Urbach energy, absorptance, quasi-Fermi level splitting and photoluminescence quantum efficiency (PLQE) values in the materials at the nanoscale by hyperspectral wide-field imaging. The observations reveal microscale heterogeneities in the pristine NPLs films that are remarkably suppressed in the treated samples. Furthermore, the optoelectronic quality of this blue emitters is boosted by means of a high PLQE surpassing 50% and an important reduction of surface defects. Interestingly, our approach allows us to achieve fine control over the work function (as revealed by Kelvin Probe) of the systems. Finally, we fabricate LEDs based on the most efficient materials and obtain maps at the diffraction limit scale demonstrating homogeneous external quantum efficiencies close to 0.3% at 460nm, which match those at the macroscale.[4]

[1] Stranks, S. D. and Snaith, H. J. Metal-halide perovskites for photovoltaic and light-emitting devices. Nat. Nanotechnol. 2015, 10, 391.
[2] Anaya, M.; et al. Best practices for measuring emerging light-emitting diode technologies. In press.
[3] Hoye, R. L. Z.; Lai, M.-L.; Anaya, M. et al. Identifying and Reducing Interfacial Losses to Enhance Color-Pure Electroluminescence in Blue-Emitting Perovskite Nanoplatelet Light-Emitting Diodes. ACS Energy Lett. 2019, 5, 1181-1188.
[4] Anaya, M.; Frohna, K.; Shamsi, J.; Stranks, S. D. to be submitted.

Colloidal quantum dots (CQDs) have been intensely pursued for applications in flexible light-emitting displays and photovoltaics. Self-assembly of CQDs into long-range ordered 3D superlattices (SL) leads to emergence of new optical and electronic properties and offers to extend their benefits portfolio beyond those afforded by size-confinement. Technological viability of these structures requires their controlled fabrication using scalable coating methods. In this study, we explore lead sulfide (PbS) CQD self-assembly during blade-coating using time-resolved X-ray scattering and optical interferometry. Blade-coating is an established prototyping tool for slot-die coating. We monitor the nucleation and subsequent evolution of the CQD SLs during the ink-to-film transformation. A rich structural diversity is observed when tuning the CQD size, concentration, solvent and substrate temperature. SLs are found to nucleate very early in the coating stage from an FCC phase at the solvent-air interface during which the individual CQDs are randomly oriented. As the solvent nears complete evaporation, the system enters a transition stage where the FCC SL contracts and morphs into a final, dry BCC phase due to ligand packing frustration. This transition is marked by an orientational ordering of the CQDs that grows stronger with the CQD size. We also observe SL twinning during coating of the largest: 8 nm diameter CQDs. This study develops a fundamental understanding of and provides design rules for fabricating and tailoring self-assembled CQD solids via scalable coating.

To fully exploit the excellent luminescence characteristics of quantum dots (QDs), researches on EL displays based on QDs have been actively conducted. To achieve this, a multi-color QD patterning technology must be established. Processing and patterning of quantum dots involve a solution-based technique, unlike the organic light emitting materials which are typically deposited through evaporation-based technique. The applicability of QDs to various solution process allows one to carry out their deposition over a large-scale at low cost, but it also causes a problme of dissolving the bottom layer if the the secondary layer is applied by solution process consecutively. To avoid this problem, several QD patterning techniques have been developed, such as inkjet printing and microcontact printing. Here we report a simple way to obtain high-resolution patterns of InP QDs using a chemical crosslinker (bis-perfluorophenyl azide)¹ that can bind to ligands surrounding the QD surface to form a bridging network upon exposure to UV, which is referred to as the photoinduced-ligand binding agents (PiLBA). Due to the chemical durability of the cross-linked QDs, non-crosslinked portions can be etched chemically to form high resolution (<3 μm) patterns of QDs. Multi-color patterns can also be stacked laterally or vertically by repeating the same solution process (see images below). The EL characteristics of the cross-linked QDs varied systematically with the loading of the PiLBA. The simple strategy converting the QD films to be photo-resistive will make significant impact enabling the production of high-resolution, high-throughput, full-color EL displays based on QDs intensively explored in the community.

Colloidal quantum dots (QDs) which are nano-sized semiconductors, have special properties such as easy emission tunability, high color purity and solution processability. With the commercial potential of QD-based displays being confirmed, quantum dot light-emitting diodes (QLEDs) have attracted attention as next generation display technology.
Over the past several decades, many groups have studied about the core / shell structure of QDs, charge transport layers and the charge balance to improve the efficiency and lifetime of QLEDs. The performance of QLEDs has dramatically improved through the studies, but it is only limited to monochromatic QLEDs. Therefore, for commercialization of full color QLEDs, studies on white emitting QLEDs should be conducted.
Herein, we fabricated white standard structured electroluminescence (EL) devices with double emitting layers (EMLs) using red QDs and blue organic phosphorescent molecules. The double EMLs were formed by stacking blue organic molecules EML directly on the red QDs EML.
Recently, organic light-emitting diodes (OLEDs) structure with the QD color filters has received huge attention in the display industry mainly applying QD to the color converting layer. This structure is different with our white QLEDs, but a blue OLEDs is commonly used between two structures. In this study, we used bis[2-(4,6-difluorophenyl)pyridinato-C2,N]iridium(III) (FIrpic) as the blue emitting organic materials. By using FIrpic, which has a broad emission spectrum, white emission with relatively high color rendering index (CRI) was obtained with less materials, only single QDs EML and organic EML.
When the applied voltage was more than 7 V, the red and blue peaks appeared simultaneously from the EMLs. Commission Internationale de I'Eclairage graph shows that the color coordinate shifts from the red region to the white region as the voltage increases. At an applied voltage of 12 V, the device exhibited a maximum luminance of 4,619 cd/m², a peak current efficiency of 2.17 cd/A and the high CRI of 66.
Our unique QLEDs with double EMLs shows the potential for white EL devices using less materials. Therefore, this result is expected to contribute to the development of practical full color display beyond OLEDs in the near future.

All-inorganic halide perovskites (CsPbX₃, X=Cl, Br and I) have emerged as one of the most prominent materials in the application of photoelectric devices due to remarkable properties such as high photoluminescence quantum yields and tunable emitting color, but toxic Pb is not environment-friendly. Therefore, the effort to develop lead-free all-inorganic perovskite QDs becomes inevitable. Herein, we report a simply, atmospheric synthesis approach for incorporating manganese (Mn) ions intocesium lead halide (CsPbX₃) perovskite quantum dots (QDs) with high luminescence and stability. Different Mn-to-Pb feed ratios are explored to study the influence on the intensity of two emission peaks, around 400 nm and 600 nm, respectively, and the optimum luminescence up to 65% is achieved. The optical properties of as- prepared nanocrystals remain consistent even after several months. Besides, we obtain high luminescence Mn-doped blue perovskite nanocrystalsand the stability has been greatly improved in comparison to those pure blue perovskite nanocrystals. Both the orange and white light-emitting diodes (LEDs) are obtained by directly employing the as-prepared CsPbxMn_1−xCl₃ QDs as color conversion materials on a commercially available 365 nm GaN LED chip and combining the orange nanocrystals with the Mn-doped blue perovskite nanocrystals on a UV LED. The excellent optical properties of the CsPb_xMn_1−xCl₃ QDs offer great potential for the application of high performance flexible displays on composite polymer substrates.

Quantum light sources in solid-state systems are of major interest as a basic ingredient for integrated quantum device technologies. The ability to tailor quantum emission through deterministic defect engineering is of growing importance for realizing scalable quantum architectures. However, a major difficulty is that defects need to be positioned site-selectively within the solid. In this work this is overcome by controllably irradiating single-layer MoS₂ using a sub-nm focused helium ion beam to deterministically create defects. Subsequent encapsulation of the ion bombarded MoS flake with high-quality hBN reveals spectrally narrow emission lines that produce photons at optical wavelengths in an energy window of one to two hundred meV below the neutral 2D exciton of MoS2 .

Based on ab-initio calculations we interpret these emission lines as stemming from the recombination of highly localized electron-hole complexes at defect states generated by the helium ion bombardment. Using a many-body approach for dephasing due to electron phonon emission, we quantify the spatial extension of the emission centers and compare them to the results of the ab-initio calculation. [1]

[1] arXiv:1901.01042 (accepted at Nature Communications)

Electrochemical devices have attracted attention as display materials owing to their properties such as simple structure, low-power driving, and variety of electrodes irrespective of work function. Among the electrochemical materials, electrochromic materials which exhibit reversible color change by redox reactions have been used in the field of smart windows, mirrors, displays, and sensors. To utilize electrochromic materials for active-matrix display applications, an electrochromic display (ECD) requires simultaneous implementation of various colors and a fine-pixelation process.
In this study, flexible ECDs with simultaneously implementable subpixelated-EC gels by sequential multiple patterning were successfully demonstrated. Ionic liquid-based EC gels of monoheptyl-viologen, diheptyl-viologen (DHV), and diphenyl-viologen (DPV) were used to create the colors of ECDs: magenta, blue, and green, respectively. Especially, to realize an improved green color, DHV–DPV composite gels were synthesized. Three EC gels exhibited stable properties without degradation during repetitive operation. The subpixelation process for multicolored-flexible ECDs was designed to facilitate both easy fabrication and rapid operation with various patterns at low cost. The subpixelated EC gels using a film mask could be implemented to a minimum size of 200 μm. Furthermore, the subpixelated flexible ECDs exhibited high durability even after 1000 cycles of mechanical bending tests at a bending radius of 10 mm. The surface characteristics of the EC materials were confirmed by optical microscopy, and the optical characteristics were analyzed by using a UV-vis spectrometer. The redox potential of the EC gel was confirmed by cyclic voltammetry using a potentiostat. After fabrication of patterned-ECD on a flexible substrate, the analyses of optical properties and reliability test were conducted according to the applied voltage.
Keywords: viologen, ion gel, patterning, electrochromic display, flexible

Strategies for controlling and enhancing the fluorescence of molecules and nanocrystals have been the subject of intense research in the past decades [1-3]. This trend is motivated by the need to develop new highly sensitive biosensors, and by the possibility to improve optoelectronic devices like light emitting diodes and solar cells.
Here, we present a study of upconversion fluorescence enhancement effects near Au nanodisks by scanning near-field optical microscopy. The enhancement and localization of light near the metallic structures are directly visualized by using a single Er/Yb-codoped fluorescent nanocrystal [4,5] glued at the end of a sharp scanning tip. Excitation was achieved in a transmission mode by a linearly polarized laser diode (l= 975 nm) focused in the tip-sample region. Fluorescence was detected in the visible range after the absorption of two photons by up-conversion. In general, the fluorescence enhancement pattern presents two lobes aligned in the direction of the incident polarization direction, but the intensity amount and the details of the enhancement distribution strongly depend on the nanodisk size. The observed patterns are in good agreement with the near-field maps calculated by finite-difference time-domain method, both in two and three dimensions above the structures. The relationship between the far-field optical response of the nanostructures and the achieved enhancement is also analyzed. [6]

[1] M. Bauch et al., Plasmonics 9, 781 (2014).
[2] J. R. Lakowicz, Plasmonics 1, 5 (2006).
[3] R. Ranjan et al., Anal. Chim. Acta 971, 1 (2017).
[4] A. Assy et al., Sens. Actuators A Phys. 250, 71 (2016).
[5] F. Auzel, Chem. Rev. 104, 139 (2004).
[6] L. Aigouy, et al., Nanoscale 11, 10365 (2019).

Solid-state lighting based on GaN and related compounds has been successfully developed and commercialized over the last few decades. Until recently, many researches have been attempted to discover new kinds of inorganic semiconductor candidates for next-generation light-emitting diodes (LEDs) that can replace GaN. Among those, ZnO has attracted much attention as a representative candidate due to its various advantages such as a wide direct transition band gap (~3.37 eV) with a large exciton binding energy (~60 meV), low-cost production, and harmlessness to the human body. However, it is challenging to obtain p-type ZnO due to native donor defect of oxygen vacancy in the ZnO lattice, thereby achieving ZnO p-n homojunction is a difficult task. To overcome this problem, many studies have been conducted on p-n heterojunction LEDs using n-type ZnO with other p-type semiconductors.
In this study, CuI possessing a direct transition band gap (~2.95 eV) with a large exciton binding energy (~58 eV) has been adopted as a p-type material for ZnO heterojunction LEDs. As an n-type material, MgZnO quantum dot (QD) which has much wider band gap than ZnO was used. Moreover, the size reduction of MgZnO QDs was observed, causing the band gap widening of MgZnO by the quantum confinement effect. The wider band gap of MgZnO can effectively suppress excess electron injection from cathode to the p-CuI layer, enhancing the electron-hole recombination efficiency by charge balance between electrons and holes. The CuI film was obtained by iodination process of Cu metal film deposited on ITO-coated glasses and the n-Mg_xZn_1-xO QDs were separately synthesized by low-temperature solution process. The p-n heterojunction was formed by spin-coating of the synthesized n-MgZnO QDs on the CuI film.
The morphology of the obtained p-CuI film and n-MgZnO QD were confirmed by scanning electron microscope and transmission electron microscope, respectively. Optical characteristics of each layer were evaluated by photoluminescence and UV-Vis-NIR spectroscopy. The Mg content of MgZnO QDs was examined by X-ray photoelectron spectra analysis. Finally, the electrical and optical characteristics of the fabricated LED were evaluated by current-voltage measurement, electroluminescence measurement with light emission images.

In the era of hyper-connection, wearable displays are attracting attention as a two-way communication method. Although there have been several demonstrations of displays on clothes, the problem of high power consumption still remained. In this work, we present wirelessly operated wearable micro light-emitting diodes (WμLEDs) fabricated on a fabric. The stability of the WμLEDs was investigated under harsh conditions like high temperature and humidity, mechanical deformation and sunlight, which means that the WμLEDs can be used for outdoor applications. Furthermore, a passive-matrix WμLEDs device was successfully demonstrated which emitted bright red light.

The emerging future engineering technology such as dark-vision displays and photobiomodulation light therapy has induced an enormous demand for the design of highly efficient, flexible, stable, and low-cost deep-red emitting luminophores. Herein, six new deep-red and near infra-red (650–800 nm) emissive iridium(III) complexes have been designed and synthesized, variation on cyclometalating (benzo[b]thiophen-2-yl)quinoline main ligand and picolinate, β-diketonate ancillary ligands. The structures and distorted octahedral coordination motifs of six iridium complexes were analyzed by single-crystal X-ray diffraction analysis. Herein, four iridium complexes were developed with electron donating or withdrawing substituents anchored on the quinoline moiety of (benzo[b]thiophen-2-yl)quinoline cyclometalating ligands. With single-crystal X-ray diffraction analysis, exact cooridination geometry, intermolecular interactions were identified. In addition, deep study on the effect of the ancillary ligands on the excited-state process was carried out and the results were compared with reference compounds containing thenoyltrifluoroacetylacetonate in place of the picolinate ancillary ligand. The picolinate ancillary ligand has rigid nature and high triplet energy level which induces a robust deep-red emission while the triplet spin density population and flexible deformation of β-diketonate ancillary ligand resulted an excited-state geometrical deformation, yielding unfavorable, nonradiative pathways. Therefore, iridium complexes which uses the picolinate as ancillary ligand show excellent emission efficiencies (Φ_PL = 0.48 and 0.37, respectively) and were effectively employed as deep-red dopants in organic light-emitting diodes, constructed via a solution-processed approach. The unoptimized synthesized iridium complexes based devices with various doping ratios performed maximum external quantum efficiency values of 5.03% and 3.41%, respectively.

PeQDs show high quantum yields (QYs) of 40−90% with a narrow full-width-at-half-maximum (FWHM) of 20−45 nm. The narrow FWHM results in a wide color gamut, making PeQDs attractive as promising materials for optoelectronic applications such as light emitting diodes (LEDs), photodetector and lasers. However, perovskite structures suffer from serious stability problems due to their ionic characteristics. Among the PeQDs, red-emitting CsPbI₃ QDs with the α-phase structure are particularly unstable and it is difficult to maintain the structural stability of these species under light, moisture, or heat conditions because iodine has a large ionic radius, weak ionic bonds, and low stability. synthesizing the perovskite quantum dots, oleic acid(OA) and oleylamine(OLA) are often used as capping ligands. However, it is difficult to stabilize the ionic perovskite due to weak surface binding, thus optical properties are easily degraded. Herein, we report highly photo-stable CsPbI₃ perovskite quantum dots after ligand exchange with thiol (-SH, X-type ligand). The success of ligand exchange from OA and OLA to thiol could be stabilize the structure. Because thiols can attach to the surface of CsPbI₃ due to their high affinity for Pb²⁺, thereby stabilizing CsPbI₃. Also, the α-phase structure could be maintained and the photoluminescence (PL) could be persisted for an extended period. As a result, structure stability and optical properties (Quantum Yields) were improved. Furthermore, the synthesized CsPbI₃ PeQDs showed that the photo-stability was highly improved under UV(365nm) irradiation. The PL intensity of thiol-uncapped CsPbI₃ decreased sharply for 12 hours, while thiol-capped CsPbI₃ QDs maintained their PL for 120 hours. For LCD color filter application, perovskite film was prepared using PeQDs and cyclo olefin copolymer (COC). The thiol-treated PeQD films improved their stability compared with the untreated PeQD films under irradiation with a 15 volt back-light unit (BLU). Due to the reason, it is predicted that it will be suitable for use as a color filter for LCD.

Perovskite have received extensive attention due to their unique crystal structure, high extinction coefficient, high carrier mobility, and long carrier migration distance. However, organic-inorganic hybrid perovskites are sensitive to light and heat, and are easily decomposed in the presence of light and heat. In view of the photosensitivity and heat sensitivity of organic-inorganic hybrid perovskite batteries, inorganic perovskites with excellent stability in light and heat have been favored. However, the efficiency of inorganic perovskite solar cells is low. In this paper, the quality of the film is improved by doping the inorganic perovskite, and the thickness of the perovskite layer is increased by appropriate temperature treatment of the inorganic perovskite solution to improve the efficiency of the inorganic perovskite solar cell and the stability. The following work:
The first part is that the manganese ions insert into the inorganic perovskite. Studies have shown that the incorporation of manganese ions significantly increases the crystallites and reduces the defects of the film. The efficiency of CsPbBrI₂ inorganic perovskite solar cells based on 2% MnCl₂ doping is 13.47%, and the average grain size is up to 1200 nm, which significantly reduces the defects of inorganic perovskite films, and the stability of device has been greatly improved. The device maintains an efficiency of over 90% for 30 days under low humidity conditions and exhibits excellent stability.
The second part increases the thickness of the inorganic perovskite film by heating the inorganic perovskite solution at a suitable temperature without adversely affecting the film quality. After heating the inorganic perovskite solution at 100 degrees for 5 minutes, the film thickness was thickened from 165 nm to 195 nm, and the current density was greatly improved. The efficiency of the CsPbBrI₂ inorganic perovskite solar cell was 14.81%, The device maintains an efficiency of over 90% for 30 days under low humidity conditions and exhibits excellent stability.

Metal halide perovskites have emerged as promising materials for light-emitting diodes (LEDs), due to high color purity, tunable bandgap, high luminescence efficiency, and solution processability. The external quantum efficiency (EQE) of green perovskite LEDs has been boosted up to more than 20%, via strategies of compositional engineering, interfacial engineering, defect passivation, and crystal grain confinement, but the value is still far behind that of the commercial organic LEDs. A primary restriction of the EQE is the total internal reflection within the planar perovskite waveguide, due to the large refractive index contrast between the perovskite layer and the surrounding carrier transport layers; and the current efficiency is limited by non-radiative energy loss attributive to electrode quenching.

Here we propose an approach of embedding plasmonic nanomesh electrodes onto nanophotonic fused silica substrates to increase the device efficiency via their synergistic effect. On one hand, the current efficiency is improved when the plasmonic nanomesh electrode not only serve as transparent electrodes with small sheet resistance, but also offer localized surface plasmonic resonance to enhance spontaneous emission rate and realize electrode quenching control. On the other hand, 2D nanopillars of fused silica substrates confine the perovskite to the complementary nanostructure, and EQE is increased by light extraction enhancement through the perovskite “antenna”. FDTD Simulations are applied to optimize the 2D nanostructure (period, duty cycle, and depth) for the highest EQE with the adopted material system.

Perovskite light-emitting diodes (LEDs) have developed very fast in the past five years, and the external quantum efficiency (EQE) higher than 20 percent has already been achieved. However, two issues remain. One is the light extraction issue; another is the stability issue. Perovskite LEDs with photoluminescent quantum yield (PLQY) higher than 80% have already been reported, but the planar structure device shows 20% closing to the limit. It means the light extraction is the next step to further enhance the EQEs of perovskite LEDs. In this work, we used nanophotonic substrate to improve the light extraction efficiency to 73% and achieved 17.5% EQE for methylammonium lead bromide (MAPbBr₃) perovskite LED, which is enhanced twice compared to the planar device. The proposed nanophotonic substrate consists of two parts, namely nanodomes as couplers and photonic crystals as optical antennas. With a geometrical design, we found out the optimal geometry was one-micron pitch and 400 nm diameter for the photonic crystals. As for the stability issue, we fabricated LED devices with perovskite nanowires (NWs) embedded in anodic alumina membrane (AAM), and the AAM prevent the water molecule diffusion on the lateral direction. With the AAM protection of perovskite NWs, the T50 of our devices had been improved from 13 mins to 37 mins with the peak luminance more than 10,000 cd m^-2. Moreover, the NWs showed enhanced field emission and could achieve 16% EQE with the cesium lead bromide (CsPbBr₃) perovskite LED. Our work demonstrates the strength of nanophotonic engineering for perovskite performance enhancement in both efficiency and stability.

Halide perovskite nanocrystals (NCs) have shown impressive advances, exhibiting optical properties that outpace conventional semiconductor NCs, such as near-unity quantum yields and ultrafast radiative decay rates. Herein, we present a block copolymer-templated synthesis for perovskite NCs providing a drastically enhanced stability. The polymer spontaneously forms micelles which act both as nanoreactors and as a protective shell. Encapsulated by this polymer shell, the NCs display strong stability against water degradation and ion migration. Heterostructures of encapsulated MAPI and MAPBr NC layers exhibiting efficient Förster resonance energy transfer (FRET), revealing a strategy for optoelectronic integration. Furthermore, we study the encapsulated NCs via single-particle spectroscopy and investigate the temperature dependence of the optoelectronic properties.

Over the last few years, there has been a steep growth in the study of lead halide perovskites, and due to their attractive optoelectronic properties, they have been incorporated into a number of devices like solar cells, lasers etc. However, due to the presence of lead as an environmentally unsustainable component and being toxic in nature, studies have been done to find alternative solutions to replace lead with other elements. Research has been done to partially dope these halide perovskites with Sn²⁺ and Ge²⁺, however these elements suffer from immediate oxidation to their respective +4 states. In the meanwhile, to obtain lead-free halide perovskites, recent studies have focussed on replacing two divalent ‘Pb²⁺’ cations with one monovalent and trivalent cation, generating quaternary A₂M⁺M³⁺X₆ compounds, which maintains the 3-D perovskite crystal structure as well as the charge neutrality. However, the problems in double perovskite structures are low photoluminescence (PL) emission and limited power efficiency (~ 2.2 %). This has further motivated the research groups around the world to search for direct-bandgap double perovskites, and a number of stable direct band gap perovskites have been computationally found to be stable.
Bulk Cs₂AgInCl₆ has been synthesised by various groups having a direct band-gap of ~ 3.3 eV and a long carrier lifetime (~ 6 µs) and exhibit excellent stability to moisture, light and heat due to the presence of inorganic cation. Recently, L. Manna et. al. have doped nanocrystals of Cs₂AgInCl₆ with Mn²⁺ owing to the enhanced optical properties and observed a red-shift in the emission spectra.^[1] Limited solubility of precursors in solvents causes a problem of solution processing in double perovskite synthesis. Herein, to solve the above problems we have undergone the synthesis of Cs₂AgInCl₆ nanocrystals by a simple colloidal hot-injection approach and further dope them with Ni to elevate their optical properties.
In this approach chloride salts of Ag and In were mixed in octadecene and size distribution control was done by injecting oleylamine and oleic acid as ligands. Nanocrystals were finally precipitated by injecting hot Cs oleate solution and the colloidal solution was re-dispersed in hexane post centrifugation and washing of the nanocrystals. 0.1 % and 0.3% Ni was doped by adding nickel chloride to the initial solution. The nanocrystals showed white photoluminescence under a UV lamp. STEM-EDS analysis was used to determine the Ni concentration in the doped nanocrystals. HRTEM micrographs of Ni-doped Cs₂AgInCl₆ nanocrystals that due to the inclusion of Ni ions, there was no disruption in the crystalline nature and no defects were formed. Optical properties were determined by UV-vis spectrometry and PL spectrometry. There was a weak PL emission (PLQY ~ 1.6 %). Absorption onset of both doped and undoped nanocrystals was at ~ 350 nm with a strong increase in the absorbance at 272 nm. Undoped nanocrystals showed an emission peak at 560 nm that was consistent with earlier reported results of Cs₂AgInCl₆ NCs in the quantum confinement regime. In the Ni-doped nanocrystals, a blue shift in the emission spectra was observed with the emission peak at 490 - 470 nm, the extent of the blue shift depending on the Ni dopant concentration. Studies were done to determine the stability of as-synthesized nanocrystals and were found to be stable in air for at least a week with no change in the photoluminescence by the naked eye. Further studies to understand the behaviour of Ni²⁺ ions in the environment of the host matrix can be done by ESR spectroscopy which can shed light on the observed blue shift in the emission spectra of the doped samples.
In conclusion, we have shown a colloidal route to synthesise Ni-doped Cs₂AgInCl₆ that gives phase-pure stable NCs in the air with increased bandgap.
References
1. J. Am. Chem. Soc.20181404012989-12995

Lead halide perovskite nanocrystals (APbX₃) are among the most exciting materials of the last 5 years due to their outstanding optoelectronic properties. These materials present narrow emission peaks and low trap state density. In cube-shaped perovskite nanocrystals, excitons are typically in weak quantum confinement regime, showing low size-dependence of the excitonic peak position as observed in the absorbance spectrum.¹ Contrarily, the so-called 2D perovskite nanocrystals exhibit strong quantum confinement effects due to their limited thickness (just a few monolayers), resulting in absorption and photoluminescence (PL) excitonic transitions strongly dependent on the thickness of the nanocrystals,² turning them promising materials for tunable optoelectronic applications.² Therefore, the development of synthetic protocols to grow more stable and with higher control over the perovskite nanoplatelets (PNPL's) thickness, is highly desirable. But to do this, it is crucial to gain insight into the mechanism that leads to the formation of PNPs.
In this work, we describe the formation of PNPs induced by the use of SnX₄ (X = Cl, Br, and I) salts as the halide source through a new protocol, inspired by two distinct methods reported by Protesescu et al.³ and by our group.⁴ All spectral features corresponding to highly confined excitons (i.e., sharp excitonic absorption and emission) can be directly related to the 2D morphology of PNPs, which is confirmed by Transmission Electron Microscopy images. Besides that, the utilization of ¹H-NMR and FTIR techniques helped to elucidate the nature of the disturbance in the acid-base equilibrium between oleic acid and oleylamine, caused by the Sn⁴⁺ cations. Our data suggest that the formation of Sn-oleylamine complexes is the cause of such disturbance, increasing the ratio [oleic acid]/[oleylamine], which was observed by the displacement in chemical shift of ¹H-α to the amine group toward higher values. Such displacement can be assigned to a high concentration of oleylammonium species, increasing the ratio [oleylammonium]/[Cs⁺], favoring the formation of PNPs even at high temperatures, as supported by our results.


REFERENCES
1. Castañeda, J. A. et al. Efficient Biexciton Interaction in Perovskite Quantum Dots Under Weak and Strong Confinement. ACS Nano 2016, 10, 8603-8609;
2. Sichert, J. A. et al. Quantum Size Effect in Organometal Halide Perovskite Nanoplatelets. Nano Lett. 2015, 15, 6521-6527;
3. Protesescu, L. et al. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX₃, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692-3696;
4. Yassitepe, E. et al. Amine-Free Synthesis of Cesium Lead Halide Perovskite Quantum Dots for Efficient Light-Emitting Diodes. Adv. Funct. Mater. 2016, 26, 8757-8763.

Recently halide double perovskites have shown promise as phosphors for light emission applications such as white light phosphor converted LED applications. Here we report on halide double perovskite Cs2NaBiCl6 doped with Mn2+ ions resulting in an orange emitting compound under UV light. Optical measurements indicate an absorption profile comprised of localized Bi3+ 6s2 to 6s16p1 transitions. These isolated absorption processes lead to efficient energy transfer to Mn2+ sites leading to emission from the 4T1g to 6A1g transition. Unlike other reported Mn2+ double perovskite hosts, the zero dimensional electronic structure of Cs2NaBiCl6 surprisingly yields enhanced energy transfer to emissive sites. Compositions are analyzed via a variety of techniques such as XRD, UV-vis diffuse reflectance, photoluminescence, thermogravimetric analysis, and electron paramagnetic resonance.
Through careful control of synthetic parameters single crystals with Br- paritally substituting for Cl- are stabilized via hydrothermal methods. By relaxing the crystal field around absorbing Bi3+ sites, exictation peaks shifted to lower energies, closer to that of commerical blue LEDs. Tailoring of photoluminescent properties demonstrates the potential of double perovskite phosphors by tuning optical properties through chemical substituion. Routes to further improve this system as well as to extend these models to other double perovskite systems are discussed.

Lead halide perovskites have emerged as promising new semiconductor materials for high-efficiency photovoltaics, light-emitting applications and quantum optical technologies. Their luminescence properties are governed by the formation and radiative recombination of bound electron-hole pairs known as excitons, whose bright or dark character of the ground state remains unknown and debated [1, 2]. While symmetry analysis predicts a singlet non-emissive ground exciton topped with a bright exciton triplet, it has been predicted that the Rashba effect may reverse the bright and dark level ordering.
Spectroscopically resolved emission from single lead halide perovskite nanocrystals at cryogenic temperatures provides unique insight into physical processes that occur within these materials. At low temperatures the emission spectra collapse to narrow lines revealing a rich spectroscopic landscape and unexpected properties, completely hidden at the ensemble level and in bulk materials.
In this talk, I will discuss how magneto-photoluminescence spectroscopy provides a direct spectroscopic signature of the dark exciton emission of single lead halide perovskite nanocrystals [3]. The dark singlet is located several millielectronvolts below the bright triplet, in fair agreement with an estimation of the long-range electron hole exchange interaction. Nevertheless, these perovskites display an intense luminescence because of an extremely reduced bright-to-dark phonon-assisted relaxation [4]. Resonant photoluminescence excitation spectroscopy allows the determination of the optical coherence lifetimes in these nanocrystals and to assess their suitability as sources of indistinguishable single photons [5].
References:
[1] M. Fu, P. Tamarat, J. Even, A. L. Rogach, and B. Lounis, “Neutral and Charged Exciton Fine Structure in Single Lead Halide Perovskite Nanocrystals Revealed by Magneto-optical Spectroscopy,” Nano Lett., vol. 17, no. 5, pp. 2895–2901, Apr. 2017.
[2] G. Nedelcu, A. Shabaev, T. Stöferle, R. F. Mahrt, M. V. Kovalenko, D. J. Norris, G. Rainò, and A. L. Efros, “Bright triplet excitons in caesium lead halide perovskites,” Nature, vol. 553, no. 7687, pp. 189–193, Jan. 2018.
[3] P. Tamarat, M. I. Bodnarchuk, J.-B. Trebbia, R. Erni, M. V. Kovalenko, J. Even, and B. Lounis, “The ground exciton state of formamidinium lead bromide perovskite nanocrystals is a singlet dark state,” Nat. Mater., pp. 1–9, May 2019.
[4] P. Tamarat, J.-B. Trebbia, M. I. Bodnarchuk, M. V. Kovalenko, J. Even, and B. Lounis, “Unraveling exciton-phonon coupling in individual FAPbI3 nanocrystals emitting near-infrared single photons.,” Nat. Commun., vol. 9, no. 1, p. 3318, Aug. 2018.
[5] L. Hou et al., to be submitted (2019)

Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies now exceeding 23%. As these devices are approaching the Shockley-Queisser limit, bimolecular (band-to-band) recombination will dominate the charge-carrier losses, withtrap-mediated charge recombination becoming less prominent.

We show that in methylammonium lead triiodide perovskite, bimolecular recombination can be fully explained as the inverse of absorption,^[1] and exhibits a dynamic that is heavily influenced by photon reabsorption inside the material.^[2,3] Such photon recycling is shown to slow charge losses from thin hybrid perovskite films, depending on light out-coupling.^[2] Interestingly, for thin films comprising a quasi-two-dimensional (2D) perovskite region interfaced with a 3D MAPbI₃ perovskite layer the blue-shifted emission originating from quasi-2D regions overlaps significantly with the absorption spectrum of the 3D perovskite, allowing for highly effective “heterogeneous photon recycling”. We show that this combination fully compensates for the adverse effects of electronic confinement, yielding quasi-2D perovskites with highly efficient charge transporting properties.^[3]

In addition, we investigate optoelectronic properties of mixed tin-lead iodide and mixed iodide-bromide lead perovskites. We show how band-gap bowing in tin-lead perovskitesis compatible with a mechanism arising from bond bending to accommodate the random placement of unevenly sized lead and tin ions.^[4] While tin-rich compositions exhibit fast, mono-exponential recombination that is almost temperature-independent, in accordance with high levels of electrical doping,^[4,5] lead-rich compositions show slower, stretched-exponential charge-carrier recombination that is strongly temperature-dependent, in accordance with a multiphonon assisted process. Finally, in the context of silicon-perovskite tandem cells, we discuss the mechanisms underlying detrimental halide segregation in mixed iodide-bromide lead perovskites with desirable electronic band gaps near 1.75eV.^[6]

[1] C. L. Davies, M. R. Filip, J. B. Patel, T. W. Crothers, C. Verdi, A. D. Wright, R. L. Milot, F. Giustino, M. B. Johnston, L. M. Herz, Nature Communications 9, 293 (2018)
[2] T. W. Crothers, R. L. Milot, J. B. Patel, E. S. Parrott, J. Schlipf, P. Müller-Buschbaum, M. B. Johnston, L. M. Herz,Nano Lett. 17, 5782 (2017)
[3] S. G. Motti, T. Crothers, R. Yang, Y. Cao, R. Li, M. B. Johnston, J. Wang, and L. M. Herz, Nano Lett. 19, doi.org/10.1021/acs.nanolett.9b01242(2019)
[4] R. L. Milot, M. T. Klug, C. L. Davies, Z. Wang, H. Kraus, H. J. Snaith, M. B. Johnston, and L. M. Herz, Adv. Mater. 30, 1804506 (2018).
[5] E. S. Parrott, T. Green, R. L. Milot, M. B. Johnston, H. J. Snaith, and L. M. Herz, Advanced Functional Materials 28, 1802803 (2018).
[6] A. Knight, A. D. Wright, J. B. Patel, D. McMeekin, H. J. Snaith, M. B. Johnston, and L. M. Herz, ACS Energy Lett. 4, 75 (2019).

Organo-lead halide perovskites are excellent candidates for applications in light emitting diodes (LEDs). Methylammonium lead bromide (MAPbBr₃) is the most investigated perovskite for visible perovskite LEDs, but its use is still limited by a low photoluminescence quantum yield (PLQY). Efforts to increase the PLQY of this material mainly consist in fine-tuning the morphology and reducing the grain size of the polycrystalline perovskite to the micro- and nanoscale. However, the origins of the increased PLQY with reduced crystal size remain unclear. Here we elucidate the physical processes underlying the light emission of MAPbBr₃ thin films using time-resolved spectroscopy.¹ A systematic correlation between the enhanced PL properties and the reduction of the crystal size, achieved by using different additives during solution-processing, is observed. The co-existence of free carriers and excitons at low excitation densities is shown in as-cast polycrystalline MAPbBr₃ (crystal size of the order of few μm), while only excitons are present at high excitation densities. Using the Burstein-Moss and Saha models, important quantities such as the exciton binding energy, the reduced exciton effective mass and the trap density are estimated. We then explain the increased PLQY upon crystal size reduction by the presence of a bright exclusively exitonic population even at low excitation densities, together with reduced surface trapping thanks to passivation by the additives.

¹N. Droseros , G. Longo, J. C. Brauer, M. Sessolo , H. J. Bolink , N. Banerji. ACS Energy Lett. (2018), 3, 1458

Metal-halide perovskites show excellent properties for photovoltaic and optoelectronic applications, with power conversion efficiencies of solar cell and LEDS now exceeding 20%. This is unexpected, because these polycrystalline, solution-processed materials are likely to contain a significant density of defects compared to melt-grown semiconductors. Yet, typical effects from defects, such as strong absorption below the bandgap, low open circuit voltage in devices and dominant non-radiative recombination were not observed. In this contribution, we investigate thin films of metal-halide perovskites CH3NH3PbX3 (X = Br,I) with multidimensional optical spectroscopy to resolve the dynamics of band and defect states on ultrafast timescales. We report an unexpected coupling between the band transitions and a continuum of sub-bandgap states, which we report to extend at least 350 meV below the band edge. We explain the comparatively large bleach signal of these dark sub-bandgap states, compared to the levels detected in linear absorption, with oscillator strength borrowing from the band-edge transition. Our results imply that, upon valence to conduction band excitation, the sub-gap states are instantaneously bleached by the presence of charges in the band for the duration of the carrier lifetime and conversely, that almost dark sub-bandgap states can be populated by light excitation. Our findings provide fundamental insights into the photophysical origin of the exceptional defect tolerance of hybrid perovskites materials.

Semiconductor quantum wells and superlattices, which are usually fabricated through metal-organic chemical vapor deposition or molecular beam epitaxy, are key building blocks in modern optoelectronics. The ability to simultaneously realize defect-free epitaxial growth and to individually fine-tune the chemical composition and band structure of each layer is essential for achieving the desired performance. Such structures are challenging to realize using organic or hybrid materials because of the difficulty of controlling the materials growth. In this talk, I will present a molecular approach to the synthesis of high-quality organic-inorganic hybrid perovskite quantum wells through incorporating widely tunable organic semiconducting building blocks. By introducing sterically tailored groups into the molecular motif, the strong self-aggregation of the conjugated organic molecules can be suppressed, and single crystalline organic-perovskite hybrid quantum wells (down to one mono-layer thick) and superlattices can be easily obtained via one-step solution-processing. Energy transfer and charge transfer between adjacent organic and inorganic layers are extremely fast and efficient, owing to the atomically-flat interface and ultra-small interlayer distance. The 2D hybrid perovskite superlattices are surprisingly stable, due to the protection of the bulky hydrophobic organic groups. The molecularly engineered 2D semiconductors are promising candidates for use in next-generation nanoelectronics, optoelectronics, and photonics.

In colloidal nanocrystal form, lead halide perovskites possess high photoluminescence quantum yields, while in bulk single-crystal form they exhibit long charge-carrier diffusion lengths. However, without proper strategies to diminish crystal surface defects and manage surface quality, the desired characteristics of perovskites cannot be effectively exploited for photovoltaic and optoelectronic devices. Here I discuss novel strategies to passivate the surface defects and improve the surface quality of perovskite nanocrystals and bulk single-crystals, enabling the fabrication of efficient devices. We demonstrate the passivation of CsPbX₃-type nanocrystals with molecular ligands and metal dopants leading to stable near-unity quantum yield emitters, as well as efficient blue and red light-emitting diodes (LEDs). We also show the importance of designing crystal growth conditions, such as solvent, temperature, and substrate in order to grow bulk single-crystals with low-defect densities and good surface quality. Depending on the composition, MAPbX₃-type single crystals grown (tens of microns thick) under optimal conditions were used to realize: a) very sensitive visible-blind UV-photodetectors with nanosecond response time; and b) single-crystal solar cells with >21% power conversion efficiency. Unlike thin film polycrystalline solar cells, efficient cells with a grain-free single-crystal absorber are an ideal unobstructed system for investigating the device physics and chemistry of perovskites.

Halide perovskites exhibit remarkable properties as solar-cell absorbers, featuring both direct bandgaps suitable for sunlight absorption and long-lived charge carriers beneficial for charge extraction. However, the intrinsic instabilities and high toxicity of these water-soluble lead salts may impede the commercialization of this technology. Notably, even the origin of the materials' superior photophysical properties remains unclear, underscoring the importance of synthesizing and studying functional analogs of the lead perovskites. However, most stable materials studied as analogs have displayed weak sunlight absorption and short carrier lifetimes.

We recently introduced halide double perovskites, which can accommodate a much greater range of metals, as solar absorbers. Armed with this substitutional flexibility, we have explored alternative metals that can be incorporated into the perovskite lattice. Studying the electronic differences between the lead perovskites and lead-free double perovskites has shown us how to synthetically tune double perovskites to efficiently absorb sunlight. I will share our understanding of how to manipulate the symmetry and energy of the bandgap transition in these materials through i) dilute impurity alloying, ii) stoichiometric metal substitution, and iii) dimensional reduction. Our recent studies have led us to double perovskites with very similar properties to the lead perovskites, with small bandgaps and long-lived carriers. I will further present a pen-and-paper method for both understanding and predicting halide double perovskite band structures based on orbital symmetry arguments.

Hybrid organic-inorganic metal halide perovskites have demonstrated remarkable properties for thin-film solar absorbers and optoelectronics, including their ease of preparation, compositional tunability, defect tolerance and high charge-carrier mobilities. The promise of these materials has also motivated the development of modified bulk and nanoscale perovskites, including reduced-dimensional perovskites (2D, 1D) and quantum dots (QDs), with emergent optoelectronic properties. Perovskite QDs, in addition to a tunable band gap and high photoluminescence quantum yield, are of interest due to unique physical and structural phenomena such as the stabilization of metastable crystallographic phases. Luther et al. reported the stabilization of the high temperature, low-band gap cubic perovskite phase of CsPbI₃ at room temperature in small QD particles via colloidal synthesis (Swarnkar et al. Science 2016, 354, 6308, 92). In addition, alloying of these CsPbI₃ QDs with FAPbI₃ (yielding Cs_1–xFA_xPbI₃) by a simple cation exchange approach allows for access to the full compositional range (i.e. x = 0–1), unlike thin-film fabrication and direct synthesis of bulk Cs_1–xFA_xPbI₃ (Hazarika et al. ACS Nano 2018, 12, 10327).
Herein, we report on structural studies of Cs_1–xFA_xPbI₃ QD films by synchrotron X-ray techniques. 15-nm Cs_1–xFA_xPbI₃ QDs (x = 0, 0.5) were deposited from colloidal solution, by both drop- and spin-casting methods, and investigated by grazing incidence small-angle and wide-angle X-ray scattering (GISAXS/GIWAXS). GIWAXS patterns indicate coherent particle ordering on the substrate for single-layer spin-coated films, while the subsequent ligand exchange and particle overcoating (2–5 layers) leads to reorganization and isotropic ordering of the particles. In addition, drop-casting and slow evaporation of the solvent from the colloidal solution also results in isotropic ordering. Interestingly, the diffraction patterns show evidence of distortion from the originally reported cubic perovskite phase, with significant tetragonal (Cs_0.5FA_0.5PbI₃) and orthorhombic (CsPbI₃) character. The distortion and phase contributions across the full range of alloyed Cs_1–xFA_xPbI₃ QDs will be reported and discussed.

Hybrid perovskite nanoparticles (PeNPs) have significant potential to be used in perovskite light-emitting diodes (PeLEDs) because of their high photoluminescence quantum efficiency and, facile color tunability and synthesis. However, the highest electroluminescence effiencies of PeLEDs based on hybrid PeNPs are still much lower than those of PeLEDs based on hybrid perovskite bulk films and all-inorganic PeNPs. Here, we suggest a strategy to improve the electroluminescence efficiency of PeLEDs based on hybrid PeNPs. We passivate the defect states of PeNPs by introducing large organic cation into the formamidinium lead bromide nanocrystals. Based on this high quality PeNPs, we were able to fabricate efficient PeLEDs. Out work provides a promising way to improve luminescent efficiency of the PeLEDs based on hybrid PeNPs.

Colloidal organic/inorganic lead halide perovskite nanocrystals (NCs) are considered promising blue and green narrow-band emitters for the next-generation light-emitting diodes. High photoluminescence efficiencies are attained in these materials without epitaxial overcoating of the NC surfaces for electronic passivation of the surface states [1]. The major practical bottleneck of these materials relates to their labile surface chemistry. In particular, typically used ling chain capping ligands are problematic due to their dynamic and loose binding as well as their highly insulating nature. We have recently rationalized the typical observation of a degraded luminescence upon aging or the luminescence recovery upon post-synthesis surface treatments using a simple surface-structure model, supported by DFT calculations [2]. Healing of the surface trap states requires restoration of all damaged PbX₆ octahedra and establishing a stable outer ligand shell. Restoration of such a structure, seen as an increase in the luminescence quantum efficiency to 90-100% and improvement in the overall robustness of CsPbBr₃ NCs, was attained using a facile post-synthetic treatment with a PbBr₂+DDAB (didodecyldimethylammonium bromimde) mixture. In our most recent work [3], we have used DDAB as a sole ligand directly in the synthesis of perovskite NCs. We then used such NCs in LEDs and demonstrate high external quantum efficiencies of up to 3.6% in blue region (460nm) and 10% in the green region (520 nm).
1. M. V. Kovalenko, L Protesescu, M. I. Bodnarchuk. Science 2017, 358, 745-750
2. M. I. Bodnarchuk,S. C. Boehme, S. ten Brinck, C. Bernasconi, Y. Shynkarenko, F. Krieg, R. Widmer, B. Aechlimann, D. Günther, M. V. Kovalenko, I. Infante. ACS Energy Letters 2018, 4, 63–74
3. Y. Shynkarenko, M. Bodnarchuk et al. submitted

The efficiency of electroluminescent perovskite devices have increased rapidly from 0.76% in earlier works to above 20% in recent reports. Such high performance, coupled with excellent spectral qualities and easy manufacturability have captured the attention of the academic and commercial communities. In this talk, we will discuss some physical principles behind device efficiency enhancement, and report our new strategies in improving the uniformity and robustness of perovskite light-emitting diodes. We will also report our recent activities in the deployment of luminescent perovskites in color-enhanced displays and other advanced consumer product applications.

In the last years, metal halide perovskites have attracted great attention as promising materials for light-emitting diodes (LED), owing to their excellent optical and electronic properties such as high charge-carrier mobility, narrow emission spectra and easily tunable colors [1,2,3]. The highest external quantum efficiencies of CsPbBr₃ LED recently exceeded 20 % [4]. Efficiencies were enhanced mainly by suppressing non-radiative recombination and establishing improved charge balance. The most efficient LED utilize hole-transport layers (HTL) of either a hydrophilic organic co-polymer (PEDOT:PSS) or an inorganic oxide (NiO_x), ensuring beneficial wetting properties for perovskite formation [5]. Despite a huge variety of organic hole-transport materials, the polar solvents required for perovskite synthesis narrow their choice leaving PEDOT:PSS as one of the few remaining options. However, PEDOT:PSS is lacking sufficient electron-blocking properties in electron-rich CsPbBr₃-based LED structures. Consequently, an additional blocking layer is required to achieve optimum charge balance as a prerequisite for high luminous efficiency.
In this work, we employ the hydrophobic hole-transporting polymer poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine)) (PTAA) as HTL. PTAA exhibits a high LUMO value of 1.8 eV [6], ensuring the desired electron-blocking properties. Thus, the requirement for an additional blocking layer is eliminated, simplifying the LED fabrication process. This HTL was examined in LED structures with a configuration of glass/ ITO (100 nm)/ PTAA (10 nm)/ CsPbBr₃ (60 nm)/ (2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) (TPBi) (30 nm) /LiF (1 nm)/Al (100 nm). In this stack, PTAA and CsPbBr₃ were deposited from solutions (PTAA – 3 mg/ml in toluene; CsPbBr₃ – 12 wt% of precursor solids PbBr₂ and CsBr with a molar ratio of 1:1.7 in dimethyl sulfoxide) by spin-coating in purified N₂ atmosphere (H₂O < 1 ppm, O₂ < 1 ppm). TPBi and a LiF/Al cathode were deposited employing organic vapor phase deposition (OVPD) and vacuum thermal evaporation, respectively. TPBi was selected as electron transport layer which also acts as a hole-blocking layer due to its deep HOMO level of 6.2 eV [7]. This combined with the PTAA electron blocking ability ensures superior carrier as well as exciton confinement in the emissive perovskite layer.
The hydrophobicity of the PTAA surface was reduced by a short exposure to remote N₂ plasma, which allowed the formation of a dense CsPbBr₃ film from solution on top. The room temperature photoluminescence spectra of pure CsPbBr₃ layer (without any additives for grain passivation and trap density minimization) exhibited a saturated color (λ_max = 520 nm) with a FWHM of 18 nm indicating a low degree of energetic disorder. The fabricated LED yielded high current densities exceeding 200 mA/cm², a turn-on voltage of 2.9 V, bright emission of 10,000 cd/m² and stable EQE (external quantum efficiency) of 1.5 %. The LED performance could be improved considerably by employing molecular pinning to enhance the morphology of CsPbBr₃ layers which leads to suppressed non-radiative recombination at interfaces and perovskite grain boundaries [8].

1 Wehrenfennig C. et al., Adv. Mater., 2014, 26, 1584–1589
2 Tan Z. K. et al., Nat. Nanotech., 2014, 9, 687–692
3 Sichert J. A. et al., Nano Lett., 2015, 15, 6521–6527
4 Lin K. et al., Nature, 2018, 562, 245-246
5 Le Q. V. et al., Small Methods, 2018, 2, 1700419
6 Xu C. et al., J. Mater. Chem. C, 2018, 6, 6975-6981
7 Zhang Y. et al., Organic Electronics, 2016, 30, 76
8 Zhang L. et al., Nature Communications, 2017, 8:15640

The organic-inorganic hybrid perovskite materials have been studied extensively owing to their excellent optical and electrical characteristics that enable a remarkable enhancement of device efficiency in light-emitting diodes (LEDs) and solar cells. High film quality of perovskite is necessary to develop highly efficient and stable perovskite optoelectronic devices. However, defect sites such as voids, pinholes, grain boundaries and under-coordinated ions, creating a large number of undesired electronic trap sites can exist in solution-processed perovskite films. Here, we observe the significant beneficial effects on the efficiency and long-term stability of perovskite light-emitting diodes (PeLEDs) through passivating the defect sites of perovskite layers.

Hybrid organic-inorganic halide perovskite materials are of considerable interest for optoelectronics. We will discuss our approach to surface passivation using an excess solution stoichiometry of organoammonium so as to maintain an ammonium terminated surface in films that consist of nanoscale crystallites. Proper passivation allows for enhanced quantum yield and stable mixed halide stoichiometries. In another aspect, we have determined that metal halide perovskites not only feature ionic motion but are also considerably redox active and heat sensitive, and are looking to understand how redox chemistry, and ionic processes more generally, dictate material and device physics and degradation.

Organic-inorganic hybrid perovskites are very promising for light emitting diodes (LEDs) due to their high color purity, tunable emission wavelength, and balanced electron and hole conduction etc. The optoelectronic properties of hybrid perovskite have shown to be very tolerant to defects, which has been demonstrated in solution processed perovskite photovoltaics. However, both lead and halide defects have been shown to be a big problem for solution processed perovskite LEDs (PeLEDs) with nanometer-sized crystallites. The defects cause obvious non-radiative recombination and short operation stability.

In this talk, we will report the molecular design of additives for dual passivation of both lead and halide defects in perovskites. We synthesized 4-fluoro-phenylmethylammonium-trifluoroacetate (FPMATFA) using a simple solution process. The C=O bond in TFA group can coordinate with lead and passivate the lead defects, while the organoammonium cations, FPMA+, can coordinate with halide and passivate halide defects. In addition, the bulky FPMA group can constrain the grain growth of 3D perovskite, which enhances electron-hole capture rates and radiative recombination rate. As a result, we achieved a decent external quantum efficiency of 20.9% for formamidinium/cesium based PeLEDs. What’s more, the operation lifetime of PeLEDs is also greatly improved due to the low trap density in the perovskite film.

Over the last 5 years, the performance of perovskite nanocrystal-based light-emitting diodes (LEDs) has improved dramatically, achieving external quantum efficiencies (EQEs) which are comparable with their chalcogenide counterparts. Crucially, perovskite nanocrystals (NCs) can be synthesised with straightforward wet chemistry techniques under ambient conditions, in stark contrast to the high temperature, inert gas environment necessary for chalcogenide core-shell structures. All-inorganic cesium lead bromide (CsPbBr₃) NCs are particularly promising, as they exhibit much better thermal and environmental stability than formamidinium (FA) or methylammonium (MA) based perovskites. However, the colloidal stability of CsPbBr₃ NCs remains poor, which severely restricts any future commercial viability.
The stability of perovskite nanocrystals is strongly reliant on surface chemistry. It is vital that the ligands bound to the nanocrystals can both passivate deleterious defects and form a robust attachment to the surface. Ligands that do not adhere strongly to the nanocrystals are prone to desorption, leading to agglomeration and rapid degradation of their initially exceptional luminescence. Until recently, labile carboxylic acid and alkylamine ligands were almost universally employed for CsPbBr₃ nanostructures. However, it has been shown that the dynamic binding equilibrium they establish limits both the colloidal and device stability. Phosphonic acids have since emerged as a promising alternative. A hot-injection synthesis of CsPbBr₃ NCs capped by octylphosphonic acid (OPA) ligands enabled LEDs with an EQE of 6.5 % and half-lifetime of over 30 mins.¹ In contrast to the standard ligands, phosphonic acids have been shown to bind irreversibly to lead sites, thus indicating their high potential for stability enhancement.²
We sought to elucidate the underlying surface chemistry of phosphonic acid binding to CsPbBr₃ NCs. Through a ligand exchange approach, we revealed that OPA binds as the monoionic phosphonate, such that P=O and P−OH functionalities remain free. Most importantly, 2D ¹H−³¹P solid-state NMR demonstrated that these functionalities allow extensive inter-ligand hydrogen bonding, such that the nanocrystals are passivated by an interconnected octylphosphonate network.³
Based on the discovery of this phenomenon, we devised a direct room temperature, open-air synthesis of CsPbBr₃ NCs passivated by an octylphosphonate network. Trioctylphosphine oxide (TOPO) aided the dissolution of PbBr₂ and OPA in toluene, such that the injection of cesium octanoate immediately formed highly luminescent green-emitting nanocrystals. Post-synthetic treatment with didodecyldimethylammonium bromide (DDAB) provided DDA⁺ ligands that bind to surface bromide, increasing the solubility, while the excess Br⁻ filled any gaps in the octylphosphonate network. This approach proved exceptionally effective; even after 3 purification cycles with methyl acetate, a photoluminescent quantum yield (PLQY) of 95 % was retained. Thus we have successfully developed an ambient, room temperature synthesis of robust near-perfect CsPbBr₃ nanocrystals. We anticipate that this method will improve the viability of these nanocrystals for high performance optoelectronic applications.

1. A. A. M. Brown, T. J. N. Hooper, S. A. Veldhuis, X. Y. Chin, A. Bruno, P. Vashishtha, J. N. Tey, L. Jiang, B. Damodaran, S. H. Pu, S. G. Mhaisalkar and N. Mathews, Nanoscale, 2019, DOI: 10.1039/C9NR02566A
2. Y. Tan, Y. Zou, L. Wu, Q. Huang, D. Yang, M. Chen, M. Ban, C. Wu, T. Wu, S. Bai, T. Song, Q. Zhang and B. Sun, ACS Appl. Mater. Interfaces, 2018, 10, 3784–3792.
3. S. R. Smock, T. J. Williams and R. L. Brutchey, Angew. Chemie - Int. Ed., 2018, 90089, 11711–11715.

The graphene quantum dots (GQDs), a zero-dimensional graphene quantum structure, have triggered an intense research worldwide. GQDs possess unique optical, chemical and physical properties as compared to conventional quantum dots (QDs), such as low toxicity, biocompatibility, optical stability, chemical inertness, high photostability and good water-solubility and therefore hold great application potential in biomedical, optoelectronics and energy storage devices. The doping of GQDs with heteroatoms is one of the most effective ways to tune their photoluminescence emission and to increase quantum yield. In this study, we developed a novel approach to synthesize high-quality Nitrogen-doped graphene quantum dots (N-GQDs) with high quantum yield, via irradiation of s-triazene in a solution with benzene by using pulsed laser. The TEM, HRTEM, XPS, XRD, Raman spectroscopy and FTIR were carried out to observe the morphology, size distribution, crystalline structure and to prove successful doping of GQDs with nitrogen atoms. To observe optical properties of as synthesized N-GQDs, the UV-vis and Photoluminescence measurements were carried out. The as-synthesized NGQDs exhibit high quality crystalline structure of graphene with an average size of about 3.7 nm. A high quantum yield was exhibited by the obtained N-GQDs as compare to the pristine GQDs. The obtained N-GQDs with oxygen-rich functional groups exhibit a strong emission. These outcomes result in an ample opportunity for the biomedical and optoelectronic applications.

Inorganic perovskite structure has emerged as optoelectronic materials as possessing advantageous optical properties for display applications used in a polymer-encapsulated form. However, insulating polymers acting as a host matrix are generally vulnerable to thermal stresses, so that softening distortion or deterioration occurs in thermoplastic polymers (generally over 150 °C). So, we develope nanostructured CsPbBr₃/Al₂O₃ composite film by aerosol deposition (AD). At first, CsPbBr₃/Al₂O₃ composite powders are synthesized by the recrystallization of CsBr and PbBr₂ ionic salts onto the surface of Al₂O₃ particles and have bright and saturated green emission. Next, the composite powder supported on submicron α-Al₂O₃ particles is effectively changed to a film-type by aerosol deposition (AD). AD, which is based on shock-loading solidification caused by countless collisions of fine ceramic particles, has many advantages for perovskite NCs, which operate at room temperature, in a vacuum system to minimize the exposure of oxygen and moisture and under an eco-friendly condition of the nonsolvent process. The ceramic composite film show high long-term stability at 150 °C for over 20 days, a pure green spectrum with a narrow full width at half maximum (522 nm and FWHM of 17 nm) and an absolute photoluminescence quantum yield of 8%–15%. General inorganic film have difficulty in flexibility and pattern. But, the films deposited by AD can be used as down-converting material for LCD backlight with a wide area of 89.2% in REC. Furthermore, it can be applied to a variety of display fields, such as high flexible chemical composites films, multilayer films, patterning of surfaces for an advanced color filter array, and deposition on protected substrates toward diverse display fields.

Lead halide perovskites (LHPs) are promising optoelectric materials, but with ionic bonding characteristics, LHPs disassemble readily in any polar solvents. This structural instability, together with dynamic lattice disorder, has prohibited the crystalline coating of LHPs using polar precursors. Here, we report a sonochemistry-based technique to synthesize poly-norepinephrine (pNE)-coated CsPbBr₃ microcrystals. The colloidal core-shell-type CsPbBr₃ microcrystals were synthesized in a polar solvent without any surfactants. High-resolution X-ray photoelectric spectroscopy suggests that the electronegative catechol group of the pNE forms Lewis acid-base adducts with under-coordinated Pb atoms. This passivation reduces surface defects, decreases hysteresis, and increases optical gain. Density functional theory (DFT) supported these observations. Furthermore, the pNE coating serves as diffusion-blocking layers, increasing the lifetime of LHPs in water by 1000-fold. This allowed us to observe lasing from a single core-shell perovskite micro-particle in water. The pNE coating also enabled us to use conventional reactions to functionalize perovskite microcrystals, for example, with fluorescent proteins and plasmonic nanoparticles. This work may open a new avenue towards environment-stable and multifunctional perovskite laser particles.

Perovskite nanoplatelets (NPls) hold great promise for light-emitting applications, having achieved high photoluminescence quantum efficiencies (PLQEs) approaching unity in the blue wavelength range, where other metal-halide perovskites have typically been ineffective. However, the external quantum efficiencies (EQEs) of blue-emitting NPl light-emitting diodes (LEDs) have typical values of 0.1% or below. In this work, we show that the performance of NPl LEDs is primarily hindered by a poor electronic interface between the emitter and hole-injector. Through Kelvin Probe and X-ray photoemission spectroscopy measurements, we reveal that the NPls have remarkably deep ionization potentials (≥6.5 eV), leading to large barriers for hole injection, as well as substantial non-radiative decay at the emitter/hole-injector interface. We find that an effective way to reduce these non-radiative losses is by using poly(triarylamine) interlayers, which lead to an increase in the EQE of the blue (464 nm emission wavelength) and sky-blue (489 nm emission wavelength) LEDs to 0.3% and 0.55% respectively. The EQEs of these devices are two orders of magnitude higher than the control devices without the poly(triarylamine) interlayer, and we elucidate the role of these interlayers through detailed spectroscopic and single-carrier device measurements. Our work also identifies the key challenges for further improvements in efficiency [1].

References
[1] R. L. Z. Hoye, M.-L. Lai, et al., ACS Energy Lett. 2019, 4, 1181

Understanding the structure of the nanocrystal cores and stabilizing ligands in colloid quantum dots presents unique set of challenges that are ideally suited to study using X-ray and neutron scattering (SAXS and SANS) techniques. There is a large electron density difference between the metal chalcogenide (e.g. lead sulfide, PbS) nanocrystal core and most organic materials. This includes the native ligands present from synthesis (typically oleic acid, OA) and solvents or polymeric matrices where present. This provides an excellent contrast scenario for studying the shape, size, distribution and packing of QD cores using SAXS either in transmission or in grazing incidence (GISAXS) while dispersed or packed in various media. Similarly, with the potential for multiple isotopic contrasts (e.g. using deuterated solvents or ligands), SANS measurements are highly sensitive to the structure of ligands and other molecules adsorbed on the QD surface and to the correlations and packing of the QD cores.

In this presentation, we explore as a case study the characterisation of PbS-OA quantum dots and their functionalisation by exchange with ligands of varying chemistry, including a variety of carboxyl-terminated small molecules. For as-synthesized QD, SAXS allows a rough estimation of OA present in the given solution and an upper bound on the amount that is bound to the PbS cores, whilst some may remain in solution. Solution SANS measurements, utilising contrast variation, can then be used to determine the thickness and density of the OA shell. In combination, SAXS and SANS are therefore also well suited to the study of ligand exchange processes. Using SAXS as a pre-characterisation, SANS resolves the changes in ligand shell scattering length density, which are related back to changes in coverage and packing density and to the degree of native ligand, if any, that is still present. This presentation follows the structural changes that occur during various ligand exchange procedures and discusses some of the self-assembling structures that emerge.

Graphene quantum dots (GQDs) are promising for a number of applications including catalysis, sensing, imaging, and photovoltaics. The advantages of GQDs over traditional inorganic quantum dots include (i) a cheap and large-scale synthesis with abundant carbon sources, (ii) excellent biocompatibility and environmental friendliness, and (iii) a high photodynamic index. One promising approach to tune the emission of GQDs and to enhance photoluminescence quantum yield (PLQY) is the incorporation of heteroatoms such as N, B, P, or S into the aromatic backbone of GQDs. Furthermore, the incorporation of heteroatoms into high-surface-area GQDs can develop highly active electrocatalysts and sensory materials. Traditional methods to produce GQDs require the use of harsh chemicals, long reaction times, and tedious purification steps and allows limited control over the functional groups in the produced GQDs. This presentation will report our recent progress in developing the synthetic technique of laser ablation in liquid (LAL) as a promising alternative method to prepare GQDs. LAL allows for fast production, use of fewer chemicals, simple purification, fewer byproducts, higher production yields, and improved control of the product by precise tuning of laser ablation parameters. We have successfully developed LAL to produce N-doped GQDs from carbon nano-onions in aqueous solutions containing three nitrogen precursors (ammonia, ethylenediamine, and pyridine). The choice of nitrogen precursor molecule allowed for the tuning of both the overall nitrogen content as well as the distribution of nitrogen-related functional groups present in the produced GQDs. The variation of LAL parameters and nitrogen precursors enabled us to tailor photoluminescence (PL) spectral properties and PL lifetimes. We have found that high concentrations of amine groups tend to red shift the emission and exhibit shorter PL lifetimes whereas pyridinic groups caused a blue shift in the emission and exhibited longer PL lifetimes. Additionally, the PL properties of these GQDs was highly dependent on the nitrogen-related functional groups while band gap emission was mainly non-radiative. This presentation will also report the promising electrochemical features of these N-doped GQDs since they demonstrated high activity for the conversion of oxygen to hydrogen peroxide, an important chemical in many industrial applications. LAL was then employed to dope GQDs with other heteroatoms including B, P, and S that further tuned the PL properties and expanded potential applications.

We explore the key factors that determine the emission bandwidth of thermally activated delayed fluorescence (TADF) emitters combining computational and experimental methods of investigation. TADF is one approach to achieve high internal quantum efficiencies (IQEs) in metal-free organic light emitting diode (OLED). In TADF the first triplet (T₁) to first singlet (S₁) reverse intersystem crossing (ISC) is promoted by configuring molecules in an electron donor-acceptor (D-A) alternation, so as to exhibit a small energy gap (DE_ST) between S₁ and T₁ levels. This allows for non-radiative triplet states to up-convert to radiative singlet states and fluoresce. However, the donor-acceptor strategy may result in molecular conformations that produce broad emission spectral bands (FWHM = 70-100 nm). Despite reports suggesting that suppressing D-A dihedral rotation can reduce the emission bandwidth, the actual reason behind the narrower-band emission is not well understood. Our results suggest that the intrinsic TADF emission bandwidth is mainly controlled by the charge transfer character, and that molecular space restriction or rotation have minimal effects. Our results can be used to design molecular organic alternatives with sharp emissions for LED applications.

Mechanoluminescence (ML) is a light emission induced by mechanical action applied to a solid material. This phenomenon has been known for a long time and observed from numerous materials in our daily life. However, unlike photoluminescence or electroluminescence, ML had received little attention as a light-emitting technology because most of previously known ML processes are one-time momentary light emission events which require irreparable deformation of materials like breaking or tearing. Then, in 2013, Jeong et al. [1] reported a unique ML phenomenon that an elastic composite, which is phosphor micropowders (Cu or Mn doped ZnS) embedded in a stretchable polymer (PDMS), exhibits unprecedentedly bright and unwearying ML under repeated (~10^5 times) elastic stresses like stretching, bending, or pressing. This groundbreaking discovery has led to the development of many distinctive ML-based light-emitting technologies such as wind-driven full-color display, handwriting recognition, or magnetic-induced luminescence.
Despite the applications are being continuously expanded, the mechanism of this unique elastic ML process has remained unknown. Most of the previous models relied on a special piezoluminescent characteristic of doped ZnS phosphors called ‘self-recovery’ effect. However, none could fully explain the incomparably high brightness and durability of the new elastic ML. In this presentation, we will introduce a set of experimental evidences that this elastic ML is not due to the characteristics of the phosphor alone but due to the interaction between the phosphor and the elastic polymer substrate. Especially, our findings claim that the interface between the phosphor and the elastic polymer plays the most essential role. We demonstrate that this elastic ML can be completely switched on and off depending on the combination between the compositions of phosphor’s surface coating layer and elastic polymer, and the combination that switches on the elastic ML is the one that maximizes the contact electrification at their interface. We also show that such interface-driven ML is not working with common piezoluminescent materials but only working with high quality electroluminescent phosphors like doped ZnS. These findings strongly suggest that the elastic composite ML is in fact a lighting up process of electroluminescent phosphors by the internal microscale contact electrification at the phosphor–polymer interface. Moreover, our conclusion implies that this can be further developed as a general strategy for designing new composite materials using various nanoscale luminescent materials that can efficiently convert mechanical energy into lights.
[1] Jeong et al., Appl. Phys. Lett. 2013, 102, 051110; Jeong et al., Adv. Mater. 2013, 25, 6194–6200.

Currently, cesium lead halide perovskite(CsPbX₃ X = Cl, Br, I) nanocrystals (NCs) have been attracted enormous attention for optoelectronics materials due to their prominent optical properties such as excellent color reproducibility and high photoluminescence quantum yield (PLQY). Herein, we demonstrate a simple coating process of perovskite NCs by adding (3-Aminopropyl) triethoxysilane (APTES) with precursor to fabricate stretchable and flexible light emitting diode (LED). The chemical, optical, and structural properties of perovskite NCs without and with APTES treatments. It was demonstrated that APTES treatment successfully creates silica shell, which improves the stability of perovskite NCs by passivating their surface. As a result, silica coated perovskite can be embedded in polymer matrix without any degradation structure and luminescence efficiency. By embedding NC in PDMS, we fabricated stretchable color filters showing high color reproducibility and remaining high luminescence, stretchable LEDs with mixture of PDMS/Cu-doped ZnS and silica coated perovskite NCs, with improved durability and stability.

Metal-halide perovskites (hereafter, perovskite) are promising materials for a light emitter of light-emitting diodes (LED) based on their narrow emission spectrum and the easy bandgap tunability. In the top-down approach using the perovskite dissolved in polar solvent such as dimethyl sulfoxide (DMSO), the growth mechanism can be easily controlled by using various additives and applying solvents, so small grain size with strong exciton confinement can be easily achieved. As the bottom-up approach, perovskite nanoparticles synthesized with passivating ligand have high photoluminescence quantum efficiency (PLQE) due to the strong exciton confinement. Both methods are promising way to develop the highly efficient PeLEDs, but these have been mainly studied by spin-coating process. Spin-coating process is a good process to easily control the morphology of perovskite film but is not suitable for fine patterned microarrays that meet criteria of high-definition display.
In this work, we induced an inkjet manipulated approach for fine patterned perovskite microarrays. Combining with a high viscosity co-solvent, the outward capillary flow as the cause of ring stains can be suppressed. Also, adding the additive into the perovskite ink, we modulated a crystal growth mechanism of perovskite ink, and finally obtained perovskite microarrays with perfect morphologies. With these results, we provide insight on how to fabricate perovskite microarrays for efficient PeLEDs.

Herein, we demonstrate the post-synthesis engineering methods to precisely control size and dimensionality (0D/1D/2D) of as-synthesized all-inorganic cesium lead halide perovskite (CsPbBr₃) nanocrystals (NCs). We investigate the chemical effects of polar solvents properties such as immiscibility, polarity, and boiling point on the surface of NCs as well as structural and optical properties. By appropriately utilizing the properties of solvent, the effect of polar solvent could be indirectly and mildly conveyed to the NCs making them lose their ligands and attached with proximal NCs without destruction. Based on our observation, we developed “Immiscible solvent phase mixing” method to induce epitaxial growth of CsPbBr₃ NC. The size of NCs can be easily tailored by controlling the engineering time. Taking advantage of the minimal effect of mild solvent, we also developed liquid-air interface self-assembly method to systematically control the dimensionality. At the interface, NCs could be horizontally assembled and grown into large area, single crystalline nanowire (1D) and nanoplate (2D) via the oriented attachment process. Finally, we discussed the origin of non-destructive epitaxial growth phenomenon with suggested model system.

Metal halide perovskites are gaining increasing interest for their light emitting properties and have become attractive for a wide range of optoelectronic devices such as light emitting diodes (LEDs), light emitting field-effect transistors (LE-FETs), lasers and scintillators. [1]
While high efficiencies have been reached with 3D dimensional perovskites, the research community keeps exploring alternative structures to boost the luminescence yield and increase the material’s tunability. In particular, the increased structural flexibility has revived great interest in low dimensional perovskites. [2] Here, the possibility to increase the exciton binding energy by spatial and dielectric confinement is believed to be beneficial to improve the radiative recombination efficiency. However, with the exception of multidimensional perovskites of the Ruddlesden-Popper series where luminescence is strongly enhanced due to energy funneling effects, the quantum yield of single-layered 2D perovskite films remains low (<3%). This is likely due to detrimental trap-assisted recombination, and has so far hampered their effective integration into electroluminescent devices.
To overcome such issues and improve the emission properties of 2D perovskites, here we identify two synthetic and defect engineering strategies, [3] one involving the use of photoactive organic cations (such as 1-naphtylmetylammonium) and one based on metallic doping involving the use of transition metals (e.g. Mn²⁺) and lanthanides (e.g. Eu³⁺, Yb³⁺). [4] While both strategies allow the tuning of perovskite luminescence from the visible down to the near infrared region (NIR), we find that highly efficient energy transfer from the perovskite to the metallic ion can be achieved in case of inorganic doping, provided that a suitable energy level alignment of the host-guest system is realized. In particular, by using temperature-dependent and time-resolved spectroscopy, we demonstrate that the presence of Mn²⁺ ions allows to overcome the funneling of the photoexcited species in inefficient recombination pathways (e.g. permanent traps) thus improving the photoluminescence quantum yield beyond 20% in perovskite films. Finally, we provide a proof-of concept demonstration of the electroluminescence properties of such doped-systems in light emitting diodes. [3]
Our work shows the potential of the doping strategy for the tuning and enhancement of the perovskite’s luminescence properties and further stimulates the investigation of a broader range of host/guest systems.

[1] S. A. Veldhuis et al, Adv. Mater. (2016), 28, 6804–6834
[2] D. Cortecchia et al, J. Mater. Chem. C (2019), 7, 4956-4969
[3] D. Cortecchia et al, Chem (2019), accepted
[4] Y. Zhou et al, Chem. Mater. (2018), 30, 6589−6613

Meeting the high demand for lanthanide-doped luminescent nanocrystals across a broad range of fields hinges upon the development of a robust synthetic protocol that provides rapid, just-in-time nanocrystal preparation. However, to date, almost all lanthanide-doped luminescent nanomaterials have relied on direct synthesis requiring stringent controls over crystal nucleation and growth at elevated temperatures. Here we demonstrate the use of a cation exchange strategy for expeditiously accessing large classes of such nanocrystals. By combining the process of cation exchange with energy migration, the luminescence properties of the nanocrystals can be easily tuned while preserving the size, morphology, and crystal phase of the initial nanocrystal template. This post-synthesis strategy enables to achieve, for the first time, upconversion luminescence in Ce³⁺ and Mn²⁺-activated hexagonal-phased nanocrystals, opening a gateway towards applications ranging from chemical sensing, biological imaging to anti-counterfeiting.

The stability of solution-processed semiconductors remains an important area for improvement on their path to wider deployment. Inorganic caesium lead halide perovskites have a bandgap well suited to tandem solar cells but suffer from an undesired phase transition near room temperature. Colloidal quantum dots (CQDs) are structurally robust materials prized for their size-tunable bandgap; however, they also require further advances in stability because they are prone to aggregation and surface oxidization at high temperatures as a consequence of incomplete surface passivation. Here we report 'lattice-anchored' hybrid materials that combine caesium lead halide perovskites with lead chalcogenide CQDs, in which lattice matching between the two materials contributes to a stability exceeding that of the constituents. We find that CQDs keep the perovskite in its desired cubic phase, suppressing the transition to the undesired lattice-mismatched phases. The stability of the CQD-anchored perovskite in air is enhanced by an order of magnitude compared with pristine perovskite, and the material remains stable for more than six months at ambient conditions (25 degrees Celsius and about 30 per cent humidity) and more than five hours at 200 degrees Celsius. The perovskite prevents oxidation of the CQD surfaces and reduces the agglomeration of the nanoparticles at 100 degrees Celsius by a factor of five compared with CQD controls. The matrix-protected CQDs show a photoluminescence quantum efficiency of 30 per cent for a CQD solid emitting at infrared wavelengths. The lattice-anchored CQD:perovskite solid exhibits a doubling in charge carrier mobility as a result of a reduced energy barrier for carrier hopping compared with the pure CQD solid. These benefits have potential uses in solution-processed optoelectronic devices.

Reference: Liu, M. et al. Lattice anchoring stabilizes solution-processed semiconductors. Nature 570, 96–101 (2019).

Recently, all-inorganic metal halide perovskite nanocrystals (PeNCs) with a general formula CsPbX₃ (X=Cl^-, Br^-, I^-) have been extensively studied for optoelectronic applications such as solar cells, light-emitting diodes (LEDs), photodetectors, and lasers due to their tunable bandgap, high photoluminescence (PL) quantum yield, long charge diffusion length, high absorption efficiency, and low-cost solution processability. The bandgap energies and PL spectra can be easily tuned over the entire visible spectral region of 410 ~ 700 nm through a halide anion exchange process. Additionally, PeNCs exhibited a narrow PL linewidths and high stability against heat and light, so they are widely studied as next-generation LED materials. [1,2] However, the instability of PeNCs to water or polar solvents due to their ionic bonding property is a major obstacle to their device applications. Several studies have been conducted on the encapsulation of PeNCs using silica or polymer materials such as polyhedral oligomeric silsesquioxane (POSS), mesoporous silica, tetramethylorthosilicate (TMOS), and polyvinylpyrrolidone (PVP). [3-5] In this study, we describe the introduction of encapsulation by modified ligand-assisted reprecipitation (LARP) method in which the perovskite precursor salts solution is dropped directly into the UV curable prepolymer solution at room temperature. It is confirmed that PeNCs encapsulated by UV cured polymer have excellent moisture stability. We will present the data of UV-Vis absorption, PL, and XRD measurement of PeNCs-polymer composite films in detail. This work provides a simple and robust method for the preparation of stable and bright PeNC films that can applicable for water-stable and highly efficient wide color gamut LED displays or other optoelectronic devices.

[1] H. Huang, M. I. Bodnarchuk, S. V. Kershaw, M. V. Kovalenko, A. L. Rogach, ACS Energy Lett., 2, 2071 (2017)
[2] Q. A. Akkerman, G. Rainò, M. V. Kovalenko, L. Manna, Nat. Mater., 17, 394 (2018)
[3] H. Huang, B. Chen, Z. Wang, T. F. Hung, A. S. Susha, H. Zhong, A. L. Rogach, Chem. Sci., 7, 5699 (2016)
[4] H. C. Wang, S. Y. Lin, A. C. Tang, B. P. Singh, H. C. Tong, C. Y. Chen, Y. C. Lee, T. L. Tsai, R. S. Liu, Angew. Chem. Int. Ed., 55, 7924 (2016)
[5] J. Hai, H. Li, Y. Zhao, F. Chen, Y. Peng, B. Wang, Chem. Commun., 53, 5400 (2017)

(Acknowledgements; This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (NRF-2019R1C1C1007569).)

In recent years, colloidal II-VI nanoplatelets have become promising gain media for lasers [1] offering properties superior than those of the conventional nanocrystals. Using the nanoplatelets, thresholds for amplified spontaneous emission were lowered to a few µJ/cm² [2] with gain lifetimes reaching up to 0.5 ns [3]. In addition, early reports pointed out that modal gain coefficients in the nanoplatelets can be large [1, 2]. However, limits of the modal gain, which are important for practical laser applications, have not been understood nor elucidated to date. Here, we systematically investigate modal gain coefficients in CdSe nanoplatelets by performing variable stripe length measurements as a function of optical pump intensity [4]. We find that modal gain coefficients in the CdSe nanoplatelets at room temperature reach a remarkable level of 6,600 cm^-1 with a material gain up to 15,300 cm^-1. The modal gain coefficient measured in the CdSe nanoplatelets is the largest among any other gain media at room temperature, and the material gain coefficient is on par with the best reports using epitaxially-grown quantum wells. We find that giant modal gain coefficients are ubiquitous to the CdSe nanoplatelet family as confirmed by samples having different vertical thicknesses and lateral sizes [4]. The large gain coefficients of the nanoplatelets indicate that gain-active species, i.e., biexcitons, are highly persistent even under high pumping levels. Therefore, nonradiative processes including Auger recombination do not rapidly deplete biexcitons in the nanoplatelets as they do in typical quantum dots, overcoming an important bottleneck against achieving efficient lasing in colloidal semiconductors.
[1] B. Guzelturk et al. ACS Nano 8, 6599 (2014), [2] C. She et al. Nano Lett. 14, 2772 (2014)
[3] B. Guzelturk et al. J. Phys. Chem. Lett. 8, 5317 (2017). [4] B. Guzelturk et al. Nano Lett. 19, 277 (2019)

Micro-scale structured dielectric substrate have been widely employed for platforms for photonic devices such as photodetectors, photovoltaic devices, and light emitters. When the platforms are integrated with metallic nano-structures, they can also work as plasmonic platform supporting strong near-field in a form of surface plasmonic resonance (SPR) or gap plasmonic resonance (GPR) In the present study, we developed a plasmonic structure composed of dielectric micro-sphere which are coated with metallic nanoparticles. Due to the spherical shape, the structure exhibited strong absorption behavior in a wide range of visible (Vis) and near infrared (NIR) spectrum. The strong absorption was found to be due to strong resonance in the structure resulting from Fabry-Perot spherical cavity resonance and extended optical path length derived by metallic nanoparticles on the surface. As well as the strong absorption, the plasmonic structure also exhibited strong light emission properties in Vis spectrum. In particular, we observed whispering gallery mode (WGM) resonance-derived facilitated light emission, which was experimentally checked by upconversion nanoparticles (UCNPs) on the surface of the structure. When aligned in a form of micro lattice, the plasmonic micro-spheres also exhibited additional advantages in light emission such as wide viewing-angle and higher haze efficiency, which are promising in developing NIR-to-Vis light emission film.

In view of optical gain application, colloidal quantum dots (CQDs) are suffering from band-edge state degeneracy which requires multiple-exciton to achieve population inversion. However, fast and efficient Auger process in the CQDs containing multiple-exciton increases the lasing threshold and limits the gain lifetime. Here, by applying the electric field to the quasi type-II CQDs (CdSe/CdS/ZnS core/shell/shell) embedded in the Sawyer−Tower circuit, we have demonstrated tunable amplified spontaneous emission (ASE) threshold in a long-sought practical device where the CQDs sandwiched between two dielectric layers retain their high quantum efficiency as in parent solution (quantum yield of > 70%). Singly-charged CQDs help building up population inversion due to pre-existing electrons while strongly enhanced Auger recombination in multiple-charged CQDs (i.e., doubly charged exciton) stymies the optical amplification. This approach allows us to fine-tune and achieve the optimal charging level to utilize the advantages of singly charged CQDs and reduce the adverse effect of doubly charged CQDs.

In addition to demonstrating tunable optical gain experimentally, we also developed a kinetic equation model to systematically analyze the electric field dependent light amplification behavior systematically. The kinetic model not only confirms our experimental results but also presents to be a reliable tool for accessing the requirements of the charging level to achieve nearly zero-threshold gain in CQDs. The implications, then, to potential applications of our robust and environment-undependable tuning method are broad, from controlling exciton recombination dynamics to continuous wave (CW) or possibly electrically pumped CQD lasers.

In atomically thin transition metal dichalcogenides (TMDs), biexcitons have sparked an unprecedented research enthusiasm since the first observation by Heinz and colleagues (Nature Phys. 11, 477–481, 2015). While TMDs are still suffering from synthesis cost, upscaling capability and quantum efficiency, colloidal nanomaterials can be an appealing complement to TMDs for biexciton sources. The specific hurdles and problems in demonstrating radiative, sustained biexcitons at room temperature in colloidal nanomaterials were earlier investigated by other groups: Klimov, Sargent, Moreels and others. Specific examples of major technical hurdles were manifested through: i) the efficient Auger recombination in multiple-carrier and quantum-confined environment, leading to ultrafast deactivation the high-order correlated excitonic states; ii) relatively small biexciton binding energy, which is comparable to the thermal energy at room temperature and much smaller than the inhomogeneous broadening of exciton emission, causing the biexciton emission only be resolved/observed at cryogenic condition.
Here, by doping the CdSe colloidal quantum wells with transition metal ions (copper) to create a new Coulomb interaction environment for band-edge excitonic resonance, we have reached and showed proof of sustained, well-resolved biexciton emission in a long-sought “all solution processable regime”- enabling the easy integration of potential quantum emitters into almost any optoelectronic quantum architectures. Suppressed Auger loss in 1D quantum confinement and reduced Coulomb screening assisted by dopant-host interaction enables a biexciton binding energy of ~64 meV, a factor of 2 greater than the values reported in other colloidal nanomaterials and comparable to the value reported in TMDs. The biexcitons are robust, optically and mechanically in ways that have no parallel in the reported literature in terms of colloidal nanomaterials.

Near-to-mid-infrared (IR) photodetection technologies have the potential to revolutionize the infrastructures of surveillance and manufacturing by enabling military or civil night vision, environmental gas monitoring, and chemical spectroscopic analysis. However, current com ercial IR photodetectors, particularly those with beyond 2000 nm spectral response, rely on expensive and size-limited epitaxial growth processes that are not compatible with silicon wafer technologies. Colloidal HgTe quantum dots (QDs), with a spectral response spanning almost the entire near-to-mid-IR range, are a promising material candidate for the new generation photodetectors.
However, at room temperature, the HgTe QD based photodetectors exhibit rather low specific detectivity of less than 109 Jones (or cm Hz1/2 W−1 in SI units); therefore, HgTe QD based photodetectors have to work at low temperatures to reduce dark currents and obtain better detecting performance. This requires the use of large cooling systems, which leads to a huge increase in the cost and limitation of the application, mak ing it difficult to achieve miniaturized photodetection systems for portable imagers, spectrometers and sensor network applications.
The limitations of HgTe QD based photodetectors intrinsically rise from the fact that HgTe QDs have low charge transfer properties and poor colloidal stability in ambient air condition. Combining HgTe QDs with a stable and highly conductive matrix, forming hybrid IR photodetectors is very promising to mitigate the limitations, as the conductive media can help with the facilitating charge transport mobility, providing great mechanical and chemical stabilities, and the HgTe QDs can act as the light absorber for sensing. P3HT, a well-established conjugated conductive polymer with excellent electronic properties, compatible band gaps and great mechanical and chemical stabilities, is an ideal candidate for the matrix material. However, achieving a controllable bicontinuous percolation network and a well-defined interface between QDs and the polymer matrix remains challenging, as it is very difficult to disperse high-density HgTe QDs uniform into P3HT matrix without aggregation, as it’s critical to tune the interface properties between HgTe and P3HT with efficient electron-hole separation and uniform phase distribution.
Here, we demonstrate for the first time a facile method to synthesize and control the nanoscale morphology and interfacial properties of poly(3-hexylthiophene) (P3HT) matrix and HgTe QDs hybrids using chemical grafting and ligand exchange methods. Solvent-assisted chemical grafting and ligand exchange were used to control the interface of P3HT/HgTe nanohybrids and the HgTe QD interparticle distance, respectively. It’s demonstrated that the formation of an interpenetrating and percolating P3HT/HgTe network gives rise to efficient charge separation and charge transport, significantly improving photovoltaic performance, which is promising for high-performance and low-cost IR photodetector applications.

In recent years, perovskite quantum dots(PeQDs) have received a lot of attention for many application. however, PeQDs were particularly unstable and the optical properties were readily degraded because of its structural instability. To overcomes these problems, We synthesized perovskite using new cations. Mixed-cation Cs_xRb_1-xPbX₃ (X = Cl, Br, I) perovskite quantum dots (PeQDs) are developed and show high quantum yields of 93% and 86% for green and blue wavelengths, respectively. The stability is significantly improved under heat, UV, and water aging conditions. We also fabricated the film by applying cyclic olefin copolymer to perovskite for the first time. The films have a wide color gamut covering up to 104.15% of the BT.2020-defined color space, with the white light color coordinates of (0.33, 0.32), luminance of 68.86 Cd/m2, and correlated color temperature of 5299 K at 20 mA

Mixed 2D/3D perovskite films with self-assembled quantum wells have significantly improved the performance of perovskite light emitting diodes (PeLEDs). In this work, such films are fabricated through a two-step interdiffusion method that is widely employed in processing of perovskite solar cells, however, remains rarely explored for PeLEDs. Mixed 2D/3D perovskites based on FAPbBr₃ and (MAPb(I/Br)₃, with the incorporation of large-cation ligands, are fabricated for green and near-infrared emission, respectively. We systematically studied the processing condition on the morphology and optoelectronic properties of the resulting perovskite films. Moreover, with pumped-probe measurements, we also provide insight on how the charge dynamics change when tuning 3D perovskite to a mixed 2D/3D one, which significantly improves the radiative recombination. The PeLEDs based on two-step fabricated mixed 2D/3D perovskites show maximum external quantum efficiency (EQE) of 7.36% (green) and over 16% (near-infrared), which are significantly higher than PeLEDs using the corresponding 3D perovskites. Finally, we present the importance of balanced charge injection on improving EQE roll-off as well as device stability. The electron injection is tuned systematically by doping the electron transport layer, reaching a balanced electron and hole injection at optimal condition. we achieve EQE> 10% beyond 100mA/cm² and a remarkable half lifetime beyond 40 h at 10 mA/cm² for the optimized device with balanced injection.

Quantum dots (QDs) have emerged as one of the most promising semiconducting nanoparticles (NPs) for high-performance information displays due to their tunability of the emission wavelength, narrow full width at half maximum (FWHM), and high quantum efficiency. In the QD-based displays, the uniform dispersion of the QDs is one of the critical issues to achieve the emission uniformity over the entire area of an individual pixel and to suppress the aggregation-induced quenching. In general, the QDs can be dispersed in a variety of host media such as organic semiconducting materials and cross-linkable polymers (hereafter referred to the QD hybridization). In the direct color conversion by the photo-luminescent (PL) QD hybridization, the color purity and the PL intensity of the QDs are limited due to the localized aggregation of the QDs in the host matrix. Thus, it remains a challenge to produce highly uniform dispersion of the QDs for the high color purity and the high PL intensity in well-defined patterns.
In this work, we developed a simple and effective method of achieving the uniform QD hybridization with the molecular field effect of the host matrix on the dispersion of the QDs. In order to utilize the molecular field effect, a reactive mesogen (RM), which exhibits the liquid crystal (LC) properties and can be photo-polymerized, was used as the host matrix. Two types of the QD hybrid films were prepared depending on whether the polymerized RM matrix was aligned or not. A solution of red QDs (with the peak wavelength of 620 nm), dissolved in the homogeneous RM solution, was spin-coated on a glass substrate with or without the homogeneous alignment and then photo-polymerized under the ultraviolet (UV) exposure. The RMs aligned along the homogeneous direction indeed suppressed the aggregation of the QDs due to the high degree of the molecular ordering of the RMs through the photo-polymerization. Based on the 3-dimensional emission profiles, probed using the confocal microscopy, in the ordered QD hybrid film, the mean peak-to-peak distance between two adjacent emission sites was found to be about 60 nm and the FWHM was comparable to the radius of an individual QD (about 40 nm). In contrast, in the non-ordered RM film, the FWHM was quite arbitrary and the distance between two emission sites was random due to the aggregation of the QDs. Note that the intensity of the emission spectrum of the ordered QD hybrid film increased with increasing the weight concentration of QDs. The aggregation of the QDs appeared at about 2 wt. % of the QDs.
We examined the electro-optical properties of the PL-based QD display consisted of the ordered QD hybrid patterns, a light modulator, and an excitation light source. A simple vertically aligned LC cell was fabricated as the light modulator. The blue LED with the peak wavelength of 455 nm was used as the excitation light. The QD hybrid film was patterned by the UV exposure through a photomask, followed by the removal of the un-polymerized region. The same process for red QDs was repeated for green QDs (the peak wavelength of 530 nm). For the blue patterns, the dummy regions containing blue absorbers were used for balancing the intensity. As the applied voltage increased, the intensity of the emission spectrum of the QDs increased according to the change in the phase retardation which converts the polarization state of the excitation light. This leads directly to the modulation of the emission spectra of the QDs with the color gamut extended to 82 % of the BT. 2020 standard. Our approach will provide a useful guideline to produce the QD hybridization for emissive displays with high color purity.
This work was supported in part by Samsung Display Co. and BK21 Plus Program funded by Ministry of Education of Korea.

Application of semiconductor nanocrystals increased over the years in fields such as solar cells, sensors, light-emitting diodes and electronics. Particularly, a form of nanocrystal known as quantum dots (QDs) has gained interest with the recent development in displays. With easy color tunability, high color purity, and high quantum yield, QDs has been selected as the candidate for next-generation displays. Because of use commercially, there is a movement for shifting the materials composition from Cd-based to Cd-free core shell QDs such as InP/ZnS. However, these attractive initial properties are difficult to maintain when undergoing fabrication and long-term operation. This is especially the case for processing QDs at elevated temperatures. During thermal processing of QDs for display application, the quantum efficiency suffers a substantial loss. From the electronic process standpoint, the loss is due to carrier ejection to trap states or electron-hole pair ejection. In addition, the materials composition and structure can vary upon thermal processing due to phenomena such as particle sintering, Ostwald ripening, or surface composition change. Furthermore, oxidation of the shell can influence the quantum yield to decrease more than 50% as well as cause blue or red shift of the photoluminescence spectra.
In order to prevent degradation from thermal processing, we believe the key is to create a protective layer for the QD. Here, we utilize block copolymer (BCP) to act as a protective layer through micelle formation. The QDs were loaded into the core of the block copolymer micelles, using self-assembly property of block copolymer. The micelle core wraps around the QD expecting to hold ligands in place while the micelle corona gives additional protection of the QD from the surrounding environment. This will provide a two-way protective layer to the Cd-free core-shell QD preventing outer shell of the QD from ligand detachment and oxidation. The block copolymer protective layer is expected to improve the thermal stability and retain PL of 90% after post thermal treatment.

Reference
1. Rowland, C. E. et al. ACS Nano 8, 977–985 (2014).
2. Watanabe, T., Iso, Y., Isobe, T. & Sasaki, H. RSC Adv. 8, 25526–25533 (2018).
3. Kim, H. Y. et al. J. Am. Chem. Soc. 138, 16478–16485 (2016).

Lead-halide perovskite APbX₃ (A=Cs or organic cation; X=Cl, Br, I) nanocrystals (NCs) are subject of intense research due to their exceptional properties as both classical¹ and quantum light sources.^2-4 Many challenges often faced with this material class concern the long-term optical stability, a serious intrinsic issue connected with the labile and polar crystal structure of APbX₃ compounds. When conducting spectroscopy at a single particle level, due to the highly enhanced contaminants (e.g., water molecules, oxygen) over NC ratio, deterioration of NC optical properties occurs within tens of seconds, with typically used excitation power densities (1-100 W/cm²), and in ambient conditions.
Here,⁵ we demonstrate that choosing a suitable polymer matrix is of paramount importance for obtaining stable spectra from a single NC and for suppressing the dynamic photoluminescence (PL) blueshift. In particular, polystyrene (PS), the most hydrophobic amongst four tested polymers, leads to the best optical stability, one-to-two orders of magnitude higher than that obtained with poly-(methyl methacrylate) (PMMA), a common polymeric encapsulant containing polar ester groups. Molecular mechanics simulations based on a force-field approximation corroborate the hypothesis that PS affords for a denser molecular packing at the NC surface. These findings underscore the often-neglected role of the sample preparation methodologies for the assessment of the optical properties of perovskite NCs at a single particle level and guide the further design of robust single photon sources operating at room temperature.

References:
1) Akkerman et al. Nature Materials 17, 394–405 (2018)
2) Becker et al. Nature 553, 189–193 (2018)
3) Rainò et al. Nature 563, 671–675 (2018)
4) Utzat et al. Science 363, 1068–1072 (2019)
5) Rainò et al. Nano Letters 19, 3648–3653 (2019)

The best perovskite LEDs have external quantum efficiencies that approach or even exceed the limit expected from classical models of optical outcoupling with perfect internal quantum efficiency. I will discuss how light is transferred from trapped modes to outcoupled modes in these devices, presenting measurements of laterally resolved electroluminescence and photoluminescence that can distinguish between scattering and other mechanisms. We find an important contribution from photon recycling, where trapped photons are absorbed in the emissive layer and re-emitted, giving another chance to enter an outcoupled mode. I will discuss the effects of parasitic absorption in non-emissive electrode layers, and will present measurements on new geometries which further enhance optical outcoupling.

Energy looping Lanthanide-doped nanoparticles were introduced recently as an attractive gain medium for continuous-wave upconverted lasing. A useful architecture for constructing microlasers with these materials involves their adsorption onto dielectric microbeads that support stable whispering gallery mode (WGM) resonances. Despite recent advances in upconverting nanoparticle microlasers, improvement in resonators’ sizes, lasing threshold and reliable processability is needed for their utilization in different imaging, sensing and photonics applications. Our previously reported microsphere resonators exhibited a broad distribution of lasing thresholds and processing variability which limit their practical applications. In this work, a more complete understanding of the nanoparticles’ deposition process, and the structural factors that determine efficient lasing was achieved. We demonstrate using correlative microscopy that the quality of WGMs is significantly influenced by structural characteristics of the beads’ coating layer of nanoparticles, such as thickness and surface roughness. With this understanding, superior lasing was achieved following delicate control over the self-assembly of nanoparticles on the microcavity’s surface. We show that 5-µm microspheres with controlled sub-monolayer UCNP coatings exhibit 25-fold lower laser thresholds (1.7 kW/cm2) and 30-fold lower variability compared to the lowest threshold UCNP lasers. WGMs are observed in the upconversion spectra for microspheres as small as 3 µm, for which optical losses had previously prevented observation. These advances will enable the fabrication of more efficient upconverting lasers for imaging, sensing, and actuation in optically complex environments.

Solution-processed perovskite semiconductors have emerged as efficient light emitters due to their bandgap tunability, high color purity, and favorable optical gain. Recently, optically pumped lasers from perovskite thin films have been demonstrated in various feedback configurations under either pulsed or continuous-wave excitation. However, unlike colloidal quantum dots or organic materials, the reports of lasing have been confined to either the green or near infrared spectral range, lacking red-emitting lasing due to undesirable halide phase separation. Through proper selection of self-assembled bulky ligand additives and the use of a cesium-mixed cation in the mixed-halide perovskite composition, we are able to suppress phase separation. In this study, we demonstrate widely tunable distributed feedback (DFB) lasers operating at room temperature. Our optimized mixed-halide perovskite films were prepared on quartz gratings and achieve single-mode lasing across the green and near infrared with a pump threshold of 4 µJ/cm² and full-width-half-maximum of 0.65 nm under pulsed optical excitation. The laser devices were stable and robust so that the lasing emission lasted for more than 40 hours (~ 10⁶ laser pulses) under sustained excitation.
Despite extensive research and progress on perovskite-based light-emitting diodes (LEDs) and lasers, realization of electrically driven solution-processed perovskite laser devices still remains elusive due to several barriers such as required high threshold current density, robust transport layers, and thermal management. Recently we have shown that hybrid perovskite LEDs can operate at the high current density regime (up to 620 A/cm²) with no pronounced non-radiative Auger recombination. Further investigations enabled us to achieve amplified spontaneous emission from a typical perovskite LED structure, where the active layer is sandwiched by organic transport films. Methylammonium lead iodide perovskite with nanometer-sized crystallites have been optimized to provide high optical gain coefficient (~ 500 cm^-1), verified by a variable-stripe-length measurement. Then, second-order DFB Bragg gratings were patterned directly on indium tin oxide, acting as both transparent electrode and optical feedback structure. From this LED device architecture, we achieve single-mode DFB lasing with a threshold of 7 µJ/cm² and full-width-half-maximum of less than 1.0 nm through pulsed optical excitation. At the same time, this device was able to operate as an LED under direct current drive with a maximum external quantum efficiency (EQE) of 0.7%. In particular, the peak EQE occurred at a current density of 1.3 A/cm². Extrapolating from our previous work, these results suggest that operation at pulsed current densities of a few kA/cm² is achievable, putting the goal of electrically stimulated lasing within reach.

Organic-inorganic hybrid perovskites in polycrystalline films have demonstrated efficient amplified spontaneous emission (ASE). Essentially, amplified spontaneous emission can be enabled by initiating the cooperative interaction between light-emitting states. Here, we consider the orbit-orbit interaction that can occur through short-range and long-range interaction channels. Specifically, the short-range and long-range orbit-orbit interactions are occurred through magnetic dipole-dipole interaction and polarization-polarization interactions, fundamentally influencing the interaction between light-emitting states. In this study, we intentionally tuned the orbit-orbit interaction between short-range and long-range channels and simultaneously study the length scale of light-emitting states by using magneto-ASE measurements during ASE characterizations in polycrystalline perovskite (MAPbBr₃) films where the grain formation are effectively managed. Interestingly, by concurrently inducing fast and slow crystallization upon mixing two precursor solutions ( PbBr₂ + MABr and Pb(Ac)₂*3H₂O + MABr), the polycrystalline perovskite films can be prepared with large grains of micrometers attached with small grains of nanometers, leading to self-doping effects. In this situation, the small and large grains functions as doping agents and light-emitting centers, respectively. Especially, the self-doping realized by attaching small grains to the surfaces of large grains provides a unique method to tune the orbit-orbit interaction between short-range and long-range channels. As a consequently, cooperative spatially extended states can be formed, leading to low-threshold ASE. This presentation will discuss the tuning of orbit-orbit interaction towards generating cooperative spatially extended states by managing grain formation for generating ASE in organic-inorganic hybrid perovskites.

There is need for efficient narrow band red and green luminescent materials for white light LEDs and LED-based display backlights to improve the energy efficiency and color gamut. Recently CdSe and InP semiconductor nanoparticles (known as quantum dots, QDs) were introduced in displays (e.g. Samsung QLED TV) because of their high efficiency and relatively narrow emission bands. A promising alternative for these QDs are CdSe nanoplatelets (2D NPLs) because of their narrower spectral linewidth compared to 0D and 1D confined nanostructures[1].
To enhance the quantum yield, stability and to tune the emission towards the red, a CdS or ZnS shell is grown around the NPLs. Unfortunately, core-shell NPLs do not exhibit the same narrow spectral linewidth. The difference in spectral width for core and core-shell NPLs has been explained by a difference in electron-phonon coupling after shell growth [2]. Here we aim to obtain insight in the origin of the difference in spectral width for core and core-shell NPLs. We investigated the temperature dependent line broadening of CdSe QDs, NPLs and core-shell NPLs . Measurements were performed over a wide temperature range, between 4 and 423K and included measurements at elevated temperatures which are relevant for practical application in displays and white light LEDs but are usually neglected.
The line broadening studies reveal that homogeneous line broadening is similar for the various systems studied (core and core-shell NPLs and QDs). Homogeneous line broadening was found to be size dependent and small QDs (2.5nm diameter) showed a noticeably stronger homogenous broadening compared to the other systems. The significantly narrower band emission of core NPLs is explained by strongly reduced inhomogeneous broadening compared to core-shell NPLs and QDs. The larger spectral linewidth of CdSe core-shell NPLs is not due to an increase in homogeneous broadening caused by differences in electron-phonon coupling as previously suggested. Instead it is caused by an increased inhomogeneous linewidth after growth of the CdS or ZnS shell, probably related to inhomogeneities of the shell and exciton emission originating from different regions in the core-shell NPLs where the local electronic structure and bandgap are affected by variations in the shell thickness. This is supported by the recent improvements in synthesis procedures for core-shell NPLs which show less inhomogeneous broadening [3].
The present observations show that the spectral linewidth of NPL-heterostructure emission can be improved and is not caused by stronger exciton-phonon coupling due to the shell material. This makes green emitting core-crown NPLs and red emitting core-shell NPLs promising candidates for narrow band emitters in future displays and LEDs. Extending measurements to a regime that matches the operating temperature for the emitters in commercial applications (displays, white light LEDs) is rarely done but highly relevant. Our measurements up to 150 °C show a continuing increase in line width for both QD and NPL emission while the NPL emission line width remains narrower: ∼34 nm for green emitting QDs and ∼15 nm for green emitting NPLs at 150 °C. The significantly narrower emission bands for CdSe NPLs at device operating temperatures give the NPLs a clear advantage and make these emitters competitive with narrow band phosphors which have ∼20-30 nm spectral bandwidths in the green-red spectral region.

[1] Ithurria, S and Dubertret, B., J. Am. Chem. Soc., 133(9), 3070-3077 (2011)
[2] Tessier, M.D. er al., Nano Letters, 13, 3321-3328 (2013)
[3] Rossinelli, A. and Norris, D., Chem. Commun., 53, 9938-9941 (2017)

Quantum dot heterostructures such as core@shell nanocrystals are used as light-emitting materials for applications such as biological imaging, lasers, displays, and solid-state lighting. The inorganic shells of these materials, which confine excitons and chemically passivate these materials, are typically grown on seed nanocrystals through continuous or layer-by-layer methods, which are tedious and time-consuming. Here, we present design rules for more scalable reactions in which uniform shells are grown on CdSe quantum dots in a single step. We leverage high-throughput synthesis along with a custom library of precursors with tunable reactivity to develop a comprehensive understanding of the role of precursor reactivity, ligands, and temperature in these one-step seeded growth reactions. These experiments reveal a narrow region of experimental parameter space that promotes the uniform, purely heterogeneous growth of shell material on the seed particles. This “ideal growth” regime is sandwiched between opposing regimes that lead to secondary nucleation or ripening during growth. Coupled with kinetic simulations, these experiments reveal that precursor reaction rate and monomer solubility determine the balance between secondary nucleation and ripening. Therefore, accelerating the rate of shell growth through the use of more reactive precursors must be accompanied by a precise increase in ligand concentration in order to maintain uniform shell growth. These design rules will be critical for the predictable growth of complex, multi-shell architectures used to engineer band gaps and mediate multi-excitonic processes in high-efficiency quantum dot optoelectronic devices.

The last two decades CdSe-based colloidal quantum dots (CQDs) and most recently perovskite nanocrystals have been under the spotlight for lasing application as exhibit excellent optoelectronic properties such as high photoluminescence quantum yield (PLQY), optical stability and wide emission tunability. Already, CQD technology has delivered ultra-low lasing and stimulated emission thresholds even with continuous wave excitation (CW).¹ Nevertheless, the research has centred in UV-Vis while very few examples has been shown in the low energy part of the electromagnetic spectrum.

On the other hand, light sources (lasers) in NIR is still the last piece missing for the realization of silicon Photonics (CMOS), while photodetectors and modulateors on Si have been already demonstrated. Apart from silicon photonics, the realization of low-cost, solution-processed materials with gain across the telecommunication window is of paramount importance for expanding the optical fiber data capacity beyond the limit of currently used Er-doped amplifiers. The realization of low-threshold and high gain amplified spontaneous emission (ASE) with high degree of tunability across the telecom band has not been achieved yet. The reason behind this challenge has been the high degeneracy of states in infrared CQDs over that of visible CQDs (twofold degeneracy of CdSe over eight-fold degeneracy for PbS-chalcogenides).

To tackle the high degeneracy of the PbS CQD we developed a doping scheme, which allow us heavily dope the PbS CQD filling with electron the conduction band. ² Both theoretical simulations (DFT) and experimental results (UPS) confirm that the Iodine substitution promotes the shift of the fermi energy (E_F) over the conduction band (CB) while via size careful size selection of the CQD we can fine-tune the doping strength. Absorption spectra of the doped PbS CQD films shows a bleach of the band-edge transition according to the doping efficiency, while it is completely bleached when the CB is fully filled with 8 electrons. The doped PbS films are stable for months under ambient conditions while is the first demonstration permanent robust doping in conductive films.

Transient absorption (TA) of both doped and undoped CQD films shows lasing threshold with occupancies <N_th> down to the single exciton regime and <N_th> = 4 respectively.² The average gain lifetime of 27ps was found in doped PbS CQD films. Interestingly, Auger processes do not prevent reaching the gain regime despite the presence of doping. Hence, we performed transient photoconductivity measurements that yield a very low value for Auger coefficient of 10^-31 cm⁶s^-1. In line with the TA results, ASE measurements shows single exciton stimulated emission in doped PbS CQD films at room temperatures. The stimulated emission wavelength ranges from 1500 – 1650 nm covering the whole telecom bands, while the FWHM is as narrow as 14meV. ² Finally, variable stripe-length experiments was performed to measure the net modal gain of 30 cm^-1 in undoped and up to 114cm^-1 for the doped films outraging Er-fibre systems (0.01-0.1 cm^-1) and compares favourably to epitaxial costly III-V quantum wells. For first time, we demonstrate nearly full coverage in telecom wavelengths with stable conductive CQD films, combined with high gain and single exciton threshold paving the way for the realization electrically pumped laser.

K. Wu et al. Nat. Nanotech. , 2017, 10,1140–1147
S. Christodoulou et al. (submitted)

Near unity nanoscale emitters have photoluminescence quantum yields (PLQY) approaching 100%, improving general optoelectronic device efficiency while enabling applications with ultrahigh PLQY requirements (>99%), such as optical refrigeration¹ or luminescent solar concentrators. The precision limit of integrating sphere PLQY measurements (~2%) is insufficient resolve PLQY improvements beyond 98% to successfully screen and thereby further optimize synthetic protocols for near unity emitters. Recently, our groups surpassed this obstacle by developing an ultra-high precision PLQY measurement technique, named photothermal threshold quantum yield (PTQY)².

Photothermal threshold quantum yield (PTQY) is a calorimetric technique, incorporating a photothermal deflection spectrometer and a photoluminescence spectrometer to simultaneously and precisely monitor the average amount of heat energy (non-radiative) and light energy (radiative) given off as a result of each incident pump photon. By removing the need to directly quantify spectral radiance, with sufficient sampling, PTQY can achieve a theoretical precision of 0.02%, an improvement of multiple orders of magnitude in precision over conventional integrating sphere approaches. PTQY was previously applied by our groups to measure near unity CdSe:CdS quantum dots, measuring a peak PLQY of 99.6 ± 0.2% at an optimal CdS shell thickness².

We will report on efforts to extend PTQY to study additional near unity nanoscale emitters, including halide perovskite nanoparticles, which can also achieve near unity PLQY,³ and have promising optoelectronic applications as single-photon sources or in optical refrigeration⁴. Ultrahigh precision PLQY measurements are required to validate such utility. Furthermore, by validating that nanoscale emitters are operating near their thermodynamic limits, we will present how this technique can be applied to more directly quantify the impact of defects on photophysical properties.

1. Sheik-Bahae, M, et. al. Nat. Photonics 2007, 1 (12), 693–699.
2. Hanifi, D. A., et. al. Science 2019, 363 (6432), 1199.
3. Koscher, B. A, et. al. J. Am. Chem. Soc. 2017, 139 (19), 6566–6569.
4. Zhang, S. et. al. Photonic Heat Engines: Science and Applications; International Society for Optics and Photonics, 2019; Vol. 10936, p 1093604.

Enhancing the light-matter interaction from low-dimensional nanoscale emitters has been attracted tremendous attentions for displays and optoelectronic devices. To achieve this goal, traditional methods have been used the plasmonic materials due to reshaping density of states (DOS) and tailing the light emission. However, the main drawbacks of the plasmonic materials are the propagation loss and narrowband resonance, which remains challenging to provide a broadband DOS to explore its functionality. Here, we develop a novel nanoscale core-shell hyperbolic structure with an extremely pronounced coupling effect inside the multishell (Au/SiO₂) nanoscale composite. Then, a giant localized electromagnetic wave of surface plasmon resonance is formed, causing a pronounced out-coupling effect than the plasmonic materials. To integrate the emerging light-emitting material, we mixed the nanoscale core-shell hyperbolic structure mixed with DCJTB dye molecules to measure the lasing spectra. DCJTB dye molecule, a kind of organic laser dye, has been used for solar-concentrator application. The lasing threshold is ultralow of ~42 μJ/cm², which was excited by a 374 nm pulsed diode laser. There is no laser action can be observed for the plasmonic-based Au nanoparticle at the low pumping power density.

Confirmed by simulation results, the nanoscale core-shell hyperbolic structure has achieved ~320 times higher scattering efficiency as compared with the plasmonic-based Au nanoparticle. Besides, the simulated mode volume is ~208 times smaller than the same components of multilayer structure, demonstrating that the nanoscale core-shell hyperbolic structure is able to strongly confine the energy, reduce propagation loss, then a tremendous feedback is formed to trigger the laser action. We believe that the nanoscale core-shell hyperbolic structure steps a great advance in lasing action from nanoscale materials, highly energy conversion efficiency for solar cells, and broadband absorption of optoelectronic devices applications.

Acknowledgments
This work was financially supported by the "Advanced Research Center for Green Materials Science and Technology" from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of Science and Technology in Taiwan (MOST 107-3017-F-002-001).

Metal halide perovskite nanocrystals (NCs) are a new class of semiconductor materials with excellent optical properties. These NCs are anticipated for use in light-emitting devices, because they exhibit high luminescence efficiencies at room temperature and their emission wavelengths can be easily controlled by the halogen composition and/or the NC size. In general, the trion, which is a stable state consisting of one electron–hole pair (exciton) and one excess charge, plays a crucial role in the optical responses of NCs. Trions usually cause photoluminescence intermittency, reduce the luminescence yield, and can also affect the threshold for optical gain. Thus, a deep understanding of the trion dynamics in NCs is important for development of NC-based light-emitting devices.
In this work, we analyze the dynamics of trions in perovskite NCs at room temperature. We studied their dynamics by means of single dot spectroscopy, femtosecond transient absorption spectroscopy, and Z-scan measurements [1-3]. Our results show that the biexciton Auger rate evaluated from femtosecond transient absorption spectroscopy can be expressed as the sum of the Auger rates of positive and negative trions obtained by single dot spectroscopy [4]. We further investigated the excitation fluence dependence of the trion generation. The trion generation in NCs under weak photoexcitation can be controlled by surface modification [5]. Under strong photoexcitation, the intrinsic nonradiative Auger recombination of biexcitons causes ionization of the perovskite NCs and trion generation. An important practical aspect is that the chemical modification of perovskites enables suppression of trion generation in light-emitting devices [6]. We discuss the trion dynamics in perovskite NCs and the mechanism of ionization of perovskite NCs.
Part of this work was supported by JST-CREST (JPMJCR16N3).
[1] N. Yarita et al., J. Phys. Chem. Lett. 8, 1413 (2017).
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[3] K. Ohara et al., submitted for publication.
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[5] S. Nakahara et al., J. Phys. Chem. C 122, 22188 (2018).
[6] H. C. Wang et al., Angew. Chem. Int. Ed. 56, 13650 (2017).

I will report on the optical properties of metal halide perovskite nano-platelets with controllable thickness down to one monolayer [1]. Pronounced quantum confinement effects, large excitonic binding energies and comparably high radiative recombination rates have been found, all depending on the number of monolayers present in the respective nano-platelets. Ultrafast optical experiments provide insight into characteristic charge carrier and spin relaxation scenarios in 2D perovskites [3]. Finally, the assembly of halide perovskite nanocrystals into ordered supercrystals [3,4] and the exchange of ligands lead to remarkable changes of their optical properties.

[1] J. Sichert et al., Quantum Size Effect in Organometal Halide Perovskite Nanoplatelets, Nano Letters 15, 6521 (2015)
[2] V. Hintermayr et al., Accelerated carrier relaxation through reduced Coulomb screening in 2D halide perovskite nanoplatelets, ACS Nano 12, 10151 (2018)
[3] Y. Tong et al., Spontaneous Self-Assembly of Perovskite Nanocrystals into Electronically Coupled Supercrystals, Advanced Materials, 30, 1801117 (2018)
[4] A. Manzi et al., Resonantly Enhanced Multiple Exciton Generation through Below Band-Gap Multi-Photon Absorption in Perovskite Nanocrystals, Nature Comm. 9, 1518 (2018)

Photon upconversion is a process in which two or more low-energy photons are sequentially absorbed in a material resulting in the emission of a higher-energy photon. Photon upconversion has a wide range of potential applications including optoelectronic devices, biotechnology, and solar energy harvesting. Semiconductor nanoparticles have recently emerged as a promising platform for upconversion due to the tunability of their absorption and emission wavelengths through control of particle shape, size, and composition. Additionally, semiconductors have inherently broadband absorption, capturing essentially all photons with energy above their bandgap, which is an important feature for solar energy harvesting applications. Previously our group developed a kinetic rate model and demonstrated upconversion could enhance solar cell performance beyond the Shockley-Queisser limit by providing additional above bandgap illumination from an upconversion layer located behind the cell. We have also demonstrated near-infrared (NIR)-to-visible upconversion photoluminescence in CdSe(Te)/CdS/CdSe core/rod/emitter colloidal quantum dot (QD) nanostructures under continuous wave illumination at solar relevant fluxes. While we and others have demonstrated the feasibility of using semiconductor nanostructures for upconversion, the reported efficiencies are rather low. We recently demonstrated that principles of semiconductor heterostructure engineering can be employed to achieve a factor of 100 improvement in upconversion efficiency. For example, by carefully controlling the core QD composition and introducing a gradient within the nanorod, we reduce nonradiative recombination rates and improve carrier transfer through the nanostructure, resulting in higher upconversion efficiency. We will report on the design, synthesis, and photophysical characterization via ultrafast and time-integrated optical spectroscopy of a range of such structures. We will describe further improvements in upconversion performance that could be realized by employing new colloidal synthesis techniques to optimize our current structures and introducing new particle shapes and elemental compositions.

The electrical generation of emissive excited states is at the heart of electroluminescent (EL) devices. The emissive excited state for colloidal quantum dots (QDs), a class of high-efficient and color-pure luminescent materials, is described as an exciton, i.e. a bound electron-hole pair. Recently, QD-based EL devices demonstrated distinctive performances at both single-dot and ensemble levels. However, the underlying mechanism associated with the electrical generation of excitons in QDs has yet to be clarified. Here we use the recently developed single-dot EL devices as a model system to identify the elementary steps responsible for the electrical generation of a single exciton confined in an isolated QD.
A new experimental technique, namely, optically probing a single emitter under electrical injection, was developed. We repetitively excited a single QD under constant electrical biases by a pulsed laser, enabling optically probing of all possible states associated with the single-dot EL. According to transient PL results, we identified the intermediate negatively-charged state (QD^-) by the characteristic lifetime of the negatively-charged trion (X^-). In addition, the occurrence of the intermediate QD^- was verified by the red-shifted spectrum of the single QD measured under simultaneous optical and electrical excitation. The identification of the intermediate state suggested that the electrical generation of a single exciton in the single QD followed a deterministic pathway comprised of the prior electron injection and the succeeding hole injection. Quantitively, the rate coefficient for hole-injection into a negatively-charged QD, which was substantially greater than that for hole-injection into a neutral QD, was determined to be comparable to that for electron-injection into QD.
This work provides a unique microscopic picture of the electrical generation of single excitons in single QDs at room-temperature. Our findings implicate the vital role of the long-lived intermediate QD^- in boosting charge balance of single-photon EL devices and shall bring new insights into operation mechanisms of state-of-the-art QLEDs.

Perovskite quantum dots (QDs) are now emerging as low-cost alternative emitters for display applications. Very recently, we developed the in-situ fabrication of hybrid perovskite QDs embedded composite films (PQDCF) with high transparency, superior photoluminescence emission and additional processing benefits for down-shifting applications. The potential use of PQDCF as color converters in LCD backlights was successfully demonstrated, showing bright potential in display technology. Moreover, we further explored the electroluminescence devices based in-situ fabricated perovskite QDs. The enhancing role of ligand-assisted reprecipitation (LARP) process was illustrated, providing uniform FAPbX₃ (X=Br, Cl, I) perovskite QDs films with photoluminescence quantum yield up to 78%. The electroluminescence devices with a maximum external quantum efficiency (EQE) of 16.3% and 15.8% were achieved, showing the promising to achieve high efficiency. In all, the in-situ fabrication strategy provides very convenient route for display technology.

Recently, InP colloidal nanocrystals draw immense attention because of their successful debut in the display market and many researches in the field focus on improving the optical properties via various synthetic scheme. Still, understanding on synthetic mechanism, surface chemistry of III-V colloidal nanocrystals is limited which results in inferior optical properties with poorer stability when compared to Cd or Pb based colloidal nanocrystals. Here, I plan to discuss the chemical and physical properties of III-V nanocrystals based on their rather covalent bonding nature. Futher, approaches to enhancing optical properties via chemically passivate deeper trap states from dangling bonds based on microscopic understanding on covalent surfaces will be shared.

Quantum dot (QD) based light-emitting diodes (QLEDs) are promising candidate for next generation display. For practical use of QLEDs, guaranteed operational stability at desired brightness is essential. Yet understanding the origin of operational instability of QLEDs is lagging behind. In this presentation, I explain the origins for the operational instability of QLEDs. The electrical characterization of QLEDs and spectroscopic analysis on the QD emissive layer within devices indicates two main mechanisms for the device degradation. The first is the luminance efficiency drop of the QD emissive layer in the running devices due to the accumulation of excess electrons in QDs, which escalates the possibility of non-radiative Auger recombination processes in QDs. The other is the electron leakage toward hole transport layers (HTLs) that accompanies irreversible physical damage to the HTL by creating non-radiative recombination centers. These processes are distinguishable in terms of the timescale and the reversibility, but both stem from a single origin, the disparity between electron versus hole injection rates into QDs. Ground on this experiment result, I offer rational guidelines that promise the realization of high performance QLEDs with proven operational stability.

A demand for more saturated color display, longer lifespan, improved economic viability, and optimal luminescence efficiency is rapidly growing in optoelectronic industries. Metal halide perovskite has been a rising candidate as our next generation semiconducting material due to the following: widely tunable bandgaps, exceptional efficiency, and relatively facile and inexpensive fabrication processes. Inorganic cesium lead halide perovskite (CsPbX₃) nanocrystals, in particular, have emerged as an ideal option for practical application due to their high performance and intrinsic stability.
Nanocrystals have challenges associated with major charge recombination sites on the surfaces. In order to overcome this problem, surface ligands can be employed to passivate charge traps. Ionic salts, given an advantage of ionic nature of perovskite, are effective at passivating either or both cationic and anionic defect sites. However, systematic understanding of the surface chemistry with the ionic salt ligands is still lacking.
Here we present our study on the nature of cation and anion pairs on the efficacy of CsPbX3 nanocrystal surface passivation. Based on a combination of optical spectroscopy, density functional theory calculations, device fabrication and testing, we show that the specific combinations of cations and anions result in superior charge trap passivation efficacy due to their different binding interactions and steric effects.

Herein, we experimentally demonstrate the successful fabrication of three-dimensional (3D) gallium nitride on silicon (GaN-on-Si) light-emitting diodes (LEDs) using 3D Si micro-scale structures on which multiple facets of {111} crystal planes are formed for effective light energy conversion. Hetero-epitaxial integration of GaN LED layers over 3D Si micro-scale structures by metal organic chemical vapor deposition not only forms continuous GaN layers on top of Si, but also achieves excellent crystal quality of GaN layer due to the effective release of the strain energy by the faceted structures of Si. Our GaN layers formed on 3D Si structures have threading dislocation densities (TDD) comparable to the GaN grown over the sapphire substrates. In addition, the formed GaN layers have unique 3D morphology favorable to the enhanced light emitting characteristics, while they exhibit broadband, multi-color emission characteristics due to the anisotropic formation of multiple quantum well (MQW) layers in the 3D structures, leading to realize full-color light emitters. While utilizing the advantages of 3D micro-scale structures, our approach to create GaN-on-Si LED with effective light energy conversion characteristics can be utilized to realize high performance optoelectronic devices for diverse applications.

Acknowledgement
This research is supported by Basic Science Research Program (NRF-2018R1C1B6007938) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT of Korea.

In this work, we introduce an asymmetric voltage waveform to operate the AC-driven organic light emitting devices (OLEDs) for an increased amount of exciton generation. The device structure is the substrate/indium tin oxide electrode / polyimide / poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) with carbon nanotube (CNT) / aluminum (Al) electrode. In our proposed asymmetric structure, two types of charge carriers are injected from Al electrodes according to the electric field conditions. The energy barrier of hole injection from Al electrodes can be reduced by introducing CNT dispersion into the poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) layer. The holes injected through the CNT path accumulate on the PI/PFO interface and the excitons are created in the inverted field conditions. The hole injection barrier has lower CNT (0.7 eV) energy barrier than PFO's highest occupied molecular orbital energy level injection barrier (1.5 eV). Therefore, the holes accumulate at the PFO/PI interface using the CNT paths. The electrons are injected into the lowest unoccupied molecular orbital energy level injection barrier (1.5 eV) of the PFO at the Al electrode. In this structure, the electrons have the mobility of PFO and the holes have the mobility of CNT. Holes are injected along the CNT channel during a positive cycle. At the same time, the charge carriers are generated in the CNT and the generated electrons generate weak light inside the PFO layer together with holes injected through the CNT. Weak light is generated due to the low density of electrons generated in the CNT during the positive cycle. The holes generated from the CNTs move to the PFO/PI interface and accumulate together with the holes injected through the electrodes. In the negative cycle, electrons can be injected and transported following the PFO paths. The injected electrons recombine with the holes generated by the electric field in the CNT, and the generated electrons and injected electrons move toward the PFO/PI interface. When the opposite polarity is applied, holes in the PFO/PI interface rapidly move toward the Al electrode. Since the hole mobility of CNT is much faster than the electron mobility of PFO, injected electrons and accumulated holes form excitons near the Al electrode. The hole mobility of CNTs is much faster than the electron mobility of PFOs of around 10^-5 cm²/Vs. Therefore, when a negative cycle is initiated, the holes at the PFO/PI interface recombine with the electrons injected near the Al electrode or rapidly escape through the electrode. More accumulated holes are needed to increase the amount of excitons that emit in combination with injected electrons. Therefore, if the positive cycle is longer than the negative cycle, the number of holes accumulated increases, and the probability of recombination the injected electrons and accumulated holes increases.
In our experiment, at the same operating voltage (40 V) and frequency (600 kHz) conditions, the electroluminescence (EL) intensities can be higher or lower according to the asymmetric wave form than that operated in conventional symmetric waveform, where the relative total EL intensities are 1.7:1:0.8 for samples operated at the waveforms having the relative polarity duration ratios.
Acknowledgement: This work was supported by the BK21 Plus project funded by the Ministry of Education, Korea (21A20131600011) and the technology development program funded by the Ministry of SMEs and Startups (MSS, Korea) (No S2672790)..

Since a couple of years, the hybrid organic-inorganic perovskites CH₃NH₃PbX₃, with X a halogen (I, Br, Cl), have emerged in the framework of photovoltaics and for light-emitting devices such as electroluminescent diodes and lasers. One of the main advantages of these materials is that they can be low-temperature and solution processed. Thus they can be deposited in large surface, which is suitable for wide-scale wafers and devices, with a low cost.

We consider here a one-dimensional planar microcavity containing a large-surface (1 cm²) spin-coated CH₃NH₃PbBr₃ thin film as the optically active material. We choose this hybrid perovskite, emitting in the green range, to address the problem of the green gap for lasers and LEDs. We demonstrate the strong coupling regime between the photon mode of the Fabry-Perot cavity and the excitonic mode of the perovskite at room temperature even with a low-quality factor of the order of 100 [1]. This leads to the formation of the so-called polaritons, which are a linear and coherent superposition of the exciton and photon states.

Up to now, the strong coupling regime with 3D halide perovskites has been obtained for bromine or chlorine-based nano/micro-platelets or nano/micro-wires. The observation of polaritons in a spin-coated microcavity is particularly significant because it opens the route to new large surface and low-cost optoelectronic devices based on polaritonic effects, operating at room temperature and compatible with electrical injection. Indeed, all-optical logical devices on a single chip constitute the perspective of polaritons at room temperature: a large-area device is required to propagate the encoded information in the polaritonic flow across macroscopic distance.

This work has been financially supported by the National French Agency for Research, in the framework of the projects EMIPERO and POPEYE. The work of P. Bouteyre is supported by the Direction Générale de l’Armement (DGA).

References

[1] Arxiv: arXiv:1810.05720 , in revision in ACS Photonics

The rational design of nanoparticle based oxide ceramics is an important prerequisite for both, materials chemistry and physics and the manufacture of advanced materials with unprecedented optical properties.[1] Moreover, the search for suitable luminescent materials of sufficient availability, low cost and biocompatibility is of great scientific and economic interest. A possible approach to substitute currently used rare earth materials or heavy metals in optical applications, which feature luminescent properties in the visible range of light is the use of highly dispersed alkaline earth oxides (MgO, CaO, SrO and BaO). The emission properties of ionic surface dopants and segregated metal oxide clusters originate from surface exciton annihilation at low coordinated surface sites.[2]
In this study, a hybrid chemical vapour synthesis approach was used to incorporate impurity ions, such as Ba(2+), into the nanocrystalline MgO host lattice within the Mg-combustion flame.[3] Subsequent vacuum annealing promotes ion diffusion within the non-equilibrium solids, leads to particle reorganization and, ultimately, to the surface and interface decoration of the MgO nanocrystals with nanometer-sized photoluminescent BaO segregates.
Structure characterization work performed with the help of X-ray diffraction and Transmission Electron Microscopy (TEM) revealed Ba-concentration dependent trends in nanoparticle growth. Moreover, annealing provides means to control impurity localization [3] and to trigger Ba-segregation within the MgO nanocrystals, as studied with Energy Dispersive X-ray Spectroscopy (EDX). The surface functionalization of the MgO nanoparticles leads to significant changes of the optical properties, showing BaO specific absorption bands (Ultraviolet-Visible-Spectroscopy) and both, a strong increase and a shift of the materials’ photoemission properties into the range of visible light (Photoluminescence).
After powder consolidation the controlled manipulation of impurity ions affects composition, energetics and, thus, the optical properties of constituent grains and grain boundaries. The here established understanding of impurity segregation including the important influence of surrounding atmospheres during processing was employed to functionalize the intergranular regions inside alkaline earth oxide nanoparticle derived ceramics. We tracked characteristic photoluminescence emission features that are specific to the surface excitonic properties of highly dispersed alkaline earth oxide ions and clusters as a diagnostic tool [2] and found that the properties could have been retained during consolidation and sintering.
[1] K. Faber et al.; J. Am. Ceram. Soc. 2017, 100, 1777-1803.
[2] A. Sternig, et al.; J. Mater. Sci. 2015, 50, 8153-8165.
[3] M. Niedermaier, C. Taniteerawong et al., ChemNanoMat 2019, 5, 634-641.

We report the synthesis and characterization of carbon nanodots (CDs) with high quantum yield (η>50%) and tailored optical absorption as well as emission properties. A well-described protocol with polyethyleneimine (PEI) as amine precursor is used as a reference to a new CD system which is stabilized by aromatic 2,3-diaminopyridine (DAP) molecules instead. The DAP stabilizer is installed in order to red-shift the absorption peak of the n-π* electron transition allowing efficient radiative recombination and light emission. Size, shape, and chemical composition of the samples are determined by (HR)TEM, EDX and FTIR-spectroscopy. Optical parameters are investigated using UV-VIS, PL and QY measurements. Several parameters such as concentration, excitation wavelength and pH are studied. Zeta-potential analysis indicate that pH-induced (de-)protonation processes of functional moieties directly affect the n-π* energy bands. This results in unique pH-dependent absorption and emission characteristics which are discussed on the specific chemical composition of each CD system.

Photoluminescent colloidal quantum dots (QDs) have been enhancing a plethora opto-electronic devices including photodetectors, light-emitting diodes, displays, and solar cells. Incorporating quantum dots into the hetero-junction solar cell process has yielded significant increases in efficiency. A highly favored, cost and time efficient fabrication method of producing optimal tunable quantum dots is a one pot microwave irradiation technique. Water soluble cadmium telluride (CdTe) quantum dots are suitable for hetero-junction solar cells due to their high band-gap photoluminescence quantum efficiencies and mono-exponential exciton decays. Additionally, quantum dot enhanced photo-electrochemical (PEC) solar cells have yielded higher efficiency in charge generation by incorporating zinc sulfide (ZnS) nanoparticles (NPs) as a passivation layer to reduce electron recombination amid the photoanode and electrolyte interface. Various characterization techniques were used to observe the opto-electronic properties of the CdTe QDs: photoluminescence spectroscopy (PL), ultraviolent-visible spectroscopy (UV-VIS), and transmission electron microscopy (TEM). Furthermore, the leakage current, capacitance, and frequency dispersion measurement show the stable performance of our CdTe QDs/Zn NPs hetero-junction solar cell.

Lead bromide perovskite quantum dots (PQDs) have emerged recently as a promising material for lighting applications. Among its properties, we can highlight the defect tolerant structure, which allows for highly emissive nanoparticles, with photoluminescence quantum yield typically higher than 70%, for core only nanoparticles. Also, its facile synthesis opened the perspective for a scalable production for the application of future technologies. Additionally to that, two intriguing properties have been reported recently on the literature: ultra-low amplified spontaneous emission (ASE) threshold (typically in the order of 5-20 μJ/cm²)¹ and an extremely large two-photon absorption cross section (typically in the order of 10⁵ GM) ². Although these properties are highly desirable for the application of PQDs in many optical systems, there is still no comprehensive study about the physical mechanism responsible for them. Here, we perform a systematic size dependence study of the two-photon absorption and threshold for two-photon induced ASE of PQDs to investigate the physical origin of these properties³. For the two-photon absorption studies, we analyzed PQDs nanoparticles with sizes ranging from 7.4 ± 0.8 to 12.5 ± 1.5 nm. From that, we report a 2PA cross section at 1.55 eV (transition at 3.1 eV) ranging from 2x10⁵ GM to 7x10⁵ GM. These values are almost one order of magnitude higher than for other visible emitting quantum dots such as CdSe and CdTe. However, by comparing the nanoparticles size, we show that this is just a matter of volume, and not due to any particular strong transition oscillator strength. We also performed size dependent two-photon excited ASE (2PA-ASE) studies on the PQDs. From that, we obtained that the 2PA-ASE threshold in terms of the fluence tends to be higher for the smallest samples, due to the reduced 2PA cross section. However, if we consider the threshold in terms of the average of excitons needed to generate ASE, the observed trend is the opposite, on which the biggest samples present the highest thresholds. In order to understand this unexpected behavior, the bi-exciton binding energies of the PQDs were investigated. From that, we observe a strong anti-correlation between the average of excitons needed to generate ASE and the bi-exciton binding energy, which goes from 41 to 61 meV. This indicates that the strong Coulombic interactions in PQDs are responsible for a red shift on the ASE energy, which causes a significant reduction on the reabsorption losses. This enables ASE with an average of excitons per dot of 0.73 for the smallest nanoparticles, which makes PQDs one of the most promising materials for future applications in the field of lasers.

1. Yakunin, S.; Protesescu, L.; Krieg, F.; Bodnarchuk, M. I.; Nedelcu, G.; Humer, M.; De Luca, G.; Fiebig, M.; Heiss, W.; Kovalenko, M. V., Low-threshold amplified spontaneous emission and lasing from colloidal nanocrystals of caesium lead halide perovskites. Nature communications 2015, 6, 8056.
2. Wang, Y.; Li, X.; Zhao, X.; Xiao, L.; Zeng, H.; Sun, H., Nonlinear absorption and low-threshold multiphoton pumped stimulated emission from all-inorganic perovskite nanocrystals. Nano letters 2015, 16 (1), 448-453.
3. Nagamine, G.; Rocha, J. O.; Bonato, L. G.; Nogueira, A. F.; Zaharieva, Z.; Watt, A. A.; de Brito Cruz, C. H.; Padilha, L. A., Two-Photon Absorption and Two-Photon Induced Gain in Perovskite Quantum Dots. The journal of physical chemistry letters 2018.

For decades, the quantum dot has been studied as an important class of tunable light emitters with narrow emission linewidths. Although the development of synthetic method has reduced the linewidth contributed from the inhomogeneity in size and shape (inhomogeneous linewidth), the linewidth of single-particle (homogeneous linewidth) still exists because of exciton-phonon coupling, exciton fine structure, and spectral diffusion. It is widely accepted that the main contribution of the homogeneous linewidth is exciton-phonon coupling (~90%), followed by exciton fine structure (~ 10%). In this study, we develop a theoretical method to calculate the contribution of exciton-phonon coupling in the homogeneous linewidth of quantum dots. Ground-state electronic structure and phonon properties of the quantum dot are studied by density functional theory (DFT) calculations, and excited state is described by delta-DFT framework. Huang-Rhys factor is obtained by projecting structural distortion of exciton structure to phonon mode, and the luminescence spectra is calculated using the parallel approximation. As a model system, we investigate the luminescence linewidth of CdSe quantum dot with various size and structures such as tetrahedron, sphere, and spheroid. Our results are compared with recent experiments that identified the homogeneous linewidths by phonon-correlation Fourier spectroscopy and 2-dimensional electron spectroscopy.

Single-walled carbon nanotubes (SWNTs) with semiconducting features emit photoluminescence (PL) in near infrared (NIR) regions. The NIR PL generates through a radiative relaxation process of exciton that is a bound electron-hole pair produced by photo-excitation of the tubes. Owing to their unique one dimensional nanostructures, the exciton is stable at room temperature and migrates through the one-dimensional structure.[1] As a new method to utilize the mobile excitons for PL enhancement, defect doping to SWNTs by local chemical functionalization is recently gathering great attention. The locally functionalized SWNTs (lf-SWNTs) show additional PL (E₁₁*) with red-shifted wavelengths and enhanced quantum yields compared to original PL (E₁₁) of pristine SWNTs.[2-11] Therein, the local functionalization of the tubes creates emissive sites whose electronic structures are modified, giving narrower bandgap and exciton trapping features. Interestingly, the modifier molecules are found to be a key tool to modulate PL of lf-SWNTs. For example, we achieved largely red-shifted PL (E₁₁²*) generation over 100 nm-shifts than typical E₁₁* PL by functionalization based on a proximal doping technique.[7]
Here, we report chemical functionalization for such longer wavelength PL generation and wavelength tuning based on molecular structure design of the modifiers. Firstly, we synthesized bis-arydiazonium modifiers (bAs) that have two reactive aryldiazonium groups connected with a methylene linker. In order to achieve wavelength tuning of E₁₁²* PL, the linker effects were examined by changing the connected positons on the aryl groups (pbAs for para and mbAs for meta) and the length of methylene chains.[11]
For local functionalization, SWNTs were solubilized in a D₂O solution containing sodium dodecyl sulfate (SDS), and then reacted with the synthesized bAs through diazonium chemistry. In PL spectra of the obtained lf-SWNTs (lf-SWNTs-pbA and lf-SWNTs-mbA), PL peaks appeared at 1245 nm and 1257 nm, respectively when the linker of five methylene units was used. Those were assigned to E₁₁²* PL that were observed in longer wavelength regions than E₁₁* PL of mono-aryl functionalized lf-SWNTs (ca. 1140 nm). When the linker lengths of bAs were changed, it’s found that the shorter linkers induced longer wavelength PL. Namely, the methylene chain length is a factor for wavelength tuning of E₁₁²* PL. In addition, the lf-SWNTs-mbA showed larger spectral shifting behavior in the linker length-dependent E₁₁²* wavelength compared to the lf-SWNTs-pbA. Thus, the molecular design of the bis-aryldiazonoum modifiers provides a wavelength modulation technique for E₁₁²* PL, which would develop new types of NIR PL nanomaterials applicable to bio-imaging/sensing and telecommunication devices. Details and other functionalization methods will be discussed at the meeting.

References: [1] Dukovic G. et al., Nano Lett. 2005, 5, 2314. [2] Weisman B. R. et al., Science 2010, 330, 1656. [3] Miyauchi, Y. et al., Nat. Photon. 2013 7, 715.[4] Schatz, G. C. and Wang, Y. et al., J. Am. Chem. Soc. 2016, 138, 6878. [5] Htoon, H. and Doorn, S. K. et al., Nat. Photon. 2017, 11, 577. [6] Shiraki, T. and Nakashima, N. et al., Sci. Rep. 2016, 6, 28393. [7] Shiraki, T. and Nakashima, N. et al., Nanoscale. 2017, 9, 16900. [8] Shiraki, T. and Nakashima, N. et al., Chem. Commun. 2017, 53, 12544. [9] Shiraki, T., Nakashima, N., Fujigaya, T. et al., Chem. Eur. J. 2018, 24, 19162. [10] Shiraki, T. and Fujigaya, T. et al., Chem. Commun. 2019, 55, 3662. [11] Shiraki, T. and Fujigaya, T. et al., Chem. Lett. 2019, in press; DOI: 10.1246/cl.190203.

Carbon dots (CDs) have unique photoluminescence (PL) properties arising from their surface functional groups and carbon core. However, unclear microscopic origin of the visible and near-infrared PL from CDs hinders attempts toward energy modification and limits their practical use. Here, we investigate the chemical origin of the visible and near-infrared PL of graphite-derived CDs (gCDs) by manipulating their chemical structures via reduction and deprotonation. The relation between chemical and PL changes resulting from these modifications are thoroughly investigated. Using time-resovled spectroscopy, we establish the roles of functional groups and carbon cores in excitation of the energy states responsible for the visible and near-infrared PL. We find that oxidized sp³ carbon matrix and small sp² carbon clusters are responsible for resonantly excited and slightly tunable emission in the visible and the near-infrared, respectively.

AlGaN-based deep ultraviolet(DUV) light-emitting diodes(LED) have ability for sterilization, air/water purification, medical treatment and so on. However, most of DUV leds have low external quantum efficiency due to inherent problem such as low internal quantum efficiency and light extraction efficiency. These factors limit their practical applications. Here, We present a remarkable enhancement in AlGaN light-emitting diodes (LEDs) through the coupling of localized surface plasmon (LSP) mediated by a high density array of Al nanoparticle (NP). The array of Al NP with the optimum resonance diameter of ~ 40 nm for 285 nm DUV emission has been achieved by utilizing block copolymer self-assembly. The internal quantum efficiency is increased by 57.7% owing to the reduced radiative recombination lifetime mediated by LSP. As a consequence, the AlGaN DUV LEDs with the array of Al NP show the enhanced electroluminescence by 33.7% with comparable electrical properties to the reference devices.

Quantum dot (QD) light-emitting diodes (QLEDs) have been gaining much attractions in display and lighting industry due to their superb characteristics such as size-dependent bandgap tunability, narrow emission spectra, and low cost solution process. Although cadmium (Cd) QLEDs show excellent device performance, the toxicity is a serious concern for applications. Among eco-friendly QD materials, indium phosphide (InP) QDs are the most promising candidate for developing high-performance displays with a wide color gamut. However, the performance of InP QLEDs is still far behind compared to Cd QLEDs. In this work, we demonstrate that the efficiency of InP QLEDs are greatly improved by adopting metal doped ZnO nanoparticle as an electron transport layer (ETL). By optimizing the Al doping concentration, we achieved a higher performance, in terms of the peak external quantum efficiency (EQE) of 3.62 % compared to the device with an undoped ZnO nanoparticle of 1.81 %. Systematic analysis on the optical and electrical properties of the metal doped ZnO films was carried out to understand the underlying mechanism for the improved device performance. Compared to the undoped ZnO film, the Al doped ZnO film has enhanced electron mobility and reduced energy barrier for the electron injection into the InP QD emissive layer. We believe that the analysis based on the metal doped ZnO will give further insight into improving and understanding InP QLEDs.

Nonresonant excitation of colloidal quantum dots (QDs) creates hot carriers that subsequently cool down to the band edges or are trapped in localized states. Carrier cooling and trapping typically happens on timescales from femtoseconds to picoseconds, orders of magnitude faster than the nanosecond to microsecond timescales of radiative recombination. Understanding cooling and trapping is relevant for (hot) carrier extraction in photovoltaics and to increase the luminesence output of QDs used as phosphor.

We investigate carrier cooling and trapping in InP QDs with a ZnSe_1-xS_x shell. Undoped QDs are compared to Cu⁺-doped QDs, where the Cu⁺ ion serves as a designed hole trap. Using pump probe transient absorption spectroscopy with femtosecond time resolution, we are able to monitor the population of electrons in the conduction band.

Our comparative study shows that hot electron cooling is almost an order of magnitude slower in the Cu⁺-doped QDs than in the undoped QDs. We ascribe this to rapid hole trapping on the Cu⁺ ion. This confirms the model in which hot electron cooling goes via an Auger-like process, where the hot electron transfers its excess energy to the hole which subsequently relaxes by phonon coupling. In our Cu⁺-doped QDs the hole is trapped on a Cu⁺ ion on sub-picosecond timescales, so the Auger cooling pathway is unavailable to the hot electron. Instead it must cool down via another, slower, pathway, most likely by coupling to high-energy vibrations at the surface of the QDs. This must also mean that hole trapping on the Cu⁺ ion is faster than the Auger cooling timescale of the undoped QDs, which is of the order of 300 fs. Our results provide insight in the behaviour of hot electrons and holes in the short time period after excitation of both Cu⁺-doped and undoped InP QDs.

Colloidal PbS nanosheets are an emerging two-dimensional material for infrared applications. In contrast to the zero-dimensional quantum dot, the anisotropic structure of the nanosheet provides an additional parameter to tune its properties. In this structure, it is possible to independently tune the energy gap (through the thickness) and the other properties (through the lateral size). The other properties include the exciton oscillator strength, the exciton-exciton interaction, and the electronic coupling of the nanosheet with other materials. All of them are important in optoelectronic applications.
The strong coupling between the nanosheet and an electron-accepting material - TiO₂- enables a fast electron transfer, favoring charge extraction for photovoltaic devices. Our time-resolved photoluminescence spectroscopy results show that it takes less than one nanosecond for the electron to transfer from PbS nanosheets to TiO₂in a solution. It is about one thousand times faster than the electron transfer from PbS quantum dots to TiO₂in the same experimental condition.

Upconverting nanomaterials are promising in several applications including bioimaging, sensing, photodynamic therapy, drug delivery, photovoltaics, and security. Specifically, NaYF₄ co-doped with Yb³⁺ as a photosensitizer and Er³⁺ as an activator is suitable for biological applications because Yb³⁺ can be excited by photons at 980 nm which have a long-penetration distance in biological tissues. Following the laser excitation of Yb³⁺, photoexcitation energy is transferred multiple times to Er³⁺ from Yb³⁺ before Er³⁺ emits both red and green photons. The overall upconversion efficiency of NaYF4:Yb³⁺/Er³⁺ is highly affected by the phase with hexagonal (β) phase allowing for much better efficiency than cubic (α) phase. One major challenge in making upconverting nanomaterials feasible for biological applications is to synthesize highly performing β-phase upconverters with good solubility in water. Traditional methods, such as solvothermal synthesis, fall short with limitations including toxic side products, poor solubility in water, high reaction temperatures, long reaction times, and poor control of phase and morphology. Additionally, the conditions required to produce the preferred β-phase may cause damage to capping agents. This presentation will address our novel approach of laser ablation in liquid as a promising alternative for upconverting nanomaterial synthesis. The laser ablation in liquid allows a fast production, fewer chemicals, fewer byproducts, and control over the product by tuning the laser parameters. NaYF₄:Yb³⁺/Er³⁺ targets are prepared by coprecipitation, and then are pressed into a pellet and annealed to convert to β-phase. This is followed by 532 nm pulsed nanosecond laser irradiation of the pellet in either water or in aqueous solutions containing capping agents including citric acid, polyethylene glycol, ascorbic acid, or albumin. The laser irradiation induces formation of smaller particles that are stable in water. The size of the particles can be controlled by the laser fluence and the solubility can be tuned by the choice of capping agents in the liquid during laser ablation. Additionally, the samples produced through laser ablation show improved β-to-α ratios when compared to the annealed pellets. Laser ablation in liquid allows for direct capping of upconverting nanomaterials and the laser conditions can be tuned to prevent damage to the capping agent in solution. Finally, time correlated single photon counting analysis revealed that NaYF₄:Yb³⁺/Er³⁺ samples with capping agents showed longer photoluminescent lifetimes in both the green and red regions due to protection from surface quenching by the capping agent.

Here we report on the photoluminescent properties of the lead-free double perovskite solid solution Cs₂AgIn_1–xBi_xCl₆. The In³⁺ end-member, Cs₂AgInCl₆, is a direct gap semiconductor that absorbs UV light (λ < 350 nm) and shows little to no photoluminescence. Incorporation of Bi³⁺ leads to a strong sub-band gap absorption that peaks in the near UV (~360 nm) and extends into the visible (λ > 390 nm). This absorption, which is thought to originate from localized 6s² → 6s¹p¹ transitions on Bi³⁺ ions, is split by a Jahn–Teller distortion of the excited state. In-rich samples (0.0 < x < 0.5) show strong photoluminescence that is attributed to radiative decay of self-trapped excitons, with a broad emission peak of significant intensity from 450 to 750 nm. The color of the emitted light is best described as white to yellow-white (CCT = 3119 K), due to the extreme breadth, FWHM ≈ 217(2) nm, of the emission peak. The excitation spectrum extends out to 450 nm for samples near x = 0.25, while the photoluminescent quantum yield (PLQY) reaches a maximum of 39 ± 3% in the x = 0.167 sample. The combination of highly efficient broadband photoluminescence that can be driven by visible photons (λ > 390 nm) emitted from a Ga_1−xIn_xN LED is attractive for solid state lighting applications.

We have developed an organic emitter that displays external stimuli-responsive emission colors ranging from yellow to orange, based on the structure of twisted donor-acceptor-donor (D-A-D) triad comprising of dibenzo[a,j]phenazine as the acceptor and phenothiazine as the conformationally-flexible donors (Chemical Science, 2017, 8, 2677). The D-A-D triad also shows orange thermally activated delayed fluorescence (TADF) in CBP host matrix, and the OLED devices fabricated with the D-A-D emitter nicely shows a high external quantum yield up to ca.17%. More recently, we have revealed that the stimuli-responsive emission color-change is based on the conformational change at the phenothiazine unit by time-resolved spectroscopy in the film (Journal of Materials Chemistry C, 2019, 7, 6616), where the thermally activated delayed fluorescence and room-temperature phosphorescence processes compete with each other. In this presentation, the effect of conformation of the molecule on emission properties will be discussed.

Enhancing the quantum efficiency and tailoring the density of states (DOS) of upconversion nanoparticles (UCNPs) have been attracted tremendous attentions due to the applications range from bio-imaging to solar battery under the excitation of near-infrared light source. Plasmonic materials, such as noble nanoparticles or metal-like materials, have been widely used to achieve these goals. However, the intrinsic loss of plasmonic materials limit themselves play the dominant roles for broader applications. Previous results mainly focused on a relatively high pumping power density (MW/cm²) to excite the emission of UCNPs, which are too large for in-vivo imaging usage. Therefore, the demand of reducing pumping power density to achieve efficient quantum efficiency is urgently emerging.

Here, we provide a first integration of UCNPs and core-shell hyperbolic meta-structure, which allows us to drastically reduced the pumping power density (~0.1 kW/cm²) to achieve a giant emission from UCNPs for more than 20 times. This core-shell hyperbolic meta-structure is composed of the core material, SiO₂, with radius of 3 nm and alternative multishells, Au (5 nm)/SiO₂ (3 nm)/Au (5 nm). A continuous coupling effect in between the multishells is created, a pronounced energy is subsequently out-coupled to enhance the emission of UCNPs. As confirmed by the simulation results, the core-shell hyperbolic meta-structure increases the DOS, confines the energy with a higher ability, and transmits these energy to the decorated UCNPs rather than being annihilated. This unique characteristic makes the emission of UCNPs only require ultra-low power of excitation light with an extremely enlarged emission intensity. Our research provides an excellent alternative for the design of hyperbolic meta-structure and offers a new platform for the enhancement of upconversion efficiency, which is very useful for the realization of practical applications in many areas.
Acknowledgments
This work was financially supported by the "Advanced Research Center for Green Materials Science and Technology" from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of Science and Technology in Taiwan (MOST 107-3017-F-002-001).

In this study, we developed a novel approach to synthesize high-quality metal-free Nitrogen-doped graphene quantum dots (N-GQDs) with high quantum yield, via irradiation of s-triazene in a solution with benzene by using pulsed laser. The TEM, HRTEM, XPS, XRD, Raman spectroscopy and FTIR were carried out to observe the morphology, size distribution, crystalline structure and to prove successful doping of GQDs with nitrogen atoms. Furthermore, for the first time, to our knowledge, their magnetic properties were investigated. The results indicate that N-GQDs exhibit superparamagnetic behavior. The specific size, shape and zigzag edge structure of N-GQDs were considered to explain the origin of the observed magnetism. The magnetization dependence led to estimating the N-GQD material magnetic permeability for different ambient temperatures. From the zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements, carried out at 50 Oe magnetic field strength, we estimated the blocking temperature T_B to be around 300 K. Based on the experimental data analysis, the magnetic permeability, number of correlated spins per single N-GQD, and number density of superparamagnetic N-GQD per gram of material were estimated. The excellent superparamagnetic properties together with optical properties manifested by N-GQDs have the potential to lead to high performance biomedical applications.

Solution-processed Organic Light-Emitting Diodes (OLEDs) have attracted great attention as a device for application in large-size OLEDs because of simple fabrication process and easy scalability of the solution process. However, the device performances of the solution-processed OLEDs are still inferior to those of vacuum-deposited OLEDs and further improvement of the device performances of solution-processed OLEDs is required. In this work, high-efficiency solution-processed blue organic light-emitting diodes (OLEDs) were developed using two thermally activated delayed fluorescence (TADF) aromatic molecules, 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)-2,5-dimethylphenyl)-10H-spiro[acridine-9,9'-fluorene] (TXSA) and 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-methylphenyl)-10H-spiro[acridine-9,9'-fluorene] (TTSA), composed of spiroacridine donor and triazine acceptor units. As a result, the blue OLEDs based on two novel TADF molecules exhibited remarkable electroluminescence with a high quantum efficiency of 14.9 % and current efficiency of 29.3 cd/A. Our results demonstrate that TXSA and TTSA TADF molecules are prospective materials to fabricate high-performance solution-processed blue OLEDs with a simple device structure. Detailed device performances using TADF molecules will be presented in the presentation.

Graphene, as a typical 2-dimensional material, becomes a promising material due to its excellent electrical and thermal conductivity, flexibility, and high transparency. While the semi-metallic property of graphene limits the compatibility with the optoelectronic devices, graphene quantum dot (GQD) facilitates its implementation for various optoelectronic devices, enabling multifunctionality in sensing and emitting devices, because the quantum size effect provides the way to tune the energy bandgap of graphene. In this regard, understanding the current transport mechanism between GQD and semiconductor materials is obviously fundamental to explore.
In this study, we investigated the contacts of GQD formed on a n-type GaN semiconductor. Blue-luminescent GQDs (QD size < 15 nm) prepared by the hydrothermal method were sprayed on a GaN wafer and the electrical and optical properties of the fabricated contacts were measured. The GQD/GaN contacts showed rectifying behavior with a typical Schottky barrier height of 0.64 eV. The spectral photoresponse and temporal response of the GQD/GaN contact revealed that the responsivity shows a sharp increase for the wavelength shorter than 375 nm. Furthermore, the metal–semiconductor–metal (MSM) ph○otodiode with interdigitated GQD contacts on n-type GaN was fabricated, which provides the lower dark current in comparison with the GQD/GaN Schottky diode. The responsivity of the MSM photodiode was remarkably improved with increasing the annealing temperature (up to 800 °C), showing good photoresponse in the ultraviolet (UV) region.

[1] P. Tian, L. Tang, K. S. Teng, and S. P. Lau, Mater. Today Chem. 10, 221 (2018).
[2] T.-N. Lin, S. R. M. Santiago, C.-T. Yuan, K.-P. Chiu, J.-L. Shen, T.-C. Wang, H.-C. Kuo, C.-H. Chiu, Y.-C. Yao, and Y.-J. Lee, Sci. Rep. 7, 7108 (2017).
[3] Q. Zhang, J. Jie, S. Diao, Z. Shao, Q. Zhang, L. Wang, W. Deng, W. Hu, H. Xia, X. Yuan, and S.-T. Lee, ACS Nano 9, 1561 (2015).

Photodetection in the mid and long-wave infrared (MWIR, LWIR) is of paramount importance for applications in night-vision, medical diagnosis, astronomy, environmental pollution monitoring, spectroscopy, and telecommunications. However, commercial infrared detectors are costly due to their epitaxial growth methods and complex non-monolithic integration with CMOS technology. Moreover, their performance is optimized upon cooling adding further complexity to their integration, power consumption, and miniaturization. The possibility of exploiting low-energy intraband transitions make colloidal quantum dots (CQD) an attractive low-cost alternative to expensive low bandgap materials for infrared applications. Unfortunately, fabrication of quantum dots exhibiting intraband absorption is technologically constrained by the requirement of controlled heavy doping, which has limited, so far, MWIR and LWIR CQD detectors to mercury-based materials. We have developed a robust doping strategy for PbS quantum dot solid-state films that allows harvesting of mid- and long-wave infrared radiation, well beyond the reach of PbS even in its bulk form. Our doping method leads to simultaneous interband bleach and an increase of intraband absorption. We show doping to be stable under ambient conditions for up to 2 months; therefore allowing, for the first time, to realize intraband PbS CQD photodetectors for energies below the bulk bandgap, in the 5-9 µm range. We also show that the doping is stable at low temperatures, which allows them to be used in sensitive photodetectors and other potential applications such as bolometers.

The synthesis of colloidal III–V quantum dots (QDs), particularly of the arsenides and antimonides, has been limited by the lack of stable and available group V precursors. In this work, we exploit accessible InCl₃- and pnictogen chloride-oleylamine as precursors to synthesize III–V QDs. Through coreduction reactions of the precursors, we achieve size- and stoichiometry-tunable binary InAs and InSb as well as ternary alloy InAs_1–xSb_x QDs. On the basis of structural, analytical, optical, and electrical characterization of the QDs and their thin-film assemblies, we study the effects of alloying on their particle formation and optoelectronic properties. We introduce a hydrazine-free hybrid ligand-exchange process to improve carrier transport in III–V QD thin films and realize InAs QD field-effect transistors with electron mobility > 5 cm²/(V s). We demonstrate that III–V QD thin films are promising candidate materials for infrared devices and show InAs_1–xSb_x QD photoconductors with superior short-wavelength infrared (SWIR) photoresponse than those of the binary QD devices.

Quantum light-matter interfaces are at the heart of quantum optics and solid-state approaches to quantum information science. In particular, single photon sources are highly desirable for photonic quantum sciences, such as quantum communication and quantum cryptography, in order to ensure physical security and protect information travelling over channels. Novel single photon sources are rare because they must satisfy a very strict set of criteria: the photons from such a source must be indistinguishable with an arbitrary high repetition rate, have near zero probability of multiple photon emission, and should operate at room temperature. An ideal single photon source should also emit photons deterministically at arbitrary times, or “on demand”.

We have discovered a potentially new single-photon source, denoted here as Fluorophore X. Single molecules of Fluorophore X exhibit a distinct three-peak photoluminescence (PL) spectrum centered at 610 nm, reminiscent of an organic dye. However, unlike a typical dye, the PL from Fluorophore X is extremely bright and robust, with a brightness comparable to small diameter CdSe nanocrystal quantum dots. PL from single molecules of Fluorophore X has a constant intensity (i.e. are nonblinking) with the ability to remain “on” for several minutes before turning “off”. This behavior is also quite different from traditional fluorophores, which typically photobleach quickly and/or exhibit fluorescence blinking. Notably, Fluorophore X exhibits photon antibunching at room temperature, with a g⁽²⁾(0) < 0.21. Interestingly, while Fluorophore X was originally observed over 20 years ago using single-molecule PL spectroscopy, in the intervening two decades it has been repeatedly misidentified as either an organic dye, a colloidal semiconductor or oxide nanoparticle, or as an “impurity” in various solvents, polymers, dyes, and substrates.In an important breakthrough, we have developed a simple method to synthesize and purify Fluorophore X in relatively large concentrations, which has enabled acquisition of the first ensemble PL, photoluminescence excitation (PLE), and absorption spectra of this molecule, at 300 K and at 10 K. While the PL spectrum and PL lifetime (few ns) are similar to a molecular dye, the unusually small Stokes shift of 10 nm (at ~ 600 nm) between absorption and PL spectra as well as the outstanding single molecule PL photostability suggest that Fluorophore X also has similarities to inorganic emitters. In fact, no small molecule is known to exhibit all these properties, making the structure and composition of this new and exciting single-photon source completely unknown. In that regard, in addition to the photophysical characterization of Fluorophore X, we will present NMR and mass spectrometry data that will shed important light on its molecular composition and structure.

Graphene quantum dots (GQDs) which are composed of few layers graphene have superior characteristics such as large surface area, tunable photoluminescence. Using these characteristics, GQDs are used for bio-imaging, biosensing, and drug delivery. Various methods have been developed to synthesize GQDs, however conventional methods have some disadvantages such as using harmful chemicals and complicated process. Thus, a facile method to produce biocompatible GQDs with simple synthesis process is a major challenge. Herein, a facile route to high-quality and luminescent GQDs based on hydrogen-assisted pyrolysis of silicon carbide (SiC) without use of harmful chemicals are developed. In pyrolysis of SiC, the morphologies of both graphene and SiC surface are significantly affected by the annealing conditions, particularly the etching rate of Si and C atoms on the SiC surface. In this study, the GQDs were created on the bumpy structure of SiC by controlling the etching rate of Si and C atoms at high temperature. Annealing temperature in study was raised up to 1500°C and maintained for 30 minutes. Fabricated GQDs was detached from the substrate using ethanol and prepared GQD solutions were centrifuged to eliminate large particles detached from the substrates.
The average size of fabricated GQDs were measured to be 2.58 nm and GQDs were composed of few layers graphene. Synthesized GQDs are proven to have highly-ordered crystalline structure by HRTEM and Raman spectra. In Raman spectra, the D to G peak intensity ratio of the GQDs is 0.79 and this value means comparatively high crystallinity. Especially, sharp 2D band were observed in the Raman spectra. X-ray photoelectron spectra and Fourier transform infrared spectra support the high purity of GQDs and suggest the hydrogen termination of GQDs. Moreover, the size of the GQDs could be controlled by adjusting the annealing temperature. As the annealing temperature increases, average size of GQDs was reduced. Synthetic mechanism of GQDs are explained by removal rate of Si and C atoms. When SiC is heated at high temperature in a vacuum, the Si atoms on surface are rapidly removed. Therefore, the surface becomes bumpy and the remained C atoms rearranged to graphene. However, a small amounts of hydrogen in the furnace etch C atoms on surface, interfering with growth, resulting in nano-sized graphene domains are produced. Thus, the GQD are formed on a rough SiC surface.
UV-vis absorption spectra of the high-quality GQDs showed the 2 peaks located at 262 and 327nm. Absorption band at 262nm is originated from a π- π* transition of aromatic C=C bond and the other band at 327 nm are reported in other researches about GQDs. GQDs aqueous solution exhibited the strong PL signals and excitation-independent PL behavior. Furthermore, 3 emission peaks located at 360, 382 and 400 nm were observed in the spectra. The PL excitation (PLE) spectrum of the GQDs showed two broad peaks centered at 290 and 343 nm. The energy difference between these two PLE peaks is comparable to that between the two UV absorption peaks centered at 262 and 327 nm. It is assumed that the photoluminescence of GQDs are attributed to intrinsic emission.
High-quality GQDs exhibited an excellent biocompatibility in living cells. Cell viability test of high-quality GQDs were performed on hepatocytes. Treatments of cells with GQDs for 48 hours at various doses did not significantly affect cell viability in comparison to the control. The microscopic observation revealed that hepatocytes treated with GQDs retained normal cellular morphology. The low cytotoxicity of high-quality GQDs is attributed to the use of no any harmful chemicals in the synthetic process and high-purity. Collectively, these results strongly suggest that high-quality GQDs exhibited the clear PL spectra and excellent biocompatibility with low cytotoxicity. Therefore, high-quality GQDs synthesized by this method are more suitable in the bioimaging fields than existing.

Production of new materials for near-infrared (NIR) sensing and hyperspectral imaging based on Si-devices with a mature design and process technology is desirable. Efficient harvesting of unutilized infrared photons can also be used to enhance the sensitivity and efficiency of Si-based solar cells and detectors. Upconversion materials absorb multiple infrared (IR) photons and emit a single visible photon obeying energy conservation. Previous results have established the vital importance of obtaining hexagonal phase in the NaYF⁴: Yb(18%), Er(2%) system. Doping with Gd(15%) results in a completely hexagonal phase with no minor cubic peaks observed in X-ray diffraction(Rigaku Miniflex benchtop X-ray diffractometer). It is important to establish minimal process conditions needed to achieve an annealed product with the highest photoluminescence efficiency. Fixing anneal time at 24 hours under ambient atmosphere, we find that after annealing at 400^oC, a mixed-phase appears. Above 500^oC, cubic phase dominates over the hexagonal phase, with a corresponding reduction in photoluminescence efficiency observed with 785 nm (intensity ~ 557 mW/cm²) excitation. A colorimetric change in upconverted emission from green (400^oC, 539 nm), to red (500^oC, 655 nm and 675 nm) is observed with a change in annealing temperature. This signifies that the mixed and cubic phase favors the red emission whereas the hexagonal phase favors the green emission. High-resolution transmission electron microscopy (HRTEM) (FEI TecnaiTF20, 200kV) scans reveal the formation of agglomerated particle clusters upon deposition from a suspension (in ethanol) at all annealing temperatures though spherical particles (~22 nm) can be observed for synthesized temperatures. Morphology of nanoparticles intrinsically changes from spherical to rod-shaped nanostructures in materials annealed at 500^oC. Work is ongoing on the functionalization of these nanoparticles to reduce possible effects of concentration quenching, and to produce printable inks. These findings are expected to establish important composition and annealing process steps for the production of efficient upconversion materials and to develop a process for fabrication of low-cost near-infrared detectors.

M₂Si₅N₈:Eu²⁺ (M=Ba, Sr) phosphor shows excellent thermal quenching characteristic and a wide wavelength range of spectrum (580–620 nm) from yellow to amber according to Ba and Sr molar ratios, which is used for automotive amber LED light sources.
In this study, the phosphor with various Sr compositions were synthesized by heat-treatment at 1,600°C for 8 h. The fabrication of M2Si5N8:Eu2+(M=Ba, Sr) phosphor ceramic plate (PCP) was carried out by dry ball milling phosphor with MgO and CaO as sintering aid. The mixture was compressed into a pellet using uniaxially pressed with a 10 mm round pellet shaped and treated again through cold isostatic pressing (CIP) at 200 MPa for 10 min. These green bodies were pressureless sintered at 1550 – 1650 °C for 4 h under a mixed gas atmosphere of 95% N2+5% H2 to compare density and optical properties. The density of the sintered samples were measured by an Archimedes method and scanning electron microscope (SEM) image. Moreover, the microstructure of sample with EDS analysis of different grains and grain boundaries were observed using a transmission electron microscopy (TEM). The crystal structure was analyzed by an X-ray diffraction system (XRD) with CuKα radiation. When the molar concentration of Sr was 0.2 mol, the wavelength shows 600 nm optical characteristics suited to amber LED. The XRD analysis appeared the crystal structure of the M₂Si₅N₈:Eu²⁺ (M=Ba, Sr) phosphor, indicating that the phase was well formed. As a result, when the ratio of MgO and CaO using the sintering preparation was 70: 30 ratio, the mixture add with 7 wt% shows the density of 4.318 g/cm3 to exhibit excellent sintering density. These sintered PCPs have excellent effect as adaptive optical characteristics for using an amber light for automotive LED applications.

The emission efficiency InGaN/GaN-based visible LEDs dramatically deteriorates with increasing the wavelength from green to red, because of the lower radiative recombination rate as well as the worsening crystal quality as the InGaN content increases. On the other hand, bottom-up grown InGaN/GaN nanocolumns (NCs), which possess columnar nanocrystals with diameters (D) of typically 30-300nm, exhibit excellent nanocrystal effects such as dislocation-free, ¹⁾ lattice strain relaxation and no generation of misfit dislocations at the InGaN/GaN hetero-interfaces. The nanocrystal effects are advantageous for improving the red-emitting InGaN/GaN NC arrays, demonstrating a high internal quantum efficiency (IQE) of 20% at 600 nm.²⁾ At the same time, the enhanced photoluminescence intensity at 600 nm via surface plasmon and exciton coupling was observed improving the IQE.³⁾ The plasmonic effect is effectively introduced in NC LEDs, by depositing the plasmonic metals on the sidewall of the NC LEDs, close to the InGaN active layer, and do not conflicting with the current injection scheme. However, for InGaN NC triangle-lattice array, the spacing between NCs is ordinarily less than 50 nm, which makes the deposition of plasmonic metals on the sidewall difficult. In this study, we investigate novel InGaN/GaN NC honeycomb-lattice arrays, in which the NCs were arranged in the geometry of a honeycomb. The honeycomb-lattice arrays possess airspaces inside honeycomb structures, revealing the sidewalls of NCs. It is, therefore easy to deposit plasmonic metals on the NC sidewalls, which enables a fabrication of plasmonic NC LEDs. The period of the honeycomb-lattice array (L’) is defined by the distance between adjacent honeycomb structure centers. In the experiment, different InGaN/GaN honeycomb-lattice arrays with 150×150 μm² area were prepared on the same substrate changing L’ from 160 to 300 nm, employing the Ti-mask selective area growth (SAG) of RF plasma assisted molecular beam epitaxy. In the SAG, Si-doped n-type GaN were grown on Ti-mask patterned GaN template, preparing GaN NCs at the nanohole mask openings. On the GaN NCs, InGaN active layers were prepared, followed by the growth of Mg-doped p type GaN/AlGaN superlattice (SL) cladding layers. Finally, p-GaN contact layers were grown on the top of NCs. The height of NCs was approximately 800 nm. Beautifully arranged NC honeycomb-lattice arrays were obtained for L’ from 160 to 300 nm. The nanocolumn diameter increased with increasing L’, the corresponding D were from 123 to 255 nm. Note that the InGaN honeycomb-lattice arrays without p-GaN/AlGaN SL cladding were evaluated by photoluminescence (PL) measurement, observing the PL peak wavelengths of 643 (red) and 577 nm (yellow) for L’ of 300 and 230 nm, respectively. Using the pn-junction NC honeycomb arrays (different sample), we made the process to form p and n type electrodes fabricating the InGaN honeycomb-lattice NC LEDs. To suppress the leakage current, SOG was embedded at the spacing between NCs and the tops of NCs were exposed by a chemical etching. A thin Ni/Al contact layer was deposited on them, followed by the deposition thick ITO transparent electrode of 300 nm thickness. Finally, Ti/Al/Pt/Au metal electrodes of 30 mm diameter were prepared at the center of the 150×150 μm² NC top surface. The emission characteristics were evaluated under the direct current injection. When the injection current was 25 mA, a honeycomb-lattice NC LED with L’=300 nm exhibited the emission spectrum with the peak wavelength of 600 nm (orange emission). This is the first demonstration of InGaN-based honeycomb-lattice NC LEDs suitable for the plasmonic LED structure.
Reference
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Replacing lead in halide perovskites is of great interest due to concerns about stability and toxicity. Recently, lead free double perovskites in which the unit cell is doubled, and two divalent lead cations are substituted by a combination of mono- and trivalent cations have been synthesized as bulk single crystals and as thin films. Here, we study stability and optical properties of all-inorganic cesium silver (I) bismuth (III) and silver (I) Indium (III) halide nanocrystals with the double perovskite crystal structure. The cube shaped nanocrystals are monodisperse in size with typical side lengths of 8 to 15 nm. The absorption spectrum of the nanocrystals presents a sharp peak, which we assign to a direct bismuth s-p transition and not to a quantum confined excitonic transition. In the case of indium nanocrystals Na+ doping increases the emission which is generally very brad and weak. Using spectroscopy combined with high resolution transmission electron microscopy (TEM) based elemental analysis, we conduct stoichiometric studies at the single nanocrystal level as well as decomposition assays in solution and observe that Ag⁺ diffusion and coalescence is one of the pathways by which this material degrades. Drying the nanocrystals leads to self-assembly into ordered nanocrystal solids, and these exhibit less degradation than nanocrystals in solution.
Our results demonstrate that Cs₂AgBiX₆ (X = Cl, Br) nanocrystals are a useful model system to study structure-function relationships in the search for stable non-toxic halide perovskites.

Replacing the traditional lighting source with white light-emitting diodes (WLEDs) could take a significant bite out of global energy consumption. According to US department of energy, 29% reduction in energy consumption will be achieved by 2025 by replacing current lighting sources with LEDs. In order to resolve the energy crisis and to address the environmental concerns, designing WLED is highly desirable but isn't completely benign or budget friendly. To help reduce the environmental footprint and cost of these lights, we have designed a continuous broadband WLED based on metal-organic framework (MOF). Advantage of MOF-based LED is that alkaline earth metal is used in designing the luminescent material which is more environmental-friendly as compared with commonly used lanthanides. Its continuous emission spectrum resembles the spectrum of ordinary sun light and hence, emits a white light similar to natural light. In addition, the continuous broadband white light originates from a single component; hence the cost of production is much lower than the current technologies. The recent developments to use MOFs as white light emitter will be presented. Our design of this natural light emitter device with advantageous features will open new perspectives for developing environmentally friendly, human-friendly, and energy-saving solid-state lighting materials.

Germanium can be converted into a purely group IV-based direct bandgap material by alloying with >~9 at% Sn requiring to overcome the solid solubility limit of < 1 at%.[1] Conventional approaches will not lead to the desired metastable compound and alternative synthesis strategies using kinetically controlled, low temperature growth conditions have to be established.[2,3] To date, notable photoluminescence in Ge_1-xSn_x nanostructures is only observed in exceptional cases, mostly limited to the low temperature regime (~100 K) or in an energy range close to elemental Ge nanostructures and not in the IR-regime expected by the nominal composition.
This contribution will demonstrate that metallic Sn can be used as both nucleation seed and source for the formation of single crystalline Ge_0.81Sn_0.19 nanowires via a CVD approach.[4] These nanostructures are epitaxially grown on Ge (111) substrates in high yield. Moreover, a clear difference in composition and morphological structure evolution of micro- to nanoscale objects can be observed. STEM-EDX elemental mapping is used to determine the Sn content, while TEM confirms the single crystalline nature of the obtained products.
The temperature and laser power dependent photoluminescence analyses verify the formation of a direct band gap material with emission in the mid-infrared region. The emission signal matches the expected energy for unstrained Ge_0.81Sn_0.19 material (e.g. band gap of 0.3 eV at room temperature) and illustrates teh excellent crystal quality of the nanowires samples. This is the first demonstration of room temperature photoluminescence in Ge_1-xSn_x nanowires and the optical properties are comparable to the best examples described for thin film samples of this metastable alloy. These materials with band gaps in the mid-IR hold promise in applications such as thermal imaging as well as photodetection and could be used as building blocks for group IV-based mid- to near-IR photonics.

[1] Wirths, S.; Geiger, R.; von den Driesch, N.; et.al., Nature Photonics 2015, 9, 88.
[2] Assali, S.; Dijkstra, A.; Li, A.; Koelling, S.; et al. Nano Lett. 2017, 17, 1538.
[3] Seifner, M. S.; Hernandez, S.; Bernardi, J.; Romano-Rodriguez, A.; Barth, S., Chem. Mater. 2017, 29, 9802.
[4] Seifner, M. S.; Dijkstra, A.; Bernardi,J.; Steiger-Thirsfeld,A.; Sistani,M.; Lugstein, A.; Haverkort, J. E. M.; Barth S.^; ACS Nano 2019 submitted.

As the counterpart of colloidal two-dimensional visible-light emitters, colloidal lead-sulfide nanosheets are the emerging efficient infrared light emitters with a tunable wavelength covering the whole fiber-optical communication band. Due to the lack of active surfaces such as {111} facets, the PbS nanosheets exhibit more efficient and stable optical properties than PbS quantum dots. Our experimental results show that colloidal PbS nanosheets dispersed in an organic solvent can achieve over 60% photoluminescence quantum yield at the wavelength around 1200 nm, exceeding the well-passivated colloidal PbS quantum dots emitting at the same wavelength.
In contrast to PbS quantum dots, the loss of the photoluminescence efficiency is greatly reduced when the PbS nanosheets are transferred from a solution phase to a solid film. The photoluminescence quantum yield of the PbS-nanosheet film reduces to 1/3 of the original nanosheets in solution, instead of around 1/10 as in the case of quantum dots. Thus a photoluminescence quantum yield ~ 20% can still be achieved in a film of PbS-nanosheets. After coupling the PbS nanosheet film with a commercial green LED, we have fabricated an infrared LED with a down-conversion efficiency of about 13%.
The photophysics study of this infrared emitter demonstrates its excellent properties. A very narrow photoluminescence emission linewidth (~ 66 meV) has been achieved in the nanosheets even at room temperature. Meanwhile, we observed a sharp optical absorption peak near the band edge. This is the first demonstration of the exciton peak from colloidal PbS nanosheets, proving the existence of excitons in this extended two-dimensional material with a large dielectric constant.

Two-dimensional (2D) halide perovskites are emerging light-emitting materials because of their high tunability and outstanding physical properties. Besides the narrow-band emission that provide high color-purity, broad-band white-light emission that originates from self-trapped excitons (STE) in the highly distorted structures is also an intriguing field. The (110)-oriented 2D perovskites are generally distorted and believed to be good candidates for white-light emitting. Here, we report that 3-aminopyrrolidinium (3APr) is a cation which permits the formation of (110)-oriented 2D perovskites for all three halides, in the form of (3APr)PbX₄ (X = I, Br, Cl). Structural characterization by single-crystal X-ray diffraction reveals that the distortion of the inorganic part is influenced by the stereochemical conformation of the cation between the perovskite layers. The high level of distortion results in the emergence of white-light emission, rarely seen in iodide perovskites, as well as the bromide and chloride isostructural analogues, which provides perfect platform to compare the broad emission mechanism for all three halides. The bromide and chloride perovskites show longer lifetimes and higher color rendering index (CRI) (83 and 85), relevant to solid-state lighting. The mechanism as studied by temperature-dependent PL suggests that a different STE mechanism is responsible for the observed broad-band emission for each halide. The detrapping energy is the highest for the chloride compound where the broad-band emission dominates even at low temperature. Comparative studies of white-light emission for all three isostructural halide perovskites as a set may advance the understanding of the mechanism for white-light emission in 2D perovskites.

The main advantage of QDs-based WLEDs (quantum dots-based white light emitting diodes) over conventional WLED is high color purity with low cost production. Although there are several ways like QLEDs, QD film in LCD to apply QDs for next generation display, QDs-based WLEDs still have several superior potentials. Compared to QLEDs, they have higher efficiency and better reliability. This design requires fewer quantities of QDs to reach target color point which attributes to reduce cost hugely. Despite of great interest in QDs-based WLEDs, the major two issues that limits the practical applications are their instability and anion-exchange reaction when different QDs mixing together. It is important to develop a suitable packaging type to overcome the anion-exchange reaction in order to get long term stability.
Therefore, we proposed a hybrid-typed structure with blue LED pumping liquid green 518 nm PQD and red 630 nm PQD film. The liquid green PQD is on the top of red PQD film. Compared to the structure using green and red PQD film (41 lm/W), this hybrid structure owned higher efficiency (51 lm/W) for PQD LEDs. It also showed outstanding color gamut that can be reached to 122 % of NTSC standard and 91 % of Rec. 2020 at a correlated color temperature (CCT) of 5516 K. The thermal resistance of hybrid-typed structure is better than the solid type structure. The device temperature vs current was measured. It shows that for hybrid-typed PQD WLED, the operating temperature will not increase too much with current increasing. The temperature gap between hybrid-typed and solid-typed PQD WLEDs is very big, which is about 15 Celsius degree. This characteristic resulted to better reliability performance at 200 hrs (12% decay).
However, the luminous efficiency and reliability performance is still not good enough to catch up conventional WLEDs' spec. We found that the drop of light intensity comes from the red QD. Thus, to improve this situation, we replaced the red QD to K₂SiF₆:Mn⁴⁺ (KSF) for hybrid-typed structure in order to get higher efficiency and reliability without the decrease of color gamut. The light performances of hybrid type using liquid green PQD and 630 nm KSF mixed with silicone was measured. The luminous efficiency can reach to 85lm/W at 10 mA driving and wide color gamut can still be kept (122 % of NTSC and 91 % of Rec. 2020). The reliability results of this hybrid-typed PQD WLEDs have been tested. The light intensity only decays by 6 % at 1000 hours. In this study, we demonstrate that the hybrid-typed PQD WLED has the higher luminous efficiency (85 lm/W) compared to the solid-typed structure and good wide color gamut performance (123 % of NTSC and 92 % of Rec. 2020) and better reliability result show that hybrid-typed structure is one of the choice for next generation display.

Gallium Nitride (GaN) based blue light emitting diode (LED) is definitely an emerging technology which already grabbed attention from various fields, including indoor and outdoor lighting systems, displays, and medical devices because of its excellent quality of light illumination. GaN LEDs in micrometer scale can be driven at higher current densities and give promising energy efficiency in light emission. [1] The outstanding performance of micro-LEDs have attracted many researches that study the potential applications of micro-LEDs. [2, 3]

In this report, we have studied method to realize full color RGB micro-LED display by patterning quantum dots (QD) on GaN micro-LED, which will convert blue light into either red or green light. We developed QD-PR that can be photolithographically patterned in micro scale using conventional method. By mixing high refractive index nanoparticles, TiO₂ as scattering enhancers into the QD-PR, the light output intensity can be improved and standard RGB can be achieved. In addition, in order to realize flexible micro-LED display, micro-LED array is released from Si substrate by wet etching process and then transferred onto ultrathin plastic substrate. Finally, flexible RGB micro-LED display was demonstrated.



References

[1] Olivier; François, et al.: Influence of size-reduction on the performances of GaN-based micro-LEDs for display application. Journal of Luminescence 191, pp. 112-116, 2017.
[2] Kim, Hoon-sik, et al.: Unusual strategies for using indium gallium nitride grown on silicon (111) for solid-state lighting. Proceedings of the National Academy of Sciences, 108.25, pp. 10072-10077, 2011.
[3] Choi, Minwoo, et al.: Stretchable Active Matrix Inorganic Light Emitting Diode Display Enabled by Overlay Aligned Roll Transfer Printing. Advanced Functional Materials, 27.11, pp. 1606005, 2017.

Magic-sized clusters (MSCs) are semiconductor nanocrystals that have well-defined structural and optoelectronic properties. These clusters are believed to grow in discrete steps from one “magic” size to the next. However, even after decades of study, the nanocrystal community does not have a detailed understanding of why “magic” sizes exist and how they grow. This is due in part to the challenges in their experimental investigation, e.g. due to their small size they can be difficult to isolate and image via electron microscopy. Here, we work to overcome these issues by developing a synthetic protocol that yields MSCs up to larger length scales. These particles can be isolated and purified easily using size-selective precipitation. We then analyse a series of these MSCs using a combination of optical (absorption, photoluminescence, and photoluminescence excitation) and structural characterisation (XRD, NMR, and TEM). The comparison of these results with previous reports suggests that the particles exhibit tetrahedral shapes. Based on our observations, we propose an atomistic model that explains the growth of MSCs and rationalizes the existence of their precise structures. This work improves our understanding of nanocrystal growth and could help expand the library of available MSCs.

Magic-sized clusters (MSCs) are small semiconductor nanocrystals with well-defined structures and characteristic optoelectronic properties. Only a few “magic” sizes with superior thermodynamic stability are believed to exist. They have been shown to grow sequentially, meaning that one “magic” size transitions directly to the next. However, the origin of their stability and the mechanism that determines their growth remain puzzling. Our experiments, in line with previous reports, suggest that MSCs are tetrahedral in shape. Based on these findings, we present a microscopic model that rationalizes their stability and growth. By combining classical nucleation theory and the tetrahedral shape of MSCs, our model explains both why MSCs correspond to local minima in a free-energy landscape and why they grow sequentially. Our model can help find other crystalline materials exhibiting the same “magic” growth behavior.

The established paradigm, that functionality and performance of a semiconductor device is critically linked to the design of the interfaces in the device, has been a major working hypothesis to access the full potential of halide perovskite (HaP)-based optoelectronics, including photovoltaics and light-emitting diodes. As an example, the rapid improvements in the performance and stability of HaP solar cells mainly originates from careful interfacial design principles and a dedicated interface layout [1]. Despite these successes, many basic physical and chemical properties of HaP thin film and crystal surfaces and at the interface to adjacent layers, e.g. carrier injection layers in the case of perovskite emitters, have been difficult to access. These challenges arise as many key parameters, such as the energy level alignment between transport layer and emitter, are convoluted with the interface chemistry of the multi-component HaP semiconductor.
In my talk, I will present how we employ surface sensitive methods, with a focus on photoelectron (PES) and X-ray spectroscopies, to measure electronic properties, energetics and chemistry at the interface between perovskite films and transport or buffer layers. In particularly PES is used to determine the energy level alignment at the interface. Yet, we face severe challenges to generalize the trends from these observations as the complex chemistry between HaP layer and adjacent semiconductor often lies at the root of the observed interfacial alignment processes and band bending [2]. Furthermore, I will give examples for typical pitfalls that occur, when characterizing HaP surfaces and interfaces with PES methods. Most importantly, beam damages effects have been identified as the perovskite layers can exhibit distinct signs of degradation under vacuum conditions and concomitant irradiation with high-energy photons. However, while we generally intend to avoid these transient effects in our measurements, we can extract additional physical and chemical parameters from the evolution of energy level positions and stoichiometry during the PES measurements with direct implications on the material stability and degradation pathways [3].
In the second half of my talk, I will discuss the use of these methods to access the effects of surface treatment and interfacial design routes for HaP quantum dot thin films. We demonstrated that treating the surface of CsPbI₃ based quantum films with a formamidinium iodide solution led to improved device characteristics of perovskite quantum dot solar cells [4]. By using PES and X-ray absorption spectroscopy techniques, we were able to identify that the surface treatment can be employed for a targeted ligand exchange, opening up a broad application space of these treatments for perovskite quantum dot based light emitting devices [5].

[1] P. Schulz. ACS Energy Lett. 2018, 3, 1287
[2] P. Schulz, D. Cahen, A. Kahn. Chem. Rev. 2019, 119, 3349-3417
[3] R. A. Kerner, P. Schulz, J. A. Christians, S. P. Dunfield, B. Dou, L. Zhao, G. Teeter, J. J. Berry, B. P. Rand. APL Materials 2019, 7, 041103
[4] E. M. Sanehira, A. R. Marshall, J. A. Christians, S. P. Harvey, P. N. Ciesielski, L. M. Wheeler, P. Schulz, L. Y. Lin, M. C. Beard, J. M. Luther. Science Adv. 2017, 3, eaao4204
[5] L.M. Wheeler, E.M. Sanehira, A.R. Marshall, P. Schulz, M. Suri, N.C. Anderson, J.A. Christians, D. Nordlund, D. Sokaras, T. Kroll, S.P. Harvey, J.J. Berry, L.Y. Lin, J.M. Luther. J. Am. Chem. Soc. 2018, 140, 10504

Cd-free quantum dot (QD) light-emitting diodes (QLEDs) such as InP QLEDs are now being actively researched for the substitute of environmentally toxic Cd-based QLEDs. We can classify the InP QLEDs based on the direction of emission, which are the bottom emission QLEDs (BQLEDs) emitting light through the substrate electrode and the top emission QLEDs (TQLEDs) emitting light through the top transparent electrode. The TQLEDs have several advantages for their use in display devices, such as high aperture ratios and efficient light extraction. In this work, we fabricated InP-based BQLEDs and TQLEDs and compared their optical and electrical characteristics. The TQLEDs showed better optical properties than BQLEDs because the outcoupling through the thin, top transparent silver anode in the TQLEDs was much higher than that through the indium tin oxide (ITO) in the BQLEDs. As a result, the maximum luminance of the TQLEDs was 29300 cd/m², which is 4 times higher than that of the BQLEDs (7400 cd/m²). The full width at half maximum of the electroluminescence (EL) spectrum was also narrowed down from 41 nm of the BQLEDs to 37 nm of the TQLEDs owing to the microcavity effect. The maximum current efficiency of the TQLEDs (9.7 cd/A) was increased by 1.4-fold compared to that of the BQLEDs (6.9 cd/A). Detailed performance and analysis on the TQLEDs and BQLEDs based on InP QDs will be presented.

Advancing the synthesis of PbS nanocrystals for infrared optoelectronic applications requires improved mechanistic understanding of the growth process. Here we demonstrate that the nucleation of PbS nanocrystals is preceded by the formation of a distinct cluster intermediate. This intermediate is characterized by a reproducible red emission spectrum (λ_peak: 740 nm) that exhibits a large Stokes shift (hν>300meV). Having revealed a two-step growth mechanism, we explore the manipulation of the cluster reaction lifetime to control nanocrystal growth kinetics. We demonstrate that the addition of diglyme (diethylene glycol dimethyl ether) gives straightforward control of PbS nanocrystal size. This occurs because diglyme accelerates cluster agglomeration, which reduces the number of nascent nanocrystals and results in a larger average size at completion. Further, the shortened cluster reaction lifetime minimizes the duration of the nucleation period, permitting the synthesis of nanocrystals with ensemble linewidths comparable to leading literature reports at lower reaction temperatures. In sum, we report that the nucleation and growth of PbS nanocrystals proceeds via a two-step mechanism involving a cluster intermediate, and open the door to the use of process additives to tune reaction kinetics and tailor the synthesis of this critical infrared-active nanostructured material.

Colloidal semiconductor nanocrystals (quantum dots) possess unique optical properties, which make them attractive emitters for diverse applications in optoelectronic devices and biomedical imaging. During their life cycle, aging of quantum dots can lead to their dissolution and induce high toxicity due to the release of toxic chemical compounds. In this context, InP-based QDs have been proposed as less hazardous alternative to widely studied CdSe-based QDs. To enhance the photoluminescence quantum yield (PLQY) and (photo-)stability, InP QDs are generally capped with a Zn(Se,S) gradient shell. ZnSe acts as a “lattice adapter” due to its lower lattice mismatch with the InP core, while the external ZnS layer(s) are chemically more inert than ZnSe. We demonstrate that this type of shell alone does not provide sufficient stability, neither under controlled ageing conditions in a climatic chamber nor in biological environment. As shown by EXAFS, ageing led to dissociation of In-P and Zn-S/Zn-Se bonds, and to complexation of In and Zn ions with carboxylate and/or phosphate moieties. Moreover, these degradation products exhibited, in contrast to the pristine QDs, significant cytotoxic effects.
To improve the stability, further growth of additional shells on top of the gradient shell was performed, namely an intermediate ZnS shell and an outer alumina shell. In the case of this core / triple shell system continuous irradiation did not induce any chemical modifications detectable by means of XPS. To the contrary, in the case of the core / single shell system with the gradient shell degradation of the ZnS outer layer and oxidation of the In(Zn)P core occurred. These effects were also at the origin of a marked decrease of the PL intensity and a hypsochromic shift of the PL maximum. In summary, the overgrowth of InP-based core/shell QDs with an inert alumina shell leads to strongly enhanced chemical stability without impacting the PLQY. The formation of toxic degradation products under ageing conditions can thus be avoided.

Colloidal quantum dots (QDs) exhibit narrow emission across the visible and NIR spectrum, and reproducible wet-chemical syntheses produce QDs with size-tunable electronic properties. While already employed in LEDs, solar cells, and biomedical imaging, QDs suffer from photoluminescence intermittency (blinking)¹, which is associated with irreversible photochemical damage². The spectroscopic manifestation of blinking is clear—even under continuous above-bandgap excitation, emission from QDs will randomly switch between ON and OFF states, with both states lacking well-defined average lifetimes due to probability distributions that are power-law distributed over many orders of magnitude. However, it has remained challenging for spectroscopic evidence to distinguish between the variety of proposed mechanisms. Here, motivated by previous demonstrations of emission recovery from QD ensembles in the dark³, we employ a two-colour experiment to modulate the lifetime of the OFF states of QDs. By optically interacting with the OFF state, its lifetime was expected to be truncated at shorter timescales than the minutes/hours observed for thermal recovery at room temperature. However, we did not observe the expected light-driven recovery, even due to local heating. Thus, we consider that the optical cross-section of the dominant OFF state must be small compared to a control experiment using the excited-state absorption of Mn-doped ZnSe QDs⁴. This result is surprising in light of previous thermal recovery kinetics, as the weak oscillator strength is consistent with an OFF state that is different than typical band-edge carriers. By shedding light on the spatial and energetic nature of the state(s) involved in blinking, we will guide modifications to synthetic procedures which produce non-blinking quantum dots, resulting in enhanced long-term photostability.

(1) Nirmal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. Nature. 1996, 802–804.
(2) Cui, J.; Beyler, A. P.; Bischof, T. S.; Wilson, M. W. B.; Bawendi, M. G. Chem. Soc. Rev. 2014, 43 (4), 1287.
(3) Jensen, R. A.; Coropceanu, I.; Chen, Y.; Bawendi, M. G. J. Phys. Chem. Lett. 2015, 6 (15), 2933.
(4) Irvine, S. E.; Staudt, T.; Rittweger, E.; Engelhardt, J.; Hell, S. W. Angew. Chemie - Int. Ed. 2008, 47 (14), 2685.

Aside from their remarkable success in photovoltaics, metal-halide perovskites are also highly promising as light emitters. For lasers, perovskites currently seed a new promise for the realization of electrically operated laser diodes that can be prepared from solution at low temperatures on virtually any substrate. The marriage of perovskite active materials with silicon (nitride) photonics holds promise to unlock substantial progress in the field of integrated optoelectronics.
As MA-based organic-inorganic halide perovskites lack intrinsic stability, all-inorganic perovskites, such as cesium lead halides, are particularly promising.
However, most attempts to use thin-films of CsPbX₃ for light emission only led to disappointing results with very low photoluminescence quantum yields (PL-QY) and amplified spontaneous emission (ASE) in CsPbX₃ thin-films has only been achieved at cryogenic temperatures. The poor performance at higher temperatures has been attributed to non-radiative recombination associated with a high number of defects. Unlike the case of thin films, CsPbX₃ nanoparticles (size < 10 nm) or nano-rods showed a high PL-QY and exhibited ASE at room temperature. These results nurtured the paradigm that for efficient light emission from lead halide perovskites at room temperature, one needs to confine the charge carriers/excitons on the nanometer scale to prevent their migration to non-radiative defects.
Here, we present results that challenge this paradigm. We demonstrate thin films of cesium lead bromide, which show a high photoluminescence quantum yield of 68% and ASE at room temperature with low threshold. The continuous films (~ 100% coverage of the substrate) are composed of large crystals with micrometer lateral extension. Our layers result from originally rough as-deposited layers, that were recrystallized by thermal imprint.[1,2] Using these layers, we demonstrate the first cesium-lead bromide thin-film distributed feedback (DFB) and vertical cavity surface emitting lasers (VCSELs) with ultra-low threshold at room-temperature, that do not rely on the use of nanoparticles.[3] Our results render this all-inorganic gain medium an excellent material platform for perovskite laser diodes in the future. The prospects of perovskite lasers as building blocks in integrated optoelectronics will be discussed. We also foresee that our results will have a broader impact beyond perovskite lasers and will possibly lead to a revision of the belief that efficient light emission from CsPbX₃ perovskites could only be achieved with nanoparticles.

[1] N. Pourdavoud et al. Adv. Mater. Technol. 2018, 3, 1700253.
[2] N. Pourdavoud et al. Adv. Mater. 2017, 29, 1605003.
[3] N. Pourdavoud et al. Adv. Mater. (submitted)

Organic-inorganic perovskites have emerged as a promising laser gain medium due to its high quantum efficiency, balanced ambipolar charge transfer characteristics, strong light absorption, and long carrier lifetime. Various types of perovskite laser have been reported since the first demonstration of amplified spontaneous emission (ASE) from perovskite in 2014 [1]. Synthesis of nanowire or microdisk cavity, or spin-coating of perovskite thin film on pre-patterned structure have been the common method of making perovskite lasers. These devices are mostly tested under pulsed pumping and/or at cryogenic temperature due to their high lasing thresholds. Because of device self-heating at high pump power, their operation under continuous wave (CW) pumping at room temperature is hindered. One way to achieve CW room temperature operation is to reduce lasing threshold by directly patterning the perovskite into a high Q cavity with large mode confinement. However, perovskite’s sensitivity in polar solvents, high temperature and high-electron energy makes direct patterning using conventional lithography challenging. Previously, we showed that direct patterning of perovskite is possible with nanoimprint lithography (NIL), and demonstrated CW lasing with 13W/cm² threshold from a MAPbI₃ distributed feedback cavity at room temperature [2]. However, lasing only sustained for ~250s when operated around lasing threshold.
To achieve stable lasing behavior, in addition to designing for even higher Q cavities to further reduce operational power, we introduce multiplex strategies that include morphological, structural, and interfacial engineering of perovskite thin film. With these considerations, we demonstrate for the first time, over 90 minute-long green CW lasing with 9.4W/cm² threshold from polycarbonate MAPbBr₃ composite in a two-dimensional photonic crystal (2D PhC) cavity without any substrate cooling. This demonstration highlights (i) synergistic effect of high crystallinity of material, high Q cavity, and defect passivation (ii) facile, high output, and controllable fabrication method which can be applied to other types of perovskite and is closely aligned with what industry seeks for mass production (iii) ultra-low lasing threshold and long lasing stability at CW room temperature, making electrically pumped perovskite laser diode look promising.

1. G. Xing, N. Mathews, S. S. Lim, N. Yantara, X. Liu, D. Sabba, M. Grätzel, S. Mhaisalkar, and T. C. Sum, "Low-temperature solution-processed wavelength-tunable perovskites for lasing," Nature Materials 13(5), 476–480 (2014).
2. Z. Li, J. Moon, A. Gharajeh, R. Haroldson, R. Hawkins, W. Hu, A. Zakhidov, and Q. Gu, "Room-Temperature Continuous-Wave Operation of Organometal Halide Perovskite Lasers," ACS Nano 12(11), 10968–10976 (2018).

To the urge for efficient semiconductor light emitters operating in the green gap, we’ve demonstrated InGaN nanowire photonic crystals, including dot-in-nanowires, nanotriangles, and nanorectangles with precisely controlled size, spacing, and morphology,
and further establishes that bottom-up InGaN photonic crystals can exhibit highly efficient and stable emission. The formation of stable and scalable band edge modes in defect-free InGaN nanowire photonic crystals is directly measured by cathodoluminescence studies. The luminescence emission, in terms of both the peak position (λ ≈ 505 nm) and spectral linewidths
(full-width-half-maximum ≈ 12 nm), remains virtually invariant in the temperature range of 5–300 K and under excitation densities of 29 W cm⁻² to 17.5 kW cm⁻². To the best of our knowledge, this is the first demonstration of the absence of Varshni and quantum-confined Stark effects in wurtzite InGaN light emitters—factors that contribute significantly to the efficiency droop and
device instability under high-power operation. Such distinct emission properties of InGaN photonic crystals stem directly from the strong Purcell effect, due to efficient coupling of the spontaneous emission to the highly stable and scalable band-edge modes of InGaN photonic crystals, and are ideally suited for uncooled, high-efficiency light-emitting-diode operation.

In this paper we study the prospect of using the hexagonal BN (h-BN) quantum dots and 2D nanosheets for generating deep ultraviolet (UV-C) emission by impact excitation and impact ionization. Our objective was to design and model alternating current driven thin electroluminescence devices (ACTEL) and alternating current driven powder electroluminescence devices (ACPEL) based on h-BN having different nano-morphologies. The recent efforts in developing dc-driven III-nitrides-based deep UV-photonic devices focused on band gap engineering of epitaxially grown heterostructures. Alternatively, one can consider developing UV-C light sources operating on principles of hot electrons impact excitation processes in h-BN material. We have shown by considering the lucky drift model and the Born Approximation (impact excitation cross section) for high field electronic transport in a single h-BN layer, when considering both ballistic and drift modes, that the electroluminescence (EL) efficiency of the ACTEL device is 0.04% when biased at 110 V. The low EL efficiency is primarily due to the long distance (~5 µm) secondary electrons need to pass between subsequent collisions in the h-BN matrix to gain sufficient energy required for electron-hole pair generation. This decreases the likelihood of an electron encountering a second collision after already being subjected to one collision within a single layer of the phosphor, significantly reducing the efficiency. To overcome this issue new ACPEL devices made of alternating h-BN phosphor-insulator layer pairs stacked together was considered. The focus was on selecting insulator material and optimizing the layers thicknesses to promote transport in ACPEL devices via ballistic mode primarily. In such a case primary electrons moving in the BN phosphor layer (ballistic mode) after encountering a collision, enter into a subsequent dielectric layer where they are reaccelerated before entering the next BN phosphor layer. The process is repeated through the ACPEL stack resulting in an efficiency enhancement as compared to a single h-BN layer ACTEL device. In the presentation we will demonstrate using COMSOL Multiphysics and analytical calculations that by taking into account the BN morphology (nano-poly-crystalline vs. single crystal) and the device structure (h-BN phosphor layer thickness, insulator layer parameters, number of layers pairs in the stack) the calculated ACPEL device emission efficiency can be as high as 20%.