Radiative Recombination Of Excitons Research Articles

Entropy-Dominated Triplet Exciton Emission in Carbon Dots.

Phosphorescence in carbon dots (CDs) from triplet exciton radiative recombination at room temperature has achieved significant advancement. Confinement and nanoconfinement, serving as valuable techniques, are commonly utilized to brighten triplet exciton in CDs, thereby enhancing their phosphorescence. However, a comprehensive and universally applicable physical description of confinement-enhanced phosphorescence is still lacking, despite efforts to understand its underlying nature. In this study, the dominance of entropy is revealed in triplet exciton emission from CDs through the establishment of a microscopic vibration state model. CDs with varying entropy levels are studied, indicatingthat in a low entropy system, the multi-energy triplet exciton emission in CDs exhibits enhanced brightness, accompanied by a corresponding increase in their lifetimes. The product of lifetime and intensity in CDs serves as a descriptor for their phosphorescence properties. Moreover, an entropy-dependent information variation system based on the CDs is demonstrated. Specifically, in a low-entropy system, information is retained, whereas the corresponding information is erased in a high-entropy system. This work elucidates the underlying physical nature of confinement-enhanced triplet exciton emission, offering a deeper understanding of achieving ultralong phosphorescence in the future.

Defect Engineering and Emission Tuning of Wide-Bandgap MAPbCl3 Perovskite.

Lead-chloride perovskites are promising candidates for optoelectronic applications, such as visible-blind UV photodetection. It remains unclear how the deep defects in this wide-bandgap material impact the carrier recombination dynamics. In this work, we study the defect properties of MAPbCl3 (MA = CH3NH3) based on photoluminescence (PL) measurements. Our investigations show that apart from the intrinsic emission, four sub-bandgap emissions emerge, which are very likely to originate from the radiative recombination of excitons bound to several intrinsic vacancy and interstitial defects. The intensity of various emission features can be tuned by adjusting the type and ratio of precursors used during synthesis. Our study not only provides important insights into the defect property and carrier recombination mechanism in this class of material but also demonstrates efficient strategies for defect passivation and engineering, paving the way for further development of lead-chloride perovskite-based optoelectronic devices.

Dual-modal optical temperature sensing based on Sb3+/Mn2+ co-doped Cs2NaYCl6 double perovskites

Optical temperature sensing based on luminescence intensity ratio shows significant potential in achieving accurate temperature detection. In this work, Sb3+/Mn2+ co-doped Cs2NaYCl6 double perovskites were synthesised via a hydrothermal method. Introducing Sb3+ ions to the host significantly enhances the radiative recombination of self-trapped excitons (STEs), resulting in a broad blue emission. The co-doping of Mn2+ ions to the composition leads to a tunable dual-emission in the spectra, originating from the energy transfer between the STEs and Mn2+ ions. The dependence of the dual-emission on temperature is employed for optical temperature sensing. The maximum relative sensitivity, optimal thermal resolution, and optimal thermal repeatability are determined as 1.40 % K−1, 0.002 K and 99.3% at 250 K, respectively. The results indicate that the synthesised Cs2NaYCl6: Sb3+/Mn2+ double perovskites are promising candidates for high-sensitivity optical thermometers.

Fine-Tuning of the Excitonic Response in Monolayer WS2 Domes via Coupled Pressure and Strain Variation.

We present a spectroscopic investigation of the vibrational and optoelectronic properties of WS2 domes in the 0-0.65 GPa range. The pressure evolution of the system morphology, deduced by the combined analysis of Raman and photoluminescence spectra, revealed a significant variation in the dome's aspect ratio. The modification of the dome shape caused major changes in the mechanical properties of the system resulting in a sizable increase of the out-of-plane compressive strain while keeping the in-plane tensile strain unchanged. The variation of the strain gradients drives a nonlinear behavior in both the exciton energy and radiative recombination intensity, interpreted as the consequence of a hybridization mechanism between the electronic states of two distinct minima in the conduction band. Our results indicate that pressure and strain can be efficiently combined in low dimensional systems with unconventional morphology to obtain modulations of the electronic band structure not achievable in planar crystals.

Blue light emissive zero-dimensional hybrid cadmium bromide as fluorescence sensor toward benzaldehyde

Organic-inorganic hybrid low-dimensional metal halides have been widely studied as a new class of luminescence materials due to abundant structural diversity and intriguing photophysical property. Herein, we explored two new Cd-based 0D halides of [BHEPZ]CdBr4 (BHEPZ = 1,4-Bis(2-hydroxyethyl) piperazine) and [EPIPZ]CdBr4 (EPIPZ = 1-ethylpiperazine) through facile wet-chemistry method. These 0D halides exhibit blue light emissions derived from radiative recombination of self-trapped excitons (STEs) based on detailed spectroscopy investigations. More interestingly, [BHEPZ]CdBr4 exhibits rapid luminescence off switching toward benzaldehyde but fails in any other organic solvents. In-depth investigations demonstrate that the detection limit can fall to 2.65 ppm indicating perfect real-time fluorescent probe to detect benzaldehyde with high sensitivity and selectivity. This work enriches the cadmium halide materials and provide a new perspective for potential applications of 0D hybrid metal halides.

Monolayer WS2 electro- and photo-luminescence enhancement by TFSI treatment

Layered material heterostructures (LMHs) can be used to fabricate electroluminescent devices operating in the visible spectral region. A major advantage of LMH-based light emitting diodes (LEDs) is that electroluminescence (EL) emission can be tuned across that of different exciton complexes (e.g. biexcitons, trions, quintons) by controlling the charge density. However, these devices have an EL quantum efficiency as low as ∼10−4%. Here, we show that the superacid bis-(triuoromethane)sulfonimide (TFSI) treatment of monolayer WS2-LEDs boosts EL quantum efficiency by over one order of magnitude at room temperature. Non-treated devices emit light mainly from negatively charged excitons, while the emission in treated ones predominantly involves radiative recombination of neutral excitons. This paves the way to tunable and efficient LMH-based LEDs.

Harnessing Conformational Disorder of Organic Cations for Efficient Blue Quasi‐2D Perovskite LEDs

AbstractDespite quasi‐2D perovskite offering great control over the optoelectronic properties, disordered organic cations are often perceived as detrimental to device performance, primarily affecting charge carrier mobility. However, it is proposed that such disordered organic cations‐enabled excellent excitonic properties can be beneficial for fabricating high‐efficiency perovskite light‐emitting diodes (PeLEDs) facilitated by reduced dielectric screening effect. Here, by incorporating acetamidinium bromide additives, the conformational disorder of organic cation is precisely manipulated and meticulously probed using sum frequency generation vibrational spectroscopy. Finally, a distinctive inverse relationship is elucidated between the degree of conformational order, characterized as relative structure ordering , and key performance metrics such as photoluminescence quantum yield and external quantum efficiency (EQE). By optimizing the configurational disorder, sky‐blue (485 nm) PeLEDs achieve a noteworthy EQE of 14.42% and exhibit significantly prolonged operational stability in open‐air conditions. This finding underscores the potential advantages of disordered organic cations in enhancing exciton properties and radiative recombination efficiency.

Anomalous temperature dependence of photoluminescence lifetime in CsPbBr3 quantum dot-polymer film for optical thermometry

Here, we report the temperature dependence of the photoluminescence (PL) lifetimes of didodecyl dimethylammonium bromide (DDAB)/tetraoctylammonium bromide (TOAB) capped CsPbBr3 QD-based films and the elucidation of their potential application to optical thermometry. Compared to the unstable oleic acid (OA)/oleylamine (OAm) capped CsPbBr3 QD film, DDAB/TOAB-CsPbBr3 QD film exhibits a reproducible temperature-dependent characteristic of PL lifetimes in the temperature range of 15–80 °C. The anomalous temperature-dependent feature of PL lifetimes, i.e., positively correlated with temperature, is observed from DDAB/TOAB-CsPbBr3 QD and DDAB/TOAB-CsPbBr3 QD-polymer films. The prolonged PL lifetime at higher temperatures pertains to the dissociation of bound excitons to long-lived free excitons in CsPbBr3 QDs by overcoming the interaction with the trapping centers using the gained thermal energy. CsPbBr3 QD-polystyrene (PS) film exhibits superior relative sensitivity and temperature resolution for temperature sensing, demonstrating its suitability to be the material for fluorescent thermometry. The high dielectric constant of the PS medium facilitates bound excitons of CsPbBr3 QD to dissociate and further participate in the radiative recombination of free excitons, resulting in the improved thermometric properties of CsPbBr3 QD-PS film.

Optical grade transformation of monolayer transition metal dichalcogenides via encapsulation annealing.

Monolayer transition metal dichalcogenides (TMDs) have emerged as highly promising candidates for optoelectronic applications due to their direct band gap and strong light-matter interactions. However, exfoliated TMDs have demonstrated optical characteristics that fall short of expectations, primarily because of significant defects and associated doping in the synthesized TMD crystals. Here, we report the improvement of optical properties in monolayer TMDs of MoS2, MoSe2, WS2, and WSe2, by hBN-encapsulation annealing. Monolayer WSe2 showed 2000% enhanced photoluminescence quantum yield (PLQY) and 1000% increased lifetime after encapsulation annealing at 1000 °C, which are attributed to dominant radiative recombination of excitons through dedoping of monolayer TMDs. Furthermore, after encapsulation annealing, the transport characteristics of monolayer WS2 changed from n-type to ambipolar, along with an enhanced hole transport, which also support dedoping of annealed TMDs. This work provides an innovative approach to elevate the optical grade of monolayer TMDs, enabling the fabrication of high-performance optoelectronic devices.

Multiphoton emission of single CdZnSe/ZnS quantum dots coupled with plasmonic Au nanoparticles.

The fluorescence blinking and low multiphoton emission of quantum dots (QDs) have limited their application in lasing, light-emitting diodes, and so on. Coupling of single QDs to plasmonic nanostructures is an effective approach to control the photon properties. Here plasmon-exciton systems including Au nanoparticles and CdZnSe/ZnS QDs were investigated at the single particle level. With the modulation of the local electromagnetic field, the fluorescence intensity of single QDs is increased, accompanied by a significant suppression in blinking behavior, and the lifetime is shortened from 15 ns to 2 ns. Moreover, the second-order photon intensity correlation at zero lag time g2(0) of coupled single QDs is larger than 0.5, indicating an increased probability of multiphoton emission. The enhancement factors of radiative and nonradiative decay rates of QDs coupled with Au nanoparticles are calculated. The sharply increased radiative decay rate can be comparable to the nonradiative Auger rate, leading to dominated multiple exciton radiative recombination with PL intensity enhancement, suppressed blinking, lifetime shortening, and multiphoton emission. The results of the exciton decay dynamics and emission properties of single QDs in this work are helpful in exploring the mechanism of plasmon-exciton interaction and optoelectronic application of single QDs.

GaSe nanoparticles synthesized by laser ablation method

GaSe nanoparticles obtained by direct interaction of laser radiation with Ga element and SeO2 solution were experimentally investigated. A pulsed Nd:YAG laser with a duration of 10 ns and an energy of 135 mJ per pulse was used as the radiation source. The formation of nanoparticles with diameters ranging from 2 to 25 nm was observed in the colloidal solution. By X-ray diffraction analysis it was found that the crystal structure of the nanoparticles is the same as that of the bulk material (hexagonal). It is shown that the photoluminescence emission of the obtained nanoparticles has a dark orange color and is associated with the radiative recombination of free excitons.

Highly flexible, green luminescent down converting and hydrophobic 0-D cesium lead bromide (Cs4PbBr6)/ poly (vinylidene difluoride) polymer nanocomposites for photonics and display applications

Halide perovskite materials have emerged as a promising new research topic in recent years, with potential applications in solar cells and optoelectronic devices, among others. These materials have significant limitations, including instability to water, heat, UV radiation, and light, limiting their research to the lab scale only. Encapsulation of perovskite with polymers is a solution for addressing the shortcoming of stability. This study illustrates the synthesis of zero-dimensional cesium lead bromide (Cs4PbBr6), as well as its poly (vinylidene difluoride) (PVDF) nanocomposites, optical, morphological, cell properties and increased stability towards the environmental stress. Different concentrations of synthesized perovskite was encapsulated in PVDF, namely 0.0, 0.5, 1.0, 1.5 and 2.0 wt/wt%. The UV–Visible spectral result depicts that the strong absorbance band ranges from 280 to 320 nm confirms the formation of Cs4PbBr6. X- ray diffraction (XRD) reveals the existence Cs4PbBr6 and good interaction between the perovskite and polymer matrix. The size of prepared perovskite is in nano scale and with high phase purity with rhombohedral unit cell with are depicted by using high-resolution transmission electron microscope (HR-TEM), selected area electron diffraction (SAED). The nanocomposites exhibit emission from 490 to 520 nm, and 2 wt/wt% of perovskite in PVDF exhibits the highest emission intensity analyzed using photoluminescence (PL) spectral results which is excited at 370 nm. PL lifetime decay measurements of the fabricated PVDF perovskite nanocomposites shows 34.744 ns for 2 % Cs4PbBr6/PVDF nanocomposites due to delayed exciton radiative recombination which infers that the fabricated nanocomposites finds potential application in the arena of photonics, UV-radiations blockers, and LEDs.

Ultrafast excitonic radiative recombination in Al-rich semipolar AlGaN quantum wells for far-UVC emitters

Ultrafast excitonic radiative recombination in Al-rich semipolar AlGaN quantum wells was characterized by photoluminescence and analyzed with a non-equilibrium Green's function (NEGF) model. The heterostructures consisted of Al0.69Ga0.31N/Al0.9Ga0.1N multiple quantum wells grown on (202¯1) bulk AlN substrates by MOVPE. The quantum wells were 3 nm thick, separated by 10-nm thick barrier layers, and possessed a low (<100 kV/cm) polarization field. Radiative lifetimes as low as ∼60 ps were recorded at 75 K. Experimental results validated the NEGF model, which incorporated excitons and free carriers and enabled calculation of the radiative lifetimes for different growth planes, temperatures, and carrier densities. Including intersubband scattering in the NEGF model significantly reduced the calculated radiative lifetimes for QW widths larger than the Bohr radius of the excitons in semipolar and nonpolar orientations, for which the polarization field is reduced or eliminated. These wide wells with low polarization fields and high radiative recombination are less susceptible to the deleterious effects of alloy and interface fluctuations, making them an attractive possibility for future devices.

CsPbBr3@Glass Nanocomposite with Green‐Emitting External Quantum Efficiency of 75% for Backlit Display

AbstractRobust amorphous glass protected CsPbBr3 (CsPbBr3@glass) perovskite quantum dots (PeQDs) with ultra‐pure green emission and superior long‐term stability are highly desirable for developing wide‐color‐gamut liquid crystal displays. However, most of the reported CsPbBr3@glass nanocomposites are subject to low external quantum efficiency (EQE). This work demonstrates that ZrO2 additive has an “accumulation” effect on the borosilicate glass network structure to promote in situ nucleation/growth of PeQDs inside glass rather than self‐crystallization. This effect is beneficial in reducing surface defects, improving the quality of PeQDs, and thus boosting radiative recombination of excitons. As a consequence, the as‐prepared CsPbBr3@glass shows a record EQE of up to 75% and can pass the accelerated aging tests at 85 °C/85% RH for 1000 h and blue light irradiation over 2000 h. Finally, a prototype display using CsPbBr3@glass‐based straight‐down backlit unit is designed and gains more favorable responses in blind selection tests for its high brightness of 2647 cd m−2 and high color purity of 88%. The findings will pave the way for realizing the commercial application of CsPbBr3@glass nanocomposite in PeQDs‐converted backlit display.

Synthesis and Optical Properties of Potassium‐Based Wide‐Bandgap Mixed–Halide Perovskite KPbF2Cl Nanorods

ABX3 halide–perovskites (HPs) have emerged as promising alternatives for optoelectronic devices owing to their excellent properties and low cost. Generally, A is either Cs or an organic molecule while B is Pb, or Sn and X = I, Br, or Cl. Here, ab initio calculations on K‐based new HP with X as a mixture of F and Cl are performed. Solid‐state synthesis leads to a mixed‐KPbF2Cl HP with an orthorhombic structure as determined from X‐ray diffraction and energy‐dispersive X‐ray spectroscopy. The high formation energy (−0.546 eV atom−1) shows excellent stability of this HP. Scanning electron microscopy reveals rodlike structures while X‐ray photoelectron spectroscopy is performed for chemical analysis. Diffuse reflectance shows it to have a wide bandgap of ≈4 eV, in good agreement with the band‐structure calculations. Further, a strong photoluminescence peak is found at ≈340 nm for radiative recombination of free excitons along with a broad emission peaking at ≈450 nm owing to the involvement of self‐trapped excitons associated with lattice distortion. The finding of this stable HP can lead to high‐frequency applications, along with other applications such as transparent and low‐loss optical windows, prisms, etc., as well as development of novel optical materials by doping.

In situ luminescence from MgOAl2O3 irradiated by swift heavy Xeq+ ions

Ion irradiation can result in luminescence emission from optical window material aluminum magnesium spinel (MgOAl2O3). In the present work, single crystals of MgOAl2O3 are irradiated with swift heavy ions. During irradiation, the emission spectra, in the range of 200–800 nm, are obtained from MgOAl2O3 irradiated by 93–609 MeV Xeq+ ions. The emission bands originated from radiative recombination of excitons and the spectral lines derived from impurity ions are observed. These emission bands, as a function of incident ion energy and irradiation dose, are presented. The results indicate that the increase of ion energy can effectively increase the spectral intensity, and due to crystal damage caused by ion irradiation, the spectral intensity decreases with increasing irradiation dose. Similar to nanohillock and ion track formation, luminescence emission occurs above a critical electronic energy loss of about 7.2 keV/nm. In-situ measurement of the luminescence emission is of great significance to study the irradiation modification and can help to understand the process of crystal damage caused by ion irradiation.

Zero‐Dimensional Gua3SbCl6 Crystals as Intrinsically Reabsorption‐Free Scintillators for Radiation Detection

AbstractThe search for efficient, re‐absorption‐free scintillators has recently focused the attention on antimony‐based halides, which exhibit largely Stokes shifted luminescence due to radiative recombination of excitons self‐trapped (STE) in strongly Jahn–Teller distorted Sb3+ color centers. Here, the synthesis of a hybrid structure is reported with chemical formula (C13H14N3)3SbCl6 consisting of spatially isolated [SbCl6]3− octahedra separated by organic N,N'‐diphenylguanidinium (Gua) molecules. The optical properties of this material are mainly determined by the inorganic component and are characterized by a pronounced Stokes shift of ≈1.3 eV and a room‐temperature photoluminescence (PL) efficiency of up to 85%. Remarkably, highly efficient radioluminescence (RL) is observed with scintillation light yields of ≈2000 ph MeV−1 using both soft X‐rays and a 124 keV gamma source. Temperature‐dependent PL and RL measurements confirm the minor role of non‐radiative channels, which are completely suppressed below 100 K. Thermally stimulated luminescence measurements suggest that the traps in Gua3SbCl6 crystals have a significantly large energy depth distribution below the absorbing state.

OPTICAL PROPERTIES OF HgGaInS4 CRYSTALS

Luminescence, transmission, reflection, and wavelength modulation transmission and wavelength modulation reflection spectra of HgGaInS4 crystals were researched in the temperature range of 300-10 K. This article discusses the various features observed in the spectra of HgGaInS4 crystals. The dependence on temperature of the energy gap is shown. Spectra of wavelength modulation transmission have minima a1, a2, and a3, which vary with temperature and are caused by indirect transitions L-Г with phonon absorption and emission. Spectra of thicker crystals have more features caused by the same indirect transitions. The photoluminescence show a strong maximum at 2.43 eV, attributed to radiative recombination of free excitons. These various features have significant implications for the understanding of the electronic and optical properties of HgGaInS4 crystals.

Measurement of the conduction band spin-orbit splitting in WSe2 and WS2 monolayers

The conduction band spin-orbit splitting ∆${}_{c}$ has a strong impact on optical, transport, and spin-valley properties of 2D materials based on transition metal dichalcogenide monolayers. Here, the authors study charge tunable devices based on WSe${}_{2}$ and WS${}_{2}$ monolayers. In addition to the well-known radiative recombination of neutral and charged excitons, photoluminescence measurements highlight a weaker intensity optical transition. This line is interpreted as an impurity-assisted recombination of the negatively charged exciton (triplet trion), whose radiative recombination energy differs by ∆${}_{c}$ from that of the trion's direct optically active recombination.

Composition engineering of lead-free double perovskites towards efficient warm white light emission for health and well-being

Lead-free halide double perovskites (DPs) [A2B(I)B(III)X6] (A: monovalent cation, B(I): monovalent cation, B(II): trivalent cation, X: halide anion) has drawn tremendous attention because of their environmental-friendliness and high stability compared to traditional lead-based perovskites ABX3 (A: monovalent cation; B: Pb2+; X: halide anion). The optoelectronic properties of DPs are, however, compromised. This work provides a methodology for improving the photophysical properties of DPs and achieving high-efficiency warm white light emission. First principles density functional theory (DFT) was used in material design and the ligand-free synthesis of halide DP Cs2Ag1−xKxIn0.875Bi0.125Cl6 (x ≤ 0.60) via a facile recrystallization process was demonstrated. The compositional tuning of K+ to Ag+ resulted in an improvement in photophysical properties. Independent of K+ content, all the samples exhibit strong absorption at a wavelength of λmax ∼ 375 nm with a direct bandgap which is consistent with our DFT calculation results. Broadband photoluminescence (PL) emission profiles covering the spectra from blue to deep red (405–820 nm at λmax ∼ 629 nm) were observed. Besides, with an optimized K+ alloying content of 0.20, the optical performance of these halide DPs was impressively regulated, demonstrating improved photoluminescence quantum yield (PLQY) of host Cs2AgInCl6 crystals from ∼ 2.70 % (x = 0) to ∼15.96 % (x = 0.20), due to increased radiative recombination of self-trapped excitons. the highest report for such structures. It also shows high stability over 180 days. Furthermore, the use of blue-emitting CsPbBr3 and DP Cs2Ag0.80K0.20In0.875Bi0.125Cl6 as color conversion layers in white light-emitting diodes (LEDs) demonstrates that high-quality white light emission with the CIE color coordinate of (0.387, 0.385) and a correlated color temperature (CCT) of 3878 K, color rendering index (CRI) of 85, and luminous efficacy of radiation (LER) of 214 lm/W. Hence, the research work provides insight into realizing environmentally friendly and high-efficiency white light emission with high color quality by using earth-abundant elements and the low-cost synthesis method.