Exciton Confinement Research Articles

Grain size control in quasi-two-dimensional perovskite thin film via intermediate phase engineering for efficient bound exciton generation

AbstractQuasi-two dimensional (2D) perovskites have emerged as a promising class of materials due to their remarkable photoluminescence efficiency, which stems from their exceptionally high exciton binding energies. The spatial confinement of excitons within smaller grain sizes could enhance the formation of biexcitons leading to higher radiative recombination efficiency. However, the synthesis of high-quality quasi-2D perovskite thin films with controllable grain sizes remains a challenging task. In this study, we present a facile method for achieving quasi-2D perovskite thin films with controllable grain sizes ranging from 500 to 900 nm. This is accomplished by intermediate phase engineering during the film fabrication process. Our results demonstrate that quasi-2D perovskite films with smaller grain sizes exhibit more efficient bound exciton generation and a reduced stimulated emission threshold down to 15.89 µJ cm−2. Furthermore, femtosecond transient absorption measurements reveal that the decay time of bound excitons is shorter in quasi-2D perovskites with smaller grain sizes compared to that of those with larger grains at the same pump density, which is 230.5 ps. This observation suggests a more efficient exciton recombination process in the smaller grain size regime. Our findings would offer a promising approach for the development of efficient bound exciton lasers.

Valley-Hybridized Gate-Tunable 1D Exciton Confinement in MoSe2.

Controlling excitons at the nanoscale in semiconductor materials represents a formidable challenge in the quantum photonics and optoelectronics fields. Monolayers of transition metal dichalcogenides (TMDs) offer inherent 2D confinement and possess significant exciton binding energies, making them promising candidates for achieving electric-field-based confinement of excitons without dissociation. Exploiting the valley degree of freedom associated with these confined states further broadens the prospects for exciton engineering. Here, we show electric control of light polarization emitted from one-dimensional (1D) quantum-confined states in MoSe2. Building on previous reports of tunable trapping potentials and linearly polarized emission, we extend this understanding by demonstrating how nonuniform in-plane electric fields enable in situ control of these effects and highlight the role of gate-tunable valley hybridization in these localized states. Their polarization is entirely engineered through either the 1D confinement potential's geometry or an out-of-plane magnetic field. Controlling nonuniform in-plane electric fields in TMDs enables control of the energy (up to five times its line width), polarization state (from circular to linear), and position of 1D confined excitonic states (5 nm V-1).

Quantum Cone-A Nano-Source of Light with Dispersive Spectrum Distributed along Height and in Time.

We study a quantum cone, a novel structure composed of multiple quantum dots with gradually decreasing diameters from the base to the top. The dot distribution leads to a dispersive radiated spectrum. The blue edge of the spectrum is determined by the quantum confinement of excitons on top of the cones, while the red edge is determined by the bandgap of a semiconductor. We observe the kinetics of photoluminescence by obeying the stretch-exponential law from quantum cones formed on the surface of diamond-like carbon (DLC). They are explained by an increase in the lifetime of excitons along the height of the cone from the top to the base of the cone and an increasing concentration of excitons at the base due to their drift in the quasi-built-in electric field of the quantum cone. The possible visualization of the quantum cone tops of DLC using irradiation by a UV light source is shown. A quantum cone is an innovative nano-source of light because it substitutes for two elements in a conventional spectrometer: a source of light and a dispersive element-an ultrafast monochromator. These features enable the building of a nano-spectrometer to measure the absorbance spectra of virus and molecule particles.

Efficient Polycrystalline Single-Cation Perovskite Light-Emitting Diodes by Simultaneous Intracrystal and Interfacial Defect Passivation.

Polycrystalline perovskite light-emitting diodes (PeLEDs) have shown great promise with high efficiency and easy processability. However, PeLEDs using single-cation polycrystalline perovskite emitters have demonstrated low efficiency due to defects within the grains and at the interfaces between the perovskite layer and the charge injection contact. Thus, simultaneous defect engineering of perovskites to suppress exciton loss within the grains and at the interfaces is crucial for achieving high efficiency in PeLEDs. Here, 1,8-octanedithiol which is a strong nucleophile, is used to increase the luminescence efficiency of a single-cation perovskite by suppressing non-radiative recombination within the grains of their polycrystalline emitter film as well as at their interface with an anode. The dithiol additive performs a multifunctional role in defect passivation, spatial confinement of excitons, and prevention of exciton quenching at the interface between the perovskite layer and the underlying hole-injection layer. Photoluminescence studies demonstrate that incorporating the dithiol additive significantly enhances the charge carrier dynamics in perovskites, resulting in an external quantum efficiency (EQE) of up to 23.46% even in a simplified PeLED that does not use a hole-injection layer. This represents the highest level of EQE achieved among devices utilizing polycrystalline single-cation perovskites.

Molten Salt Modulation of Potassium-Nitrogen-Carbon for the Breaking Kinetics Bottleneck of Photocatalytic Overall Water Splitting and Environmental Impact Reduction.

Sluggish interfacial water dissociation and the O2 evolution reaction (OER) kinetics are the main obstacles that limit the photocatalytic overall water-splitting performance. A molten salt modulation of potassium-nitrogen-carbon is herein demonstrated as the formation of highly crystalline potassium-doped poly(triazine imide) (KPTI). The in situ X-ray diffraction patterns and theoretical calculation show that the KCl melt can significantly reduce the free energy for the polycondensation of triazine building blocks owing to the formation of a kinetically stable KPTI. Benefiting from the presence of potassium-carbon-nitrogen moiety, the catalyst not only weakens the excitonic confinement to improve the separation efficiency of photogenerated charge carriers but also enhances the stability of carbon sites by suppressing the undesired C═O formation. Moreover, KPTI accelerates water dissociation by forming interfacial K·H2O clusters with an ordered structure, which supplies sufficient protons for the H2 evolution reaction and lowers the energy barrier to enhance the kinetics of OER. Therefore, a stable photocatalytic overall water-splitting performance can be achieved over KPTI with a stoichiometric generation of products (H2 and O2). Life cycle assessment shows that a carbon-neutral scenario can be achieved on KPTI production in the near term with an increase in green power in the electricity grid.

Manipulating exciton confinement for stable and efficient flexible quantum dot light-emitting diodes

Flexible quantum dot light-emitting diodes (QLEDs) show great promise for the next generation of flexible, wearable, and artificial intelligence display applications. However, the performance of flexible QLEDs still lags behind that of rigid substrate devices, hindering their commercialization for display applications. Here we report the superior performance of flexible QLEDs based on efficient red ZnCdSe/ZnS/ZnSe QDs (A-QDs) with anti-type-I nanostructures. We reveal that using ZnS as an intermediate shell can effectively confine the exciton wavefunction to the inner core, reducing the surface sensitivity of the QDs and maintaining its excellent emission properties. These flexible QLEDs exhibit a peak external quantum efficiency of 23.0% and a long lifetime of 63,050 h, respectively. The anti-type-I nanostructure of A-QDs in the device simultaneously suppresses defect-induced nonradiative recombination and balances carrier injection, achieving the most excellent performance of flexible QLEDs ever reported. This study provides new insights into achieving superior performance in flexible QD-based electroluminescent devices.

Confinement of excited states in two-dimensional, in-plane, quantum heterostructures

Two-dimensional (2D) semiconductors are promising candidates for optoelectronic application and quantum information processes due to their inherent out-of-plane 2D confinement. In addition, they offer the possibility of achieving low-dimensional in-plane exciton confinement, similar to zero-dimensional quantum dots, with intriguing optical and electronic properties via strain or composition engineering. However, realizing such laterally confined 2D monolayers and systematically controlling size-dependent optical properties remain significant challenges. Here, we report the observation of lateral confinement of excitons in epitaxially grown in-plane MoSe2 quantum dots (~15-60 nm wide) inside a continuous matrix of WSe2 monolayer film via a sequential epitaxial growth process. Various optical spectroscopy techniques reveal the size-dependent exciton confinement in the MoSe2 monolayer quantum dots with exciton blue shift (12-40 meV) at a low temperature as compared to continuous monolayer MoSe2. Finally, single-photon emission (g2(0) ~ 0.4) was also observed from the smallest dots at 1.6 K. Our study opens the door to compositionally engineered, tunable, in-plane quantum light sources in 2D semiconductors.

Sub-second long lifetime triplet exciton reservoir effect—A novel paradigm on exciton confinement in organic light-emitting diodes

Sub-second long lifetime triplet exciton reservoir effect—A novel paradigm on exciton confinement in organic light-emitting diodes

Pressure-triggered intense natural white emission via controllable distortions of antimony chloride dimers

Zero-dimensional (0D) hybrid metal halides (HMHs), featured with unique self-trapped exciton (STE) emissions, are emerging photoluminescence (PL) materials for photoelectronic applications. Compared with metal-halogen monomers, dimeric units in 0D HMHs possess more structural flexibility and less spatial confinement of excitons, creating new opportunities for PL engineering. Herein, high pressure is employed on 0D (C3H12N2)2Sb2Cl10 to controllably distort inter- and intra-octahedral structures within [Sb2Cl10]4− dimers. Intense natural white emission is achieved with dramatically increased photoluminescence quantum yield from less than ≈1% to 74.2 %. The high-pressure emission of (C3H12N2)2Sb2Cl10 is proven to originate from the STE recombination from triplet states with varied emission equilibrium, as well as the promoted excitonic trapping with restrained nonradiative recombination. This work offers valuable insights into the structure effects on STE emission and the transition mechanisms of metal-halogen dimers, which are crucial for developing novel metal halides for illumination.

Regulating photoluminescence through single-crystal-to-single-crystal transformation of solvent-containing zero-dimensional hybrid metal halide isomers

Regulating photoluminescence (PL) properties induced by single-crystal-to-single-crystal (SCSC) transformation has sparked significant interest in the research of zero-dimensional organic–inorganic metal halides (0-D OIMHs). To best of our knowledge, the SCSC transformation of solvent-containing 0-D OIMH isomers hasn’t been reported yet. In this work, we report the first example of SCSC transformation between two isomers containing lattice solvent molecules, namely α- and β-[EtPPh3]2[SbCl5]·MeCN (α- and β-1·MeCN; EtPPh3 = ethyltriphenylphosphonium; MeCN = acetonitrile). Differential scanning calorimetry (DSC) results show that the SCSC transformation from α- to β-1·MeCN is an exothermic process, implying that β-1·MeCN is a more stable phase than α-1·MeCN. Moreover, α- and β-1·MeCN exhibit bright orange (emission peak: 610 nm) and red (emission peak: 635 nm) emission under 360/365 nm excitation, respectively. DFT calculations suggest that the negligible self-absorption and a strong exciton confinement contribute to their effective PL emission. By analyzing temperature-dependent PL spectra, β-1·MeCN has been found to exhibit stronger electron-phonon coupling under excitation than α-1·MeCN. Thus, the broader emission with full width at half maximum (FWHM: 134 vs. 147 nm) and the higher photoluminescence quantum yield (PLQY: 40.37% vs. 70.50%) have been realized by SCSC transformation from α- to β-1·MeCN by heating at 92 °C. Benefitting from the regulation of PL performances brought by SCSC transformation, the white light-emitting-diode (LED) devices assembled by using β-1·MeCN as red phosphor could be optimized, and the CRI reaches as high as 95.3.

Local tuning of Rydberg exciton energies in nanofabricated Cu2O pillars

Rydberg excitons in Cu2O feature giant optical nonlinearities. To exploit these nonlinearities for quantum applications, the confinement must match the Rydberg blockade size, which in Cu2O could be as large as a few microns. Here, in a top-down approach, we show how exciton confinement can be realised by focused-ion-beam etching of a polished bulk Cu2O crystal without noticeable degradation of the excitonic properties. The etching of the crystal to micron sizes allows for tuning the energies of Rydberg excitons locally, and precisely, by optically induced temperature change. These results pave the way for exploiting the large nonlinearities of Rydberg excitons in micropillars for making non-classical light sources, while the precise tuning of their emission energy opens up a viable pathway for realising a scalable photonic quantum simulation platform.

A tight-binding model for illustrating exciton confinement in semiconductor nanocrystals.

The Brus equation describes the relation between the lowest energy of an electron-hole pair and the size of a semiconductor crystallite. However, taking the strong confinement regime as a starting point, the equation does not cover the transition from weak to strong confinement, the accompanying phenomenon of charge-carrier delocalization, or the change in the transition dipole moment of the electron-hole pair state. Here, we use a one-dimensional, two-particle Hubbard model for interacting electron-hole pairs that extends the well-known tight-binding approach through a point-like electron-hole interaction. On infinite chains, the resulting exciton states exhibit the known relation between the Bohr radius, the exciton binding energy, and the effective mass of the charge carriers. Moreover, by introducing infinite-well boundary conditions, the model enables the transition of the exciton states from weak to strong confinement to be tracked, while straightforward adaptations provide insights into the relation between defects, exciton localization, and confinement. In addition, by introducing the dipole operator, the variation of the transition dipole moment can be mapped when shifting from electron-hole pairs in strong confinement to delocalized and localized excitons in weak confinement. The proposed model system can be readily implemented and extended to different multi-carrier states, thus providing researchers a tool for exploring, understanding, and teaching confinement effects in semiconductor nanocrystals under different conditions.

Sub-Second Long Lifetime Triplet Exciton Reservoir for Highly Efficient and Stable Organic Light-Emitting Diode.

In organic light-emitting diode (OLED), achieving high efficiency requires effective triplet exciton confinement by carrier-transporting materials, which typically have higher triplet energy (ET) than the emitter, leading to poor stability. Here, an electron-transporting material (ETM), whose ET is 0.32 eV lower than that of the emitter is reported. In devices, it surprisingly exhibits strong confinement effect and generates excellent efficiency. Additionally, the device operational lifetime is 4.9 times longer than the device with a standard ETM, 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl) phenyl (whose ET 0.36 eV is higher than the emitter). This anomalous finding is ascribed to the exceptionally long triplet state lifetime (≈0.2 s) of the ETM. It is named as long-lifetime triplet exciton reservoir effect. The systematic analysis reveals that the long triplet lifetime of ETM can compensate the requirement for high ET with the help of endothermic energy transfer. Such combination of low ET and long lifetime provides equivalent exciton confinement effect and high molecular stability simultaneously. It offers a novel molecular design paradigm for breaking the dilemma between high efficiency and prolonged operational lifetime in OLEDs.

Red quantum-dot common layer for organic-quantum-dot hybrid light-emitting diodes for full-color displays

Organic and quantum dot (QD) light-emitting diodes (OLEDs and QLEDs, respectively) have been known for their practical application in full-color displays. Each primary color-emitting layer (EML) needs to be patterned via fine metal shadow masks (FMMs) or inkjet printing. However, the FMM method suffers from a shadow effect, misalignment, and size limitation, while inkjet printing hinders fine patterning, which disrupts high-resolution, large-scale fabrication. Here, we show a novel organic–QD hybrid device architecture by introducing a red QD common layer (QDCL). Since the QDCL is deposited onto all sub-pixels in a solution process without patterning, one patterning step for the red sub-pixel is reduced for full-color display fabrication. The green and blue OLEDs incorporating a red-emitting QDCL emitted their own color, owing to the effective exciton confinement within the EML by adopting a buffer layer, while the QDs in the QDCL emitted saturated red light. As a result, we could demonstrate a full-color organic–QD hybrid light-emitting device fabricated on a single substrate using the QDCL. We believe our strategy would help fabricate full-color and high-resolution displays with reduced manufacturing costs and time.

Confinement of Excitons within GaN 1D Nanoarchitectures Formed on AlN Molecular Steps

AbstractThe fabrication and the optical properties of ultrathin GaN quantum wells on AlN (0001) vicinal surfaces with macrosteps are studied. The employed growth condition spontaneously restricts the GaN thickness to one monolayer (ML) on a planar plane. However, because of a higher density of dangling bonds at macrosteps, 2 ML GaN can be stabilized only at step edges, which creates quantum‐wire‐like 1D nanostructures with a dimension of a few nanometers for the lateral confinement. Exciton (carrier) confinement within the 1D GaN nanostructure is predicted theoretically and is confirmed experimentally by strongly polarized photoluminescence with in‐plane twofold symmetry at a wavelength of ≈250 nm at room temperature. This study demonstrates one path to realize quantum confinement within a fine nanoarchitecture composed of nitride semiconductors and might provide clues to the future creation of novel low‐dimensional nanostructures for the UV–vis spectral range.

High-performance warm-white OLEDs using interfacial exciplex energy transfer with external quantum efficiency exceeding 30%

To achieve energy-saving and environmentally friendly warm white lighting through the simplification of white organic light-emitting device (WOLED) structure and manufacturing process, we designed and optimized a WOLED by incorporating bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III)(FIrpic)-doped 4,4′,4′-tris(N-carbazolyl)triphenylamine(TCTA) and bis(4-phenylthieno[3,2-c]pyridinato-N,C2')(acetylacetonate)iridium(III)(PO-01)-doped 2,2',2′'-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)(TPBI) thin films as the blue and yellow emissive layer (EML), respectively, and forming an exciplex at TCTA and TPBI interface. By adjusting the doping concentration and position within the EML and the thickness of the transport layer, we achieved the optimal emission balance between the blue and yellow colors in the proposed WOLED. We successfully fabricated a highly efficient and structurally simplified WOLED without the use of light extraction coupling techniques, which exhibited high external quantum efficiency (EQE) of 32.6% and current efficiency (CE) of 80.3 cd/A, along with stable emission color with minimal CIE variation of ΔCIE(0.0027, 0.0023) from 500 cd/m² to 5000 cd/m². The experimental results revealed that the designed device structure with the triple host system (TCTA, TPBI, and interface exciplex) facilitated the widening of the recombination zone (RZ), enhanced exciton confinement and utilization, reduced triplet-triplet annihilation (TTA) effects, and achieved efficient host-guest energy transfer. Furthermore, the designed WOLED emitted warm white light spectrum suitable for healthy lighting applications. Therefore, we believe that our work contributes to the further development of ideal and simplified WOLED structure designs for achieving high-efficiency, simplified manufacturing, low-cost, energy-saving, and environmentally friendly healthy lighting.

Defect analysis and microstructural characterization of zinc-substituted cadmium oxide nanocrystallites by positron annihilation and supplementary methods

Pure and zinc-doped cadmium oxide nanocrystallites of sizes in the range 25 nm to 16 nm are synthesized by adopting a chemical precipitation method and by varying the doping concentration from 0.0 to 0.25. The decrease in nanocrystallite sizes with increasing substitution is expected from the smaller ionic radii of Zn2+. But more revealing is the interfacial defects formation at higher concentration of doping, which is attributed to the dissimilar crystalline structure of ZnO and CdO. X-ray diffraction patterns show well defined peaks and additional characterisation is done through transmission electron microscopy. The optical band gap measurements indicate the dominance of substitution-induced disorder over the confinement of excitons, leading to a decrease in the band gap energies. The results of positron annihilation studies confirm the cancellation of the existing vacancy type defects in the initial stage, followed by the substitution. Photoluminescence spectra reveal the distinct peaks of optical plasmonic excitations and the defect population in the bandgap and the intensity variations agreed with that of the defect related positron annihilation lifetime intensity. The segregation of ZnO phase leading to the formation of interfacial boundaries is found as a strong deterrent against the success of continued substitution.

Universal Wet‐Chemistry‐Methods Synthesized Novel Halide‐Intercalated Perovskites with Reduced Exciton Confinement for Low‐Dose X‐ray Scintillation Imaging

AbstractX‐ray radiography, playing a crucial role in the daily life of humans, extends the application landscapes of perovskites. Lead halide perovskites remain superior candidates because of strong X‐ray absorption and effective conversion of X‐ray to visible photons. Unfortunately, the perovskites exhibiting efficient band‐edge emission (3D, quasi‐2D or 2D) usually suffer from severe self‐absorption, while the perovskites showing broadband emission with large Stokes shift (0D, 1D or 2D) usually have low quantum yield, long afterglow or poor solution‐processability. Halide‐intercalated perovskites, with incorporated halides in organic layers, exhibit attractive optoelectronic properties due to reduced confinement of excitons, but their development falls far behind due to limited molecular design strategies and wet‐chemistry methods. Here, the use of universal‐wet‐chemistry‐synthesized novel halide‐intercalated perovskites as an efficient X‐ray scintillator for X‐ray imaging is reported. These novel perovskites exhibit superior luminescence by exploiting both free and self‐trapped excitons via halide intercalation. Accordingly, such perovskite scintillators present higher radioluminescence performance compared with conventional perovskite scintillators through enhanced radiative recombination and suppressed self‐absorption simultaneously. With such a scintillator screen, high‐performance X‐ray imaging is demonstrated. The results not only represent a universal synthesis route for the development of perovskites, but also provide valuable guidance for high‐performance X‐ray radiography.

Edge-Confined Excitons in Monolayer Black Phosphorus.

Quantum confinement of two-dimensional excitons in van der Waals materials via electrostatic trapping, lithographic patterning, Moiré potentials, and chemical implantation has enabled significant advances in tailoring light emission from nanostructures. While such approaches rely on complex preparation of materials, natural edges are a ubiquitous feature in layered materials and provide a different approach for investigating quantum-confined excitons. Here, we observe that certain edge sites of monolayer black phosphorus (BP) strongly localize the intrinsic quasi-one-dimensional excitons, yielding sharp spectral lines in photoluminescence, with nearly an order of magnitude line width reduction. Through structural characterization of BP edges using transmission electron microscopy and first-principles GW plus Bethe-Salpeter equation (GW-BSE) calculations of exemplary BP nanoribbons, we find that certain atomic reconstructions can strongly quantum-confine excitons resulting in distinct emission features, mediated by local strain and screening. We observe linearly polarized luminescence emission from edge reconstructions that preserve the mirror symmetry of the parent BP lattice, in agreement with calculations. Furthermore, we demonstrate efficient electrical switching of localized edge excitonic luminescence, whose sites act as excitonic transistors for emission. Localized emission from BP edges motivates exploration of nanoribbons and quantum dots as hosts for tunable narrowband light generation, with future potential to create atomic-like structures for quantum information processing applications as well as exploration of exotic phases that may reside in atomic edge structures.

Bulk Passivation Enables Hundredfold‐Enhanced Electroluminescence of Monophosphine Cu4I4 Cubes

AbstractElectroluminescent (EL) clusters emerged rapidly, owing to their organic–inorganic hybrid character useful for comprehensive performance integration and the potential for large‐scale display and lighting applications. However, despite their good photoluminescent (PL) properties, until present, no efficient EL monodentate ligand‐based clusters were reported due to structural variation during processing and excitation and exciton confinement on cluster‐centered quenching states. Here we demonstrate an effective bulky passivation strategy for efficient cluster light‐emitting diodes with a monophosphine Cu4I4 cube named [TMeOPP]4Cu4I4. With terminal pyridine groups, an active matrix named TmPyPB supports an effective host‐cluster interplay for configuration fixation, structural stabilization, and exciton‐confinement optimization. Compared to common inactive hosts, the passivation effects of TmPyPB markedly reduce trap‐state densities by 24–40 % to suppress nonradiative decay, resulting in state‐of‐the‐art PL and EL quantum yields reaching 99 % and 15.6 %, respectively, which are significantly improved by about 7‐fold. TmPyPB simultaneously increases EL luminance to 104 nits, which is ≈100‐fold that of the non‐doped analogue.