Polarized Raman Measurements Research Articles

Polarization‐Sensitive Artificial Optoelectronic Synapse Based on Anisotropic β‐Ga2O3 Single Crystal for Neuromorphic Vision Systems and Information Encryption

AbstractA novel polarization‐sensitive artificial optoelectronic synapse based on β‐Ga2O3 single‐crystal is proposed in this work, featuring reconfigurable anisotropic vision. A series of polarization‐sensitive synaptic activities and polarization‐sensitive image recognition functions are successfully simulated using this device. The intriguing performance of this device, stems from the crystal anisotropy in β‐Ga2O3, which is confirmed through polarization Raman measurements and first‐principles theoretical calculations. Furthermore, a comprehensive analysis of the persistent photoelectric properties of the device unveils that the adaptability of the optoelectronic synapse device stems from the ionization and dissociation of oxygen vacancies. Ultimately, the device is utilized in the fields of image recognition and information encryption. A four‐layer artificial neural network with two hidden layers is constructed for recognizing handwritten digits. After training, the recognition accuracy reaches over 91.5% for both unpolarized and polarized light. Information encryption is achieved by controlling polarization states. The device enables data generation and encryption to be conducted on the same platform, mitigating exposure risks during transmission and significantly enhancing data security and confidentiality. This work presents new opportunities for future applications of polarization‐based perception systems.

Observation of band gap bowing effect vanishing in graded-composition monolayer Mo1−xWxS2 alloy

Over the past decade, tremendous effort has been put into developing 2D semiconductor materials with a tunable bandgap by alloying different individual components. However, the bandgap bowing effect has hindered the ability to arbitrary control the emission of these alloys. In this study, we report the chemical vapor deposition growth of a graded-composition Mo1−xWxS2 monolayer alloy, in which the photoluminescence emission energy exhibits nearly linear variation in the bandgap, indicating the vanishing of the bandgap bowing effect. Polarized Raman measurements show that the polarization is composition dependent, and a large symmetry breaking occurs at the point where the bandgap bowing effect vanishes. This suggests that the vanishing of the bowing effect may be attributed to the symmetry breaking induced by compressive strain. Our findings demonstrate a significant advancement in the synthesis of alloys for future use.

Analysis of diamond dislocations by Raman polarization measurement

Raman peaks with weak intensities at low wavelength were detected in dislocation areas of diamond substrates. Polarized Raman measurements were performed, and the origins of peaks were estimated. Based on the appearance of the Raman modes, the three peaks were estimated to be non-diamond owing to their structural asymmetries. Mapping measurements were carried out using one of the peaks and a small area of a high intensity region was estimated as the core of dislocation threading to the crystal surface. This method showed two dislocations on the surface, approximately 1.2 μm apart. The imaging using the peaks will be a powerful tool for investigating the dislocations of diamond epitaxial layers.

Anisotropic Phonon Behavior and Phase Transition in Monolayer ReSe2 Discovered by High Pressure Raman Scattering.

Re-based transition metal dichalcogenides have attracted extensive attention owing to their anisotropic structure and excellent properties in applications such as optoelectronic devices and electrocatalysis. The present study methodically investigated the evolution of specific Raman phonon mode behaviors and phase transitions in monolayer and bulk ReSe2 under high pressure. Considering the distinctive anisotropic characteristics and the vibration vectors of Re and Se atoms exhibited by monolayer ReSe2, we perform phonon dispersion calculations and propose a methodology utilizing pressure-dependent polarized Raman measurements to explore the precise structural evolution of monolayer ReSe2 under the stress fields. Varied behaviors of the Eg-like and Ag-like modes, along with their specific vector transformations, have been identified in the pressure range 0-14.59 GPa. The present study aims to offer original perspectives on the physical evolution of Re-based transition metal dichalcogenides, elucidating their fundamental anisotropic properties and exploring potential applicability in diverse devices.

Vibrational Coupling and Hydrogen-Bond Structure of Water in the Extended Hydration Shell of Metal Ions.

Intra- and intermolecular vibrational coupling (VC) and hydrogen bonding (H-bonding) of water are sparsely understood in the hydration shell (HS) of a metal ion, though the corresponding knowledge for an anion is quite extensive. This is primarily due to the overwhelming effect of anions on water, which masks the subtle perturbing influence of most of the cations. Using Raman difference spectroscopy with simultaneous curve fitting (Raman-DS-SCF) in combination with isotopic dilution and polarized Raman spectroscopy, we have elucidated the VC and H-bonding of water in the HS of bi- and trivalent metal ions─Mg2+, Ca2+, La3+, Gd3+, Dy3+. Polarized Raman measurement of the HS water with VC "turned on" and "turned off" (using isotopically diluted water, HOD) reveals that water retains the intra- and intermolecular vibrational coupling in the HS of high-charge-density metal ions, which is in stark contrast to that of an anion. Hydration shell spectroscopy in HOD unambiguously shows that the average H-bonding of water becomes stronger in the HS than that of bulk water. The first HS water strongly donates two H-bonds to the second HS water (ν̅max ≈ 3200 cm-1) but weakly accepts a H-bond from the second HS water (ν̅max ≈ 3590 cm-1), which makes the HS water heterogeneous in terms of its H-bond structure. The weakly interacting OH (ν̅max 3585 cm-1 in HOD) red-shifts by ∼ 15 cm-1 while the VC is "turned on" (ν̅max ≈ 3600 cm-1 in H2O), revealing the intramolecular coupling of water in the HS of metal ions.

Phase Transition and Energy Storage Density in Lead-Free Ferroelectric Ba1−xSrxTiO3 (x = 0.1, 0.3, and 0.7) Capacitors

Structure, phonon, and energy storage density in Sr2+-substituted lead-free ferroelectric Ba1−xSrxTiO3 (BSTx) for compositions x = 0.1, 0.3, and 0.7 were investigated using X-ray diffraction, Raman, and ferroelectric polarization measurements as a function of temperature. The samples were tetragonal for x = 0.1 with a large c/a ratio. The tetragonal anisotropy was decreased upon increasing x and transforming to cubic for x = 0.7. The changes in structural and ferroelectric properties were found to be related to the c/a ratios. The temperature-dependent phonon spectroscopy results indicated a decrease in tetragonal–cubic phase transition temperature, Tc, upon increasing x due to a reduction in the lattice anisotropy. The intensity of ~303 cm−1 E(TO2) mode decreased gradually with temperature and finally disappeared around the tetragonal ferroelectric to cubic paraelectric phase at about 100 ℃ and 40 ℃ for x = 0.1 and 0.3, respectively. A gradual reduction in the band gap Eg of BSTx with x was evident from the analysis of UV-visible absorption spectra. The energy storage density (Udis) of the ferroelectric capacitors for x = 0.7 was ~0.20 J/cm3 with an energy storage efficiency of ~88% at an applied electric field of 104.6 kV/cm. Nearly room temperature transition temperatures TC and reasonably fair energy storage density of the BSTx capacitors were found.

Vibrational and electronic properties of Na2Ti6O13

AbstractSodium hexatitanate, Na2Ti6O13, in the form of elongated microcrystals, were synthesized by solid‐state reaction of titanium dioxide and sodium carbonate. The monoclinic structure was confirmed by X‐ray powder diffraction and selected area electron diffraction. Electron diffraction was used to determined zone axis and the crystal‐grown direction, which allowed to perform polarized Raman measurements. Energy band gap and the electronic transition were studied by UV–Vis diffuse reflectance and using Tauc relation. Density functional theory (DFT) calculations also revealed an indirect allowed bandgap transition and a good agreement between theoretical and experimental energy band gaps were demonstrated. For the first time, the assignment of the vibrational modes was carried out in the samples and thoroughly compared with theoretical data obtained from DFT calculations. The most relevant of the 60 predicted optical phonon modes of Na2Ti6O13 were experimentally observed and assigned to their symmetries.

Birefringence in the Polarized Raman Scattering of Biaxial van der Waals α‐MoO3

AbstractThe effect of birefringence in anisotropic materials has been a long‐term issue for polarized Raman scattering. In this work, the polarization‐dependent Raman scattering in anisotropically birefringent materials is modeled with birefringence considered in the fundamental polarization selection rule. The birefringence‐induced polarization transformation is treated as a tensor in the calculation of Raman scattering intensity. The validity of this theory is further demonstrated in experiments by taking angle‐resolved polarized Raman measurements on the basal and cross planes of α‐MoO3—a typical biaxial van der Waals crystal with strong in‐plane and out‐of‐plane anisotropy. The anomalous angular dependency of polarized Raman scattering intensity can be well reproduced by the modified theoretical model with fitted real‐valued Raman tensors and birefringence‐related parameters. It can be concluded that this work can provide a valuable reference and guidance for quantitative analyses of anisotropic materials by using polarization‐resolved Raman scattering spectroscopy.

Thin Films Deposition and Energy Storage Density in Lead-Free Ferroelectric Ba1-XSrxTiO3 Thin Film Capacitors (x = 0.1, 0.3, 0.7)

Structure, phonon, and energy storage density in Sr2+-substituted lead-free ferroelectric Ba1-xSrxTiO3 (BSTX) thin films (x = 0.1, 0.3, and 0.7) were prepared using RF sputtering technique, and investigated using x-ray diffraction, Raman, and ferroelectric polarization measurements as a function of temperature. The thin film is tetragonal for x = 0.1 with large c/a ratio. The tetragonal anisotropy (c/a ratio) decreases upon increasing x and that turns cubic for x = 0.7. All these structural and ferroelectric properties change due to change in the c/a ratio. Temperature dependent phonon spectra analyses (80-500 K) indicate decrease in tetragonal to cubic phase transition temperature upon x due to reduction in anisotropy. Overdamping of ~90 cm-1 E soft phonon mode along with a dramatic decrease of intensity of ~160 cm-1 A-phonon mode was observed around the tetragonal ferroelectric to cubic paraelectric phase (< 300 K). The energy storage densities of these thin film capacitors (metal-insulator-metal) were estimated from the measured polarization hysteresis loops and were compared. For x = 0.1, a larger energy storage density (Ure) of ~29 J/cm3 with efficiency of ~48 % was estimated at an applied voltage of 1.1 MV/cm. Nearly room temperature transition temperature Tc, larger dielectric constant and encouraging energy density values of our lead-free BSTX thin film capacitors suggest their possible application in high energy storage and pulse power technology. These results will be presented in detail in the ECS meetings.

Low-energy electronic structure of perovskite and Ruddlesden-Popper semiconductors in the Ba-Zr-S system probed by bond-selective polarized x-ray absorption spectroscopy, infrared reflectivity, and Raman scattering

Chalcogenides in perovskite and the related layered Ruddlesden-Popper crystal structures (chalcogenide perovskites for brevity) are an exciting family of semiconductors but remain experimentally little studied. Chalcogenide perovskites share crystal structures and some physical properties with ionic compounds such as oxide and halide perovskites, but the metal-chalcogen bonds responsible for semiconducting behavior are substantially more covalent than in these more-studied perovskites. Here, we use complementary experimental and theoretical methods to study how the mixed ionic-covalent Zr-S bonds support the electronic structure and physical properties of perovskite ${\mathrm{BaZrS}}_{3}$ and Ruddlesden-Popper ${\mathrm{Ba}}_{3}{\mathrm{Zr}}_{2}{\mathrm{S}}_{7}$. We apply theoretical methods to assign features of experimentally measured x-ray absorption spectroscopy (XAS) to particular orbital transitions, enabling a clear physical interpretation of angle-dependent, polarized XAS data measured on single-crystal samples, and an atomistic view of the covalent bonding network that facilitates charge transport. Polarized Raman measurements identify signatures of crystalline anisotropy in ${\mathrm{Ba}}_{3}{\mathrm{Zr}}_{2}{\mathrm{S}}_{7}$ and enable the first assignments of mode symmetry in this material. Infrared reflectivity reveals electronic transport properties that augur well for the use of chalcogenide perovskites in optoelectronic and energy-conversion technologies.

Raman tensor of zinc-phosphide (Zn3P2): from polarization measurements to simulation of Raman spectra.

Zinc phosphide (Zn3P2) is a II–V compound semiconductor with promising photovoltaic and thermoelectric applications. Its complex structure is susceptible to facile defect formation, which plays a key role in further optimization of the material. Raman spectroscopy can be effectively used for defect characterization. However, the Raman tensor of Zn3P2, which determines the intensity of Raman peaks and anisotropy of inelastic light scattering, is still unknown. In this paper, we use angle-resolved polarization Raman measurements on stoichiometric monocrystalline Zn3P2 thin films to obtain the Raman tensor of Zn3P2. This has allowed determination of the Raman tensor elements characteristic for the A1g, B1g and B2g vibrational modes. These results have been compared with the theoretically obtained Raman tensor elements and simulated Raman spectra from the lattice-dynamics calculations using first-principles force constants. Excellent agreement is found between the experimental and simulated Raman spectra of Zn3P2 for various polarization configurations, providing a platform for future characterization of the defects in this material.

Novel strategies of Raman imaging for monitoring intracellular retinoid metabolism in cancer cells

We developed a label-free Raman method for whole-cell biochemical imaging to detect molecular processes that occur in normal and cancer brain cells due to retinol transport in human cancers at the level of isolated organelles. Our approach allows to create biochemical maps of retinoids localization in lipid droplets, mitochondria and nuclei in single cells. The maps were capable of discriminating triglycerides (TAG) from retinoids (RE) in lipid droplets (LD), and mitochondria providing an excellent tool to monitor intracellular retinoid metabolism. We detected spectral changes that arose in proteins and lipids due to retinoid metabolism in human cell lines of normal astrocytes and high-grade cancer cells of glioblastoma as well as in human medulloblastoma and glioblastoma tissue. Raman imaging is an effective tool for monitoring retinoids and retinol binding proteins involved in carcinogenesis by detecting unique spectral signatures of vibrations. We found two functionally distinct lipid droplets: TAG-LD, for energy storage, and RE-LD, for regulating mechanisms of signal transduction. Raman polarization measurements revealed the occurrence of conformational changes affecting discrete regions of proteins associated with retinol binding. Aberrant expression of retinoids and retinol binding proteins in human tumours were localized in lipid droplets, and mitochondria.

Elucidating the interfacial nucleation of higher-index defect facets in technologically important GaP/Si(0 0 1) by azimuthal angle-resolved polarized Raman spectroscopy

Structural complexities evolving at hetero-polar interface of III-V/Si have been critical obstacles to high-quality and cost-effective epitaxial integration of wide-bandgap GaP on closely lattice-matched Si. Unveiling the nature of interface-originated defect structures is quintessential for efficientintegration of III-V semiconductorson Si. In this work,investigation of surface and interface of technologically important GaP/Si(001)hetero-structuresis presented, through unique implementation of combiningtheinformation onvariation in spatially resolved polarized Raman spectra and surface topography of grown epilayer.The mechanisms responsible for variation in ratio of Raman scattering cross-section of symmetry forbidden to allowed optical phonons of GaP aredelineatedusingazimuthal angle-resolvedpolarizedRaman measurements by allowing for the contribution of energetically favorable higher-index {111} and {112} crystallographic facets, nucleating near the hetero-interface, to light scattering. This is reasserted from polarized Raman spectroscopy performed on cross-sectional surface of GaP/Si(001) hetero-structures.To the best of our knowledge, this is first of it’s kind work wherein angle-dependent polarized Raman measurements are employed for ascertaining the nature and orientations of interfacial defect facets in advanced hetero-structures. Non-invasive and expeditious nature of this optical technique open pathways for fabrication of efficient device structures of III-Vsemiconductors on Si and Geplatforms.

Polarized Raman spectroscopy of phosphorous doped diamond films

Polarized Raman measurements were performed on P-doped diamond films to investigate the origin of the peaks in the lower wave numbers, which are unknown to date, and a comparative study of several P-doped substrates was conducted. The measurements with four configurations of the polarized geometries were carried out for five samples with different P and H concentration, [H]/[P] ratios, and film thickness. For four highly P-doped films, several Raman peaks were observed for all four configurations; retaining the Oh7 cubic structure of diamond, presumably relate to substitutional sites. For a sample of high [H]/[P] ratio (=1.17) and a very thick film, several other peaks were observed. The peak appearances for four configurations indicate that the origin is not in the Oh7 diamond structure but presumably interstitial sites. In the future, it may be possible to realize effective P doping by the improvement of film synthesis by utilizing the results of Raman polarization measurements, which are simple and nondestructive.

The growth of a large GdPO4 crystal guided by theoretical simulation and the study of its phonon properties.

It has remained a big challenge to grow large crystals of compounds at very anisotropic crystal growth rates. GdPO4 is such an example, which usually grows into a needle-like shape. This work solved this problem by using Li+ ions, predicted by first-principles calculations, to adjust the crystal morphology of GdPO4. Based on the calculated surface energies and the polar properties of the surfaces, we identified two crystal planes, i.e. (-102) and (-111), which can be used to adjust the morphology of the GdPO4 crystal by using extrinsic cations in the flux system. With a predicted Li containing flux system, Li2O-MoO3-B2O3, a large crystal with a record size, 10.0 × 9.0 × 18.2 mm3, was grown by using the top-seeded solution growth (TSSG) method. Based on the grown large crystal and the polarized Raman measurements, four new Raman bands missing in early studies and other bands were unambiguously assigned. By introducing a new formula, the phonon-phonon interactions were shown to be weak in the temperature range from 83 to 803 K. Our study indicates that the crystal of GdPO4 can be used as a promising laser host material working under relatively high temperature conditions.

Raman spectroscopy and lattice dynamics calculations of tetragonally-structured single crystal zinc phosphide (Zn3P2) nanowires

Earth-abundant and low-cost semiconductors, such as zinc phosphide (Zn3P2), are promising candidates for the next generation photovoltaic applications. However, synthesis on commercially available substrates, which favors the formation of defects, and controllable doping are challenging drawbacks that restrain device performance. Better assessment of relevant properties such as structure, crystal quality and defects will allow faster advancement of Zn3P2, and in this sense, Raman spectroscopy can play an invaluable role. In order to provide a complete Raman spectrum reference of Zn3P2, this work presents a comprehensive analysis of vibrational properties of tetragonally-structured Zn3P2 (space group P42/nmc) nanowires, from both experimental and theoretical perspectives. Low-temperature, high-resolution Raman polarization measurements have been performed on single-crystalline nanowires. Different polarization configurations have allowed selective enhancement of A1g, B1g and Eg Raman modes, while B2g modes were identified from complementary unpolarized Raman measurements. Simultaneous deconvolution of all Raman spectra with Lorentzian curves has allowed identification of 33 peaks which have been assigned to 34 (8 A1g + 9 B1g + 3 B2g + 14 Eg) out of the 39 theoretically predicted eigenmodes. The experimental results are in good agreement with the vibrational frequencies that have been computed by first-principles calculations based on density functional theory. Three separate regions were observed in the phonon dispersion diagram: (i) low-frequency region (<210 cm−1) which is dominated by Zn-related vibrations, (ii) intermediate region (210–225 cm−1) which represents a true phonon gap with no observed vibrations, and (iii) high-frequency region (>225 cm−1) which is attributed to primarily P-related vibrations. The analysis of vibrational patterns has shown that non-degenerate modes involve mostly atomic motion along the long crystal axis (c-axis), while degenerate modes correspond primarily to in-plane vibrations, perpendicular to the long c-axis. These results provide a detailed reference for identification of the tetragonal Zn3P2 phase and can be used for building Raman based methodologies for effective defect screening of bulk materials and films, which might contain structural inhomogeneities.

Vibrational Properties and DFT Calculations of Perovskite-Type Methylhydrazinium Manganese Hypophosphite.

Recently discovered hybrid perovskites based on hypophosphite ligands are a promising class of compounds exhibiting unusual structural properties and providing opportunities for construction of novel functional materials. Here, we report for the first time the detailed studies of phonon properties of manganese hypophosphite templated with methylhydrazinium cations ([CH3NH2NH2][Mn(H2PO2)3]). Its room temperature vibrational spectra were recorded for both polycrystalline sample and a single crystal. The proposed assignment based on Density Functional Theory (DFT) calculations of the observed vibrational modes is also presented. It is worth noting this is first report on polarized Raman measurements in this class of hybrid perovskites.

Anisotropic optical properties of single Si2Te3 nanoplates

We report a combined experimental and computational study of the optical properties of individual silicon telluride (Si2Te3) nanoplates. The p-type semiconductor Si2Te3 has a unique layered crystal structure with hexagonal closed-packed Te sublattices and Si–Si dimers occupying octahedral intercalation sites. The orientation of the silicon dimers leads to unique optical and electronic properties. Two-dimensional Si2Te3 nanoplates with thicknesses of hundreds of nanometers and lateral sizes of tens of micrometers are synthesized by a chemical vapor deposition technique. At temperatures below 150 K, the Si2Te3 nanoplates exhibit a direct band structure with a band gap energy of 2.394 eV at 7 K and an estimated free exciton binding energy of 150 meV. Polarized reflection measurements at different temperatures show anisotropy in the absorption coefficient due to an anisotropic orientation of the silicon dimers, which is in excellent agreement with theoretical calculations of the dielectric functions. Polarized Raman measurements of single Si2Te3 nanoplates at different temperatures reveal various vibrational modes, which agree with density functional perturbation theory calculations. The unique structural and optical properties of nanostructured Si2Te3 hold great potential applications in optoelectronics and chemical sensing.

Lattice and spin excitations of YFeO3 : A Raman and density functional theory study

We report a Raman scattering study of $\mathrm{Y}\mathrm{Fe}{\mathrm{O}}_{3}$ orthoferrite. Polarized Raman measurements in wide temperature range and first-principles calculations provided the assignment of the observed phonon modes to vibrational symmetries and atomic displacements. We did not observe previously reported anomalies in the behavior of phonon frequencies near the temperature of the antiferromagnetic transition. Temperature behavior of frequencies and linewidths of most phonons may be described by known expressions for cubic and quartic anharmonicity, which implies a significant contribution of phonon-phonon interaction. At the same time, several phonons exhibit asymmetric profiles and anomalies in the behavior of their widths, which indicates an interaction with a continuum of some excitations. The most striking evidence of this is the observation of the coupling of the ${B}_{2g}$ phonon at 640 ${\mathrm{cm}}^{\ensuremath{-}1}$ with the two-magnon continuum, expressed in anomalous phonon self-energies in the vicinity of ${T}_{N}$. The appearance of a broad continuum of low-frequency excitations when approaching ${T}_{N}$, which is possibly induced by the damping of spin excitations due to interaction with the lattice, has been revealed. This interaction can lead to additional contributions to the renormalization of phonon self-energies, formally described by formulas for anharmonic effects. Phonon scattering induced by defects has been identified. The temperature behavior of resonant second-order phonon and two-magnon scattering was studied in detail.

Anisotropic optical phonons in MOCVD grown Si-doped GaN/Sapphire epilayers

Micro-Raman spectroscopy is employed to study the anisotropic optical phonons of Si-doped GaN/Sapphire epifilms grown by metal organic chemical vapor deposition method. In an undoped 3.6 μm thick sample – our polarized Raman measurements in the backscattering geometry revealed major first order modes of GaN and sapphire. Careful analyses of the second-order Raman spectra using critical-point-phonons from a rigid-ion-model fitted inelastic X-ray spectroscopy data with appropriate selection rules helped us attain expedient data for the lattice dynamics of GaN. In Si-doped films, a modified phonon confinement model is used for simulating Raman line shapes of E2high phonons to monitor crystalline quality. While the optical phonons in lightly doped samples are coupled to electron plasma– at higher carrier concentration the over-damped A1(LO) mode vanished in the background. For each sample we assessed the transport parameters by simulating Raman profiles of A1(LO) line shape with contributions from plasmon-LO-phonon and Lorentzian shaped Eg sapphire mode. A realistic Green’s function theory is adopted to study the vibrational modes of Si donors and Mg acceptors in GaN by including force constant changes estimated from lattice relaxations using first-principles bond-orbital model. Theoretical results of impurity-activated modes compared favorably well with the existing Raman scattering data.