Exciton Emission Research Articles

Enhanced UV Light Responsivity in <110>‐Oriented 2D Perovskites Realized by Pressure‐Induced Ultrafast Exciton Transport

AbstractTwo‐dimensional (2D) <100>‐oriented perovskites exhibit superior optoelectronic properties, offering significant potential in photovoltaic, light‐emitting, and photodetection applications. Nevertheless, their enlarged interlayer spacing restricts longitudinal carrier transport, thereby limiting its potential applications. While <110>‐oriented 2D perovskites provide a prospective solution with their compact interlayer spacing, their inherent structure, characterized by octahedra tilting, indirectly hinders carrier transport due to the generation of self‐trapped excitons (STEs) caused by strong electron‐phonon coupling. Here, we adeptly regulate the photoluminescence (PL) from STEs to free excitons (FEs) emission within the 2D <110>‐oriented (API)PbBr4 single crystal through structure optimization under pressure treatment. Besides, we observed anisotropic FE transport with an anisotropy ratio of 4.97. The exciton mobility reaches a peak of 93.6 cm2 V−1 s−1 at 2.7 GPa, a value comparable to those of their three‐dimensional (3D) counterparts, which is attributed to a reduction in electron‐phonon coupling and exciton reduced mass. Additionally, this ultrafast exciton transport significantly enhances UV light responsivity, exhibiting an increase of approximately 5000 times at 2.7 GPa in comparison to ambient conditions. These findings highlight the prospective application of 2D <110>‐oriented perovskites in high‐performance optoelectronic devices through intrinsic structural modulation.

Regulating Second‐Harmonic Generation in 2D Chiral Perovskites Through Achiral Organic Spacer Cation Alloying Strategy

AbstractThe inherent structural flexibility and chiroptical activity of 2D chiral perovskites make them promising for the nonlinear optical (NLO) application. A comprehensive understanding of the second‐harmonic generation (SHG) mechanism in 2D chiral perovskites is essential for developing NLO devices. However, the rational design of 2D chiral perovskite structures to regulate SHG properties remains challenging. Herein, to regulate SHG response, an achiral organic spacer cation alloying strategy is employed to construct a series of 2D chiral perovskites. Through the measurement of temperature‐dependent photoluminescence (PL) spectra, it is revealed that the material design strategy can effectively modulating self‐trapped exciton (STE) emission. More importantly, it is confirmed that there is a competitive relationship between STE emission and SHG in 2D chiral perovskites. Meanwhile, the microscopic imaging of circularly polarized‐SHG is demonstrated in chiral perovskites. This work will not only advance the understanding of the SHG mechanism in 2D chiral perovskites but also provide inspiration for the rational design and synthesis of perovskites for NLO devices.

Enhanced UV Light Responsivity in -Oriented 2D Perovskites Realized by Pressure-Induced Ultrafast Exciton Transport.

Two-dimensional (2D) <100>-oriented perovskites exhibit superior optoelectronic properties, offering significant potential in photovoltaic, light-emitting, and photodetection applications. Nevertheless, their enlarged interlayer spacing restricts longitudinal carrier transport, thereby limiting its potential applications. While <110>-oriented 2D perovskites provide a prospective solution with their compact interlayer spacing, their inherent structure, characterized by octahedra tilting, indirectly hinders carrier transport due to the generation of self-trapped excitons (STEs) caused by strong electron-phonon coupling. Here, we adeptly regulate the photoluminescence (PL) from STEs to free excitons (FEs) emission within the 2D <110>-oriented (API)PbBr4 single crystal through structure optimization under pressure treatment. Besides, we observed anisotropic FE transport with an anisotropy ratio of 4.97. The exciton mobility reaches a peak of 93.6 cm2 V-1 s-1 at 2.7 GPa, a value comparable to those of their three-dimensional (3D) counterparts, which is attributed to a reduction in electron-phonon coupling and exciton reduced mass. Additionally, this ultrafast exciton transport significantly enhances UV light responsivity, exhibiting an increase of approximately 5000 times at 2.7 GPa in comparison to ambient conditions. These findings highlight the prospective application of 2D <110>-oriented perovskites in high-performance optoelectronic devices through intrinsic structural modulation.

The Influence of Temperature and Stoichiometry on the Optical Properties of CdSe Nanoplatelets

Colloidal quasi-two-dimensional cadmium chalcogenide nanoplatelets have attracted considerable interest due to their narrow excitonic emission and absorption bands, making them promising candidates for advanced optical applications. In this study, the synthesis of quasi-two-dimensional CdSe NPLs with a thickness of 3.5 monolayers was investigated to understand the effects of synthesis temperature on their stoichiometry, morphology, and optical properties. The NPLs were synthesized using a colloidal method with temperatures ranging from 170 °C to 210 °C and optimized precursor ratios. Total reflection X-ray fluorescence (TXRF) analysis was employed to determine stoichiometry, while high-resolution transmission electron microscopy (HRTEM) and UV-Vis spectroscopy and photoluminescence spectroscopy were used to analyze the structural and optical characteristics. The results showed a strong correlation between increasing synthesis temperature and the enlargement of nanoscroll diameters, indicating dynamic growth. The best results in terms of uniformity, stoichiometry, and optical properties were achieved at a growth temperature of 200 °C. At this temperature, no additional optical bands associated with secondary populations or hetero-confinement were observed, indicating the high purity of the sample. Samples synthesized at lower temperatures exhibited deviations in stoichiometry and optical performance, suggesting the presence of residual organic compounds.

Unlocking Full-Spectrum Brilliance: Dimensional Regulation in Lead-Free Metal Halides for Superior Photoluminescence.

Lead-free metal halides with tunable structures have emerged as a new class of optoelectronic materials. The arrangement of metal halide polyhedra defines their structural dimensionality and serves as a key factor influencing their optical properties. To investigate this, we synthesized four different antimony (Sb)-doped indium (In)-based metal halides, all of which possess zero-dimensional (0D) electronic structures but exhibit 3D, 2D, 1D, and 0D structural dimensionality at the molecular level. With a decreasing of structural dimensionality, their self-trapped exciton (STE) emission shows a red shift with peak position from 496 to 663 nm. We revealed that the red shift is caused by increased distortion of [SbCl6]3- octahedra as the structural dimensionality decreases, leading to lowered energy levels of STE and a corresponding red shift. The tunable STE emission makes these metal halides promising for anticounterfeiting and white LED applications. These findings provide a new strategy for tuning STE emission in lead-free metal halides.

Liquid-Phase Fabrication of Janus 2D Materials: Defect-Rich MoS2 Ultrathin Layers Asymmetrically Decorated with Au Nanoparticles.

Asymmetrically decorated nanoparticles (NPs), also known as "Janus nanoparticles", possess at least two differently functionalized surfaces. This coexistence results in novel features that surpass the inherited benefits of the initial counterparts. Despite significant advances in spherical morphologies, research on Janus two-dimensional (2D) materials is limited, as fabrication strategies primarily focus on dry deposition techniques. To produce Janus 2D materials in large quantities, solution-based techniques are proposed. However, this approach remains largely unexplored for 2D materials other than graphene and its derivatives, and it yields Janus 2D materials in very low amounts. This study develops a liquid-phase fabrication strategy for the asymmetric decoration of MoS2 ultrathin layers with gold nanoparticles. This approach builds on previous advances in the asymmetric functionalization of spherical nanoparticles, using SiO2 microbeads as a masking template. Interestingly, the photoluminescence (PL) spectrum of the processed material is unusually dominated by the B exciton emission. The reported versatile method has proven to be scalable, enabling the production of 2D Janus flakes in appreciable quantities, whether as 1T or 2H-polytypes. Overall, the novel synthetic strategy is highly adaptable and can be extended to a variety of other 2D materials and functionalizing agents.

Achieving Luminous Efficiency Enhancement and Near‐Infrared Emission in Moisture‐Stable 0D Zinc‐Based Halides

AbstractZero‐dimensional (0D) metal halides are attractive due to their structure‐dependent and tunable photoluminescence properties. Herein, a new 0D organic–inorganic hybrid Zn‐based halide, (C4H12N2)ZnBr4, featuring a long‐term stable crystal structure and moisture‐stable PL emission under various extreme conditions is reported. A strong electron–phonon coupling effect enables the Zn‐based halide to display highly efficient blue light at 472 nm with a large Stokes shift of 7385 cm−1. Intriguingly, heterovalent substitution of Cu+‐Zn2+ further enhances the photoluminescence quantum efficiency to 60% as the introduction of Cu+ effectively suppresses the nonradiative recombination process. Besides, the formation of twisted [SbBr4]− tetrahedra via Sb3+‐Zn2+ substitution help to achieve a broadband near‐infrared (NIR) emission (760 nm) with full width at half maxima (FWHM) of 203 nm, enabling the potential applications in night‐vision and nondestructive fruit damage inspection. Detailed structural and optical analyses are used to investigate the photophysical processes of different self‐trapped exciton (STE) emission for pristine and Cu+/Sb3+‐doped 0D metal halides. These findings advance the understanding of spectral regulation mechanism via heterovalent substitution and initiate more exploitation of luminescent metal halides for emerging applications.

Controllable Multi‐Exciton Zero‐Dimensional Antimony‐Based Metal Halides for White‐light Emission and β‐Ray Detection

AbstractSelf‐trapped exciton (STE) emission, typified by antimony (Sb), with broadband characteristics, represents the next generation of materials for solid‐state lighting and radiation detection. However, little is known about the multiexciton behavior of the Sb emission center. Here, we proposed a general approach for designing antimony‐centered multi‐exciton emitting materials through self‐assembly. Benefitting from controllable multiexciton behavior, dual‐band white light emission spanning the entire visible spectrum was achieved. Relying on the reduction of an effective atomic number brought by self‐assembly, excellent scintillation response to β‐rays was attained. This study offers unprecedented insight into hybrid single/triple STE emission and unveils new avenues for single‐emitter white‐light emission, as well as radiographic testing using low‐risk β‐rays as sources.

Controllable Multi-Exciton Zero-Dimensional Antimony-Based Metal Halides for White-light Emission and β-Ray Detection.

Self-trapped exciton (STE) emission, typified by antimony (Sb), with broadband characteristics, represents the next generation of materials for solid-state lighting and radiation detection. However, little is known about the multiexciton behavior of the Sb emission center. Here, we proposed a general approach for designing antimony-centered multi-exciton emitting materials through self-assembly. Benefitting from controllable multiexciton behavior, dual-band white light emission spanning the entire visible spectrum was achieved. Relying on the reduction of an effective atomic number brought by self-assembly, excellent scintillation response to β-rays was attained. This study offers unprecedented insight into hybrid single/triple STE emission and unveils new avenues for single-emitter white-light emission, as well as radiographic testing using low-risk β-rays as sources.

Reversible Structural Phase Transitions in Zero‐Dimensional Cu(I)‐Based Metal Halides for Dynamically Tunable Emissions

AbstractExploring structural phase transitions and luminescence mechanisms in zero‐dimensional (0D) metal halides poses significant challenges, that are crucial for unlocking the full potential of tunable optical properties and diversifying their functional capabilities. Herein, we have designed two inter‐transformable 0D Cu(I)‐based metal halides, namely (C19H18P)2CuI3 and (C19H18P)2Cu4I6, through distinct synthesis conditions utilizing identical reactants. Their optical properties and luminescence mechanisms were systematically elucidated by experiments combined with density functional theory calculations. The bright cyan‐fluorescent (C19H18P)2CuI3 with high photoluminescence quantum yield (PLQY) of 77 % is attributed to the self‐trapped exciton emission. Differently, the broad yellow‐orange fluorescence of (C19H18P)2Cu4I6 displays a remarkable PLQY of 83 %. Its luminescence mechanism is mainly attributed to the combined effects of metal/halide‐to‐ligand charge transfer and cluster‐centered charge transfer, which effects stem from the strong Cu−Cu bonding interactions in the (Cu4I6)2− clusters. Moreover, (C19H18P)2CuI3 and (C19H18P)2Cu4I6 exhibit reversible structural phase transitions. The elucidation of the phase transitions mechanism has paved the way for an unforgeable anti‐counterfeiting system. This system dynamically shifts between cyan and yellow‐orange fluorescence, triggered by the phase transitions, bolstering security and authenticity. This work enriches the luminescence theory of 0D metal halides, offering novel strategies for optical property modulation and fostering optical applications.

Warm-White Light Emission of Lead-free CsAg2I3 Single Crystal Scintillator with a One-Dimensional Electronic Structure.

Presently, the exploration of novel inorganic lead-free perovskite scintillators has emerged as a prominent topic in the field of perovskite materials. Extensive attention has been garnered by materials such as Cs3Cu2I5 due to their notable advantage in scintillation intensity, but the response time constants in the microsecond or even millisecond range severely constrain their potential applications in scintillators. In this study, large-sized (5-6 mm) CsAg2I3 single crystals with an ultrafast warm-white light emission on a nanosecond time scale are presented. Specifically, upon X-ray excitation, the single crystal demonstrates a broad-spectrum white light emission with a color temperature as high as 5129 K, attributed to its self-trapped exciton emission. The 137Cs energy spectrum reveals that CsAg2I3 possesses an ultrafast response for γ rays with a time constant of 15 ns, which is significantly faster than that of Cs3Cu2I5. Furthermore, time-resolved photoluminescence unveils a subnanosecond component with a response time of 0.9 ns. The characteristics of ultrafast warm-white light emission exhibit the significant potential of CsAg2I3 in radiation scintillation detection and its probability of playing a pivotal role in future radiation detection technology.

The role of BaTiO3 nanoparticles as photocatalysts in the synthesis and characterization of novel fruit dyes is investigated.

In the present work, the photocatalytic activity against the natural dye extracted from the novel fruits has been studied by the BaTiO3 nanoparticles (NPs) under a ultra-violet (UV) light source. The large concentrations of an essential phenolic agent present in this phytochemical extract superimposed with cloths fibers make strong stain and degrade into another form of toxic, which is excluded from the many textiles industries as the colorful waste waters without recycling and removal of that dye pigments have been investigated using both photodegradation and photoluminescence techniques. The entitled nanoparticles (NPs) were prepared using the soft chemical root-modified solvothermal synthesis combo method and exposure to heat treatment such that the annealing process has been done for different temperatures ranging from 100°C to 250°C. As for as concern the characterization, as a start, structural and morphology studies have been reported here that highly crystalline oriented peaks data using powder x-ray diffraction techniques (PXRD) as well as the surface morphology including the size, shape, and mass distribution using the field emission scanning electron microscopy (FESEM) techniques, which purely belong to rutile tetragonal structure of the crystal system and circular and noncircular flakes like rough surface morphology materials respectively. The lattice dissociation constant 'ε' value of the BaTiO3 NPs has determined to be ~2.71 × 10-3 using the Williamson-Hall (W-H plot) analysis of crystallographic data. In the UV visible spectroscopy findings, since the extreme quantum confinement of BaTiO3 nanoflakes/nanodisc, the optical energy bandgap has been estimated to be a range of 1.98 to 2.67 eV (~2.48 eV) found from the Tauc plot analysis, which contributes to the significantly owing to the enhanced photocatalytic efficiency with excellent performance along exciton formation, superoxide ions, and hydroxyl free radicals generations under UV-vis light irradiation resulting in efficient degradation of typical novel fruit organic dye. Photoluminescence spectra observed at room temperature and low temperature have been observed for the BaTiO3 nanoflakes, which exhibit the blue emission due to the crystalline defects such as the appearance of Ba vacancies leads to the conceivable beginning of p-type conductivity and the origination of free exciton emission reveals the direct bandgap transition nature of nanoflakes. RESEARCH HIGHLIGHTS: According to our findings, 89.71% of the natural syzygium cumin is degraded by photocatalysis reaction. As a plausible mechanism for the destruction of natural dyes under solar light, photocatalytic destruction has been proposed. The reaction between these reactive free radical species leads to high efficiency photodegradation with a short decay time. In addition to water treatment and environmental cleaning applications, the excellent performance of this photocatalyst makes it a promising candidate for other applications. Hence, the synthesized BaTiO3 nanoflakes showcase a highly significant advancement towards the development of a textiles dye recycling method.

Lattice distortion enhanced self-trapped excitons emission in antimony halide crystalline clusters

Lattice distortion enhanced self-trapped excitons emission in antimony halide crystalline clusters

Simultaneous achieving color‐tuning long persistent luminescence and phosphorescent quantum yield of 81.05% in 2D organic metal halide perovskite

AbstractIonically bonded organic metal halide perovskite‐like luminescent materials, which incorporate organic cations and metal halides, have emerged as a versatile multicomponent material system. However, these materials still face challenges in terms of low phosphorescence quantum yields and limited long persistent luminescence (LPL) colors. Herein, we present the design and synthesis of an intraligand charge‐transfer organic‐based metal halide perovskite‐like material, in which organic cations form a compact supramolecular hydrogen‐bonded organic framework (HOF) structure, exhibiting crystallization‐induced phosphorescence emission of ligand, while metal halides form a unique two‐dimensional (2D) structure that displays intrinsic self‐trapped excitons (STE) emission under the radiation of UV light. Notably, the metal halide hybrid is found to exhibit enhanced phosphorescent photoluminescence efficiency of up to 81.05% and tunable LPL from cyan to orange compared to the pristine organic phosphor, due to the structural distortion and scaffolding effects of 2D metal halides as well as a well‐packed HOF structure. Optical characterizations and theoretical calculations reveal that charge transfer from organic cations and halogen to ligand as well as STE from inorganic layers are responsible for the tunable LPL. Meanwhile, the high‐efficiency phosphorescent quantum yield is attributed to stronger hydrogen bond stacking as well as structural distortion of metal halogen bands. Thus, the obtained LPL provides potentials in anti‐counterfeiting, security systems, and so on.

Unveiling Mechanism of Temperature‐Dependent Self‐Trapped Exciton Emission in 1D Hybrid Organic–Inorganic Tin Halide for Advanced Thermography

AbstractLead‐free hybrid metal halide phosphors/crystals showing self‐trapped exciton (STE) emission have been recently explored for thermography due to the strong temperature dependence of their photoluminescence (PL) lifetime (τ). However, realizing high‐spatial‐resolution thermography using polycrystalline powders or crystals presents a challenge. Moreover, the underlying mechanism of temperature‐dependent STE remains elusive. Herein, a homogeneous 1D ODASn2I6 (ODA, 1,8‐octanediamine) nm‐scale thin film exhibiting efficient STE emission is investigated. The PL decay shows a strong temperature dependence from 275 K (τ ≈ 1.31 µs) to 350 K (τ ≈ 0.65 µs) yielding a thermal sensitivity of 0.014 K−1. By employing temperature‐dependent transient absorption spectroscopy, detailed information is obtained about the relaxation processes prior to the STE formation. Simultaneous analyses of steady‐state and time‐resolved spectroscopies lead to a self‐consistent model where the thermally activated phonon‐assisted nonradiative pathway explains the temperature dependence of the PL lifetime via a conical intersection between the ground state and STE potential energy surfaces. Finally, a discernible 50 ns variation in PL lifetimes across different heated regimes over a distance of 1.15 mm is successfully demonstrated with fluorescence lifetime imaging microscopy, underscoring the substantial potential of ODASn2I6 thin film for high‐spatial‐resolution thermography.

Low-temperature photoluminescence and Raman study of monolayer WSe2 for photocarrier dynamics and thermal conductivity

Low-temperature PL analysis reveals an intriguing temperature-dependent emission pattern in WSe2: excitonic dominance above the 150 K Debye temperature, a balance between excitonic and trionic emissions at 150 K, and trionic dominance below this threshold. At lower temperatures, both excitons and trions display linearly polarized emissions, with polarization increasing from 0% at 300 K to 23% (excitons) and 7% (trions) at 150 K, and 12% for trions at 90 K. Moreover, the synthesized monolayer of WSe2 exhibits high thermal conductivity (246 W m−1 K−1 for A1g and 185 W m−1 K−1 for E2g1 modes). This property is attributed to Se vacancies and defects at triangle edges, which redirect phonons, reducing scattering and enabling efficient heat transport along boundaries. The unveiling of these novel insights within the synthesized 2D WSe2 material holds significant promise for its potential applications in nano-optoelectronics. Its demonstrated efficiency in dissipating heat, coupled with improved thermal stability, suggests the possibility of employing it in future devices. This could facilitate compact designs and the miniaturization of advanced technological tools, showcasing the material's potential for practical implementation.

Quasi-One-Dimensional Fibrous Phosphorus: An Air-Stable Low-Symmetry Semiconductor with High Anisotropy.

Due to their novel optical and optoelectronic properties, 2D materials have received increasing interest for optoelectronics applications. However, when the width of the channel layer decreases to the nanoscale, the properties of 2D materials can be seriously influenced due to the boundary defects and the approximation of physical dimensions and mean free path of the electron. That brings many challenges to developing novel electronics. Hence, researchers began to maintain their focus on 1D semiconductors without boundary defects and surfaces. Herein, fibrous phosphorus, another Quasi-1D layered semiconducting phosphorus allotrope with air-stable low symmetry, is reported. We found the in-plane anisotropic Raman response and excitation and exciton emission at room temperature. Moreover, the raw materials of fibrous phosphorus are nontoxic and abundant on the earth. These excellent properties will make it a highly competitive material for future applications in the optoelectronic area.

Excitonic properties of deep-blue emitting fragmented CsCl:Eu2+ nanocrystals

In this work, polycrystalline CsCl:Eu2+ nanocrystals were grown by the hot injection method. The CsCl:Eu2+ nanocrystals were fragmented into particles of a few nanometers by dispersion in organic solvents, and they exhibited strong deep-blue emission under ultraviolet light. Excitonic emission at room temperature was observed near 410 nm, with a full width at half-maximum of 37.8 nm and a photoluminescence quantum yield of 4.1%. Additionally, the average exciton lifetime was determined to be 6.45 ns. In the temperature range of 10 K to 300 K, the excitonic emission intensity gradually decreased while the energy of the peak position slightly increased, corresponding to an increase in the temperature. From the result, the exciton binding energy and average longitudinal phonon energy were calculated to be about 29 and 13 meV, respectively, comparable to the energy calculations of similar Cs-series perovskites. Thermal expansion and electron-phonon interaction also increased steadily with the temperature, and these two phenomena gave rise to a monotonic blue-shift.

Synthesis of β-Ga2O3:Mg Thin Films by Electron Beam Evaporation and Postannealing

Doping divalent metal cations into Ga2O3 films plays a key role in adjusting the conductive behavior of the film. N-type high-resistivity β-Ga2O3:Mg films were prepared using electron beam evaporation and subsequent postannealing processing. Various characterization methods (X-ray diffraction, X-ray photoelectron spectroscopy, photoluminescence, etc.) revealed that the Mg content plays an important role in affecting the film quality. Specifically, when the Mg content in the film is 3.6%, the S2 film’s resistivity, carrier content, and carrier mobility are 59655.5 Ω·cm, 1.95 × 1014 cm3/C, and 0.53682 cm2/Vs. Also, the film exhibits a smoother surface, more refined grains, and higher self-trapped exciton emission efficiency. The Mg cation mainly substitutes the Ga+ cation at a tetrahedral site, acting as a trap for self-trapped holes.

Robust excitonic light emission in 2D tin halide perovskites by weak excited state polaronic effect

2D perovskites hold immense promise in optoelectronics due to their strongly bound electron-hole pairs (i.e., excitons). While exciton polaron from interplay between exciton and lattice has been established in 2D lead-based perovskites, the exciton nature and behavior in the emerging 2D tin-based perovskites remains unclear. By combining spin-resolved ultrafast spectroscopy and sophisticated theoretical calculations, we reveal 2D tin-based perovskites as genuine excitonic semiconductors with weak polaronic screening effect and persistent Coulomb interaction, thanks to weak exciton-phonon coupling. We determine an excited state exciton binding energy of ~0.18 eV in n = 2 tin iodide perovskites, nearly twice of that in lead counterpart, despite of same large value of ~0.2 eV from steady state measurement. This finding emphasizes the pivotal role of excited state polaronic effect in these materials. The robust excitons in 2D tin-based perovskites exhibit excitation power-insensitive, high-efficiency and color-purity emission, rendering them superior for light-emitting applications.