Symposium EQ09—Emerging Light Emitters for Photonics and Optoelectronics—Hybrid Perovskites and Other Low-Dimensional Emitters

Organic metal halide hybrids (OMHHs) are an emerging class of photoactive materials with exceptional structure and property tunability, which have seen applications in a wide range of areas, from light emitting diodes (LEDs) to photovoltaic cells (PVs) and radiation detectors. By choosing appropriate organic and metal halide components, the crystallographic structures of OMHHs can be finely controlled with metal halide units, for instance, metal halide octahedrons, forming distinct 3D, 2D, 1D, and 0D structures at the molecular level. The photophysical properties of OMHHs are found to exhibit strong dependence on their compositions and dimensionalities, with narrow direct band emissions often obtained in 3D metal halide perovskites and broadband molecule like emissions observed in low dimensional OMHHs. In this talk, I will present our recent efforts on the development and study of luminescent OMHHs beyond perovskites, by synthetically controlling the crystallographic structures of OMHHs and the interactions between organic and metal halide components in the hybrids. The applications of developed OMHHs in various types of optoelectronics will be discussed.

Heterostructures are created by combining two different materials and are the building blocks for the advanced semiconductor devices that are being used today such as multijunction photovoltaics and light emitting diodes and lasers. Halide perovskites have emerged as a new class of semiconductors and have demonstrated near-ideal properties, which have led to high effiiciency optoelectronic devices. However, despite more than a decade of progress, it has been challenging to create heterostructures with sharp interfaces and arbitrary thickness between two perovskites (3D/3D or 3D/2D largely due to solvent incompatibility, which destroys or degrades the underlying perovsite.
We have developed a novel approach to create heterostructures of 3D/2D perovskites with arbitrary thickness and chemical composition for 3D or 2D perovskites. Characterization of the 3D/2D interface using ToF-SIMS, angle dependent GIWAX, absorbance and photoluminescence reveal the presence of a phase-pure 2D perovskite with an interface sharpness of <10 nm. By tailoring the band-alignment between the 3D and 2D heterointerfaces, we show that we can use the 2D perovskite as either a hole of electron transport layer. Moreover, tuning the 2D perovskite also allows us to create either a type I or type II heterstrostructures. Finally, we demonstrate photovoltaic devices with 23% efficiency and LEDs with >10% EQE with excellent stability.

Quasi-two-dimensional (quasi-2D) halide perovskite are emerging candidates for the next-generation light-emitting diodes (LEDs) because of their superior optoelectronic properties with remarkable tunability. The external quantum efficiency of quasi-2D perovskite-based LEDs has recently exceeded 20% in both green and near-infrared regions. Nevertheless, owing to the weak ionic bonding of halide perovskites, the quasi-2D perovskites suffer from ion diffusion-controlled phase disproportionation that causes the formation of multiple-quantum-well structures with broad and unpredictable phase distributions. This limits device efficiency, stability, and reproducibility. Here, we report a molecular design strategy to suppress phase disproportionation. Specifically, we extend the pi-conjugation length and fine-tune the dihedral angle between two phenyl rings within the ligand molecules to increase the effective bulkiness. When incorporated into the quasi-2D perovskite lattice, the novel ligands substantially inhibit ion transport and kinetically stabilize metastable phases in the perovskite solids. This leads to high-quality thin films with narrow phase distributions, low defect densities, and enhanced radiative recombination efficiencies. Molecular dynamics simulations suggest that the free energy barrier for ion transport through the new organic ligand layer is twice that of the conventional ligands, in agreement with experimental observations. Using the improved materials, we demonstrate efficient, stable, and wavelength-tunable red LEDs with a record peak external quantum efficiency of 25.8% (average 22.7% among 60 devices). Importantly, in contrast to existing perovskite LEDs, our devices exhibit ideal current-voltage characteristics completely free from hysteresis and efficiency overshoot, suggesting suppressed ion migration and diffusion under operational conditions. These discoveries provide critical insights into the crystallization pathways of solution-processed low-dimensional perovskite semiconductors, thus opening up a new avenue towards high-performance optoelectronic devices via molecular engineering.

Perovskite nanocrystal-based light-emitting diodes (NC PeLEDs) have gained tremendous attention due to their superior optoelectronic properties, which are considered as the leading candidates for the next generation of low-cost and high-performance LEDs for display, lighting and optical communication applications. Although green and near-infrared PeLEDs have achieved external quantum efficiencies (EQEs) higher than 20%, there are still challenges with fabricating spectrally stable and highly efficient pure red and blue PeLEDs (colour coordination (0.708, 0.292) and (0.131, 0.046), respectively). Compositional engineering is one of the common methods to produce red and blue NC PeLEDs, low dimensional perovskites provide an alternative route to achieve spectrally stable devices with desirable color emission coordinates. However, the almost unavoidable drawback of mixing halide NCs is the spectral instability (red-shifted emission) associated with halide migration under the application of an electric field. In contrast, 2D low dimensional colloidal perovskites nanoplatelets (NPls) provide an alternative route to achieve spectrally stable devices with desirable color emission coordinates. Another limitation of the current PeLEDs is that their intrinsic light outcoupling efficiency remains considerably lower than the organic counterpart because it is not yet possible to control the transition-dipole-moment (TDM) orientation in QD solids at the device level. Here, we reported orientation controllable CsPbI₃ NPls red LEDs with a stable pure-red emission from 3-7 V, with an EQE of 2.8% (one of the highest EQE among the strongly confined colloidal PeLEDs). The edge-up (the lateral axis of NPls are vertical to the substrate) and face-down configuration (the lateral axis NPls are parallel to the substrate) are studied with TEM and GIWAXS, and potentially indicate that the improvement of LED performance for face-down configuration is due to an increase in horizontal TDMs at a device level. Furthermore, we demonstrate that the NPls exhibit polarized emission compared with their bulk NC counterparts, which can lead to further optoelectronic and photonic applications associated with polarized light-emitting diodes.

Metal halide perovskites are inexpensive semiconductor materials promising for high performance solar cells and light emitting diodes (LEDs) because they are easy to make and tolerant of defects. A fundamental understanding of the factors controlling the carrier transfer mechanisms in heterostructures of halide perovskites is crucial for guiding the synthetic strategies to improve properties and device applications. We developed new methods for synthesizing nanostructures of both three-dimensional (3D) perovskites and two-dimensional (2D) Ruddlesden–Popper (RP) layered perovskites, and using them to create novel and arbitrary heterostructures, such as 2D/3D perovskite heterostructures, vertical 2D/2D heterostructures and lateral 2D heterostructures, with high quality interface and tunable band alignments. We use various structural characterization and time-resolved spectroscopic methods to study the carrier transfer mechanisms between these well-defined 2D and 3D perovskites and between different RP phases. We are trying to fabricate optoelectronic devices using such heterostructures. The excellent properties of these single-crystal perovskite nanostructures of diverse families of perovskite materials with different cations, anions, and dimensionality make them ideal model systems for fundamental physical studies of carrier transport and decay mechanisms and for enabling high performance solar cells, lasers, LEDs, and other optoelectronic applications.

Layered metal halide perovskites are rising stars in the photovoltaic community due to their increased thermodynamic stability compared to 3D perovskites. Simultaneously, they provide an exciting playground to investigate structural versatility and excitonic physics. The majority of layered perovskites are of the <100>-type, i.e. planar sheets of the inorganic perovskite octahedra that are sandwiched by large organic cations. However, another version of this layered system is possible, namely the <110>-type, which is characterised by corrugated inorganic sheets. However, the <110>-type is scarcely reported and key design principles that enable this type of perovskite are still missing. In this work, we studied the impact of two diammonium cations (2,2′- (ethylenedioxy)bis(ethylammonium) and 2,2'-oxybis(ethylammonium)) on the structure and photophysics of tin iodide perovskites. We show that the shorter 2,2'-oxybis(ethylammonium) cation forms <100>-type perovskite, while the longer 2,2′- (ethylenedioxy)bis(ethylammonium) stabilizes the <110>-type perovskites. The change from a planar to a corrugated structure changes the photoluminescence from a relatively narrow emission with a small Stokes shift to a broad emission with a large Stokes shift. Intriguingly, thin films made with 2,2'-oxybis(ethylammonium) show photoluminescence that is red-shifted compared to the emission from single crystals. We will reveal the impact of the structure of these layered perovskites on their photophysics, showing that layered tin compound using 2,2'-oxybis(ethylammonium) as cation is a promising new active material for optoelectronic devices.

Lead-free double halide perovskite, Cs2AgBiBr6, which exhibits better stability and lower toxicity than lead counterparts, has been considered as promising alternative for optoelectric application. However, intrincially indirect bandgap of Cs2AgBiBr6 has been a hindrance to be used as light-emitter. Recently it was reported that electron-phonon coupling can be weakened by decrasing dimenson of perovskite emitters.
Here, we effectively tackled low PL intensity issue by decreasing grain size by synergistic control of annealing temperature and additive mixing into antisolvent. PL intensity from double halide perovskite film has been doubled at 100nm-sized homogenous grain compared to control smaple with larger grain size(~300nm). We anticipate these techniques would pave the way of Cs2AgBiBr6 as new candidates for lead-free light-emitters.

Hybrid organic-inorganic materials have been employed in a variety of applications including photovoltaics, optoelectronics, and nonlinear optics. Despite their unique and diverse properties, there is still potential to employ the organic molecules as more active contributors to the optoelectronic properties of these materials. We have built towards this goal by exploring three lead halide hybrid materials all containing the organic molecule 4,4’-methylenedianline (MDA). This organic molecule was targeted as part of an objective to determine how and if organic molecules with a clear molecular dipole can induce a dipole in the solid state. In this work, we present the (MDA)Pb₂I₆, (MDA)Pb₂Br₆, and (MDA)PbCl₄ hybrids which display diverse properties within the series. Although they all contain the same organic moiety, only (MDA)Pb₂I₆ and (MDA)Pb₂Br₆ adopt the same polar structure and are SHG active with (MDA)Pb₂Br₆ having a slightly larger SHG intensity than α-SiO₂. (MDA)Pb₂Br₆ and (MDA)PbCl₄, are photoluminescent at room temperature. Photophysics and SHG intensities will be discussed along with diffuse reflectance, dielectric measurements, and DFT calculations. This work highlights that organic molecules can take a more active role in directly affecting the properties of hybrid materials.

In this talk I will discuss about the origin of the broad band emission (BE) in two-dimensional metal halide perovskites of Ruddlesden-Popper type. These compounds have recently moved into the centre of attention of perovskite research due to their potential for light generation and for stabilisation of their 3D counterparts. It has become widespread to attribute luminescence with a large Stokes shift to self-trapped excitons (STEs), forming due to strong carrier-phonon interactions in these compounds.
In contrast to most reports, we focus on single crystals in order to exclude grain boundaries and interfaces as the origin of the BE. Through a combination of spectroscopic techniques, we show that intrinsic self-trapping is not responsible for the broad emission in these materials. Instead, we show that extrinsic factors determine the presence or absence of the broad emission band and we relate them to in-gap states caused by defects. Temperature dependent analysis and low energy excitation are used to highlight that several emitting states are present
within these materials' band gap. We hence show that exciton self-trapping and its importance in halide perovskites cannot be postulated a priori, but its relevance has to be demonstrated experimentally case by case. I

The Rashba effect has been proposed to give rise to a bright exciton ground state in halide perovskite nanocrystals (NCs), resulting in very fast radiative recombination at room temperature and extremely fast radiative recombination at low temperature. [1] In this work we find the dispersion of the "Rashba exciton", i.e., the exciton whose bulk dispersion reflects large spin-orbit Rashba terms in the conduction and valence bands and thus has minima at non-zero quasi-momenta. [2] Placing Rashba excitons in quasi-2D cylindrical quantum dots, we calculate size-dependent levels of confined excitons and their oscillator transition strengths. We consider the implications of this model for two-dimensional hybrid organic-inorganic perovskites, discuss generalizations of this model to 3D NCs, and establish criteria under which a bright ground exciton state could be realized.
[1] Nature 553, 189 (2018) DOI: 10.1126/science.aau7392
[2] Nanoscale 13, 16769 (2021) DOI: 10.1039/d1nr04884h

Halide perovskite nanocrystals have emerged as excellent light sources for light-emitting devices and lasers due to quantum yields approaching unity and an emission wavelength tunable throughout the visible range. As recently shown, it is the only material currently known that exhibit Rashba effect-induced bright-dark exciton inversion, greatly enhancing radiative efficiency. Nanoplatelets, colloidal semiconductor quantum wells, have additional advantages for light emission, as quantum confinement and dielectric confinement substantially enhance their radiative rates and outcoupling efficiencies.
Here, we investigate the excitonic fine structure of thickness tunable halide perovskite nanoplatelets by merging time- and temperature-resolved photoluminescence spectroscopy with an effective mass model. We find a thickness-dependent bright-dark exciton splitting reaching up to 33 meV, the largest ever reported value. Additionally, we find a splitting of the bright exciton into in-plane and out-of-plane polarized states by up to 16 meV. Due to these immense splitting energies, the radiative properties of the nanoplatelets will also be effected at room temperature. The experimental results obtained in this paper are in excellent agreement with the novel theory, which takes quantum confinement and dielectric confinement anisotropy into account. Notably, the model we develop is applicable to any semiconductor nanoplatelet and generalized for any nanostructure.
We further confirm the model through single NPL PL spectroscopy at low temperatures. Additionally, we investigate a giant oscillator strength transition (GOST) of the dark exciton state and quantify multiexcitonic effects (trions, biexcitons, exciton-exciton annihilation).

Spin-orbit coupling (SOC) of electrons is responsible for a number of quantum phenomena in solids, including spin Hall effect and topological insulators. Such SOC phenomena also emerge in photonic systems. In planar microcavities, the natural splitting between transverse electric (TE) and transverse magnetic (TM) modes behaves as a winding in-plane magnetic field B_T on the photon spin and results in the photonic spin-Hall effect. If in-plane optical anisotropy is present to break rotational symmetry, there is an effectively constant magnetic field B_XY which, in combination with B_T, leads to a non-Abelian gauge field for photons or exciton-polaritons. While such photonic systems have been demonstrated for synthetic Rashba-Dresselhaus Hamiltonians, an exciting prospect is forming exciton-polariton condensates under such a gauge field to simulate a number of phenomena in SOC quantum fluids, in analogy to what have been demonstrated in SOC Bose-Einstein condensates (BECs) in cold atoms. Here we demonstrate SOC exciton-polariton condensation in microcavities composed of single crystal lead halide perovskites (LHPs) known for strong light-matter coupling. We take advantage of low-symmetry phases of LHPs with strong optical anisotropy, which is tunable by composition and/or temperature, and measure the spin textures produced by the effective magnetic fields using polarization resolved Fourier space photoluminescence (FS-PL) imaging. In the region near where B_T and B_XY cancel each other, the results are well-described by the Rashba-Dresselhaus Hamiltonian. As the SOC exciton-polaritons undergo condensation with increasing density, as is evidenced by a two-threshold lasing behavior, B_T vanishes towards k_|| = 0 and the dominance of B_XY gives rise to two competing condensates with mutually orthogonal polarizations that are separated in space and momentum. These exciton-polariton condensates offer a platform for the exploration of many-body SOC phenomena in quantum fluids.

In the last 30 years, inorganic semiconductor nanocrystals have been extensively researched because of their size-dependent optoelectronic properties, thus successfully debuting in the commercial sector. The synthetic development of nanocrystals, however, has perplexed from the complex energy landscape of the reaction. Unlike intermediates that can be isolated and characterized with atomic precision in organic synthesis, the intermediates in nanocrystal synthesis are difficult to resolve because of the co-existence of metastable states with similar energy. If one can control the energy of the metastable intermediates originating from various factors including shapes and surface-ligand interaction and isolate the singular intermediate species, the nanocrystal growth with atomic precision could be achievable.

In this presentation, I will briefly discuss the tetrapod InAs nanocrystals as a crystalline “late intermediate” that warrants controlled colloidal nanocrystal growth. The use of the late intermediate with well-defined facets at the sub-10 nm scale for directional growth with atomic control and highlight the potential for the new directed approach of nanocrystal synthesis. Secondly, a route of synthesizing highly mono-dispersed InAs colloidal quantum dots in wide size-range by introducing chemical reactivity-controlled “early intermediate” InAs nanoclusters will be presented. Finally, n- and p- doping polarity tuned InAs quantum dot film based optoelectronic device characteristics will be discussed.

Due to high emission efficiencies and size-controlled emission wavelengths, colloidal quantum dots (QDs) are attractive materials for the realization of solution-processable laser diodes [1, 2]. In addition to facile spectral tunability, QD gain media benefit from a wide separation between their atomic-like states, which inhibits thermal depopulation of the band-edge ‘emitting’ levels and thereby reduces lasing thresholds and improves temperature stability [3]. Despite these advantageous features, colloidal QD lasers are yet to reach the stage of technologically viable devices. A primary obstacle is nonradiative Auger recombination, which leads to very fast relaxation of optical gain [4]. This represents an especially serious challenge in the case of inherently slow electrical pumping when multiexciton states, required for optical gain, are generated via step-by-step injection of individual carriers. Another complication is poor stability of QD solids under high current densities needed to enact the lasing effect. Recently, these challenges have been successfully tackled via novel approaches to Auger decay engineering and special strategies for boosting current densities to ultrahigh values of ~1,000 A cm^-2 [5]. These advances have brought the QD lasing community very close to its ultimate objective, which is the realization of a QD laser diode (QLD). In this presentation, I will overview the principles of light amplification with colloidal QDs, assess the status of the QD lasing field, examine the remaining challenges on a path to a QLD, and, finally, discuss practical strategies for attaining electrically pumped QD lasing.

[1] Y.-S. Park, J. Roh, B.T. Diroll, R.D. Schaller, V.I. Klimov, Colloidal quantum dot lasers, Nature Reviews Materials 6, 382 (2021).
[2] H. Jung, N. Ahn, V.I. Klimov, Prospects and challenges of colloidal quantum dot laser diodes, Nature Photonics 15, 643 (2021).
[3] Y. Arakawa, H. Sakaki, Multidimensional Quantum Well Laser and Temperature-Dependence of Its Threshold Current, Appl. Phys. Lett. 40, 939 (1982).
[4] V.I. Klimov, A.A. Mikhailovsky, A. Malko, J.A. Hollingsworth, C.A. Leatherdale, H.J. Eisler, M.G. Bawendi, Optical gain and stimulated emission in nanocrystal quantum dots, Science 290, 314 (2000).
[5] J. Lim, Y.-S. Park, V.I. Klimov, Optical Gain in Colloidal Quantum Dots Achieved by Direct-Current Charge Injection, Nature Materials 17, 42 (2018).

Colloidal quantum dots (QDs) are nanometer-scale crystals of inorganic semiconductors that are capped by organic or inorganic ligands. These QDs form an excellent, solution-processable materials class for thin-film electronics and optoelectronics. In this talk, I will describe and compare post-synthesis surface- and cation-exchange processes to engineer the optoelectronic properties QD thin films. In the first example, starting with electronically-coupled, thiocyanate-capped, wurtzite CdSe QD thin films, we use sequential cation exchange processes to substitute Cu⁺ for Cd²⁺ to realize Cu₂Se nanocrystal thin-film intermediates. Subsequent partial cation exchange with the liquid-coordination-complex trioctylphosphine-indium chloride yields n-doped CuInSe₂ NC thin films [1]. These CuInSe₂ nanocrystal films are used to form the channel of field-effect transistors (FETs), which upon Al₂O₃-encapsulation, are air-stable devices with high electron mobilities of ~10 cm²/Vs and current modulation of 10⁵, comparable to those of high-performance Cd- and Pb-containing QD FETs. In the second example, we begin with epitaxially-fused PbSe QD thin films, and using sequential Cd⁺ and Cd²⁺ cation exchange, realize epitaxially-fused, zinc-blende CdSe QD films [2]. We study the detrimental influence of Pb²⁺ impurities inherited from the starting QDs on the characteristics of CdSe QD FETs and how engineering of the device architecture can suppress these detrimental effects [3]. We realize FETs with high electron mobilities of 35 cm²/Vs and current modulation of 10⁶, after doping [2,3].
1. H. Wang, D. J. Butler, D. B. Straus, N. Oh, F. Wu, J. Guo, K. Xue, J. D. Lee, C. B. Murray, C. R. Kagan, “Air-Stable CuInSe2 Nanocrystal Transistors and Circuits via Post-Deposition Cation Exchange,” ACS Nano 13, 2324-2333 (2019).
2. Q. Zhao, G. Gouget, J. Guo, S. Yang, T. Zhao, D. B. Straus, C. Qian, N. Oh, H. Wang, C. B. Murray, C. R. Kagan, “Enhanced Carrier Transport in Strongly Coupled, Epitaxially Fused CdSe Nanocrystal Solids,” Nano Lett. 21. 3318-3324 (2021).
3. Q. Zhao, S. Yang, J. J. Ng, J. Xu, Y. C. Choi, C. B. Murray, C. R. Kagan, “Impurities in Nanocrystal Thin-Film Transistors Fabricated by Cation Exchange,” J. Phys. Chem Lett. 12, 6514-6518 (2021).

Nanostructured semiconductors, or quantum dots (QDs), are heavily investigated for their applications in light emission such as light emitting diodes and lasers. The premise of cost-effective solution processing of such devices based on nanocrystals has recently driven research towards electrically pumped population inversion in laser diode structures. Challenges however remain to achieve net light amplification in the cavity due to a balance between limited material gains and lossy electrical contacts. Further reductions in threshold current densities, mainly limited by the non-radiative cap of ca. 1 nanosecond on the gain lifetime, are also required to achieve stable operation. Finally, color tunability is limited to the red by the gain bandwidth of the red-emitting CdSe/CdS QDs or even core/shell nanoplatelets used.
Here, we show that weakly confined charge carriers in giant CdX (X=S,Se,Te) quantum dots display disruptive optical gain metrics that could alleviate these remaining issues. Being active in the green and near-infrared part of the spectrum, their properties match and even outcompeting state-of-the-art colloidal materials in the red. Material gain coefficients up to 50.000 cm^-1 combined with a broad gain window of >100 nm and gain lifetimes close to 3 ns are found. Gain thresholds of 9 uJ/cm2 are also on-par with the best performances reported so-far for nanomaterials. Invoking a model of stimulated emission based on bulk semiconductor physics, we are able to explain all of these remarkable gain metrics if a large band gap renormalization effect is invoked. Our results show that weakly confined nanomaterials are excellent gain materials, combining straightforward wet chemical synthesis and the promise of solution processability with beyond state-of-the-art gain metrics.

Single-photon sources are in great demand for quantum information technologies. A perfect single photon source emits one and only one photon at a time. This photon 'anti-bunching' is characterized by the second-order intensity correlation function, g⁽²⁾(τ), with g⁽²⁾ at time zero of < 0.5. Single-photon emission has been observed in a wide variety of systems (1), but there is still a need for electrically-pumped single-photon sources capable of operating at room temperature with stable emission. Other desired characteristics include: 1) ease of operation, 2) suppression of background noise, 3) scalability using established micro-fabrication techniques, and 4) ease of integration with photonic resonators and waveguides for on-chip manipulation of single photons. However, current room temperature electrically-driven single photon sources (based, e.g., on defects in diamond or silicon carbide) suffer from poor single-photon purity and low determinicity in photon emission.

Here we describe the fabrication and characterization of colloidal quantum dot (QD) LEDs (QLED) which exhibit high-purity single photon emission at room temperature. QDs have received extensive attention due to their size-tunable emission color, reproducible and scalable solution-based synthesis, narrow emission linewidths and high emission quantum yields approaching 100%. In our devices, single photon emission is achieved by dilution of the QD concentration by ~5 orders of magnitude below that of a normal QLED. Further, to meticulously control electron/hole injection we apply a novel LED design that features an insulating interlayer embedded with QDs into which current is focused, while emission from a single QD is isolated via an aperture in the optical path to the detectors.

At room temperature, stable non-blinking electroluminescence is observed with a single QD linewidth as narrow as ~25 meV, which is equivalent to the room-temperature thermal energy (k_BT = 25 meV). At higher biases, we realize dual-band emission due to the band-edge (1S) and the excited-state (1P) transitions. This unusual emission regime is a direct result of a novel device design which allows us to flow ultrahigh currents through a single QD and achieve high steady-state QD occupancies of more than 2 excitons per QD. The measured g⁽²⁾(0) values are of ~0.2 under low biases, indicating high single-photon purity. The g⁽²⁾(0) dip shows an exponential decay with a lifetime of 4.5 ns, indicating that emitting species are negatively charged excitons (negative trions) as dictated by intentionally imbalanced charge injection. As was demonstrated previously, negative charging helps suppress intensity fluctuations and accelerates emission rates (2).

Finally, to establish that our single-dot QLEDs are suitable for the deterministic, on-demand generation of single photons, we employ pulsed electrical excitation with pulse widths varied from 3-50 µs. We are able to vary the number of photons emitted per pulse from << 1 to >> 1 by changing the pulse width, the repetition rate and the applied voltage. This wide-range control over the photon number signifies a transition from a regime of quasi-random photon emission to the deterministic generation of single photons. This proof-of-concept demonstration paves the way for the near-future use of single-dot QLEDs as inexpensive, spectrally tunable, room temperature, and deterministic sources of single photons for a range of emerging quantum technologies.

1. Lin, X., Dai, X., Pu, C. et al. Electrically-driven single-photon sources based on colloidal quantum dots with near-optimal antibunching at room temperature. Nature Communications 8, 1132 (2017). https://doi.org/10.1038/s41467-017-01379-6
2. Galland, C., Ghosh, Y., Steinbrück, A. et al. Two types of luminescence blinking revealed by spectroelectrochemistry of single quantum dots. Nature 479, 203–207 (2011). https://doi.org/10.1038/nature10569

The demonstration of a new-generation of laser diodes employing solution-processable materials has been actively pursued across multiple fields including organic molecules, perovskites, and colloidal quantum dots (QDs). The latter structures are especially attractive due to their outstanding light-emission properties and sparce atomic-like electronic states with size-controlled energies. A primary obstacle towards a QD-based laser diode is very fast Auger decay of multiexciton states required for a lasing action. Recently, continuously graded QDs (cg-QDs) with suppressed Auger recombination demonstrated optical gain achieved with electrical injection in a “current-focusing” light-emitting diode (LED)^1,2. However, light amplification from QDs in a complete LED stack has not been yet realized because modal gain has not been sufficiently high to outcompete optical losses. Here, we demonstrate a new LED architecture, which employs a newest generation of rigorously optimized cg-QDs incorporated into a two-dimensional current focusing device. By tuning the thicknesses and refractive indices of various elements of the LED stack, we maximize a mode confinement factor for the gain-active QD layer and simultaneously minimize the extent of the optical mode into lossy charge transport layers. Using this approach, we are able to achieve the regime when modal gain exceeds optical losses and realize amplified spontaneous emission (ASE) within both the band-edge (1S) and the higher-energy (1P) transitions using pulsed optical excitation. The same devices also demonstrate strong optical gain under electrical pumping. In particular, we are able to invert both the 1S and 1P transitions using current densities of >500 A cm^-2, which leads to generation of more than 4 excitons per dot on average. The experimental observations are rationalized through numerical COMSOL modeling with input optical-gain and loss parameters derived from variable stripe length measurements. These studies prove once again the feasibility of colloidal QD laser diodes and clear some of the last obstacles on the path to a functional laser diode based on colloidal QDs.
1. J. Lim, Y.-S. Park, V. I. Klimov, Nature Materials, 17, 42-49 (2018)
2. H. Jung, N. Ahn, V. I. Klimov, Nature Photonics, 15, 643-655 (2021)

Luminescent solar concentrators (LSCs) are semi-transparent light harvesting devices, comprised of nanocrystal (NC) luminophores embedded inside optically transparent waveguides with small photovoltaic (PV) cells mounted on the edges. LSCs harvest solar energy by absorbing broadband sunlight, downshifting the light to a narrow spectrum, and concentrating the light toward the PV cells. One promising application space for LSCs is in agrivoltaics, where the LSC is mounted in a greenhouse, and simultaneously generates electricity and modifies the spectral quality of the transmitted light, promoting plant growth. The PV efficiency of LSCs can be limited by low luminophore absorption, however, and increasing the NC concentration in a polymer matrix leads to significant scattering that is detrimental to LSC performance. For agrivoltaics, the transmitted spectrum of LSCs also needs to have exceptional tunability to control the spectral quality. Therefore, we studied a bilayer LSC utilizing two different semiconductor quantum dots for enhanced absorption efficiency, and a tunable transmission spectrum.

A nanocomposite bilayer LSC comprised of a film of narrow band gap Si NCs embedded in poly(methyl methacrylate) (PMMA) and a film of wider band gap CdSe/CdS NCs embedded in poly(cyclohexylethylene) (PCHE) was fabricated. The Si NCs were synthesized through a non-thermal plasma synthesis method, and the film of the Si NCs embedded into PMMA was fabricated using doctor blade deposition. The CdSe/CdS NCs were synthesized using a colloidal synthesis method, then dispersed in octane. A solution of CdSe/CdS and PCHE in octane was spin-coated directly on the Si-PMMA film to create the bilayer LSC. Diffuse transmission measurements indicated low levels of haze in the bilayer devices.

Complementary Monte Carlo ray-tracing simulations were used to evaluate device efficiency. Compared to a single layer of Si-PMMA, the addition of the CdSe/CdS-PCHE film increased the overall absorption, which enhanced the optical efficiency of the device. Additionally, the CdSe/CdS nanocrystals act as sensitizers, increasing the absorption of the Si nanocrystals.

Light propagation through the LSC device was characterized experimentally by measuring the concentrated PL while changing the position of a 405 nm light source. The Si photoluminescence (PL) propagates through the bilayer LSC device with an attenuation coefficient close to zero, which is a necessary property for high efficiency LSCs. This finding emphasizes the benefit of sensitizing Si NC absorption through the incorporation of the CdSe/CdS NCs.

To demonstrate the tunable transmission spectrum of the bilayer device, the Soret and Q band transmission of chlorophyll was calculated while changing the optical density of the CdSe/CdS and Si NCs. The transmission through either band could be controlled by changing the optical density of either nanocomposite, thus this device can be used to design a transmission spectrum tailored for more ideal crop germination and growth.

This study demonstrates that incorporating two nanocrystal quantum dots into a bilayer LSC design can enhance absorption efficiency and spectral tunability, while also revealing the nature of the interactions between the luminophores.

Hybrid inorganic-organic lead halide perovskites have captured significant interest in the thin film optoelectronics community due to their impressive optical and electrical properties. For light emitting diodes (LEDs), we are exploring their operation under very high current densities, for high-brightness applications and, eventually/hopefully, lasers. Our findings suggest that Joule heating and voltage-induced electrochemical interface reactions present substantial challenges to these operating conditions.

Short-wave infrared stimulated emission and lasing action at room temperature have been recently reported in uncharged and heavily doped PbS colloidal quantum dots (CQDs). ^[1,2] However, due to the presence of active in-gap trap states in the gain medium, excessive nonradiative Auger recombination in the order of ~200 ps outcompetes stimulated emission in PbS CQDs. ^[2,3] This results in high optical gain thresholds, where amplified spontaneous emission (ASE) and lasing thresholds are in the range of 800-1500 μJ.cm^-2 for neutral PbS CQDs films. ^[1,2] Here, we exploit a binary blend of PbS CQDs and ZnO nanocrystals in order to electronically passivate the trap states of PbS CQDs. ^[3] Owing to successful remote passivation of the trap states in the binary blend, overall trap states density dropped by an order of magnitude to 2.36×10¹⁶ cm^-3. As a consequence, photoluminescence quantum yield (PL-QY) of 35% is observed for PbS/ZnO binary blend films, significantly increased over the PL-QY of ~2% in bare-PbS CQDs solid film. Using transient absorption spectroscopy, the Auger recombination lifetime is prolonged five-fold up to 1000 ps. Owing to high suppression of trap-assisted Auger process and the reduction of self-absorption losses in the gain medium, an ASE threshold of 430 μJ.cm^-2 is achieved, demonstrating a greater than two-fold reduction. Finally, coupling our proposed binary blend of PbS/ZnO to an alumina DFB resonator, we present a room temperature single-mode lasing emission at 1650 nm within a linewidth of 1.23 nm (0.62 meV). Our short-wave infrared laser operates at a record low threshold of 385 μJ.cm^-2, exhibiting a record lasing stability up to 5 hours continuous pumping at 10 kHz under ambient conditions. Overall, these remarkable results suggest a novel method for supressing Auger recombination, paving the way to use PbS CQDs as an efficient gain medium for the infrared lasers.
References:
[1] S. Christodoulou et al., Nano Letter 2020, 20, 5909-5915.
[2] G. L. Whitworth et al., Nature Photonics 2021, 15, 738-742.
[3] N. Taghipour et al., under review.

Metal halide perovskites are attracting increasing interest for applications in photonics, light emitting devices and lasers. However, the need to overcome trap-mediated non-radiative losses, increase the radiative recombination efficiency, improve the material stability, and find non-toxic alternatives to the standard lead-based materials urgently calls for the study of different perovskite structures.
Currently, perovskite lasers remain restricted to 3D architectures with limited chemical tunability, or multidimensional perovskites of the Ruddlesden-Popper series where the radiative recombination still relies on the phases with highest dimensionalities due to energy funneling processes. 2D perovskites have long been proposed as promising alternatives to improve the luminescence efficiency, thanks to their extremely high exciton binding energy which allows the formation of stable excitons and favours fast radiative decay. At the same time, their layered architecture can grant increased environmental stability, block detrimental ionic diffusion, and relax geometrical constraints giving access to a wider range of lead-free compositions. Nevertheless, compared to the parental 3D structures, lasing in single-layered 2D perovskites has been very scarcely reported, [1-3] and their ability to sustain coherent emission remains unclear and highly debated. [4-6] Notably, to our knowledge, amplified spontaneous emission (ASE) in single-layered 2D perovskites has never been reported.
In this work, we investigate the gain properties of 2D tin-based perovskites. By combining temperature-dependent structural characterization and optical spectroscopy, we show that the choice of the templating cation critically affects the structural rigidity, defectivity, photostability and morphology of the perovskite film determining its ASE properties. Through proper synthetic design, we observe intense ASE with very low threshold (down to 30 μJ/cm²), optical gain coefficient comparable to that of the archetypical methylammonium lead iodide [7] (up to 3500 cm^-1 at 77 K) and ASE operational stability up to room temperature in tin layered perovskites. We further discuss the integration of such 2D perovskites with optical resonators to achieve optically-pumped lasing.
Our findings represent the first characterization of the gain properties of single-layered perovskites, providing fundamental synthetic guidelines to improve their coherent emission properties and opening the way to the design and exploitation of a new class of lead-free perovskite lasers.
[1] T. Kondo et al, Solid State Commun. (1998), 105, 253-255.
[2] E. P. Booker et al, Adv. Optical Mater. (2018), 6, 1800616.
[3] C. M. Raghavan et al, Nano Lett. (2018) , 18, 3221-3228.
[4] W. K. Chong et al, Phys. Chem. Chem. Phys. (2016), 18, 14701.
[5] M. R. Layden et al, Phys. Chem. Chem. Phys. (2018), 20, 15030.
[6] Y. Liang et al, Adv. Mater. (2019), 31, 1903030.
[7] A. L. Alvarado-Leaños et al, Adv. Opt. Mater. (2021), 9, 31, 1903030.

Foldable ultrathin light sources and displays can play an important role in realizing future optoelectronic devices such as body attachable devices due to their excellent form factor. Perovskite light-emitting diode (PeLED) is a promising candidate as a light source to be applied to next-generation electronics thanks to its superior optoelectronic properties, such as high color purity. Due to its excellent material properties, several studies have been reported to realize a highly flexible light source using perovskite as a light source. However, these studies have been mainly focused on implementing highly flexible perovskite light-emitting diodes by securing mechanical stability. Since realizing low power consumption and long-term device lifetime is essential for use in body-attachable and wearable devices, it is also necessary to focus on improving the optical efficiency of foldable light sources.
Here, we improved the light extraction efficiency by introducing an external light extraction structure to perovskite light-emitting diodes fabricated on an ultrathin substrate. In addition, to ensure the long-term operation of the device, the additional UV-curable resin was spin-coated onto the surface of the devices to ensure mechanical robustness by the formation of the mechanical neutral plane. Through this, it was confirmed that the entire structure of the perovskite light-emitting diode, particularly, the oxide electrode having a high young's modulus, which was typically used in the flat devices, could be used as it is. As a result, the EQE of the foldable perovskite light-emitting diode with these light extraction structures was improved by 60% compared to the reference device. Also, the foldable perovskite light-emitting diodes show no degradation in performance even after repeating 10,000 bending cycles at a bending radius of 250 micrometers. Foldable light sources with such external light extraction structures are expected to play a key role in building body-attachable and wearable platforms.

While lead-free halide (Cs₃Cu₂X₅) perovskites have attracted a great deal of attention in the research community, size- and shape-controlled synthesis of these materials in the nanometer regime remains rather unexplored. Here, we present an efficient ligand-mediated hot injection method for fabricating Cs₃Cu₂Br₅ nanocrystals with controlled morphology and properties. Use of different chain-length ligands resulted in different morphologies such as irregularly shaped nanoparticles, nanocuboids, nanospheres, and nanodisks. In addition, the nanocrystals had band gap energies and optical absorptions that depended on their shape. Finally, this work provides a fundamental understanding of the factors that are important for predictive synthesis of perovskite nanocrystals with the desired properties.

Metal-halide perovskite nanocrystals (PeNCs) have garnered great scientific attention as promising light-emitting materials due to their excellent optical and electrical properties such as high-color purity light-emission, high photoluminescence quantum yield, halide composition-dependent tunable emission wavelength, and high charge carrier mobility. Over the past few years, considerable advancement has been made on green and red perovskite LEDs with external quantum efficiencies exceeding 20%. However, the performance of blue PeLEDs still lags far behind that of red and green counterparts. In this study, we have demonstrated efficient blue perovskite LED (PeLED) employing dually passivated perovskite nanocrystals of CsPbBr_xCl_3-x with thiocyanate and neodymium Ions. By adopting thiocyanate and neodymium Ions-assisted dual passivation, chlorine vacancies were effectively passivated, leading to the greatly enhanced efficiency and stability of PeNCs. The obtained PeNC was inserted into LED devices, and the fabricated blue PeLEDs in this manner showed significantly improved device performance.

Quantum dot light-emitting diodes(QLEDs) has emerged as a one of the next-generation displays due to their outstanding advantages such as high efficiency, high color purity, color tunability and low energy consumption. Process benefits like solution processability and comparability with flexible substrates are also advantages of QLEDs. However, despite many advantages, QLEDs have a critical problem of efficiency drop at high current levels, as known as efficiency roll-off phenomena. Among the various causes of efficiency roll-off, charge imbalance is considered one of the biggest reasons. In general, hole injection barrier in QLEDs is much larger than that of electron injection barrier because of Cd-based QD’s deep HOMO(Highest Occupied Molecular Orbital). As a result, excess electrons are injected into the device and the device becomes electron-rich state which increases the possibility of non-radiative recombination like Auger-recombination. This charge imbalance affects not only efficiency roll-off but also decrease of lifetime. Therefore, it is very important to control the charge injected into the device so that the holes and the electrons could be balanced well. To make improvement of charge balance, various studies have been conducted mainly by boosting hole injection. The most widely known way to boost hole injection into QLED is adopting inverted structure which are fabricated by deposition of monomer with appropriate energy level on solution processed quantum dot layer. When inverted structure is applied, there is an advantage that various materials which are difficult to utilize the solution process can be used. However, this method includes additional thermal evaporation process, increasing the fabrication complexity.
In this work, we reported QLEDs with improved efficiency roll-off by controlling charge injection based on all-solution process. To make improvement of charge balance, we applied two different methods, hole injection enhancement and electron injection suppression, respectively. Specifically, HTL material (TFB) having high hole mobility was applied as a method for enhancing hole injection characteristics. For controlling the electron injection characteristics, we fabricated QLEDs with mixed-ETL consist of ZnO with LiQ (8-hydroxyquinolatolithium). By mixing LIQ with ZnO, electron mobility was controlled according to the mixing concentration. In order to evaluate the efficiency roll-off phenomena quantitatively, we normalized the current efficiency of each device and compared the numerical values at a specific luminance (5,000 nit) to see under what conditions the characteristics was improved. As this value approached to 1, it could be judged that the efficiency roll-off was improved. As a result, the normalized current efficiency of devices with enhanced hole injection and suppressed electron injection were 0.98 and 0.97, respectively, while that of the reference showed only 0.89. These results confirmed that by controlling each charge to improve the charge balance state, the efficiency roll-off of the QLED was changed significantly. However, in the case of the device that applied hole injection enhancement and electron injection suppression simultaneously, the charge balance was collapsed again. Finally, the normalized current efficiency was found to be 0.90.
The detailed methods and results will be discussed at the conference.

Perovskite solar cells (PSCs) research has been projected over the past decade due to its promising photovoltaic properties, increasing efficiency and low cost. With a need to eliminate toxic lead from prevailing halide perovskite solar cells, several low-toxicity cations have been proposed, among them Sn(II) is considered as one of the most suitable replacements due to its similar electronic configuration and ionic radius to Pb(II). Here we demonstrate the use of atmospheric solution-processed partially Pb-substituted all inorganic cesium tin-lead triiodide (CsSnPbI₃) as the light absorber in PSCs. The present work aims at analyzing the effect of pyrazine and guanidinium thiocyanate (GuaSCN) in fabrication of low bandgap (1.5 eV) films when used separately as a mediator during film fabrication. Effect of CsCl as an additive and DMSO as one of the solvents in perovskite morphology optimization has also been studied. The presence of excess SnF₂ in precursor solution assists phase separation. Pyrazine reduces Sn vacancy thus restricts phase separation and GuaSCN improves the structural and optoelectronic properties of low bandgap perovskites. Initial results suggest guanidinium thiocyanate gives absorption onset at higher optical wavelength (>1000nm) and low bandgap (~1.3-1.5 eV) while Pyrazine and films with no additive tend to have wider bandgap. GuaSCN induced films show higher crystallinity and lower oxidation tendency. Films both with and without additives have dendritic morphology which comes from insolubility of Pb. Addition of CsCl as an additive in the precursor partially suppresses dendrite formations. Addition of DMSO in mixed solvents completely removes the dendrites and optimizes the film surface. All the samples with GuaSCN after dark testing for over 100 hours at 65°c retain their properties and show similar absorption edge and bandgap pattern implying high stability and reproducibility achieved with GuaSCN incorporation.

CsPbBr₃ perovskite nanorods (NRs) are emerging as a new class of semiconductor nanomaterial with remarkable optical properties, such as high quantum fluorescence yields (FQYs) and tunable emission wavelengths. In addition, these materials display high emission anisotropy, making them potential candidates for applications that require oriented photon emission, such as luminescent solar concentrators and LCD backlights. However, unlike more well-established cadmium chalcogenide NRs, the origin of emission anisotropy is less well understood. To date, there are two major sources of emission anisotropy in semiconductor NRs: (1) classical dielectrics effects and (2) quantum confinement of electronic transition dipoles. Here, we measure the emission anisotropy of an ensemble of CsPbBr₃ NRs and apply a dielectric model based on the dielectric constant of the NRs and their environment to analyze the interplay between electronic and dielectric contributions to their emission anisotropy. An excitation photoselection method was used to measure the emission anisotropy of NRs ensembles. The contribution of dielectric effects to the emission anisotropy of CsPbBr₃ NRs was analyzed by modeling the NR as a dielectric prolate ellipsoid and calculating the electric field distribution inside the NR using the finite difference time domain (FDTD) method. Ensemble emission anisotropy measurements of NRs show an energy-independent anisotropy spectrum with a constant value close to 0.1. Moreover, the determination of the emission anisotropy of NRs based on the dielectric model reveals an anisotropy value close to 0.1, indicating the significant role of the dielectric effect on the obtained anisotropy. These results suggest that the emission anisotropy in CsPbBr₃ NR is mainly due to the dielectric confinement of the optical electric field and not the result of quantum confinement of transition dipoles. These findings provide a fundamental understanding of the origin of emission anisotropy in perovskite NRs and inform possible strategies to control anisotropy in these materials by changing their dielectric environment for applications that require the emission of oriented photons.

InP colloidal quantum dots(CQDs) have 1.35 eV of bulk bandgap that can cover the visible spectral region by controlling their size owing to the quantum confinement effect. In addition, InP CQDs are applied to the visible optoelectronic devices as an alternative for heavy metal (Cd-, Pb-) containing materials. Recently, single-crystalline InP tetrapod nanoparticles through the surface energy-driven growth have been reported.[1] Single-crystalline InP tetrapod QDs have only zinc blende structure, it is clearly different crystal structure from typically reported heterostructure nanotetrapods (e.g., tetrapod has a zinc blende core and four wurtzite arms).[2] Although novel optical and physical properties are expected in single-crystalline InP tetrapod QDs, some challenges, such as low photoluminescence (PL) and stability in ambient condition, limit the optoelectronic analyses. In this study, we especially focus on the shell passivation approach using ZnSe and ZnS on the tetrapod InP core with maintaining their shape to overcome these issues. Various reaction conditions are investigated for conformal shell growth on the core. Finally, the optical properties, structure, and shape of the tetrapod-shaped InP/ZnSe/ZnS core/shell/shell are analyzed through absorbance, PL, TEM, and XRD measurements.
[1] S. Jeong et al. Nat Commun. 2021, 12, 4454
[2] A. P. Alivisatos et al. Nano Lett. 2007, 7, 10

Lanthanide doped nanoparticles have been extensively researched and shown to be promising for a variety of optical applications including upconversion, bioimaging, super resolution imaging, photocatalysis, lasing and more. In the last decade, dye sensitised versions have bypassed the poor absorption of lanthanide species (both in spectral range and extinction coefficient) mainly based on the near-IR dye IR806¹ (or dyes of a similar structure). However, these systems still suffer from large losses leading to low efficiencies. By combining the strongest performing NIR dyes² with recent work on surface passivation³ using rigid bidentate ligands we outline the synthesis and behaviour of novel bidentate NIR sensitizers. The key modification being a change from benzoic acid type linker group to picolinic acid. The efficiency of these systems is compared to their traditional counterparts, deepening the understanding of the energy transfer mechanisms and loss pathways.

1. Zou, W., Visser, C., Maduro, J. et al. Broadband dye-sensitized upconversion of near-infrared light. Nature Photon 6, 560–564 (2012).
2. Wu, X., Lee, H., Bilsel, O. et al. Tailoring dye-sensitized upconversion nanoparticle excitation bands towards excitation wavelength selective imaging. Nanoscale 7 18424–8 (2015).
3. Xu, H., Han, S., Deng, R. et al. Anomalous upconversion amplification induced by surface reconstruction in lanthanide sublattices. Nat. Photon. 15, 732–737 (2021).

The development of single photon emitters (SPEs), which are critical to quantum communication and information processing, has largely utilized atomically-thin transition metal dichalcogenides (TMDs). Although TMDs are promising for their high light extraction efficiency, it has been challenging to modulate the emission sites and emission energy of TMD based SPEs. Strain engineering allows precise control of band structure and photon energy of TMDs. In particular, mechanical instability driven wrinkle on stretchable substrates is an excellent platform for tuning TMDs’ electronic structures via local strain manipulation. Here, we report strain-induced exciton energy modulation and single photon emission of biaxially wrinkled monolayer tungsten diselenide (WSe₂). We applied biaxial strains on monolayer WSe₂ by placing WSe₂ on a pre-stretched polydimethylsiloxane (PDMS) substrate with 10 to 50 % of prestrain and releasing the pre-stretch. The different types of biaxial wrinkle structure, such as herringbone or labyrinth shapes, were observed as functions of skin layer stiffness, prestrain, and encapsulation layer. We performed Raman, photoluminescence (PL), and absorption imaging via optical Raman/PL spectroscopy, tip-enhanced Raman/PL spectroscopy (TERS/TEPL) and photo-induced force microscopy (PiFM) to demonstrate modulation of PL energy and bandwidth with respect to the wrinkle topography. Furthermore, time-resolved PL and Hanbury Brown Twist (HBT) interferometry were performed at varying temperature to obtain exciton lifetime and degree of exciton antibunching as a critical parameter of SPEs. These results pave the way for mechanically reconfigurable SPEs and provide a new platform for biaxial local strain engineering of 2D materials and heterostructures.

Colloidal Quantum Dots (QDs) have attracted attention as a high definition and large area display for decades due to their outstanding optical characteristics, such as tunable bandgaps, solution processability, narrow emission spectra, and high photoluminescence quantum yield (PLQY). Since the first colloidal QDs light-emitting diode (QD-LED) is reported, various attempts are being made regarding the material design and device structures to make efficient devices.
In QD-LED devices, the carrier injection and transport balance are significant factors for improving device efficiency and lifetime. Typically, the QDs emission layer (EML) is caught in the middle of the electron transport layer (ETL) and hole transport layer (HTL), through which electrons and holes are transferred from the cathode and anode respectively. Therefore, the material used for these transport layers is very crucial to balance carrier injection and transport, and improve the radiative recombination in QDs EML.
As ETL, zinc oxide (ZnO) is widely used owing to its excellent transparency, high electron mobility, and ease of the process. However, In terms of charge balance, the fast ZnO mobility (~1cm²V^-1s^-1) is not balanced with the mobility of organic material as HTL (~10^-3cm²V^-1s^-1). This charge imbalance causes phenomena such as accumulation of electrons in EML, Auger recombination, and shifting emission zone, resulting in the QD-LED device's efficiency and lifetime loss. So there are many strategies for adjusting ETL to slow down the injection of electrons into QDs. As part of that, we placed passivation layers, poly(4-vinyl pyridine) (PVPy), in our devices between ZnO and QD to delay electron injection into EML.
Even if the injection of ZnO is delayed, matching the injection rate of electrons and holes depends on the HTL materials. It is important to use material with a suitable highest occupied molecular orbital (HOMO) level to facilitate hole injection into the deep valance band of Green QDs. Compared to ETL, more research is needed on the mechanism of hole injection according to various HTL in the inverted structure.
In this study, We compared 4 HTL materials, Poly(9,9-dioctylfluorene-alt-N-(4-sec-butylphenyl)-diphenylamine) (TFB), Poly(9-vinylcarbazole) (PVK), Poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA), and N4, N4'-Di(naphthalen-1-yl)-N4, 4'-bis(4-vinylphenyl) biphenyl-4,4'-diamine (VNPB) widely used for HTL. To find out the suitable HTL, we coated HTL materials on the QD layer combining various solvents like p-Xylene, Chlorobenzene, and Dioxane. Our inverted device structure is ITO/PVPy/Al-doped ZnO (AZO)/QDs/HTL/MoO₃/Ag. Furthermore, the device was manufactured using the various HTL to evaluate Electroluminescence (EL) characteristics through IVL measurement. In addition, Hole-only Device (HOD) was made ITO/MoO₃/HTL/MoO₃/Ag to observe the difference in hole conductivity of each HTL material. Moreover, We get the surface image of each HTL material using an atomic force microscope (AFM) to determine the effect of surface morphology on hole injection mechanism and device performance.
Through this study, we compared common HTL materials, TFB, PVK, PTAA, and VNPB, to fabricate the Cd-based inverted QD-LED devices. And we identified the most suitable HTL material for our inverted device structure through the process condition and various analyses of HTL materials.

Metal halide perovskite (MHP) is a promising candidate to achieve Rec. 2020 with size independently high color purity when their size is over exciton Bohr diameter. Ligands are widely accepted to control the crystallization of MHP. However, since the binding of ligands is based on weak electrostatic attraction, the stability of the MHP crystal is limited. Also, the ionic nature of MHP reduced their stability against the ambient environment. Here, we report the facile ligand-free synthesis method comprising perhydroxypolysilazane (PHPS). PHPS crystalized to SiO2-SiNx inorganic polymer composites and interact with MHP precursor to control the crystallization of MHP crystal. Synthesized MHP crystal was effectively encapsulated by an inorganic polymer matrix and showed high stability against high-temperature, water, and ambient environment.

Multiphoton upconversion of lanthanide-doped materials has proven the potential for future applications, such like solar energy harvesters, deep brain optogenetics, and near-infrared photoreceptors for mammalian retina. Behind the great success of the upconversion materials, there has been continued efforts to enhance upconversion efficiency such like hybridizing with nanoantennas, core-shell structuring and coupling with plasmonic nanostructures. However, one of the most promising approaches is to distort the lattice symmetry of the host materials to increase non-centrosymmetric crystal fields, which is crucial for deceiving the parity-forbidden nature of 4f-4f optical transitions, because it dominates all the optical properties of the upconversion materials including the fate of the synergetic effects with other enhancement principles. Distorting the lattice symmetry has been demonstrated by applying electric fields or pressure, doping interstitial ions, and rapid quenching of the molten host materials. However, the previous techniques have the limitations in practicability because they demand cumbersome operating conditions or ruin the hexagonal morphology of the NaYF₄-based upconverters. In particular, the hexagonal morphology of the NaYF₄-based upconverters is important for the potential applications, for example, micro-optical devices like microlasers or waveguides. Here, we describe a new post-synthetic modification method for NaYF₄-based upconverters which not only enhances upconversion efficiency but also maintains the high-quality hexagonal morphology of the initial materials. The post-synthetic modification was performed by low-pressure annealing. We found that the low-pressure annealing thermodynamically promotes hexagonal-to-cubic phase transformation of NaYF₄ to realize lattice disarrangement within few minutes for the upconversion enhancement, therefore, not to ruin the initial hexagonal morphology during the annealing process. We believe that our finding provides a clue for the breakthrough in upconversion efficiency by expanding the capability of the upconversion material itself to pave the way for next-generation applications.

Halide-based perovskite materials have been spotlighted in light-emitting diodes and perovskite solar cells applications owing to their excellent optical and electrical characteristics that enable a remarkable enhancement of device efficiency. Excellent film quality of perovskite with defect passivation is required to obtain highly efficient and stable perovskite optoelectronic devices. However, defect sites such as voids, pinholes, grain boundaries and under-coordinated ions, creating many undesired electronic trap sites can exist in solution-processed perovskite films. Here, we deal with the various problems that originated from surface defects of MHPs and suggest strategic approach of surface defect passivating materials for device efficiency and stability enhancement of perovskites.

Surface chemistry primarily defines colloidal nanomaterials and is thus quintessential for their entire life cycle and practical utility. Metal and covalent semiconductor nanomaterials have accumulated rich surface chemistry knowledge, including organic and inorganic capping ligands, or stability in solvents with various polarity.[1,2,3,4] Highly ionic surfaces, although ubiquitous among inorganic nanomaterials, remain a formidable challenge in the view of inherently non-covalent surface bonding of surface ligands.[5,6] Here we propose zwitterionic phospholipid molecules as universal capping ligands for various functional metal halide nanomaterials. Ligands comprising variety of aliphatic, aromatic, heteroatomic or even polymeric tails anchored to phosphocholine, phosphaethanolamine or phosphapropanolamine head groups are effective in the direct synthesis of nanocrystals or can be surface-attached post-synthetically. Due to readily available synthetic versatility of phospholipids, we demonstrate nascent efforts in transferring metal halide nanomaterials from apolar to more polar solvents by designing ligands to match specific solvents. To gain deeper insight into the surface chemistry of zwitterionic ligands, we conduct molecular dynamics and solid-state NMR experiments, which evidence that the ligand surface affinity is primarily governed by the geometric fitness of its anionic and cationic moieties. We specifically target improvement in softer and more labile metal halide nanomaterials, e.g., lead-free Cs₃B₂X₉ (where B=Sb, Bi, X=Cl, Br, I), FAPbX₃ and MAPbX₃ (FA-formamidinium; MA-methylammonium), a challenge that previously was out of reach with classical monodentate ammonium ligands. Capped with novel ligands, FAPbX₃ nanomaterials specifically exhibit superb performance in various demanding applications, i.e., single dot spectroscopy, electroluminescence, and self-assembly.

[1] Science 2015, 6222, 347, 615-616
[2] J. Am. Chem. Soc. 2013, 135, 49, 18536–18548
[3] Acc. Chem. Res. 2021, 54, 7, 1555–1564
[4] Science 2009, 5933, 324, 1417-1420
[5] ACS Nano 2016, 10, 2, 2071–2081
[6] Chem. Mater. 2021, 33, 15, 5962–5973

Halide perovskite has achieved a remarkable progress in its application to light-emitting diodes (LEDs) by taking advantages of high color purity with an extremely narrow spectral width (< 20 nm) and tunable band gap by adjusting the composition of halide anions. Most of the state-of-the-art perovskite light-emitting films were fabricated by a solution process, namely, spin-coating. This has the intrinsic limitation in the fine control of the emitter thickness and in mass production, necessary for commercialization. Here, we fabricate highly efficient vacuum-evaporated CsPbBr₃ perovskite LEDs with poly-ethylene oxide (PEO) passivation layer. A perovskite film was deposited on a PEO layer via thermal co-evaporation of CsBr and PbBr₂. The photoluminescence quantum yield (PLQY) of perovskite film improved as the thickness of PEO increased: from 0.6% with no PEO to 18.0% and 31.9% with a 15 nm and 40 nm PEO layer, respectively. This enhancement is attributed to the passivation of nonradiative defects via PEO-Pb²⁺ interaction, as suggested by x-ray photoelectron spectroscopy (XPS) and temperature-dependent photoluminescence measurements. Although a thicker PEO layer (> 40 nm) can further improve PLQY, the electrically insulating nature of PEO led to the failure of device operation. We improved the electrical conductivity of PEO by doping with MgCl₂. The doping strategy allowed us to take advantage of the passivation effect by PEO without losing the performance of final EL devices. The application of MgCl₂-doped PEO led to the luminance of 6887 cd m^-2 and a record external quantum efficiency of 7.6% for vacuum-deposited perovskite LEDs. Details of the results and analysis will be discussed.

Organic-inorganic hybrid halide perovskites are exciting new semiconductors that show great promising in low cost and high-performance optoelectronics devices including solar cells, LEDs, photodetectors, etc. However, the poor stability is limiting their practical use. In this talk, I will present a molecular approach to the synthesis of a new family of organic-inorganic hybrid material - Organic Semiconductor-incorporated Perovskite (OSiP). Energy transfer and charge transfer between adjacent organic and inorganic layers are extremely fast and efficient, owing to the atomically-flat interface and ultra-small interlayer distance. Furthermore, this rigid conjugated ligand design dramatically enhances materials’ chemical stability and suppresses solid-state ion migration and diffusion, making them promising for real-world applications. Using this stable hybrid materials, we demonstrate the fabrication of high quality polycrystalline thin films and highly stable and efficient LED devices with suppressed ion migration and improved external quantum efficiency, color purity, and operational lifetime. Optically-driven nano lasers will be discussed briefly as well.

Excitonic PbS nanoparticles have shown excellent infrared photosensitivity, but their limited diffusion lengths of photoexcited charge carriers result in poor device efficiencies. They also have slow radiative recombination rates that limit the performance of light-emitting diodes. To address these limitations, we work to couple PbS excitons with localized surface plasmons on the nanoscale in order to increase the radiative rate and absorption cross section. Combining infrared plasmonic Cu_2-xS nanocrystals with infrared excitonic PbS nanocrystals has the potential to improve the optical properties of PbS nanocrystals via exciton-plasmon coupling. We develop the first synthetic method to deposit a PbS shell onto Cu_2-xS nanocrystals.¹ This structure is confirmed to have a Cu_2-xS core with a patchy PbS shell using high-angle annular dark-field imaging scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy spectrum (EDS). Addition of the PbS shell changes the crystallographic phase of the Cu_2-xS cores and blueshifts and enhances their infrared plasmonic resonance. The shell also provides chemical stability to the Cu_2-xS nanocrystals such that they no longer completely chemically quench the photoluminescence of neighboring excitonic PbS nanocrystals in solution; consequently, these Cu_2-xS/PbS core/shell nanocrystals will enable future studies of infrared exciton-plasmon coupling at the nanoscale.

Increasing the PbS precursor concentration in the reaction used to deposit PbS shells onto the Cu_2-xS cores leads to cation exchange of some of the Cu_2-xS cores into an alloyed Cu_xPb_yS nanocrystals that have a new excitonic signature. Time-resolved photoluminescence spectroscopy shows a shorter photoluminescence lifetime relative to PbS nanocrystals but a comparable photoluminescence quantum yield, indicating faster radiative decay rates. This is particularly attractive for infrared single-photon emission. Initial HAADF-STEM EDS maps show two populations of nanocrystals: Cu_2-xS/PbS core/shell nanocrystals and Cu_xPb_yS alloyed nanocrystals. The Cu_2-xS/PbS core/shell nanocrystal population can be controlled and even removed when smaller Cu_2-xS cores are used. With decreasing initial Cu_2-xS core size in a given PbS shell reaction, the plasmonic signature decreases and the exciton energy begins to blueshift. These new Cu_xPb_yS alloyed nanocrystals lie on a sizing curve (1^st exciton transition energy vs nanocrystal diameter) distinct from that of pure PbS nanocrystals, while also showing shorter photoluminescence lifetimes and faster radiative decay rates. Thus, these Cu_xPb_yS alloyed nanocrystals are promising as a new material for infrared single-photon emitters.

1. Yee, P. Y.; Brittman, S.; Mahadik, N. A.; Tischler, J.G.; Stroud, R.M.; Efros, A. L.; Sercerl, P.C.; and Boercker, J. E. Cu_2-xS/PbS Core/Shell Nanocrystals with Improved Chemical Stability. Chem. Mater. 2021, 33, 17, 6685-6691.

With the (re)discovery of solution-processable perovskite materials, their remarkable properties have led to many novel applications in optoelectronics. Most prominantly, the Lead Halide Perovskites (LHP) have revolutionised photovoltaics [1]. However, also for LEDs, LHPs have great potential owing to easy colour tuning, narrow emission bands and high quantum yields, when produced as nanocrystals enclosed by ligands. With the ligand’s protection, stable dye solutions can be created [2].
The most common ligands contain long alkyl chains, leading to surface passivation and protecting the inside of the nanocrystals effectively. However, when processed as a thin film these ligands cause a significant electric resistance, which limits their practical use in LEDs [3]. Approaches to deal with this drawback are being presented.
The high absorbance of LHPs promotes using them as a colour converter. As proof of concept, we demonstrate that blue light emitted by an organic pump LED is turned into green very efficiently. This back-lighting technique eliminates the necessity of electrical charge injection completely. By applying a combination of ligands and polymer micelle nanoreactors a powerful shelter against ambient conditions is achieved [4].
If direct emission without a pump LED is desired, the ligand’s resistance has to be overcome. Two ligands with different coupling moieties and sizes are presented that increase the performance LHP nanocrystal LEDs. They exhibit higher efficiency along with higher driving currents and luminances.
Another approach features the doping of LiTFSI (Lithiumtrifluorosulfoimide) to increase conductivity. Along with that, a significant increase in photoluminescence quantum yield (PLQY) is observed for various LHPs. The underlying mechanisms are currently being investigated. Possible scenarios range from Lithium ion migration as well as intercalation as reported within bulk LHPs used as electrodes in Lithium ion batteries [5][6]. This opens a new field of research on the interplay of Lithium ions with LHP nanocrystals.


References
[1] W. S. Yang et al, Science 356, 6345, 1376-1379 (2017).
[2] L. Protesescu et al, Nano Lett. 15, 6, 3692-3696 (2015).
[3] C. Bi et al, Adv. Func. Mater. 29, 1902446 (2019).
[4] Q. Xue et al, in revision (2021).
[5] Q. Jiang et al, ACS Nano 11, 1073-1079 (2017).
[6] N. Vicente et al, J. Phys. Chem. Lett. 8, 1371-1374 (2017).

Confinement of lead-iodide perovskites to nanoscale dimensions using colloidal synthesis has enabled several emergent properties, notably including the stabilization of crystallographic phases known to be thermodynamically unstable in the bulk [1]. These halide perovskite quantum dots exhibit desirable optoelectronic properties for light emission and quantum optics applications, including efficient photoluminescence and superfluorescence arising from coherent coupling between neighboring nanocrystals. We investigated the structural phase and coherent particle orientation phenomena in thin films of 15-nm FA_xCs_1–xPbI₃ nanocrystals (NCs) using synchrotron grazing incidence wide-angle X-ray scattering (GIWAXS).

We established that three distinct perovskite phases exist at room temperature within perovskite NCs of mixed A-site composition, FA_xCs_1–xPbI₃ (x = 0, 0.1, 0.25, 0.5, 0.75, 1) [2]. Upon the addition of Cs (decreasing x), symmetry-lowering phase transitions are observed between x = 0.75-0.5 (cubic-to-tetragonal) and between x = 0.25-0.1 (tetragonal-to-orthorhombic), along with large-amplitude octahedral tiling evidenced by structural refinement. Thus, we demonstrate a unique ability to target perovskite crystal symmetry with facile synthetic control over the A-site composition in lead-iodide perovskite nanocrystals.

GIWAXS patterns collected on thin films of these FA_xCs_1–xPbI₃ NCs show a degree of inter-particle orientation (i.e., superlattice formation) that is highly dependent on the method of deposition. We employed time-resolved in situ GIWAXS to investigate the inter-particle orientation during spin-casting; texture analysis shows evidence of superlattice formation within a few seconds of spinning, concurrent with the loss of solvent. Finally, we introduce surface-ligand engineering approaches to achieve optimal superlattice formation, evidenced by GIWAXS texture analysis and microwave conductivity measurements. The design principles connecting colloidal synthesis to nanocrystal structure and ongoing efforts to achieve highly oriented NC thin films directly from the colloidal solution will be discussed.

[1] A. Swarnkar et al., Science 2016, 354, 6308, 92.
[2] J. A. Vigil et al., ACS Energy Lett. 2020, 5, 8, 2475–2482.

Low dimensional hybrid organic-inorganic lead-halide structures are gaining interest due to their structural flexibility and tunability compared their parent 3-D perovskite structure. By combining the tunability of organic chromophores and the desirable optical properties of perovskites, we aim to develop a new portfolio of hybrid organic-inorganic lead-halide materials. Deriving from the well-known 2-D hybrid perovskite structure PEA₂PbI₄, we introduced net acceptor and donor molecules into the organic bilayer to study the effects of charge transfer within the layered perovskite. We will present work on deposition of these hybrid perovskites using vapor deposition method and spin coating method, which are subsequently characterized by X-ray diffraction, fluorescence microscopy, and optical measurements such as UV-vis, photoluminescence and lifetime.

Metal-halide perovskite particles embedded within an insulating host matrix has proven to be a potential emerging class of light emitters [1]. Particularly, outstanding luminescence properties observed in a nominally pure zero-dimensional cesium–lead–bromide perovskite (Cs₄PbBr₆) have been intensively studied due to their high quantum yield, narrow full width at half maximum, in addition to an elusive structural origin behind their bright green emission [2,3,4]. However, the rapid nature of conventional solution-based synthesis methods has limited access to mechanistic insights underlying the reaction process. In this study, we investigated the formation mechanism of Cs₄PbBr₆ by investigating the time evolution of mechanochemical synthesis process [5]. A full characterization of the phase competition between various Cs–Pb–Br intermediates during the synthesis supports the three-dimensional emitters embedded in the Cs₄PbBr₆ host as the origin of the bright emission. Furthermore, the growth mechanism proposed in our study enabled us to design synthesis routes that can enhance emission efficiency by promoting a directional growth of Cs₄PbBr₆, and therefore providing key insights for understanding and designing guided-syntheses of highly luminescent perovskite light emitters.
Reference:
[1] Akkerman, Q. A. et. al. Nat. Mater. 17, 394–405 (2018).
[2] Wang, L. et. al. ACS Energy Lett. 5, 87–99 (2020).
[3] Qin, Z. et al. Chem. Mater. 31, 9098–9104 (2019).
[4] Akkerman, Q. A. et. al. J. Phys. Chem. Lett. 9, 2326–2337 (2018).
[5] Baek, K.-Y. et. al. (In preparation)

Vacuum-based deposition of halide perovskites has received a lot of attention for its proven scalability and conformal depositions. Co-evaporation in particular has had remarkable success, but efficiencies continue to lag behind its solution-based counterparts. This is in part attributed to complex sublimation behavior of some organic ammonium precursor salts and the increased complexity of the absorber materials, requiring the use of four or more sources in a conventional co-evaporation setup. The latter, as well as the promise of scalability, has driven work into single-source or sequential deposition solutions. Flash evaporation has been presented as one such single-source method, enabling the stoichiometric transfer of preformed perovskite powders or precursors. This is possible with temperatures sufficiently high to overcome the relative volatility or degradation reactions of the constituent components. However, flash evaporation suffers from poor process control, largely because all the necessary precursors are evaporated in a single batch (i.e. all the precursor powders are placed at once into/onto a heated crucible, boat, or foil). This imposes a trade-off between the deposition time and temperature; short deposition times with high temperatures are beneficial to avoid degradation reactions, but sputter effects seen at high temperatures drive researchers towards low temperatures where prolonged exposure causes degradation. Using a patented method, we obtain stable flash-evaporation rates for >1 hour, with controlled rates of around 1.5 Å per second, enabling film thicknesses >500 nm. We demonstrate the feasibility of this method for three different perovskite compositions FAPbBr₃, MAPbI₃, and FA_1-xMA_xPbI₃. Furthermore, we have integrated FAPbBr₃ and MAPbI₃ into all vacuum processed thin-film solar cells leading to power conversion efficiencies of ~4% and 10% respectively.

Metal-halide perovskites (MHPs) hold great potential for next-generation optoelectronic technologies due to their remarkable characteristics. While the influence of strain on the intrinsic properties of MHPs is gaining interest, its combined effect with an external electric field (E-field) has been largely overlooked. Using an electric-field-dependent Photoluminescence (PL) intensity study on heteroepitaxially strained surface guided CsPbBr₃ nanowires, we reveal an unexpected linear coupling between the alternating field and the PL intensity with a significant modulation depth (up to 40%), stemming from an induced internal dipole. Low-frequency polarized-Raman spectroscopy shows structural modifications in the nanowires under an external electric field, associated with the observed polarity. These results reflect the important interplay between an external E-field and strain in MHPs and offer new insight into the design of novel MHP-based nano-optoelectronics.

Organic-inorganic and all-inorganic lead halide perovskites (APbX₃) have continuously attracted research interest and went through significant improvements towards highly efficient photovoltaic technologies. Recently, these materials were shown to have room temperature electroluminescence. Most lead-halide perovskite devices are based on thin films or quantum dots while reports on alternative morphologies – e.g. fibers – are rare. In our research, we prepared CsPbBr₃@PVP composite nanofibers by one-step electrospinning and characterized them by scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, UV/vis and photoluminescence spectroscopy. We subsequently integrated the fiber mats as active layers in proof-of-concept electrically driven light emitting devices. While the concept of perovskite nanofibers is not new, to the best of our knowledge we would be the first to report on electroluminescence of such fibers. In addition, all preparations were conducted under ambient atmosphere and the perovskite precursor ink was prepared with “green” solvents (H₂O/EtOH/ionic liquid). This work could pave the way towards cost effective and flexible optoelectronic fiber- or yarn-based lead-halide perovskite devices by a facile up-scalable method.

As a form of frequency conversion, photon downshifting involves luminescent materials that convert high-energy photons into low-energy ones, which requires (i) high photoluminescence quantum yield (PLQY) of the phosphor and (ii) spectral overlap between the phosphor and the light absorber. The strategy of photon downshifting has been widely studied in the field of photovoltaics, especially perovskite solar cells (PSCs), to overcome inefficient photocurrent generation efficiency in the UV region as well as severe UV-induced degradation. However, the low PLQY of conventional photon downshifting layers (PDLs) and lack of versatility in the film fabrication process are still challenges. Moreover, additional design strategies for photon downshifting materials are necessary to address the remaining humidity-stability issue of PSCs.
Herein, to enhance UV and humidity stability as well as photocurrent generating efficiency, a highly luminescent and water-repellent platinum(II) complex, Pt-F, was developed as a multifunctional PDL for PSCs. The Pt-F PDL was fabricated on glass substrates of PSCs using ultrasonic spray deposition (USD) to suppress aggregation-induced emission quenching, leading to considerably high crystallinity and PLQY of 77%. A maximum device performance of 22.0% was achieved through the addition of a PDL coating to a 21.4% efficient PSC owing to the long-range photon downshifting effect of Pt-F, confirmed by the enhanced spectral response of the device in the UV region. Moreover, remarkable improvements in UV and humidity stability were observed in Pt-F-coated PSCs. The versatile effects of the Pt-F-based PDL suggest wide ranging applicability that can improve the performance and stability of other optoelectronic devices.

Perovskite thin films have been of great interest in the area of photovoltaics, mainly due to the rate at which their power conversion efficiency (PCE) has increased since their inception in the late 2000s, as well as for their low production price. Perovskite quantum dots (PQDs) have also garnered much interest for similar applications. Despite this, both PQDs and thin films have their drawbacks. Thin films have stability issues, particularly in the presence of moisture and oxygen, and PQDs have not demonstrated high enough PCE, mostly owing to low charge conductivity due to the presence of ligands. Some of these issues could be improved by combining the two into a tandem structure, composed of PQDs layered atop a perovskite thin film. Such a composite could result in a device with increased stability, since rationally designed aromatic ligands have demonstrated the ability to structurally stabilize PQDs, while also allowing efficient inter-dot charge transfer. We investigate the energy and charge transfer mechanisms between spin-coated methylammonium lead iodide (MAPI) thin films and PQDs, where both PQD composition and surface ligands are varied for a systematic study. Using photoluminescence (PL) spectroscopy, we find that emission from both CsPbBr and MAPbBr PQDs, functionalized with aliphatic ligands, is quenched in the tandem structures, while emission from the MAPI thin film is spectrally blue-shifted, indicating energy transfer between the PQD and thin film layers. Further, we find that charge carrier recombination lifetimes in the MAPI thin films are significantly longer after PQD deposition. However, while the structures with MA-based PQDs increase the PL emission intensity of the MAPI film by a factor of nearly 12, Cs-based PQDs did not have the same effect. In addition to these studies, further investigations are in progress to optimize charge transfer between PQD and thin film layers by substituting the insulating ligands with conducting ones.
The authors acknowledge funding from NASA grant no. NNH18ZHA008CMIROG6R through MACES.

Metal halide perovskites (MHPs) are promising materials for active layers in solar cells (SCs) because they have outstanding advantages such as low exciton binding energy (~14 meV for CH(NH₂)₂PbI₃)^[1] and high carrier mobility (100 cm²/(Vs))^[2]. Despite the superior properties of MHPs, MHP polycrystalline (PC) bulk film based SCs had bad reproducibility due to bad surface morphology of MHP PC films. Furthermore, controlling the surface morphology of MHP PC films is very difficult because the environmental conditions such as temperature and moisture have a strong influence on the morphology. Here, we present mixed-cation MHP nanoparticles (NPs) synthesized by hot injection method and purified with ligand-engineering method. Moreover, perovskite NP layers with uniform morphology were fabricated by using a ligand washing method. The ligand engineering methods which reduce the amount of ligand improved the charge extraction/transport characteristics in MHP NP films. Also, the morphology of perovskite film was enhanced by using mixed-cation perovskite nanoparticles. Our research suggests a promising way to increase PCE of perovskite NP SCs.
[1] K. Galkowski et al., Energy Environ. Sci., 2016, 9, 962.
[2] L. M. Herz, ACS Energy Lett., 2017, 2, 1539-1548.

Metal halide perovskites have received tremendous interest in the past few years and become very promising candidates for next-generation high-performance optoelectronic devices. Perovskite solar cells and LEDs suffer from the instability in practical use, they are either extremely prone to decomposition or transform to photo-insensitive non-perovskite phase. By performing self-consistent phonon calculations for CsPbI3 and MAPbI3, we find that soft lattice and the incurring vibration energy play the key role in Pb-I perovskite formation. This stabilization mechanism is shared by both inorganic and hybrid ones. Specifically, for MAPbI3, the porous structure of its lattice allows large diffusive motion of MA cation, which provides additional stabilization mechanism to the material, which helps it maintain the perovskite phase at room temperature. Strong exciton-phonon interaction in low dimensional halide perovskites promotes the formation of self-trapped excitons (STEs) which induces the widely observed broadband emission. In halide perovskites, STE favors spatially separated configuration due to the bonding and antibonding characters of the electron and hole. Homovalent doping could facilitate the STE formation which tunes the light emission properties. The intrinsic dark nature of STE due to spin-forbidden transition can be broken by introducing defect bound states, which greatly enhances the radiative recombination rates.

Exciton-polaritons - hybrids of strongly-coupled cavity photons and semiconductor excitons - are emerging optoelectronic quasiparticles with promising applications in low-threshold lasing, LEDs, and sources of light with spatial coherence or tailored photon-statistics.[1] Owing to their high exciton binding energy, high oscillator strengths, small Stoke's shift, and high emission quantum yields, lead-halide perovskite (LHP) nanomaterials have demonstrated particular promise for the realization of polaritons.[2] However, establishing hybrid-fabrication techniques to integrate low-loss photonic cavities with chemically-made LHPs still throttles scientific advances. Here, we demonstrate an exciton-polariton platform based on dielectric metasurfaces and CsPbBr3 perovskite quantum dots (PQDs). We show that square arrays of circular silicon meta-atoms integrated with densely packed PQDs superlayers support photonic Surface Lattice Resonances (SLRs). Conversely to localized Mie resonances, the mode-profiles of SLR exhibit field-localizations largely inside the PQD-layer. By spectrally overlapping the SLR with the PQD exciton, we observe the absorption-line splitting of 250meV and angular mode dispersion consistent with polariton-formation in the strong light-matter coupling regime. Our results point to an all-dielectric platform for large-area polariton devices combining the low photonic losses and versatile mode-tunability inherent to dielectric metasurfaces.
[1]: The road towards polaritonic devices, Nature Materials, 15, 1061-1073 (2016).
[2]: Perovskite semiconductors for room-temperature exciton-polaritonics, Nature Materials, 20, 1315-1324 (2021).

In this presentation I will discuss our studies of controlling the charge carrier dynamics, light/matter interactions, and spin populations in metal-halide organic/inorganic 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. In a separate effort we have explored the use of chiral copper-halide hybrid systems for circular light polarized detection. Chiral based copper-halide systems combined with highly conductive carbon nanotube networks can be employed to detect circular polarized light with the use of polarizers. Our chiral heterostructure shows high photoresponsivity of 452 A/W, a competitive anisotropy factor of up to 21%, a current response in microamperes, and low working voltage down to 0.01 V. Finally, we have developed novel photocatalyst based upon the unique properties of metal-halide semiconductor nanocrystals and show that such systems can drive multiple electron reactions. These results demonstrate that the emergent properties of organic−inorganic hybrid systems offer unique opportunities in controlling light, charge and spin.

This presentation will present our recent studies on orbit-orbit interaction effects of light-emitting properties. The hybrid perovskites are known as strong-orbital light-emitting materials. particularly, using orbit-orbit interaction provides a new approach to control the light-emitting properties in hybrid perovskites. In general, the orbit-orbit interaction can occur through electrical polarization and spin polarization between light-emitting excitons, leading to long-range and short-range orbit-orbit interactions. Here, we use circularly polarized and linearly polarized photoexcitations to manipulate the long-range and short-range orbit-orbit interactions between light-emitting excitons. Specifically, a circularly polarized photoexcitation generates in-phase electrical polarizations with parallel spin polarizations between orbitals, while a linearly polarized photoexcitation creates out-phase electrical polarizations with antiparallel spin polarizations. When electrical polarizations govern the orbit-orbit interaction, the in-phase electrical polarizations generated by circularly polarized photoexcitation provides a precondition to initiate the coherent interaction between light-emitting excitons towards developing a spontaneous amplified emission (ASE). When spin polarizations govern the orbit-orbit interaction, the antiparallel spin polarizations generated by linearly polarized photoexcitation provides the possibilities to decrease the spin flipping between bright and dark excitons, increasing the photoluminescence quantum yield (PLQY). Clearly, using electrical polarizations and spin polarizations between orbitals provides a vital method to control the light-emitting properties in hybrid perovskites. On the other hand, the light-emitting excitons can be generated both within single perovskite structures and heterostructured perovskites, leading to intrinsic excitons and charge-transfer excitons. Especially, the charge-transfer excitons formed with heterostructured perovskites can be considered as artificially engineered light-emitting excitons with facile property-tuning capabilities. This presentation will discuss the orbit-orbit interaction effects of both intrinsic excitons and charge-transfer excitons in hybrid perovskites.

The environmental stability and toxicity challenges associated with luminescent lead halides have recently led a push in the advancement of ternary group 11 metal halides. As a representative family, A₂MX₃ (A=Rb, K, NH₄; M= Cu, Ag; X= Cl, Br, I) will be discussed in this presentation. A₂MX₃ have been prepared through a variety of solid-state and solution synthesis techniques, yielding large single crystals and polycrystalline powders. The all-inorganic A₂CuX₃ are found to consistently produce blue photoluminescence with record-high efficiencies of up to 100%. The high-efficiency emission in A₂CuX₃ is attributed to self-trapped excitons (STEs) localized around Cu atoms within the 1D chains of [CuX₃]^2-. In contrast, A₂AgX₃ are found to be broadband emitters. The broadband emission in these compounds is attributed to STEs and defect-bound excitons (DBEs). This dual-band emission mechanism is supported by comprehensive experimental data and density functional theory (DFT) calculations. Our results demonstrate the strong potential of A₂MX₃ and related ternary group 11 halides for optical applications.

Colloidal CdSe quantum dots (QDs) designed with a high degree of asymmetric internal strain have recently been shown to possess a number of desirable optical properties including subthermal room-temperature line widths, suppressed spectral diffusion, and high photoluminescence (PL) quantum yields. It remains an open question, however, whether they are well-suited for applications requiring emission of identical single photons. In comparison to conventional colloidal CdSe/ZnS core/shell QDs, we find that, in asymmetrically strained CdSe QDs, over six times more light is emitted directly by the bright exciton. We measure the low-temperature PL dynamics and the polarization-resolved fluorescence line narrowing spectra from ensembles of these strained QDs. Our spectroscopy reveals the radiative recombination rates of bright and dark excitons, the relaxation rate between the two, and the energy spectra of the quantized acoustic phonons in the QDs that can contribute to relaxation processes. These results are therefore encouraging for the prospects of chemically synthesized colloidal QDs as emitters of single indistinguishable photons.

Lead halide perovskites exhibit outstanding optical and electronic properties, making them fascinating materials for light-emitting applications. Whereas near-infrared and green-emitting perovskites are already well-established, blue and red perovskite emitters suffer from low quantum yields, halide ion migration, and insufficient colloidal stability. Here, we present a facile room-temperature synthesis for quasi-two-dimensional CsPbBr3 nanoplatelets (NPLs) with distinct thicknesses in the quantum confinement regime resulting in tunable photoluminescence between 430 to 510 nm. We employ a machine-learning algorithm based on Bayesian optimization to enhance the quality of the resulting NPLs. Not only are the resulting PL spectra significantly narrower, but for the first time, we synthesized monodisperse 6ML, 7ML, and 8ML NPLs.
As the colloidal instability of perovskite nanocrystals mainly stems from a highly dynamic binding mode of organic capping ligands, we then utilize a post-synthetic ligand exchange strategy with didodecyldimethylammonium bromide (DDAB) for a stronger binding to the NPL surface. Compared to commonly used primary alkylamines, the quarternary alkylammonium ligand DDAB has multiple alkyl chains attached to the same headgroup, causing an additional kinetic barrier inhibiting ligand detachment. The ligand exchange procedure results in stable colloids with up to 90% photoluminescence quantum yield, among the highest values reported for pure blue-emitting perovskite materials.

Combining the advantageous electrical and optical properties of semiconductors with magnetic characteristics gives access to extraordinary phenomena and applications. Fully inorganic dilute magnetic semiconductors (DMS) have been known for decades, which show diverse functionalities like control of magnetism by electrical fields. Typically, the material class of DMS is obtained by introducing a substantial number of magnetic ions to an otherwise non-magnetic host semiconductor.
In this work we investigate the magneto-optical properties of the heavily doped hybrid semiconductor MAPbBr₃ (CH₃NH₃PbBr₃) with circularly polarized broadband femtosecond transient absorption (CTA) and pump-probe Faraday rotation, from which we find strong interactions between the excited charge carriers’ angular momentum and the dopants’ spin.
Due to their outstanding optoelectronic properties and high defect tolerance, organo-metal halide perovskites form an ideal system for efficient magnetic doping. We prepared a selection of magnetic perovskites using simple solution processing techniques by doping of the host MAPbBr₃ perovskite with three different magnetic ions Mn²⁺, Co²⁺, Ni²⁺. We investigated the fundamental optical, structural and magnetic properties of the perovskites with absorption and photoluminescence spectroscopy, XRD and SQUID measurements. Doping of the host lattice leads in all cases to structural changes and an increase or decrease of the bandgap of several 10 meV for Co²⁺ and Mn²⁺/Ni²⁺, respectively. We observe paramagnetic properties for all dopants and different doping concentrations.
To reveal the impact of doping on excitation dynamics, we employ the technique of CTA at cryogenic temperatures down to 4 K and magnetic fields up to 700 mT to gain a better understanding of possible ultrafast processes and coupling mechanisms between the angular momentum of photoexcited charge carriers and the d-electron spin of the dopants. Measuring CTA with different combinations of right- and left-handed circularly polarized pump (λ_ex = 515 nm) and probe (super-continuum white light) at variable temperatures and magnetic fields with an exceptional spectral resolution of 0.1 nm enabled us to precisely compare the energetic shifts and ultrafast dynamics of the charge carriers. For all investigated samples we observed an energy splitting of the CTA spectra for two different circular polarizations of the pump pulse even at zero magnetic field, which is substantially larger for the doped samples. The lifetime of this splitting is temperature-dependent and exceeds our investigable time span of 2 ns for the doped samples at 4 K. Pump-probe Faraday rotation measurements at 4 K underpin the different spin lifetime of undoped and doped samples. Both, lattice deformations and remanent magnetism due to high doping concentrations could lead to an energetically preferred spin alignment of the photoexcited charge carriers mediated via spin-orbit coupling. We plan to employ transient circularly polarized photoluminescence spectroscopy (CPL) to show if this spin polarization is long-lived and if it will lead to a significant degree of circularly polarized PL, which would be of high interest for future light emitting diode applications.

The control of intraband carrier relaxation in semiconductor quantum dots (QDs) is a longstanding topic of interest due to its central role in QD optoelectronic technologies. Applications such as hot carrier solar cells, mid-infrared detection and mid-infrared emission require a slow intraband relaxation. While the importance of multiphonon processes (“phonon bottleneck”) on intraband relaxation has been a debate over the past three decades, recent simulations [1-2] and experiments [1] suggest that QDs suffer from a fast intrinsic phonon decay lifetime of <~1 nanosecond. This bottleneck is a serious limitation, as it sets a fundamental limit on performance of QD mid-infrared optoelectronics.

In order to slow the nonradiative decay and study the underlying relaxation mechanism, we report [3] the synthesis of thick CdS shells on HgSe QDs. To access the intraband transition, we n-dope the QDs by introducing an electric dipole on the QD surface. At low doping and large shell thickness, photoluminescence lifetime and quantum yield (QY) measurements show that HgSe/CdS QDs achieve an intraband QY of 2% and lifetime in excess of 10 ns. These results suggest that multiphonon relaxation in QDs can be much slower than previously predicted, and highlight the promise of QDs for mid-infrared detection and emission.

[1] Han, P.; Bester, G. Phys. Rev. B 2015, 91, 085305
[2] Bozyigit, D. et. al. Nature 2016, 531, 618–622
[3] A. Kamath, C. Melnychuk, P. Guyot-Sionnest, J. Am. Chem. Soc. 2021, in press

Metal-organic chalcogenolate assemblies (MOCHAs) are an emerging class of crystalline materials. In particular, silver benzeneselenolate, with the chemical formula [AgSePh]_∞, which forms a two-dimensional multiple-quantum-well structure, has sparked interest recently.^1,2 The layered system exhibits strong quantum confinement with properties similar to two-dimensional materials like transition metal dichalcogenides or layered perovskites, while still exhibiting high material stability at ambient conditions.³ This makes silver benzeneselenolate a suitable candidate for applications such as excitonic lasers or light-emitting diodes. To facilitate the integration of MOCHAs into such optoelectronic device architectures, a need exists for simple and efficient fabrication methods.
Here, we present a method to synthesize silver benzeneselenolate crystals.⁴ The method is broadly tunable, and we can synthesize crystals with lateral sizes ranging from tens of micrometers to < 50 nm. These smallest particles are colloidally stable in polar solutions. The modularity of the material system also allows the fabrication of different variants of MOCHAs, with different chalcogenides and organic ligands.
Based on these demonstrations, we envision that our novel fabrication approach can also enable the fabrication of new semiconductor materials and facilitate their incorporation into optoelectronic devices.
References
[1] E. Schriber et al., ACS Appl. Nano Mater. 2018, 1, 3498–3508.
[2] B. Trang et al., J. Am. Chem. Soc. 2018, 140, 13892-13903.
[3] K. Yao et al., ACS Nano 2021, 15, 4085-4092.
[4] A. C. Hernandez Oendra et al., in preparation.

Avalanches are steeply nonlinear events in which outsized responses arise from a series of minute inputs. For steeply nonlinear optics, photon avalanching (PA) enables technologies such as optical phase-conjugate imaging, IR quantum counting, and efficient upconverted lasing. However, PA has been observed only in bulk materials and aggregates, often at cryogenic temperatures, preventing its application to imaging. We recently reported the first nanoscale realization of PA in sub-30 nm Tm³⁺-doped upconverting nanoparticles (UCNPs) and demonstrate their use in high-resolution imaging at wavelengths that fall within NIR spectral windows of maximal biological transparency.¹ Avalanching nanoparticles (ANPs) can be pumped by either continuous-wave or pulsed lasers and exhibit all of the defining features of PA, including a dominant excited-state absorption that is >10,000 times larger than ground-state absorption and a non-linear order of s ≥ 15, where emission increases as the s power of pump intensity. Beyond the avalanching threshold, Tm³⁺-doped ANP emission scales with up to the 31^st power of pump intensity, an extreme nonlinearity caused by the induced positive optical feedback within each nanocrystal. NaYF₄ ANPs with 8-20% Tm³⁺content can be excited at either 1064 or 1450 nm, with avalanching emission at 800 nm. We find that b-phase core/shell NaYF₄ UCNPs with core 8-10% Tm³⁺ content offer the steepest non-linearity, while >5-nm undoped NaYF₄ shells show the lowest thresholds. ANPs enable the experimental realization of sub-70 nm spatial resolution using only simple scanning confocal microscopy and before any computational data analysis. Paired with superresolution techniques and computational methods, ANPs allow for imaging with higher localization accuracy and at ~100-fold lower excitation intensities than possible with other probes.
1. Lee, C. et al. Giant nonlinear optical responses from photon-avalanching nanoparticles. Nature 589, 230–235 (2021).

Exciton-polaritons are coupled quantum particles resulting from a strong interaction between electron-hole pairs (excitons) inside a material and photonic modes of a microcavity. The unique features of these quantum particles can be engineered for new technological applications such as inversionless lasers, all-optical logic gates and others. Recently researchers have been exploring ultra-strong coupling (USC) which is a new light-matter interaction regime that exceeds both weak and strong coupling schemes. Here, we introduce a solid-state organic polariton microlaser, for the first time, based on pristine and single-crystalline microplates of a dye-coordinated Metal-Organic Framework (MOF) operating under USC regime. From optical measurements we extract a record-high Rabi splitting value of 1.15 eV, which is 51% of the molecular transition energy. Further, the percentage of virtual photons in the polariton ground state is determined to be 1.79 %, indicating MOF as a promising platform for quantum vacuum emission. A miniaturized form of such light sources have a wide variety of applications in science, technology and medicine. More importantly, we propose a novel physical mechanism for the first time, named ‘self-grouping of exciton-ensemble’, to explain the anomalous thickness dependence of multimode Rabi splitting. All the experimental results can only be explained using the correction factor introduced by this new mechanism.
An ultra-low threshold and a high degree of emission polarization have been achieved by stimulated polariton scattering occurred in single-crystallites of as-synthesized MOF microplates. With combined experiments and theoretical modelling, we found that the exciton-polaritons are formed at room temperature because of the strong coupling between Fabry-Perot cavity modes inherently formed by two parallel surfaces of a microplate and Frenkel excitons provided by the 2D layers of the dye linkers in the MOF. Such a cost-effective exciton-polariton laser cavity is realized at room temperature, without the support of any external mirrors made by metal coatings, or distributed Bragg reflectors (DBRs). Our findings confirm that MOFs, which can be synthesized readily from easily available materials under relatively mild conditions, are the potential candidates to unveil the previously unexplored fundamental physics and advanced applications of USC regime without needing any complicated material fabrication procedures.

The Stokes shift is an important property of luminescent materials, defined as the energy difference between the absorption band maximum and the emission spectrum maximum frequencies. The Stokes shift value is crucial in photonic devices and applications because, at a first approximation, it enables to estimate if a specific emitter would be affected by significant reabsorption of the generated light. If its value is lower or similar to the bandwidth of the absorption and emission spectra, the consequent intrinsic extensive ‘inner-filter’ effect can heavily limit the lighting performance of bulk photonic devices, and, in the worst cases, it can also affect the kinetics of the luminescence generation. Conversely, if the Stokes shift is larger than the spectral bandwidths the system the inner filter effects are avoided. These reabsorption-free materials are highly desirable for several applications. For example, in fluorescence imaging large Stokes shift optical probes allow to obtain high contrast images with limited excitation stray light, avoiding the use of expensive filtering component or time-consuming image post-processing. For solar applications, large Stokes shift emitters are undoubtedly the most promising materials to realize luminescent solar concentrators without reabsorption of the condensed radiation. Similarly, the sensitivity of scintillating detectors for ionizing radiation would greatly benefit from the use of fast emitters with no reabsorption showing good light output intensity without effects on the scintillation pulse timing, as required by the most advanced medical imaging techniques such as time-of-flight positron emission tomography and high-rate high-energy physics experiments.
High efficiency, large Stokes shift emission is obtained by realizing hetero-ligand Metal-Organic Framework (MOF) nanocrystals. Two fluorescent conjugated polyacene ligands of equal molecular length and connectivity, yet complementary photophysical properties, are co-assembled by zirconium oxy-hydroxy clusters, generating highly crystalline MOF nanoparticles. The fast diffusion of singlet molecular excitons in the framework, coupled to the achieved fine matching of co-ligands absorption and emission properties, enables to achieve an ultrafast activation of the low energy emitting ligands by diffusion-mediated, non-radiative energy transfer in the 100 ps time scale. In the optimized composition, MOF nanocrystals show a fluorescence quantum efficiency of about 70% with an actual Stokes shift of 750 meV. This large Stokes shift suppresses the reabsorption of fast emission issue in bulk devices, pivotal for a plethora of applications in photonics and photon managing spacing from solar technologies, imaging, and detection of high energy radiation. These features allowed to realize a prototypal fast nanocomposite scintillator [1] that shows an enhanced performance with respect to the homo-ligand nanocrystals,[2] achieving benchmark values competing with those of some commercial inorganic and organic systems.
[1] Hajagos, T. J., at al. Adv. Mater. 30, 1706956 (2018).
[2] J. Perego, I. Villa, at al. Nature Photonics 15, 393–400 (2021).

We have developed a generic electroluminescent device that can be used to generate and study electroluminescence (EL) from arbitrary luminescent materials, such as small molecules, conjugated polymers, quantum dots and more. The device has a metal-oxide-semiconductor capacitor structure with a carbon nanotube network contact and is operated by applying alternating current voltage. Unlike conventional light-emitting devices, our device can produce EL from materials with a variety of physical forms, chemical compositions and optoelectronic properties. The device relies on transient band bending at the source contact, which obviates the need for energy level alignment and enables emission from materials across a wide range of band gaps. By studying how device and material parameters affect device performance, we show how low voltage, efficient emission can be achieved. EL is observed at voltages near the optical gap of various emissive materials when a high-k gate oxide is used. Furthermore, bright EL can be obtained even in materials with large film thicknesses or extremely low carrier mobilities by tuning the carbon nanotube network density. Finally, the device can be driven resonantly to reduce the conventionally high input voltages. These results elucidate the performance limits of an electroluminescent device based on alternating carrier injection from a single contact and inform the capabilities of our device as a platform for new optoelectronic applications.

Carbon-based luminescent nanoparticles, which are usually referred to as carbon dots (CDs), are popular for light-emitting applications due to their fascinating optical properties and low-cost synthesis. In recent years, a desire to improve the photoluminescence (PL) quantum yield (QY) of CDs has led to the popularity of bottom-up synthesis methods. However, recent studies highlight the presence of fluorescent by-products in CD dispersions, especially in CDs synthesized by bottom-up methods. Here, the effects of fluorescent by-products and the structure of CDs are investigated. Additionally, the suitability of CDs for use in light-emitting applications is examined.
The CDs are synthesized by a solvothermal method using citric acid and 1,5-diaminonaphthlene. The as-synthesized product (unpurified CDs) shows a strong blue PL emission in addition to a weak green PL emission. Transmission electron microscopy images show that the average size of as-synthesized CDs is ~23 ± 9.7 nm. A significant increase in particle size is observed as a function of time after synthesis when the dispersion is stored in a water and ethanol mixture. The average size of the CDs increases from 23 nm to 63 nm over a period of 13 months after the synthesis. With storage time, the blue emission is substantially reduced; PL QY decreases from 30% to 3% after 13 weeks. From these observations it is concluded that the blue emission is mostly from free-floating molecular fluorophores that slowly attach to the CDs and cause an increase in particle size [1]. Furthermore, the free-floating fluorophores are removed by dialysis and a detailed characterization of purified CDs is also carried out. X-ray photoelectron spectroscopy shows that the CDs also contain oxygen (31%) and nitrogen (9%) in addition to carbon (60%). Fourier-transform infrared and nuclear magnetic resonance spectroscopy analysis shows that the CDs are composed of aromatic as well as aliphatic carbon. Purified CDs show two distinct excitation-wavelength-independent PL emission bands: one at ~400 nm and another green emission band at ~520 nm.
As in polar solvents, like water and ethanol, fluorophores can adsorb on CDs and cause particle size growth, the CDs in low polarity solvents can unravel into individual fluorophores. Detailed optical characterization of the CDs are carried out in different organic solvents. As solvent polarity decreases more fluorophores release from the CDs and redissolve in the solvent. It is observed that as solvent polarity decreases PL QY increases (e.g., PL QY in methanol is 3.5 % while in benzene it increases to 53.6%). PL lifetime also decreases with polarity of the solvents. Finally, the CDs are dispersed in two different solid matrices (epoxy and poly (methyl methacrylate) (PMMA)) and films are prepared. The PL QY of the films is 15.3 % and 8.0 % in PMMA and epoxy, respectively. In addition to promising PL QY values, the CDs films show broadband, white color PL emission with CIE (International Commission on Illumination) coordinates of (0.34, 0.42) in PMMA and (0.33, 0.35) in epoxy. White color PL emission, high PL QY of the films, and dominating aromatic structure of the CDs suggest that they can be excellent candidates for light emitting device applications. Electroluminescence measurements and device fabrication will also be discussed.
[1] N. Javed and D. M. O'Carroll, Nanoscale Adv., 2021,3, 182-189

Alkaline-earth rare-earth fluoride (MLnF) nanoparticles have been extensively studied over the past decade for a plethora of applications ranging from energy storage to biolabeling and bioimaging. Here, we investigate MLnF core-shell nanoparticles doped with lanthanide ions with a fast spontaneous emission rate as promising building blocks for scintillators. Scintillators are a key component of various medical imaging systems including positron emission tomography (PET) and they consist of materials that can efficiently convert high energy photons to the UV-Vis regime, more specifically from gamma rays to optical frequency light in the case of PET. To tune the emission to the UV-Vis regime and minimize the decay lifetime, we systematically investigate the MLnF nanoparticles and the impact of various compositional factors including host lattice, choice of dopant and concentration of dopant.
To fulfill the criteria for an ideal scintillator (high Z, high light yield, short rise, and decay time), we synthesize monodisperse, sub 20nm diameter MLnF core-shell nanoparticles doped with high Z trivalent lanthanides with concentrations systematically varying from 5%-45%. We select SrLuF with effective Z number of 54.5 as the host lattices of choice, an undoped SrLuF as the inert shell composition of choice and Ce3+, Pr3+ trivalent lanthanide ions with allowed interconfigurational f-d transitions (fast spontaneous emission rate) as choice of dopants. To reduce surface quenching, we shell these nanoparticles with varying thicknesses of 5-15nm. During the two-step synthesis, the core nanoparticles serve as seed crystals with surface nucleation sites for epitaxial growth of shell layers via dropwise hot injection approach.
We employ a range of optical characterization techniques to identify the compositional dependence of Pr3+, Ce3+ 4f→5d (2F7/2+ 2F 5/2) decay lifetime and emission wavelength via photoluminescence (PL) and time-correlated single photon counting (TCSPC) techniques. The measured decay lifetime (τ) for Ce³⁺ 4f→5d varies between ~20ns to ~288ns with the shortest lifetime belonging to the 5% Ce3+ sample and longest lifetime belonging to the 45% Ce3+ sample.In addition, high resolution transmission electron microscopy (HRTEM) in conjunction with cathodoluminescence (CL) are used to correlate the microstructural properties with localized optical response. Finally, various assembly pathways for building 3D artificially designed nanophotonic structures using the MLnF nanoparticles as basic building blocks with the ability to manipulate light and create a highly directional scintillation process will be discussed. This understanding is a key step to develop next-generation highly efficient scintillator materials.

It is well known that the photoluminescence (PL) of lanthanides (Ln = Eu, …) centers in inorganic matrices depends on their oxidation states. Therefore, it is desirable to control that factor at the stage of synthesis of the PL materials which can be promising phosphors in many applications, e.g., in lighting. The experience of our group with controlling the oxidation states of transition metals present in prospective glassy and nanostructured cathode materials for Li-ion and Na-ion [1], has helped us to use similar methods to take control of the oxidation states of lanthanides centers embedded in transparent glassy matrices. Lanthanides are widely used in today's technology – among others, in lasers based on Ln-doped crystals or glasses. This is due to a large variety of their 4f configurations of electrons in Ln atoms, which leads to a wide range of fluorescent states and wavelengths [2, 3].
Our recent work [4] proved that it is possible to synthesize Eu-doped glassy materials whose photoluminescence spectra can be tuned depending on synthesis conditions by using melt-quenching process. During that work, we found out that one can control the relative Eu³⁺/Eu²⁺ ions concentrations. As both Eu³⁺/Eu²⁺ are photoluminescent in different parts of the visible range, the possibility of controlling their relative concentrations gives an opportunity to "tune” photoluminescence spectrum of the material and makes it similar to that of natural daylight. It is also possible to adjust phosphors, based on glassy matrices doped with Eu, to other desired colors. However, further studies are needed to improve this method of europium ion reduction and to specify the exact influence of melting time and temperature on the final properties of the samples.
In order to do that, a series of glassy matrices (10 samples in total) with nominal composition Na₃Al₂(PO₄)₂F₃ doped with 1 wt% of Eu₂O₃ were successfully synthesized by a melt-quenching process, using a double crucible method [5] – 9 samples, and 1 sample without reducing atmosphere. Samples synthesis condition differed, temperature was controlled in the range of 1000 °C, 1100 °C and 1200 °C and time was set as 5 min, 10 min or 15 min. Obtained glasses were carefully investigated using X-ray diffractometry (XRD), photoluminescence spectroscopy (PL), photoluminescence excitation (PLE) and absorption spectroscopy.
X-ray diffractometry confirmed amorphousness of all samples. Photoluminescence spectroscopy results (samples excited with 270 nm and 396 nm) showed gradual increase of Eu²⁺ emission line in the spectrum together with increase of time and temperature of melting. At the same time, the opposite was observed for Eu³⁺ line – it decreased with time and temperature. Photoluminescence excitation and absorption spectroscopy confirmed photoluminescence spectroscopy data with the difference of increase/decrease of characteristic absorption peaks for Eu³⁺/Eu²⁺.
References
[1] T.K. Pietrzak et al., Materials Science and Engineering B 213 (2016) 140-147.
[2] M. C. Goncalves et al., Comptes Rendus Chimie, 5 (2002) 845–854.
[3] B. M. Walsh: Advances in Spectroscopy for Lasers and Sensing. Judd-ofelt theory: principles and practices. [red.] Baldassare Di Bartolo, Ottavio Forte. Erice Italy, Springer, 2005
[4] T.K. Pietrzak, A. Golebiewska et al., Journal of Luminescence 208 (2019) 322–326.
[5] K. Hirose et al., Solid State Ionics, 178 (2007) 801–807.
Acknowledgments: This research was funded by POB FoTech of Warsaw University of Technology within the Excellence Initiative: Research University (IDUB) program.

Cesium iodide is not just a source of iodide or of a simple, size-appropriate perovskite A⁺-site cation; instead, it plays a key role in DMF solution through previously unexplored coordination chemistry of iodoplumbate species, PbI₃^-. Nuclear magnetic resonance (NMR) ¹³³Cs spectroscopic analysis found that Cs⁺ and methylammonium (CH₃NH₃⁺, MA⁺) compete for coordination with PbI₃^-. Cation-PbI₃^- coordination was further studied by ²⁰⁷Pb NMR, which showed unprecedented groupings: Perovskite forming cations Cs⁺, MA⁺, formamidinium (HC(NH₂)₂⁺; FA⁺) and, to a lesser extent Rb⁺, formed one such group, and non-forming ones Li⁺, Na⁺ and K⁺ fell into another. Molecular dynamics analysis yielded two striking results: Cs⁺ interacts more strongly with PbI₃^- in DMF than does MA⁺: whereas MA⁺ and FA⁺ interact with PbI₃^- through typical hydrogen bonding, Cs⁺ (and presumably Rb⁺) ion pair directly with all three iodides of PbI₃^-. Calculational and experimental data indicate the key role that solvation energies of A⁺-site cations play in perovskite precursor solutions: The stronger coordination energy of Cs⁺ vs. MA⁺ with PbI₃^- in DMF demonstrates the critical role that Cs⁺ plays in solution as an additive in MA⁺- or FA⁺-targeted perovskite crystallization processes: Strong Cs⁺ coordination with iodoplumbate complexes may facilitate the organization of a perovskite framework precursor that transforms from solution into the solid-state perovskite structure.

Luminescent semiconductor quantum dots (QDs) have attracted wide attention due to their multimodal abilities to absorb and emit light, perform photo- and electro-catalysis, and more. These materials can exhibit unique properties belonging to their nanoscale building blocks, as well as ensemble behavior at the micro- or macro-scales. However, typical semiconductor QDs contain toxic heavy metals which are not always environmentally friendly – hence, non-toxic and benign QDs like carbon nanomaterials are preferred for practical applications. In the present study luminescent carbon QDs are synthesized in a non-thermal plasma reactor using methane and argon as the reactant species. Radiofrequency power at 13.56 MHz was supplied through ring electrodes to a flow-through reactor tube held at 4.7 Torr. The QDs formed in the plasma were deposited onto arbitrary substrates via inertial impaction. The synthesized material was characterized using Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Raman spectroscopy, and UV-Vis optical absorption. X-ray Diffraction revealed the presence of crystalline graphite peak around 2θ = 24° and the same was corroborated from the lattice fringes visible from the TEM micrographs. From these lattices, the lattice spacing in the micrographs was around 0.33 nm, corresponding to the (002) plane of graphite, and the average QD diameter was 4 nm. Raman spectra exhibited sharp D and G peaks as expected from carbon nanostructures. The UV-Vis absorption spectrum showed a peak around 278 nm, which has been correlated with the ∏ → ∏* transition of the carbon aromatic rings. On ultraviolet excitation of the carbon quantum dots suspended in toluene, the photoluminescence emission peak comes around 500 nm (green region of the visible spectrum) which can then be red shifted with increasing residence time of QDs in the plasma. For other semiconductors produced in plasmas, an increased residence time has resulted in a larger QD size – and PL from carbon QDs is known to shift to lower energies with increasing QD size. We hypothesize that the redshift in PL emission peak is thus an indication of an increasing QD diameter, which we investigate using further TEM and XRD analysis. On adding hydrogen to the gas mixture, we observed that the PL emission intensity increased remarkably, possibly due to passivation of the defect states on the surface with a simultaneous change in emission wavelength. Finally, we found the carbon QD samples to be hydrophobic with a water contact angle of 135° due to the surface C-H bonds, further illustrating their multifunctionality. These exciting properties signify the future prospects for plasma-produced carbon QDs in LEDs, circuits, coatings, and more.

The external quantum efficiency (EQE) of perovskite LEDs (PeLEDs) was dramatically improved to above 20% in the past few years. Reducing crystal size has been shown to be an effective way of enhancing electron-hole capture rates and the performance of PeLEDs.
In this talk, we will present our efforts on the development of efficient PeLEDs with solution-processed nanocrystals including: 1) In-situ formation of perovskite nanocrystals with bulky organoammonium ligands, 2) Surface engineering of the perovskite nanocrystals with minimized traps, 3) Dual trap passivation of both lead and halide-induced traps for PeLEDs with EQE reaching 20.9%. and 4) Design of polymerized perovskites with reduced electron-phonon coupling and non-radiative recombination rate, which leads to an EQE of 23.2%. We will then introduce our recent work on large-area fabrication of PeLEDs using blade-coating approach. Moreover, we will report our design of quantification of the contribution of photon recycling in PeLEDs. Our result shows that photon recycling can increase the outcoupling efficiency significantly, and contribute 2.4% to 40.4% of total emission depending on perovskite thickness.

Perovskite light-emitting diodes (PeLEDs) have showed significant progresses in recent years, the external quantum efficiency (EQE) of electroluminescence in green and red regions have been beyond 20%, and the efficiency in blue is lagged out. Here, I will represent our two recent progresses in efficient blue PeLEDs via energy-band engineering. 1) We introduced a large cation CH₃CH₂NH₂⁺ (EA) into the Cs⁺ site in PEA₂(CsPbBr₃)₂PbBr₄ perovskite with green emission to decrease the Pb-Br orbit coupling and increase the bandgap for blue emission. And as high as 12.1% EQE of sky-blue electroluminescence located at 488 nm has been demonstrated. 2）By doping the hole injection layer with alkali metal salt, a more matching energy level arrangement is obtained, and the hole injection efficiency is improved. At the same time, the defect states in the perovskite films are passivated and/or suppressed, and the electroluminescence efficiency of blue PeLEDs is up to 16.07%.

Colloidal quantum dots (CQDs) are promising emitters for near-infrared light emitting diodes (NIR LEDs) due to their highly-valued properties such as size-tunable emission and high photoluminescence quantum efficiencies (PLQE). However, NIR LEDs based on CQDs generally employ restricted heavy metals such as lead (PbSe, PbS, CsPbI₃) or cadmium (CdTe), and their performance remain modest compared to their visible counterparts. In this talk, I will discuss new strategies to prepare highly-efficient NIR LEDs based on heavy-metal-free indium arsenide (InAs) core-shell CQDs with long emission wavelengths of >1000 nm. These devices could find significant applications in eye-safe depth sensing, LiDAR and health-monitoring technologies.

Significant progress has been made in emitters based on low-dimensional perovskites, with both 2D perovskites and 0D (quantum dot) perovskites showing significant advances. I will review chemical control over the energetic landscape in these materials, as well as passivation/surface management with localized electronic states in mind. In this context I will summarize progress in quantum efficiency, emission linewidth, brightness, and reliability.

The bright emission observed in cesium lead halide perovskite nanocrystals (NCs) has recently been explained in terms of a bright exciton ground state, a claim that would make these materials the first known examples in which the exciton ground state is not an optically forbidden dark exciton. This unprecedented claim has been the subject of intense experimental investigation that has so far failed to detect the dark ground-state exciton in CsPbBr₃ NCs. I will discuss the exciton fine structure created by the crystal feld and the short-range and long-range electron−hole exchange interaction and will show that the only Rashba terms provide an explanation for the observed bright exciton level order in CsPbBr₃ NCs. The size dependence of the exciton fine structure calculated for perovskite NCs shows that the bright−dark level inversion caused by the Rashba effect is suppressed by the enhanced electron−hole exchange interaction in small NCs. [1]
Another interesting phenomenon connected with presence of the Rashba spin-orbit terms is creation of helical exciton states in orthorhombic perovskites, which are split from each other. The splitting can be described as a Zeeman effect in an effective magnetic field, whose direction and magnitude depend on the exciton momentum. The selective excitation of these states by helical light gives rise to CD. Using experimentally determined material parameters, we calculate significant circular dichroism of order 30% in orthorhombic perovskites under off-normal, top illumination. These calculations suggest the effect is observable and CD can be measured in non-chiral perovskite nanostructures such as layered-2D perovskites or nanoplatelets. [2]
Large spin-orbit Rashba terms in the conduction and valence bands leads to the exciton with complex dispersion – Rashba exciton. The Rashba terms in case when they have different signs flip the order of the bright and dark excitons at zero exciton momentum. They also affect the exciton dispersion at momentum not equal to zero and creates the exciton dispersion minima at momentum not equal to zero. [3] However, the exciton dispersion minima occur at nonzero momentum.
Finally, I will discuss the thickness-dependent fine structure of excitons in perovskite nanoplatelets. Our theoretical model introduces the effect of the strong special confinement on the band-edge Bloch functions. The predicted fine structure is very different from that observed in 3D nanocrystals, and is in good agreement with experimental observation. [4]

[1] M. A. Becker, et al. “Bright triplet excitons in caesium lead halide perovskites,” Nature, 553, 189-193 (2018)
[2] P. C. Sercel, Z. V. Vardeny, Al. L. Efros, “Circular dichroism in non-chiral metal halide perovskites” Nanoscale 2020, DOI/10.1039/D0NR05232A
[3] M. W. Swift, J. L. Lyons, Al. L. Efros, P. C. Sercel “Rashba exciton in a 2D perovskite quantum dot” Nanoscale 2021, doi.org/10.1039/D1NR04884H
[4] M. Gramlich, M. W. Swift, C. Lampe, J. L. Lyons, M. Döblinger, Al. L. Efros, P. C. Sercel, and A. S. Urban, “Dark and Bright Excitons in Halide Perovskite Nanoplatelets” Advanced Science 2021, to be published.

Colloidal lead halide perovskite nanocrystals (LHP NCs_, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) have become a research spotlight owing to their spectrally narrow (<100 meV) fluorescence, tunable over the entire visible spectral region of 400-800 nm, as well as facile colloidal synthesis. These NCs are attractive single-photon emitters as well as make for an attractive building block for creating controlled, aggregated states exhibiting collective luminescence phenomena. Attaining of such states through the spontaneous self-assembly into long-range ordered superlattices (SLs) is a particularly attractive avenue. In this regard also the atomically-flat, sharp cuboic shape of LHP NCs is of interest, because vast majority of prior work had invoked NCs of rather spherical shape. Long-range ordered SLs with the simple cubic packing of cubic perovskite NCs exhibit sharp red-shifted lines in their emission spectra and superfluorescence (a fast collective emission resulting from coherent multi-NCs excited states). When combined with spherical dielectric NCs, perovskite-type ABO₃ binary NC SLs form, wherein CsPbBr₃ nanocubes occupy B- and/or O-sites, while with spherical dielectric Fe₃O₄ or NaGdF₄ NCs reside on A-sites. When truncated-cuboid PbS NCs are added to these systems, ternary ABO₃-phase form (PbS NCs occuoy B-sites). Such ABO₃ SLs, as well as other newly obtained SL structures (binary NaCl- and AlB₂-types, columnar assemblies with disks etc.), exhibit a high degree of orientational ordering of CsPbBr₃ nanocubes. These mesostructures exhibit superfluorescence as well, characterized, at high excitation density, by emission pulses with ultrafast (22 ps) radiative decay and Burnham-Chiao ringing behaviour with a strongly accelerated build-up time.

Hybrid perovskites like (C₄H₉NH₃)₂PbI₄ have fascinating layered crystal structure with periodic nanoscale interfaces between the inorganic {PbI₄}^2- and organic C₄H₉NH₃⁺ layers. Because of these interfaces, electron and hole are confined in atomically thin {PbI₄}^2- inorganic well layers. Therefore, these layered perovskites are considered as electronically 2D systems, irrespective of their crystallite sizes.^1,2 Importantly, the crystal structure is flexible, allowing a number of combinations of different organic cations and inorganic anions. So a rational design of the nanoscale interfaces, and hence, tunable optoelectronic properties are feasible. For example, excitonic binding energy can be controlled over an order of magnitude from a few tens of meV to a few hundreds of meV, with simple variation of composition of organic cations. An appropriate choice of chiral organic cation can induce chirality in optoelectronic properties arising from the inorganic sublattice. Furthermore, one can introduce new intermolecular chemical interactions between organic cations, resulting into completely water stable low dimensional hybrid perovskite.³ In this talk, I will discuss about how controlling nanoscale interface between organic and inorganic layers can yield interesting optical, optoelectronic and chemical properties.⁴ But note that the nanoscale properties will be discussed using millimeter sized single crystals.

References:
1. Sheikh, T.; Shinde, A.; Mahamuni, S.; Nag, A. Possible Dual Bandgap in (C₄H₉NH₃)₂PbI₄ 2D Layered Perovskite: Single-Crystal and Exfoliated Few-Layer. ACS Energy Lett. 2018, 3, 2940.
2. Sheikh, T.; Nawale, V.; Pathoor, N.; Phadnis, C.; Chowdhury, A.; Nag, A. Molecular Intercalation and Electronic Two Dimensionality in Layered Hybrid Perovskites. Angew. Chem. Int. Ed. 2020, 59, 11653.
3. Sheikh, T.; Maqbool, S.; Mandal, P.; Nag, A. Introducing Intermolecular Cation-p Interactions for Water-Stable Low Dimensional Hybrid Lead Halide Perovskites. Angew. Chem. Int. Ed. 2021, 60,18265.
4. Nag, A. “Plenty of Room” at the Interface of Hybrid Metal Halide Perovskite Single Crystals. Nano Lett. 2021, 21, 8529.

Metal halide perovskites have shown promising optoelectronic properties suitable for light-emitting applications. The development of perovskite light-emitting diodes (PeLEDs) has progressed rapidly over the past several years, and molecular additives have played an important role in these advances. By rational design of these molecular additives, we can significantly enhance the interaction with defects sites and minimize non-radiative recombination losses. In addition, we also demonstrate that the molecular additives can manipulate the perovskite crystallization and help to enhance the radiative recombination. As such, we have been able to demonstrate perovskite LEDs with high external quantum efficiencies over 20%. We find that our devices can also work efficiently in an emitting/detector switchable mode, with tens-megahertz speed for both functions. We further demonstrate the potential of the dual-functional diode for biomedicine diagnosis applications (as a monolithic heart pulse sensor) and for inter- and intra-chip bidirectional optical communications.

In some ways, the properties of colloidal CsPbBr₃ nanocrystals resemble those of conventional semiconductor nanocrystals. In other ways, CsPbBr₃ is wholly unique. In this talk, I will present experimental results from lab using time-resolved emission microscopy to probe the spatiotemporal dynamics of exciton transport in disordered CsPbBr₃ nanocrystal films and nanocrystal superlattices. CsPbBr₃ nanocrystals exhibit a range of novel and unconventional transport behavior, including excitation memory effects, collective phenomena, and record-high values of exciton diffusivity.

Halide perovskites are remarkably clean semiconductors with versatile emission properties. They are showing exceptional promise in light emitting applications including as colour converters and light-emitting diodes in different colours. Here I will outline recent efforts in our group towards different forms of white light emission. I will first cover new perovskite and perovskite-like materials that exhibit broad but tunable white light. I will then show integration of perovskite emitters into LED structures capable of producing tunable and high quality white light, including mixtures of nanocrystals and bulk organic emitters, leading to promising efficiencies and stabilities. Ongoing efforts to improve these metrics further and to enable new device designs will be outlined.

Silver phenylselenide (AgSePh) has recently generated excitement as a new hybrid organic-inorganic two-dimensional (2D) semiconductor. AgSePh shows multiple intriguing properties such as narrow blue emission, in-plane anisotropy, large exciton binding energy, chemical robustness and a scalable synthesis. Unlike 2D layered perovskites and transition metal dichalcogenides, AgSePh features strong covalent bonding between organic and inorganic components, forming a truly hybrid organic-inorganic semiconductor. In this presentation, we will show a new strategy to synthesize AgSePh thin films with controllable grain size from <200 nm to >5 µm and orientation from vertical to parallel to substrate. Systematic structural, optical and electrical characterization to demonstrate improved crystalline quality with lower defect densities as well as controllable morphology will be presented. Furthermore, tunable electronic and optical properties of AgSePh by substitution of Se with Te will be presented. Finally, different physical mechanisms underlying light emission in AgSePh, AgTePh, and their alloys will be illustrated through time-resolved and temperature-dependent optical spectroscopy. Overall, this work highlights properties of a novel class of 2D hybrid organic-inorganic semiconductors, and lays the foundation for understanding photophysical processes in these light-emitting materials.

2D hybrid lead iodide perovskites are light-harvesting semiconductors where the nature of the organic cations permits a vast compositional space to tailor the structural and technological features of environmentally robust materials for optoelectronic devices. Herein, we present three new homologous series of (100) lead iodide perovskites with organic spacers that carry functional groups, allylammonium (AA) and iodopropylammonium (IdPA): (AA)₂MA_n-1Pb_nI_3n+1 (n=3-4), [(AA)_x(IdPA)_1-x]₂MA_n-1Pb_nI_3n+1 (n=1-4) and (IdPA)₂MA_n-1Pb_nI_3n+1 (n=1-4), as well as their perovskite-related substructures. We report the in-situ synthetic transformation of the AA organic layers into IdPA and the incorporation of these cations simultaneously into the 2D hybrid perovskite structure. Single crystal X-ray analysis shows (AA)₂MA₂Pb₃I₁₀ crystallizes in the space group P2₁/c, with a unique stacking arrangement with the inorganic layer offset (0, < ½), comprising the first example of halide perovskite with a monoammonium spacer cation that deviates from the classic Ruddlesden-Popper (RP) hybrid halide structure type. (IdPA)₂MA₂Pb₃I₁₀ and the alloyed [(AA)_x(IdPA)_1-x]₂MA₂Pb₃I₁₀ crystallize in the RP structure, both in space group P2₁/c. The adjacent I-I interlayer distance in (AA)₂MA₂Pb₃I₁₀ is ~ 5.6 Å, drawing the [Pb₃I₁₀]^4- layers closer together among all reported n=3 RP lead iodides. (AA)₂MA₂Pb₃I₁₀ presents band-edge absorption and photoluminescence (PL) emission at around 2.0 eV that is redshifted in comparison to (IdPA)₂MA₂Pb₃I₁₀, with band-edge absorption and PL at around 2.03 eV. The band structure calculations suggest both (AA)₂MA₂Pb₃I₁₀ and (IdPA)₂MA₂Pb₃I₁₀ have in-plane electron and hole effective masses around 0.04.m₀ and -0.08.m₀, respectively. IdPA cations have a greater dielectric contribution from to the iodine atom in the organic cation, than AA. The excited-state dynamics were investigated by transient absorption (TA) spectroscopy, which reveal a long-lived (~100 ps) trap state ensemble with broadband emission in the visible region.

Recently, there is significant interest in 2D materials for electronic and optoelectronics devices due to their attractive properties [1]. Molybdenum Disulfide (MoS₂) is attractive due to its layer-dependent optoelectronic properties. It has a tuneable band gap, where the layered bulk MoS₂ crystal has an indirect band gap of 1.2 eV while monolayer has a direct band gap of 1.9 eV [2]. In this work, MoS₂ nanoparticles were synthesized through a chemical exfoliation and used to fabricate a MoS₂/p-Si junction photodiode, to study the effect of the MoS₂ NPs on the optical responsivity.
The chemical exfoliation process starts with dispersing a 0.5 gram of the MoS₂ powder in a 50 mL of N-Methyl-2-pyrrolidone (NMP). After that we place it in an ice bath maintained at 0 °C and sonicate at 50% amplitude, and 10 s / 2 s duty cycle for 6 hours. To remove unexfoliated particles from the resulted dispersion, it was centrifuged at 1500 rpm for 60 min, followed by a 30 min of centrifuging at higher speed of 7500 rpm to remove the soluble impurities. After that, the NMP was removed, and the synthesized MoS₂ NPs was filtered and dispersed in a 25 mL of IPA.
To test the synthesized MoS₂ NPs, a MoS₂ NPs/p-Si junction photodiode was fabricated. First, a lightly doped p-type Si wafer is cut to a size of 1.5 cm x 1.5 cm and passivated with acetone, IPA, deionized (DI) water, followed by dip in a solution of 10:1 of DI water to Hydrofluoric acid (HF) for 30 second to remove the native oxide. After the cleaning process, two drops of the MoS₂ NPs with 100 µL is drop casted on the substrate to create the MoS₂ Si junction using a unform coat. Next, we sputter Au/Pl through metal fingers for the front contact and uniform layer for the back side
The fabricated MoS₂/Si photodiode IV is measured under dark and under 1 Sun in a solar simulator. The results show a working fabricated photodiode that has large increase between dark and light current. And compared to a metal/Si photodiode a much larger responsivity. This shows the semiconductor nature of MoS₂ in nanoparticle form with significant light absorption. Moreover, MoS₂ nanoparticles are promising material for future low cost photodetectors and integration of optics and electronics.

References
[1] A. U. Rehman, M. F. Khan, M. A. Shehzad, S. Hussain, M. F. Bhopal, S. H. Lee, J. Eom, Y. Seo, J. Jung, and S. H. Lee, “n-MoS2/p-Si Solar Cells with Al2O3 Passivation for Enhanced Photogeneration,” ACS Applied Materials & Interfaces, vol. 8, no. 43, pp. 29383–29390, 2016.
[2] C. P. Veeramalai, F. Li, Y. Liu, Z. Xu, T. Guo, and T. W. Kim, “Enhanced field emission properties of molybdenum disulphide few layer nanosheets synthesized by hydrothermal method,” Applied Surface Science, vol. 389, pp. 1017–1022, 2016.

In the past decade, halide perovskites have shown tremendous promise in photovoltaics, light-emitting devices, miniaturized lasers and other diverse applications in optoelectronics. Perovskite semiconductors exhibit high optical gain properties, large oscillator strength, strong exciton binding energy and so forth, therefore they provide a unique platform to study strong light-matter coupling in microcavity devices. Here in this talk, I will present an overview of the historical development of exciton polariton research in halide perovskite materials. Particular focus will be given to the comparison between different optical gain materials and their advantages (and disadvantages); then I will discuss our recent progress in demonstration of room-temperature exciton polariton lasing and condensate in all-inorganic perovskite microcavities and their nonlinear optical properties, which drives the coherent propagation in condensate in waveguides for ultrafast optical switching applications. Finally, I will discuss the realization of artificial lattices towards quantum control of exciton polariton condensate at room temperature and their potential applications in 2D quantum simulators.

References:

R. Su<span style="font-size:10.8333px"> et al.</span>, “Perovskite semiconductors for room-temperature exciton-polaritonics”, Nature Materials, DOI: 10.1038/s41563-021-01035-x (perspective) (2021)
Q. Zhang et al., “Halide perovskite semiconductor lasers: materials, cavity design, and low threshold”, Nano Lett. 21 DOI: 10.1021/acs.nanolett.0c03593 (2021) (invited minireview)
R. Su et al., “Optical switching of topological phase in a perovskite polariton lattice”, Science Advances 7, eabf8049 (2021)
R. Su et al., “Observation of exciton polariton condensation in a perovskite lattice at room temperature”, Nature Physics 16, 301-306 (2020)
R. Su et al., “Room-temperature polariton lasing in all-inorganic perovskite nanoplatelets”, Nano Lett. 17, 3982–3988 (2017)

Halide perovskites possess outstanding optoelectronic properties favorable for applications as highly efficient emitters in perovskite light-emitting devices (PeLEDs), lasers as well as quantum emitters. Their versatile structures and diverse dimensionalities afford new levers for tunning their photophysical properties. In this talk, I will distill some of the underpinning photophysical mechanisms in low-dimensional emitters such as layered Ruddlesden Popper perovskites and colloidal perovskite nanocrystals.

While there are advances in the external quantum efficiency (EQE) and film photoluminescence quantum yields (PLQY) of perovskite light emitting diodes (PeLEDs), one of the key challenges of red color PeLEDs based on perovskite-nanocrystals (NCs) is the operational stability because of the easy transformation from cubic to orthorhombic structure and the weak bond energy between inorganic surfaces and long-chain capping ligands. In this work, we demonstrate highly efficient and stable PeLEDs with a peak wavelength of 689 nm by adopting thermodynamically stable tetragonal phase CsPbI₃ (b-CsPbI₃) NCs as the emitting layer [1]. The stable b-CsPbI₃ was successfully synthesized via incorporating the poly(maleic anhydride-alt-1-octadecene) (PMA) into the precursor. The PMA can interact with PbI₂ in the precursors to regulate the crystallization kinetics of b-CsPbI₃ NCs and finally adsorb on the surface of b-CsPbI₃ NCs through strong Pb-O bond to significantly stabilize the b-CsPbI₃ NCs without any crystalline deformation on the NCs crystal structure. Additionally, these chemically crosslinked PMA also significantly reduces deep defects of Pb_Cs via increasing the formation energy and passivating the Pb_Cs deep defects. Moreover, the PMA-incorporated β-CsPbI₃ NCs exhibit increasing exciton binding energy and reducing longitudinal-optical (LO) phonon energy. Benefiting from the improved crystal phase quality with PMA incorporation, the PLQY of b-CsPbI₃ NCs yields an increase from 34% to 89%. The corresponding PeLEDs achieve a high EQE of 17.8% and exhibited superior operational stability with operation time for a device decay to 50% of the initial EL intensity (T₅₀) of 317 h.
[1] H. Li, H. Lin, D. Ouyang, C. Yao, C. Li, J. Sun, Y. Song, Y. Wang, Y. Yan, Y. Wang, Q. Dong, W. C. H. Choy, "Efficient and Stable Red Perovskite Light Emitting Diodes with Operational Stability > 300 h", Adv. Mater., 2008820, 2021.

Colloidally synthesized CdSe-based quantum wells (CQWs) exhibiting narrow emission linewidth (full-width-at-half-maximum, FWHM), giant oscillator strength, large absorption coefficient and reduced Auger recombination have been introduced as an important quasi-2D class of solution-processed nanocrystals for next-generation photonic applications. It is possible to tune the emission wavelength by designing the heterostructures of these CQWs, to improve their photoluminescence quantum efficiencies (PLQY) and photostability. These outstanding features make them a promising candidate for low-cost and high-efficiency LEDs. In early reports, red- and green-emitting CQW-LEDs were achieved with high colour purity. However, to date, there has been only one report related to LED devices with cyan-emitting CQWs. To obtain blue emission, 3 ML CdSe CQWs suffer from poor optical properties because of their low PLQY and rolled-up morphology in LED device. Therefore, blue CQW-based LEDs are of great interest to achieve pure coloured LEDs using CQW-based red, green and blue LEDs.

In this study, instead of using 3 ML CdSe CQWs as a blue emitting layer, we have proposed and developed alloyed CdSe_1-xS_x CQWs core/crown heterostructures emitting in the blue region for LED applications. We have synthesized alloyed 4 ML CdSe_1-xS_x CQWs having CdS crown with PLQY as high as ~60% in the blue region (462-487 nm). We have demonstrated the formation of the crown on the core using x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD) measurements. The absence of Se peaks in the alloyed 4 ML CdSe_1-xS_x core/crown CQWs confirmed the formation of the crown in alloyed 4 ML CdSe_1-xS_x core CQWs. Furthermore, XRD measurements indicated that with the formation of the CdS crown region, the diffraction peaks were slightly shifted to higher angles towards CdS CQWs, verifying the formation of a crown and also XRD analysis showed that both CdSe and CdS phases have zinc-blende structures. In addition, the elemental analysis with energy dispersive x-ray (EDX) spectroscopy showed the increase in the concentration of sulphur with the crown deposition, confirming the formation of core/crown heterostructures. Finally, blue-emitting CQW-LEDs have been fabricated using these CQWs as the emitting layer between a hole injection layer (PVK) and an electron injection layer (ZnO). Our NPL-LEDs exhibited a low turn-on voltage of ~4 V while operating with a narrow FWHM of 34 nm at the Commission Internationale de L’Eclairage (CIE) colour coordinates of (0.23, 0.14). We believe the proposed heterostructure will potentially open up the opportunity for full-colour displays using only CQW-based blue LEDs along with the red and green ones.

In recent years, new display technologies using mini-LEDs and micro-LEDs have attracted much attention, and improvement of emission efficiency of miniaturized LEDs with a size of below 100 um, which are the basis of these technologies, has become one of the serious issues in their application. In this paper, we propose an InGaN/GaN nano-porous membrane LED structure and its fabrication technique, which are expected to significantly improve the emission efficiency of mini/micro-LEDs.
We have developed a simple fabrication method for InGaN/GaN nanoporous membrane, that is an air-bridge structure of thin InGaN MQW LED film with randomly formed high-density vertical nanoholes with diameters of several tens of nanometers. Surprisingly, the InGaN/GaN nanoporous membrane exhibited a strong room-temperature (RT) blue photoluminescence (PL) emission with 23-fold enhancement compared to that of the as-grown wafer. The mechanism of the remarkable emission enhancement is also discussed using experiments and simulation with FDTD method.
For the experiments, an epitaxial wafer with the layer structure of p-GaN(60nm)/InGaN-MQW(85nm)/n-GaN(45nm)/n-AlInN(350nm)/n-GaN(4um) on (0001)Al₂O₃ substrate was used. On the surface of this wafer, SiO₂ (15 nm) was deposited by the ALD method, and some areas were removed by etching to expose the p-GaN layer. The sample was heated at 875°C for 60 min in a H₂:NH₃=100:9.5 mixture atmosphere at a total pressure of 1000 Pa. We call this process hydrogen environment anisotropic thermal etching (HEATE) [1]. Under these HEATE conditions, high-density random vertical nano-holes with diameters of several tens of nm were formed from the p-GaN to the interface between n-GaN/n-AlInN in the exposed area. By immersing this sample in hot nitric acid at 110^oC for 3 hours, only the n-AlInN layer at the bottom of the porous layer was selectively etched away, forming a air-bridge nanoporous membrane structure.
As an optical characterization, we compared the RT-PL properties of as-grown wafers, nanoporous and nanoporous membranes. The PL intensity of the nanoporous was reduced to about half of that of the as-grown wafer, which is considered to be the result of the balance between the reduced active layer volume, the reduction of the emission efficiency due to surface non-radiative recombination, increase of the emission efficiency due to strain relaxation, and increase of light extraction efficiency. It was also confirmed that thermal nitric acid treatment suppressed the non-radiative recombination at the MQW side wall due to the oxidation passivation effect, and increased the PL intensity by about 1.8 times. On the other hand, in the nanoporous membrane structure, the PL intensity was 23 times higher than that of as-grown wafers.
To investigate the mechanism of this remarkable increase in emission intensity, 2D-FDTD analysis was performed using a cross-sectional and top-view model of the nanoporous membrane. It was found that the intensity of radiation from the active layer to the surface side increased by a factor of 5 to 6, and the amount of excitation light absorbed by the active layer increased by a factor of about 2.1 for the nanoporous membrane compared with the as-grown wafer. The increase in the light extraction efficiency can be explained by the fact that the nanoporous structure suppressed the coupling of light into the waveguide mode due to light localization effect, and the light emitted downward was reflected upward with high efficiency at the GaN surface under the AlInN layer. In addition, there is a difference in the far-field radiation patterns between the nanoporous structure and the flat surface of as-grown sample.
The results of a comprehensive analysis of the mechanism of the 23-fold enhancement of PL intensity of the InGaN nanoporous membrane and the possibility of micro-LED applications will be discussed more precisely.
[1] R. Kita et al. Jpn. J. Appl. Phys. 54 (2015) 046501.

In the world of perovskite halides, the lower dimensionality is showing high structural tunability and more stability with the introduction of hydrophobic organic cations by blocking the moisture penetration in comparison to 3D structure. This result in attractive optoelectronic properties for different field of applications such as solar cells, lasers, photodetectors, memory devices and LEDs. The progress and field of applications of perovskite halides in the last decade is tremendous, however they contain toxic lead which is restricting the applications due to stability, health and ecological concerns. So, there is an urgent need to include the non-toxic elements.
Here, we are reporting a new photoluminescent lead-free class of materials, BZA₃BiX₆ [BZA is benzylamine, Bi is bismuth and X is bromine and iodine]. They have been synthesized by a solution-based approach with a monoclinic structure that has isolated MX₆ octahedra surrounded by the organic cations. The powder morphology of the grown crystals is analysed by HR-SEM which concluded the layered growth. BZA₃BiBr₆ and BZA₃BiI₆ exhibits semiconducting behaviour with a bandgap of 2.7eV and 2.2eV respectively. Upon 350 nm excitation, BZA₃BiBr₆ exhibits broad emission centered at 415 nm and 450 nm with a long tail. On the other hand, BZA₃BiI₆ is not an emitter. We also synthesized the intermediate perovskites and they confirmed that the iodine presence is reducing the emission prooperties. We consider them as 0D perovskite halides as the octahedral cations are completely surrounded by the organic cations. In this work we are doing a systematic study of the crystal structure, degree of distortion and its purity using the single crystal X-ray diffraction, powder X-ray diffraction, Fourier-Transform Infrared Spectroscopy (FTIR) and Raman analysis. The thermal stability of the sample has been examined by TGA/DTA analysis. The optical studies have been thoroughly studied on UV-spectrophotometer and spectrofluorometer. This study will open the doors for a new type of materials by exploring the individual sites in the structure.