Polarization-dependent Raman Spectroscopy Research Articles

Surface Phonons and Possible Structural Phase Transition in a Topological Semimetal PbTaSe2

AbstractTopological insulators are characterized by protected gapless surface or edge states but insulating bulk states which is due to the presence of spin‐orbit interactions and time‐reversal symmetry. Here, an in‐depth investigation of a topological nodal line semimetal PbTaSe2 via temperature, polarization dependent Raman spectroscopy, and temperature dependent single crystal X‐ray diffraction (SC‐XRD) measurements is reported. The analysis shows signature of electron‐phonon coupling as reflected in the Fano asymmetry in line shape of M1‐M4 modes and anomalous temperature variation of line‐width of P3‐P4 modes. Further polarization dependent phonon symmetry changes at different temperature (6K and 300K), discontinuities in bulk phonon dynamics for P2‐P5 modes, and disappearance of phonon modes, i.e., M1‐M5, on decreasing temperature indicates toward a thermally induced structural phase transition which is also supported by the SC‐XRD results. Hence based on the findings, it is proposed that M1‐M4 modes are surface phonon modes, the material undergoes a thermally induced structural phase transition from α to β phase at Tα→β ≈ 150 K or is in close proximity to the β phase and another transition below TCDW+β ≈ 100K which is possibly due to the interplay of remanent completely commensurate charge density wave (CCDW) of 1H‐TaSe2 and β phase.

Infrared Optical Anisotropy in Quasi‐1D Hexagonal Chalcogenide BaTiSe3

AbstractPolarimetric infrared (IR) detection bolsters IR thermography by leveraging the polarization of light. Optical anisotropy, i.e., birefringence and dichroism, can be leveraged to achieve polarimetric detection. Recently, giant optical anisotropy is discovered in quasi‐1D narrow‐bandgap hexagonal perovskite sulfides, A1+xTiS3, specifically BaTiS3 and Sr9/8TiS3. In these materials, the critical role of atomic‐scale structure modulations in the unconventional electrical, optical, and thermal properties raises the broader question of the nature of other materials that belong to this family. To address this issue, for the first time, high‐quality single crystals of a largely unexplored member of the A1+xTiX3 (X = S, Se) family, BaTiSe3 are synthesized. Single‐crystal X‐ray diffraction determined the room‐temperature structure with the P31c space group, which is a superstructure of the earlier reported P63/mmc structure. The crystal structure of BaTiSe3 features antiparallel c‐axis displacements similar to but of lower symmetry than BaTiS3, verified by the polarization dependent Raman spectroscopy. Fourier transform infrared (FTIR) spectroscopy is used to characterize the optical anisotropy of BaTiSe3, whose refractive index along the ordinary (E ⊥ c) and extraordinary (E ‖ c) optical axes is quantitatively determined by combining ellipsometry studies with FTIR. With a giant birefringence Δn ∼ 0.9, BaTiSe3 emerges as a new candidate for miniaturized birefringent optics for mid‐wave infrared to long‐wave infrared imaging.

In situ tracking anisotropic photocarrier dynamics in two-dimensional ternary Ta2NiSe5 via digital micromirror device-based pump-probe microscopy

In two-dimensional (2D) material studies, tracking the anisotropic ultrafast carrier dynamics is essential for the development of optoelectronic nano-devices. Conventionally, the anisotropic optical and electronic properties are investigated via either polarization-dependent Raman spectroscopy or field-effect transistors measurements. However, study of the anisotropic transient carrier behaviors is still challenging, due largely to the lack of picosecond-resolved acquisition or programmable scanning capabilities in the current characterization systems. In this work, we select Ta2NiSe5 as a model system to investigate the ultrafast anisotropic transportation properties of photo-excited carriers and transient polarized responses via a digital micromirror device (DMD)-based pump-probe microscope, where the probe beam scans along the armchair and zigzag directions of a crystal structure via binary holography to obtain distinct carrier diffusion coefficients, respectively. The results reveal the nonlinear diffusion behaviors of Ta2NiSe5 in tens of picoseconds, which are attributed to the interplay between excited electrons and phonons. The trend of the measured local polarization-dependent transient reflectivity is consistent with the polarized Raman spectra results. These results show that the DMD-based pump-probe microscope is an effective and versatile tool to study the optoelectronic properties of 2D materials.

Raman scattering spectroscopy of MBE grown thin film topological insulator Bi2-xSbxTe3-ySey.

BSTS epitaxial thin film topological insulators were grown using the MBE technique on two different types of substrates i.e., Si (111) and SiC/graphene with Bi0.7Sb1.6Te1.8Se0.9 and Bi0.9Sb1.5Te1.8Se1.1, respectively. The crystallographic properties of BSTS films were investigated via X-ray diffraction, which showed the strongest reflections from the (0 0 l) facets corresponding to the rhombohedral phase. Superior epitaxial growth, homogeneous thickness, smooth surfaces, and larger unit cell parameters were observed for the films grown on the Si substrate. Polarization dependent Raman spectroscopy showed a weak appearance of the Ag mode in cross--polarized geometry. In contrast, a strong Eg mode was observed in both parallel and cross-polarized geometries which correspond to the rhombohedral crystal symmetry of BSTS films. A redshift of Ag and Eg modes was observed in the Raman spectra of BSTS films grown on the Si substrate, compared to those on SiC/graphene, which was directly associated with the unit cell parameter and composition of the films. Raman spectra showed four fundamental modes with asymmetric line shape, and deconvolution of the peaks resulted in additional modes in both the BSTS thin films. The sum of relative ratios of linewidths of fundamental modes (Ag and Eg) of BSTS films grown on Si substrate was lower, indicating a more ordered structure with lower contribution of defects as compared to BSTS film grown on SiC/graphene substrate.

Phonon dynamics in lead free perovskite (1-x)KNN-xBAN (x = 0.0–0.1): a temperature dependent raman study

K0.5Na0.5NbO3 (KNN) has been under extensive focus in recent times due to it being an alternative to lead-free multifunctional materials and properties like room temperature ferroelectricity, stability in air and technology friendly applications. In this work, we report a comprehensive temperature dependent, compositional change induced structural variation and polarization dependent Raman spectroscopy on BaAl0.5Nb0.5O3 (BAN) doped KNN in the spectral range of 10–1000 cm−1. Multiple phase transitions are marked by the strong renormalization of the phonon self-energy parameters, i.e. mode frequencies and linewidths, in the temperature range of 83 K to 623 K. Change in the phase transition temperature is tracked via phonon anomalies with varying doping concentration, x = 0.0, 0.02, 0.05, 0.07 and 0.1, and is found to be as large as ∼100 K for orthorhombic to tetragonal phase transition for x = 0.1. Raman extracted phase diagram shows that the stability of the ferroelectric orthorhombic phase in the vicinity of room temperature increases with the increasing doping concentration. Also, from our polarization-dependent measurements we could decipher the symmetry of the observed phonon modes for KNN system.

Anisotropic phonon dispersion and optoelectronic properties of few-layer HfS2

Few layer HfS2flakes are studied by temperature, excitation and polarization-dependent Raman spectroscopy. In contrast, the fabricated device shows considerable photodetection properties under light laser power illumination.

Optical study on crystal symmetry of two-dimensional WTe<sub>2</sub>

Two-dimensional WTe<sub>2</sub> possesses a special crystal symmetry, leading to novel properties such as quantum spin Hall effect and nonlinear Hall effect. Determining the details of its crystal structure is essential for understanding these interesting properties. Here, we report an optical study on the crystal symmetry of monolayer, bilayer, and trilayer WTe<sub>2</sub>, using temperature and polarization dependent Raman spectroscopy and optical second harmonic generation (SHG). We find that monolayer WTe<sub>2</sub> is noncentrosymmetric as indicated by its sizable SHG, in contrast to the commonly believed centrosymmetric 1<i>T'</i> structure. The polarization dependence of the SHG is consistent with the <i>C</i><sub>s</sub> point group. Bilayer WTe<sub>2</sub> exhibits SHG signal more than one order of magnitude higher than in the monolayer and trilayer samples, with its temperature dependence reflecting the ferroelectric phase transition, evidencing strong inversion symmetry breaking induced by layer stacking and interlayer-sliding ferroelectricity. We also observe prominent second-order resonant Raman scattering peaks only in monolayer and bilayer WTe<sub>2</sub>, but not in thicker samples, and their temperature dependence indicates an electronic structure highly sensitive to interlayer coupling. These results will be useful for further exploring the properties of atomically thin WTe<sub>2</sub>.

CVD growth of layered Cr2O3 hexagonal flakes for optoelectronic applications

Chromium (III) oxide (Cr2O3) is a unique material which possesses a layered structure in the corundum structure phase, quite similar to the graphene and other transition metal dichalcogenides (TMDCs). Systematically, layered 2D-Cr2O3 hexagonal flakes were synthesized by chemical vapor deposition (CVD) method with Iodine as an assisting material. Under the controlled conditions (temperature, N2:H2 in carrier gas, etc), Iodine is playing a key role in transforming the crystal structure into hexagonal phase and grows in the form of layers. Incident light polarization dependent Raman spectroscopy was employed to see the coupling of in-plane Eg mode with spin to predict the anti-ferromagnetic ordering of spin in Cr2O3 hexagonal flake. Due to layer stacking and ordering, strong feature of creation of magnon states has been observed in photoluminescence (PL) spectroscopy. Finally, photo-detector response was measured to test the stability and reliability of the as-grown flakes for their use in spintronic devices.

Polarization-tunable nonlinear absorption patterns from saturated absorption to reverse saturated absorption in anisotropic GeS flake and an application of all-optical switching

Due to the unique anisotropic chemical and physical properties, two-dimensional (2D) layered materials, such as IV-VI monochalcogenides with puckered honeycomb structure, have received considerable interest recently. Among the IV-VI layered MX (M = Ge, Sn; X = Se, S) compounds, germanium sulfide (GeS) stands out for its strongest anisotropic thermal conductivities and figure-of-merit values. Additionally, the layer-independent direct energy bands( E g ~1.6 eV , E 1 ~ 2.1 eV ) of GeS flake provide excellent insights into further applications as visible photodetectors. Herein, the polarization-tunable nonlinear absorption (NA) patterns of GeS flake have been systematically investigated. Specifically, both the polarization-dependent Raman spectroscopy and the linear absorption (LA) spectroscopy were employed to characterize the lattice orientation and absorption edges of the251-nm GeS flake. Considering the low damage threshold of GeS flake, the GeS/graphene heterostructure was fabricated to increase the threshold without changing the nonlinear properties of GeS. Our NA results demonstrated that a 600-nm femtosecond laser with different polarizations would excite the saturated-absorption (SA) effect along armchair and reverse-saturated-absorption (RSA) effect along zigzag in theGeS/graphene heterostructure. Moreover, the function of the polarization-based GeS/graphene heterostructure all-optical switch was experimentally verified. Notably, thanks to the polarization-dependent NA patterns (SA/RSA) of GeS, the “ON” and “OFF” states of the all-optical switch can be accomplished by high and low transmittance states of continuous-wave laser (532 nm, 80 nW), whose state can be controlled by the polarization of femtosecond switching laser (600 nm, 35 fs, 500 Hz, 12 GW cm−2) . The ON/OFF ratio can achieve up to 17% by changing polarization, compared with the ratios of 3.0% by increasing the incident power of switching light in our experiment. The polarization-tunable absorption patterns introduced in this work open up real perspectives for the next-generation optoelectronic devices based on GeS/graphene heterostructure.

Quasi-Two-Dimensional Magnon Identification in Antiferromagnetic FePS3via Magneto-Raman Spectroscopy.

Recently it was discovered that van der Waals-bonded magnetic materials retain long range magnetic ordering down to a single layer, opening many avenues in fundamental physics and potential applications of these fascinating materials. One such material is FePS3, a large spin (S=2) Mott insulator where the Fe atoms form a honeycomb lattice. In the bulk, FePS3 has been shown to be a quasi-two-dimensional-Ising antiferromagnet, with additional features in the Raman spectra emerging below the Néel temperature () of approximately 120 K. Using magneto-Raman spectroscopy as an optical probe of magnetic structure, we show that one of these Raman-active modes in the magnetically ordered state is actually a magnon with a frequency of ≈3.7 THz (122 cm-1). Contrary to previous work, which interpreted this feature as a phonon, our Raman data shows the expected frequency shifting and splitting of the magnon as a function of temperature and magnetic field, respectively, where we determine the g-factor to be ≈2. In addition, the symmetry behavior of the magnon is studied by polarization-dependent Raman spectroscopy and explained using the magnetic point group of FePS3.

Spin-phonon coupling in epitaxial SrRuO3 heterostructures.

Spin-phonon coupling is one of the fundamental interactions in functional materials, indispensable for understanding their unexpected magnetic ground states. Ferromagnetic SrRuO3 is a correlated metal with the potential for utilization in novel spintronic devices and serves as a promising platform for studying spin-phonon interactions. In this study, we used Raman spectroscopy to identify spin-phonon coupling in SrRuO3 heterostructures. We deliberately decreased the exchange interactions within SrRuO3 by reducing system dimensions, which was coherently observed in both temperature-dependent magnetization measurements and phonon spectra. To collect the Raman signals from the very thin (quasi-2D) SrRuO3 layers while maintaining the layer thickness, we fabricated epitaxial oxide superlattices with 50 repetitions of the layers. We also present polarization-dependent Raman spectra of SrRuO3, with accurate identification of the Raman modes. These results show that the phonon dynamics of SrRuO3 is strongly influenced by the spin ordering, which can be efficiently tailored via atomically controlled epitaxial heterostructuring.

Orientation analysis of pentacene molecules in organic field-effect transistor devices using polarization-dependent Raman spectroscopy

Pentacene, an organic molecule, is a promising material for high-performance field effect transistors due to its high charge carrier mobility in comparison to usual semiconductors. However, the charge carrier mobility is strongly dependent on the molecular orientation of pentacene in the active layer of the device, which is hard to investigate using standard techniques in a real device. Raman scattering, on the other hand, is a high-resolution technique that is sensitive to the molecular orientation. In this work, we investigated the orientation distribution of pentacene molecules in actual transistor devices by polarization-dependent Raman spectroscopy and correlated these results with the performance of the device. This study can be utilized to understand the distribution of molecular orientation of pentacene in various electronic devices and thus would help in further improving their performances.

Enhancement of interlayer exchange in an ultrathin two-dimensional magnet

Following the recent isolation of monolayer CrI3, there has been a surge of new two-dimensional van der Waals magnetic materials, whose incorporation in van der Waals heterostructures offers a new platform for spintronics, proximity magnetism, and quantum spin liquids. A primary question in this burgeoning field is how exfoliating crystals to the few-layer limit influences their magnetism. Studies on CrI3 have shown a different magnetic ground state for ultrathin exfoliated films but the origin is not yet understood. Here, we use electron tunneling through few-layer crystals of the layered antiferromagnetic insulator CrCl3 to probe its magnetic order, finding a ten-fold enhancement in the interlayer exchange compared to bulk crystals. Moreover, temperature- and polarization-dependent Raman spectroscopy reveal that the crystallographic phase transition of bulk crystals does not occur in exfoliated films. This results in a different low temperature stacking order and, we hypothesize, increased interlayer exchange. Our study provides new insight into the connection between stacking order and interlayer interactions in novel two-dimensional magnets, which may be relevant for correlating stacking faults and mechanical deformations with the magnetic ground states of other more exotic layered magnets, such as RuCl3.

Thin films of tetrafluorosubstituted cobalt phthalocyanine: Structure and sensor properties

In this work, thin films of tetrafluorosubstituted cobalt phthalocyanine (CoPcF4) were prepared by organic molecular beam deposition and their structure was studied using UV–vis, polarization dependent Raman spectroscopy, XRD and atomic force microscopy. Quantum chemical calculations (DFT) have been employed in order to determine the detailed assignment of the bands in the CoPcF4 IR and Raman spectra. The electrical sensor response of CoPcF4 films to ammonia vapours was investigated and compared with that of unsubstituted cobalt phthalocyanine films. In order to explain the difference in sensitivity of the unsubstituted and fluorinated phthalocyanines to ammonia, the nature and properties of chemical binding between CoPc derivatives and NH3 were described by quantum-chemical calculations utilizing DFT method. The effect of post-deposition annealing on surface morphology and gas sensing properties of CoPcF4 films was also studied.

Strain-induced direct–indirect bandgap transition and phonon modulation in monolayer WS2

In situ strain photoluminescence (PL) and Raman spectroscopy have been employed to exploit the evolutions of the electronic band structure and lattice vibrational responses of chemical vapor deposition (CVD)-grown monolayer tungsten disulphide (WS2) under uniaxial tensile strain. Observable broadening and appearance of an extra small feature at the longer-wavelength side shoulder of the PL peak occur under 2.5% strain, which could indicate the direct-indirect bandgap transition and is further confirmed by our density-functional-theory calculations. As the strain increases further, the spectral weight of the indirect transition gradually increases. Over the entire strain range, with the increase of the strain, the light emissions corresponding to each optical transition, such as the direct bandgap transition (K-K) and indirect bandgap transition (Γ-K, ≥2.5%), exhibit a monotonous linear redshift. In addition, the binding energy of the indirect transition is found to be larger than that of the direct transition, and the slight lowering of the trion dissociation energy with increasing strain is observed. The strain was used to modulate not only the electronic band structure but also the lattice vibrations. The softening and splitting of the in-plane E’ mode is observed under uniaxial tensile strain, and polarization-dependent Raman spectroscopy confirms the observed zigzag-oriented edge of WS2 grown by CVD in previous studies. These findings enrich our understanding of the strained states of monolayer transition-metal dichalcogenide (TMD) materials and lay a foundation for developing applications exploiting their strain-dependent optical properties, including the strain detection and light-emission modulation of such emerging two-dimensional TMDs.

Gold(III) phthalocyanine chloride: Optical and structural characterization of thin films

In the present work, the investigation of gold(III) phthalocyanine chloride (AuClPc) which received very little attention before is progressed further using a range of optical spectroscopic techniques. Thin films of gold(III) phthalocyanine chloride were produced by organic molecular beam deposition deposition and their structure was investigated by UV–visible spectroscopy, polarization dependent Raman spectroscopy and atomic force microscopy. Experimental observations were confirmed by theoretical analysis using quantum chemical calculations; the latter were used for the detailed assignment of different bands in IR spectra of the gold(III) phthalocyanine chloride. The orientation of gold(III) phthalocyanine chloride molecules relative to the substrate surface was studied in detail using techniques based on the polarization dependent Raman spectroscopy. It was shown that gold(III) phthalocyanine chloride films deposited on quartz substrates at 100 °C exhibit a co-facial structure with the average tilt angles of Pc moieties of ∼70°.

Valence band splitting in wurtzite InGaAs nanoneedles studied by photoluminescence excitation spectroscopy.

We use low-temperature microphotoluminescence and photoluminescence excitation spectroscopy to measure the valence band parameters of single wurtzite InGaAs nanoneedles. The effective indium composition is measured by means of polarization-dependent Raman spectroscopy. We find that the heavy-hole and light-hole splitting is ∼95 meV at 10 K and the Stokes shift is in the range of 35-55 meV. These findings provide important insight in the band structure of wurtzite InGaAs that could be used for future bandgap engineering.

Polarized Raman scattering of single ZnO nanorod

Polarized Raman scattering measurement on single wurtzite c-plane (001) ZnO nanorod grown by hydrothermal method has been performed at room temperature. The polarization dependence of the intensity of the Raman scattering for the phonon modes A1(TO), E1(TO), and E2high in the ZnO nanorod are obtained. The deviations of polarization-dependent Raman spectroscopy from the prediction of Raman selection rules are observed, which can be attributed to the structure defects in the ZnO nanorod as confirmed by the comparison of the transmission electron microscopy, photoluminescence spectra as well as the polarization dependent Raman signal of the annealed and unannealed ZnO nanorod. The Raman tensor elements of A1(TO) and E1(TO) phonon modes normalized to that of the E2high phonon mode are |a/d|=0.32±0.01, |b/d|=0.49±0.02, and |c/d|=0.23±0.01 for the unannealed ZnO nanorod, and |a/d|=0.33±0.01, |b/d|=0.45±0.01, and |c/d|=0.20±0.01 for the annealed ZnO nanorod, which shows strong anisotropy compared to that of bulk ZnO epilayer.

The three A symmetry Raman modes of kesterite in Cu_2ZnSnSe_4

We investigate CZTSe films by polarization dependent Raman spectroscopy. The main peaks at 170 cm(-1), and 195 cm(-1) are found to have A symmetry. The Raman signal at 170 cm(-1) is found to be composed of two modes at 168 cm(-1) and 172 cm(-1). We attribute these three Raman peaks to the three A symmetry modes predicted for kesterite ordered Cu(2)ZnSnSe(4). The main Raman peak is asymmetrically broadened towards lower energies. Possible sources of the broadening are tested through temperature and depth dependent measurements. The broadening is attributed to phonon confinement effects related to the presence of lattice defects.

Giant Optical Response from Graphene–Plasmonic System

The unique properties of graphene when coupled to plasmonic surfaces render a very interesting physical system with intriguing responses to stimuli such as photons. It promises exciting application potentials such as photodetectors as well as biosensing. With its semimetallic band structure, graphene in the vicinity of metallic nanostructures is expected to lead to non-negligible perturbation of the local distribution of electromagnetic field intensity, an interesting plasmonic resonance process that has not been studied to a sufficient extent. Efforts to enhance optoelectronic responses of graphene using plasmonic structures have been demonstrated with rather modest Raman enhancement factors of less than 100. Here, we examine a novel cooperative graphene-Au nanopyramid system with a remarkable graphene Raman enhancement factor of up to 10(7). Experimental evidence including polarization-dependent Raman spectroscopy and scanning electron microscopy points to a new origin of a drastically enhanced D-band from sharp folds of graphene near the extremities of the nanostructure that is free of broken carbon bonds. These observations indicate a new approach for obtaining detailed structural and vibrational information on graphene from an extremely localized region. The new physical origin of the D-band offers a realistic possibility of defining active devices in the form of, for example, graphene nanoribbons by engineered graphene folds (also known as wrinkles) to realize edge-disorder-free transport. Furthermore, the addition of graphene made it possible to tailor the biochemical properties of plasmonic surfaces from conventional metallic ones to biocompatible carbon surfaces.