Power-dependent Photoluminescence Spectra Research Articles

Wavelength-tunable photoluminescence of CsPbBr3 microcrystal via in situ halogen doping

Lead halide perovskite semiconductors have received significant attention in recent years as promising candidates for optoelectronic applications. Their controllable bandgap in nanostructures (nanocrystals, nanowires) has been widely and successfully investigated. Intentionally manipulating the atomic behavior with dopants in their macroscopic structure with high quality and stability is still challenging. We studied the crystal structure and photoluminescence (PL) behavior of the anion (Cl) doped CsPbBr3 microcrystals. The doped microcrystal with a low doping concentration has shown a large wavelength-tunable PL emission from 535 nm to 457 nm. Two separate emission peaks (blue and green) are carefully studied in the excitation power dependent PL spectra. The doping and excitation endow the perovskite microcrystal adjustable PL emission band and peak intensities, which may serve as a new degree of manipulation freedom in the color-tunable display and functional optoelectronics.

Anomalous layer-dependent photoluminescence spectra of supertwisted spiral WS2.

Twisted stacking of two-dimensional materials with broken inversion symmetry, such as spiral MoTe2 nanopyramids and supertwisted spiral WS2, emerge extremely strong second- and third-harmonic generation. Unlike well-studied nonlinear optical effects in these newly synthesized layered materials, photoluminescence (PL) spectra and exciton information involving their optoelectronic applications remain unknown. Here, we report layer- and power-dependent PL spectra of the supertwisted spiral WS2. The anomalous layer-dependent PL evolutions that PL intensity almost linearly increases with the rise of layer thickness have been determined. Furthermore, from the power-dependent spectra, we find the power exponents of the supertwisted spiral WS2 are smaller than 1, while those of the conventional multilayer WS2 are bigger than 1. These two abnormal phenomena indicate the enlarged interlayer spacing and the decoupling interlayer interaction in the supertwisted spiral WS2. These observations provide insight into PL features in the supertwisted spiral materials and may pave the way for further optoelectronic devices based on the twisted stacking materials.

Vapor growth of V-doped MoS2 monolayers with enhanced B-exciton emission and broad spectral response

Dynamically engineering the optical and electrical properties in two-dimensional (2D) materials is of great significance for designing the related functions and applications. The introduction of foreign-atoms has previously been proven to be a feasible way to tune the band structure and related properties of 3D materials; however, this approach still remains to be explored in 2D materials. Here, we systematically demonstrate the growth of vanadium-doped molybdenum disulfide (V-doped MoS2) monolayers via an alkali metal-assisted chemical vapor deposition method. Scanning transmission electron microscopy demonstrated that V atoms substituted the Mo atoms and became uniformly distributed in the MoS2 monolayers. This was also confirmed by Raman and X-ray photoelectron spectroscopy. Power-dependent photoluminescence spectra clearly revealed the enhanced B-exciton emission characteristics in the V-doped MoS2 monolayers (with low doping concentration). Most importantly, through temperature-dependent study, we observed efficient valley scattering of the B-exciton, greatly enhancing its emission intensity. Carrier transport experiments indicated that typical p-type conduction gradually arisen and was enhanced with increasing V composition in the V-doped MoS2, where a clear n-type behavior transited first to ambipolar and then to lightly p-type charge carrier transport. In addition, visible to infrared wide-band photodetectors based on V-doped MoS2 monolayers (with low doping concentration) were demonstrated. The V-doped MoS2 monolayers with distinct B-exciton emission, enhanced p-type conduction and broad spectral response can provide new platforms for probing new physics and offer novel materials for optoelectronic applications.Graphical abstract

Visible upconverted and NIR luminescence properties of Er3+ and Er3+/Yb3+-doped Lu3Sc2Ga3O12 nanocrystals

Trivalent erbium and ytterbium embedded in Lu3Sc2Ga3O12 nanocrystals in the garnet phase were synthesised employing the citrate sol-gel method. The crystalline phase and size, grain size, and effect of ion concentration and laser pump power on the photoluminescence properties of lanthanide ions were systematically characterised. XRD results showed that the present phosphor was in the cubic garnet phase with an average crystallite size of 27 nm and a lattice constant of 12.325 Å. Morphology images revealed the particles to be nearly spherical and agglomerated. Partial energy level diagrams have been constructed using diffused reflectance and excitation spectra. Stokes and anti-Stokes photoluminescence spectra were observed under 980 nm laser excitation. The near-infrared (NIR) photons (980 nm) were successfully converted into visible radiation. Upconverted luminescence spectra consisted of two strong peaks located in the green and red regions, corresponding to the 2H11/2,4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of the Er3+ ion. The intensity ratio of red to green emission bands increased with increasing Er3+ or Yb3+ concentrations as well as with the pump power of laser. Power-dependent photoluminescence spectra revealed that there were two photons involved in the upconversion mechanism. The decay profiles of the thermally coupled 2H11/2,4S3/2 levels of Er3+ showed a non-exponential nature accompanied by a shortening of lifetime as the concentration increased. CIE colour co-ordinates were located in the green region and shifted to the yellow region with increasing Yb3+ ion concentration. NIR emission spectra exhibited an intense peak at around 1535 nm, attributed to the 4I13/2 → 4I15/2 transition of Er3+ ion, the intensity of which was observed to increase with the incorporation of Yb3+ ions up to 5 mol%, confirming the existence of Yb3+-to-Er3+ energy transfer processes. Therefore, the present phosphors can be used for photovoltaic cells, solid-state light emitting diodes, and display devices.

Room-temperature exciton-polariton and photonic lasing in GaN/InGaN core-shell microrods

Room-temperature exciton-polariton is observed in GaN/InGaN core–shell microrods grown by metal-organic vapor phase epitaxy. We demonstrate a large Rabi splitting in the core–shell microrod structure over 265 meV. Room-temperature lasing in core–shell microrods is confirmed by power-dependent photoluminescence spectra. The lasing in the shell layer results to a modulated lasing wavelength and takes one step further to more stable polariton lasing in MQW core–shell microrods.

The dependence of structural, optical and electrical properties on substrates for GaAs nanowires grown by metal organic chemical vapor deposition

GaAs nanowires (NWs) play important roles in the field of low-dimensional infrared photodetector and laser. In this work, the GaAs NWs are directly grown on Al2O3, Si and SiO2 substrates by metal organic chemical vapor deposition. The structural, optical and electrical properties of these NWs are further studied to reveal the effect of substrates. Using transmission electron microscopy characterization, the wurtzite/zinc-blend (WZ/ZB) mixed-phase structure and plenty of defects are directly observed in the GaAs NWs grown on Si and SiO2 substrates. But the GaAs NWs grown on Al2O3 substrates are high-quality, uniform and single-phase. The peak shapes of the photoluminescence (PL) spectra of the three samples are quite different, which is essentially attributed to the structural difference. Temperature- and power-dependent PL spectra are performed to analyze the origin of peaks. The luminescence of GaAs NWs on Al2O3 substrate originates from exciton related recombination, while that of the GaAs NWs on Si and SiO2 substrates come from free exciton emission, the defect-related emission and the radiative recombination at the WZ/ZB mixed-phase interface. The electron mobility of GaAs NWs on Al2O3 substrate is higher than that on Si and SiO2 substrates, which is due to the small carriers scattering of pure phase nanowires. This work has a great influence on the fabrication of GaAs NWs and their application in optoelectronic devices.

Excitonic optical properties and lasing mode shifts in square CsPbBr3 nanoplate cavities

Cesium lead bromide (CsPbBr3) perovskite micro/nanostructures have potential applications in miniaturized integrated light sources due to their confined modal volume and high luminous efficiency. In this work, we synthesized CsPbBr3 nanoplates with regular square shapes and smooth surface by the chemical vapor deposition method. The temperature-dependent and excitation power-dependent photoluminescence spectra reveal the excitonic photoluminescence properties. The whispering gallery mode (WGM)-like lasing in CsPbBr3 nanoplates was observed at room temperature. We systematically investigated the pump fluence-dependent multi-mode and single-mode lasing actions in square microcavities. The blueshift of individual lasing modes can be attributed to the variation of refractive index resulting from the formation of the electron-hole plasma (EHP) state under highly injected carrier density. The results reveal the dependence of emission properties on excitation intensity, providing further understanding of the carrier-induced mode shift mechanism in the WGM-like lead halide perovskite microcavities.

Effect of localized states on the optical properties in InGaAs/GaAs multiple quantum wells grown by MOCVD

The InGaAs/GaAs multiple quantum wells (MQWs), as the most commonly used semiconductor laser epitaxial material, have important optical properties and wide-spread applications. However, the segregation of In atoms in the InGaAs/GaAs MQWs with the high In component has always been a serious problem. In this paper, the In0.29Ga0.71As/GaAs MQWs were prepared by metal-organic chemical vapor deposition (MOCVD), and then the temperature and excitation power-dependent photoluminescence spectra of MQWs were performed and studied. In the variable temperature measurement, the position peak of PL and full width at half maxima (FWHM) showed an "S"-shaped change from 20 to 300 K. Our systematic theoretical and experimental studies were performed that the peak before 50 K was considered to be the lasering caused by the localized states which play a leading role. And the α parameter value obtained by fitting the theoretical formula was less than 1, which also proved the existence of the localized states at low temperature in the variable power measurement at a fixed temperature. Through this research, it is of great significance to comprehensively understand the radiation emission mechanism in InGaAs materials and further develop the epitaxial growth of InGaAs quantum wells, and can deepen the understanding of the luminescence properties of InGaAs quantum well materials.

Orientation-Mediated Luminescence Enhancement and Spin-Orbit Coupling in ZnO Single Crystals.

Temperature-, excitation wavelength-, and excitation power-dependent photoluminescence (PL) spectroscopy have been utilized to investigate the orientation-modulated near band edge emission (NBE) and deep level emission (DLE) of ZnO single crystals (SCs). The near-band-edge emission of ZnO SC with <0001> orientation exhibits strong and sharp emission intensity with suppressed deep level defects (mostly caused by oxygen vacancies Vo). Furthermore, Raman analysis reveals that <0001> orientation has dominant E2 (high) and E2 (low) modes, indicating that this direction has better crystallinity. At low temperature, the neutral donor-to-bound exciton (DoX) transition dominates, regardless of the orientation, according to the temperature-dependent PL spectra. Moreover, free-exciton (FX) transition emerges at higher temperatures in all orientations. The PL intensity dependence on the excitation power has been described in terms of power-law (I~Lα). Our results demonstrate that the α for <0001>, <1120>, and <1010> is (1.148), (1.180), and (1.184) respectively. In short, the comprehensive PL analysis suggests that DoX transitions are dominant in the NBE region, whereas oxygen vacancies (Vo) are the dominant deep levels in ZnO. In addition, the <0001> orientation contains fewer Vo-related defects with intense excitonic emission in the near band edge region than other counterparts, even at high temperature (~543 K). These results indicate that <0001> growth direction is favorable for fabricating ZnO-based highly efficient optoelectronic devices.

Room-temperature polariton lasing in GaN microrods with large Rabi splitting.

Room-temperature polariton lasing is achieved in GaN microrods grown by metal-organic vapor phase epitaxy. We demonstrate a large Rabi splitting (Ω = 2g0) up to 162 meV, exceeding the results from both the state-of-the-art nitride-based planar microcavities and previously reported GaN microrods. An ultra-low threshold of 1.8 kW/cm2 is observed by power-dependent photoluminescence spectra, with the linewidth down to 1.31 meV and the blue shift up to 17.8 meV. This large Rabi splitting distinguishes our coherent light emission from a conventional photon lasing, which strongly supports the preparation of coherent light sources in integrated optical circuits and the study of exciting phenomena in macroscopic quantum states.

Kinetic Molecular Cationic Control of Defect-Induced Broadband Light Emission in 2D Hybrid Lead Iodide Perovskites.

In this study, we examine the effects of changing organic cation concentrations on the efficiency and photophysical implications of exciton trapping in two-dimensional hybrid lead iodide self-assembled quantum wells (SAQWs). We show that increasing the concentration of alkyl and aryl ammonium cations causes the formation of SAQWs at a liquid-liquid interface to possess intense, broadband subgap photoluminescence (PL) spectra. Electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopic studies suggest that materials formed under these cation concentrations possess morphologies consistent with inhibited crystallization kinetics but exhibit qualitatively similar bulk chemical bonding to nonluminescent materials stabilized in the same structure from precursor solutions containing lower cation concentrations. Temperature- and power-dependent PL spectra suggest that the broadband subgap light emission stems from excitons self-trapped at defect sites, which we assign as edge-like, collective I vacancies using a simple model of the chemical equilibrium driving material self-assembly. These results suggest that changes to the availability of molecular cations can suitably control the light emission properties of self-assembled hybrid organic-inorganic materials in ways central to their applicability in lighting technologies.

Multiple Hydrogen Bond-Induced Structural Distortion for Broadband White-Light Emission in Two-Dimensional Perovskites

Low-dimensional hybrid lead-halide perovskites with broadband white-light emission upon near-UV excitation have attracted immense scientific interest due to their potential application for the next...

Investigation of localized state emissions in quaternary InGaAsSb/AlGaAsSb multiple quantum wells grown by molecular beam epitaxy

As an essential structure of infrared semiconductor lasers, the optical properties of InGaAsSb/AlGaAsSb multiple quantum wells need to be fully investigated. In this paper, the temperature and excitation power-dependent photoluminescence (PL) spectra of the InGaAsSb/AlGaAsSb MQWs are measured. A strong free exciton emission with a photon energy of 0.631 eV was observed at room temperature. Besides the main peak, an obvious shoulder peak, located at the low photon energy position under low temperature range (T≤90 K), was confirmed to be an emission of defect related localized carriers by power-dependent PL spectra. The power-dependent PL spectra were dominated by the localized carriers emission under low excitation power (Iex≤20 mW), while the free-exciton emission dominated the PL spectra gradually when excitation power higher than 40 mW. This phenomenon is ascribed to the dissociation of localized states. Our work is of great significance for the device applications of InGaAsSb/AlGaAsSb multiple quantum wells.

Multiple Self-Trapped Emissions in the Lead-Free Halide Cs3Cu2I5.

Low-dimensional copper halides with high luminance have attracted increasing interest as heavy-metal-free light emitters. However, the optical mechanisms underpinning their excellent luminescence remain underexplored. Here, we report multiple self-trapped emissions in Cs3Cu2I5. Power-dependent photoluminescence spectra reveal the appearance of multiple self-trapped emission peaks with increasing excitation power, and this emission behavior is explored across a temperature range of 80-420 K. The zero-dimensional structure and soft crystal lattice contribute to the multiple self-trapped emissions in Cs3Cu2I5: this explains the origin of the broad emission and the luminescence mechanism in Cs3Cu2I5 and will assist in improving our understanding of the optical properties of other metal halides. We incorporate the Cs3Cu2I5 in light-emitting diodes that achieve a peak luminance of 140 cd/m2 and an external quantum efficiency of 0.27%.

Optical properties demonstrating strong coupling of compactly arranged Ge quantum dots.

Coupled quantum dots (QDs) have been extensively investigated for their unique collective properties and potential applications in optoelectronic devices. Herein, dense Ge QDs with a compact arrangement in-plane are readily obtained. Systematic studies on power-dependent photoluminescence (PL) from the QD ensemble demonstrate a PL peak with a superior intensity, a constant peak energy and width as a function of the excitation power. Moreover, the temperature-dependent PL spectra exhibit a pronounced red-shift and a rapid PL quenching with increasing temperature. Such PL properties are attributed to the formation of miniband and the delocalization of holes in the QD ensemble due to the strong coupling between closely adjacent QDs.

Low-temperature study of neutral and charged excitons in the large-area monolayer WS2

We present a low-temperature optical study of the large-area monolayer WS2 grown by chemical vapor deposition (CVD). Power-dependent photoluminescence (PL) measurements were conducted, and temperature-dependent PL spectra were measured in the range of 3 to 300 K. With the comparative PL bands obtained, a stronger trion emission in the edge region was detected to be the key difference. Sulfur vacancies (SVs) were observed to increase in density along the growth direction and found to be the main source of the large population of local charge carriers. The monolayer WS2 exhibited an upper bound for the trion binding energy of 18 meV in the edge region.

Role of Hole Trap Sites in MoS2 for Inconsistency in Optical and Electrical Phenomena

Because of strong Coulomb interaction in two-dimensional van der Waals-layered materials, the trap charges at the interface strongly influence the scattering of the majority carriers and thus often degrade their electrical properties. However, the photogenerated minority carriers can be trapped at the interface, modulate the electron-hole recombination, and eventually influence the optical properties. In this study, we report the role of the hole trap sites on the inconsistency in the electrical and optical phenomena between two systems with different interfacial trap densities, which are monolayer MoS2-based field-effect transistors (FETs) on hexagonal boron nitride (h-BN) and SiO2 substrates. Electronic transport measurements indicate that the use of h-BN as a gate insulator can induce a higher n-doping concentration of the monolayer MoS2 by suppressing the free-electron transfer from the intrinsically n-doped MoS2 to the SiO2 gate insulator. Nevertheless, optical measurements show that the electron concentration in MoS2/SiO2 is heavier than that in MoS2/h-BN, manifested by the relative red shift of the A1g Raman peak. The inconsistency in the evaluation of the electron concentration in MoS2 by electrical and optical measurements is explained by the trapping of the photogenerated holes in the spatially modulated valence band edge of the monolayer MoS2 caused by the local strain from the SiO2/Si substrate. This photoinduced electron doping in MoS2/SiO2 is further confirmed by the development of the trion component in the power-dependent photoluminescence spectra and negative shift of the threshold voltage of the FET after illumination.

Optical properties of n- and p-type modulation doped GaAsBi/AlGaAs quantum well structures

In this work, optical properties of n- and p-type modulation doped GaAsBi/AlGaAs single quantum well (QW) heterostructures are investigated via temperature- and excitation power-dependent photoluminescence (PL) and integrated photoluminescence (IPL) studies and the results are compared to the n- and p-type GaAs/AlGaAs QW structures to determine the influence of Bi and doping type on the optical properties. The results of the temperature dependent PL peak energy reveal that, as an effect of doping type, the temperature dependence of the PL peak energy exhibit different characteristic for n- and p-type samples. The temperature dependence of the PL peak energy reveals S-shaped trend for the n-type GaAsBi/AlGaAs QW sample. On the other hand, the characteristic follows Varshni law for the p-type GaAsBi/AlGaAs QW sample. The observed S-shaped behaviour for the n-type sample is explained by considering the contribution of the Bi-induced states above valence band (VB) to the PL. As for p-type sample, the localised states-related contribution to the PL signal is drastically diminished, resulting in an almost S-shape free temperature dependence of the optical transition energy, which can be explained by the compensation of acceptor-like states. The observed PL spectra of n- and p-type samples successfully reconstructed by two Gaussian peaks, which are assigned to optical transitions due to recombination of free and localised excitons. The localised exciton-related peak is observed to be very weak in p-type sample compared to that in n-type one. The allowed transitions in GaAsBi QW is calculated by self-consistently solving the Schrödinger-Poisson equation to identify the origin of the observed transitions. A comparison of the PL results of the Bi-containing samples with the results of the Bi-free ones is exhibited approximately 80 meV/Bi% decrease in the fundamental optical transition. Using the excitation-dependent IPL measurements, it is found that at low temperatures and under low excitations, recombination process is under the effect of Shockley-Read-Hall (SRH) non-radiative process. When the excitation power density increases, radiative recombination becomes dominant. At higher-temperatures and in the high-intensity regime, we observed Auger effect in recombination process for n- and p-type GaAsBi samples, but the effect of Auger loss is observed to be much less in the p-type GaAsBi sample due to enhanced spin orbit split-off energy as a result of incorporation of bismuth. Furthermore, when we compare the result for Bi-free and Bi-containing p-type samples, again, Auger recombination is found to be less effective for the Bi-containing sample.

TlSbS2: a Semiconductor for Hard Radiation Detection

We report the quasi-2D semiconductor compound TlSbS2 as a new hard radiation detection material. This compound crystallizes in the triclinic P-1 space group, with a direct bandgap of 1.67 eV and high chemical stability. Thanks to its congruent melting at 484 °C, 1 cm-sized single crystals were grown from stoichiometric melts by the Bridgman method. The device exhibits a high resistivity of >1010 Ω·cm, and responds to 22.4 keV Ag X-rays and 5.5 MeV a-particles from 241Am at room temperature. Power-dependent photoluminescence spectra at 17 K reveal that the near-band emission bands peaked at 1.61 and 1.53 eV can be ascribed to donor–acceptor pair recombination. The mobility-lifetime product for electrons along the perpendicular direction with respect to the (0k0) cleavage planes was estimated as 2.4 × 10–6 cm2·V–1, based on spectral response against a-particles. Drift mobility measurements based on a time-of-flight technique using a-particle response reveals an electron mobility of 13.2 ± 2.6 cm2·V–1·s–1. E...

Photoluminescence of localized excitons in ZnCdO thin films grown by molecular beam epitaxy

Abstract We have investigated the luminescence characteristics of Zn 1− x Cd x O thin films with different Cd contents grown by molecular beam epitaxy system. The temperature-dependent photoluminescence (PL) and excitation power-dependent PL spectra were measured to clarify the luminescence mechanisms of the Zn 1− x Cd x O thin films. The peak energy of the Zn 1− x Cd x O thin films with increasing the Cd concentration is observed as redshift and can be fitted by the quadratic function of alloy content. The broadened full-width at half-maximum (FWHM) estimated from the 15 K PL spectra as a function of Cd content shows a larger deviation between the experimental values and theoretical curve, which indicates that experimental FWHM values are affected not only by alloy compositional disorder but also by localized excitons occupying states in the tail of the density of states. The Urbach energy determined from an analysis of the lineshape of the low-energy side of the PL spectrum and the degree of localization effect estimated from the temperature-induced S-shaped PL peak position described an increasing mean exciton-localization effects in ZnCdO films with increasing the Cd content. In addition, the PL intensity and peak position as a function of excitation power are carried out to clarify the types of radiative recombination and the effects of localized exciton in the ZnCdO films with different Cd contents.