Angle-resolved Polarized Raman Spectroscopy Research Articles

Anisotropies of angle-resolved polarized Raman response identifying in low miller index β-Ga2O3 single crystal

Beta-phase gallium oxide (β-Ga2O3) with a monoclinic lattice structure is a research hotspot in the field of ultra-wide bandgap semiconductors in the world and displays intriguing anisotropic properties. Here, we synthesize β-Ga2O3 single-crystal (SC) using the edge-defined film-fed growth method and systematically investigate the anisotropic structural and vibrational properties. The anisotropic nature of β-Ga2O3 SC is revealed by angle-resolved polarized Raman spectroscopy under linearly polarized excitations, in which different vibration modes exhibit pronounced periodic variations in intensity. In the detection of different excitation wavelengths, not only the Raman intensity show significant change, but also the strongest main peak of the Raman modes also shows significant difference. In addition, the angular dependence of the peak intensities of the Ag(4), Ag(6), Ag(7), Ag(10) and Bg modes will also be affected by different low Miller index. All the experimental results above are beneficial to the understanding of inelastic light-scattering process of β-Ga2O3 SC.

Raman spectroscopy studies of black phosphorus

Black phosphorus has important applications in many fields such as optics, optoelectronics and thermals. Many of its excellent properties are related to its special anisotropy. In this work, we adopted Raman spectroscopy, which can obtain fast response optical signals without destroying the structure of the sample, to identify its crystal orientation and explore its thermal and SERS properties. We successfully distinguished the armchair and zigzag directions of black phosphorus by angle-resolved polarized Raman spectroscopy of Ag mode in a less studied orthogonal polarization configuration. Then we used temperature dependent Raman spectroscopy to study its thermal properties. It is found that the first order temperature coefficients of its three Raman vibration modes Ag1, B2g, and Ag2 are −0.0133 cm−1 K−1, −0.0232 cm−1 K−1 and −0.0229 cm−1 K−1, respectively for the 3.2 nm sample. Furthermore, we studied the surface enhancement effect of black phosphorus with different thicknesses as SERS substrates. We found that few-layer black phosphorus has better enhancement effects and its limit of detection for MB and CV are both 10-6M. The analytical enhancement factor of black phosphorus substrates on CV can achieve 1.2 × 103 by calculation. These methods can be extended to other similar two-dimensional materials.

Anomalous polarization pattern evolution of Raman modes in few-layer ReS2 by angle-resolved polarized Raman spectroscopy.

Low-symmetry of ReS2 has not only in-plane but also out-of-plane anisotropic light scattering, which is complicated, yet interesting with intrinsic strong electron-phonon coupling. In such a case, the Raman tensor also gets sophisticated with nine non-zero elements, which is layer-dependent for different Raman modes. Herein, we systematically investigated the polarization pattern evolution of both in-plane and out-of-plane Raman modes of few-layer ReS2 by angle-resolved polarized Raman spectroscopy. We found that in-plane Raman modes with less layer-dependence could be used to determine the crystal orientation (Re-chain direction) due to the weak electron-phonon interaction between layers. However, the out-of-plane and mixed vibration Raman modes demonstrate much evident layer-dependence due to the obvious electron-phonon interaction between layers. As such, the polarization patterns for the out-of-plane vibration Raman modes are distorted with layers in not only petal types but also maximum Raman intensity directions. This distortion is mainly due to the phase difference between Raman elements, which are complex values due to the near bandgap excitation laser. The results reveal that deep insights into anisotropy in low-symmetry two-dimensional materials could afford not only rich physics but also potential polarized optoelectronic devices.

Revealing the anisotropic phonon behaviours of layered SnS by angle/temperature-dependent Raman spectroscopy.

Tin sulfide (SnS), a IV-VI group layered compound, has attracted much attention because of its excellent thermoelectric properties along the crystallographic b-axis. However, there are few reports on the identification of its in-plane orientation. We observe a strong anisotropy of the in-plane Raman signal in bulk SnS. With the help of ab initio calculations, the vibrational symmetry of each observed Raman mode in the cleaved (00l)-plane is consistent with the experimental values. The angle-resolved polarized Raman spectroscopy, combined with electron backscattered diffraction technology, is utilized to systematically investigate the in-plane anisotropy of the phonon response and then determine the in-plane orientation. Furthermore, the temperature-dependent and laser-power-dependent Raman scattering analyses reveal that the adjacent layers in the SnS crystals show a relatively weak van der Waals interaction. These findings could provide much-needed experimental information for future applications related to the anisotropic transport properties of SnS single crystals.

A Polarization-Sensitive Self-Powered Photodetector Based on a p-WSe2/TaIrTe4/n-MoS2 van der Waals Heterojunction.

Polarization-sensitive photodetection is highly appealing considering its great important applications. However, the inherent in-plane symmetry of a material and the single structure of a detector hinder the further development of polarization detectors with high anisotropic ratios. Herein, we design a p-WSe2/TaIrTe4/n-MoS2 (p-Ta-n) heterojunction. As a type-II Weyl semimetal, TaIrTe4 with an orthorhombic structure has strong in-plane asymmetry, which is confirmed by angle-resolved polarized Raman spectroscopy and second-harmonic generation. Due to the specific structure of the p-Ta-n junction with two vertical built-in electric fields, the device obtains a broadband self-powered photodetection ranging from visible (405 nm) to telecommunication wavelength (1550 nm) regions. Further, an optimized device containing 50-70 nm-thick layered TaIrTe4 has been realized. What is more, high-resolution imaging of "T" based on the device with clear borders illustrates excellent stability of the device. Significantly, the photocurrent anisotropic ratio of the p-Ta-n detector can reach 9.1 under 635 nm light, which is more than eight times that of the best known TaIrTe4-based photodetector reported before. This p-Ta-n junction containing a type-II Weyl fermion semimetal can provide an effective approach toward highly polarization-sensitive and high-performance integrated broadband photodetectors.

Thickness-dependent in-plane anisotropy of GaTe phonons

Gallium Telluride (GaTe), a layered material with monoclinic crystal structure, has recently attracted a lot of attention due to its unique physical properties and potential applications for angle-resolved photonics and electronics, where optical anisotropies are important. Despite a few reports on the in-plane anisotropies of GaTe, a comprehensive understanding of them remained unsatisfactory to date. In this work, we investigated thickness-dependent in-plane anisotropies of the 13 Raman-active modes and one Raman-inactive mode of GaTe by using angle-resolved polarized Raman spectroscopy, under both parallel and perpendicular polarization configurations in the spectral range from 20 to 300 cm−1. Raman modes of GaTe revealed distinctly different thickness-dependent anisotropies in parallel polarization configuration while nearly unchanged for the perpendicular configuration. Especially, three Ag modes at 40.2 ({text{A}}_{text{g}}^{1}), 152.5 ({text{A}}_{text{g}}^{7}), and 283.8 ({text{A}}_{text{g}}^{12}) cm−1 exhibited an evident variation in anisotropic behavior as decreasing thickness down to 9 nm. The observed anisotropies were thoroughly explained by adopting the calculated interference effect and the semiclassical complex Raman tensor analysis.

Synthesis of ultrathin PdSe2 flakes for hydrogen evolution reaction

Two-dimensional (2D) palladium diselenide (PdSe2) has demonstrated great promise in electronics and optoelectronics due to its remarkable air stability, appealing carrier mobility, and controllable bandgap. However, realizing the synthesis of 2D PdSe2 remains still a daunting challenge. Herein, high-quality, ultrathin (∼2.2 nm) 2D PdSe2 flakes are achieved on mica substrates through a NaCl-assisted ambient-pressure chemical vapor deposition method. We systematically probe the crystal quality and optical characteristics in PdSe2 using scanning transmission electron microscopy, angle-resolved polarized Raman spectroscopy, and second harmonic generation characterizations. Impressively, the transferred PdSe2 flakes on Au foil, featuring lower overpotential of ∼ 150 mV at 10 mA/cm2, lower Tafel slope of ∼ 70 mV/dec, higher exchange current density of ∼ 85 μA/cm2, and superior stability, display outstanding electrocatalytic performance for hydrogen evolution reaction (HER). This work shows a promising prospect of PdSe2 in electrocatalysis for HER.

Centimeter-Scale Few-Layer PdS2: Fabrication and Physical Properties

To develop next-generation electronic devices, novel semiconductive materials are urgently required. The transition metal dichalcogenides (TMDs) hold the promise of next generation of semiconductor materials for emerging electronic applications. As a member of the group-10 TMDs, PdS2 has a notable layer-number-dependent band structure and tremendously high carrier mobility at room temperature. Here, we demonstrate the experimental realization of centimeter-scale synthesis of the few-layer PdS2 by the combination of physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods. For the first time, the optical anisotropic properties of the few-layer PdS2 were investigated through angle-resolved polarized Raman spectroscopy. Also, the evolution of Raman spectra was studied depending on the temperature in the range of 12-300 K. To further understand the electronic properties of the few-layer PdS2, the field-effect transistor (FET) devices were fabricated and investigated. The electronic measurements of such FET devices reveal that the PdS2 materials exhibit a tunable ambipolar transport mechanism with field-effect mobility of up to ∼388 cm2 V-1 s-1 and the on/off ratio of ∼800, which were not reported before in the literature. To well understand the experimental results, the electronic structure of PdS2 was determined using density functional theory (DFT) calculations. These excellent physical properties are very helpful in developing high-performance opto-electronic applications.

Morphotropic phase boundaries of (1−x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 probed by Raman spectroscopy at high temperature

Morphotropic phase boundaries (MPBs) in relaxor Pb(Zn1/3Nb2/3)O3–ferroelectric PbTiO3 systems were investigated by means of angle-resolved polarized Raman spectroscopy at 800 K in the paraelectric phase. The features of the spectra represented by contour maps were extracted through the multivariate curve resolution (MCR) method, rendering the comparison among the data with six compositions more accurate and convenient. The result indicates that the structure becomes unstable at x = 0.07, which is slightly smaller than the MPB occurring at x = 0.09. The additional analysis using the MCR method to investigate the x dependence of the extracted Raman spectra shows anomalies at the MPB. The information related to the MPB near room temperature could be extracted through Raman spectroscopy even in a simple high-temperature paraelectric phase.

Identification of vibrational mode symmetry and phonon anharmonicity in SbCrSe3 single crystal using Raman spectroscopy

Developing an understanding of the physics underlying vibrational phonon modes, which are strongly related to thermal transport, has attracted significant research interest. Herein, we report the successful synthesis of bulk SbCrSe3 single crystal and its thermal transport property over the temperature range from 2 to 300 K. Using angle-resolved polarized Raman spectroscopy (ARPRS) and group theory calculation, the vibrational symmetry of each observed Raman mode in the cleaved (00l) crystal plane of SbCrSe3 is identified for the first time, and then further verified through first-principles calculations. The ARPRS results of some Raman modes (e.g., A g 2 ∼64 cm−1 and A g 7 ∼185 cm−1) can be adopted to determine the crystalline orientation. More importantly, the temperature dependence of the lattice thermal conductivity (κL) is revealed to be more accurately depicted by the three-phonon scattering processes throughout the measured temperature range, substantiated by in-situ Raman spectroscopy analysis and the model-predicted κL. These results reveal the fundamental physics of thermal transport for SbCrSe3 from a completely new perspective and should thus ignite research interest in the thermal properties of other low-dimensional materials using the same strategy.

Laser Tuning in Layered h-BN Crystals.

Integrated optics shows great potential in the current optical communication systems, sensor technology, optical computers, and other fields. Tunable laser technology within a certain range is the key to achieving on-chip optical integration; to realize which, Raman scattering is a competitive method that can effectively transfer incident laser energy to optical phonons due to the photon-phonon interaction. Here, we take hexagonal boron nitride as the energy conversion medium, and based on the angle-resolved polarized Raman spectroscopy, it is found that when laser polarization vector ei ⊥ c axis, the spectrum obtains maximal scattering across the cross section and a minimal depolarization ratio. At room temperature, h-BN obtains an output signal with a wavelength of 522.8 nm and a full-width at half-maximum of 0.24 nm under the excitation of 488 nm pump laser, and the depolarization ratio is 0.09 (theoretically, it is 0, and this difference is due to experimental errors). And then, within the temperature range of 80∼420 K, the scattered light wavelength shows a high-precision shift of 0.006 nm/25 K, indicating that continuous wavelength tuning has been successfully achieved in h-BN.

Anisotropic Raman scattering and intense broadband second-harmonic generation in tellurium nanosheets.

Tellurium (Te) is a novel elementary material, which has recently attracted extensive attention due to its intriguing physical properties, such as topological, thermoelectric, and photoelectric properties. Further study on Te crystal structures will help to understand its properties and facilitate its application. Here, the angle-resolved polarized Raman spectroscopy has been employed to study Te crystal symmetry. Three different Raman vibration modes were obtained, each of which possess a different polarization dependence. Furthermore, it is revealed that Te nanosheets show a second-order harmonic response over a wide spectrum and have the greatest conversion efficiency at an excitation wavelength of 880 nm. Its second-order nonlinear susceptibility is estimated to be 2049pmV-1. This substantial nonlinear optical response endows Te nanosheets with the potential for developing nonlinear photonic and optoelectronic nanodevices with high efficiency.

Synthesis of 2D ternary layered manganese phosphorous trichalcogenides towards ultraviolet photodetection

Manganese phosphorous selenium (MnPSe3), as a representative of layered metal phosphorus trichalcogenides (MPTs), has gained significant attention due to its direct bandgap, high carrier mobility, large absorption coefficient, which indicate great potential in photoelectric application. Herein, high-quality two-dimensional (2D) MnPSe3 flakes were mechanically exfoliated from the corresponding bulk crystals synthesized by chemical vapor transport (CVT) methods. The systematic investigation was applied to the lattice vibrations of MnPSe3via angle-resolved polarized Raman spectroscopy (ARPRS), and the Raman vibration modes were determined based on Raman selection rules and crystal symmetry. Impressively, the photodetectors based on 2D MnPSe3 flakes exhibit excellent photoresponse to the ultraviolet light with a responsivity up to 22.7 A W−1 and a detectivity of 2.4 × 1011 Jones. The high performance in the ultraviolet range signifies that 2D MnPSe3 is expected to be a powerful candidate for future ultraviolet photodetection.

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.

2D layered SiP as anisotropic nonlinear optical material

Two-dimensional (2D) material of silicon phosphide (SiP) has recently been shown as a promising optical material with large band gap, fast photoresponse and strong anisotropy. However, the nonlinear optical properties of 2D SiP have not been investigated yet. Here, the thickness-dependent in-plane anisotropic third-harmonic generation (THG) from the mechanically exfoliated 2D layered SiP flakes is reported. The crystal orientation of the SiP flake is determined by the angle-resolved polarized Raman spectroscopy. The angular dependence of the THG emission with respect to the incident linear polarization is found to be strongly anisotropic with the two-fold polarization dependence pattern. Furthermore, the effect of the SiP flake thickness on the THG power is analyzed.

Flux Method Growth of Large Size Group IV–V 2D GeP Single Crystals and Photoresponse Application

Two-dimensional (2D) materials driven by their unique electronic and optoelectronic properties have opened up possibilities for their various applications. The large and high-quality single crystals are essential to fabricate high-performance 2D devices for practical applications. Herein, IV-V 2D GeP single crystals with high-quality and large size of 20 × 15 × 5 mm3 were successfully grown by the Bi flux growth method. The crystalline quality of GeP was confirmed by high-resolution X-ray diffraction (HRXRD), Laue diffraction, electron probe microanalysis (EPMA) and Raman spectroscopy. Additionally, intrinsic anisotropic optical properties were investigated by angle-resolved polarized Raman spectroscopy (ARPRS) and transmission spectra in detail. Furthermore, we fabricated high-performance photodetectors based on GeP, presenting a relatively large photocurrent over 3 mA. More generally, our results will significantly contribute the GeP crystal to the wide optoelectronic applications.

Femtometer atomic displacement, the root cause for multiferroic behavior of CuO unearthed through polarized Raman spectroscopy

Recently, CuO has been proposed as a potential multiferroic material with high transition temperature. Competing models based on spin current and ionic displacements are invoked to explain ferroelectricity in CuO. The theoretical model based on ionic displacement predicted very small displacement (∼10−5 Å) along the b axis. Experimentally detecting displacements of such a small amplitude in a particular direction is extremely challenging. Through our detailed angle resolved polarized Raman spectroscopy study on single crystal of CuO, we have validated the theoretical study and provided direct evidence of displacement along the b axis. Our study provides important contribution in the high temperature multiferroic compounds and showed for the first time, the use of the polarized Raman scattering in detecting ionic displacements at the femtometer scale.

Unambiguous determination of crystal orientation in black phosphorus by angle-resolved polarized Raman spectroscopy.

Angle-resolved polarized Raman spectroscopy (ARPRS) is widely used to determine the crystal orientations of anisotropic layered materials (ALMs), which is an essential step to study all of their anisotropic properties. However, the understanding of the ARPRS response of black phosphorous (BP) as a most widely studied ALM is still unsatisfactory. Here, we clarify two key controversies about the physical origin of the intricate ARPRS response and the determination of crystal orientations in BP. Through systematic ARPRS measurements, we show that the degree of anisotropy of the response evolves gradually and periodically with the BP thickness, eventually leading to the intricate response. Meanwhile, we find that using the Raman peak intensity ratio of the two Ag phonon modes, the crystal orientations of BP can be unambiguously distinguished via a concise inequality . Comprehensive analysis and first-principles calculations reveal that the external anisotropic interference effect and the intrinsic electron-phonon coupling are responsible for the observations.

Multivariate curve resolution for angle-resolved polarized Raman spectroscopy of ferroelectric crystals

Raman spectra from crystals contain a mixture of peaks from various phonon modes. Angle-resolved polarized Raman spectroscopy, in which numerous spectra are obtained with different polarization angles of incident and scattered lights, is widely used to separate peaks by considering the symmetries of the Raman tensor. In the present study, to efficiently process large amounts of spectral data, we apply multivariate curve resolution (MCR) to Raman spectra from a typical ferroelectric PbTiO3 crystal to reduce the number of spectra to be fitted. The spectra to fit are thus reduced to three, and their fits determine all the phonon parameters, such as frequency shift, width, and integrated intensity. The angle dependence of the integrated intensity was determined by multiplying the integrated intensity of the three peaks by the angle profiles obtained by MCR. Thus, angle-resolved polarized Raman spectroscopy combined with MCR provides an efficient way to determine phonon parameters.

Raman Tensor of WSe2 via Angle-Resolved Polarized Raman Spectroscopy

Tungsten selenide (WSe2) as a van der Waals layered crystal has anisotropic light absorption and light scattering on its out-of-plane surface. However, the Raman tensor of WSe2, which determines the anisotropy of inelastic light scattering, is still unknown without being carefully studied. Here, via establishing angle-resolved polarized Raman spectroscopy measurement, we obtained the experimental Raman tensor of WSe2. Additionally, theoretical Raman tensors under different incident laser line excitations from ultraviolet to infrared were calculated via the first-principles density functional approach, coinciding fairly well with the experimental ones. The first-principles results also reveal that WSe2 has different Raman tensor forms under different laser line excitations, especially the phase difference between Raman tensor elements of the A1g mode. It is suggested that the photon-energy dependent phase differences are determined by the dispersion and absorption of WSe2 on different pump lights.