Symposium NM12 : Transitioning Quantum Dots from Benchtop to Industry

Colloidal Quantum Dots (CQD) have been historically studied in the visible region, but they have the potential to transform infrared technologies by providing much lower processing cost and possibly better performance than traditional HgCdTe of InSb bulk detectors. CQDs can display strong infrared electronic transitions using either interband or intraband transitions. HgTe is a semi metal with a light electron such that nanoparticles of sizes between 20 and 6 nm have absorption edges from 12 to 2 microns. Thus, HgTe CQDs cover all the mid-infrared. As synthesized, HgTe CQDs are nearly intrinsic, films readily exhibit photoconductance from room temperature to low temperatures, and photovoltaic structures achieve Background Limited Performance (BLIP). The main challenge is to raise the operation temperature which requires to reduce non-radiative losses and to control accurately the doping, both stemming from surface chemistry. The other approach to the mid-infrared is to use the intraband transitions of doped quantum dots. HgTe and HgS are two systems that show stable n-doping in ambient conditions and that have been demonstrated as photoconductors. Potential advantages of the intraband approach are the elimination of Auger processes, the increased absorption per unit length, a narrower photodetection spectral range, and the possibility of using a wider range of starting materials, including wide band gap visible transparent semiconductors. The talk will review the progress and challenges of this new material approach to infrared technology.

Among colloidal nanomaterials, 2D nanoplatelets^1,2 (NPLs) show remarquable optical properties due to their atomic flatness in the only confined dimension. Those objects are typically strongly confined and thus usually address the visible range. Recently, the synthesis of mercury telluride nanoplatelets with optical features in the near infrared has been reported.³

We have investigated the electronic transport of thin films made of HgTe NPL in an electrolytic filed effect transistor configuration⁴. We have demonstrated that the majority carrier can be tuned from p-type to n-type using ethanedithiol (EDT) or sulfide (S^2-) capping, respectively⁵. This change of behavior was confirmed by looking at the electronic structure of the material using X-ray photoemission and attributed to a change of hybridization of the ligands with the Hg rich surface of NPL.

We then probed the photophysics of HgTe NPL films, focusing on carriers' dynamics. Using pulsed near-infrared illumination, we measured a response time around 100 µs at the device scale, and wonder whether this value is a material limitation. The answer has been found using time-resolved photoemission where relaxation occurs on a time scale of 100 ns, suggesting that current device performances are limited by geometrical factors rather than physical processes.

References

1. M. Nasilowski, B. Mahler, E. Lhuillier, S. Ithurria, and B. Dubertret, Chem. Rev. 116, 10934 (2016).

2. E. Lhuillier, S. Pedetti, S. Ithurria, B. Nadal, H. Heuclin, and B. Dubertret, Acc. Chem. Res. 48, 22 (2015).

3. E. Izquierdo, et al., J. Am. Chem. Soc. 138, 10496 (2016).

4. E. Lhuillier, S. Pedetti, S. Ithurria, H. Heuclin, B. Nadal, A. Robin, G. Patriarche, N. Lequeux, and B. Dubertret, ACS Nano 8, 3813 (2014).

5. C. Livache, E. Izquierdo, B. Martinez, M. Dufour, D. Pierucci, S. Keuleyan, H. Cruguel, L. Becerra, J.L. Fave, H. Aubin, A. Ouerghi, E. Lacaze, M.G. Silly, B. Dubertret, S. Ithurria, and E. Lhuillier, Nano Lett. 17, 4067 (2017)

The use of solution-synthesized semiconductor nanocrystals (NCs) in optoelectronic devices inevitably requires postsynthetic chemical surface treatments to allow for efficient charge transport in NC films. In the case of lead chalcogenide (PbE; E = S, Se, Te) NCs, such treatments are invariably applied after the NCs are already deposited, typically requiring the film to be built up by repetitive deposition steps to achieve device-relevant thicknesses. This represents a critical hurdle in progress toward exploiting fast, inexpensive and highly scalable solution-based fabrication processes, like spray or knife-edge deposition, especially for intriguing applications such as solar cells and sensors that require films >1 μm thick. Such considerations have driven the development of in-solution surface treatments for a range of other NC materials, but corresponding advances for PbE NCs have lagged dramatically behind. Here, we describe a universal method for fast, complete, in-solution exchange of the surface ligands of PbE NCs with a wide range of anions and small ligands, resulting in a NC “ink” suitable for single-step deposition of conductive films of unprecedented thickness. Optical studies of these inks reveal fascinating complexity in the interactions of the NCs and ligands that suggest that far from being the enemy of NC devices, ligands offer a facile and versatile means for fine tuning of NC and NC film electronic properties. In this talk, we discuss how concepts borrowed from organometallic complexes help to elucidate the complex effects ligands have on NC properties, and how we might exploit this understanding toward realization of high-performance devices.

Arrays of electronically coupled colloidal quantum dots (QDs) present a unique model system for exploring charge transport mechanisms and energy conversion in strongly disordered inorganic films. In this work, we present the results of dynamic junction capacitance spectroscopies (drive level capacitance profiling (DLCP) and thermal admittance spectroscopy (TAS)) on PbS QD solar cells. We study properties critical to the charge transport of both dark and photo-excited carriers in these devices, such the spatial uniformity of bulk defects and their energetic depth within the semiconductor band gap, carrier densities, and illumination-induced meta-stabilities.

Our results indicate a deep bulk trapping response present throughout the PbS film, regardless of whether halide (I^-) or thiol (MPA) ligand treatments are used. Fermi level pinning at the heterojunction interface is also observed, and both the density of bulk and interface states is determined using novel DLCP analysis. The conductivity of these deep states is also calculated and suggests weakly conductive activated transport channels within a deep defect band. Upon oxygen exposure, films of halide-capped QDs exhibit a pronounced shift in activation energy not observed in thiol treated films, which we use to explore the chemical origins of the deep state response. Finally, capacitance spectroscopy is also performed under infrared illumination of the solar cells, and indicates that photo-excited charge in the QDs is preferentially trapped in deep bulk defects, providing direct insight into the role of defects in device operation.

Since the development of colloidal methods of quantum dot (QD) synthesis in the early 1990s, there has been strong interest in the potential applications of these materials. The initial barrier to commercialisation was scale-up of the QD synthesis. Further, while early research focussed on cadmium-based II-VI QDs, concerns over the toxicity of these materials has led to the search for visible-emitting QDs free of cadmium and other toxic heavy metals. To address this, Nanoco Technologies Limited (Nanoco) has developed and patented a “molecular seeding” method of nanoparticle synthesis. Following over a decade of research, the method can be used to produce high quality heavy-metal free QDs in large volumes. While display products incorporating QD backlight units are now firmly established on the consumer market, we will discuss how QDs initially developed for displays are now being adapted and integrated into new applications, such as lighting, electroluminescent displays, and bio-imaging.

Hot-injection technique approaches are convenient and fast one-pot processes, which are capable of providing colloidal nanocrystals with ultra-narrow size distributions. Effective time separation between nucleation and growth processes is facilitated by fast addition (i.e., injection) of an elemental precursor or reducing agent to the hot reaction mixture. However, it is this fast addition of large volumes that presents a serious challenge for upscaling hot-injection protocols.
Here we focus on the possibility to upscale injection-based syntheses of colloidal nanocrystals without modifying the original protocol or using specially designed jet equipment. This work presents an easy and universal solution for linear upscaling of hot-injection synthesis. Applying a mild vacuum to the reaction mixture prior the injection enables an injection rate of 100-150 mL/s such that large volumes of 200-500 mL can be introduced into the reaction flask within few seconds. We apply this underpressure-assisted approach to successfully upscale synthetic protocols for metallic (Sn) and semiconductor (PbS, CsPbBr₃ and Cu₃In₅Se₉) nanocrystals by one-to-two orders of magnitude to obtain tens of grams of nanocrystals per synthesis. We provide the technical details of how to carry out underpressure-assisted upscaling and demonstrate that nanocrystal quality is maintained for the large-batch syntheses by characterizing the size, size distribution, composition, optical properties, and ligand coverage of the nanocrystals for both small- and large-scale syntheses.
This work shows that fast addition of large injection volumes does not intrinsically limit upscaling of hot injection-based colloidal syntheses. An underpressure-governed hot-injection method enables a systematic optimization of nanocrystals and nanocrystal-based devices from a single source batch for research and development purposes and reinforce the commercial viability of electronic, photonic, and electrochemical devices that use large numbers of colloidal nanocrystals (e.g., solar cells, lithium-ion batteries, thermoelectrics, phase-change memories, etc.).

For a number of decades, the study and development of colloidal semiconductor nanocrystals has become a rich field toward quantum dot (QD) integration into applications ranging from photovoltaics and photocatalysis to inks for printed electronics to biomedical imaging and drug delivery. Cadmium-based colloidal QDs are some of the most extensively studied semiconductor nanocrystals, and thus represent a model system for much needed surface chemistry investigations/descriptions toward any potential application. Essentially, the challenges in providing a full thermodynamic profile for nanocrystal surface chemistry must be overcome if QD-based technologies are to reach their full potential. This work will highlight how even in such a well-studied QD system, there are several caveats for appropriately compiling a thermodynamic profile in situ for the dynamic nature of their surfaces. It is imperative to consider the impact on the QD surface in the various surrounding media from the purification stage and on through surface modification reactions. Our lab has focused on further developing metrics for investigating colloidal nanoparticle surfaces while perturbing the environment as little as possible. We have established a highly effective and novel gel permeation chromatography approach to nanoparticle purification, which is ideal for investigating QD surface interactions with common spectroscopic tools; as well as investigating QD-ligand thermodynamics with isothermal titration calorimetry (ITC). In fact, where spectroscopic techniques have been limited in providing a full description of QD-ligand interactions, we have demonstrated the capability of ITC to detect binding phenomena responsible for drastic effects on QD photo-physical properties.

Quantum dots have made it from Benchtop to Industry, but the journey was never straight and simple, and there are lots of lessons to be learned from the experience. This talk will endeavor to tell one version of the story, extract some lessons, and seek to apply them to a new lab to market arc in holographic optical elements.

Time correlated single photon counting spectroscopy coupled with fluorescence microscopy has emerged as a powerful tool that reveals the complex photophysics of colloidal quantum dots. Although the nanocrystals can be interrogated individually, their behavior is often linked to an ideal structure derived from ensemble optical spectroscopy and conventional high resolution transmission electron microscopy (HRTEM). Further, dim or dark particles can go completely undetected while small aggregates may appear spectroscopically as a single particle. Recently, we’ve developed an intuitive and reproducible method to correlate the atomic and chemical structure of individual colloidal nanocrystals with the same particle’s fluorescence dynamics.¹ Nanocrystal samples for correlation are prepared by first spin coating a solution of 1 mm polystyrene spheres onto an insulating 8 nm thick SiO₂ TEM grid followed by drop-casting a pM concentration of quantum dots. The polystyrene forms unique arrays that are visible in both the fluorescence microscope and in the TEM allowing for unambiguous identification of single quantum dots. Utilizing a Tecnai Osiris in HRSTEM mode in conjunction with its advanced SuperX energy dispersive spectroscopy (EDS) system, high quality lattice images and chemical maps can be obtained and paired with that same quantum dots’ fluorescence dynamics. It can now be possible to identify sub-populations of structures that exhibit the desired photophysics then use that knowledge to direct chemistry to produce quantum dots with specifically chosen optical behavior. We have used this method to directly identify dark structures in commercial quantum dots while identifying shell defects and stacking faults which result in low on-time intermittency.¹ In contrast, we found no significant dark fraction in giant-shelled CdSe/CdS quantum dots and instead confirmed charge-state emission and the culprit for low ensemble quantum yields.² HRSTEM, advanced STEM-EDS and initial correlation results will be presented for thick shelled InP/ZnSe colloidal quantum dots. Additionally, correlation data of seeded CdSe/CdS nanorods and thick-shelled green emitting quantum dots will be presented.


1. Orfield, N.J.; McBride, J.R.; Keene, J.D.; Davis, L.M.; Rosenthal, S.J. Correlation of Atomic Structure and Photoluminescence of the Same Quantum Dot: Pinpointing Surface and Internal Defects That Inhibit Photoluminescence ACS Nano 2015, 9 (1), 831-839.
2. Orfield, N.J.; McBride, J.R.; Wang, F.; Buck, M.R.; Keene, J.D.; Reid, K.R.; Htoon, H.; Hollingsworth, J.A.; Rosenthal, S.J. Quantum Yield Heterogeneity among Single Nonblinking Quantum Dots Revealed by Atomic Structure-Quantum Optics Correlation ACS Nano 2016, 10 (2), 1960-1968.

Growth of monodisperse quantum dots (QDs) is a pressing requirement in the context of commercial application as well as academic study, as the size uniformity is directly related to color purity in display products which have been commercialized. In the sense, understanding on the generation of active species via thermolysis of precursors, diffusion of precursors from bulk solution onto nanocrystal surface, and surface growth reaction kinetics has been ever more important. In this presentation, I will present our recent findings pertaining to these issues on the basis of different material examples.
First, our study on the colloidal synthesis of InP QDs in the presence of Zn precursors will be discussed, in which size uniformity is markedly enhanced as compared to the case of InP QDs synthesized without Zn precursors. The nuclear magnetic resonance spectroscopy and mass spectrometry analyses on aliquots taken during the synthesis allow us to monitor the appearance of metal-phosphorus complex intermediates in the growth of InP QDs. In the presence of zinc carboxylate, intermediate species containing Zn-P bonding appears. The Zn-P intermediate complex with P(SiMe₃)₃ exhibits lower reactivity than In-P complex, which is corroborated by our prediction based on density functional theory and electrostatic potential charge analysis. The formation of stable Zn-P intermediate complex results in lower reactivity, hence monodisperse QDs. Insights from the experimental and theoretical studies advance the mechanistic understanding and controlling of nucleation and growth of InP QDs, key to the preparation of monodisperse InP-based QDs in meeting the demand of display market.
Second, we have investigated diffusion of active species monomers through ligand layers using CdSe nanorods (NRs) as a model system. Colloidal NRs are of special interest for optoelectronic applications because its shape anisotropy leads to unique optical and physical characteristics, expandable with morphological and structural deviation. Previous studies focused on the development of diverse NR structures. However, synthesis relied on empirical observations under specific conditions, and general NR growth process remained elusive. I present a new answer for detailed growth mechanism of colloidal semiconductor NRs. For this, we developed dual-diameter nanorod (DDNR) structure via colloidal synthesis, where two sections along the long axis in each NR have different diameters at a few nanometer scale. The vivid segmentation is an ideal platform for monitoring the growth process of NRs, presenting important determinants in the reactivity of distinguishable NR facets. The lesson obtained from DDNR is universally applied to nanocrystal growth in any colloidal batch. By controlling the discovered factors, single-diameter NRs with controllable core position also became available. I will put the findings in perspective by outlining the effect of diffusion of monomers and surface growth reactions.

Solar devices based on semiconductor nanoparticles require the use of conductive ligands; however, replacing the native, insulating ligands with conductive metal chalcogenide complexes introduces structural defects within the crystalline nanostructure that act as traps for charge carriers. We utilize atomically thin semiconductor nanoplatelets (NPs) as a convenient platform for studying both microscopically, and spectroscopically, the development of defects during ligand exchange with conductive ligands Na₄SnS₄ and (NH₄)₄Sn₂S₆. Using calibrated HAADF STEM imaging we quantify the extent of ligand exchange induced damage on NPs. Remarkably, we show that these defects can be repaired via mild chemical or thermal routes, through the addition of L-type ligands or wet annealing, respectively. Photoluminescence quantum yield and conductivity studies confirm this picture and further stress the importance of choosing a non damaging ligand exchange protocol. This results in a higher quality, conductive, colloidally stable nanomaterial that can be used as active film in optoelectronic devices such as transistors, solar cells or light emitting diodes.

E. Marino, T. E. Kodger, R. W. Crisp, D. Timmerman, K. E. MacArthur, M. Heggen, P. Schall, Angew. Chem. 2017, 129, 13983.

The Quantum Dots (QDs) have gained enormous interests in the display industry in recent years due to their tunable optical properties depending on size, narrow emission spectra, and high photoluminescence quantum yield (PLQY). However, the QDs have some limitations derived from their high surface areas, such as thermal stability and oxidation problem under high temperature/high humidity conditions. For stabilization of surface and dispersion in organic solvents, the organic ligands such as oleic acid, trioctylphosphine (TOP) or trioctylphosphine oxide (TOPO) are usually used during synthesis process. Despite the ligand capping, the organic ligands are easily detached and decomposed by heat, air, and moisture. Many groups have studied about silica modification of the colloidal QDs or QD/polymer composites to protect QD surface from heat and moisture. [1]
Recently, we reported a siloxane encapsulated QD (CdSe/CdZnS core/shell, ligand: oleic acid, from Ecoflux, Korea)) resin through in-situ sol-gel condensation reaction of 3-metharyloxypropyltrimethoxy silane (MPTS) and diphenylsilanediol (DPSD) in the QD existence. [2] The QDs were uniformly dispersed in the photo-cured QD-siloxane film, and the QD-siloxane film showed long-term stability in harsh environment (85--> in air and 85-->/85% RH conditions).
To increase thermal stability of the QD-siloxane film, we additionally silica-modify the oleic acid-capped QDs using various silanes such as tetraethylorthosilicate (TEOS) and MPTS, which could be reacted during sol-gel condensation reaction. The oleic acid-capped QDs are clearly modified by formation the additional silica shell. The silica-modified QDs are well dispersed in siloxane matrix because QD-siloxane film is fabricated by in-situ sol-gel condensation reaction of MPTS and DPSD in the existence of QDs, and the silica-modified QD-siloxane film is also stable under the harsh condition (85--> in air and 85-->/85% RH). The silica-modified QD-siloxane film is even enhanced the thermal stability under 120--> in air condition, compared to previously reported oleic acid-modified QDs encapsulated in siloxane. Moreover, the silica modification of QDs passivated by organic-ligands will be further studied to improve photo-stability.

References
[1] A. Biermann, T. Auber, P. Baumeister, E. Drijvers, Z. Hens, and J. Maultzsch, J. Chem. Phys. 2017, 146, 134708
[2] H. Y. Kim, D.-E. Yoon, J. Jang, D. Lee, G.-M. Choi, J. H. Chang, J. Y. Lee, Doh C. Lee, and B.-S. Bae, J. Am. Chem. Soc., 2016, 138, 16478-16485

Near-Infrared (NIR) emitting fluorophores attract significant attention due to their potential use in a wide variety of applications including biolabeling, solar windows, and medical diagnostics. Currently available NIR fluorophores lack wavelength tunability, do not cover the entire NIR spectrum, cover only a small portion of the NIR, suffer from low brightness, have poor stability, low quantum yield (QY), small extinction coefficients, high self-absorption, and/or are composed of Restriction of Hazardous Substances Directive prohibited elements such as Cd, Hg, or Pb. UbiQD solves the problems of existing NIR fluorophores with I-III-VI semiconductor-based quantum dots (QDs). Moreover, these CuInSe_xS_2-x/ZnS QDs have a tunable emission ranging from 550 nm to 1300 nm (visible-to-NIR) with QY >90% for most of that range, coupled with a large absorption cross section, and inherently large Stokes shifts. However, for these QDs to be used in almost any application, their surface must be modified for better compatibility with the host matrix or biological media. The conventional surface modification methods found in literature often create surface defects significantly reducing QY and brightness, we developed a surface modification method that allows our QDs to be soluble in different media while retaining most of their original optical properties. The results of the surface modification approach together with optical characterizations of QDs in different media will be presented.

CuInS₂/ZnS quantum dots (QDs) have the potential to be disruptive in the emerging QD industry owing to their wide-range tunable bright emission from visible to near-infrared spectrum, lack of toxic heavy elements, air and moisture stability, and scalable low-cost manufacturing process. UbiQD, a New Mexico-based quantum dot manufacturer spun out from Los Alamos National Lab, has achieved greater than 85% quantum yield, or optical efficiency, for its quantum dots over most of its accessible spectrum from the visible to the near infra-red (550 nm to 1100 nm peak emission). The company also manufactures QDs with peak emissions further into the near infra-red (out to ~1300 nm), but typically with QYs ~ 60%. For some colors between orange (600 nm) and NIR (1000 nm), the company manufactured optimized quantum dots with near 100% quantum yield.

With this milestone, UbiQD's materials now have the highest reported photon conversion efficiency for quantum dots that do not contain cadmium, an element known for its toxicity and which is widely used by many quantum dot manufacturers. At the same time, the quantum yields reported by the company are also comparable to the best cadmium-containing nanomaterials that currently exist. The benefit of high quantum yield positively impacts all quantum dot applications including lighting, displays, security, biotechnology, and design. While there are many potential markets for UbiQD's patented technology, the company's primary focus is enabling windows to generate electricity using products known as luminescent solar concentrators.

This poster presentation will describe results from our QD optimization and scale-up efforts, together with an overview of the markets enabled by high performance QDs.

A major issue facing colloidal quantum dot (CQD) solar cells is the large open-circuit voltage (V_oc) deficit due to sub-bandgap trap states. Recent advances in architecture engineering using bulk nano-heterojunction (BNH) active layers achieved by mixing ZnO and PbS in layer-by-layer processing have dramatically reduced V_oc deficit. In addition, high concentration, single-step active-layer deposition has yielded record devices with enhanced ligand exchange and higher packing density that exhibit a sharper bandtail. We explore the role of ligands at the BNH using single-step deposition to further decrease V_oc deficit.

Quantum dot (QD) films are under an intense spotlight in the display or other optical construction area due to their high color purity from the narrow full width at half maximum (FWHM) and wide color gamut. On this QD film, recently, a light diffusion film (or haze film) is laminated on it in order to increase the light extraction efficiency and achieve uniform emission through the entire surface. Alternatively, a method of incorporating inorganic oxide particles such as silica, alumina or polymer particles as a scattering agent has been introduced. However, the method of further laminating the haze film has several problems such as necessity of complicated process and causing cost-effect. Incorporating particles as scattering agent has also disadvantage such as non-uniform light emission distribution in matrix due to aggregation.
Previously, we reported rollable glass-fabric reinforced siloxane hybrid composite (GFRHybrimer) film for substrate of flexible devices [1]. GFRHybrimer film was fabricated by impregnation and lamination of glass-fabric with photo-curable siloxane material (Hybrimer) as matrix. Also, we fabricated siloxane encapsulated QD composite, which is exceptionally stable against heat and moisture [2]. Herein, we fabricated QD incorporated GFRHybrimer (QD-GFRHybrimer) film, which is fabricated by impregnation of glass-fabric with siloxane encapsulated QD resin as matrix. Haze of QD-GFRHybrimer film can be easily controlled by several methods; (i) tuning the refractive index by changing the composition of precursors, (ii) changing the number of glass-fabric, (iii) incorporating inorganic particles or graphene oxide as scattering agent. Thanks to haze controllable QD-GFRHybrimer film, it can achieve uniform emission through the entire surface, enhancing light extraction efficiency without an additional haze film. Furthermore, our QD-GFRHybrimer film has high thermal stability in harsh environment such as 85 ^oC in air and 85 ^oC & 85 % relative humidity conditions. Therefore, our QD-GFRHybrimer film has great potential for color enhancement film with controllable haze and thermal stability. Additionally, we will more study our QD-GFRHybrimer for improving photo-stability.
[1] J. Jin, J. -H. Ko, S. Yang, and B. -S. Bae, Adv. Mater. 2010, 22, 4510-4515
[2] H. Y. Kim, D.-E. Yoon, J. Jang, D. Lee, G.-M. Choi, J. H. Chang, J. Y. Lee, Doh C. Lee, and B.-S. Bae, J. Am. Chem. Soc., 2016, 138, 16478-16485.

Solution-processed semiconductor colloidal quantum dots (CQDs) gradually attract attentions as low-cost, printing-compatible materials for flexible systems. They have been employed for optoelectronics for their tunable absorption spectrum and narrow emission linewidth brought by quantum size-effect. The amorphous nature of CQD thin-films also enables them to be mass-produced on lightweight, flexible plastic substrates using reel-to-reel printing, spray painting and ink-jet printing. Till now, CQDs have been widely used in solar energy harvesting, logic devices and displays. However, CQDs haven’t been widely used on strain and pressure sensing systems, since CQDs lack proper transduction mechanisms (i.e. piezoelectric, piezoresistive) to respond to strain or pressure.

Here we report a novel application of quantum dots – introducing triboelectric phenomenon into CQD devices to create a CQD triboelectric field-effect transistor (TFET) for high-performance flexible touch sensors. In our touch sensors, we use a polydimethylsiloxane (PDMS) film to generate triboelectric voltage in response to touching and a floating-gate CQD FET to produce source-drain current modulated by the triboelectric voltage. This design makes it possible to generate a much larger current change compared to the free-standing PDMS touch sensor. The fabrication process of CQD TFET doesn’t require expensive equipment (reactive ion etching etc.) which enables the potential of our devices to be mass produced by printing technologies at low cost in industry. We further improve the performance of our TFET touch sensors by the surface modification strategy. This strategy will largely enhance the mobility of CQD solid, thereby increase the current output of CQD TFET touch sensors.

: Now display technology is heading to give reality-like experience, for example, give a feeling as if we are watching a real object simply through window, not watching a display. Therefore, since around 2010, there have been continuous increases in the demand for large sized display products with higher resolution. Nowadays, UHD (Ultra High Definition), which is also called as 4K, is a new standard resolution of large sized display over 55 inch size. To give a more cinematic and immersive experience, TV size is increasing up to 110” and the curved design is also suggested to provide wider field view. These displays approach the limits of human perception in resolution and produce more than enough luminance in most cases. Differentiation strategy is now wide color gamut display bringing high color reproducibility.
For high color reproducibility, QD-display might be the most practical solution at this stage, because the quantum dot showed many advantages as light emitting materials over conventional technology - narrow spectra, high quantum efficiency and easy color tunability. Most works have been performed on Cd-based QDs, however the toxicity problem has restricted their commercial application even though their superior properties.
Therefore, we have focused the development of more environmentally friendly QD, InP based material that exhibited the high quantum efficiency over 90% compared to Cd-based QD and narrow spectrum below 40 nm FWHM. We fabricated Cd free green and red emitting quantum dots as film form without loss of their optical properties. The quantum dot display adopted Cd free quantum dot film on blue LED as back light shows the sharp and pure color spectrum covering more than 96% of DCI (Digital Cinema Initiatives) color space. The talk will cover the broad range of work from QD material synthesis, composite fabrication and characterization to application to display, especially focusing on the characteristics of the internally developed Cd-free product in Samsung, and the commercialized product from the first Cd-free QD adopted SUHD TV to upgraded QLED TV.

Although the efficiency of quantum dot (QD) light-emitting diodes (QLEDs) has been greatly improved recently as a result of remarkable advances in the QD synthesis and device engineering, the lifetime of QLEDs is still far behind the requirement of practical display applications. In order to improve the lifetime it is necessary to understand the intrinsic degradation mechanism of QLEDs and optimize the device structure. The performance of QLEDs is strongly affected by the balance of electrons and holes injected into the QD emissive layer. Unbalanced charge injection leads to poor efficiency and stability as well as efficiency roll-off at high current density due to Auger recombination. Here, we systematically studied the correlation between the device performance and charge transport properties of electron transport layer (ETL) and hole transport layer (HTL). We will demonstrate that the lifetime and efficiency of QLEDs can be enhanced through optimizing charge transport properties of ETL and HTL. Furthermore, we will show that the device lifetime increases significantly by reducing the leakage current at low bias voltage.

The ability to efficiently separate, recombine, and direct charge carriers is central to a wide range of applications, including electronics, photovoltaics, displays and solid-state lighting. Engineering band structure and heterointerfaces with atomic precision is an obvious route to achieving such capabilities. To do so through widely-accessible and cost-effective means is not. But such a means would allow rapid advances in these critical application areas. The evolution of semiconductor nanocrystals from single-composition, spherical particles to complex heterostructures of diverse shapes provides many opportunities for precision band structure engineering through scalable solution synthesis. With anisotropic shapes that can be exploited for assembly, charge carrier manipulation and optical anisotropy, incorporating heterojunctions and other functional interfaces into colloidal nanorod heterostructures represents an especially promising direction. In this talk, general challenges to the synthesis of complex-yet-well-defined colloidal nanorod heterostructures will first be discussed. Approaches such as spatially selective solution epitaxy, catalytic growth, cation exchange and combinations thereof can be exploited to achieve unique heterostructures with useful properties. A specific example of double-heterojunction nanorods (DHNRs) will then be highlighted. Their engineered band structure with shape anisotropy improves charge injection, enhances light outcoupling and increases device lifetime of their light-emitting diodes (LEDs). At the same time, these features of DHNRs facilitate photo-induced charge separation, leading to useful photovoltaic response in high-performance, solution-processed LEDs. Emerging anisotropic colloidal heterostructures such as DHNRs can not only radically improve existing function but also impart new capabilities that could open up new directions for future generation of devices without adding complexity in manufacturing.

The successful commercialization of QDs requires simultaneous development of stable QDs plus accompanying composite systems to ensure dispersion and air stability. This presentation covers advancements in developments of our custom polymer systems and fumed silica dispersions, as well as discussion of Cd-free formulations.

These advanced systems take advantage of Crystalplex’s unique Sapphire™ QDs with alumina coatings to provide air and heat stability in films as well as other form factors.
–Thermoplastic acrylic for injection molded components, extruded films, or other extruded shapes
–Solvent cast films for coated components
–Thermoset acrylics for ink-jet printing
–New silica composites enable dispersion in silicones

The authors also discuss the parallel development of Cd-free formulations which will take advantage of both our alloy gradient intellectual property plus all of the added benefits of the alumina Sapphire™ shell technology.

Semiconducting nanocrystals, also known as quantum dots (QDs), have great potential to apply wavelength converting materials for next-generation display applications due to their unique optical properties such as narrow emission spectra, color tenability depending on size, wide color garmut, and high photoluminescence quantum yield (PLQY). However, QDs have various limitations for practical applications because of weakness to oxygen and moisture, which can cause degradation of their luminescence. Mostly, QDs are mixed with polymer resins to coat on the film or dispense on the LED packaging. Thus, uniformly dispersed QD in matrix resin should be prepared and its cured product of coating or bulk films should have thermal and moisture stability to be practically applied. However, currently used QD/polymer resin is hard to be uniformly dispersed due to low compatibility between QDs and polymer resin. Also, its cured products are vulnerable to heat and moisture since polymer has low thermal stability and QDs are easily attacked by oxygen and water in the polymer matrix.
Recently, we reported new siloxane encapsulated QD (core: CdSe / shell: CdZnS / ligand: oleic acid, from Ecoflux) (SE-QD) resin through in-situ sol-gel condensation reaction of 3-methacryloxypropyltrimethoxysilane (MPTS) and diphenylsilanediol (DPSD) in the existence of QDs. [1] The photo-cured SE-QD resin containing methacrylate groups shows almost permanent dispersion stability of QD in siloxane matrix and its cured films show long-term stability in harsh environment (85 ^oC in air and 85 ^oC & 85 %RH). However, photo-curing of SE-QD resin is not desirable for practical application. Because it is difficult for completely curing of the resin due to UV light absorption of QDs and their photo-oxidation degradation. Also, thermal stability is limited due low degradation temperature of methacrylate. Herein, we fabricate new siloxane encapsulated QD (TSE-QD) resin which is thermally cured (especially hydrosilylation curing). The siloxane matrix for TSE-QD was already confirmed to show higher thermal stability over 180 ^oC and better optical transparency to be used as the LED encapsulant. [2] The fabricated TSE-QD resin is successfully cured by hydrosilylation reaction with no degradation of QD even during high temperature sol-gel reaction and thermal curing process. Especially, the cured films of TSE-QD resin represent better thermal stability (at 120 ^oC in ambient condition) compared to our previous photo-cured SE-QD. It is also stable at high temperature & high humidity condition (85 ^oC & 85 %RH) and better photo-stability. Therefore, the siloxane encapsulated QD resin has great potential for QD based display applications.
<Reference>
[1] H. Y. Kim, D.-E. Yoon, J. Jang, D. Lee, G.-M. Choi, J. H. Chang, J. Y. Lee, Doh C. Lee, and B.-S. Bae, J. Am. Chem. Soc., 2016, 138, 16478-16485.
[2] J.-Y. Bae, Y. H. Kim, H. Y. Kim, Y.-W. Lim, and B.-S. Bae, RSC Advances, 2013, 3, 8871-8877.

Core/shell type quantum dots (QDs) have attracted much attention for the application to thin film display and solid-state lighting, owing to their narrow band emission with high photoluminescence quantum yields (PLQY), color tunability, and solution processability.
We synthesized deep blue emitting CdZnS/ZnS QDs by conventional hot-injection method. Generally, QDs are purified by precipitation/dispersion method. In this study, we purified the QDs by gel permeation chromatography (GPC) with toluene eluent. PLQY of the QDs purified by GPC was higher (73%) than that of the QDs purified by typical precipitation/dispersion method (64%), due to the thorough removal of free ligands and high boiling point solvent such as 1-octadecene. The GPC purification also enabled to selectively separate the QDs with different ligand density, which was confirmed by NMR and thermogravimetric analysis. Difference of ligand density between the QDs with fewer ligands and the QDs with more ligands was 1.53 times. The both QDs showed almost same photoluminescent properties with 71-73% of high PLQY, 444 nm of deep blue emission maximum, and 22 nm of narrow FWHM. Usually, deligand or ligand exchange process largely deteriorate photoluminescent properties because of the formation of quenching defects at surface of QDs. In this case, we controlled surface ligand density of the blue emitting QDs without changing the photoluminescent properties.
We applied these QDs to solution-processed LEDs with the following device structures: ITO/ PEDOT: PSS (40 nm)/ cross-linked PVK (20 nm)/ QDs (25 nm)/ ZnO (60 nm)/ Al. The cross-linked PVK was used as a hole transport layer with instant low-temperature method, and the QD layer was spin-coated from toluene solution onto PVK. The device showed saturated deep blue electroluminescence with high luminance.

Luminescent solar concentrators (LSCs) use down-converting luminophores embedded in a waveguide to absorb sunlight and deliver high irradiance, narrowband output light for driving photovoltaic (PV) and other solar energy conversion devices. Achieving a technologically useful level of optical gain requires bright, broadly absorbing, large-Stokes-shift luminophores incorporated into low-loss waveguides, a combination that has long posed a challenge to the development of practical LSCs. We will present a combined theoretical and experimental study of LSCs based on giant effective Stokes shift CuInS2 nanocrystal (NC) phosphors, which demonstrate best-in-class performance as large-area, semitransparent concentrators suitable for use in energy-harvesting window layers and other applications. A new analytical optical model will be discussed that allows accurate determination of each major loss mechanism without the need for time-consuming Monte Carlo ray-tracing simulations. We will show that nanocrystal clustering in polymer composite waveguides leads to light scattering losses that ultimately limit efficiency at large geometric gain. By optimizing NC concentration, we demonstrate optical power efficiencies up to 5.7% under AM1.5 illumination for devices having a geometric gain 6.7×, with limiting achievable efficiencies predicted to exceed 10%.

The development of highly photoluminescent, copper-indium-sulfide (CIS) quantum dots (QD) has broadened the scope of QD applications owing to their lowered toxicity and reduced manufacturing costs. Recent efforts by UbiQD, the leading manufacturer of low-cost CIS QDs, have been focused toward the development of robust and efficient luminescent solar concentrator (LSC) technology. The key material for LSC technology is a QD-polymer nanocomposite (PNC) with no scattering and/or surface defects. In developing PNCs at UbiQD, acrylate polymers made via cast-in-place methods have the best performance. The primary challenge for optimizing acrylate PNCs for use as LSCs lies in the dispersion of QDs in monomer and the subsequent polymerization of the material while maintaining dispersion. Ample consideration must be given to a number of nanocomposite production variables including (i) choice of monomer, (ii) method of polymerization, and (iii) nanoparticle surface chemistry. Depending on the monomer choice, the glass transition temperature and shrinkage behavior upon polymerization can vary from material to material, thus dictating the surface properties of the resultant PNC. With regard to the method of polymerization, there exist a number of trade-offs between polymerization by exposure to ultraviolet light versus heat. Most notable are the differences in speed and cost between the two methods. UV light polymerization can significantly reduce PNC manufacturing time making it an ideal method for large scale polymer curing; however, absorption of UV light by the quantum dots can slow polymerization and damage the QDs. While thermal polymerization is superior in preserving the optical properties of the QDs and may be cheaper to implement than UV exposure systems, it can prove a costly method of PNC manufacturing at larger scales owing to substantially longer material processing times. Finally, the surface chemistry of the QDs controls, to a great extent, the solubility of the nanoparticles in monomer and polymer. We demonstrate that additives and native ligand modification are potential strategies for improving dispersion.
UbiQD’s QDs have been shown to exhibit PLQY up to 95%, enabling highly efficient energy conversion. When incorporated into polymers, we maintain up to 8% external quantum efficiency at 60% transmittance, the highest external quantum efficiency reported for LSC technology with polymer sheets to our knowledge.

Luminescent solar concentrators (LSCs) improve the performance of solar cells by concentrating sunlight onto small photovoltaic devices. A LSC consists of a polymer slab embedded with luminophores such as dyes or luminescent nanocrystals. Incident sunlight is absorbed by the luminophore and emitted at a longer wavelength, which propagates to solar cells on the edges of the concentrator via total internal reflection. This allows the solar cell to be optimized for a spectrally narrow light source. Moreover, the luminophores in the LSC can be designed to form colorful architectural components, making them well-suited for the building integrated photovoltaic industry.

To reach the thermodynamic performance limit for a LSC, a photonic mirror that both transmits high energy light and traps luminescent light is necessary (Rau 2005). However, due to the overlap between the absorption and emission spectrum of most luminophores, it is challenging to design ideal mirrors. Here we design photonic mirrors for LSCs that can be placed on the front or back, and show how the design choices change for different luminophores or concentrator geometries.

The photonic mirrors on the front of the LSC consist of one-dimensional, aperiodic layers of HfO₂ and SiO₂. CdSe/CdS nanocrystals are used as the luminophore. To examine the tradeoff between incident light transmission and trapping of luminescent light, an optimization algorithm was used to design the mirrors. Using Monte Carlo simulations we predict that mirrors designed to trap luminescent light are preferred for quantum yield (QY) greater than 85%, collection areas greater than 10 cm, loading fraction less than an optical density of 1.4 at 450 nm, and low absorption and emission spectral overlap. For QY less than 85%, mirrors that are optimized for transmission and trapping are comparable. For the remaining cases the mirror weighted toward incident light transmission is preferred. These mirrors all significantly outperform open top concentrators. We analyze the mirrors by tracking the loss processes inside the concentrator, revealing how changes in the mirror design influence the escape cone losses and reabsorption fractions during propagation.

We also design metasurface-based mirrors for the back of the LSC to be combined with the top mirror. These metasurface mirrors are designed to change the angle of a propagating photon upon reflection, guiding it more effectively toward the edge of the concentrator and away from steep angles that require multiple passes to reach the edge. We show that LSCs incorporating metamirrors can achieve high efficiency with lower QY nanocrystals.

This work provides a pathway to high performance LSCs through enhanced optical transport and improved trapping efficiency. By controlling the emission angle or trapping emitted photons we work to design LSCs that efficiently guide light to the edges, an important property needed for future commercialization.

The newly rediscovered perovskite semiconductor system has the potential to be extremely transformative for all optoelectronic devices, especially photovoltaics (PVs). Perovskite semiconductors of the form APbI₃ where A is a large +1 charged cation, typically Cs, methylammonium, or formamidinium have had a huge resurgence among materials scientists for outstanding PV properties despite being overlooked for decades. Semiconductors containing the latter two A-site cations listed are hybrid organic-inorganic materials, and as such, are far less understood compared to conventional all inorganic or even organic material systems. Regardless of this spotty formal understanding, lead-halide perovskites have very rapidly been optimized to power conversion efficiency levels on par with all other materials even with extensive history of research. Perovskites show a unique tolerance to crystalline defects that cause trouble in most other semiconductors. Therefore the potential offered is that very high efficiency PVs can be fabricated in extremely fast and inexpensive ways, thus offering a revolution for the solar industry and a direct route toward producing the world’s energy with a simple and clean technology. Long-term durability of the devices is the critical remaining challenge to be solved. Two examples of major instabilities in device performance are the volatility of the organic cation and the specific crystal habit in which the material embodies.
Nanoscale versions (often termed quantum dots (QDs)) of the all-inorganic metal halide perovskite (CsPbI₃) tend to retain the desired cubic phase due to strain effects at the surface of the QDs whereas conventional films of the same material “relax” to an orthorhombic structure at room temperature. Therefore these QDs potentially solve both of the instability issues. The cubic CsPbI₃ QD cells operate with a rather remarkable open-circuit voltage of >1.2 volts and have produced power conversion efficiencies over 13%. This customizable new nanomaterial system has incredible potential for many applications in optoelectronics, including photovoltaics, LEDs, displays and lasers. We describe the formation of α-CsPbI₃ QD films with long range electronic transport that retain the high temperature phase in ambient conditions making up the active layer in optoelectronic devices. Perspectives on how this technology can become transformative will be discussed.

Mid-infrared (MIR) photodetection has been dominated by state-of-the-art molecular beam-processed materials such as HgCdTe and InAs. However, the cost of materials and processing restricts the utility of these detector technologies to research and defense applications. In the MIR, HgTe colloidal quantum dots (CQD) are a cheap, solution-processed, and MIR tunable semiconductor nanomaterial, which has demonstrated the potential for next generation infrared photodetection.
We will show that HgTe CQD MIR photodetectors have improved more than 10-fold with respect to previous reports. We will discuss the influence of interfacial layers for device performance with characterization by current-voltage-temperature measurements, FTIR, photoluminescence, EDX, and XPS. When operating under zero bias, we achieve a specific detectivity (D^*) of 1x10¹¹ Jones at 90K with Background Limited Infrared Photodetection (BLIP) and cutoff wavelength between 3-5 μm. HgTe CQD MIR photodetectors achieve D^* above 10⁹ Jones at 230K and 4.2μm cutoff, approaching the performance of epitaxial HgTe.

Color-tuned and semi-transparent solar cells, photovoltaic devices with controlled and tunable reflection and transmission spectra, are of significant interest due to their potential applications in building-integrated photovoltaics, vehicular heat and power management, and multijunction photovoltaics. Solution-processed solar cells are suitable for these potential applications due to their ease and flexibility of fabrication, thin film and lightweight nature, associated low costs, and high efficiency potential. Strategic and selective reflection or transmission in the visible spectral region results in tunable apparent device color, including visual transparency. Typically, color-tuning and semitransparency are achieved at the expense of absorption, resulting in lowered photocurrent in solar cells based on solution-processed materials such as perovskites and organic polymers that absorb primarily in the visible. Two advantages of colloidal quantum dots (CQDs) for color-tuned photovoltaics are their tunable band gap, a result of quantum confinement effects, and their large spectral absorption range that can extend into and beyond the near infrared (NIR) portion of the spectrum. If suitable spectral engineering techniques could be employed in CQD solar cells, the optical losses experienced in the visible portion of the spectrum could be compensated for by strong absorption in the NIR.

Using thin film interference methods and optimization algorithms, we developed a technique for generating arbitrary spectral profiles in multilayered solar cell structures. We use the Transfer Matrix Method (TMM), which takes layer thicknesses and refractive indices as inputs and outputs normalized electric field profiles within multilayer stacks, to calculate the optical properties of our devices. Device “transparency” is computed by averaging transmittance results over the visible wavelength range (420 nm – 680 nm) output by the TMM calculations. To achieve an objective apparent color or transparency level, we use population-based multi-objective optimization algorithms to maximize reflection or transmission over a relevant wavelength range while maximizing photocarrier generation. We fabricate color-tuned devices with photocurrents of 10-15 mA/cm² and semitransparent devices with ~ 30% visible transparency. Our results indicate that designs with minimum transparency do not necessarily correspond to the highest attainable device photocurrent, thus providing a route for achieving high efficiency in color-tuned devices. Experimentally, we fabricated proof-of-principle blue, green, yellow, red and semi-transparent devices with measured reflectance and transmittance spectra that matched well with the predicted color and transparency levels. Our optimization technique, based on multilayer interference, provides a viable foundation for the custom-design of spectrally selective optoelectronic devices.

Quantum dots (QDs) were first proposed for biological imaging by the groups of Shuming Nie and Paul Alivisatos in 1998, yet nearly 20 years later, there are no commercial QD products in the clinical market. This talk will address the challenges that have prevented rapid translation of QDs to the clinic.

Current commercial offerings, while widely used in research applications, display significant variability in quality. We will show that QD quality can vary by vendor, product line, and even from lot-to-lot. All of these challenges not only prevent commercialization in a highly regulated clinical market, but also call into question published research results employing these products.

QDs with the best optical properties, including high quantum yield and narrow emission bands, are typically produced via organic synthesis. Yet, biology requires an aqueous presentation. Unfortunately, most methods of aqueous phase transfer result in a reduction in photoluminescence quantum yield. Even when transfer is achieved, QD stability can be poor with further reductions in quantum yield resulting from dilution, transfer to biological buffers and medium, and bioconjugation protocols.

In addition to these challenges, many bench scale processes are suitable for only small volumes of material processing. In our case, the initial batch method employed produced only 1/100^th the amount of material required for a single mouse study. Thus, scalable methods of manufacturing are required.

This talk will explore current issues in commercialization of QDs for the biological and particularly clinical market.

Antimicrobial resistance (AMR) occurs when microorganisms change in ways that render them unresponsive to previously effective medications. AMR is especially amplified in the hospital where excessive and often unnecessary use of antibiotics increases selective pressure in bacterial populations, allowing bacteria that can acquire resistance to thrive. This has led to a rise of hospital-acquired infections (HAIs) that are progressively more challenging to treat and increase morbidity and mortality as well as costing billions in healthcare costs for patients and healthcare systems worldwide.

Bacterial contamination on hospital touch surfaces such as door handles, computer keyboards and telephones is extremely common, facilitating the spread of HAIs. One strategy to reduce HAIs is the use of antimicrobial surfaces containing quantum dots (QDs) – highly fluorescent inorganic semiconducting nanoparticles usually ranging from 2 to 10 nm in diameter – which exhibit antibacterial activity through light-activated generation of cytotoxic reactive oxygen species (ROS).

In this interdisciplinary project, new commercial non-toxic cadmium-free QD nanoparticles are incorporated into polymer along with a clinically approved photosensitising dye, crystal violet (CV) using a simple, non-covalent and up-scalable ‘swell-encapsulation-shrink technique’. The combination of QDs and CV dye enhances antibacterial activity by boosting ROS production via Forster Resonance Energy Transfer (FRET). Moreover, unlike dyes used on their own, QDs absorb energy broadly in the visible range, enabling activation of QD-CV materials by ambient lighting, much safer than UV radiation.

The antibacterial activity of QD-CV samples was tested against Staphylococcus aureus and Escherichia coli as representative Gram-positive and Gram-negative bacteria under dark and light conditions. QD-CV samples displayed potent antibacterial activity, resulting in complete kill of a laboratory strain of Staphylococcus aureus after 1 hour irradiation at 6000 lux light intensity and 99.99% (4 log) reduction of a laboratory strain of Escherichia coli. QD-CV samples were also effective against clinical strains of bacteria, inducing a 99.97% (3.5 log) reduction in MRSA and 99.85% reduction (2.8 log) in E.coli 1030, a carbapenemase-expressing multi-drug resistant strain of Escherichia coli.

This work demonstrates that the effectiveness of QD-photosensitiser-incorporated polymer surfaces and offer a viable alternative to reduce contamination of frequently touched surfaces in hospital wards thus potentially decreasing the risk of hospital-acquired infections.

Nanocrystals with controlled size, shape, and chemical composition can be readily synthesized and have demonstrated various new phenomena due to quantum confinement. Hierarchical assemblies of nanocrystals with different composition and sizes lead to new functionalities. With the increase in chemical precision, a need has developed for better techniques to measure photoluminescence quantum yield. Right now, the best photoluminescence measurement techniques utilize a photons-in-to-photons-out method, where the spectral sensitivity of the detector must be calibrated with a standard reference lamp, with an uncertainty budget between 2% to 10%. This makes it impossible to differentiate between a 95% quantum yield and a 100% quantum yield. Therefore, we have developed a technique called Photothermal Threshold Quantum Yield (PTQY) operates using the mirage effect, which measures the non-radiative heat produced per given photon absorbed of monochromatic light. Instead of measuring photons in and photons out, we measure heat out and photons out. The amount of heat emitted per luminesced photon can be used to determine the quantum yield without the use of any standard reference or calibrant. We will show samples of nanocrystals prepared with luminescence quantum yields well exceeding 95% measured by conventional integrating sphere techniques (2 to 10% uncertainty), compared to our PTQY measurements allowing for uncertainties less than 1%. We will also explore the meteorology and statistical analysis that allows us to approach theoretical limits of core shell nanocrystals, as well as what we can learn about crystalline defects that allow us to differentiate perfect emitters from one another.

We have shown that the descent of the nanocrystal symmetry from spherical to point group C_s, which is characterized by just one mirror plane symmetry element, leads step by step to activation of all five F =2, F_z=±2, ±1, 0 excitons. Even the ground exciton becomes optically active, which should be observable in low-temperature photoluminescence measurements. For several intermediate symmetries the band edge exciton fine structure consists of sets of three linearly polarized mutually orthogonal dipoles plus a dark exciton, one of which is always the ground state. We quantify the effect of symmetry descent on the exciton fine structure by introducing a charged Coulomb impurity in the nanocrystals. The calculations show that the nanocrystal symmetry breaking by a Coulomb impurity, particularly a positively charged center, shortens the radiative decay of nanocrystals even at room temperatures in qualitative agreement with the increase in PL efficiency observed in nanocrystals doped with positive Ag charge centers.¹
[1]. P. C. Sercel, A. Shabaev, and Al. L. Efros, Nano Lett. 17, 4820−4830 (2017)

Colloidal CdSe nanoplatelets (NPLs), which have atomically flat geometry with well-defined thickness of several monolayers, exhibit unique optical properties, such as narrow emission band width, giant oscillator strength transition and linearly polarized emission. Considering the isotropic zincblende crystal structure of CdSe NPLs, the linearly polarized emission is particularly intriguing. In general, geometric anisotropy is responsible for the degree of polarization from anisotropic nanocrystals. Since NPLs have three-dimensionally anisotropic morphology, the lateral aspect ratio between two lateral edges should also play an important role.
In this study, we report the shape-dependence of fluorescence polarization from colloidal CdSe NPLs. We controlled morphology of CdSe NPLs with lateral aspect ratios ranging from 1.1 to 4.5 by varying the ratio of precursor, acetate hydrate salt, which triggers lateral growth of NPLs. From measurement of fluorescence polarization of CdSe NPLs with controlled morphology, we revealed that elongated NPLs with higher lateral aspect ratio exhibit higher optical anisotropy at the same thickness. In order to identify the origin of the shape dependence of optical anisotropy, we measured emission and absorption polarization separately with NPLs aligned by electric field. As a result, only absorption polarization depends on the lateral aspect ratio, while the emission polarization stays nearly unchanged regardless of shape anisotropy of NPLs. Now that these results allude to a likelihood that absorption polarization is responsible for the shape-dependence of fluorescence polarization, we design a model to assess the correlation between the geometry of NPLs and the optical transition polarization by way of the local field effect. Theoretically estimated absorption polarization also shows shape dependence similar to experimental data, which suggests that the anisotropic local field effect is a primary denominator of shape-dependent fluorescence polarization in CdSe NPLs.

The design of surface ligand shells of nanocrystals (NCs) is of great importance for their applications. Recently, we developed a mild flocculation procedure allowing us to exchange bulky organic NC ligands with short inorganic species, such as halide ions or metal-halide-complexes [1,2]. This kind of ligand exchange is a key step in the NCs design necessary for their applications in solution processable (e.g. printable) electronics. While the all-inorganic ligands provide very short interparticle distances, which is desired for an efficient coupling and charge transfer, their NCs solids outperform organic semiconductors in many critical parameters, such as carrier mobility and chemical stability. Moreover, all-inorganic shells can act as dopants thus ensuring the fine-tuning of the carrier concentration [2].
Nowadays, a variety of successful examples of all-inorganic capping is known. Nevertheless, the formation of 2D or 3D ordered nanostructures from the all-inorganic-capped NCs appears to be extremely challenging. The introducing of a NC hybrid capping by utilizing short-chain amines (e.g. n-butylamine) allows not only an efficient ordering of the NCs, but also extends their processability to common solvents such as chloroform [3]. Finally, the applicability of all-inorganic- and hybrid-capped NCs and their solids for solution processable electronics (e.g. field-effect transistors) is demonstrated.
In addition to applications in electronics, our method can be extended to achieve phase transfer of organically soluble NCs into aqueous media, including biologically relevant buffers. This innovative approach involves the intermediate hybrid ligand capping with e.g. chloride ions and butylamine and the replacement of the short-chain organic compound with functional polyethylene glycols (PEGs). This procedure is quantitative, widely applicable and it significantly preserves original photoluminescence quantum yields of the NCs.
References:
[1] Sayevich, V.; Gaponik, N.; Plötner, M.; Kruszynska, M.; Gemming, T.; Dzhagan, V. M.; Akhavan, S.; Zahn, D. R. T.; Demir, H. V.; Eychmüller, A., Chem. Mater. 2015, 27, 4328.
[2] Sayevich, V.; Guhrenz, C.; Sin, M.; Dzhagan, V. M.; Weiz, A.; Kasemann, D.; Brunner, E.; Ruck, M.; Zahn, D. R. T.; Leo, K.; Gaponik, N.; Eychmüller, A., Adv. Funct. Mater. 2016, 26, 2163.
[3] Sayevich, V.; Guhrenz, C.; Dzhagan, V. M.; Sin, M.; Werheid, M.; Cai, B.; Borchardt, L.; Widmer, J.; Zahn, D. R. T.; Brunner, E.; Lesnyak, V.; Gaponik, N.; Eychmüller, A., ACS Nano 2017, 11, 1559.

Near-infrared chromophores and fluorophores are useful for many types of imaging applications and could be useful for fiberoptic communications, but it is challenging to find such species with high emission quantum yields and controllable exciton dynamics. Several types of colloidal quantum dots have >50% quantum yields on the near infrared, but their long radiative lifetimes make them poor energy donors. In this talk, mechanisms for extraction of near-infrared excitons from quantum dots within assemblies with molecules are explored, and some applications, such as photocatalysis, are highlighted.

The rapidly growing near-infrared (NIR) spectrum-based industries like luminescent solar concentrating (LSC) windows, deep tissue bio-imaging, security inks, NIR sensors, and medical diagnostics demand high-performance, stable, tunable, and non-toxic NIR materials. At present, the NIR fluorophores available in the market such as organic dyes, fluorescent diamonds, fluorescent proteins severely lack wavelength tunability, have low quantum yield (QY), poor photostability, small extinction coefficient, and/or insignificant Stokes shifts. On the other hand, NIR emitting CdTe, CdHgTe or PbS/Se quantum dots have low QY and they are composed of toxic and RoHS prohibited elements such as Cd, Hg or Pb which hamper their use in practical applications. At UbiQD, we address these problems with low-toxic CuInSe_xS_2-x/ZnS QDs at a much lower cost than alternatives. Importantly, CuInSeS/ZnS QDs have a tunable emission covering from visible (550 nm to 650 nm) to NIR (650 nm to 1300 nm) spectral regions with over 90% QYs for most of the region coupled with good stability, large absorption cross-sections, and large Stokes shifts. As an example this technology’s potential, we recently demonstrated a record solar window conversion efficiency of 2.9% at 60 % transmittance using CuInSeS/ZnS NIR QDs. This presentation will discuss our development of NIR-emitting QDs together with challenges and progress of various emerging applications of NIR QDs.

Band offsets of semiconductors is critical for their functionality in numerous optoelectronic applications such as photovoltaics, photoelectrochemistry and light emitting diodes. Here, it is the first time that around 0.2 nm R-CF₃ and 2 nm CF₃(CF₂)₁₄-R (R is the binding group) perfluorocarbon ligands have been applied as capping ligand for lead sulfide (PbS) quantum dots(QD). We show energy of band edges of lead sulfide (PbS) colloidal semiconductor nanocrystals, specifically quantum dots (QDs), can be tuned in a big range through surface chemistry engineering via a simple and robust ligand exchange in organic solution. Not only dipole moment, but also electronegativity of ligand affect the absolute band energy shifts of PbS QD. It shows that dielectric constant of ligand shell can tune the bandgap of semiconductor QD.

Furthermore, strongly electronic coupled 3-D, ordered QD arrays, superlattices will be fabricated and characterized with field-effect transistor measurements. As fluorinated materials have low surface energy and low steric hindrance, it is hypothesized that the superlattice of PbS QD has low permeability to moisture/ air as the organic ligand shell forming a kinetic barrier on QD surface. Moreover, different length of fluorinated ligands will be applied as QD surface ligand to tune electronic coupling of QD and investigate the effect of ligand length and dipole moment in the carrier mobility. An increased QD packing density and conductivity is expected due to wavefunction overlap of neighboring QDs, resulting from the shorter length of those fluorinated ligands, ~ 0.1 nm, compared to the original ligands on QD surface, ~2 nm. Finally, a significant enhancement of charge transfer efficiency may be obtained with air-stable and band energy tunable PbS QD thin films.