Low-temperature Photoluminescence Research Articles

Uncovering Multiple Intrinsic Chiral Phases in (PEA)2PbI4 Halide Perovskites.

Two-dimensional (2D) halide perovskites offer a unique platform for investigating the ground state of materials possessing significant anharmonicity. In contrast to three-dimensional perovskites, their 2D counterparts offer substantially fewer degrees of freedom, resulting in multiple well-defined crystal structures. In this work, we thoroughly investigate the anharmonic ground state of the benchmark (PEA)2PbI4 compound, using complementary information from low-temperature X-ray diffraction (XRD) and photoluminescence spectroscopy, supported by density functional theory calculations. We extrapolate four crystallographic configurations from low-temperature XRD. These configurations imply that the ground state has an intrinsic disorder stemming from two coexisting chiral sublattices, each with a bioriented organic spacer molecule. We further show evidence that these chiral structures form unevenly populated ground states, portraying uneven anharmonicity, where the state population may be tuned by surface effects. Our results uncover a disordered ground state that may induce intrinsic grain boundaries, which cannot be ignored in practical applications.

Ultrafast photocarrier dynamics of CdSexTe1-x polycrystalline films under low illumination

Understanding the effect of Se in CdTe on photocarrier dynamics is important to improve the performance of devices based on Se-containing absorber. In this study, the photocarrier dynamics of CdSexTe1-x polycrystalline films with different Se contents was investigated at low illumination comparable to AM1.5 G by a transient absorption (TA) test system with ultra-high sensitivity. It was found that the photocarrier trapping was suppressed and lifetime prolonged by incorporating Se into CdTe, and this became more pronounced as xSe increased from 0.2 to 0.55. The pump intensity-dependent TA test and low-temperature photoluminescence (PL) test suggested that this is attributed to the passivation of defects in the bandgap of CdTe by Se-component. These results enriched the knowledge of photocarrier dynamics of CdTe with incorporation of Se components.

Influence of polarities on optical properties of Mg-doped GaN films grown on GaN free-standing substrates by MOCVD

Time-resolved photoluminescence measurements were performed to investigate the minority carrier recombination mechanism of p-GaN films grown on c-plane, m-plane and (10–11)-plane GaN freestanding substrates by MOCVD. The fast and slow decay lifetimes of the three samples were fitted by the bi-exponential function, in which the slow decay lifetime of the m-plane Mg-doped GaN reached a record long value of 493.7 ps? The effects of stress-induced non-radiative recombination centers and edge dislocations on the slow decay lifetime were ruled out using Raman and XRD. The reason for the longer slow decay lifetime of the m-plane sample is the reduction of the non-radiative recombination centers due to the decrease of the VGa concentration, ensuring by the magnitude of the IYL/INBE ratio in the PL spectra. The low-temperature PL spectra of the m-plane sample were investigated. According to Haynes' rule, the position of acceptor level was calibrated through combining with the position of the bound excitons at 5 K.

Interface engineering of charge-transfer excitons in 2D lateral heterostructures

The existence of bound charge transfer (CT) excitons at the interface of monolayer lateral heterojunctions has been debated in literature, but contrary to the case of interlayer excitons in vertical heterostructure their observation still has to be confirmed. Here, we present a microscopic study investigating signatures of bound CT excitons in photoluminescence spectra at the interface of hBN-encapsulated lateral MoSe2-WSe2 heterostructures. Based on a fully microscopic and material-specific theory, we reveal the many-particle processes behind the formation of CT excitons and how they can be tuned via interface- and dielectric engineering. For junction widths smaller than the Coulomb-induced Bohr radius we predict the appearance of a low-energy CT exciton. The theoretical prediction is compared with experimental low-temperature photoluminescence measurements showing emission in the bound CT excitons energy range. We show that for hBN-encapsulated heterostructures, CT excitons exhibit small binding energies of just a few tens meV and at the same time large dipole moments, making them promising materials for optoelectronic applications (benefiting from an efficient exciton dissociation and fast dipole-driven exciton propagation). Our joint theory-experiment study presents a significant step towards a microscopic understanding of optical properties of technologically promising 2D lateral heterostructures.

Strain-tunable valley polarization and localized excitons in monolayer WSe2.

Monolayer transition metal dichalcogenides (TMDs) have a crystalline structure with broken spatial inversion symmetry, making them promising candidates for valleytronic applications. However, the degree of valley polarization is usually not high due to the presence of intervalley scattering. Here, we use the nanoindentation technique to fabricate strained structures of WSe2 on Au arrays, thus demonstrating the generation and detection of strained localized excitons in monolayer WSe2. Enhanced emission of strain-localized excitons was observed as two sharp photoluminescence (PL) peaks measured using low-temperature PL spectroscopy. We attribute these emerging sharp peaks to excitons trapped in potential wells formed by local strains. Furthermore, the valley polarization of monolayer WSe2 is modulated by a magnetic field, and the valley polarization of strained localized excitons is increased, with a high value of up to approximately 79.6%. Our results show that tunable valley polarization and localized excitons can be realized in WSe2 monolayers, which may be useful for valleytronic applications.

Low-threshold lasing of optically pumped micropillar lasers with Al0.2Ga0.8As/Al0.9Ga0.1As distributed Bragg reflectors

We report on the design, realization, and characterization of optically pumped micropillar lasers with low-absorbing Al0.2Ga0.8As/Al0.9Ga0.1As dielectric Bragg reflectors (DBRs) instead of commonly used GaAs/AlGaAs DBRs. A layer of (In, Ga)As quantum dots is embedded in the GaAs λ-cavity of as an active medium. We experimentally study the lasing characteristics of the fabricated micropillars by means of low-temperature photoluminescence with varying pump laser wavelength between 532 and 899 nm. The incorporation of 20% Al content in the DBRs opens an optical pumping window from 700 to 820 nm, where the excitation laser light can effectively reach the GaAs cavity above its bandgap while remaining transparent to the DBRs. This results in a substantially improved pump efficiency, a low lasing threshold, and a high thermal stability. Pump laser wavelengths outside of the engineered spectral window lead to low pump efficiency due to strong absorption by the top DBR or inefficient excitation of pump-level excitons. The superiority of the absorption-free modified DBRs is demonstrated by simply switching the pump laser wavelength from 671 to 708 nm, which crosses the DBRs absorption edge and drastically reduces the lasing threshold by more than an order of magnitude from (363.5 ± 18.5) to (12.8 ± 0.3) μW.

Ultra-sharp-Line Emission in Isotopic c-10BP

Cubic boride crystals with pure boron isotopes have attracted great attention in recent years. Here, high-quality cubic boron phosphide single crystals with isotopic 10B and a maximum width reaching 1.5 mm are grown via the high-temperature molten salt method. An interesting phenomenon that there are many discrete sharp lines in the low-temperature (4–80 K) photoluminescence spectra is observed. Besides, as temperature increases, the luminescence intensity of these sharp lines shows a nonmonotonic decay with two different trends exhibited. Based on analyses, the sharp-line emission originates from phonon-assisted donor–acceptor pair radiative recombination. The nonmonotonic decay is affected by the number of assisting phonons and the degree of thermal ionization under the effect of temperature, and the different decay trends are attributed to the effect of thermal ionization dependent on the distance between the donor and acceptor. Results obtained in this research are expected to enrich the knowledge concerning cubic boride crystals in the aspect of physical properties.

Generation of cylindrical vector beam from GaAs/InGaAs/GaAs core-multishell nanowire cavity

We investigated the beam profiles and polarization states in the low-temperature photoluminescence from vertical GaAs/InGaAs/GaAs core-multishell nanowire (NW) under continuous-wave and pulsed excitations. In the beam profile under pulsed excitation, a doughnut-shaped intensity distribution was confirmed. The beam was shown to exhibit an axisymmetric distribution in the polarization. These observations indicate that cylindrical vector beams were generated from the NW. The observed polarization did not correspond to low-order vector beams but suggested the generation of higher-order beams.

Defect Passivation Results in the Stability of Cesium Lead Halide Perovskite Nanocrystals

Nanoheterostructures (NHSs) based on lead halide perovskites (LHPs) and chalcogenide quantum dots have proved to be promising candidates for photovoltaic device applications. However, understanding the defect chemistry at the interfaces of LHPs and chalcogenides is essential to stabilize them and further tune their optoelectronic properties. Here, we demonstrate a route for designing CsPbBr3–PbSe NHSs and other derivatives of LHP-based NHSs using defect-rich MoSe2 nanosheets (NSs) and study the effect of the size of PbSe NPs on their optical properties. In this synthesis route, PbSe nanoparticles (NPs) are formed at an early stage of the reaction through a unique cation displacement reaction, over which CsPbBr3 nanocrystals (NCs) are epitaxially grown. Using this methodology, a nearly 3-fold enhancement in photoluminescence (PL) is achieved, whereas other selenium precursors, which form larger PbSe NPs, result in negligible PL enhancement with respect to the pure CsPbBr3 NCs. Detailed density functional theory (DFT) calculations suggest that the PbSe NPs are responsible for passivating the surface defects that consequently enhance the PL intensity. However, in the case of larger PbSe NPs, the associated valence and conduction bands lie within the band-gap region of CsPbBr3, creating a type-I heterostructure between the two materials, thereby affecting the luminescence properties. Strong passivation of surface defects in CsPbBr3–PbSe NHSs is also evidenced from low-temperature PL studies. Furthermore, the resulting CsPbBr3–PbSe NHSs demonstrate enhanced stability in the presence of water and do not degrade under ambient conditions for several months.

Morphological studies of Si nanowires effect on the photovoltaic s

Silicon nanowires (SiNWs) were produced by an electroless method on FZ-Si (100) wafer, in HF/AgNO3 solution. The influence of etching time and temperature on SiNWs morphology were studied using FESEM images. Optical properties were also investigated by optical absorption spectroscopy and low-temperature photoluminescence at 4.2 K. Considering their role as active regions, photo- voltaic properties of SiNWs solar cells were studied for their different lengths. Photovoltaic measurements were taken in 1 sun condition under AM 1.5 illumination supplied by a solar simulator. Measurements indicated a reduction in effi ciency as SiNWs length increased, which might be attributed to increased dangling states on nanowires surfaces.

Characterization, room and low temperature photoluminescence of yttrium aluminium borate activated with Sm3+ ions

In this study, the combustion method assisted by urea that is ideally suited to economic and time saving was used for the synthesizing of reddish orange emitting YAl3(BO3)4 phosphor samples doped with various Sm3+ ions (from 0.01 wt% to 7wt%). A detailed study of the structural and luminescence properties at room/low temperature of the synthesized samples was performed. XRD analysis revealed a rhombohedral structure with an R32 space group (155). The particle size was determined by the Scherrer's method to be 48 nm. The visible PL emission spectra upon excitation at 359 nm are recorded and four emission peaks around 564, 599, 646, and 707 nm with transitions 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4G5/2 → 6H9/2 and 4G5/2 → 6H11/2 are observed. Concentration quenching was mainly caused by dipole-dipole interactions between neighbouring trivalent Sm3+ ions. Through the CIE chroma coordinates (0.606, 0.382), the optimized sample (x = 0.03) demonstrates admirable luminous performance. YAl3(BO3)4:Sm3+ can be a good candidate for use as a red component for lighting applications.

Morphology-Dependent Optoelectronic Properties of Pentacene Nanoribbon and Nanosheet Crystallite.

Organic, single crystals have emerged as unique optoelectrical materials due to their highly ordered structure and low defects. In this work, pentacene nanoribbons and nanosheets were selectively fabricated by controlling their growth temperature. The results show that their photoluminescence (PL) activity and electrical properties were strongly dependent on their geometrical morphology and molecular stacking mode such as the degree of π-orbital overlap and intermolecular interaction. The pentacene nanoribbon crystal exhibited a higher PL intensity compared with the nanosheet configuration; conversely, its electrical conductivity was poor. The low-temperature PL measurement indicated that there are stronger π-π stacking interactions in the nanosheet crystal than in the nanoribbon crystal, leading to exciton quenching and higher conductivity. Our study demonstrated that a unique optoelectronic property of organic crystals can be obtained by controlling the crystal's morphology, which offers potential guidance for the future design and development of organic crystal optoelectronics.

Surface Quantum-Dimensional Photocarrier Recombination in <i>CdTe</i> Microcrystals

The scope of the study is the spectra of low-temperature (T = 2K) photoluminescence of a p-CdTe/n-CdS film heterostructure comprising a monolayer of CdTe microcrystals, where a single microcrystalline particle is typically one micron in size. Focus is made on the dominant band of “super-hot” emission appearing in the spectral region located in energy above the fundamental absorption edge of a CdTe bulk crystal. A theoretical model has been developed that assumes the existence of a space-charge layer inside a microcrystal, which leads to the formation of a triangular potential well for an electron near the surface. The anomalous emission band arises as a result of the optical transitions of electrons from near-surface levels of spatial quantization to valence band states.

Thickness-dependent excitonic properties of WSe2/FePS3 van der Waals heterostructures.

van der Waals heterostructures (vdWHs), with their flexible combination of various two-dimensional (2D) materials, are continuously revealing new physics and functionalities. 2D magnetic materials have recently become a focus due to their fascinating electronic and spintronic properties. However, there has rarely been any investigation of the optical properties of 2D magnetic materials-based heterostructures. Herein, we construct a new WSe2/FePS3 heterostructure, in which WSe2 works as a "sensor" to visualize the thickness-dependent properties of FePS3. As characterized by photoluminescence (PL) spectra, whether under or on top of the FePS3, the PL intensity of the monolayer WSe2 is strongly quenched. The quenching effect becomes more obvious as the FePS3 thickness increases. This is because of the efficient charge transfer process occurring at the WSe2/FePS3 interface with type II band alignment, which is faster for thicker FePS3, as is evident from transient absorption measurements. The thickness-dependent charge transfer process and corresponding excitonic properties are further revealed in low-temperature photoluminescence spectra of WSe2/FePS3 heterostructures. Our results show that the thickness of 2D magnetic materials can work as an experimental tuning knob to manipulate the optical performance of conventional 2D semiconductors, endowing van der Waals heterostructures with more unexpected properties and functionalities.

Impact of the organic cation on the band-edge emission of two-dimensional lead-bromide perovskites.

Organic-inorganic low-dimensional layered metal-halide perovskites are semiconductors in which the optoelectronic properties can be tuned by the material composition and the design of the layered architecture. While the electronic band structure is mainly determined by the inorganic octahedra lattice, the binding and conformation of the organic cations induces related lattice distortions that can break the symmetry and lead to the splitting of the exciton energy levels, and influence the dielectric confinement. Furthermore, organic-induced lattice deformations lead to offsets in k-space (where k is the wavevector) that go along with the exciton energy level splitting. Hence, the electronic transitions between these levels require the momentum contribution of phonons, and contributions of phonons in the exciton recombination dynamics result in thermal broadening of the emission linewidth. In this work, we investigate the band-edge emission of two-dimensional Ruddlesden-Popper lead-bromide perovskites synthesized with different organic cations that vary in their binding head group and their alkyl chain length. We find several peaks in the low-temperature photoluminescence spectra, and the number of peaks in the band-edge emission and their decay dynamics depend strongly on the type of organic cation in the material, which we relate to the difference in the inorganic lattice distortions that the cations induce. For two-dimensional layered perovskites with mainly in-plane distortions, induced by short primary ammonium molecules, we find a two-fold splitting of the band edge emission at low temperatures. If also out-of-plane distortions are present, as for the long-chain primary ammoniums, a three-fold splitting is observed. Interestingly, the low-energy peaks of the split series merge into the highest energy peak with increasing temperature. Thermal broadening analysis of the temperature-dependent photoluminescence linewidth in the structures with out-of-plane distortions yields energies that are larger than those reported for the inorganic lattice phonons. This indicates the involvement of either high-frequency oscillations involving the organic cations, or the broadening might be related to higher order phonon scattering processes in the excitonic recombination process. The strong directionality of the phonon modes in the octahedral lattice could promote the involvement of multiple electron-phonon scattering processes in the exciton relaxation dynamics, for example involving modes with orthogonal directionality.

Диполярные биэкситоны в латеральных ловушках в гетероструктурах Si/SiGe/Si

Si/SiGe/Si heterostructures with long-range lateral potential fluctuations that appear in the capping Si layer near the SiGe/Si heterointerface due to the presence of relaxed areas in the SiGe layer have been studied. Analysis of the low-temperature photoluminescence spectra indicates that, upon the photoexcitation of the structure, the accumulation of nonequilibrium charge carriers, formation of dipolar excitons, and their recombination take place in large-scale lateral traps formed by these fluctuations. It is found that, at temperatures T < 10 K, a new narrow line appears with an increase in the excitation level at the blue side of the broad photoluminescence band of dipolar excitons localized by short-range potential fluctuations. At temperatures T ~ 2 K, this line is dominant in the spectra even at the lowest excitation levels. It is shown that, at moderate excitation levels, this line is caused by the recombination of free dipolar biexitons in large-scale traps. At high excitation levels, the width of the new line increases by more than a factor of 2 compared to that at lower excitation levels, and under these conditions this line is associated with the recombination of dipolar electron–hole plasma in large-scale traps.

Highly efficient and tunable broadband UV excitable Ba9Lu2Si6O24:Eu2+, Mn2+ single-phase white-light-emitting phosphors

Based on crystal site engineering, a series of inorganic Ba9Lu2Si6O24: Eu2+ (BLSO: Eu2+) and Ba9Lu2Si6O24:Eu2+, Mn2+ phosphors were synthesized. These phosphors exhibit broad emission throughout the whole visible spectrum range. More specifically, a systematic cation substitution favored efficient energy transfer. Hence, the produced Ba9Lu2−zSczSi6O24: 0.2Eu2+, 0.4Mn2+ (BLSSO: 0.2Eu2+, 0.4Mn4+) were effective single-phase white-light-emitting phosphors. Particularly, Ba9Lu2Si6O24: Eu2+ phosphors emit a broadband blue emission peaking at 462 nm with a long tail. The results of the emission spectrum's Gaussian fitting (and deconvolution), low-temperature photoluminescence, and decay time observations point to the existence of many luminescence centers in the host lattice of BLSO. Partial cation substitution favors the occupation of Eu2+ ions in other available crystallographic sites, i.e., Ba(2) and Ba(3), resulting in the emission spectrum broadening in the green spectral region. The energy transfer from the doped Eu2+ to the co-doped Mn2+ ions broadens the emission in the red spectral region (618 nm). Accordingly, the broadband emission covers the entire visible spectral region. Furthermore, the make-up of these phosphors provides a powerful method for obtaining continuous tuning throughout the whole visible spectrum. The above features, in conjunction with the color coordinates and the remarkable thermal stability, qualify the optimized Ba9Lu2−zSczSi6O24: 0.2Eu2+, 0.4Mn2+ phosphor for potential use in single-phase white-light-emitting phosphors.

Investigation of charge carrier dynamics in beaded ZnO nanowire decorated with SnS2/IrOx cocatalysts for enhanced photoelectrochemical water splitting

We report the synthesis of shape-modulated ZnO nanowires (NWs) and their role to achieve competent photoelectrochemical water splitting (PEC-WS) performance. The carrier dynamics of smooth (s-) and beaded ZnO NW (b-ZnO NW) photoanodes were investigated using time-resolved, temperature-controlled (room-temperature and low-temperature) photoluminescence spectroscopy. The enhanced PEC-WS performance obtained with the b-ZnO NWs was related to the reduced density of surface recombination states, superior crystallinity, and enhanced carrier lifetime, which improved carrier separation. Furthermore, an effective type-II aligned structure with b-ZnO NWs encapsulated in SnS2 and IrOx co-catalyst for enhanced PEC-WS performance and viability have been demonstrated. The optimized IrOx/15 nm SnS2/b-ZnO NW photoanodes demonstrated higher PEC-WS performance (0.58 mA/cm2 at 1.23 V vs RHE) with feasible photostability retention. We believe that our structural modulation approach, in-depth understanding of the carrier dynamics, and demonstration of a co-catalyst-enabled core–shell structure will be beneficial for obtaining highly efficient PEC-WS performances in ZnO-based photoanodes.

Synchrotron-Excited Luminescence and Converting of Defects and Quantum Dots in Modified Silica Films

Optically active defects in modified silicon oxide films on a silicon substrate have been studied by low-temperature photoluminescence (PL) spectroscopy using excitation by synchrotron radiation in the vacuum ultraviolet region. Films of dry thermal silicon oxide obtained by treatment of stoichiometric SiO2 in hydrogen plasma, films of wet thermal silicon oxide, and films with low dielectric constant (so called "low-k" dielectrics) have been studied. Investigations of various type SiO2 films detect the PL centers, which can be conditionally divided into two groups. The first group includes intrinsic point defects, the different variants of oxygen-deficient centers (ODCs). The second group contains centers like the spatially confined excitons in silicon quantum dots (SiQDs). It is shown that SiQDs differ in spectral characteristics, are formed in different ways, due to the transformation and clustering of point defects such as E'-centers, ODC(I) and ODC(II). In this case, the size and spectral properties of quantum dots depend on the mechanism of their formation. Schemes for the conversion of these defects are proposed.It was found that for wet films and nonstoichiometric SiOx films treated in hydrogen plasma, "metastable" centers ODC(I) and ODC(II) arise only under the action of synchrotron radiation in the region of interband or exciton absorption and then they decay because of radiative relaxation. The absence of stable ODC centers in these films is due to their clustering and the formation of SiQDs. In "low-k" mesoporous films, on the contrary, silicon quantum dots SiQDs are not formed, but ODC(I) and ODC(II) centers are clearly manifested. A scheme of electronic transitions upon indirect excitation of silicon quantum dots through exciton states of the SiO2 matrix is considered. The presented results demonstrate the possibility of controlling the optical properties of silicon thin-film structures by creating in them both oxygen-deficient point defects and quantum dots of various sizes.

Copper-related defects in ZnTe thin films grown by the close space sublimation method

Low-temperature photoluminescence (PL) is used to study defects evolution via immersion technique and annealing in vacuum of ZnTe thin films. In this paper we studied how copper doping from solutions of different molar concentrations affects PL of ZnTe thin films grown by close space sublimation (CSS) method. Undoped ZnTe thin films showed PL emission in the (520-680) nm wavelength region. The incorporation of copper in ZnTe produce a number of broad emission bands that correspond to an electron transition from the conduction band to spin-orbit states of the localized level of Cu2+ ions. All the studied samples had variable concentrations of oxygen and the possibility of the formation of auxiliary oxides is discussed.