Angle-resolved Polarized Raman Spectroscopy Research Articles

Linear Dichroism Transition and Polarization-Sensitive Photodetector of Quasi-one-dimensional Palladium Bromide

Abstract Perpendicular optical reversal of the linear dichroism transition has promising applications in polarization sensitive optoelectronic devices. We perform a systematical study on the in-plane optical anisotropy of quasi one-dimensional PdBr2 by using combined measurements of the angle resolved polarized Raman spectroscopy (ARPRS) and anisotropic optical absorption spectrum. The analyses of ARPRS data validate the anisotropic Raman properties of the PdBr2 flake. And anisotropic optical absorption spectrum of PdBr2 nanoflake demonstrates distinct optical linear dichroism reversal. Photodetector constructed by PdBr2 nanowire exhibits high responsivity of 747 A W-1 and specific detectivity of 5.8×1012 Jones. And the photodetector demonstrates prominent polarization-sensitive photoresponsivity under 405 nm light irradiation with large photocurrent anisotropy ratio of 1.56, which is superior to those of most of previously reported quasi one-dimensional counterparts. Our study offers fundamental insights into the strong optical anisotropy exhibited by PdBr2, establishing it as a promising candidate for miniaturization and integration trends of polarization-related applications.

Abnormal Linear Dichroism in Anisotropic Van der Waals Crystal CrOCl Revealed by Phase‐Dependent Polarized Raman Scattering

AbstractWithin angle‐resolved polarized Raman spectroscopy (ARPRS) experiments, ensuring accurate configurations is crucial for obtaining reliable results, especially when dealing with anisotropic materials like CrOCl with anisotropic crystal structure and phonon properties. However, comprehensive understanding of phase‐dependent ARPRS and the calibration of phase angle on anisotropic phonon modes from a crystallographic perspective are still lacking. Herein, detailed investigation on phase‐dependent ARPRS and looked into the anisotropic photo‐phonon interaction in CrOCl through in situ ARPRS is conducted. The Raman tensors and crystal orientation are acquired under two typical configurations. Phase‐dependent ARPRS are thoroughly analyzed for the calibration of analyzer deviation. The anomalous anisotropic response in phase‐dependent ARPRS is closely related to the intrinsic linear dichroism (LD) effect in CrOCl, which is further confirmed by angle‐resolved polarization absorption. Moreover, the strong localized rules for various parallel energy bands of CrOCl have generated anomalous LD conversion effect near 470 nm, which expands the potential application of CrOCl in polarization‐wavelength selected devices. This findings can offer in‐depth comprehension on the anisotropic phonon physics of CrOCl and a feasible approach to utilize phase‐dependent ARPRS for probing the undiscovered abundant photo‐induced effects in low‐symmetry 2D materials.

Polarized Raman scattering and magnetic ordering in Mn lightly doped van der Waals Cr1−xMnxGeTe3 crystals

A layered van der Waals ferromagnet CrGeTe3 has attracted considerable interest due to the great potential for exploring low-power functional spintronic devices. However, research on the incorporation of chemical substitution in CrGeTe3 to tune the lattice vibrations and magnetic interactions is still lacking. In this work, an understanding of the phonon property and magnetic orders in Mn lightly doped Cr1−xMnxGeTe3 single crystals has been reported. It is found that the phonon symmetries are preserved over the studied Mn substitution content, as confirmed by angle-resolved polarized Raman spectroscopy and first-principles calculations. Furthermore, clear signatures of improved ferromagnetic Curie–Weiss temperature and magnetic moment are observed by detailed magnetic measurements. The higher magnetic moment of Mn ions, the variation of Mn–Te–Cr angle, Mn-induced hole doping, and the concomitant strain effect may all contribute to the enhanced ferromagnetism. Overall, our results demonstrate that chemical substitution is an effective pathway to engineering the lattice and magnetism of CrGeTe3.

In-Plane Anisotropy in the Layered Topological Insulator Ta2Ni3Te5 Investigated via TEM and Polarized Raman Spectroscopy.

Layered Ta2M3Te5 (M = Pd, Ni) has emerged as a platform to study 2D topological insulators, which have exotic properties such as spin-momentum locking and the presence of Dirac fermions for use in conventional and quantum-based electronics. In particular, Ta2Ni3Te5 has been shown to have superconductivity under pressure and is predicted to have second-order topology. Despite being an interesting material with fascinating physics, the detailed crystalline and phononic properties of this material are still unknown. In this study, we use transmission electron microscopy (TEM) and polarized Raman spectroscopy (PRS) to reveal the anisotropic properties of exfoliated few-layer Ta2Ni3Te5. An electron diffraction and TEM study reveals structural anisotropy in the material, with a preferential crystal orientation along the [010] direction. Through Raman spectroscopy, we discovered 15 vibrational modes, 3 of which are ultralow-frequency modes, which show anisotropic response with sample orientation varying with the polarization of the incident beam. Using angle-resolved PRS, we assigned the vibrational symmetries of 11 modes to Ag and two modes to B3g. We also found that linear dichroism plays a role in understanding the Raman signature of this material, which requires the use of complex elements in the Raman tensors. The anisotropy of the Raman scattering also depends on the excitation energies. Our observations reveal the anisotropic nature of Ta2Ni3Te5, establish a quick and nondestructive Raman fingerprint for determining sample orientation, and represent a significant advance in the fundamental understanding of the two-dimensional topological insulator (2DTI) Ta2Ni3Te5 material.

Thermal transport and optical anisotropy in CVD grown large area few-layer MoS2 over an FTO substrate

Atomically thin MoS2 is a promising candidate for its integration into devices due to its strikingly unique electronic, optical, and thermal properties. Here, we report the fabrication of a few-layer MoS2 thin film over a conducting fluorine-doped tin oxide-coated glass substrate via a one-step chemical vapor deposition method. We have quantitatively analyzed the nonlinear temperature-dependent Raman shift using a physical model that includes thermal expansion and three- and four-phonon anharmonic effects, which exhibits that the main origin of nonlinearity in both the phonon modes primarily arises from the three-phonon anharmonic process. We have also measured the interfacial thermal conductance (g) and thermal conductivity (ks) of the synthesized film using the optothermal Raman spectroscopy technique. The obtained values of g and ks are ∼7.218 ± 0.023 MW m−2 K−1 and ∼40 ± 2 W m−1 K−1, respectively, suggesting the suitability of thermal dissipation in MoS2 based electronic and optoelectronic devices. Furthermore, we performed a polarization study using the angle resolved polarized Raman spectroscopy technique under non-resonance and resonance excitations to reveal the electron–photon–phonon interaction in the prepared MoS2, based on the semi-classical theory that includes deformation potential and Fröhlich interaction. Our study provides much needed experimental information about thermal conductivity and polarization response in a few-layer MoS2 grown over the conducting substrate, which is relevant for applications in low power thermoelectric and optoelectronic devices.

Optical properties of ferroic Fe2O(SeO3)2 and Fe2(SeO3)3·3H2O.

Ferroic compounds Fe2O(SeO3)2 (FSO) and Fe2(SeO3)3·3H2O (FSOH) prepared by the hydrothermal method are characterized and their optical properties are investigated by combining with first-principles calculations. The results show that (i) FSO is antiferromagnetic below ∼110 K and becomes ferromagnetic at elevated temperatures, while FSOH is antiferromagnetic at low temperatures probably due to a change in the spin state from Fe3+ (S = 5/2) to Fe2+ (S = 2); (ii) the optical bandgap is determined to be ∼2.83 eV for FSO and ∼2.15 eV for FSOH, consistent with the theoretical calculation; and (iii) the angle-resolved polarized Raman spectroscopy results of both crystals demonstrate the strong anisotropic light absorption and birefringence effects, and the unconventional symmetricity of some Raman modes is observed, which can be interpreted from the variation of Raman scattering elements. This work can provide not only an understanding of the structure and physical properties of iron selenites, but also a strategy for exploring the anomalous Raman behaviors in anisotropic crystals, facilitating the design and engineering of novel functional devices with low-symmetry ferroic materials.

Polarization‐Sensitive Photodetectors Based on Highly In‐Plane Anisotropic Violet Phosphorus with Large Dichroic Ratio

AbstractAnisotropic 2D layered materials with distinctive anisotropic electronic band structures and optical response are promising for practical applications in polarization imaging and sensors. Nonetheless, many of the currently reported polarized photodetectors based on anisotropic 2D materials are constrained by the low polarization responsivity and low linear dichroism ratio. Here a high‐performance polarization‐sensitive photodetector based on the novel in‐plane anisotropy 2D violet phosphorus (VP) crystal is reported. The angle‐resolved polarized Raman spectroscopy (ARPRS) study and the anisotropic electrical transport measurements revealed the highly in‐plane anisotropy phonon vibration and electrical properties of VP crystal. The anisotropic ratio of electron mobility is calculated to be 2.7 at room temperature. Moreover, a polarization‐sensitive photodetector based on the VP nanosheet shows high photoresponsivity of up to 341 A W−1 and a linearly dichroic ratio of up to 3.9, which is much larger than most other anisotropic 2D materials. These results indicate that the anisotropic VP crystal would be suitable for polarization‐sensitive photodetectors, which leads to a promising perspective for future novel electronic and optoelectronic devices.

The Spatial Correlation and Anisotropy of β-(AlxGa1-x)2O3 Single Crystal.

The long-range crystallographic order and anisotropy in β-(AlxGa1-x)2O3 (x = 0.0, 0.06, 0.11, 0.17, 0.26) crystals, prepared by optical floating zone method with different Al composition, is systematically studied by spatial correlation model and using an angle-resolved polarized Raman spectroscopy. Alloying with aluminum is seen as causing Raman peaks to blue shift while their full widths at half maxima broadened. As x increased, the correlation length (CL) of the Raman modes decreased. By changing x, the CL is more strongly affected for low-frequency phonons than the modes in the high-frequency region. For each Raman mode, the CL is decreased by increasing temperature. The results of angle-resolved polarized Raman spectroscopy have revealed that the intensities of β-(AlxGa1-x)2O3 peaks are highly polarization dependent, with significant effects on the anisotropy with alloying. As the Al composition increased, the anisotropy of Raman tensor elements was enhanced for the two strongest phonon modes in the low-frequency range, while the anisotropy of the sharpest Raman phonon modes in the high-frequency region decreased. Our comprehensive study has provided meaningful results for comprehending the long-range orderliness and anisotropy in technologically important β-(AlxGa1-x)2O3 crystals.

Identification of phonon symmetry and spin-phonon coupling in van der Waals antiferromagnetic FePSe3

For engineering of van der Waals antiferromagnetic material FePSe3 and expanding its potential technological promise in spintronic fields, a detailed knowledge of the underlying physics of vibrational phonon modes and their coupling with the spin degree of freedom are undoubtedly needed. In this work, by angle-resolved polarized Raman spectroscopy in corroboration with first-principles calculation, the Raman modes of bulk FePSe3 have been rationally assigned, which clarify the ambiguity about symmetry identification of some Raman-active modes in previous studies. Moreover, through detailed temperature-dependent Raman scattering experiments, the abnormal shift of the frequency, linewidth, and integrated intensity across the Néel temperature have been identified for phonon modes associating with a non-magnetic [P2Se6]4- cluster. This can be well explained with the assistance of the spin-phonon coupling mechanism that involves the variation of the d electron transfer with lattice vibration in magnetic materials. Our results are helpful for uncovering the rich physics in FePSe3 and also for enriching the further understanding of magnetic van der Waals materials down to the 2D limit.

Optical Effect Modulation in Polarized Raman Spectroscopy of Transparent Layered α-MoO3.

Optical anisotropy, which is quantified by birefringence (Δn) and linear dichroism (Δk), can significantly modulate the angle-resolved polarized Raman spectroscopy (ARPRS) response of anisotropic layered materials (ALMs) by external interference. This work studies the separate modulation of birefringence on the ARPRS response and the intrinsic response by selecting transparent birefringent crystal α-MoO3 as an excellent platform. It is found that there are several anomalous ARPRS responses in α-MoO3 that cannot be reproduced by the real Raman tensor widely used in non-absorbing materials; however, they can be well explained by considering the birefringence-induced Raman selection rules. Moreover, the systematic thickness-dependent study indicates that birefringence modulates the ARPRS response to render an interference pattern; however, the amplitude of modulation is considerably lower than that by linear dichroism as occurred in black phosphorous. This weak modulation brings convenience to the crystal orientation determination of transparent ALMs. Combining the atomic vibrational pattern and bond polarizability model, the intrinsic ARPRS response of α-MoO3 is analyzed, giving the physical origins of the Raman anisotropy. This study employs α-MoO3 as an example, although it is generally applicable to all transparent birefringent ALMs.

Strong Anisotropic Two-Dimensional In2Se3 for Light Intensity and Polarization Dual-Mode High-Performance Detection

Detecting the light from different freedom is of great significance to gain more information. Two-dimensional (2D) materials with low intrinsic carrier concentration and highly tunable electronic structure have been considered as the promising candidate for future room-temperature multi-functional photodetectors. However, current investigations mainly focus on intensity-sensitive detection; the multi-dimensional photodetection such as polarization-sensitive photodetection is still in its early stage. Herein, the intensity- and polarization-sensitive photodetection based on α-In2Se3 is studied. By using angle-resolved polarized Raman spectroscopy, it is demonstrated that α-In2Se3 shows an anisotropic phonon vibration property indicating its asymmetric structure. The α-In2Se3-based photodetector has a photoelectric performance with a responsivity of 1936 A/W and a specific detectivity of 2.1 × 1013 Jones under 0.2 mW/cm2 power density at 400 nm. Moreover, by studying the polarized angle-resolved photoelectrical effect, it is found that the ratio of maximum and minimum photocurrent (dichroic ratio) reaches 1.47 at 650 nm suggesting good polarization-sensitive detection. After post-annealing, α-In2Se3 in situ converts to β-In2Se3 which has similar in-plane anisotropic crystallinity and exhibits a dichroic ratio of 1.41. It is found that the responsivity of β-In2Se3 is 6 A/W, much lower than that of α-In2Se3. The high-performance light intensity- and polarization-detection of α-In2Se3 enlarges the 2D anisotropic materials family and provides new opportunities for future dual-mode photodetection.

F4-TCNQ on Epitaxial Bi-Layer Graphene: Concentration- and Orientation-Dependent Charge Transfer at the Interface

Bi-layer epitaxial graphene (BLG) on 6H-SiC(0001) (EG/SiC) was grown and modified by thermal deposition of the molecular electron acceptor tetrafluoro-tetra cyano quinodimethane (F4-TCNQ). The surface-modified system, F4-TCNQ/EG/SiC, was studied by X-ray photoelectron spectroscopy (XPS) and angle-resolved polarized Raman spectroscopy (ARPRS). XPS results indicate that bonding of deposited F4-TCNQ molecules depends on their concentration. Although bonding through the cyano groups is present at all concentrations, charge transfer from graphene to fluorine is evident only at sub-monolayer concentrations. The corresponding change in bond character is coupled with a change in molecular orientation. Raman spectroscopy not only provides results consistent with the findings from the XPS study but also reveals a significant degree of molecular stacking above the monolayer concentration. Thus, both the variation of the acceptor concentration and the number of graphene layers provide further handles to manipulate charge and doping that may be useful in device applications.

Misfit-generated structural and optical anisotropies of the natural MoS2−PbS van der Waals heterostructure merelaniite

Significant efforts developing ${\mathrm{MoS}}_{2}\text{\ensuremath{-}}\mathrm{PbS}$-based heterostructure devices with several different architectures show promise for photosensor, solar cell, and chemical sensor applications. Merelaniite $({\mathrm{Mo}}_{4}{\mathrm{Pb}}_{4}{\mathrm{VSbS}}_{15})$ is a newly discovered natural van der Waals heterostructure of the cylindrite type, composed predominantly of heavily modulated pseudotetragonal PbS layers and pseudohexagonal ${\mathrm{MoS}}_{2}$ layers with large misfit-induced anisotropy. For an incommensurate modulated structure, the refined structural model from single-crystal x-ray diffraction analysis is in reasonable agreement with the results obtained by high-resolution scanning transmission electron microscopy, especially in light of the fact that that the two isolated single-crystal domains used for the x-ray and electron diffraction experiments were extracted from two different whiskers and subjected to different sample preparation methods. The effects of the misfit-induced structural anisotropy are studied using angle-resolved polarized Raman spectroscopy. The intensities of 12 Raman modes are studied as a function of incident polarization angle relative to merelaniite's whisker axis, and show maximal intensity with the polarization direction perpendicular to the whisker axis. Polarization-dependent anisotropic third-harmonic generation from ultrathin mechanically exfoliated flakes reveals the anisotropy of the third-order nonlinear susceptibility tensor. Merelaniite demonstrates an expanded structure-chemistry space for engineering stable layered materials for potential device applications.

Identifying the crystal orientation of epitaxially grown MoO2 nanoflakes on c-sapphire

Identifying the crystallographic orientations and understanding the growth mechanism of MoO2 nanocrystals are important for device applications. Here, we investigate the effect of precursor concentrations in chemical vapor deposition on the orientation of MoO2 nanoflakes with respect to the substrate surface. X-ray diffraction, optical microscopy, (tilted) scanning electron microscopy, high resolution transmission electron microscopy, selected area electron diffraction, and angle-resolved polarized Raman spectroscopy were used to determine the crystal orientations of the nanoflakes and their epitaxial relationship with c-sapphire substrates: lying-down MoO2 nanoflakes for MoO2(010)∥sapphire(0001) and MoO2 <001>//sapphire<11¯00>, and standing-up ones for MoO2(100)∥sapphire(0001) and MoO2 <001>∥sapphire<11¯00>. Tentative atomic epitaxial models are proposed to explain the orientation and alignment. The investigations also demonstrated that angle-resolved polarized Raman spectroscopy is a quick and nondestructive method to identify the crystallographic orientations of nanoflakes.

Excitation-Dependent Anisotropic Raman Response of Atomically Thin Pentagonal PdSe2.

The group-10 noble-metal dichalcogenides have recently emerged as a promising group of two-dimensional materials due to their unique crystal structures and fascinating physical properties. In this work, the resonance enhancement of the interlayer breathing mode (B1) and intralayer Ag 1 and Ag 3 modes in atomically thin pentagonal PdSe2 were studied using angle-resolved polarized Raman spectroscopy with 13 excitation wavelengths. Under the excitation energies of 2.33, 2.38, and 2.41 eV, the Raman intensities of both the low-frequency breathing mode B1 and high-frequency mode Ag 1 of all the thicknesses are the strongest when the incident polarization is parallel to the a axis of PdSe2, serving as a fast identification of the crystal orientation of few-layer PdSe2. We demonstrated that the intensities of B1, Ag 1, and Ag 3 modes are the strongest with the excitation energies between 2.18 and 2.38 eV when the incident polarization is parallel to PdSe2 a axis, which arises from the resonance enhancement caused by the absorption. Our investigation reveals the underlying interplay of the anisotropic electron-phonon and electron-photon interactions in the Raman scattering process of atomically thin PdSe2. It paves the way for future applications on PdSe2-based optoelectronics.

One-Step Growth of Bilayer 2H-1T' MoTe2 van der Waals Heterostructures with Interlayer-Coupled Resonant Phonon Vibration.

2H-1T' MoTe2 van der Waals heterostructures (vdWHs) have promising applications in optoelectronics due to a seamlessly homogeneous semiconductor-metal coupled interface. However, the existing methods to fabricate such vdWHs involved complicated steps that may deteriorate the interfacial coupling and are also lacking precise thickness control capability. Here, a one-step growth method was developed to controllably grow bilayer 2H-1T' MoTe2 vdWHs in the small growth window overlapped for both phases. Atomic-resolution low-voltage transmission electron microscopy shows the distinct moiré patterns in the bilayer vdWHs, revealing the epitaxial nature of the top 2H phase with the lattice parameters regulated by the underneath 1T' phase. Such epitaxially stacked bilayer vdWHs modulate the interlayer coupling by resonating their vibration modes, as unveiled by the angle-resolved polarized Raman spectroscopy and first-principles calculations.

Single-crystalline transition metal phosphide superconductor WP studied by Raman spectroscopy and first-principles calculations

We present a study of phonon modes and electron-phonon coupling in superconductor WP using angle-resolved polarized Raman spectroscopy. Seven out of twelve Raman-active modes are detected, with frequencies in good accordance by the first-principles calculations. We analyze parallel- and cross-polarized configurations of ${\mathrm{A}}_{\mathrm{g}}$ and ${\mathrm{B}}_{3\mathrm{g}}$ phonon modes with polarization- and angle-resolved Raman measurements. For the observed seven modes, the total electron-phonon coupling parameter $\ensuremath{\lambda}$ is only about 0.07, which is too small to achieve superconductivity. Our results elucidate that the low-frequency phonon modes play a key role in the superconductivity within the Bardeen-Cooper-Schrieffer theory framework.

Comparative Raman spectroscopy of magnetic topological material EuCd2X2 (X = P, As)

EuCd2X2 (X = P, As) is a new class of magnetic topological materials discovered recently. The electronic structure and the band topology are intimately coupled with its magnetism, giving rise to interesting properties such as spin fluctuation and colossal magnetoresistance. Phonon excitation can contribute to the quasi-particle response of the topological matters through spin-lattice and electron–phonon coupling. However, the phonon properties of this material family remain unexplored. Here we report a comparative study of Raman-active vibration modes in EuCd2X2 (X = P, As) by means of angle-resolved, temperature-resolved, and magnetic-field-resolved Raman spectroscopy together with the first-principle calculations and Raman tensor analysis. The phonon properties can be tuned by chemical potential and temperature within the material family. All the phonon modes are softened with increased chemical pressure by replacing P with As. Angle-resolved polarized Raman spectroscopy reveals the configuration-sensitive Raman activity and the isotropic intensity response. In addition, the magneto-Raman spectrum indicates the stability of Raman-active vibration modes against the magnetic field at room temperature. Our work sheds light on the phonon dynamics of magnetic topological matters, which are potentially coupled with the topological charge and spin excitation.

Wide-spectrum polarization-sensitive and fast-response photodetector based on 2D group IV-VI semiconductor tin selenide

Tin selenide (SnSe) has attracted considerable interest recently on account of its low-symmetry lattice structure, great compatibility with key semiconductor technology, and remarkable electrical and optical performance. SnSe-based polarization-sensitive photodetectors show promising application prospects because of their fast response and excellent photoelectric performance. Here, an in-plane anisotropic SnSe nanosheet was synthesized and reported in detail by applying angle-resolved polarized Raman spectroscopy (ARPRS), polarization-resolved optical microscopy(PROM), angle-resolved optical absorption spectroscopy (AROAS), and other crystal structure characterization methods. Moreover, SnSe crystals exhibit superior polarization detection performance with a high anisotropic photocurrent ratio (2.31 at 1064 nm) due to the structure formed by the Van der Waals superposition of covalently bonded atomic layers. Furthermore, SnSe-based photodetectors have high responsivity (9.27 A/W), high detectivity (4.08 × 1010 Jones), and fast response (in the order of nanoseconds). These results suggest a new method for fabricating 2D fast-response polarization-sensitive photodetectors in the future.

Zenith-angle resolved polarized Raman spectroscopy of graphene

Angle resolved polarized Raman (ARPR) spectroscopy has been widely used to study basic properties of two-dimensional materials (2DMs), such as underlying symmetry, mode assignment, crystallographic orientation and optical anisotropy. The substrate effect has never been uncovered in ARPR spectroscopy. In this work, we investigated zenith-angle resolved polarized Raman (ZRPR) spectra of the G mode of graphene (1LG) deposited on different substrates as well as graphite under in-plane and out-of-plane configurations. In contrast to the independent behavior in normal incidence geometry, the G mode intensity exhibits obvious zenith-angle dependence. In particular, this polarization behavior is sensitive to the underlying substrates underneath 1LG. The ZRPR intensity of the G mode can be well understood by a model considering both Raman selection rule and zenith-angle resolved interference effect. This work enriches the understanding of polarized Raman spectroscopy of 2DMs and the approach can be applicable to other 2DMs, especially in-plane anisotropic 2DMs.