Low-temperature Photoluminescence Spectroscopy Research Articles

Role of passivation ligand in exciton diffusion and emission mechanism of metal halide perovskite nanocrystal solid

Shorter-ligand passivated metal halide perovskite nanocrystal (PNC) solids possess smaller inter-NC edge distance which can potentially activate shorter-range nonradiative routes of energy transfer. However, these PNCs are prone to develop surface defects due to ligand detachment and the photogenerated excitons also have high probability to undergo dissociation. It remains unclear how these factors influence the exciton diffusion and emission mechanism of PNC solids. Herein, we prepare two PNC solids passivated with alkylamine and alkylacid ligands of different chain lengths and investigate the exciton diffusion and emission properties by combining time-resolved photoluminescence (PL) and low temperature PL spectroscopy. Our results show that due to the easy ligand detachment and rich surface defects, no prominent exciton transfer occurs in PNC solid passivated with short ligands, despite of the shorter distance between adjacent PNCs. In comparison, the PNC solid passivated with long ligands shows significant occurrence of exciton diffusion from high energy sites towards low energy sites. Accordingly, the PL spectra of thick PNC solid passivated with different ligands show a strikingly different evolution trend with decreasing temperature: in short ligand passivated PNC solid, an intrinsic emission is observed along with a defect-related emission arising from carrier trapping and subsequent radiative recombination, and both emissions are enhanced at lower temperature; in long ligand passivated PNC solid, high energy emission quenches while low energy emission is strengthened at lower temperature due to enhanced exciton transfer. This work reveals that the exciton diffusion and emission properties of PNC solid is strongly mediated by the passivation ligands, providing important insights for their applications in optoelectronic devices.

Luminescent manifestations of ytterbium ions in the crystal structure of silicate apatite

The manifestation of ytterbium ions in the microcrystals of the Sr2Y6.8YbSi6O26:Er0.2 solid solution with apatite structure was studied on the basis of low-temperature photoluminescence (PL) spectroscopy and spectral-kinetic measurement data. Intracenter down-conversion luminescence of Er3+ ions in the visible and IR ranges was detected. Up-conversion luminescence of Er3+ ions is observed upon IR excitation due to excitation of Yb3+ ions and subsequent energy transfer Yb3+ → Er3+. Overlapping bands at 412 and 430 nm with a Stokes shift of less than 0.3 eV are observed in the PL spectra at room temperature. These emission bands are assigned to the spin-allowed and spin-forbidden 4f135d → 4f14 transitions from the low-spin and high-spin exciting states of Yb2+ ion, respectively. The PL decay kinetics for transitions from the low-spin states is characterized by a dominant component τ = 0.39 ns. An alternative model of Yb3+ luminescence center associated with a charge transfer band does not explain the data of low-temperature (5K) PL spectroscopy. At T = 5 K, a new wide emission band with a maximum at 696 nm and a Stokes shift of 1.1 eV appears in the PL spectrum. Two options of this PL band nature are considered. The first is the luminescence of defects having a vacancy nature. The second possible option is the manifestation of anomalous luminescence of Yb2+ ions, which occupy a different crystallographic position in the apatite crystal structure.

Strong Exciton-Phonon Coupling as a Fingerprint of Magnetic Ordering in van der Waals Layered CrSBr.

The layered, air-stable van der Waals antiferromagnetic compound CrSBr exhibits pronounced coupling among its optical, electronic, and magnetic properties. As an example, exciton dynamics can be significantly influenced by lattice vibrations through exciton-phonon coupling. Using low-temperature photoluminescence spectroscopy, we demonstrate the effective coupling between excitons and phonons in nanometer-thick CrSBr. By careful analysis, we identify that the satellite peaks predominantly arise from the interaction between the exciton and an optical phonon with a frequency of 118 cm-1 (∼14.6 meV) due to the out-of-plane vibration of Br atoms. Power-dependent and temperature-dependent photoluminescence measurements support exciton-phonon coupling and indicate a coupling between magnetic and optical properties, suggesting the possibility of carrier localization in the material. The presence of strong coupling between the exciton and the lattice may have important implications for the design of light-matter interactions in magnetic semiconductors and provide insights into the exciton dynamics in CrSBr. This highlights the potential for exploiting exciton-phonon coupling to control the optical properties of layered antiferromagnetic materials.

Temperature/laser dependence of N3 optical centers in natural type Ia diamonds through photoluminescence spectroscopy

In this work, low-temperature photoluminescence spectroscopy was employed to investigate the N3 optical color center in natural type Ia diamond. The optical properties of the N3 center with regular changes in testing temperature and laser power were studied, and the energy level transition was also discussed. The results showed that the PL intensity of the N3 center enhanced sub-linearly with laser power increased, indicating that the electron transition of the N3 center was mainly radiative recombination with weak Auger recombination participation. In addition, the physical model was employed to analyze the change in the zero phonon line with the increase in the test temperature; it obtained thermal quenching activation energy (25.2 meV), bond softening, and strong interaction with acoustical phonons of the N3 center. The theoretical analysis of the broadening parameters revealed that the longitudinal optical phonon energy was 55.1 meV, and the electron–optic phonon coupling strength was 2.3 meV.

Improvement of crystallinity of CdZnTe epilayers on GaSb substrates by ZnTe buffer layer

Thick CdZnTe epitaxial film is a promising alternative for X-ray detectors, especially in the case of large-area imaging application. One of the key issues in growing thick heteroepitaxial film with high crystallinity is to reduce the mismatch dislocations that appear at the interface between CdZnTe epilayer and the substrate. In this paper, we prepared CdZnTe films without and with ZnTe buffer layers on GaSb (001) substrates by the closed space sublimation. All preparation conditions for ZnTe buffer layers, CdZnTe epilayers on ZnTe buffers and GaSb substrates, as well as the pretreatment of GaSb surface have been optimized through tremendous experiments. The results of electron backscatter diffraction and transmission electron microscope observation prove that ZnTe buffer layers suppress (1‾1‾1)M twins extending from GaSb. The deposition temperature for inserting ZnTe buffer about 495 K is optimized for getting smooth ZnTe surface, on which high crystalline quality CdZnTe epilayer with a narrower full-width at half-maximum (85 arcsec) of double crystal X-ray rocking curve for (004) reflection and lower intensity ratio IDcomplex/I(D0, h) of low-temperature photoluminescence spectroscopy were produced.

Stacking-Order-Dependent Excitonic Properties Reveal Interlayer Interactions in Bulk ReS2.

Rhenium disulfide, a member of the transition metal dichalcogenide family of semiconducting materials, is unique among 2D van der Waals materials due to its anisotropy and, albeit weak, interlayer interactions, confining excitons within single atomic layers and leading to monolayer-like excitonic properties even in bulk crystals. While recent work has established the existence of two stacking modes in bulk, AA and AB, the influence of the different interlayer coupling on the excitonic properties has been poorly explored. Here, we use polarization-dependent optical measurements to elucidate the nature of excitons in AA and AB-stacked rhenium disulfide to obtain insight into the effect of interlayer interactions. We combine polarization-dependent Raman with low-temperature photoluminescence and reflection spectroscopy to show that, while the similar polarization dependence of both stacking orders indicates similar excitonic alignments within the crystal planes, differences in peak width, position, and degree of anisotropy reveal a different degree of interlayer coupling. DFT calculations confirm the very similar band structure of the two stacking orders while revealing a change of the spin-split states at the top of the valence band to possibly underlie their different exciton binding energies. These results suggest that the excitonic properties are largely determined by in-plane interactions, however, strongly modified by the interlayer coupling. These modifications are stronger than those in other 2D semiconductors, making ReS2 an excellent platform for investigating stacking as a tuning parameter for 2D materials. Furthermore, the optical anisotropy makes this material an interesting candidate for polarization-sensitive applications such as photodetectors and polarimetry.

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.

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.

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.

Low temperature photoluminescence study for identification of intersubband energy levels inside triangular quantum well of AlGaN/GaN heterostructure

A non-destructive method for extracting the true signatures of intersubband energy level states inside a single triangular quantum well for AlGaN/GaN heterostructure is presented. Herein, we report experiments showing light interaction with high-mobility two-dimensional electron gas (2DEG) in Al0.3Ga0.7N/AlN/GaN based HEMT structure using low-temperature photoluminescence spectroscopy. An interband transition from two-dimensional electron gas (2DEG) subbands to the valence band is identified. These observed transitions were confirmed by comparing the PL spectra of the as grown and top barrier layer etched samples. The luminance peak related to 2DEG disappeared when the top AlGaN barrier layer was removed using reactive ion etching (RIE) system. Furthermore, the emission peak data are also supported by a calculation based on a self-consistent solution of one-dimensional Poisson and Schrodinger equations. The three PL spectra peaks corresponding to the interband transitions from 2DEG subbands to the valence band were reported at 3.269, 3.356 and 3.438 eV respectively. The corresponding intersubband energy states inside the quantum well were extracted (simulated) with 87 (91) and 178 (186) meV energy spacing between E0 & E1 and E0 & E2 respectively. The temperature and excitation power-dependent PL measurement makes it easy to identify the transitions from the 2DEG subbands to valence bands.

Performance optimization of CZT thick films by CdTe buffer layers for solar-blind ultraviolet photoelectric detectors

CdZnTe (CZT) thick films with CdTe buffer layers (CZT/CdTe) were studied for high-performance ultraviolet photoelectric detector. CZT thick films were prepared on AlN/Si substrates via close-spaced sublimation (CSS) and CdTe buffer layers were introduced to optimize the interface quality of CZT thick films. Results of x-ray diffraction showed that the full width at half maximum (FWHM) and dislocation density (δ) of interface of optimized samples were 0.156 ° and 2.543 × 1014 lin/m2, respectively. Interface detect analysis of free-standing CZT and CZT/CdTe thick films was studied by x-ray photoelectron spectroscopy (XPS) and low-temperature photoluminescence (PL) spectroscopy, and mechanism of in-situ annealing in CdTe atmosphere to reduce interfacial defect states was discussed. Moreover, the photo-dark current ratio of the CZT/CdTe thick film metal-semiconductor-metal (MSM) structural photodetector was as high as 5530, which was nearly double that of the CZT thick film detector, and the carrier mobility lifetime product could reach 5.59 × 10−4 cm2/V at 100 V bias, confirming the improvement of carrier transport capacity and detection efficiency. CZT-based detectors have inestimable prospect in the field of solar-blind ultraviolet detection with the participation of CdTe buffer layers.

Microstructure and optical properties of CdTe thin films prepared by close spaced sublimation method at various growth temperatures

Cadmium telluride (CdTe) is an important Ⅱ-Ⅵ multifunctional semiconductor, often used as the light absorption layer in photovoltaic solar cells or the core layer in γ-ray detection. The deposition temperature has a great greatly influences on the microstructure and morphology of the thin films during their growth. Here, close spaced sublimation method is used for depositing CdTe thin films on glass substrate at various temperatures conditions. The microstructure, morphology, optical and luminescent properties of the CdTe thin films were systematically characterized. CdTe thin films with a high preferred crystal orientation, large grain size, low micro-stress, and strong luminescence performance were obtained. And the origin of luminescent peaks is analyzed in detail by low-temperature photoluminescence spectroscopy.

Unveiling the phonon effect on the narrow-band deep-red emission from solution-combustion synthesized Mn4+ doped CaYAlO4 microcrystals

In this study, we report a unique solution-combustion synthesis of Mn4+ doped CaYAlO4 microcrystals and demonstrate a narrow-band deep-red emission. Through the simulation of low-temperature photoluminescence (PL) spectroscopy and theoretical fitting on temperature-dependent PL intensity at T < 400 K, we obtained a dominant phonon mode with energy of 40 meV during the vibronic coupling process, with a small Huang-Rhys factor of S= 0.68. From the analysis of the measured PL lifetime evolution of Mn4 + doped CaYAlO4, it is found that luminescence thermal quenching effect was mainly attributed to four-dominant phonon interaction with Mn4+ electronic states. More interestingly, the deviation of luminescence decay curve from the single-exponential mode at T > 400 K suggests that the occurrence of two radiative transitions is a consequence of ionically thermal excitation to the second excited state of 4T2. These findings provide not only a general approach to identify the predominant phonon vibration, but also deep insights into the effect of weak electron-phonon coupling on PL properties of solid-state luminescent.

Evaluation of crystalline quality of traveling heater method (THM) grown Cd0.9Zn0.1Te0.98Se0.02 crystals

Because of its excellent opto-electronic properties, CdZnTe (CZT) has been the material of choice for x- and gamma-ray detectors operable at room temperature. CZT is the leading commercially available room-temperature radiation detector material today. Although much progress has been made over the past three decades, today's CZT crystals still face certain challenges, especially the presence of the performance-limiting materials defects and the associated relatively high production cost. In this regard, CdxZn1−xTeySe1−y (CZTS) is emerging as a next-generation compound semiconductor, which overcomes some of the limitations of CZT technology for the stated applications. Here, we conducted a study to evaluate the crystalline quality of the traveling heater method grown CZTS with an optimized alloy composition, i.e., Cd0.9Zn0.1Te0.98Se0.02. The as-grown samples were evaluated by low-temperature photoluminescence (PL) spectroscopy and high-resolution x-ray diffraction using the synchrotron light source at Brookhaven National Laboratory. The full width at half maximum of both the PL and x-ray rocking curves was observed to be broadened due to the lattice disorder of the quaternary compound, eventually degrading the crystalline quality. This was consistent with density functional theory calculations.

Passivation efficacy study of Al2O3 dielectric on self-catalyzed molecular beam epitaxially grown GaAs1-x Sb x nanowires

This work evaluates the passivation efficacy of thermal atomic layer deposited (ALD) Al2O3 dielectric layer on self-catalyzed GaAs1-x Sb x nanowires (NWs) grown using molecular beam epitaxy. A detailed assessment of surface chemical composition and optical properties of Al2O3 passivated NWs with and without prior sulfur treatment were studied and compared to as-grown samples using x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and low-temperature photoluminescence (PL) spectroscopy. The XPS measurements reveal that prior sulfur treatment followed by Al2O3 ALD deposition abates III–V native oxides from the NW surface. However, the degradation in 4K-PL intensity by an order of magnitude observed for NWs with Al2O3 shell layer compared to the as-grown NWs, irrespective of prior sulfur treatment, suggests the formation of defect states at the NW/dielectric interface contributing to non-radiative recombination centers. This is corroborated by the Raman spectral broadening of LO and TO Raman modes, increased background scattering, and redshift observed for Al2O3 deposited NWs relative to the as-grown. Thus, our work seems to indicate the unsuitability of ALD deposited Al2O3 as a passivation layer for GaAsSb NWs.

Investigation on low temperature photoluminescence properties of GO-ZnO composite for UV detection application

The effects of different concentrations of graphene oxide (GO) on the optical properties of ZnO particles (Ps) were investigated. A novel sol-gel method was utilized to synthesis ZnO nanopowder as pure and in the presence of different surfactants zinc acetate dihydrate (Zn (CH3COO)2,2H2O), sodium hydroxide (NaOH), Methanol (CH3OH) at calcination temperature 400 °C. The decrease in crystallite size above 11 pH level is evidence of the acceleration of ZnO dissolution during competitive GO-ZnO composite formation. GO-ZnO composite was investigated by low-temperature photoluminescence spectroscopy (PL), we achieved the bandgap shift to 90 meV.

Oxygen-Driven Growth Regulation and Defect Passivation in Chemical Vapor Deposited MoS2 Monolayers

Due to the lowest formation energies, sulfur vacancies are inevitable in the vapor-phase chemical vapor deposition (CVD) of MoS2, which act as deep donors and induce midgap defect states, making the material intrinsically n-type. The postgrowth oxygen passivation of such defects has been the subject of a large number of recent studies because passivation of defects augments the photoluminescence quantum yield by several orders. In this study, by introducing an SiO2/Si wafer in close proximity to the growth substrate, we were able to supply trace oxygen in situ during the growth while simultaneously enabling chemisorption of oxygen at defect sites on the basal plane of large-area MoS2 monolayers. Low-temperature photoluminescence spectroscopy allowed us to distinguish clearly the nature of oxygen bonding in defective MoS2 grown with and without the trace oxygen. Chemisorption of oxygen enabled elimination of defect-related bound exciton emission at the near band edge transition of MoS2, leading to about 300% enhancement in the photoluminescence. We observed unusual splitting of the first-order A1g Raman mode in monolayer MoS2 films when the sulfur vacancies are not compensated by oxygen. The present study provides new experimental evidence to better distinguish between chemisorption and physisorption of oxygen and may serve as an effective way to tune the optical properties of van der Waals crystals during the large-area CVD process.

Double Perovskite Single-Crystal Photoluminescence Quenching and Resurge: The Role of Cu Doping on its Photophysics and Crystal Structure.

Cs2AgBiBr6 is a potential lead-free double perovskite candidate for optoelectronic applications; however, its large and indirect band gap imposes limitations. Here, single crystals of Cs2AgBiBr6 are doped with Cu2+ cations to increase the absorption range from the visible region up to 0.5 eV in the near-infrared region. Inductively coupled plasma spectroscopy confirms the presence of 1.9% of copper in the Cs2AgBiBr6 structure. Structural and optical changes caused by Cu doping were studied by Raman spectroscopy combined with X-ray diffraction, heat capacity measurements, and low-temperature photoluminescence spectroscopy. Along with the 1.9 eV emission typical of the pristine Cs2AgBiBr6 single crystals, we report a novel low-energy emission at 0.9 eV related to deep defects. In the doped crystals, these peaks are quenched, and a new emission band at 1.3 eV is visible. This new emission band appears only above 120 K, showing that thermal energy is necessary to trigger the copper-related emission.

Polarized upconversion luminescence from a single LiLuF4:Yb3+/Er3+ microcrystal for orientation tracking

Polarized upconversion luminescence (UCL) of lanthanide-doped micro/nano-crystals has shown great promise in single-particle tracking and super-resolution bioimaging. However, because of the spectral line broadening and multiple sites of lanthanide in upconversion particles (UCPs), the crystal-field (CF) polarization components of UCL are usually undistinguishable. Herein, we report the linearly polarized UCL in LiLuF4:Yb3+/Er3+ single microcrystals with resolvable CF transition lines and a polarization degree up to 0.82. The CF levels and CF transition lines of Er3+, as well as their emission polarization anisotropy, are unraveled for the first time through low-temperature and high-resolution photoluminescence (PL) and UCL spectroscopies. By taking advantage of the well-resolved and highly-polarized CF transition lines of Er3+, we demonstrate the application of LiLuF4:Yb3+/Er3+ single microcrystals as anisotropic UCL probes for orientation tracking. These findings provide fundamental insights into the polarization anisotropy of UCL in lanthanide-doped single particles, thus laying a foundation for the future design of anisotropic luminescent probes towards versatile applications.