Symposium EQ12—Optical Probes of Nanostructured, Organic and Hybrid Materials

Many common electron transport layer (ETL) materials employed in organic light-emitting devices (OLEDs) show a preferred orientation of molecular permanent dipole moments. This phenomenon is known as spontaneous orientation polarization (SOP), and leads to the formation of bound polarization charge, with polarons (typically holes) accumulating at the ETL/emissive layer interface to balance this charge. Previous study of phosphorescent OLEDs constructed using a polarized electron transport layer of tris-(1-phenyl-1H-benzimidazole) (TPBi) found that SOP-induced hole accumulation led to substantial exciton-polaron quenching prior to device turn-on. This quenching was found to reduce peak device external quantum efficiency by ~ 20%. This is in contrast with the prevailing understanding of exciton kinetics in OLEDs, where it is generally assumed that exciton-polaron quenching is only significant under high injection. This work examines the generality of this result by systematically probing polaron accumulation and quenching in phosphorescent OLEDs constructed using archetypical ETLs with varying degrees of SOP. Exciton quenching is quantified by the lock-in measurement of emitter photoluminescence during device operation. The use of a lock-in method allows emitter photoluminescence to be probed in situ even after turn-on by rejecting device electroluminescence. In comparison to device-efficiency based analyses of quenching, this technique decouples luminescence loss due to exciton-polaron quenching from inefficiencies in exciton formation. We find SOP-induced luminescence quenching to be a general phenomenon, here observed for devices based on several polar ETL materials. The onset-voltages for exciton-polaron photoluminescence quenching are in good agreement with conventional displacement current measurements, suggesting that this optical technique can be generally used to assess the magnitude of SOP. It is found that exciton quenching is a major limiting factor for determining peak internal quantum efficiency. These findings contrast the common practice of attributing lower-than-expected peak efficiency to charge balance losses and highlight the impact of SOP on OLED device efficiency, and the need to consider SOP-induced quenching in device design and analysis.

Singlet fission (SF) is the photophysical process where an excited singlet state is converted into two triplets on the ultrafast timescales [1]. SF gained increasing attention in recent years due to the capability to overcome the Shockley-Queisser limit of single junction solar cells [2]. Different approaches to control the key parameters of SF (molecular arrangement, electronic coupling or energy balance) are often based on covalently bound dimers of SF chromophores. Another promising way to control the energy balance of SF and the optical bandgap is to combine two different chromophores. Here we focus on two prototypical SF chromophores pentacene (PEN, exothermic SF) and tetracene (TET, endothermic SF), and we exploit the advantage of intermolecular heterofission where two triplets are formed on different chromophores. In this work we investigate thin film blends with 5%, 25%, 50% and 75% of PEN with ultrafast transient absorption (TA) spectroscopy to study the impact of neighbouring TET molecules on the photophysics of crystalline PEN. We consider the TA of the neat compounds excited resonantly as references for understanding the interaction in the blends.
The results for PEN:TET blends performed with PEN excitation (to rule out energy transfer from TET to PEN) show a change in the relative position and intensity of the Davydov-components of the PEN photobleaching (PB) until the energetically higher one dominates at 5% PEN blend. The presence of excited state absorption (ESA) of triplets in all blends indicates that they are still being formed via SF despite the presence of TET. The 5% PEN blend shows a build-up of a negative signal around TET PB, as well as a disappearing SE at long delays. Global Analysis (GA) performed on the TA data, assuming SF to be the dominating channel, showed two different processes for the 5% PEN blend. The first one, occurring with 165 fs time constant was attributed to homofission of PEN, while a second 26 ps process was attributed to heterofission of PEN isolated among TET neighbours. The low heterofission rate can be explained by the endothermicity of the process. For PEN concentrations exceeding 5% there is no evidence of this process, since homofission outcompetes heterofission thanks to its exothermicity. Time constants from GA of the other blends confirm the robustness of PEN homofission [3] against the incorporation of weakly interacting compounds due the fact that SF in PEN proceeds via a coherent pathway where the triplet pair state directly mixes into the photoexcited bright state.
The study is completed with TA measurements exciting the TET, where the interpretation of TA and GA becomes complex as TET homofission and Förster resonance energy transfer (FRET) from TET to PEN additionally can occur. For high PEN concentration, FRET followed by PEN homofission outcompete TET homofission and heterofission [4]. The TA data for the 5% PEN blend present complex multi-exponential dynamics that reflect the relaxation of excited PEN and TET molecules via a multitude of channels. The persistence of the TET GSB at long delays is again evidence for TET triplets, which can be populated either via heterofission or TET homofission.
In conclusion we observe heterofission of a singlet from PEN to two triplets on one PEN and one TET, after direct excitation of PEN chromophores, showing that heterofission is observable in weakly interacting systems bridging the gap between studies of doped single crystals [5] and strongly coupled systems such as heterodimers [6]. Our approach allows continuous tuning of the optical bandgap along with the singlet and triplet energies by varying the mixing ratio without significant impact on PEN homofission time constants.

[1] J. Phys. Chem. C 123, 3923–3934 (2019)
[2] J. Appl. Phys. 32, 510–519 (1961)
[3] Nat. Commun. 9, 954 (2018)
[4] Angew. Chem. Int. Ed. 59, 19966-19973 (2020)
[5] Phys. Status Solidi B 83, 249–256 (1977)
[6] J. Am. Chem. Soc. 141, 17558–17570 (2019)

Understanding the opto-electronic properties of conducting polymers under external strain are essential for their applications in flexible opto-electronic and photovoltaic devices. While prior studies have highlighted the impact of static strain applied on a macroscopic length scale, accessing structural relaxation under the local transient deformation was challenging due to the lack of spatiotemporal resolution using conventional methods. Here, we employ the newly developed scanning ultrafast electron microscopy (SUEM) to image the dynamical effect of the photo-excited transient strain in poly(3-hexylthiophene) (P3HT) facilitated by high space-time resolutions. We observe that the photo-induced SUEM contrast, corresponding to the local change of secondary electron emission, exhibits a ring-shaped spatial profile with a rise time of around 500 ps, beyond which the profile persists near the absence of spatial diffusion. We attribute the observation primarily to the electronic structure modulation induced by a local strain field owing to high elastic modulus, as supported by a finite-element numerical analysis. Our work provides insights into tailoring opto-electronic properties using transient mechanical deformation in conducting polymers, and demonstrates the versatility of SUEM to study photo-physical processes in various materials.
This work is based on research supported by the Army Research Office under the award number W911NF-19-1-0060.

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.

Tin halide perovskites (THPs) have emerged as a promising alternative to Lead Halide Perovskites (LHPs). The interest has grown not only because of the reduced environmental toxicity, but also due to the narrow bandgap (between 1.3 and 1.4 eV). We have recently reported that the optoelectronic quality of Sn-based perovskites is inherently comparable to LHP.¹ Despite this, device efficiency and stability are still limited. ² Sn-based perovskites show a very interesting interplay of shallow and deep defects that can be either stable in the bulk or at the surface of the material. The dominant effect of one or the other can be tuned by differentiating the fabrication process. Even though there is general agreement on the beneficial effect of additives (mostly SnF₂ ^3,4) the mechanisms at the basis of the observed improvement in optoelectronic properties are yet not understood.

We have unraveled the optoelectronic properties of FA_0.85Cs_0.15SnI₃ thin films providing a comprehensive picture of the competing radiative and non-radiative recombination processes with a combination of steady-state, time-resolved, and pump-probe spectroscopic techniques. We also discuss the origin of these competing processes in relation to the presence of additives. The interpretation of experimental results is supported by simulations describing free and trapped-carrier dynamics in a simplified system.

We show that the content of Sn⁴⁺ in the precursors' solution has a critical effect on optoelectronic properties drastically reducing the radiative efficiency. We suggest that surface Sn(IV), incorporated in the film from solution, might act as an electron trap enhancing non-radiative recombination that results dominant over doping-induced monomolecular radiative recombination.⁵ A moderated improvement of optoelectronic properties upon SnF₂ addition could still not match the performances of the pristine sample. This suggests that the introduction of the additive on one side might reduce the doping but conversely introduces additional non-radiative decay channels that fundamentally limit the radiative efficiency. The optimal percentage is not generally fixed but depends on the initial oxidation state of the precursors. We eventually show that an excess of the additive creates a Sn-rich growth condition that might result in the formation of deep defects⁶, with an overall detrimental effect on optoelectronic performances.

(1) Poli, I.; Kim, G.; Wong, E. L.; Treglia, A.; Folpini, G.; Petrozza, A. High External Photoluminescence Quantum Yield in Tin Halide Perovskite Thin Films. ACS Energy Letters 2021, 6, 609–611. https://doi.org/10.1021/acsenergylett.0c02612.
(2) Milot, R. L.; Klug, M. T.; Davies, C. L.; Wang, Z.; Kraus, H.; Snaith, H. J.; Johnston, M. B.; Herz, L. M. The Effects of Doping Density and Temperature on the Optoelectronic Properties of Formamidinium Tin Triiodide Thin Films. Advanced Materials 2018, 30 (44). https://doi.org/10.1002/adma.201804506.
(3) Gupta, S.; Cahen, D.; Hodes, G. How SnF2 Impacts the Material Properties of Lead-Free Tin Perovskites. Journal of Physical Chemistry C 2018, 122 (25), 13926–13936. https://doi.org/10.1021/acs.jpcc.8b01045.
(4) Savill, K. J.; Ulatowski, A. M.; Farrar, M. D.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. Impact of Tin Fluoride Additive on the Properties of Mixed Tin-Lead Iodide Perovskite Semiconductors. Advanced Functional Materials 2020, 2005594, 1–13. https://doi.org/10.1002/adfm.202005594.
(5) Ricciarelli, D.; Meggiolaro, D.; Ambrosio, F.; De Angelis, F. Instability of Tin Iodide Perovskites: Bulk p-Doping versus Surface Tin Oxidation. ACS Energy Letters 2020, 5 (9), 2787–2795. https://doi.org/10.1021/acsenergylett.0c01174.
(6) Meggiolaro, D.; Ricciarelli, D.; Alasmari, A. A.; Alasmary, F. A. S.; Angelis, F. De. Tin vs . Lead Redox Chemistry Modulates Charge Trapping and Self Doping in Tin/Lead-Iodide Perovskites. J. Phys. Chem Lett. 2020, 11, 3546–3556. https://doi.org/10.1021/acs.jpclett.0c00725.

Two-dimensional, organic-inorganic hybrid perovskites (2DHPs) are stoichiometric compounds composed of alternating sheets of corner-sharing, metal-halide octahedra and organoammonium cationic layers. We study 2DHPs containing single lead iodide layers separated by intervening substituted, phenethylammonium (PEA) cations with the chemical structure (x-PEA)₂PbI₄, where x=F, Cl, Br, or CH₃. These 2DHPs form type-I heterojunctions in which excitons and carriers are strongly confined to the lead halide layers with exciton binding energies > 150 meV. We use x-ray diffraction and variable-temperature steady-state and time-resolved absorption and photoluminescence (PL) measurements to uncover the correlation between their structure and photophysical properties. (PEA)₂PbI₄ excitonic absorption and PL spectra at 15 K show splittings into regularly spaced resonances every 40-46 meV [1]. Anti-Stokes hot exciton PL is observed at the same energy as the optical absorption resonances. Replacing a single atom in the para position of the PEA-cation phenyl group increases its length and therefore the interlayer spacing, but leaves the cross-sectional area unchanged and results in structurally similar metal halide frameworks [2]. As the cation length increases, the absorption spectra broaden and blueshift, but the PL spectra remain invariant. Substitution in the ortho position with progressively larger cations increasingly distorts and strains the inorganic framework [3]. Ortho substitutions change the number of and spacing between the discrete excitonic resonances and increase the hot exciton PL by >10X. By correlating the atomic substitutions on the cation with changes in the excitonic structure, we show that the origin of the discrete excitonic resonances is consistent with a vibronic progression caused by strong exciton-phonon coupling to a phonon on the organic cation. The properties of 2DHPs can be tailored by the selection of the cation without directly modifying the inorganic framework.

(1) Straus, D. B.; Hurtado Parra, S.; Iotov, N.; Gebhardt, J.; Rappe, A. M.; Subotnik, J. E.; Kikkawa, J. M.; Kagan, C. R. Direct Observation of Electron-Phonon Coupling and Slow Vibrational Relaxation in Organic-Inorganic Hybrid Perovskites. J. Am. Chem. Soc. 2016, 138 (42). https://doi.org/10.1021/jacs.6b08175.
(2) Straus, D. B.; Iotov, N.; Gau, M. R.; Zhao, Q.; Carroll, P. J.; Kagan, C. R. Longer Cations Increase Energetic Disorder in Excitonic 2D Hybrid Perovskites. J. Phys. Chem. Lett. 2019, 10 (6), 1198–1205. https://doi.org/10.1021/acs.jpclett.9b00247.
(3) Straus, D. B.; Hurtado Parra, S.; Iotov, N.; Zhao, Q.; Gau, M. R.; Carroll, P. J.; Kikkawa, J. M.; Kagan, C. R. Tailoring Hot Exciton Dynamics in 2D Hybrid Perovskites through Cation Modification. ACS Nano 2020, 14 (3), 3621–3629. https://doi.org/10.1021/acsnano.0c00037.

Perovskite nanocrystals have attracted much attention recently because of their high luminescence quantum efficiency compared to traditional nanocrystals. We have used multidimensional coherent spectroscopy to study excitonic states in perovskite nanocubes and nanoplatelets at low temperature.

The studies reveal coherences involving triplet states of a CsPbI₃ nanocube ensemble [1]. Picosecond time scale dephasing times are measured for both triplet and inter-triplet coherences, from which we infer a unique exciton fine structure level ordering composed of a dark state energetically positioned within the bright triplet manifold.

In an ensemble of colloidal CsPbI₃ perovskite nanoplatelets, we determine the homogeneous line broadening [2]. We demonstrate a dependence of not only their intrinsic line widths but also of various broadening mechanisms on platelet geometry. We find that decreasing nanoplatelet thickness by a single monolayer results in a 2-fold reduction of the inhomogeneous line width and a 3-fold reduction of the intrinsic homogeneous line width to the sub-millielectronvolts regime. In addition, our measurements suggest homogeneously broadened exciton resonances in two-layer (but not necessarily three-layer) nanoplatelets at room-temperature.

These results provide new insight into the coherent dynamics of excitonic states in perovskite nanocrystals as well as information about the underlying electronic states.

[1] A. Liu, D. B. Almeida, L. G. Bonato, G. Nagamine, L. F. Zagonel, A. F. Nogueira, L. A. Padilha, and S. T. Cundiff, “Multidimensional coherent spectroscopy reveals triplet state coherences in cesium lead-halide perovskite nanocrystals,” Science Advances 7, eabb3594 (2021).

[2] A. Liu, G. Nagamine, L. G. Bonato, D. B. Almeida, L. F. Zagonel, A. F. Nogueira, L. A. Padilha, and S. T. Cundiff, “Toward Engineering Intrinsic Line Widths and Line Broadening in Perovskite Nanoplatelets,” ACS Nano 15, 6499–6506 (2021).

Halide perovskites perform remarkably in optoelectronic devices including tandem photovoltaics ^1,2. However, this exceptional performance is striking given that perovskites exhibit deep charge carrier traps and spatial compositional and structural heterogeneity^3-5, all of which should be detrimental to performance. In this presentation, I will discuss how we resolve this long-standing paradox by providing a global visualisation of the nanoscale chemical, structural and optoelectronic landscape in halide perovskite devices⁶. This was made possible through the development of a new suite of correlative, multimodal microscopy measurements combining quantitative optical spectroscopic techniques and synchrotron nanoprobe measurements. We show that compositional disorder dominates the optoelectronic response, while nanoscale strain variations, even of large magnitude (~1 %), have only a weak influence. Nanoscale compositional gradients drive carrier funneling onto local regions associated with low electronic disorder, drawing carrier recombination away from trap clusters associated with electronic disorder and leading to high local photoluminescence quantum efficiency. These measurements reveal a global picture of the competitive nanoscale landscape, which endows enhanced defect tolerance in devices through spatial chemical disorder that outcompetes both electronic and structural disorder.

1 Al-Ashouri, A. et al. Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction. Science 370, 1300, doi:10.1126/science.abd4016 (2020).
2 Xu, J. et al. Triple-halide wide–band gap perovskites with suppressed phase segregation for efficient tandems. Science 367, 1097, doi:10.1126/science.aaz5074 (2020).
3 Doherty, T. A. S. et al. Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites. Nature 580, 360-366, doi:10.1038/s41586-020-2184-1 (2020).
4 Correa-Baena, J.-P. et al. Homogenized halides and alkali cation segregation in alloyed organic-inorganic perovskites. Science 363, 627, doi:10.1126/science.aah5065 (2019).
5 Feldmann, S. et al. Photodoping through local charge carrier accumulation in alloyed hybrid perovskites for highly efficient luminescence. Nat. Photonics 14, 123-128, doi:10.1038/s41566-019-0546-8 (2020).
6 Frohna, K. et al. Nanoscale Chemical Heterogeneity Dominates the Optoelectronic Response over Local Electronic Disorder and Strain in Alloyed Perovskite Solar Cells. arXiv e-prints, arXiv:2106.04942 (2021).

Nanocrystals based on halide perovskites offer a promising material platform for highly efficient lighting and as quantum emitters. Using transient optical spectroscopies supported by ab initio calculations, we study excitation recombination dynamics in atomically doped lead halide perovskite nanocrystals which exhibit dramatically improved luminescent properties.
Unexpectedly, we find an increase in the intrinsic excitonic radiative recombination rate upon doping, which is typically a challenging material property to tailor. We can attribute the enhanced emission rates to increased exciton localization through lattice periodicity breaking from dopants, which increases exciton effective masses and overlap of electron and hole wavefunctions and thus the oscillator strength.
The important interplay between electronic and structural effects involving dopant and halide ions will be elucidated. Our report of a generalizable strategy for improving luminescence efficiencies in perovskite nanocrystals will be valuable for maximizing the performance of light-emitting applications.

Utilization of the interaction between spin and heat currents is the central focus of the field of spin caloritronics. The recent emergence of chiral phonons possessing angular momentum arising from the broken symmetry of the lattice creates the potential for generating spin currents at room temperature in a non-magnetic material in response to a thermal gradient, precluding the need for an applied magnetic field or polarized ferromagnetic contacts. In this talk, we will show the observation of spin currents generated by chiral phonons in a two-dimensional layered hybrid organic-inorganic perovskite semiconductor implanted with chiral cations when subjected to a thermal gradient. Identified by transient magneto-optical Kerr effect measurements, the spin polarization direction shows a strong dependence on the chirality of the semiconductors. Our findings indicate the potential of chiral phonons for spin caloritronic applications and offer a new route toward spin generation in the absence of magnetic materials.

Liquid phase exfoliation (LPE) of graphene nanosheets from bulk graphite powders is of interest in materials science for graphene production because of the potential for environmentally benign production using green and non-toxic solvents in addition to well defined and controllable chemistry. Graphene flakes are suitable for many solution processing and fabrication methods such as direct ink writing for applications ranging from printable electronics and energy storage to optoelectronics and sensing. Because many of the desired applications for graphene are centered around its electrical conductivity and hence carrier transport, it is important to measure carrier conductivity and mobility, and correlate the finds with the LPE procedures for optimizing and tailoring the synthesis. To this end, terahertz time-domain spectroscopy is employed as a non-contact method of probing the carrier transport for a variety of graphene layers and graphite powders. Fitting the data to transport models, such as variants of the Drude model, provide conductivity and mobility characteristics that can be linked to microscopic scattering mechanism that are related to the LPE process. This characterization is expected to inform the preparation of exfoliated flake materials for more desirable properties for electronic and optoelectronic applications.

Scintillators are materials that produce light pulses upon interaction with ionizing radiation, therefore they are widely employed in radiation detectors. In advanced medical-imaging technologies, fast scintillators enabling a time resolution of tens of picoseconds are required to achieve low-noise and fast imaging at the millimeter length scale. [1] Because, commonly used bulk and nanostructured materials, both organic and inorganic, does not offer the synthetic versatility required to have full control over the properties affecting the time resolution, composite systems are currently intensively investigated with the aim to overcome the performance traditional scintillators. [2]

We demonstrate that composite materials based on fluorescent metal–organic framework (MOF) nanocrystals can work as fast scintillators. We present a prototype scintillator fabricated by embedding MOF nanocrystals in a polymer. The MOF comprises zirconium oxo-hydroxy clusters, high-Z linking nodes interacting with the ionizing radiation, arranged in an orderly fashion at a nanometric distance from 9,10-diphenylanthracene ligand emitters. Their incorporation in the framework enables fast sensitization of the ligand fluorescence, thus avoiding issues typically arising from the intimate mixing of complementary elements. This proof-of-concept prototype device shows an ultrafast scintillation rise time of ~50 ps, thus supporting the development of new scintillators based on engineered fluorescent MOF nanocrystals. We further developed the scintillating MOF concept by realizing hetero-ligand fluorescent nanocrystals with large Stokes shift of 750 meV showing a fluorescence quantum efficiency as a large as 70%. The Stokes shifted emission is successfully activated by diffusion-mediated non-radiative energy transfer within the crystalline framework between the co-ligands, thus obtaining a fast scintillator with zero re-absorption effects that it strictly required for high-resolution medical imaging and other applications involving the detection of high energy radiation and particles.



REFERENCES

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A mechanistic understanding of the photo-excited states in colloidal quantum dots (QDs) is the essential step toward unleashing their full potential in various optoelectronic applications. Fluorescence spectroscopy at the single-particle level is a powerful tool to study the excited states in QDs and has revealed a number of phenomena such as photoluminescence (PL) intensity blinking and lifetime blinking in recent decades. However, interpreting fluorescence data of single QDs is still challenging to date. For one thing, excited state lifetimes and PL intensities of single QDs are correlated, but unfortunately the fluctuation of PL intensity in single QDs is non-ergodic. Moreover, the PL signal from single QDs is often weak and noisy because the single QD emission is intrinsically weak, and current detection efficiency of single-photon detectors is far from perfect. These effects become even more pronounced when studying excited state lifetimes of single QDs, for the accuracy and reliability of derived lifetimes depend upon the purity and size of the data. So far, most studies analyzed excited state lifetimes of single QDs without preselecting the data or preselected the data by setting subjective bins and/or thresholds. As a result, the excited state lifetimes were actually derived from mixed states and lack reproducibility, from where the true photophysics may be blurred. To overcome these difficulties, we utilize statistical methods to derive well-defined excited state lifetimes of single QDs. We employ change-point analysis to break down the arrival time series of compact CdSe/CdS core/shell QDs so that the consecutive bins that belong to the same state could be analyzed altogether. We then perform multi-exponential fitting and tests of significance to verify that in the single QD PL decay curves there is an additional lifetime component whose value is roughly half of that of the single exciton lifetime. The additional lifetime component in this type of QDs is assigned to lifetime blinking that happens too fast to be detected by traditional binning analysis methods. We propose that lifetime blinking happens when a negative trion forms in the core of a QD after photoexcitation while non-radiative processes are not activated. However, the easy accessibility to efficient non-radiative processes results in the short durations of lifetime blinking events in this type of QDs.

We have studied the anisotropic optical and electronic properties of ReS₂ flakes using ultrafast pump-probe microscopy. In contrast to other highly symmetric TMDCs like MoS₂ and WS₂, ReS₂ remains a direct bandgap semiconductor regardless of the number of layers due to its low-symmetry distorted 1T crystal structure. This low symmetry also gives rise to anisotropic optical and electronic properties. We have used polarization-resolved pump-probe microscopy to investigate the anisotropic carrier dynamics in ReS₂ nanoflakes. In these experiments, a localized region on a single nanoflake is excited with a linearly polarized 440 nm femtosecond laser pulse focused through a microscope objective. A second 880 nm laser pulse with variable arrival time and polarization probes how the excitation evolves in time. Carrier recombination dynamics were studied by measuring the difference in the transmission of the 880 nm probe as its time delay increases with respect to the initial 440 nm pump. At fixed time delays, changes due to polarization were resolved by rotating the probe polarization. The resulting polar plots illustrate how changes in excited carrier population influence the anisotropic optical properties in ReS₂ nanoflakes.

We study the optical conductivity response of simple ferromagnetic alloys driven by spin-orbit coupling. The spin-orbit coupling leads to a band splitting near the Fermi surface in the range of 10-100 THz. This band-splitting energy range provides an opportunity to study the optical conductivity in the THz regime. The optical conductivity response is sensitive to the details of the structure and chemical composition. For simple alloys, like Fe-Co alloys, we find that there is not much dependence of optical conductivity response in the THz range. However, we find the optical conductivity response changes sign from positive in Fe to negative in Fe-Al alloys. Furthermore, those features are washed away at 10 THz but remain only at 100 THz because of the short electron lifetime.

NIR-Vis up-conversion (UC) plays a key role in many emerging applications such as theranostics, solar energy harvesting, sensing and others.^[1] While rare earth (RE) up-conversion luminescence in fluoride nanoparticles is extensively studied,^[2] fewer reports are available on RE-doped metal oxide UC nanoparticles, despite their high chemical stability. In particular, HfO₂ nanocrystals seem to be a promising alternative because of their high inertness and biocompatibility.^[3] Nevertheless, the polymorphism observed in hafnia when high RE doping levels are used, prevents a clear understanding of the parameters relevant for controlled and improved UC. Therefore, a finer control over doping levels and crystal structure is necessary to optimize the optical performance.

In this work, we studied the influence of structural and compositional modifications on the up-conversion luminescence (UCL) recorded in Er/Yb co-doped hafnia nanocrystals obtained by a colloidal solvothermal synthesis. The up-conversion characteristics of the materials were monitored in dependence of structural parameters such as crystal size and lattice symmetry, determined by means of X-ray diffraction and transmission electron microscopy (TEM), and originating from the multifunctional doping.^[4] We report that the cubic polymorph of hafnia expresses much higher up-conversion efficiency with respect to monoclinic nanoparticles. On the other hand, the elemental analysis carried out by TEM energy diffusive X-ray spectroscopy (EDS) allowed us to correlate the composition of Er/Yb/Lu:HfO₂ with the main UC features like efficiency and the ratio between green and red emission deriving from radiative recombination on Er³⁺ centres. These results give useful insights about the phenomena responsible for UCL features in hafnia nanocrystals, enabling their employment in various fields, thanks to improved UC intensities and control over the emission profile. Moreover, the experimental platform described here permitted to possibly explain the inhomogeneity of doping in mixed polymorphs by analysing UCL profiles as probes of the local rare earth environment.

[1] B. Zhou, B. Shi, D. Jin, X. Liu, Nat. Nanotechnol. 2015, 10, 924.
[2] S. Wilhelm, ACS Nano 2017, 11, 10644.
[3] I. Villa, C. Villa, A. Monguzzi, V. Babin, E. Tervoort, M. Nikl, M. Niederberger, Y. Torrente, A. Vedda, A. Lauria, Nanoscale 2018, 10, 7933.
[4] A. Lauria, I. Villa, M. Fasoli, M. Niederberger, A. Vedda, ACS Nano 2013, 7, 7041.

In this talk we will describe recent advances in our group towards developing efficient infrared emissive devices using nanostructured materials. First, we will discuss the realizing of ultrastrong light-matter coupling in microcavities containing dense films of single chirality semiconducting nanotubes. We will show that this allows for hybridization with the K-momentum dark exciton and the bright direct exciton mediated by the optical field. As a result, new opportunities arise for enhancing the luminescence efficiency by bypassing losses associated to the optically forbidden dark exciton transition.

We will then describe our work on the spectroscopy and realization of LEDs using the two-dimensional (2D) semiconductor black phosphorus (BP). Black phosphorus is unique amongst 2D materials in that it possesses a direct bandgap, which is highly tunable from 0.3 eV in the bulk, to ~2 eV at the monolayer level. In addition, it possesses significant in-plane anisotropy, with the transition dipole moment strongly oriented along the armchair direction. We will describe the first realization of LEDs based on BP, which rely on the use of BP/Molybdenum disulfide heterojunctions. These emit at 3.8 um with an internal quantum efficiency of ~1%. Then, we will discuss strategies for increasing the scalability of black phosphorus devices through the use of solution-processing technique and the realization of cm x cm-scale homogeneous thin films of BP-fullerene via the Langmuir-Blodgett technique. This will highlight an outstanding problem in the community in understanding the strong discrepancy between the optical response of nanosheets as compared to bulk crystals. We will postulate on the likely origin of this discrepancy.

Recently, the fluid-like behavior of charge carriers in condensed matter systems has attracted much interest. For example, the ultraclean graphene can host a hydrodynamic charge flow at low temperatures, when momentum conserving carrier-carrier scattering is much faster than momentum relaxing scattering. In analogy with whirlpools in flowing water, the viscous electronic flow generates vortices in the electronic fluid in graphene. The vortices drive electric current against an applied field, resulting in a negative nonlocal voltage near current sources and drains. The alternating, sign-flipping pattern in the voltage close to the contacts is an experimentally detectable signature of the electronic viscous behavior.
We observe visually similar charge-alternating accumulation patterns in tungsten ditelluride in the semimetallic phase at room temperature by using a polarization-sensitive laser microscopy technique with an electrical bias. To observe charge distribution, we utilize an electro-optical effect, which is a polarization variation resulting from optical refractive index change due to the local carrier density variation. The spatial charge distribution shows exceptionally long relaxation profiles, ~1.4 μm, which are much more extended than a charge screening length in conventional semimetals (usually a few nanometers). We analyze that the nonlocal negative charge current and the sizeable spatial size of the charge domains are supported by the long recombination time of electron-hole pairs in this nearly-compensated two-component Weyl semimetal.
We show theoretically that these peculiar charge accumulation patterns appear due to the unbalanced diffusive behavior of electrons and holes. Our analytical calculations in a two-component model indicate that the Ohmic electric current can produce hydrodynamic-like whirlpool structures in the neutral electron-hole current close to the injection contacts. The whirlpools in the neutral current spatially correlate with the charge accumulation regions. We argue that it is the backflow of a neutral current which leads to the experimentally observed exotic alternating charge distribution near the contacts. Moreover, the fluid-like behavior of the neutral quasiparticle current can be described in terms of a modified Navier-Stokes equation equipped with higher-derivative terms.
We find that the extremely long relaxation length and alternating distribution of charge in semimetal WTe₂ can be described by the pseudo-hydrodynamic flow of neutral quasiparticle. We expect that the unconventional flow of neutral quasiparticle in WTe₂ is responsible for many interesting physical effects at room temperature.

Coupling electronic states of molecules or nanoparticles to the quantized radiation field inside an optical cavity creates a set of new photon-matter hybrid excitations, called polaritons. As opposed to atoms, the vibrational modes of molecules provide new degrees of freedom to mediate the quantum transduction between electronic and photonic states, offering new paradigms for quantum materials and chemical science. We will discuss the formation of CdSe nanoplatelet (NPL) exciton-polaritons in a distributed Bragg reflector (DBR) cavity [1]. The molecule-cavity hybrid system is in the strong coupling regime with an 83 meV Rabi splitting, characterized from angle resolved reflectance and photoluminescence measurements. Mixed quantum-classical dynamics simulations are used to investigate the polariton photophysics of the hybrid system by treating the electronic and photonic degrees of freedom (DOF) quantum mechanically and the nuclear phononic DOF classically. Our numerical simulations of the angle-resolved photoluminescence (PL) agree extremely well with the experimental data, providing a fundamental explanation of the widely-observed asymmetric intensity distribution of the upper and lower polariton branches. Our results also provide mechanistic insights into the importance of phonon-assisted nonadiabatic transitions among polariton states, which are reflected in the various features of the PL spectra. This work provides a facile and direct path for the coupling of nanoparticle electronic states with the photon states of a dielectric cavity to form a hybrid system, and provides a new platform for investigating cavity-mediated physical and chemical processes, providing the emerging quantum information science field the knowledge for preparing next generation quantum systems.

(1) L. Qiu, A. Mandal, O. Morshed, M. T. Meidenbauer, W. Girten, P. Huo, A. N. Vamivakas, and T.D. Krauss J. Phys. Chem. Lett. 12, 5030−5038 (2021).

Semiconducting carbon nanotubes (CNTs) are promising candidates for next-generation optoelectronic and photovoltaic devices due to their strong absorptivity and easily tunable photophysical properties. However, device performance is typically limited by sub-10 nm exciton diffusion lengths as well as energy loss via trap states and non-radiative pathways. An emerging approach that has the potential to overcome these limitations is to utilize hybrid light-matter states known as exciton-polaritons, where the strong coupling between photons and excitons leads to energy delocalization over the length of the entire cavity, which is typically hundreds of nanometers. We demonstrate the fabrication of a donor-acceptor polaritonic system bearing semiconducting single-walled CNTs with two different band gap energies, and characterization of the energy flow therein using transient reflection and two-dimensional white-light (2DWL) spectroscopies. The CNT layers are spatially separated by a 150-nm thick insulating polymer layer, ruling out the possibility of energy transfer due to direct donor-acceptor contact. The transient and 2DWL spectra reveal a 300-fs growth of the lower-polariton excited-state population upon photoexcitation into the higher-lying upper-polariton state, indicative of a polariton-assisted long-range energy transport process over a ~300-nm distance. These results highlight the potential of polaritonic microcavities as a tool for tailoring and optimizing material properties toward desired device performance.

Experiments at the intersection of condensed matter physics and chemical dynamics are now able to control and alter the properties of quantum matter on demand in regimes of strong-coupling. In such experiments, strongly coupling the complex matter system to a mode of an optical high-Q cavity leads to the emergence of hybrid light-matter states that influence the physical, chemical and spectroscopic properties of the matter system. An example of a physical process that can be modified is second-harmonic generation (SHG) where the efficiency of the process can be increased by strongly coupling to photons [1, 2]. To date, an ab initio approach to this process has not been investigated in the context of strong light-matter coupling. In this context, we present an ab initio strong light-matter description of SHG from a molecular system within the framework of quantum-electrodynamical density functional theory (QEDFT) [3, 4]. We investigate the efficiency of the process for a single molecular system coupled to a cavity mode as well as for the case of collectively coupled molecules. From these first principles calculations, we deduce how collective strong coupling influences non-linear properties of the matter system. Also, we show the spectrum of the photon field of the corresponding SHG in this talk.

[1] J. Schmutzler, et al., Phys. Rev. B 90, 075103 (2014),
[2] T. Chervy, et al., Nano Lett. 16, 12, 7352-7356 (2016),
[3] M. Ruggenthaler et al., Phys. Rev. A 90, 012508 (2014),
[4] J. Flick and P. Narang, Phys. Rev. Lett. 121, 113002 (2018).

Organic electrochemical transistors (OECTs) are highly sensitive sensors used in increasingly challenging biologic applications such as wearable textiles with integrated biosensors and in vivo recording of brain activity.[1,2] They can be described as an ionic circuit embedded with an electronic circuit. When gating the OECT with a voltage bias, ions penetrate and modifiy the doping level of the organic channel thereby changing its conductivity at both macro- and microscopic scales.

One of today’s challenges to further develop this promising technology lies in the fundamental understanding of the interplay between doping level and conductivity. [3] Previous investigations have addressed this question via macroscopic approaches which are inherently sensitive to the device geometry and morphology.[3-4] In this work, we propose an innovative in situ bottom-up approach that alows the determineation of the intrinsic conductivity of bipolarons and polarons.

We use UV-VIS-NIR absorption spectroelectrochemistry combined with a multivariance curve resolution (MCR) analysis [4] to extract the neutral, polaron and bipolaron populations at different voltages. The microscopic conductivity is then probed via in situ THz spectroscopy [5], which represents the first measurement of this kind on OECTs. For archetypal PEDOT:PSS-based OECTs, we find that bipolarons are slightly more localized than the polarons and thus demonstrate a lower microscale mobility. However, the higher density of charges in bipolarons counterbalances their lower mobility and explains why bipolarons largerly contribe to the total microscale conductivity.

Finally, we will present the two main perspectives of this new THz measurement technique in the field of OECTs:

1. Combined with macroscopic conductivity measurements, microscopic conductivity offers the prospect to disentangle underlying mechanisms limiting long-range charge transport such as device geometry and/or morphological features.

2. Determining the fundamental conductivity properties of doped species like bipolarons and polarons represents a necessary step to push microscale modelling of devices by linking the doping level and conductivity at the smallest possible level.

[1] Gualandi I., Marzocchi M., Achilli A. et al., Sci Rep, 2016, 6, 33637.
[2] Khodagholy D., Doublet T., Quilichini P. et al. Nat Commun, 2013, 4, 1575.
[3] Colucci R., de Paula Barbosa, H. F., Günther, F., Cavassin, P., & Faria, G. C. (2020). Recent advances in modeling organic electrochemical transistors. Flexible and Printed Electronics, 5(1), 013001.
[4] Rivnay J., Inal S., Salleo A. et al., Nat Rev Mater, 2018, 3, 17086
[5] De Juan A., Jaumot J., Tauler R. Multivariate Curve Resolution (MCR). Solving the mixture analysis problem. Anal. Methodes 6, 4964-4976 (2014)
[6] Unuma T., Yamada N., Nakamura A. et al. Direct observation of carrier delocalization in highly conducting polyaniline. Appl Phys Lett 103, 053303 (2013)

Mixed ionic-electronic conducting polymers have recently emerged as a promising material choice for bioelectronic devices due to their low impedance, soft mechanical properties, and ability to transduce ionic signals to electronic currents.¹ The unique behaviour of mixed conducting polymers arises from ion intercalation through the bulk of the material which can modify the oxidation state, and therefore charge carrier concentration, of the semiconducting polymer backbone. However, the physics of ion-doping in polymers is still unclear² with increasingly complex experiments and models being developed to capture the internal electric fields,³ ionic distributions,⁴ and structure-property relationships.⁵ Here, we utilize the electrochromic response of mixed conducting polymers to monitor ion motion optically at the length scales (from 1 μm to 500 μm) and timescales (from 1 ms to 10 s) relevant to device operation. We fabricate devices with polymer/electrolyte interface orthogonal to the substrate to limit ionic penetration to single plane, similar to previously investigated “ionic moving front” geometries.⁶ Using spectrally- and time-resolved transmission microscopy, we monitor the change in visible light absorption of mixed conducting polymer backbone as it is doped/de-doped when ions flow out of/into the bulk. Contrary to previous work at much larger distances (several mm) and longer times (from 5 s to 1 min),^6,7 we observe a linear relationship between time and the distance of the doping front from the polymer/electrolyte interface. Furthermore, the results reveal unintuitive relationship between the phase-front velocity and the applied electrochemical potential, with larger de-doping potentials resulting in slower electrochromic transitions. We explore a range of organic mixed conducting materials, both p-type and n-type, with varying degrees of swelling during bulk doping to identify the structure-property relationships for ion dynamics in mixed conducting polymers. Finally, using finite element analysis, we reconstruct the internal electric field evolution near the polymer/electrolyte interface based on the experimental results. This work helps resolve the fundamental physics of electrochemical doping in mixed conducting polymers and provides guidance for optimizing the polymer microstructure and chemical functionality for improved bioelectronic devices.

[1] J. Rivnay et al. Nat. Rev. Mater. 3, 17086 (2018)
[2] B.D. Paulsen et al. Nat. Mater. 19, 13-26 (2020)
[3] K. Tybrandt et al. Sci. Adv. 3, eaao3659 (2017)
[4] V. Kaphle, et al. Nat. Comm. 11 (2020)
[5] I. Bargigia et al. J. Am. Chem. Soc. 143, 294-308 (2021).
[6] E. Stavrinidou et al. Adv. Mater. 25, 4488-4493 (2013)
[7] J. Rivnay et al. Nat Comm. 7, 11287 (2016)

Conductive polymers exhibit hybrid electronic-ionic transport properties that are critical to understanding functionality in electrochemical devices for energy conversion and storage and biosensing. Electrochemical impedance spectroscopy enables the resolution of charge transport in both the frequency and energy domains, but often is complicated by complex circuit analysis necessary to estimate non-Faradaic (double layer) and Faradaic (charge transfer) contributions, often where these two events are coupled together. Optical spectroscopies offer specificity but often lack the sensitivity of electrochemical techniques. Ultimately, neither EIS nor spectroscopic methods alone afford the sensitivity and specificity to confidently isolate charge transfer and transport processes in conductive polymer/electrolyte systems.
Herein we demonstrate the use of color impedance spectroscopy (CIS) to resolve electrolyte effects on electrical charge transport in a prototypical material: poly(3-hexylthiophene) (P3HT). In CIS, the modulated transmittance tracks the motion of Faradaic charges in the films via the optical signatures attributed to P3HT polarons with resolution in both energy and frequency. This approach allows for characterization of the dynamics of charge transport in the context of Coulombic binding effect between counterions and polarons. Using CIS, we observe that at low doping potentials, only a small fraction of polarons are actually mobile and respond to electric field modulation at moderate frequencies. We attribute this to the localization of the polarons via long-range Coulombic interactions with counterions; i.e. Coulombic trapping. Increasing the electrochemical doping increases the steady-state carrier density and increases the fraction of mobile carriers. This is consistent with a more homogenous energetic environment, i.e. the reduction in quantity and/or trap energy. The chemical information gained and the CIS method development described here should be instructive for future efforts to quantify charge transfer/transport structure-property relationships, with applications to energy conversion and storage and biosensing.

CuO and Cu₂O, both being small and direct bandgap materials, have been studied and implemented individually as an active layer of solar cell from early age of manufacturing. A mixture of these two-phase contents at different ratios can bring variance in absorbance and electrical conductivity which can present new and diverse aspect in solar cell fabrication with added efficiency. Previous works was successful to vary the phase mixture and the nanoparticle (NP) structure provided very high absorption co-efficient but poor electrical conductivity. By controlling the PLD parameters and subsequent processing steps we figured out a way to vary the phase mixture of copper oxide thin films to maximize the electrical conductivity and absorption in the solar spectrum. Phase mixture with different CuO/Cu2O ratios was obtained using pulsed laser deposition through varying growth parameters such as oxygen pressure in the chamber and temperature of the substrate. i. To obtain the structure-property correlation we have performed the X-ray diffraction, Raman spectroscopy, scanning electron microscopy, electrical measurements and optical transmittance measurements by UV-VIS spectroscopy on the samples. The TOPAS analysis of the XRD data from all the samples has shown various percentage of CuO and Cu₂O and one of the samples also had metallic Cu phase. Owing to the change in the phase content in the copper oxide thin films we have observed some consistent variations in the electrical and optical properties. And so far, a thin film copper oxide phase mixture (CuO 61.60%-Cu₂O 38.15%) with a thickness of 200 nm yielded an absorption co-efficient of 7.02E+6 cm^-1 which is greater than that of commercially available pure CuO (1.19E+5 cm^-1). The measured resistivity of this sample was obtained to be 0.034 Ω-m indicating a doped semiconducting composite. This phase mixture, with the maximum absorption coefficient and suitable electrical property, can be implemented as the absorption layer of a solar cell which would help attaining maximum efficiency in the copper oxide based low-cost heterojunction solar cell. Moreover, the presence of Cu in one of the sample hints the possibility of NP incorporation with more controlled and reduced amount of copper in the film.

Silver phenylselenide (AgSePh) is a blue-emitting two-dimensional (2D) semiconductor with in-plane anisotropy, large exciton binding energy, non-toxic and earth-abundant elemental composition, and a scalable synthetic method that is compatible with modern microelectronics. Unlike transition metal dichalcogenides and 2D layered perovskites, AgSePh features covalent bonding between its organic and inorganic components, becoming a truly hybrid organic-inorganic semiconductor with chemical stability and a unique bandgap tunability through organic modification. Although AgSePh contains multiple desirable characteristics for modern electronics, its performance is still limited by the crystal size and quality produced by conventional synthetic methods. In this presentation, we will show two synthetic advancements to produce AgSePh thin films with controllable grain size from <200 nm to >5 µm and AgSePh microcrystals with improved crystal size from ~2 µm to >1mm. Systematic optical and electrical characterizations to confirm higher crystalline quality with lower defect densities in these samples will be presented. Furthermore, mechanistic insight into the improved syntheses will be elucidated, and an application of this knowledge to grow single crystals will be shown. We expect these improved synthetic methods to facilitate the studies on the fundamentals and applications of this exciting new material.

Over 17% of water contamination in the world is due to the discharge of wastewater from textile and dyeing industries. These industries use a large portion of organic dyes, which are found to be toxic, carcinogenic, and can be difficult to remove. Conventional treatments have been studied and found to be ineffective for the removal of the dyes. However, one effective way to remove them from our water sources is by a photocatalytic degradation process using semiconductor nanoparticles. Titanium dioxide (TiO₂) is a semiconductor material with intrinsic optical and electronic properties and is one of the most used photocatalyst for the degradation of these organic contaminants. The photocatalytic capacity of TiO₂ nanostructures is based on the generation of electron-hole pairs due to the Ultraviolet (UV) excitation of TiO₂. The objective of this investigation is to assess the organic dyes individually and combined, in the presence of Titanium Dioxide nanoparticles. Photodegradation studies were evaluated in the presence of dyes like Malachite Green Chloride (MG) and Methyl Violet (MV) at different concentrations of TiO₂ nanoparticles (10, 15, 25, and 50 ppm). Following the UV irradiation of the dyes at certain time intervals, it was found that 15 ppm TiO₂ degraded MG and MV more swiftly than the other concentrations, with a result of ~90% and ~98%, respectively. Our future studies include evaluating the photocatalysis of the mixture of both dyes (multi-component) in the presence of TiO₂.

Generally, ITO thin film is deposited by sputtering which is required a high-vacuum and high-temperature process, resulting in high manufacturing costs. On the other hand, spin coating method has advantages such as efficient cost, simplicity, fast and mass production. So, it can be another method for manufacturing ITO thin films. However, Since the ITO thin films deposited by spin coating method has a porous surface, electrical conductivity is lower than that of ITO thin films deposited by sputtering.
The electronic conductivity of nickel-plated ceramic is improved by electroless Nickel plating, and it is verified that nickel increase conductivity. [1]. In this study, We confirmed with 4-point probe that spin-coating Nickel plated ITO (ITO@Ni) increase electrical conductivity. [2-4] The composition and morphological of nickel deposited on ITO were characterized by transmission electron microscope (TEM) and SEM-EDX analysis. ITO@Ni powder becomes the source of ITO@Ni colloid ink to be used in spin coating. Particle size and ink dispersion were measured by electrophoresis light scattering (ELS). ITO@Ni was deposited by spin coater. Transmittance and conductivity was respectively measured with UV-Visible Spectrophotometer and 4-point probe. When pure ITO and ITO containing 10wt% of nickel were deposited as one layer, the transmittance was lowered from about 97% to 90%. Sheet resistance of ITO@Ni multilayer appears less than 30ohm/square.
As a result, Transmittance and conductivity had a trade-off relationship according to nickel ratio. It is expected that spin-coating ITO@Ni can be used as a transparent electrode without high vacuum and high temperature process.

[1] Pratihar, S. K., Sharma, A. D., & Maiti, H. S., Materials Chemistry and Physics, (2006), 96(2-3), 388-395.
[2] Alavi, B., Aghajani, H., & Rasooli, A.,Thin Solid Films, (2019), 669, 514-519.
[3] Uysal, M., Karslioğlu, R., Alp, A., & Akbulut, H., Ceramics international, (2013), 39(5), 5485-5493.
[4] Kumar, D. S., Suman, K. N. S., & Kumar, P. R., International Journal of Advanced Science and Technology, (2016), 97, 59-68.

When mentioning emerging fields, nanoparticle research is at the forefront due to their unique physical and chemical properties. Tin dioxide (SnO₂) is a n-type semiconductor with potential photocatalytic properties, based on the fact that they exhibit a large surface area. Additionally, tin is a nontoxic material and a safe alternative in the cleaning of bodies of water. Considering these facts, this research aims at the production of SnO₂ nanoparticles in aqueous phase to study their spectroscopic properties using characterization techniques, specifically UV-vis and spectrofluorometer analysis.
Tin dioxide synthesis was made in the presence of tin chloride (SnCl₂), cetyltrimethylammonium bromide (CTAB), ammonium hydroxide and deionized water. The reaction time was optimized to 6-hours and the fluorescence was studied by using a spectrophotometer evidencing a peak at 403nm when excited at a wavelength of 280nm. While, UV-vis analysis denoted a peak at 303nm.
Taking into consideration SnO₂ spectroscopic properties and expected photocatalytic tendencies, future studies will be focused on the degradation of organic dyes.

Recently, research on nanostructured photovoltaic cells based on organic materials has been actively conducted to improve power conversion efficiency through the development of new device structures, synthesis of new organic materials and applied to charge extraction layer. This study applies various ZnO thin films (sol-gel process based on high and low temperature and nanoparticle process) as electron extraction layer in nanostructured photovoltaic cells based on PBDB-T:ITIC, and the correlation between ZnO and photovoltaic cells performance was analyzed. When the ZnO thin film was formed by the high temperature(450°C) sol-gel process, the sheet resistance of ITO electrode was increased up to 5 times. As a result, the power conversion efficiency was low at 4.12%. In the nanoparticle process, butanol-based ZnO has better dispersion and surface properties than IPA-based ZnO, resulting in improved polymer solar cell performance (PCE of 6.35% and 4.58% with butanol and IPA-based ZnO respectively). In addition, ZnO precursor solution with a low-temperature (150°C) sol-gel process was developed, and as a result of applying it as an electron extraction layer of a nanostructured photovoltaic cells, the device performance was greatly improved (PCE of 8.89%). The main reason for the improvement of the device performance is that the ripple-shaped surface is formed, which facilitates extraction of electrons and has excellent surface roughness.

Investigating the optical properties of transition metal dichalcogenide (TMDs) monolayers has recently become a promising research direction towards direct bandgap two-dimensional (2D) semiconductor applications. Monolayer 2D-TMDs emit light efficiently in the form of two distinct excitonic emission features, referred to as the A–peak (ground state) and B–peak (higher spin-orbit split state). These emission peaks can be tuned to promote a higher intensity ratios and therefore, to reduce the opportunity for undesirable intervalley scattering. Our work provides insight into the degree of valley polarization in MoS2 monolayers and of MoS2/plasmonic/heterostructures that can help maximize the optically–induced valley polarization by inducing a population imbalance between the two inequivalent valleys. Our monolayers are produced by lithium intercalation as well as by epitaxial growth on sapphire, while the heterostructures are fabricated using plasmonic nanostructures such as Ag nanoparticles. We observe that high quality samples produce enhanced emission from the B–exciton relative to the A–exciton, which corresponds to an increased degree of valley polarization. As it turns out, the relative optical orientation of these peaks, occurring at 612 nm and 675nm in MoS2 monolayers, remains efficient even at room temperature. We take advantage of this fact and make use of a modified circular polarization luminescence (CPL) emission spectroscopy system. More specifically, our system consists of a static achromatic quarter wave plate (QWP) that converts L–CPL and R–CPL light to orthogonal linearly polarized light; later both L–CPL and R–CPL states are analyzed by automated rotation of a linear polarizer prior to whole-spectra detection with a CCD spectrometer. We present the analysis of the A– and B–exciton photoluminescence intensity ratio of both intrinsic and induced circular polarization emission in these structures.

Metal-organic frameworks (MOFs) are crystalline materials consisting of organic building blocks and metal clusters. Traditionally known as a versatile system for manipulating chemical processes, MOFs’ potential has recently been extended beyond controlling the movement of molecules to structuring the flow of energy. The promise of MOF as a platform for photophysical studies lies in its ability to lock chromophores into a variety of packing distances and geometries. Since photophysical processes are highly sensitive to the communication among neighboring molecules, MOFs provide a synthetic handle for structuring exciton dynamics. Taking advantage of this unique opportunity, herein we present a strategy that allows inhibition of singlet exciton transfer in a spin-selective fashion. The angular arrangement of chromophores inside the donor-acceptor MOF is visualized with high resolution transmission electron microscopy, and the exciton dynamics are probed by time-resolved spectroscopy. Connecting the structural and photophysical properties demonstrates that spin-selective control over exciton transfer can be synthesized with long-range structural order. We envision that this strategy can be applied to increasing the efficiency of photophysical processes that involve interplay between transfer of excitons of different spin states, such as solid-state triplet-triplet annihilation upconversion.

The lack of materials with large ultrafast nonlinearities currently limits the rise of optical information processing as a solution to efficiently process the ever-growing amount of data collected for big data and artificial intelligence applications. Optical processing schemes still need to interface with electronics for key functions which require nonlinear functions such as switching. Due to the weak nonlinear response of bulk materials, current optical switching technologies function at high threshold power which makes them unpractical. The development of efficient switching would be greatly helped by materials showing a fast and strong nonlinear optical response. Low-dimensional nanostructured materials like carbon nanotubes are predicted to have large and fast nonlinearities. Several groups have attempted to measure optical nonlinearities in single-walled carbon nanotube (CNT) thin films using the Z-scan technique. However, the use of a mixture of chiralities masked the resonant excitonic behavior and the analysis didn’t account for the limited validity of common approximations when z-scan is used to study strongly nonlinear or absorbing films. In fact, the common approximations used in Z-scan fail drastically near electronic resonances.

In this work, we study the nonlinear optical properties of single chirality carbon nanotubes thin films using the Z-scan technique at wavelengths ranging from 917 nm to 1550 nm covering the near, on, and off-resonant excitation regime. Our investigation reveals a large enhancement of the nonlinear response near the excitonic resonance (1003 nm) of the film. In this regime, we observe a 15-fold enhancement of the nonlinear parameters. A nonlinear refractive index (n₂) as large as 3.4 +/- 0.2 cm²/GW and -0.50 +/- 0.06 cm²/GW are measured on the blue and red sides of the resonance respectively. Large changes in the imaginary refractive index attributable to saturable 1-photon absorption are also observed for the near and on-resonance regimes together. The changes are well into the non-perturbative regime, where variations of the real and imaginary refractive index are on the order of the linear index, for intensities of less than 5 GW/cm². To account for the non-perturbative nature of the response, we developed a numerical beam propagation code that extracts the nonlinear properties of the thin film from the transmittance measurement, for an arbitrary nonlinear refractive index model, free of approximations. The strength of the nonlinearities in high-density single chirality CNT thin films, together with their compatibility with the silicon photonics fabrication processes, makes them a promising material for the development of nonlinear devices for optical signal processing.

Enhanced light-matter interactions and high surface to volume ratio in two-dimensional (2D) materials makes them promising candidates for application in solar energy conversion and optoelectronics. Recently, we have reported on a scalable, near ambient bottom-up synthesis of 2D transition metal oxides and carbo-oxides from non-layered precursors. Here we investigate the nature and dynamics of photoexcitations in 2D TiC_1-xO_1+x and 2D Mn_xO_y using a combination of time-resolved photoluminescence spectroscopy and pump-probe spectroscopy. Both classes of materials are indirect gap semiconductors with the gap as high as 4.1 eV for Ti-based films and as low as 2.37 eV for Mn-based films. However, even high band gap films have considerable optical absorption across the visible range due to pronounced band tail states. Some of the films also exhibit efficient optical emission. Tuning optical excitation from the ultraviolet to the infrared range, we study the transient changes in optical properties and the lifetime of photoexcitations to uncover the nature and origin of the optically active states that play critical roles in solar energy conversion processes.

Over the years, luminescence spectroscopy has established itself as one of the fundamental methods for analyzing the photophysical properties of a variety of samples, ranging from simple organic molecules to semiconductor materials and photovoltaic (PV) devices. Combining spectral and lifetime information of a sample’s luminescence signals provide a deeper insight into its photophysical processes. This can be further enhanced by including spatial information. Acquiring steady-state and time-resolved spectroscopic data at multiple, well defined points of interest (POI) of the sample can help in inferring structural-to-photophysical relationships in PV materials. Gathering such information is an important step toward the optimization of structure as well as preparation process of such materials in order to increase the performance of PV devices.

Here we will demonstrate the performance of a spectrometer-microscope assembly based on the FluoTime300 spectrometer and FluoMic add-on for investigating a Copper Indium Gallium Selenide (CIGS) based solar cell in terms of steady-sate and time-resolved luminescence spectroscopy at different micrometer sized POIs. We will show that acquiring data at well defined excitation / detection areas can provide deeper insights into relationships between structure and photophysical behavior of CIGS solar cells. This additional information are not readily available when investigating such a sample with a conventional spectrometer as the luminescence signal is averaged over a much lager area (typically 1 mm² or more).

Converting solar energy into chemical energy is a promising strategy towards the production of solar electricity and stored fuels. Until now, most of the supramolecular complexes that undergo symmetry breaking charge separation (SB-CS) process, mimicking the photosynthetic special pair, reveal strong competing pathways of long-lived excitons and/or excimeric traps. Recently, a new supramolecular framework was developed taking inspiration from the minimal component in the photosynthetic organisms, defined as “Quantasome” (QS) [1]. The artificial QS is composed of a self-assembled perylene bisimide (PBI) and a polyoxometalate (POM) catalyst as a guest, building highly ordered three-dimensional (3D) noncovalent assemblies in water.
We study a series of QS assemblies, with a redox-inactive and a redox-active POM catalyst as guest, Q-1 and Q-2 complexes respectively. Structural information extracted from Powder X-Ray Diffraction (PXRD) proves that the QS structures are mimicking the organization of the thylakoids in the granum domain of the chloroplasts.
We perform Ultrafast Transient Absorption (TA) spectroscopy to unveil the photo-physical pathways after photo-excitation. The samples are pumped with a sub-100 fs laser pulse peaked at 520 nm, while the probe pulse covers a broad spectral window spanning from 560 nm to 1000 nm. Q-1 and Q-2 complexes exhibit excited state absorption (ESA) bands originating exclusively from the superposition of PBI radical anion and cation. Our TA data show ultrafast (< 150 fs) and quantitative formation of symmetry-broken charge transfer (SB-CT) excitonic states, delocalized over a number of PBI units. After excitation, there is an evident sharpening of the line-widths accompanied by partial peak-shifts over 0.8 ps timescale, suggesting electronic relaxation towards a more localized lower-energy Charge Separated (CS) state. After the localization of the charges into CS states, the Q-1 and Q-2 complexes follow different excited-state pathways. The Q-1 undergoes bi-exponential recombination, involving photo-generated PBI+ and PBI-, with 80 ps and 1 ns lifetime components. In Q-1, the quantitative SB-CS is followed by long-lived charges with an unprecedented k_CS/k_CR ratio with respect to geminate rylene-based systems in literature [2]. On the contrary, the Q-2 undergoes 8 ps hole transfer from PBI+ to POM which is followed by bi-exponential recombination, involving photo-generated POM+ and PBI-, with 60 ps and >1 ns lifetime components. In this case, the presence of long-lived charges in the catalytic sites can lead to efficient water splitting.
To investigate the mechanism in QS structures leading to long-lived charges, we performed TA Anisotropy measurements. Anisotropy is sensitive to the relative polarization displacement of the photo-generated charges. We track the anion motion due to the anisotropy of the signal detected at the anion ESA peak (820 nm). The prominent depolarization for Q-1 and Q-2 complexes, suggests that the charges are not localized in the space they were created after photoexcitation, and instead they rapidly move in the 3D space [3]. The overall high anisotropic depolarization denotes that charge pairs in QS undergo an efficient dissociation and have a high mobility, consistent with the observation of long-lived CS states.
Hence, the ability to control the outcome application of the long-lived charges can be obtained through the suitable choice of the guest catalyst in the Quantasome-like self-assembly organization.
References
[1] Bonchio, M. et al. Nat. Chem. 11, 146–153 (2019).
[2] Spenst, P., Young, R. M., Wasielewski, M. R. & Würthner, F. Chem. Sci. 7, 5428–5434 (2016).
[3] Moia, D. et al. J. Am. Chem. Soc. 138, 13197–13206 (2016).

The field of organic photovoltaics (OPV) has undergone a resurgence in recent years owing to the swift increase in efficiency values of the solar cells. Highly efficient OPV systems demand a concomitant increase in the short-circuit current density (j_SC) and the open-circuit voltage (V_OC). In order to optimize the V_OC losses, majority of the high-efficiency OPV systems employ donor:acceptor combinations with near-zero energetic offset for the interfacial charge transfer reaction. In our study, we have investigated a blend of a low optical-gap diketopyrrolopyrrole (DPP) polymer and a fullerene derivative: PC[70]BM, with a near-zero driving force of ~50 meV for the interfacial electron transfer. In order to probe a sequence of events that ultimately lead to the formation of free charges, we employ a series of complementary spectroscopic and theoretical tools. Using femtosecond transient absorption (TA) and electroabsorption (EA) spectroscopy, we quantify the charge transfer (CT) and recombination dynamics and as well as the charge transport at early timescales. Firstly, we observe unusually short S₁ and CT state lifetimes in the investigated system (~13-14 ps) and significant geminate charge recombination (gCR). Secondly, electron transfer is found to be ultrafast with a time constant of ~ 240 fs. The finding of ultrafast electron transfer was found to be consistent with a semiclassical Marcus-Levich-Jortner (MLJ) description at low driving force and low reorganization energy (λ_reorg.). At low S₁-CT offset, a short-excited state lifetime mediates charge recombination because i) back-transfer from the CT to the S₁ state followed by S₁ recombination can occur and ii) additional S₁-CT hybridization can decrease the CT lifetime as a result of intensity borrowing effects. Both effects were confirmed by density functional theory (DFT) calculations. In addition, we employed time-resolved electromodulated differential absorption (EDA) spectroscopy to probe the transport of charge carriers on a sub-nanosecond time scale. We observed relatively slow dissociation of charges (tens of picoseconds) from the interfacial CT state, in contrast to polymer:fullerene blends with high CT driving force. We identify low local charge carrier mobility as a primary reason for the slow rise of the free charge population. Lastly, we performed simulations using a four-state kinetic model using Gaussian manifold of states and thus entailing the effects of energetic disorder (σ). These simulations enabled us to discern the role of interfacial CT state disorder (σ_CT) and the energetic disorder in the bulk of the film (σ_bulk). Firstly, the simulations reveal that the free charge yield could be increased from the observed 12% to 60% by increasing the S₁ and CT lifetimes to ∼ 150 ps. Alternatively, by decreasing the interfacial CT state disorder (σ_CT) and increasing the bulk disorder (σ_bulk) the yield of free charge generation can be enhanced to 65% despite short excited state lifetimes.

The development of new molecular and macromolecular materials for organic light emitting devices (OLEDs) and organic solar cells (OSCs) is constantly evolving thanks to new insights into the interactions between light and matter. There is a great deal of work currently underway towards harnessing useful excited state properties of organic molecules and polymers such as room temperature phosphorescence,[1] thermally activated delayed fluorescence,[2] and singlet fission [3], making use of intermolecular interactions between individual donor and acceptor molecules which can yield efficient luminescence and simple devices.
The influence of three-dimensional molecular topology on the photophysical and electrochemical behaviour of organic semiconducting materials can be truly defining in how well such a molecule performs in any given application. Small changes in, for example, the dihedral angle between two rings in a molecule can make the difference between efficient or poor luminescence, charge transport, processability etc.[4-5]
Structure property relationships which take consideration of molecular geometry in three dimensions are required.
Here, the results of studies on new organic molecules which have been designed to make use of twisted, contorted or otherwise three-dimensionally defined structures will be presented. Using such 3D structures allows us to exploit or enhance photophysical processes towards obtaining higher efficiencies in organic electronic applications.
Particular focus will be given to the exploitation of homoconjugation – the through-space overlap of molecular orbitals – on the enhancement of defining photophysical parameters of new donor-acceptor compounds for use in OLEDs and OSCs. [6]

References:

[1] B. Xu, H. Wu, J. Chen, Z. Yang, Z. Yang, Y-C. Wu, Y. Zhang, C. Jin, P-Y. Lu, Z. Chi, S. Liu, J. Xu and M. P. Aldred, Chem. Sci., 2017 8, 1909
[2] M. Zhang, C. J. Zheng, H. Lin and S.L. Tao, Mater. Horiz., 2021, 8, 401
[3] T. Ullrich, D. Munz, D. M. Guldi; Chem. Soc. Rev., 2021, 50, 3845
[4] R. Huang, J. S. Ward, N. A. Kukhta, J. Avó, J. Gibson, T. Penfold, J. C. Lima, A. S. Batsanov, M. N. Berberan-Santos, M. R. Bryce and F. B. Dias, J. Mater. Chem. C, 2018, 6, 9238
[5] I. A. Wright, A. Danos, S. Montanaro, A. S. Batsanov, A. P. Monkman, M. R. Bryce, Chem. Eur. J., 2021, 27, 6545
[6] S. Montanaro, D. G. Congrave, M. K. Etherington, I. A. Wright, J. Mater. Chem. C, 2019, 7, 12886

Radical organic semiconductors have unpaired electrons, leading to unusual physics that can be utilised in next-generation optoelectronics. High stability and luminescence can be achieved by molecular design, thus taming the typical reactivity of radical systems. In particular, emitters based on the tris(2,4,6-trichlorophenyl)-methyl (TTM) radical moiety have recently been shown to fluoresce within the spin-doublet manifold with near-unity internal quantum efficiency. The ground state open-shell electronic structure, in combination with strong light absorption and emission, makes these materials promising for organic light-emitting diodes and photovoltaics with eliminated triplet losses. The integration of radicals into devices requires a detailed understanding of their fundamental energy and electron transfer mechanisms.

Here, a model system of closed-shell energy donor and open-shell acceptor is used to study energy transfer into radical materials via time-resolved optical spectroscopy, and signatures of direct triplet to doublet energy transfer are observed. In combination with temperature-dependent time-resolved photoluminescence measurements and modelling, these results allow us to propose the mechanism and yield for this process. Additionally, we present new insights into the photophysics of TTM derivatives on picosecond timescales. This work demonstrates a new method of harvesting dark triplet states, enabling lowering efficiency losses in optoelectronic devices.

Investigations of photovoltaic devices and semiconductors are essential to enhance the efficiency of preparation methods as well as their electronic and optical properties. Different parameters define these properties, e.g., number of defects and trap states, interface interactions, energy and electron transfer behavior and, of course, absorption properties and response to photon stimulation.

Surface characterization of such materials is usually done with scanning electron, scanning tunnel and atomic-force microscopes, which can gather information about homogeneity, conductivity, carrier mobility and defect center of films, semiconductors or within devices. But there still is a lack of information about important parameters that are necessary for understanding and optimizing the preparation and efficiencies of these materials.

We present here how combining time-resolved laser scanning microscopy – offering a broad range of techniques – with a spectrometer results in a valuable and powerful toolbox. This combination of microscopic (e.g., FLIM, PLIM, fast switch between widefield and confocal resolution, or carrier diffusion measurements) and spectroscopic methods (such as time-resolved photoluminescence or wavelength dependent emission scanning) allows investigating the photophysical properties of semiconductors, nanoparticles, QDs, polymers, solid-states as well as nanostructures on a whole new level. This additional information enables gaining a deeper understanding of both photophysical processes as well as structure-property relationships, which will help for the optimization of properties and efficiencies in practical applications.

The energetic interaction between low-temperature plasma and the surface of materials produces a phenomenon commonly refer as plasma heating. This phenomenon, which plays a critical part in many plasma-driven processes [1], is very difficult to characterize and therefore poorly understood. Low-temperature plasma is used to activate several important surface reactions, since it can accelerate the surface kinetics, leading to processes such as reactive ion-etching, atomic layer deposition and heterogeneous chemistry, among others. One interesting example is the heat induction due to the recombination of atomic hydrogen at the surface nanoparticles in dusty plasmas to synthetized refractory materials [2]. However, the low rate of heat dissipation from the particles to the background gas, leads to a significant temperature increment and therefore, leaving unknow the real contribution of plasma heating into the temperature profile.
In this work, we present a Raman spectroscopic approach to quantify plasma-induced heating on a two-dimensional material as a surface-sensitive temperature probe. We use graphene because it is atomically thin, and it has a well characterized Raman signature. In addition, is well known that graphene has been implemented into a variety of fields such as sensing, graphene-based catalysis or electrochemical energy conversion and others [3]. The temperature of the plasma- exposed graphene surface is obtained by Raman thermometry characterization [4]. We simultaneously measure the signal intensity of the Stokes and anti-Stokes G Raman band, while plasma impinging in the surface. The measurements are calibrated bringing the graphene temperature up 200 °C, by means of a vacuum chamber-heater, to increase the thermal population of the vibrational modes and therefore enhance the anti-Stokes signal, which is significantly low compared to the Stokes signal at room temperature. When graphene is exposed to a radiofrequency (RF) argon plasma, its temperature raises to approximately 350 °C and its linearly dependent on the plasma input power. We strength our experimental results using a heat transfer model in COMSOL, where we discard the possibility of temperature increment due to gas heating by the plasma. In addition, we find a heating dependence on the plasma composition, where hydrogen-diluted argon decreases the surface temperature as hydrogen concentration increases in exponential-like behavior. Finally, we show evidence of anisotropic etching by hydrogenation, increasing disorder in the material structure and reducing the size of graphene domains. This study highlights the potential of two-dimensional materials, particularly graphene, to understand better plasma related phenomena.

[1] G. S. Oehrlein and Y. H. Lee, Reactive ion etching related Si surface residues and subsurface damage: Their relationship to fundamental etching mechanisms. J. Vac. Sci.Technol., A5, 1585 (1987).
[2] L. Mangolini and U. Kortshagen, Selective nanoparticle heating: Another form of nonequilibrium in dusty plasmas, Phys.Rev. E79, 026405 (2009).
[3] K. M. F. Shahil and A. A. Balandin, Thermal properties of graphene and multilayer graphene: Applications in thermal interface materials. Solid State Commun.152, 1331(2012)
[4] A. G. Souza Filho, A. Jorio, J. H. Hafner, C. M.Lieber, R. Saito, M. A. Pimenta, G. Dresselhaus, and M. S. Dresselhaus, Electronic transition energy Eiifor anisolated (n,m) single-wall carbon nanotube obtained by antiStokes /Stokes resonant Raman intensity ratio, Phys.Rev. B63, 241404 (2001).

Carbon-based luminescent nanoparticles, which are usually referred to as carbon dots (CDs), are becoming increasingly popular for light-emitting applications due to their attractive optical properties and low-cost synthesis. CDs exhibit strong excitation-wavelength-dependent and -independent photoluminescence (PL) emission that can be tuned throughout the visible spectrum. However, their structure and the origin of their PL emission is still debated. In this study, CDs are synthesized by a solvothermal method using citric acid and 1,5-diaminonaphthlene, and X-ray photoelectron spectroscopy (XPS), UV-visible absorption, PL emission, Fourier transform infrared (FTIR), and Raman spectroscopies are used to investigate their structure and the origin of PL emission. The average size of the purified CDs is 19.5 nm with the majority of particles in the size range of 10 nm to 25 nm. XPS shows that the CDs also contain oxygen (31%) and nitrogen (9%) in addition to carbon (60%). FTIR and nuclear magnetic resonance (NMR) spectroscopy analysis show that the CDs are composed of aromatic as well as aliphatic carbon.
To investigate the photonic properties of the CDs, PL emission from the CDs is studied using different excitation wavelengths in different water-miscible solvents. Two distinct excitation-wavelength-independent PL emission bands are observed from the CDs: one deep violet emission band at 400 nm and another green emission band at ~520 nm. The PL quantum yield (QY) varies from 3.5% to 9.4 % with polarity of the solvents. Time-resolved PL (TRPL) emission measurements reveal that the CDs have short PL lifetime (≤ 8 ns) with a double exponential decay. The excitation-wavelength-independent PL emission, and the short PL lifetime imply that the CDs are likely to be composed of molecular clusters instead of graphitic crystals. Double exponential decay suggests that the CDs may be composed of two different types of fluorophores. The structure and optical properties of individual fluorophores are studied for the first time by dissolving the CDs in low-polarity, water-immiscible solvents, and separating the green-emitting molecules by solvent extraction. The PL QY of the green-emitting molecules after separation increases to 53.6%, and TRPL emission of these molecules decays with a single exponential with a lifetime of 5.8 ns. The structure of the green-emitting molecules is determined by using 1D (¹H and ¹³C) and 2D (¹H-¹H correlation spectroscopy, ¹H-¹³C heteronuclear multiple quantum coherence and ¹H-¹³C heteronuclear multiple bond correlation) NMR spectroscopies, which reveal that the molecules mainly have aromatic structure with pyridinic nitrogen doping. Finally, time-dependent density functional theory (TD-DFT) calculations are carried out to further study the excited states in the green-emitting molecules and to confirm that the green emission arises from the same molecules. From this study, it is concluded that the CDs are mainly composed of molecular fluorophores and that PL arises from individual molecules within the CD clusters.

With a 1.8 eV band gap in the red region of the visible spectrum, bismuth oxyiodide (BiOI) may be viable as a top absorber in tandem-junction photovoltaics or a non-toxic photocatalyst for degradation of environmental pollutants. Viability requires understanding the interplay between interfacial chemical states, ambient atmosphere, and the resulting dynamics of photoexcited carriers in this 2D layered material. We synthesized single-crystal BiOI(001) via vapor transport and explored the behavior of optically excited carriers using time-resolved THz spectroscopy (TRTS) and time-resolved photoluminescence spectroscopy (TRPL). Under inert environments or chemical passivation, above-band-gap excitation with UV or blue light yields free carriers with mobilities that rival crystalline silicon, and hundreds-of-picosecond lifetimes that recombine with both band-edge and defect-related emission. In contrast to in vacuo behavior, PL intensities show a rapid and irreversible attenuation with increasing exposure to the 485 nm excitation light. We discuss the results in the context of BiOI for energy harvesting and sensing applications, and highlight pathways for chemical passivation for stable, efficient operation under illumination in air.

Light trapping in arrays composed of subwavelength light funnel arrays (LF arrays) is a promising approach towards efficient broadband absorption of the solar radiation and surface arrays of subwavelength structures has an additional advancement towards ultra-thin photovoltaic (PV) cells. In the following we examine the origin of light trapping enhancement in LF arrays as compared to nanopillar (NP) arrays and we show that light trapping enhancement is due to favorable strong optical coupling between adjacent light funnels which is not realized in NP. We suggest that the enhanced light trapping and absorption in dense LF arrays is governed by strong modal excitation coupled with high filling ratio, unlike the absorption in dense optimized nanopillar arrays which is governed by weak modal excitation and high filling ratio. Finally, we make the distinction between two types of optical overlap: weak overlap in which the coupling between the sparse array modes and the impinging illumination increases with array densification, and strong overlap where the array densification introduces new highly absorbing modes. Finally, the study of the resonant behaviour of silicon non-imaging light concentrators (NLCs) provide a new route for achieving efficient control of both electric and magnetic components of light. We use near-field scanning optical microscopy (NSOM) to measure the near-field light intensity as function of array geometry.

Semiconductor nanocrystals (NCs) exhibit bright, narrow, and size-tunable emission across the visible and infrared spectrum with large absorption cross-sections and high quantum yields. A recent use for NCs gaining interest is triplet fusion based incoherent photon upconversion (TUC), which is a rising strategy to generate emissive spin-singlet states from two spin-triplet excitons. While TUC motifs have found success in biomedical imaging applications and have recently been employed to initiate photochemical transformations, an understanding of sensitization mechanisms is required.

Here NCs are promising sensitizers due to their small exchange splitting and facile surface functionalization routes. These factors allow for minimal energy loss and direct injection of triplets to surface bound transmitters, along with the ability to load >1 transmitter per NC. In the archetypal core-only CdSe/9-anthracenecarboxylic acid (9-ACA), the role of the commonly observed trap during triplet sensitization remains unclear, as do the energetics of this surface-oriented trap state.

Here, we use time-resolved photoluminescence to probe energy transfer in hybrid CdSe/10-(4-methoxyphenyl)anthracene-9-carboxylic acid (MeOPh-ACA) systems. We observe rapid transfer from both band-edge and trap states and show that transfer occurs from states which would appear to be thermodynamically uphill if their photon energy was a faithful proxy of chemical potential. Notably, we observe comparable quenching from band-edge and trap states, with similarly rapid kinetics. Further, we synthesize MeOPh-ACA, a 9-ACA derivative with stabilized frontier orbitals with respect to bare 9-ACA. When paired with a sufficiently small (<2 nm) CdSe NC sensitizer, both Dexter-like correlated energy transfer and sequential charge transfer are thermodynamically favourable mechanisms to sensitize the spin-triplet state on the organic ligand. Even with this new available pathway, we observe no change in the rate of energy transfer, while continuing to observe comparable quenching from band-edge and trap states. Finally, we find that these trap states are not pathological for TUC, and observe upconverted photon emission when our system is paired with free-floating annihilator.

Our results suggest that the deeply Stokes-shifted photoluminescence from these nanocrystals arises from energetically shallow surface-oriented states which localize the exciton shortly after irradiation, but that selection rules for exchange-mediated transfer qualitatively differ. Our observation of commensurate quenching from band-edge and trap states suggests that the band-edge and trap band are in rapid thermal equilibrium and supports the hypothesis that traps in CdSe NCs are 'shallow' i.e., of comparable energy to the band-edge. Taken together, these observations have the practical implication that the trap band can be used as a surrogate to monitor the population of band-edge excitations. These findings indicate that surface traps in core-only CdSe NCs may not hinder energy transfer during triplet sensitization and that the trap photon energy is not an honest representation of its free energy.

Accurate description of finite-temperature vibrational dynamics is indispensable in the computation of two-dimensional electronic spectra. Such simulations are often based on the density matrix evolution, statistical averaging of initial vibrational states, or approximate classical or semiclassical limits. While many practical approaches exist, they are often of limited accuracy and difficult to interpret. Here, we use the concept of thermo-field dynamics to derive an exact finite-temperature expression that lends itself to an intuitive wavepacket-based interpretation. Furthermore, an efficient method for computing finite-temperature two-dimensional spectra is obtained by combining the exact thermo-field dynamics approach with the thawed Gaussian approximation for the wavepacket dynamics, which is exact for any displaced, distorted, and Duschinsky-rotated harmonic potential but also accounts partially for anharmonicity effects in general potentials. Using this new method, we directly relate a symmetry breaking of the two-dimensional signal to the deviation from the conventional Brownian oscillator picture.
[1] T. Begušić and J. Vaníček, J. Phys. Chem. Lett., 12, 2997 (2021).
[2] T. Begušić and J. Vaníček, J. Chem. Phys. 153, 184110 (2020).
[3] T. Begušić and J. Vaníček, J. Chem. Phys. 153, 024105 (2020).

Understanding the photogenerated charge carrier transport in semiconductors is crucial, particularly for applications toward high-efficiency photovoltaic cells and photosensors. Scanning ultrafast electron microscopy (SUEM) is an emerging technique that can visualize photocarrier transport by measuring the spatial-temporal change of secondary electron emission as a result of photoexcitation. Despite the increasing demonstrations of SUEM capability, the physical picture of the image contrast mechanisms of SUEM remains incomplete. In this work, we implement Monte Carlo simulation based on the Mott elastic scattering cross section and Landau’s theory of statistically distributed energy loss for inelastic collision; we simulate the secondary electron yield (SEY) of doped silicon under photoexcitation. Photoexcitation will induce both photocarriers in the bulk and the surface photovoltaic voltage (SPV) effect that modifies the surface band bending. Photoexcited carriers result in the change of the inelastic scattering cross section which is determined by the electron energy loss functions (ELF). Time-dependent density functional theory (TDDFT) is applied to evaluate the ELF for both n-type and p-type Si with different doping concentrations and transient electronic temperatures, but only marginal changes of SEY are observed. In contrast, SPV effect changed the surface transmission probability for secondary electrons, and SEY of n-type Si had a 3% ~5% increase while p-type Si had a 3%~5% decrease. This study provides a detailed understanding of SUEM image contrasts and lays the foundation for precise interpretation of ultrafast electron microscope images.

This work is based on research supported by the Army Research Office Young Investigator Program under the award number W911NF-19-1-0060.

Transparent electrodes for electronic devices have to unique properties such as transmittance of more than 80% in visible light and sheet resistance of less than 10³Ω. ITO (Indium Tin Oxide) is the TCO (Transparent Conductive Oxide) used in almost transparent electrodes. ITO uses indium, which is a rare earth, and has the drawbacks of high cost and low mobility.[1] Consequently, the many research of TCO like ZnO, STO (SrTiO₃), BSO (BaSnO₃) to replace ITO were being studied. LBSO (La_xBa_x-1SnO₃) is a transparent conductive oxide candidate to replace ITO due to its wide bandgap of 4.05 eV, high transmittance in visible light, mobility, and carrier density. [2], [3] For high crystallinity, these properties of LBSO required a vacuum process that had drawbacks such as high cost, low deposition rate, shadow effect, etc. Thus, a thin film deposition method using a non-vacuum process has been studied, but there are problems in depositing several layers and having lower performance compared to vacuum processes [4] High resistance of thin film using non-vacuum process can be improved by electroless silver plating, and it is confirmed that silver nano particle increase conductivity. [5] In this research, using non-vacuum process improve of low deposition rate and high equipment cost in vacuum process, and overcome low thin film quality and high sheet resistance in non-vacuum process deposition method through electroless silver plating. It has been verified that LBSO thin films deposited using non-vacuum process have a sheet resistance of less than 10³ Ω, a work function of more than 5 eV, and a transmittance of more than 80% in visible light. The above-mentioned LBSO can also open new ways toward intrinsically next-generation transparent electrodes to replace ITO.
[1] Terzini, E., P. Thilakan, and C. Minarini. Materials Science and Engineering: B 77.1 (2000): 110-114.
[2] Luo, X., et al. Applied Physics Letters 100.17 (2012): 172112.
[3] Sanchela, Anup V., et al. Journal of materials chemistry C 7.19 (2019): 5797-5802.
[4]. Wang, Zhaokui, et al. ACS applied materials & interfaces 3.7 (2011): 2496-2503.
[5] Babaahmadi, Vahid, Majid Montazer, and Wei Gao. Carbon 118 (2017): 443-451.
This research was funded and conducted under the Competency Development Program for Industry Specialists of the Korean Ministry of Trade, Industry and Energy (MOTIE), operated by Korea Institute for Advancement of Technology (KIAT). (No. P0012453, Next-generation Display Expert Training Project for Innovation Process and Equipment, Materials Engineers)

The development of new photoluminescent (PL) materials with simultaneous room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) features is highly desirable for bio-imaging, security applications and sensors due to the involvement of both singlet and longer lived triplet states.¹ In this work we have systematically designed two multiple donors and single acceptor based molecular systems which can simultaneously harness the triplet state via two different channels. We designed the molecules by taking cue from the idea that the free lone pair of electron containing phenoxy donors when linked to the phthalonitrile (PN) acceptor gives rise to RTP emission. When carbazole connects angularly with PN acceptor, it give rise to TADF emission. In this work we have connected both the donors to the single acceptor PN. Here we discuss the photophysical studies of two carbazolyl-phenoxy-phthalonitrile conjugates (CPPN, CPPNF).² The detailed spectroscopic studies of CPPN and CPPNF along with two daughter compounds of phenoxy-phthalonitrile (PPN, PPNF) in polar and non-polar hosts confirmed efficient blue-RTP from the higher-energy locally excited (LE) triplet state (T_PPN) due to the phenoxy-phthalonitrile (PPN) part. While the blue-TADF was observed via the reverse intersystem crossing from the low-lying charge transfer triplet state (T_CzPN) of the carbazolyl-phthalonitrile (CzPN) part to the singlet (S₁) charge transfer state of the same CzPN part. Moreover, it was observed that the higher lying LE triplet state (T_PPN) acts as an intermediate for spin–vibronic coupling^3,4 to cause the reverse intersystem crossing. Such PL characteristics are observed due to the energetic proximity of ³LE_PPN, ¹CT_CzPN and ³CT_CzPN.⁵ In the hydrogen-bonded matrix and crystals, we found faint persistent green-RTP characteristics of the PPNF due to supramolecular interactions and aggregation of the molecule. This study paves the way to understand the involvement of different excited states associated with TADF and RTP processes of asymmetric-donor–acceptor systems.

References:
1. Data, P.; Takeda, Y.; Chem. Asian J. 2019, 14, 1613-1636.
2. Bhatia, H.; Ray, D.; Mater. Adv. 2020, 1, 1858-1865.
3. Chen, X, -K.; Kim, D.; Bredas, J. -L. Acc. Chem. Res. 2018, 51, 2215-2224.
4. Etherington, M. K.; Gibson, J.; Higginbotham, H. F.; Penfold, T. J.; Monkman, A. P. Nat. Commun. 2016, 7, 13680.
5. Samanta, P. K.; Kim, D.; Coropceanu, V.; Bredas, J. -L. J. Am. Chem. Soc. 2017, 139, 4042-4051.

The performance of Organic Light-Emitting Diode (OLEDs) has considerably improved with the discovery of Thermally-Activated Delayed Fluorescence (TADF) materials that can harvest dark triplet excitons. In TADF molecules, the triplet excitons are transferred to emissive singlet excitons by a process called reverse intersystem crossing (rISC). However, the collection of triplet excitons remains a challenge for OLEDs that suffer from a poor efficiency at high current (roll-off). Indeed, the drop of the performance is mainly due to polaron-exciton deactivation and exciton-exciton annihilation coming from the high density of triplet excitons and their relatively long lifetime. In order to overcome that problem, the rISC process can be faster in order to decrease the density of triplet excitons to limit their interactions. It is shown that this process can be enhanced by a stronger spin-orbit coupling, generated by a heavy-atom effect in the host molecules.

Derivatives of 1,3-Bis(N-carbazolyl)benzene (mCP) were synthesized by adding bromine or iodine atoms in order to increase the heavy-atom effect in the mCP host. It results in a greater phosphorescence of the host due to a better transfer between singlet and triplet states. Then, photophysical studies were carried out on different TADF guest molecules. The lifetime of the excitons has been reduced while keeping a high photoluminescent quantum yield when using halogenated molecules as host and the rate of rISC has been multiplied by a factor of 10. The use of spin-orbit coupling through the heavy-atom effect in the host molecule could be a solution to the strong roll-off and degradation observed in OLED devices.

Silver phenylselenolate (AgSePh) is an emerging hybrid organic-inorganic two-dimensional (2D) semiconductor due to its unique properties such as narrow blue emission, in-plane anisotropy and high exciton binding energy. To expand an application of this novel 2D semiconductor, a systematic method for precisely controlling its band gap, which determines electronic and optical properties, is required. One of the simplest and the most effective way to tune the band gap is making an alloy and controlling its composition. Herein, we show that the chemical transformation reaction between silver film and the mixture of diphenyl diselenide (Ph₂Se₂) and diphenyl ditelluride (Ph₂Te₂) vapors produces 2D silver phenylchalcogenolate alloy (AgSe_nTe_1-nPh) thin films with controllable Se to Te ratio depending on partial pressure of Ph₂Se₂ and Ph₂Te₂ vapors during reaction. Systematic structural and optical characterization to confirm tunable structural and optical properties as well as homogeneity of alloys will be presented. Furthermore, the emission mechanisms of AgSe_nTe_1-nPh alloy films depending on chalcogen ratio will be discussed. This work provides an insight to design novel 2D hybrid organic-inorganic semiconductors with desirable optical properties.

Monitoring the viscosity of polymers in real-time remains a challenge, especially in confined environments where traditional rheological measurements are hard to apply. In this study, we have utilized [Cu(diptmp)₂]⁺ (diptmp = 2,9-diisopropyl-2,4,7,8-tetramethyl-1,10-phenanthroline) as an optical probe for real time sensing of viscosity in various adhesives during the curing process (viscosity increases) via changes in luminescence. The emission lifetime of the triplet metal to ligand charge transfer (³MLCT) state of [Cu(diptmp)₂]⁺ in epoxy adhesive increased exponentially similar to viscosity values obtained from oscillatory rheology. The longer lifetime in higher viscosity materials was attributed to changes in the excited state deactivation processes from a known Jahn-Teller distortion in the Cu(I) geometry from tetrahedral in the ground state to square planar in the excited state. The real-time viscosity was also monitored reversibly by emission lifetime during polymer swelling (viscosity and lifetime decrease) and unswelling (viscosity and lifetime increase). Monitoring emission lifetime, unlike absorption lifetime in our previous study, allowed us to measure viscosity in opaque samples which scatter light. The optical probe [Cu(diptmp)₂]⁺ in Gorilla Glue^TM adhesive, showed a clear correlation of emission intensity or lifetime to viscosity during the curing process. We have also compared these lifetime changes using [Ru(bpy)₃]²⁺ (bpy = bipyridine) as a control. [Cu(diptmp)₂]⁺ showed not only higher emission lifetime but also more ubiquity as a real-time viscosity sensor.

Studying charge transport in emergent semiconducting materials is particularly challenging as structural inhomogeneities in the material can play an influential role, leading to dynamics that require resolution of sub-diffraction limited length-scales and femtosecond time-scales. Transient photoluminescence (PL) microscopy can provide the spatial and temporal resolution required to image diffusion of excitons and free charge carriers in optoelectronic materials. In excitonic materials, extraction of key material parameters such as diffusion coefficients can be simplified as the bound electron and hole move together. However, in materials where transport is dominated by free charge carriers, extracting diffusivities accurately from this multi-dimensional data requires good physical models coupled with statistically robust fitting algorithms. In this work, I present a detailed numerical framework for modeling the free charge carriers in transient PL microscopy experiments, and provide model and parameter analyses using a Markov Chain Monte Carlo sampler as part of the fitting algorithm. This approach is applied to lead-halide perovskites and transition metal dichalcogenides to obtain statistically confident estimates of ambipolar charge carrier diffusivities for these materials.

Nanostructured metal oxides such as TiO₂ or ZnO represent state-of-the-art photocatalysts for water splitting and waste treatment. While highly efficient, they are limited by the need for ultraviolet illumination. Recently, we have developed a simple one pot approach for fabrication of nanostructured titanium carbo-oxide TiC_1-xO_1+x (x=0.25-0.5) which is photocatalytically active under visible excitation. Here we investigate the photophysical properties of TiC_1-xO_1+x films using time-resolved photoluminescence spectroscopy. UV-VIS spectroscopy shows an indirect band gap at ~ 4.1 eV, higher than that of TiO₂, and sub-gap states that absorb in the visible range. Excitation with below band gap, 485 nm light results in a broadband optical emission centered around ~ 2.1 eV. Its decay is bi-exponential, with the ~ 1ns fast component in all studied films, and the slower component that varies in 5-12 ns range depending on synthesis duration. We hypothesize that radiative recombination at the nanostructure surfaces is responsible for the slower process, with longer processing yielding smaller nanostructures and higher surface recombination rates. Lifetime of optically excited carriers has major implications for photocatalysis and other applications, and we demonstrate here that it can be tuned it during one-pot titanium carbo-oxide nanostructure synthesis.

I will describe our transient optical approaches to directly image the nanoscale transport dynamics of various types of quasiparticles in a wide range of hierarchical semiconducting and conducting materials. For example, by characterizing the mean squared expansion of initially localized charge carriers, bound electron-hole pairs, heat, and sound, we elucidate the impact of various material heterogeneities on electronic and thermal transport and also the interplay between heat and charge distributions.

Charge-transfer (CT) excitons at hetero-interfaces play a critical role in light to electricity conversion using nanostructured materials. However, how CT excitons migrate at these interfaces is poorly understood. Atomically thin and two-dimensional (2D) nanostructures provide a new platform to create architectures with sharp interfaces for directing interfacial charge transport. Here we investigate the formation and transport of interlayer CT excitons in van der Waals (vdW) heterostructures based on semiconducting transition metal dichalcogenides (TMDCs) employing transient absorption microscopy (TAM) with a temporal resolution of 200 fs and spatial precision of 50 nm.
We have investigated interlayer exciton dynamics and transport modulated by the moiré potentials in WS₂-WSe₂ heterobilayers in time, space, and momentum domains using transient absorption microscopy combined with first-principles calculations. Experimental results verified the theoretical prediction of energetically favorable K-Q interlayer excitons and unraveled exciton-population dynamics that was controlled by the twist-angle-dependent energy difference between the K-Q and K-K excitons. Spatially- and temporally-resolved exciton-population imaging directly visualizes exciton localization by twist-angle-dependent moiré potentials of ~100 meV. Exciton transport deviates significantly from normal diffusion due to the interplay between the moiré potentials and strong many-body interactions, leading to exciton-density- and twist-angle-dependent diffusion length. These results have important implications for designing vdW heterostructures for exciton and spin transport as well as for quantum communication applications.
We have also imaged the transport of interlayer CT excitons in 2D organic-inorganic vdW heterostructures constructed from WS₂ layers and tetracene thin films. Photoluminescence (PL) measurements confirm the formation of interlayer excitons with a binding energy of ~ 0.3 eV. Electron and hole transfer processes at the interface between monolayer WS₂ and tetracene thin film are very rapid, with time constant of ~ 2 ps and ~ 3 ps, respectively. TAM measurements of exciton transport at these 2D interfaces reveal mobile CT excitons, with diffusion constant of ~ 1 cm²s^-1. The high mobility of the delocalized CT excitons could be the key factor to overcome large CT exciton binding energy in achieving efficient charge separation.

Using time-space resolved ultrafast microscopy on 2D materials or single-crystal singlet-fission crystals, we show how long-range interlayer electronic coupling can be selectively enhanced either by applying an E-field or by twisting the layer stacking orientation. In both cases, transient interlayer exciton states form and drive the optoelectronic material response. Considering first twisted bilayer graphene (tBLG), we discovered how stacking-angle tunable absorption resonances form a strongly-bound exciton state owing to the symmetrized rehybridization of the interlayer 2p orbitals. Using two-photon photoluminescence and intraband-transient absorption microscopies, we have recently imaged the photoemission and exciton dynamics from single-grains of tBLG. After resonant excitation, our results suggest the formation of strongly bound (up to 690 meV), metastable interlayer exciton states. Our observation of resonant PL emission from twisted bilayer graphene materials is best explained by the theoretically predicted coexistence of strongly bound interlayer excitons and metallic graphene continuum states. We show how stacking angle-tunable interlayer excitons states may permit new photocurrent extraction routes from quasi-stable interlayer excitons.

Unlike stacked graphene, semiconducting 2D transition metal dichalcogenides (TMDCs) have diffuse interlayer d-orbital overlap. To enhance interlayer electronic coupling in TMDCs, we apply an interlayer-directed E-field, inducing electron-hole dissociation. Time-resolved photocurrents show that stacked WSe₂ devices can have both IQE >50% and fast (<60 ps) picosecond electron escape times. Our ultrafast photocurrent rates kinetics give the same E-field-dependent electronic escape and dissociation rates seen from optical ultrafast microscopy. To rationalize these fast electronic escape rates, we show the ratio of the electronic rates accurately predicts the actual WSe₂ device photocurrent generation efficiency. Collectively, we show how optical and photocurrent-based ultrafast microscopies together provide an analytical extraction of the photocurrent-generating ultrafast dynamics in van der Waals stacked materials and organic singlet fission materials.
References
[1] H. Patel, L. Huang, C.J. Kim, J. Park, M. W. Graham, Stacking Angle-Tunable Photoluminescence from Interlayer Exciton States in Twisted Bilayer Graphene, Nature Comm, 10, 1445 (2019).
[2] K. T. Vogt, S.-F. Shi, F. Wang, M. W. Graham, Ultrafast photocurrent and absorption microscopy of few-layer TMD devices isolate rate-limiting dynamics driving fast and efficient photoresponse, J Phys Chem C, 124, 28, 15195–15204 (2020)

The impact of annealing at different temperatures and times on the morphology, composition, the transformation of crystal phases, and emission of Si-rich-HfO₂ films prepared by radio-frequency magnetron sputtering and doped with the rare-earth Pr element has been investigated. The follows methods: scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Raman scattering and photoluminescence (PL) have been used. Thermal annealing was carried out at 800, 900, 1000 and 1100C for 30 min or for times 5, 15, 20 and 60 min at 1000^oC in nitrogen atmosphere.
The films are characterized by the fine grain structures with the maximum grain size of 40nm. The composition of the films varied significantly at annealing with the oxidation increasing at high annealing temperatures. The annealing causes the transformation and separation of the crystal phases from the hafnia silicates at low annealing temperatures to the tetragonal HfO₂ and SiO₂ phases detected after high temperature (1000 or 1100^oC) treatments. The great variety of the visible and infrared PL bands related to the optical transitions in the 4f inner shells of the Pr³⁺ ions and via native host matrix defects were detected in the PL spectra. Their contribution depends on the annealing temperature and governs the shape of total PL spectra. The impact of crystalline phase transformation at annealing on the peculiarities of the PL excitation and emission spectra of rare-earth Pr³⁺ ions are analyzed and discussed.

The conventional high-speed semiconductor-based photodetectors involve limited wavelengths of operation and usually incorporate complex and expensive fabrication. Vanadium dioxide is a phase change material that undergoes an insulator to metal transition near room temperature. This material has drawn attention to broadband optoelectronic applications due to its unique band structure and versatile electronic and optical properties. However, vanadium dioxide-based photodetectors reported so far involve complex structures that lack scalability and constrained wavelength response similar to commercial photodetectors. Herein, two-terminal planar devices based on vanadium dioxide thin film are presented where devices were fabricated by using standard micro-/nano-fabrication. To enhance the photoresponse of our photodetector devices, we use two novel techniques. Firstly, we explore the effect of the size of photodetector devices in photoresponse at broadband wavelengths ranges from ultra-violet to near-infrared. With the miniaturization of device size, the photoresponsivity increases. Finally, we investigated photoresponse in the insulator-to-metal transition slope (50-60 ^oC) and beyond insulator-to-metal transition slope (65 ^oC) regions. The co-existence of the insulator and metallic domains in these regions significantly enhances the photoresponse. The ability to manipulate the phase transition and the photoresponse with device size opens opportunities for designing and controlling functional domains of vanadium dioxide for scalable micro- and nano-scale devices and sensor applications. This work enables a technology where phase change oxides can contribute to ultra-fast and compact optoelectronic applications.

In recent years, sensitization of molecular triplets using inorganic semiconductor nanocrystals via triplet energy transfer (TET) has emerged as a new area with potential applications ranging from photochemical photon upconversion to organic synthesis. We investigated the fundamental mechanisms of the inorganic/organic TET by building well-defined model systems and applying state-of-the-art time-resolved spectroscopy tools. In doing so, we uncovered the essential role of quantum confinement of nanocrystals in facilitating electronic coupling required for triplet energy transfer. We also established a unified picture of charge-transfer-mediated triplet energy transfer mechanisms, which greatly expanded the scope of molecular triplet sensitization using nanocrystals. Additionally, we demonstrated that nontoxic nanocrystals such as CuInS₂ and InP were also capable of sensitizing molecular triplets for efficient photon upconversion. These contributions will serve as a roadmap for the design of nanocrystal-molecule triplet sensitization systems for photon upconversion as well as many other photochemical applications.

It is shown through comparison with experimental results that the efficiency of quantum dot (QD) film-based photoemission would be impacted by an inadequate supply of electrons from an electron source. An explanation for the related photocurrent droop, as arising from restricted electron transmission at the substrate-QD interface as well as between the QDs, is proposed. It is suggested that interfacial potential based biasing schemes could considerably enhance electronic coupling for improved transmission and quantum efficiency. Modeling of the system through application of a 1-dimensional transfer matrix algorithm (TMA) to smoothly varying potential profiles indicates that electric fields of ∼450 MV/m would be necessary for ensuring electron transmission coefficients close to unity.

Triplet-triplet annihilation upconversion is a promising approach for application in solar cell devices since it allows the conversion of sub-bandgap infrared photons into visible photons that can be absorbed by the underlying cell. Most examples of photochemical upconversion are either in solution or glassy polymer matrices, which is undesirable from a technological point of view. In this contribution we demonstrate a solid state approach in which absorption of near infrared light is followed by direct electron injection into an inorganic substrate. We use time-resolved microwave photoconductivity experiments to study the injection of electrons into the electron-accepting substrate (TiO2) in a trilayer device consisting of a triplet sensitizer (fluorinated zinc phthalocyanine), triplet acceptor (methyl sub- situted perylenediimide) and smooth polycrystalline TiO2. Absorption of light at 700 nm leads to the almost quantitative generation of triplet excited states by intersystem crossing. This is followed by Dexter energy transfer to the triplet acceptor layer where triplet annihilation occurs and concludes by injection of an electron into TiO2 from the upconverted singlet excited state.

The effect of temperature on the triplet upconversion process in the solid is largely unexplored. We have investigated the effect of temperature on exciton diffusion and overall charge injection efficiency into metal oxide TiO₂ resulting from upconversion in a triple-layer system composed of TiO₂, methyl substituted perylene diimide (PDI-CH₃) and hexadecafluorinated zinc phthalocyanine (F₁₆ZnPc). We used the time resolved microwave photoconductivity technique to probe free mobile charge carriers that are generated at a planar heterojunction interface from upconverted singlet excitons. The charge yield is found to be thermally activated, which is attributed to thermally activated exciton diffusion, in combination with a phase change in the PDI-CH₃ crystalline material at higher temperature. These results show that crystal engineering can be a valuable approach to optimization of solid state upconversion.

[1] Felter, K. M.; Fravventura, M. C.; Koster, E.; Abellon, R. D.; Savenije, T. J.; Grozema, F. C., Solid-State Infrared Upconversion in Perylene Diimides Followed by Direct Electron Injection. ACS energy letters 2019, 5(1), 124-129.
[2] Kevin M. Felter, Jetsabel M. Figueroa-Tapia, John W.A. Suijkerbuijk and Ferdinand C. Grozema, Optimizing the efficiency of solid state trilayer upconversion systems: Effects of temperature and morphology. Submitted for publication, 2021.

Multidimensional coherent techniques in the visible range, 2D electronic spectroscopy (2DES) in particular, have been developed starting from the beginning of 2000, and historically they have been mainly exploited for the investigation of subtle dynamic mechanisms of energy and charge transport in biological complexes. Only later, these techniques have been recognized to be particularly valuable also for the study of transport processes in artificial nanomaterials and nanodevices.
Recent results obtained applying 2DES (including ‘action-based techniques’) to the study of nanomaterials will be overviewed, focusing the attention on semiconductor nanocrystals (‘quantum dots’) in solid-state devices and metal-organic hybrid systems.
In particular, the characterization of coherent mechanisms active in the transport of excitation energy in these materials will be presented. This is particularly meaningful towards the possible application of these nanomaterials for quantum technology.

Second harmonic generation (SHG), a nonlinear optical process where two photons of the same energy generate a single photon of twice the energy, is an optical fingerprint to probe an inversion symmetry of crystal structure of nonlinear materials. This symmetry-dependent characteristics of SHG are particularly important in the study of chemical properties in functional nanostructures whose crystal structure can be altered by chemical reactions facilitated by their high surface-to-volume ratio. SHG also provides a simple, non-invasive, and vacuum-free optical method, which can be easily integrated with various gas flow and temperature control systems for in-operando study. However, the fundamental limitations such as the intrinsically low SHG efficiency and the limited spatial resolution have rendered the nanoscale nonlinear optics studies for the functional nanostructures challenging.
Here, we developed a tip-enhanced in-situ nano-imaging of second harmonic generation (SHG) for nanoscale corrosion process studies based on the scanning near-field optical microscopy (SNOM) platform. We proposed that strong nonlinear responses of Au tips can be used as an optical probe for SHG signals of a nonlinear sample. This possibly originates from a resonant energy transfer between the Au tip and the sample in nonlinear process, while neither the Purcell effect nor the plasmonic field enhancement can explain the SHG enhancement in our tip-enhanced microscopy. Integrated with environmental control system, we probed in-situ nanoscale corrosion process occurred at the surface of zinc oxide (ZnO) nanowires, which was supplemented by correlative tunnelling electron microscopy (TEM) studies. Further, our tip-enhanced SHG nano-mapping can visualize multipolar SHG by Mie-resonance in dielectric silicon nanowires, manifested by the characteristic fringe patterns apparent in nanoscale SHG images. Thus, we believe that the presented tip-enhanced SHG microscopy for in-operando test can provide powerful means for nanoscale probing of optical and chemical properties of nanostructures.

Antiferromagnets (AFMs) are promising materials for spintronic and opto-electronic applications due to natural spin dynamics in the THz range and, at the same time, no net magnetization. This leads to the absence of stray fields which is a key property for data storage applications. A promising approach here is to combine the unique magnetic properties of AFMs with the strong coupling of optical and magnetic properties of semiconductors. Recently, semiconducting AFMs attract a lot of attention with several studies focusing on van der Waals layered AFMs from metal phosphotrichalcogenides such as NiPS3, FePS3, and MnPS3. For our studies, we have chosen NIPS3 because of its non-trivial exciton behavior of singlet and triplet states. To investigate light-induced dynamics in the excited, state spin, and electronic populations in semiconducting NiPS3, we use time-resolved transient absorption (TA). This technique provides access to spectral information about changes in absorption induced by excitation of non-equilibrium carrier populations and the dynamics of the carrier and exciton recombination. We will further report our results on time and spectrally resolved optical detection of the Neel vector at cryogenic temperatures through complex Zhang-Rice excitonic states of NiPs3 antiferromagnet by using Faraday rotation experiments. We also demonstrate the manipulation of the spin structure via application of the external magnetic field in the range of 1 T. Application of the external magnetic field leads to changes in polarization properties of PL. Finally, we provide a comparison of such dynamics for chemically exfoliated NiPS3 from scalable solution-based fabrication, by varying the size of flakes and their thickness from monolayers, up to bulk materials.

We have developed spectroscopic techniques to probe unprecedented properties of a 2D layered semiconductor, CrSBr. Using second harmonic generation (SHG), we find that monolayers are ferromagnetically (FM) ordered below a Curie temperature T_C = 146 K, an observation enabled by the discovery of a large magnetic dipole SHG effect in the centrosymmetric structure. In multilayers, the ferromagnetic monolayers are coupled antiferromagnetically (AFM) with a Néel temperature of T_N = 148 - 132 K (decreasing with increasing thickness). Symmetry analysis establishes magnetic dipole and magnetic toroidal moments as order parameters of FM monolayer and AFM bilayer, respectively. The embodiment of both magnetic and semiconducting properties in this material allows the magnetic control of interlayer electronic coupling, as manifested in tunable excitonic transitions. Excitonic transitions in bilayer CrSBr and above can be drastically changed when the magnetic order is changed from paramagnetic to AFM as T is lowered below T_N or as it is switched from layered AFM to the field-induced FM state. The magnetic-excitonic coupling is attributed to the spin-allowed interlayer hybridization of electron and hole orbitals in the FM, but not AFM state. Our work uncovers a magnetic approach to engineer electronic and excitonic effects in layered magnetic semiconductors.

During the past decade halide perovskites have risen to prominence as a novel class of energy and optoelectronic materials. A fundamental question in the materials science of halide perovskites is how lattice vibrations influence their electronic and transport properties. In this talk I will describe first-principles investigations of electron-phonon couplings in hybrid organic-inorganic lead halide perovskites [1,2] and all-inorganic lead-free halide double perovskites [3], and I will analyze the atomic-scale mechanisms underpinning charge transport in these systems. I will show how the lower mobilities of halide perovskites, as compared to standard inorganic semiconductors, are linked with the heavy atomic masses and the resulting low vibrational frequencies. Furthermore, I will show that lead-free halide double perovskites exhibit lower carrier mobilities than their lead-halide counterparts because they possess much heavier band effective masses. For both lead-based and lead-free perovskites, carrier scattering by longitudinal optical polar phonons is found to be the main source of mobility degradation. Building on this understanding, I will discuss a universal scaling law for the carrier mobility in these compounds, and illustrate possible avenues to design new halide perovskites with improved carrier transport properties.

[1] M Schlipf, S Poncé, F Giustino, Phys. Rev. Lett. 121, 086402 (2018).
[2] S. Poncé, M. Schlipf, F. Giustino, ACS Energy Lett. 4, 456 (2019).
[3] J. Leveillee, G. Volonakis, F. Giustino, J. Phys. Chem. Lett. 12, 4474 (2021).

Quasi-two-dimensional (2D) perovskites are promising optoelectronic materials for lighting due to their excellent luminescent properties and color tunability. The recent demonstration of optically pumped continuous-wave (CW) lasing at room temperature stipulated the promise that these materials are also candidates for realizing electrically driven lasers from solution-processed materials. Excitonic emission from the quantum wells (QWs) of quasi-2D perovskite accounts for their superior light emission at low excitation fluences and can enhance LED performance. However, at high excited-state densities (necessary for lasing) the emission efficiency of 3D samples overtakes that of quasi-2D samples whose excited-state population is dominated by excitons. In this contribution, it is explained by the 2^nd order rate of bimolecular radiative recombination increasing with fluence in the 3D sample (whose excited-state population is dominated by free charge carriers) making the emission more efficient and the radiative rate faster with increasing excitation density. Before the lasing threshold, the bimolecular radiative rate in the 3D materials apparently exceeds the radiative rate in the excitonic materials. The efficiency of the excitonic emission is also reduced by exciton-exciton annihilation, leading the 3D samples to become brighter at high fluences. In terms of amplified spontaneous emission (ASE) threshold, these effects mean that the ASE threshold is higher for the quasi-2D samples (~600 μJ cm^-2) whose excited-state population is dominated by excitons than for the 3D samples (~130 μJ cm^-2).

Furthermore, we then demonstrate that the ASE threshold in quasi-2D samples is significantly affected by the type and concentration of the 2D spacer used. The 2D spacer content affects the ASE threshold by two mechanisms. Firstly, the spacer content has a large influence on the surface roughness. The smoother the film, the longer the photon lifetime within the film, and therefore the higher the photon density within the film. As the ASE threshold is related to the product of the photon and excited-state density in the film, these higher photon densities work to reduce the ASE threshold. Secondly, the spacer nature and content influence the excited states that are present in the quasi-2D films. Moving from butylamine to napthymethylammonium (NMA) 2D spacers and decreasing the spacer concentration in the film shifts the emission mechanism towards free carrier radiative recombination and away from excitonic emission. This shift towards free carrier recombination based emission is associated with a reduction in the ASE threshold (from > 600 μJ cm^-2 to ~200 μJ cm^-2) of the quasi-2D materials. The best ASE thresholds in quasi-2D materials that we achieved used low concentrations of 40% NMA spacer, and thermal imprinting to reduce the roughness. In this case, we achieved a threshold of 200 μJ cm^-2 similar to those in the 3D material. Our findings are closely relevant to many of the widely concerned 2D/3D perovskite materials these days.

In terms of optimizing quasi-2D materials for gain applications, our results suggest that careful consideration of the excited-state population and emission mechanism should be made. We propose that an excited-state population in the quasi-2D material dominated by free charge carriers may be the most favorable for their gain performance, and should be the focus of further material engineering efforts. Irrespective, a close examination of the excited states present in the quasi-2D materials and their influence on the optical gain in these materials will be of continued relevance to their development.

Structural and material inhomogeneities in microstructures can have large effects on the optical properties and functional performance [1]. In this study, we demonstrate a novel way to harness this variation to probe the optoelectronic properties of the material. This is achieved using high-throughput spectroscopy to gather statistically significant, large experimental datasets of microstructures [2]. The methodology was applied to in-plane CsPbBr3 nanowires (NWs), which are promising for nano-photonic applications, showing good waveguiding [4] and luminescence properties [3] as well as demonstrating lasing at elevated carrier densities [5].

Optical microscopy with a machine vision camera was used to identify and measure the width and length of >10,000 NWs. The optoelectronic properties of each nanowire were characterised using room temperature photoluminescence (PL) spectroscopy and time correlated single photon counting techniques. The measurements were repeated when exciting carriers at the air/NW interface and the substrate/NW interface to probe the inhomogeneity in each NW. Differences in the bandgap were observed due to tensile effects at the NW/substrate interface [6]. Two carrier lifetimes were observed: a fast lifetime (~2ns) associated with carriers at the air/NW interface and a slow lifetime (~8ns) associated with carriers in the bulk. A single recombination model was developed to explain correlations between the NW geometry and the results. This model, when applied to the large volume of experimental data, is unique in that it provides a large breadth of information about the growth, and can simultaneously determine values for the diffusion length, carrier mobility, trap densities and internal quantum efficiency (IQE). These values are compatible with those from small-scale studies in the literature, and the IQE was verified independently using temperature dependent PL. Importantly, the model requires minimal a-priori knowledge and can therefore be applied to other material ensembles in a statistically rigorous way, including those which have not been previously studied.

[1] Al-Abri, R.et al. (2021). Photonics 3(2), 22004.
[2] Parkinson, P. et al. (2020) Proc. SPIE 112910K.
[3] Oksenberg, E. et al. (2021) Adv. Funct. Mater. 31, 2010704.
[4] Shoaib, M. et al. (2017) J. Am. Chem. Soc. 139, 15592–15595.
[5] Schlaus, A. P. et al. (2019) Nat. Commun. 10, 265.
[6] Oksenberg, E. et al (2018). Nano Letters 18(1), 424.

One of the most challenging tasks for LED applications is emitting 100% polarized light from the device. Typically, this is achieved by introducing an additional layer of polarization filter which leads to losing half of the light intensity. To overcome this issue, one has to find a system with a high degree of photoluminescence (PL) polarization. A promising approach here is using magnetic metal doping in combination with a highly efficient semiconductor. Metal-halide perovskites have attracted the attention of many scientific groups around the world as one of the most promising candidates for solar cell applications and light-emitting diode (LED) technology. In our work, we will present how doping of methylammonium lead bromide (CH₃NH₃PbBr₃) with the transition metals Ni, Co, and Mn induces dramatic changes in the magneto-optical properties and carrier dynamics due to interactions between the magnetic moments of the dopants and Coulomb interactions of the excitons.
We have chosen to use transient absorption (TA) spectroscopy at cryogenic temperatures with applied magnetic fields of up to 1 T to investigate changes in the optical properties induced by magnetic metal doping in CH₃NH₃PbBr₃, since it gives spectral information about the energies of electronic states and dynamic properties of the photoexcited carriers. We find a change in the main ground state bleach (GSB) peak position in doped CH₃NH₃PbBr₃, which depends on the transition metal used. The main GSB peak of pure CH₃NH₃PbBr₃ at 4 K is at 2.32 eV. Doping CH₃NH₃PbBr₃ with Mn and Ni leads to a shift of the main peak to lower energies by 0.04 eV and 0.08 eV, respectively, while doping with Co leads to a shift to higher energies by 0.02 eV. The modifications of the TA spectra are associated with changes of the bandgap energy, which is the result of doping-induced lattice expansion or compression.
For all investigated samples, we observed energy splitting of the TA spectra for two different circular polarizations of the pump pulse. We interpret this splitting as the result of a light-induced Zeeman effect which arises from the spin-polarized photoexcited charge carriers due to angular momentum selection rules of the optical transitions. For the pure CH₃NH₃PbBr₃ case, we observed splitting in the range of the 8 meV, while for the Ni-doped material, this splitting is around 17 meV. We attributed this significant enhancement in the splitting energy to the exchange coupling of the photoexcited charge carrier spins with the Ni ion spins. The presence of the Ni spins in the crystal lattice also affects the lifetime of the photo-induced Zeeman splitting. Undoped CH₃NH₃PbBr₃ shows a lifetime of the Zeeman splitting in the range of 30 ps, while for the Ni-doped samples this lifetime is exceeding 1300 ps which is an indication of one order of magnitude longer spin lifetime in the Ni-doped sample, compared to undoped CH₃NH₃PbBr₃. The reason for this significantly different behavior could be a long-range order of the Ni spins, potentially indicating magnetic spin order, which we are currently exploring. The long lifetime of the Zeeman splitting could also indicate a long lifetime of circularly polarized PL.

Control over the conformation of conjugated polymers using nucleic acids (NAs) as templates can be very advantageous in inducing unique and readily controllable properties. Complexation of NAs with conjugated polyelectrolytes (CPEs) such as cationic polythiophenes (CPT) is facilitated by the presence of charged side groups on the backbone.¹ Previous studies on the biosensing ability of a certain CPT has revealed a tremendous impact of the ssDNA sequence on the conformational and optical response of the polymer.^2,3 The complex formed of CPT and oligocytosine (dC₂₀) strands is especially noteworthy due to the formation of highly ordered and extended chains due to π-stacking between the cytosine bases and thiophene rings, further stabilized by π-stacking with the imidazole side chain.^4,5 This stands in contrast to the complex with oligoadenosine (dA₂₀), where π-stacking between the bases limits any stacking interactions with the polymer resulting in a flexible and torsionally disordered conformation. Interestingly, though, our previous ultrafast transient absorption (TA) work showed that both complexes showed only excitonic behavior, with similar and much faster decay dynamics compared to the polymer alone, which forms interchain aggregates.⁵

Therefore, we used TRIR to directly probe the role of NA templating in the excited state species formed in the two complexes, which is responsible for the faster dynamics observed with vis-nIR probes, as well as compare the photophysics of loosely bound complexes with the more rigid and ordered systems. We find that with excitation of the polymer in the visible (532 nm) both complexes form the P₀ intrachain polaron evidenced by a broad absorption background that decays faster than the delocalized polaron DP₁ formed in the case of CPT alone. The lower intensity of the P₀ absorption in the CPT/dA₂₀ case is consistent with increased intrachain disorder, while higher interchain long range order leads to the higher DP₁ absorption.⁶ The rigid complex formed in the case of CPT/dC₂₀ limits the geometric relaxation observed in the C=C stretch Fano antiresonance, in agreement with the reduced reorganization energy calculated from resonance Raman intensity analysis and the limited evolution of the stimulated emission in the TA data.⁵ Excitation in the UV (266 nm) shows that a charge transfer complex is formed between CPT and dC₂₀ with the observation of the cytosine anion and a broad absorption background, in contrast to CPT/dA₂₀ case, where the TRIR spectra solely reflect the excited state behavior of the oligoadenosine. These results show that non-covalent interactions between NAs and CPTs can determine the excited state species formed, which has implications to the applicability of these complexes in molecular electronics.

Rubio-Magnieto, J. et al, Soft Matter 2015, 11, 6460–6471.
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Controlling the behavior of molecular excitons in solids is crucial for the expression of optical functions. We have been interested in the triplet diffusion and upconversion of triplet into singlet in solid materials, especially in densely packed dyes in solid environments where molecular diffusion is limited.
As an example, we present a system of emitter dyes densely integrated in epoxy resins. Epoxy resins have a high oxygen barrier property, which is used as an encapsulant for OLEDs. We have succeeded in dispersing the dye in epoxy resins with a high concentration of more than 1 M by appropriate molecular design, and achieved a high upconversion efficiency in air. We will show how the emitter density plays an important role not only in the triplet diffusion between emitters, but also in the circumvention of back energy transfer from emitter to sensitizer.
The introduction of singlet energy collectors to more actively prevent back energy transfer will also be presented. We show that the singlet energy collectors play an important role in effectively suppressing the back energy transfer from emitter to sensitizer, and can also compete with the free triplet generation by the singlet fission.

Organic light emitting diodes (OLEDs) are consider the most recent commercially available lighting and display technology. Until now, only the first two generations of OLEDs were used in the market but not until recently the 3rd generation made huge steps towards real life applications. Using a thermally activated delayed fluorescence (TADF) sensitizer and a multiple resonance (MR) TADF emitter in a host matrix, hyper-fluorescence (HF) OLEDs already achieved higher efficiencies and comparable lifetimes with the ones already in the market. In this work, we focus on understanding the mechanism of the MR-TADF emitters using steady-state and time-resolved spectroscopic methods. Evidence of hot transitions from vibrational excited states is shown experimentally along with a temperature dependant singlet-triplet state energy gap (ΔE_ST). Combining the two, a reverse intersystem crossing (rISC) mechanism that involves thermally activated reverse internal conversion (rIC) from the T₁ to the T_n (n ≥ 2) state followed by rapid (r)ISC to the S₁ state, is deduced. The existence of an intermolecular specie, due to the planarity/rigidity of the molecule, is assigned to an excimer which affects the optical performance even at concentrations below those used in devices. HF-OLEDs, using v-DABNA as the emitter, exhibit an increase of maximum external quantum efficiency (EQE), compared to a control device. Forster resonance energy transfer (FRET) studies show that although FRET efficiency decreases with decreasing emitter concentration, the overall device efficiency is higher, indicating that excimer quenching affects the OLED's performance.

Thermally activated delayed fluorescence (TADF) emitters are considered a novel class of molecules for applications in organic light-emitting diodes (OLEDs). Due to the small energy gap between triplet and singlet state, reverse intersystem crossing (rISC) is allowed. This efficient mechanism has the ability to convert triplet states into an emissive singlet state, which enhances the internal quantum efficiency (IQE) towards 100%. In the context of an OLED, the TADF emitters are mainly incorporated in a matrix of host molecules. Apart from a better control of the electrical transport properties in the emission layer, the host molecules also prevent emitter self-quenching, a great source of non-radiative loses. The aim of this work is the use of DMAC-TRZ as a TADF emitter ‘probe’ to differentiate the environmental effects of a range of solid state host materials in guest-host systems. A variety of typical TADF OLED hosts with different characteristics was studied. This provides a clearer understanding of guest-host interactions and what effects emitter performance in solid state. Using time-resolved photoluminescence (PL) measurements, the CT state energy distribution was obtained from the full width at half maximum (FWHM) of the emission band. By correlating FWHM with other photophysical properties such as the apparent dynamic redshift of CT emission onset, we obtain the disorder-induced heterogeneity of D-A dihedral angles. Concentration dependence studies show that aggregation contributes to reducing photoluminescence efficiency. In our study, we explore the hosting potential of two host materials of similar chemical structure: mCPCN and mCBPCN, demonstrating the divergent photophysical behavior of DMAC-TRZ as a guest molecule.
References:
Stavrou, K.; Franca, L. G.; Monkman, A. P. Photophysics of TADF Guest–Host Systems: Introducing the Idea of Hosting Potential. ACS Appl. Electron. Mater. 2020, 2 (9), 2868–2881.

Nanoscale infrared (IR) resonators with sub-diffraction limited mode volumes and open geometries have emerged as new platforms for implementing cavity quantum electrodynamics (QED) at room temperature [1,2]. The use of infrared (IR) nano-antennas and tip nanoprobes to study strong light-matter coupling of molecular vibrations with the vacuum field can be exploited for IR quantum control with nanometer and femtosecond resolution. In order to accelerate the development of molecule-based quantum nano-photonic devices in the mid-IR, we developed a generally applicable semi-empirical quantum optics approach to describe light-matter interaction in systems driven by mid-IR femtosecond laser pulses.

The theory is shown to reproduce recent experiments on the acceleration of the vibrational relaxation rate in infrared nanostructures [3], and also provide physical insights for the implementation of coherent phase rotations of the near-field using broadband nanotips [4]. We then apply the quantum framework to develop general tip-design rules for the experimental manipulation of vibrational strong coupling and Fano interference effects in open infrared resonators. We finally propose the possibility of transferring the natural anharmonicity of molecular vibrational levels to the resonator near-field in the weak coupling regime, to implement intensity-dependent phase shifts of the coupled system response with strong pulses. Our semi-empirical quantum theory is equivalent to first-principles techniques based on Maxwell’s equations, but its lower computational cost suggests its use a rapid design tool for the development of strongly-coupled infrared nanophotonic hardware for applications ranging from quantum control of materials to quantum information processing.

[1] T. W. Ebbesen. Acc. Chem. Res. 49, 2403, 2016.
[2] F. Herrera and J. Owrutsky. J. Chem. Phys. 152, 100902, 2020.
[3] B. Metzger et al. Phys. Rev. Lett. 123, 153001, 2019.
[4] E. A. Muller et al. ACS Photonics, 5, 3594, 2018.

Strong light-matter coupling reveals itself in dramatic changes in linear and nonlinear response spectroscopy. In particular, the emergence of so-called polariton (hybrid light-matter) modes that are well-separated in energy from bare material transitions indicates the onset of strong coupling. However, a more subtle question arises as to the extent to which polariton modes can alter photochemical and photophysical processes of interest. An important aspect that is not highlighted enough in molecular polaritonics studies is that whether the strong coupling happens with one or a few molecules (as in plasmonic nanocavities) or with a thermodynamic ensemble (of >10^6 molecules/photon mode) dramatically affects the outcome of the molecular process of interest. Here I will discuss effective strategies to take advantage of the various flavors of strong coupling phenomena to control photochemistry and photophysics.

We establish a phase-field model for coupled electron and lattice dynamics in correlated material systems under ultrafast excitation. The model integrates a phase-field description of phase transitions with other notable mechanisms at picosecond-to-nanosecond timescales to deliver a comprehensive spatiotemporal profile of excitation and relaxation transients of the system. Combined with first-principles calculation of electronic and phonon properties, we theoretically examine the light-excited electron and lattice dynamics in strongly correlated material Ca₃Ru₂O₇ involving a coupled electronic and magnetic phase transition. We demonstrate that the excited-state dynamics contains a three-component relaxation process of carrier concentration across picosecond-to-nanosecond timescales, which is supported by evidence from previous experimental works. A strong temperature dependence of all relaxation components within a wide range across the phase transition temperature is revealed. This model provides in-depth theoretical insights of the dynamical properties of complex electronic systems and ultrafast phenomena that lies within them. It can be applied to various material systems for predicting the electronic and lattice dynamics under different ultrafast external stimuli, including light pulses, electric fields, and currents, etc.

During the past decade halide perovskites have risen to prominence as a novel class of energy and optoelectronic materials. A fundamental question in the materials science of halide perovskites is how lattice vibrations influence their electronic and transport properties. In this talk I will describe first-principles investigations of electron-phonon couplings in hybrid organic-inorganic lead halide perovskites [1,2] and all-inorganic lead-free halide double perovskites [3], and I will analyze the atomic-scale mechanisms underpinning charge transport in these systems. I will show how the lower mobilities of halide perovskites, as compared to standard inorganic semiconductors, are linked with the heavy atomic masses and the resulting low vibrational frequencies. Furthermore, I will show that lead-free halide double perovskites exhibit lower carrier mobilities than their lead-halide counterparts because they possess much heavier band effective masses. For both lead-based and lead-free perovskites, carrier scattering by longitudinal optical polar phonons is found to be the main source of mobility degradation. Building on this understanding, I will discuss a universal scaling law for the carrier mobility in these compounds, and illustrate possible avenues to design new halide perovskites with improved carrier transport properties.

[1] M Schlipf, S Poncé, F Giustino, Phys. Rev. Lett. 121, 086402 (2018).
[2] S. Poncé, M. Schlipf, F. Giustino, ACS Energy Lett. 4, 456 (2019).
[3] J. Leveillee, G. Volonakis, F. Giustino, J. Phys. Chem. Lett. 12, 4474 (2021).

We seek to understand the lower limits of non-radiative recombination losses from charge transfer states in organic donor/acceptor systems. These factors underpin applications ranging from organic photovoltaics to organic LEDs and lasers. As an extreme case, we study a model thermally activated delayed fluorescence (TADF) system that exhibits both a high photoluminescence quantum efficiency (PLQY = ~22%) and comparatively long PL lifetime, while simultaneously yielding appreciable amounts of free charge generation (photocurrent external quantum efficiency EQE of 24%). In solar cells, this blend exhibits non-radiative voltage losses of only ~0.1 V, among the lowest reported for an organic system. Notably, we find that the non-radiative decay rate, knr, is on the order of 1E-5 /s, roughly 4-5 orders of magnitude slower than typical OPV CT states. Surprisingly however, this rate is still orders of magnitude too *fast* compared to predictions by Marcus-Levich-Jortner two-state theory. By comparing experimental PL and electroabsorption spectroscopy with theory, we conclude that CT-local exciton (LE) hybridization is present and that this mixing greatly increases the non-radiative recombination rate of the CT state. We discuss implications of this finding for different optoelectronic devices.

The electronic structure and exciton dynamics of the molecules and polymers that form the active layer in organic electronic devices can change dramatically during solution deposition and thermal annealing. As solvent vaporizes, molecules aggregate and become electronically coupled, sometimes dramatically changing the exciton dynamics and thus the suitability of the material for electronic devices. Similar changes occur during thermal annealing, as the relative orientation of molecules change. The exciton dynamics of molecules in solution and in films of aggregates can be measured using transient absorption spectroscopy. However, the progression of exciton dynamics during film formation and thermal annealing is unknown, as measurements typically cannot be performed quickly enough to collect accurate transient absorption spectra of the evolving molecular aggregates. The exciton dynamics of evolving material systems can be measured by increasing the speed of data collection. Single-shot transient absorption spectroscopy using tilted pulses can measure transient spectra with up to a 60 ps pump-probe time delay in 8 ms, with an adequate signal-to-noise ratio achieved in just a few seconds. The exciton dynamics of intermediate aggregation states are revealed during the formation of an organic film and during thermal annealing. This type of measurement provides insight into the complex process of organic film formation and how the excitonic properties of a film emerge.

Activated carbon is used as an adsorbent material for various hazardous materials. Typically, active carbon is impregnated with metal oxides in order to improve its adsorbing capabilities towards inorganic low molecular weight gases, and high vapor pressure compounds by chemisorption mechanism. Exposure of activated carbon to humid air leads to water adsorption. As a result, the carbon surface and the impregnants are changing over time. The impregnants are dissolve and migrate to the external surface of the carbon following gradual recrystallization. The aging process may reduce the chemisorption capacity of the activated carbon.
In this study, the effect of aging on activated carbon from coconut husks source, impregnated with zinc oxide and the organic additive triethylenediamine, was studied by non-destructive Raman spectroscopy as well as energy dispersive spectroscopy. Raman spectroscopy enables the chemical characterization of a packed activated carbon samples. Energy dispersive spectroscopy was used to characterize the surface elements of the native and aged activated carbon. Accelerated aging was achieved by water adsorption on the carbon, followed by storage at 50^oC in a closed container. The study shows that the aging process increased the oxygen content on the carbon surface, both by introducing carboxyl and epoxides groups on the carbon surface, as well as migration of the metal oxide impregnants to the carbon granules’ external surface. The aging process was studied by two laser excitation wavelengths (532 nm and 785 nm) Raman spectroscopy. Raman spectroscopy is commonly used to study carbonaceous materials molecular structure. It allows to characterize the graphitic sp² versus disordered sp² carbon, amorphous carbon sp³ degree, and their stress state. In general, the G-band at ∼1582 cm^-1, is of E_2g mode symmetry, which is common to all sp² carbon systems, and results from the in-plane bond stretching of the C-C bond in graphitic materials. The full-width at half-maximum of the G band increases monotonically with the degree of disorder in the graphitic lattice. The D-band at ∼1350 cm^-1, is an A_1g breathing mode symmetry, it is associated with carbon atoms having disordered sp³ carbon atoms. The intensity ratio (R = I_D/I_G) of D and G bands, is used to assess the sp³/sp² carbon ratio, which represents the degree of disordered carbon atoms. An increase in this ratio results from a reduction in the crystalline size. The Raman D and G band shifts were followed by double Lorentzian model fitting. The aging outcome indicated by a D band position upshift from 1337 cm^-1 to 1340 cm^-1 and 1342 cm^-1, that can be explained by the compression of the activated carbon structure. The aging process leads to I_D/I_G increase indicating a reduction in the size of the crystal. In addition, the low frequency Raman bands were used to study the aging process of the carbon. Higher oxygen content on the carbon external surface was presented by elevated Raman intensities of the epoxide’s groups at 643 cm^-1. This finding indicates oxidation of the carbon surface during the aging. Furthermore, higher content of the sp-sp² amorphous carbon at 405 cm^-1 and 418 cm^-1 was found in aged carbon, indicating carbon structure configuration changes with aging. The increased content of zinc on the carbon granules external surface, due to its migration during the aging, was portrayed by the creation of new vibration bands at 333 cm^-1 and 991 cm^-1 with aging. In addition, the higher content of oxygen on the carbon surface with aging, and the higher content of the impregnated metal was demonstrated by energy dispersive spectroscopy characterization.
Raman spectroscopy was found to be a powerful non-destructive method for analyzing the aging process of impregnated activated carbon.

Spherical titanium barium nanoparticles with an average size about 100 nm in the tetragonal crystal phase were obtained by peroxide synthesis and then with formation of OH groups on its surface by hydrogen peroxide. To reduce the toxic effect, the surface of nanoparticles was chemically modified with hydrolyzed chitosan obtained from shrimp exoskeleton (Penaeus vannamei). The chitosan obtained and the modified surface were analyzed by X-ray diffraction, Raman spectroscopy and Infrared. Influence of surface modification on the optical response intensity of the second harmonic was investigated using a Nd:YAG laser 1064 nm (2.8, 4.5 and 8.1 mJ/pulse).

Controlling charge and energy flow in functional systems can be beneficial for solar energy conversion and utilization. The marriage of colloidal semiconductor nanocrystals and functional organic molecules has brought unique opportunities in emerging photonic and optoelectronic applications. In this presentation I will discuss our studies of controlling the charge carrier dynamics, light/matter interactions, and spin populations in these novel hybrid systems. In one effort we are exploring the use of novel organic hybrid systems at and near interfaces to control the carrier dynamics and reduce surface recombination but also to protect grain boundary surfaces from degradation. With respect to controlling spins we have recently studied and developed a novel class of chiral hybrid semiconductors based upon layered metal-halide perovskite 2D Ruddlesden-Popper type structures. These systems exhibit chiral induced spin selectivity whereby only one spin sense can transport across the film and the other spin sense is blocked. From these systems we can achieve a high degree of spin current polarization and injection when used as a contact layer. We have developed novel spin-based LEDs using mixed NCs as the light emitting layer that promotes light emission at a highly spin-polarized interface. The LED spin-polarization is limited by spin-depolarization within the MHP NCs. Thus understanding and controlling the spin-depolarization is an important goal. Here we discuss the charge carrier recombination rate and spin-coherence lifetimes in single crystals of 2D Ruddlesden-Popper perovskites. Layer thickness dependent charge carrier recombination rates are observed with the fastest rates for n = 1 due to the large exciton binding energy and the slowest rates for n = 2. Room temperature spin-coherence times also show a nonmonotonic layer thickness dependence with an increasing spin-coherence lifetime with increasing layer thickness from n = 1 to n = 4, followed by a decrease in lifetime from n = 4 to bulk.The longest coherence lifetime of ~7 ps is observed in the n = 4 sample. We also studied the spin-coherence lifetimes as a function of exciton binding energy and find that the spin-lifetime increases with decreasing exciton binding energy.