Excitation Power-dependent Photoluminescence 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.

An efficiently excited Eu3+ luminescent site formed in Eu,O-codoped GaN

For the development of III-nitride-semiconductor-based monolithic micro-light-emitting diode (LED) displays, Eu,O-codoped GaN (GaN:Eu,O) is a promising material candidate for the red LEDs. The luminescence efficiency of Eu-related emission strongly depends on the local atomic structure of Eu ions. Our previous research has revealed that post-growth thermal annealing is an effective method for reconfiguring luminescent sites, leading to a significant increase in light output. We observed the preferential formation of a site with a peak at ∼2.004 eV by the annealing process. In this study, we demonstrate that it is a previously unidentified independent site (OMVPE-X) using combined excitation–emission spectroscopy and time-resolved photoluminescence measurements. In addition, we perform excitation power-dependent photoluminescence measurements and show that this OMVPE-X site dominates the emission at a low excitation power region despite its small relative abundance, suggesting a high excitation efficiency. Most importantly, applying our annealing technique to an LED exhibits a reasonably increased electroluminescence intensity associated with OMVPE-X, confirming that this site has a high excitation efficiency also under current injection. These results demonstrate the importance of OMVPE-X as a notable luminescent site for brighter and more efficient GaN:Eu,O-based LEDs.

Realizing Fluorescence Blinking in Monolayer MoSe2 with the Formation of 1D/2D Heterostructure

AbstractThe understanding and manipulating of photoluminescence blinking is essential in nano‐photonics and related device applications. The formation of mixed‐dimensional semiconductor heterostructures provides an important platform for the generation of blinking phenomena, which can be utilized to realize quantum emission properties of the heterostructures that are beyond the pure single material. Here, one‐dimensional (1D)/two‐dimensional (2D) heterostructures consisting of MoSe2 monolayer and a single CsPbBr3 perovskite nanowire are constructed. By studying the photoluminescence (PL) intensity time trajectory and lifetime, MoSe2 in heterostructure exhibits fluorescent blinking behavior, which has a negative correlation relation with the PL blinking in CsPbBr3 nanowire due to its super‐trap states. The excitation power dependent blinking and PL lifetime measurements of the heterostructure, as well as the low‐temperature PL spectroscopy, are performed to conclude that the bright state of MoSe2 in heterostructure is due to the suppressed interfacial charge transfer. In addition, MoSe2/hBN/PVK heterostructure is prepared and investigated, showing no blinking phenomena due to the dominated energy transfer process. This study not only provides a deeper understanding of interfacial charge transfer process in 1D/2D heterostructure, but also offers guidance for the design of photonic device with extended functionality.

Anisotropic Excitons Reveal Local Spin Chain Directions in a van der Waals Antiferromagnet.

A long-standing pursuit in materials science is to identify suitable magnetic semiconductors for integrated information storage, processing, and transfer. Van der Waals magnets have brought forth new material candidates for this purpose. Recently, sharp exciton resonances in antiferromagnet NiPS3 have been reported to correlate with magnetic order, that is, the exciton photoluminescence intensity diminishes above the Néel temperature. Here, it is found that the polarization of maximal exciton emission rotates locally, revealing three possible spin chain directions. This discovery establishes a new understanding of the antiferromagnet order hidden in previous neutron scattering and optical experiments. Furthermore, defect-bound states are suggested as an alternative exciton formation mechanism that has yet to be explored in NiPS3 . The supporting evidence includes chemical analysis, excitation power, and thickness dependent photoluminescence and first-principles calculations. This mechanism for exciton formation is also consistent with the presence of strong phonon side bands. This study shows that anisotropic exciton photoluminescence can be used to read out local spin chain directions in antiferromagnets and realize multi-functional devices via spin-photon transduction.

Low threshold lasing in Al-decorated GaN microdisk on silicon substrate

In spite of the unique advantages of nitride microcavities laser, the optical loss and threshed values are still high in most GaN microdisk laser integrated on silicon substrate. Herein, we fabricated a GaN microdisk cavity pivoted on Si substrate using standard semiconductor process. Al nanoparticles (NPs) with diameter below 100 nm were then decorated on the GaN cavity as surface plasmons (SPs) gain to enhance the lasing performance. SPs coupling properties and photoluminescence (PL) enhancement of Al decorated cavities were studied via excitation power-dependent PL and time-resolved PL measurements. Low optical loss caused by device suspension and SPs coupling induced Purcell Factor enhancement significantly improve the lasing properties. A spontaneous enhancement in PL (by 1.75 folds) along with altered lasing characteristics, including accelerated exciton recombination, reduced lasing threshold value (by 5 folds approximately), slight lasing intensity improvement and redshift of the resonant mode, were observed.

Pressure-induced core defects and photoluminescent quenching in carbon quantum dots

Carbon quantum dots (CDs) with favorable luminescent features for biphotonic applications have attracted much interest in modulating their photoluminescence (PL) efficiency. A surface state with various defects is believed to play a key role in the emissive intensity. Here, pressure-induced quenching of PL is observed in red emissive CDs (R-CDs) and is ascribed to defects in carbon cores upon compression. In the power-law fitting to the excitation power-dependent PL of R-CDs at high pressure, the coefficient k parameter related to the emissive mechanism decreases from 1 under ambient pressure to much less than 1 under the application of pressure, suggesting a transition from single exciton recombination to defect-related emission. With the k parameter decreasing to 0.69 at 1.6 GPa, the pressure-induced defects reduce the PL intensity by approximately one order of magnitude. Furthermore, the attenuation and broadening of the G band characterizing the sp2 hybrid structure of carbon cores in the Raman spectra for R-CDs at high pressure support that the pressure-induced lattice relaxation impairs the crystalline symmetry of the carbon core and results in the dramatic quenching of PL. Our results highlight the importance of the well-crystallized carbon core in designing CDs with high quantum yields.

Twist-angle-dependent momentum-space direct and indirect interlayer excitons in WSe2/WS2 heterostructure.

Interlayer excitons (ILEs) in the van der Waals (vdW) heterostructures of type-II band alignment transition metal dichalcogenides (TMDCs) have attracted significant interest owing to their unique exciton properties and potential in quantum information applications. However, the new dimension that emerges with the stacking of structures with a twist angle leads to a more complex fine structure of ILEs, presenting both an opportunity and a challenge for the regulation of the interlayer excitons. In this study, we report the evolution of interlayer excitons with the twist angle in the WSe2/WS2 heterostructure and identify the direct (indirect) interlayer excitons by combining photoluminescence (PL) and density functional theory (DFT) calculations. Two interlayer excitons with opposite circular polarization assigned to the different transition paths of K-K and Q-K were observed. The nature of the direct (indirect) interlayer exciton was confirmed by circular polarization PL measurement, excitation power-dependent PL measurement and DFT calculations. Furthermore, by applying an external electric field to regulate the band structure of the WSe2/WS2 heterostructure and control the transition path of the interlayer excitons, we could successfully realize the regulation of interlayer exciton emission. This study provides more evidence for the twist-angle-based control of heterostructure properties.

Achieving excitation wavelength-power-dependent colorful luminescence via multiplexing of dual lanthanides in fluorine oxide particles for multilevel anticounterfeiting.

The optical characteristics of multimode luminescent materials like multimode luminescence (photoluminescence, afterglow, thermoluminescence) and a multi-excitation source (light, thermal, mechanical force) play crucial roles in optical data storage and readout, document security and anticounterfeiting. A higher level of advanced anticounterfeiting may rely on multimode anticounterfeiting materials that can realize multicolor luminescence. Here, a highly integrated multimode and multicolor Y7O6F9:Er3+,Eu3+ material is developed through multiplexing of dual lanthanides in fluorine oxide particles. In photoluminescence and photoluminescence/up-conversion luminescence modes, the material Y7O6F9:Er3+,Eu3+ has the characteristic of excitation wavelength and power dependence. In the photoluminescence mode, under excitation at 254 nm and 365 nm, Y7O6F9:Er3+ and Y7O6F9:Eu3+ showed bright red and green emissions, respectively. In the photoluminescence/up-conversion mode, under the increased excitation power from 0.2 to 2.0 W cm-2, the color of luminescence emission can be finely tuned from red to orange, yellow and green. Taking this unique excitation wavelength-power-dependent luminescence property into account, a multilevel anticounterfeiting device with the Lily pattern was designed. The device readily integrates the advantages of the excitation wavelength-dependent photoluminescence emissions and excitation power-dependent photoluminescence emissions in one overall device. These findings offer unique insight for designing highly integrated multimode, multicolor luminescence materials and advanced anticounterfeiting technology toward a wide variety of applications, particularly multilevel anticounterfeiting devices.

Spectroscopic Characteristics of Dual - Color CdSe/CdS/CdSe1-xSx/CdS Quantum Dot - Quantum Wells

In this work, preparation and spectroscopic characteristics of CdSe/CdS/CdSe1-xSx/CdS core/shell 1/well/shell 2 structure were presented. The obtained quantum dot - quantum well (QDQW) samples exhibited two emission peaks at 1.97 and 2.2 eV. Excitation power dependent photoluminescence (PL) of the QDQWs indicated that the integrated emission intensities of these peaks increase linearly with excitation power densities up to 1.3×102 mW/cm2. The temperature dependence of the bandgap energies of the QDQW’s core and well layer was well described using the Varshni expression. Evidence of the existence of “local wells” in the QDQW’s well layer that leads to a more significant change in the maximum position and full width at half maximum of the well’s emission peak compared to the core’s counterpart was discovered. Annealing samples resulted in the considerable increase of the well layer’s emission efficiency.
 Keywords: Quantum dot - quantum wells, dual - color, spectroscopic characteristics.

Redistribution of carrier localization in InGaN-based light-emitting diodes for alleviating efficiency droop

We propose an efficient method for reducing efficiency droop in InGaN QWs by redistributing carrier localization via thermal annealing, which results in increased internal quantum efficiency (IQE) in this study. Entire structures of InGaN-based light-emitting diode (LED) were grown using metal-organic chemical vapor deposition. After thermal annealing, the photoluminescence (PL) intensity of the InGaN LED was gradually increased up to 900 °C, as confirmed by room-temperature macro-PL spectroscopy. The temperature-dependent PL spectra indicated that the In-rich InGaN cluster's local energy band was flattened by the thermal annealing treatment. Moreover, excitation power-dependent PL spectroscopy confirmed that redistribution of carrier localization in InGaN QWs alleviated the efficiency droop phenomenon. Furthermore, the flattened local energy band of the In-rich InGaN cluster and the increased carrier localization site implied improved IQE.

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.

Study of the optical properties of Sb2(Se1-xSx)3 (x = 0–1) solid solutions

This study presents a detailed analysis of the optical properties of the Sb2(Se1-xSx)3 (x = 0–1) polycrystals. Four antimony selenosulfide solid solutions Sb2(Se1-xSx)3 together with Sb2Se3 and Sb2S3 were synthesized from elemental precursors at the same synthesis conditions, only varying the x = S/(Se + S) value with a step of Δx = 0.2. Successful formation of the Sb2(Se1-xSx)3 solid solutions was determined by Raman spectroscopy and X-ray diffraction. As expected for the same type of crystal structure of Sb2Se3 and Sb2S3, the bimodal behavior of Ag Raman mode in Sb2(Se1-xSx)3 was detected. Temperature and excitation power dependent photoluminescence (PL) analysis of Sb2(Se1-xSx)3 was performed in order to look into the electronic and defect structure of these promising semiconductor materials for optoelectronic applications. The shift of the near band edge PL emission in Sb2(Se1-xSx)3 towards higher energies from 1.309 eV to 1.728 eV with increasing sulfur content was detected at T = 3 K. A change in the radiative recombination mechanism was detected being of excitonic origin in samples with x ≤ 0.2 and resulting from deep donor-deep acceptor pair recombination in samples with x > 0.2.

High precision parabolic quantum wells grown using pulsed analog alloy grading technique: Photoluminescence probing and fractional-dimensional space approach

The designed GaAs/AlGaAs parabolic quantum well (PQW) was grown using a pulsed analog alloy grading (PAAG) technique with molecular beam epitaxy by setting a significantly lower growth rate, growth time variation with growth rate change, and growth interruptions for stabilization of Al source temperature. The growth conditions allowed to achieve high precision parabolic potential of the PQW due to the arrangement of crystalline lattice by migration of group-III atoms and smoothing of interfaces under As overpressure. The structure was probed by scanning transmission electron microscopy and investigated via excitation power and temperature dependent photoluminescence (PL). The origin of the PL spectra lines was corroborated by fractional-dimensional space approach with dimensionality factor varying from 2.46 for confined to 3 for bulk semiconductor radiative transitions. Up to 5 PQW equidistant subbands in conduction and valence bands are observed by filling the states with photoexcited carriers. The PL of the excited PQW states is resolved up to 240 K temperature. The results indicate excellent agreement between luminescent properties of the designed PQW using numerical Schrödinger equation solver and the produced PQW using PAAG technique. It validates the developed PAAG technique as a powerful tool for PQWs growth to tailor a basis for more sophisticated quantum systems for near-infrared to terahertz emitters employing interband and intersubband transitions.

Systematic study of the emission spectra of nanowire quantum dots

A systematic study of the emission spectra of single InAsP nanowire quantum dots in position-controlled InP photonic nanowire waveguides is presented. Using excitation power-dependent photoluminescence and correlation measurements, we distinguish between the different excitonic complexes responsible for s-shell emission. From measurements of over 40 nominally identical devices, we obtain a standard deviation of ∼5 meV in the emission energy of excitons, biexcitons, and charged exciton photons. The mean biexciton binding energy was 1.9 meV with a standard deviation of ∼0.8 meV. The experimental spectra are understood using atomistic multi-million atom theory of neutral and charged multi-exciton complexes implemented in the design tool QNANO.

Combined effects of carrier scattering and Coulomb screening on photoluminescence in InGaN/GaN quantum well structure with high In content* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51672163 and 51872167) and the Major Research Plan of the National Natural Science Foundation of China (Grant No. 91433112).

Photoluminescence (PL) spectra of two different green InGaN/GaN multiple quantum well (MQW) samples S1 and S2, respectively with a higher growth temperature and a lower growth temperature of InGaN well layers are analyzed over a wide temperature range of 6 K–330 K and an excitation power range of 0.001 mW–75 mW. The excitation power-dependent PL peak energy and linewidth at 6 K show that in an initial excitation power range, the emission process of the MQW is dominated simultaneously by the combined effects of the carrier scattering and Coulomb screening for both the samples, and both the carrier scattering effect and the Coulomb screening effect are stronger for S2 than those for S1; in the highest excitation power range, the emission process of the MQWs is dominated by the filling effect of the high-energy localized states for S1, and by the Coulomb screening effect for S2. The behaviors can be attributed to the fact that sample S2 should have a higher amount of In content in the InGaN well layers than S1 because of the lower growth temperature, and this results in a stronger component fluctuation-induced potential fluctuation and a stronger well/barrier lattice mismatch-induced quantum-confined Stark effect. This explanation is also supported by other relevant measurements of the samples, such as temperature-dependent peak energy and excitation-power-dependent internal quantum efficiency.

Intrinsic excitation-dependent room-temperature internal quantum efficiency of AlGaN nanowires with varying Al contents

Aluminum gallium nitride (AlGaN) nanowires have become an emerging approach for semiconductor deep ultraviolet light-emitting devices. To further improve the device performance, it is critical to understand the optical quality of AlGaN nanowires. However, today, the room-temperature internal quantum efficiency (IQE) of AlGaN nanowires is predominantly analyzed by the temperature-dependent photoluminescence (PL) approach under one excitation power or taking the PL intensity ratio at the room temperature and low temperature with different excitation powers. In both cases, one needs to assume the low temperature IQE to be 100%, which is not always valid, in particular when the excitation power changes at the low temperature. In this work, we study the room-temperature IQE of AlGaN nanowires through the detailed excitation power-dependent PL experiments and theoretical analysis. This allows us to derive the intrinsic room-temperature IQE of AlGaN nanowires as a function of the excitation power. It is found that for an Al content in the range of 22%–54%, the IQE of all samples increases as the excitation increases, followed by an efficiency droop. Moreover, comparing different samples, the IQE at low excitations increases as the Al content increases, whereas the peak IQE reduces from 73% to 56% as the Al content increases. The underlying mechanisms are also discussed in this paper.

Research on the photoluminescence of spectral broadening by rapid thermal annealing on InAs/GaAs quantum dots

Photoluminescence (PL) test was conducted to investigate the effect of rapid thermal annealing (RTA) on the optical performance of self-assembled InAs/GaAs quantum dots (QDs) at the temperatures of 16 and 300 K. It was found that after RTA treatment, the PL spectrum of the QDs sample had a large blue-shift and significantly broadened at 300 K. Compared with the as-grown InAs QDs sample, the PL spectral width has increased by 44.68 meV in the InAs QDs sample RTA-treated at 800 °C. The excitation power-dependent PL measurements showed that the broadening of the PL peaks of the RTA-treated InAs QDs should be related to the emission of the ground state (GS) of different-sized InAs QDs, the InAs wetting layer (WL) and the In0.15Ga0.85As strain reduction layer (SRL) in the epitaxial InAs/GaAs layers.

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.