Low-energy Photoluminescence Research Articles

Amplification of interlayer exciton emission in twisted WSe2/WSe2/MoSe2 heterotrilayers

Van der Waals heterostructures based on transition metal dichalcogenides exhibit physical properties that depend on their monolayer constituents’ twisting angle and stacking order. Particularly in type-II heterostructures, low-energy photoluminescence is dominated by interlayer excitons, resulting in low emission yields, which drastically hampers their optoelectronic applicability. This study reports on the photoluminescence quantum yield of heterostructures consisting of WSe2/WSe2/MoSe2 twisted layers. Our findings show that the additional WSe2 monolayer in the trilayer system enhances the low-energy photoluminescence by more than an order of magnitude depending on the WSe2/WSe2 twist-angle in comparison to their WSe2/MoSe2 heterobilayer counterpart. Furthermore, combining density functional theory calculations and extracted degree of circular polarization, we identify excitonic signatures arising from hybridized states that originate from the additional WSe2 layer. In addition to providing an additional understanding of hybridization effects in 2D semiconducting heterostructures, our findings provide a viable method to enhance emission in van der Waals heterostructures, relevant for studying the fundamental properties of excitons and enabling optoelectronic applications with high luminescence yield.

Quantum features of low-energy photoluminescence of aluminum nitride films

Photoluminescence of aluminum nitride films at the below bandgap excitation has been studied. It has been found that low-energy (up to 2.02 eV) photoluminescence spectra of the AlN films contain a series of equidistant maxima, the intensities of which decrease with energy. Theoretical analysis has shown that the observed photoluminescence features may be caused by strong electron-phonon interaction (long-range interaction of electrons in the band gap with Al3+ ions in the lattice sites). This interaction presumably leads to appearance of quasi-particles in the band gap of AlN, which are a bound state of an electron with an ion in a crystal lattice site. Such quasi-particles have been called “elions”. The energy of an elion is quantized. An elion quantum is equal to the longitudinal optical phonon energy. The low-energy photoluminescence is based on the elion generation and subsequent annihilation mechanism.

Enhanced interlayer electron transfer by surface treatments in mixed-dimensional van der Waals semiconductor heterostructures

We investigate the excitonic species in WS2 monolayers transferred onto III–V semiconductor substrates with different surface treatments. When the III–V substrates were covered with amorphous native oxides, negatively charged excitons dominated the spectral weight in low-temperature near-resonance photoluminescence (PL) measurements. However, when the native oxides of the III–V substrates were reduced, neutral excitons began to dominate the spectral weight, indicating a reduction in the electron density in the WS2 monolayers. The removal of the native oxides enhanced the electron transfer from the WS2 monolayer to the III–V substrate. In addition, an additional shoulder-like PL feature appeared ∼50 meV below the emission of neutral excitons, which can be attributed to the emission of localized excitons. When the III–V substrate surface was passivated by sulfur after the reduction of the native oxides, neutral excitons still dominated the spectral weight. However, the low-energy PL shoulder disappeared again, suggesting the effective delocalization of excitons through substrate surface passivation. Surface engineering of the semiconductor substrates for two-dimensional (2D) materials can provide a novel approach to control the carrier density of the 2D materials, implement deterministic carrier localization or delocalization for the 2D materials, and facilitate the interlayer transfer of charge, spin, and valley currents. These findings open the avenue for novel device concepts and phenomena in mixed-dimensional semiconductor heterostructures.

Mid-infrared detectors for space electronics based on InAs-core/InP-shell nanowires

The Mid-Infrared (MIR) spectral range is most important for free-space communications and astronomy. It contains radiation emitted by astrophysical objects during evolution of the planets, stars, galaxies and in particular by prebiosignature molecules on exoplanets. Nanowires (NWs) are expected to improve various optoelectronic devices, including IR photodetector technology. The bandgap of the catalystless InAs NWs can be tuned by introducing mechanical strain due to lattice mismatches in the core/shell NWs structures. Passivation with wider bandgap InP shell provide highly tunable functionality for future electronic devices. Temperature dependence of photoluminescence (PL) spectra of NWs InAs and InAs-core/InP-shell was acquired using Fourier-spectrometer. The position of the high-energy PL peak between calculated values of Eg for wurtzite and sphalerite structures confirms the formation of NWs into a combined polytype. Low energy PL peak is connected with parasitic bulk islands. Surface passivation successfully eliminates surface states and provides nontrivial temperature dependence of high-energy PL peak due to tensile and compressive strain in InAs core. Thus, detectors based on NWs InAs and InAs-core/InP-shell structures can operate in the MIR range of the spectrum with a wavelength from 2 to 5 μm.

Understanding Electron-Phonon Interactions in 3D Lead Halide Perovskites from the Stereochemical Expression of 6s2 Lone Pairs.

The electron-phonon (e-ph) interaction in lead halide perovskites (LHPs) plays a role in a variety of physical phenomena. Unveiling how the local lattice distortion responds to charge carriers is a critical step toward understanding the e-ph interaction in LHPs. Herein, we advance a fundamental understanding of the e-ph interaction in LHPs from the perspective of stereochemical activity of 6s2 lone-pair electrons on the Pb2+ cation. We demonstrate a model system based on three LHPs with distinctive lone-pair activities for studying the structure-property relationships. By tuning the A-cation chemistry, we synthesized single-crystal CsPbBr3, (MA0.13EA0.87)PbBr3 (MA+ = methylammonium; EA+ = ethylammonium), and (MHy)PbBr3 (MHy+ = methylhydrazinium), which exhibit stereo-inactive, dynamic stereo-active, and static stereo-active lone pairs, respectively. This gives rise to distinctive local lattice distortions and low-frequency vibrational modes. We find that the e-ph interaction leads to a blue shift of the band gap as temperature increases in the structure with the dynamic stereo-active lone pair but to a red shift in the structure with the static stereo-active lone pair. Furthermore, analyses of the temperature-dependent low-energy photoluminescence tails reveal that the strength of the e-ph interaction increases with increasing lone-pair activity, leading to a transition from a large polaron to a small polaron, which has significant influence on the emission spectra and charge carrier dynamics. Our results highlight the role of the lone-pair activity in controlling the band gap, phonon, and polaronic effect in LHPs and provide guidelines for optimizing the optoelectronic properties, especially for tin-based and germanium-based halide perovskites, where stereo-active lone pairs are more prominent than their lead counterparts.

Photoluminescence Induced by Substitutional Nitrogen in Single-Layer Tungsten Disulfide.

The electronic and optical properties of two-dimensional materials can be strongly influenced by defects, some of which can find significant implementations, such as controllable doping, prolonged valley lifetime, and single-photon emissions. In this work, we demonstrate that defects created by remote N2 plasma exposure in single-layer WS2 can induce a distinct low-energy photoluminescence (PL) peak at 1.59 eV, which is in sharp contrast to that caused by remote Ar plasma. This PL peak has a critical requirement on the N2 plasma exposure dose, which is strongest for WS2 with about 2.0% sulfur deficiencies (including substitutions and vacancies) and vanishes at 5.6% or higher sulfur deficiencies. Both experiments and first-principles calculations suggest that this 1.59 eV PL peak is caused by defects related to the sulfur substitutions by nitrogen, even though low-temperature PL measurements also reveal that not all the sulfur vacancies are remedied by the substitutional nitrogen. The distinct low-energy PL peak suggests that the substitutional nitrogen defect in single-layer WS2 can potentially serve as an isolated artificial atom for creating single-photon emitters, and its intensity can also be used to monitor the doping concentrations of substitutional nitrogen.

Compositional Variation in FAPb1-xSnxI3 and Its Impact on the Electronic Structure: A Combined Density Functional Theory and Experimental Study.

Given their comparatively narrow band gap, mixed Pb–Sn iodide perovskites are interesting candidates for bottom cells in all-perovskite tandems or single junction solar cells, and their luminescence around 900 nm offers great potential for near-infrared optoelectronics. Here, we investigate mixed FAPb1–xSnxI3 offering the first accurate determination of the crystal structure over a temperature range from 293 to 100 K. We demonstrate that all compositions exhibit a cubic structure at room temperature and undergo at least two transitions to lower symmetry tetragonal phases upon cooling. Using density functional theory (DFT) calculations based on these structures, we subsequently reveal that the main impact on the band gap bowing is the different energy of the s and p orbital levels derived from Pb and Sn. In addition, this energy mismatch results in strongly composition-dependent luminescence characteristics. Whereas neat and Sn-rich compounds exhibit bright and narrow emission with a clean band gap, Sn-poor compounds intrinsically suffer from increased carrier recombination mediated by in-gap states, as evidenced by the appearance of pronounced low-energy photoluminescence upon cooling. This study is the first to link experimentally determined structures of FAPb1–xSnxI3 with the electronic properties, and we demonstrate that optoelectronic applications based on Pb–Sn iodide compounds should employ Sn-rich compositions.

A Study on the Effects of Gallium Droplet Consumption and Post Growth Annealing on Te-Doped GaAs Nanowire Properties Grown by Self-Catalyzed Molecular Beam Epitaxy

In this work, the effects of arsenic (As) flux used during gallium (Ga) seed droplet consumption and the post-growth annealing on the optical, electrical, and microstructural properties of self-catalyzed molecular beam epitaxially grown tellurium (Te)-doped GaAs nanowires (NWs) have been investigated using a variety of characterization techniques. NWs using the same amount of As flux for growth of the seed droplet consumption demonstrated reduced density of stacking faults at the NW tip, with four-fold enhancement in the 4K photoluminescence (PL) intensity and increased single nanowire photocurrent over their higher As flux droplet consumption counterparts. Post-growth annealed NWs exhibited an additional low-energy PL peak at 1.31 eV that significantly reduced the overall PL intensity. The origin of this lower energy peak is assigned to a photocarrier transition from the conduction band to the annealing assisted Te-induced complex acceptor state (TeAsVGa−). In addition, post-growth annealing demonstrated a detrimental impact on the electrical properties of the Te-doped GaAs NWs, as revealed by suppressed single nanowire (SNW) and ensemble NW photocurrent, with a consequent enhanced low-frequency noise level compared to as-grown doped NWs. This work demonstrates that each parameter in the growth space must be carefully examined to successfully grow self-catalyzed Te-doped NWs of high quality and is not a simple extension of the growth of corresponding intrinsic NWs.

Doublet luminescence due to coexistence of excitons and electron-hole plasmas in optically excited CH3NH3PbBr3 single crystal

Doublet luminescence from hybrid metal trihalide perovskite semiconductors is observed along with materials processing when high-quality single crystals are obtainable. Yet, the underlying physical mechanism remains poorly understood. Here, we report controllable solution-processed crystallization that affords high-quality CH3NH3PbBr3 single crystals with atomically flat pristine surfaces. Front-face photoluminescence (PL) shows doublet luminescence components with variable relative intensities depending on the crystal surface conditions. We further find that the low-energy PL component with asymmetric spectral line-shape becomes predominant when the atomically flat crystal surfaces are passivated in the ion-abundant saturated solutions, while poor-quality single crystal with visually rough surface only gives the high-energy PL with symmetric line-shape. The asymmetric spectral line-shape of the low-energy PL matches perfectly with the simulated bandedge emission. Therefore, the low-energy PL component is attributable to the intrinsic bandedge emission from the crystal bulk while the high-energy one to surface-specific emission. Elliott fitting to the absorption data and multi-exponential fitting to the time-resolved photoluminescence traces jointly indicate the coexistence of excitons and electron–hole plasmas in optically excited CH3NH3PbBr3 single crystals, thereby catching the physical merit that leads to the occurrence of doublet luminescence.

Phosphorescent Soft Salt Based on Platinum(II) Complexes: Photophysics, Self-Assembly, Thermochromism, and Anti-counterfeiting Application.

A new platinum(II) complex-based soft salt S1, ([Pt(tpp)(ed)]+[Pt(pba) (CN)2]-) (tpp = 2-(4-(trifluoromethyl)phenyl)pyridine, ed = ethane-1,2-diamine, pba = 4-(2-pyridyl)benzaldehyde), was designed and synthesized. UV-visible absorption and photoluminescence (PL) spectra were studied to elucidate the nature of ground and excited states. The soft salt complex was found to show self-assembly properties with the assistance of electrostatic, π-π stacking, and Pt···Pt interactions, resulting in the remarkable emergence of low-energy absorption and PL bands. Morphological transformation of S1 from undefined nanosized aggregates to nanofibers with different solvent compositions has been demonstrated. Interestingly, a luminescent polymer film was prepared by doping S1 into a polyethylene glycol matrix. The film displayed distinctive emission color change from yellow to red upon heating. Eventually, a high-level anti-counterfeiting application was accomplished using a time-resolved imaging technique based on the thermochromic luminescence property and long emission decay time displayed by S1. It is anticipated that this work can provide deep insights into the control of intermolecular interactions between cationic and anionic complexes of soft salt upon exposure to different external stimuli, resulting in the development of smart luminescent materials for various applications.

Carbon Nanotube Photoluminescence Modulation by Local Chemical and Supramolecular Chemical Functionalization.

ConspectusCarbon nanotubes (CNTs) have been central materials in nanoscience and nanotechnologies. Single-walled CNTs (SWCNTs) consisting of a cylindrical graphene show a metallic (met) or semiconducting (sc) property depending on their rolling up manner (chirality). The sc-SWCNTs show characteristic chirality-dependent optical properties of their absorption and photoluminescence (PL) in the near-infrared (NIR) region. These are derived from their highly π-conjugated structures having semiconducting crystalline sp2 carbon networks with defined nanoarchitectures that afford a strong quantum confinement and weak dielectric screening. Consequently, photoirradiation of the SWCNTs produces a stable and mobile exciton (excited electron-hole pair) even at room temperature, and the exciton properties dominate such optical phenomena in the SWCNTs. However, the mobile excitons decrease the PL efficiency due to nonradiative relaxation including collision with tube edges and relaxation to lower-lying dark states. A breakthrough regarding the efficient use of the mobile exciton for PL has recently been achieved by local chemical functionalization of the SWCNTs, in which the chemical reactions introduce local defects of oxygen and sp3 carbon atoms in the tube structures. The defect doping creates new emissive doped sites that have narrower band gaps and trap the mobile excitons, which provides locally functionalized SWCNTs (lf-SWCNTs). As a result, the localized exciton produces E11* PL with red-shifted wavelengths and enhanced PL quantum yields compared to the original E11 PL of the nonmodified SWCNTs.In this Account, we describe recently revealed fundamental properties of the lf-SWCNTs based on the analyses by photophysics, theoretical calculations, and electrochemistry combined with in situ PL spectroscopy. The new insight allows us to expand the wavelength regions of the NIR E11* PL derived from the localized exciton, in which upconversion generates a higher energy PL through thermal activation and proximal doped site formation using bis-aryldiazonium modifiers provides a much lower energy PL than typical E11* PL. Moreover, owing to the chemical reaction-dominant doping process, the molecular structure design of modifiers succeeds in producing functionalized lf-SWCNTs; namely, molecular functions are incorporated into the doped sites for their PL modulation. The wavelength changes/switching in the E11* PL selectively occurs by a supramolecular approach using molecular recognition and imine chemistry. Therefore, the local chemical functionalization of the SWCNTs is a key to designing the properties and creating their new functions of the lf-SWCNTs. Fundamental understanding of the doped site properties of the lf-SWCNTs and molecularly driven approaches for exciton and defect engineering would unveil the intrinsic natures of these materials, which is crucial for elevating the SWCNT-based nanotechnologies to the next stage. The resulting materials are of interest in the fields of high performance NIR-II imaging and sensing for bio/medical analyses and single-photon emitters in quantum information technology.

Complex evolution of photoluminescence during phase segregation of MAPb(I1-xBrx)3 mixed halide perovskite

Under illumination, the photoluminescence of a mixed-halide perovskite such as MAPb(I1-xBrx)3 is known to undergo a significant intensity enhancement while spectrally shifting to lower energies. The evolution of low energy photoluminescence is attributed to the formation of iodide rich domains due to phase segregation. This process is detrimental for optoelectronic devices however the mechanism is not well-understood. Here we present a real-time study of the photoluminescence evolution in MAPb(I1-xBrx)3 samples during light-induced phase segregation. We show that the evolution of photoluminescence proceeds via several intermediate stages making it more complex than previously suggested. Within the first few seconds of photo-excitation, we found a very rapid formation of a short-lived intense photoluminescence band with a peak energy even lower than the final emission of the fully segregated sample. We propose that this emission comes from small pure iodide nano-domains formed during the initial stage of photo-induced phase-segregation.

GaInAsSb/AlGa(In)AsSb type I quantum wells emitting in 3 μm range for application in superluminescent diodes

In this paper, we present results of Fourier-transformed photoluminescence measurements of quaternary GaInAsSb quantum wells with quinary AlGaInAsSb barriers grown on GaSb substrate, designed for spectral range of mid-infrared. Here, we show an emission shift towards longer wavelength as a result of incorporation of indium into the quantum wells reaching up to 3 μm at room temperature (RT). Additionally, we have observed an additional low-energy photoluminescence signal with maximum wavelength of 3.5 μm at RT, which we have attributed as states localised on the layer interfaces. The activation energy of carriers trapped in those states is estimated to be 35 meV.

Direct and Indirect Interlayer Excitons in a van der Waals Heterostructure of hBN/WS2/MoS2/hBN.

A van der Waals (vdW) heterostructure composed of multivalley systems can show excitonic optical responses from interlayer excitons that originate from several valleys in the electronic structure. In this work, we studied photoluminescence (PL) from a vdW heterostructure, WS2/MoS2, deposited on hexagonal boron nitride (hBN) flakes. PL spectra from the fabricated heterostructures observed at room temperature show PL peaks at 1.3-1.7 eV, which are absent in the PL spectra of WS2 or MoS2 monolayers alone. The low-energy PL peaks we observed can be decomposed into three distinct peaks. Through detailed PL measurements and theoretical analysis, including PL imaging, time-resolved PL measurements, and calculation of dielectric function ε(ω) by solving the Bethe-Salpeter equation with G0 W0, we concluded that the three PL peaks originate from direct K-K interlayer excitons, indirect Q-Γ interlayer excitons, and indirect K-Γ interlayer excitons.

Direct-Gap Photoluminescence from a Si-Ge Multilayer Super Unit Cell Grown on Si0.4Ge0.6

Both Si and Ge possess indirect band gaps, which makes them very inefficient light emitters. One way to overcome this limitation is through bandgap engineering. M. d'Avezac et al. predicted in 2012 that a strained SiGe2Si2Ge2SiGen super unit cell on Si0.4Ge0.6 would have a direct and dipole-allowed gap of 0.863 eV, which is suited for optical fiber applications. Here we report on the epitaxial growth of such a structure and its optical properties, for which purpose two similar samples were prepared by molecular beam epitaxy and solid phase epitaxy. Photoluminescence (PL) spectra were obtained at low temperatures (6–25 K) with excitation at wavelengths of 405 and 458 nm, which emphasize the light emission from the sample superstructure. A strong low-energy PL quadruplet is seen, with peaks near 727, 758, 792 and 822 meV at 6 K, together with a much weaker peak at 871 meV. The ratio of intensities of the strong and weak peaks is the same in both samples. The weak peak at 871 meV is assigned to the dipole-allowed direct-gap transition associated with the super unit cell. The four strong peaks are attributed to dislocation related emission lines of the thick relaxed Si0.4Ge0.6 transition layer.

Imaging Atomic-Scale Clustering in III-V Semiconductor Alloys.

Quaternary alloys are essential for the development of high-performance optoelectronic devices. However, immiscibility of the constituent elements can make these materials vulnerable to phase segregation, which degrades the optical and electrical properties of the solid. High-efficiency III-V photovoltaic cells are particularly sensitive to this degradation. InAlAsSb lattice matched to InP is a promising candidate material for high-bandgap subcells of a multijunction photovoltaic device. However, previous studies of this material have identified characteristic signatures of compositional variation, including anomalous low-energy photoluminescence. In this work, atomic-scale clustering is observed in InAlAsSb via quantitative scanning transmission electron microscopy. Image quantification of atomic column intensity ratios enables the comparison with simulated images, confirming the presence of nonrandom compositional variation in this multispecies alloy.

Modification of defects and potential fluctuations in slow-cooled and quenched Cu2ZnSnSe4 single crystals

Recent literature reports have shown the ability to manipulate Cu-Zn cation ordering for Cu2ZnSnSe4 (CZTSe) via low temperature treatments. Theoretical arguments suggest that one of the major roadblocks to higher VOC—significant band tailing—could be improved with increased cation order; however, few direct measurements have been reported and significant device improvements have not yet been realized. This report investigates electrical properties, defects, and devices from quenched and slow-cooled single crystals of CZTSe. The extent of disorder was characterized by Raman spectroscopy as well as x-ray diffraction, where the change in Cu-Zn order can be detected by a changing c/a ratio. Quenched samples show higher acceptor concentrations, lower hole mobilities, and a lower-energy photoluminescence (PL) peak than crystals cooled at slower rates, consistent with a reduction in the bandgap. In addition, samples quenched at the highest temperatures showed lower PL yield consistent with higher quantities of deep defects. Devices fabricated using slow-cooled CZTSe single crystals showed improved efficiencies, most notably with increased VOC; however, low temperature intensity-dependent photoluminescence measurements continue to indicate the existence of potential fluctuations. We discuss the possibility that potential fluctuations in slow-cooled samples may be related to the inability to achieve a long range order of the Cu-Zn sub-lattice resulting in local regions of high and low levels of cation order, and consequent local variations in the bandgap. The presence of significant potential fluctuations, even after the slow-cooling step, suggests the difficulty in eliminating band-tailing in this system, and thus, additional approaches may be needed for significant reduction of the VOC deficit.

Effects of thickness layer on the photoluminescence properties of InAlAs/GaAlAs quantum dots

We investigated the effect of InAlAs layer thickness on exciton-spin relaxation and optical properties of In0.62Al0.38As/Al0.67Ga0.33As QDs. The luminescence properties and carrier dynamics of QDs were studied by the temperature-dependent photoluminescence (PL) and pump-probe measurements. As the total amount of deposited In0.62Al0.38As alloy increased, the central position of the low-energy PL signal decreases, while its full width at half maximum (FWHM) increases. A monotonous redshift of the PL peak was observed with increasing temperature due to the electron–phonon scattering. From the pump-probe measurement, the spin relaxation time decreases with the monolayers at higher temperatures, in agreement with the phonon energy determinate by PL measurements.

Controlling the compositional inhomogeneities in AlxGa1−xN/AlyGa1−yN MQWs grown by PA-MBE: Effect on luminescence properties

Al0.35Ga0.65N/Al0.55Ga0.45N MQWs were grown by PA-MBE using a range of group III/V flux ratios. TEM images indicate sharp interfaces and well/barrier widths of 1.5/2nm. We observe that small variations of group III/V flux ratio cause dramatic variations in the room temperature photoluminescence (PL) spectra. In addition to band edge luminescence, multiple low energy PL peaks are observed for growths under excess group III conditions, which are absent for near-stoichiometric growth. Temperature dependent PL measurements indicate that at room temperature, emission occurs due to transitions at potential fluctuations generated by the presence of compositional inhomogeneity. These effects are dominant for growth under excess group III conditions due to the presence of a metallic layer on the growth surface during deposition. This can be eliminated by the use of an Indium surfactant during growth, which modifies the diffusion length of Ga and Al adatoms. Under these conditions, the optical properties of MQWs are relatively insensitive to variations in group III to V flux ratio and hence substrate temperature, thus making them suitable for industrial-scale fabrication of optoelectronic devices in the ultraviolet range.

Dynamic Optical Properties of CH₃NH₃PbI₃ Single Crystals As Revealed by One- and Two-Photon Excited Photoluminescence Measurements.

The dynamic optical properties of perovskite CH3NH3PbI3 single crystals were studied by means of time-resolved photoluminescence (PL) spectroscopy at room temperature. The PL peak under one-photon excitation exhibits a red-shift with elapsing time, while two-photon PL is time-independent and appears at lower energy levels. The low-energy two-photon PL can be attributed to emissions from the localized states because of strong band-to-band absorption and photon re-absorption of the emitted light in the interior region. We revealed that the PL behaviors can be explained by the diffusion of photocarriers generated in the near-surface region to the interior region. The excitation fluence dependence of the one-photon PL dynamics is also discussed in terms of the electron-hole radiative recombination and carrier diffusion effects.