Raman-active Vibrational Modes Research Articles

Electronic properties, anisotropic elasticity, theoretical strengths, and lattice vibrational mode of orthorhombic YSi

We investigated the crystal structure, electronic structure, elastic stiffness constants, responses to tensile and shear deformations, lattice vibrational modes as well as vibration heat capacity and vibration entropy of the orthorhombic YSi compound using ab initio approach. The relaxed ground-state lattice constants a, b and c of YSi exhibit fair agreement with the available experimental and theoretical data. Nine independent elastic stiffness constants have been computed by increasingly varying minute strains based on “energy-strain” approach, which confirm that the orthorhombic YSi is mechanically stable. Some polycrystalline moduli involving bulk modulus, Young's modulus and shear modulus, Poisson's ratio, brittle/ductile characteristic have also been systematically derived, and YSi can be classified as brittle compound. Both three-directional Young's modulus anisotropy and bulk modulus anisotropy are calculated in detail, and the calculations indicate that the orthorhombic YSi takes on significant elastic anisotropy. We further studied the theoretical tensile and shear strengths under different loads. Also, the phonon spectra, total and partial phonon densities of states are researched by employing Density Functional Perturbation Theory (DFPT) and the finite-displacement method, and the infrared-active and Raman active vibration modes at the center of Brillouin zone are classified using the group theory. Moreover, we predicted the constant volume heat capacity of YSi under high temperature relying on the calculated phonon densities of states as well as quasi-harmonic approximation (QHA) and Debye model.

First‐Principles Calculations of Phonons and Thermodynamic Properties of Zr(Hf)S2‐Based Nanotubes

Comparative hybrid density functional calculations on the structure, stability, and phonon frequencies of monolayers and single-walled nanotubes are performed for Zr(Hf)S2 disulfides. The first-principles calculations of HfS2 -based nanotubes are made for the first time. The symmetry analysis of infrared and Raman active vibrational modes in ZrS2 and HfS2 nanotubes is made using the induced representations of the isogonal point groups of line groups. It is shown that the number of infrared and Raman active modes is constant for NTs with the same chirality type. The correlation of the phonon modes of the nanotubes of relatively large diameters with those of monolayer is analyzed. The thermodynamic functions of monolayers and nanotubes with various chirality and diameters are calculated on the basis of the obtained phonon frequencies. It is established that the phonon contribution to the nanotube strain energy is small, but may be important for an accurate estimate of the stability of the nanotubes of small diameters. The calculated results show that the thermal contributions to Helmholtz free energy are positive; thereby they slightly reduce the stability of ZrS2 and HfS2 nanotubes at elevated temperatures. © 2019 Wiley Periodicals, Inc.

Fabrication of high photosensitivity nanostructured n-Fe2O3/p-Si heterojunction photodetector by rapid thermal oxidation of chemically sprayed FeS2 film

In this study, we have reported a novel route for the preparation of α-Fe2O3 by rapid thermal oxidation RTO of chemical sprayed FeS2 film at 550 °C/20 s condition under oxygen ambient. The direct optical band gap of Fe2O3 was 2.6 eV, while its indirect energy gap was 1.7 eV. The x-ray diffraction XRD studies show that the FeS2 film has pyrite cubic structure and changed to rhombohedral α-Fe2O3 film after oxidation. Scanning electron microscope SEM and transmission electron microscope TEM investigations confirm the formation of Fe2O3 nanorods with a diameter of 65 nm. Energy dispersive x-ray analysis EDX confirms complete conversion of the FeS2 to Fe2O3 film after RTO process. Raman shift data of Fe2O3 shows the presence of five Raman active vibration modes indexed to A1g and E1g modes. The dark and illuminated current-voltage properties of n-Fe2O3/p-Si heterojunctions have been investigated in the absence of the buffer layer and the ideality factor of n-Fe2O3/p-Si was 2.7. The responsivity of Fe2O3/Si photodetector was 0.51 A W−1 at 500 nm and 0.37 A W−1 at 850 nm. The specific detectivity of the photodetectors was measured as a function of wavelength. The rise time of the photodetector was measured and it was around 60 ns. The energy band line-up of n-Fe2O3/p-Si heterojunction was demonstrated under illumination condition.

Role of CuAlO2 as an absorber layer for solar energy converter

A delafossite semiconductor, namely CuAlO2, with a lower band gap is a potential candidate for the absorption of solar radiation due to its excellent electronic and optical properties. In this paper, structural, electronic, vibrational and optical properties of delafossite CuAlO2 have been calculated using state-of-the-art first principles calculations based on density functional theory (DFT). The optimized structural parameters, electronic properties and Raman active vibrational modes namely the Eg and A1g are in good agreement with experimental and other theoretical data. The full phonon dispersion curves (PDC) together with the phonon density of states (PHDOS) of CuAlO2 depict its dynamical stability due to non-existence of any imaginary phonon mode in the entire Brillouin zone (BZ). To understand the mechanism of optical transition properties, we have further calculated the dielectric functions, absorption coefficient and joint density of states (JDOS) for delafossite, CuAlO2. The dielectric constant and absorption coefficient show a significant anisotropic nature in the components of polarization directions. Solar cell parameters of CuAlO2 are also calculated and the highest theoretical efficiencies of 14.8% and 12.5% have been observed with Shockley-Queisser (SQ) limit and spectroscopic limited maximum efficiency (SLME) respectively. The SLME predicted efficiency agrees better to the experimental value as it includes the thickness dependent absorbance coefficient in account of non-radiative recombination and carrier loss mechanism.

Preparation and Decay of a Single Quantum of Vibration at Ambient Conditions

A single quantum of excitation of a mechanical oscillator is a textbook example of the principles of quantum physics. Mechanical oscillators, despite their pervasive presence in nature and modern technology, do not generically exist in an excited Fock state. In the past few years, careful isolation of GHz-frequency nano-scale oscillators has allowed experimenters to prepare such states at milli-Kelvin temperatures. These developments illustrate the tension between the basic predictions of quantum mechanics that should apply to all mechanical oscillators existing even at ambient conditions, and the complex experiments in extreme conditions required to observe those predictions. We resolve the tension by creating a single Fock state of a vibration mode of a crystal at room temperature using a technique that can be applied to any Raman-active system. After exciting a bulk diamond with a femtosecond laser pulse and detecting a Stokes-shifted photon, the 40~THz Raman-active internal vibrational mode is prepared in the Fock state $|1>$ with $98.5\%$ probability. The vibrational state is read out by a subsequent pulse, which when subjected to a Hanbury-Brown-Twiss intensity correlation measurement reveals the sub-Poisson number statistics of the vibrational mode. By controlling the delay between the two pulses we are able to witness the decay of the vibrational Fock state over its $3.9$ ps lifetime at room temperature. Our technique is agnostic to specific selection rules, and should thus be applicable to any Raman-active medium, opening a new generic approach to the experimental study of quantum effects related to vibrational degrees of freedom in molecules and solid-state systems.

Structural, electronic, optical properties and first-principles calculations of Sr1-xCaxWO4 ceramics

A series of strontium calcium tungstates Sr1-xCaxWO4 powders with (x = 0; 0.25; 0.5; 0.75 and 1.0) were prepared by solid-state reaction method and analyzed by X-ray diffraction (XRD). All these compositions possess a tetragonal scheelite structure with I41/a space group. Raman active vibrational modes in the range from 20 to 1000 cm-1 of the series Sr1-xCaxWO4 with tetragonal structure exhibit 13 modes in arrangement with the Group theory analysis of structural Raman-active modes. The optical properties were investigated using the diffuse reflectance UV–visible absorbance spectrum. Based on Density Functional Theory (DFT) and using full Potential-linearized Augmented Plane Wave (FP-LAPW) method with the Local Density Approximation and the Generalized Gradient Approximation (GGA), implemented in the Wien2k package, we have investigated electronic and optical properties of all the compositions. The results indicate a decrease in the values of the optical direct bandgap (from 4.29 to 3.87 eV) with the increase of Ca into SrWO4 lattice, which is in good agreement with our experimental results.

A Universal Length-Dependent Vibrational Mode in Graphene Nanoribbons

Graphene nanoribbons (GNRs) have attracted considerable interest, as their atomically tunable structure makes them promising candidates for future electronic devices. However, obtaining detailed information about the length of GNRs has been challenging and typically relies on low-temperature scanning tunneling microscopy. Such methods are ill-suited for practical device application and characterization. In contrast, Raman spectroscopy is a sensitive method for the characterization of GNRs, in particular for investigating their width and structure. Here, we report on a length-dependent, Raman-active low-energy vibrational mode that is present in atomically precise, bottom-up-synthesized armchair graphene nanoribbons (AGNRs). Our Raman study demonstrates that this mode is present in all families of AGNRs and provides information on their length. Our spectroscopic findings are corroborated by scanning tunneling microscopy images and supported by first-principles calculations that allow us to attribute this mode to a longitudinal acoustic phonon. Finally, we show that this mode is a sensitive probe for the overall structural integrity of the ribbons and their interaction with technologically relevant substrates.

Intriguing of two-dimensional Janus surface-functionalized MXenes: An ab initio calculation

MXenes, two-dimensional (2D) transition metal carbides and nitrides displaying astonishing properties are emerged as a new class of two-dimensional (2D) layered materials. Motivated by the recent successful synthesis of Janus monolayers of transition metal dichalcogenides, we investigated the structural, electronic, magnetic and vibrational properties of Janus structure of pristine M′MC and functionalized M′MCT2 (where T = F, O, OH) MXene by means of first-principles calculations. Here M′MC as a representative Janus MXene material is designed. The ground state structures of Janus pristine M′MC and functionalized M′MCT2 are found in four different configurations. Our calculated electronic band structures indicate that the Janus M′MC MXenes are metallic and their functionalized M′MCT2s display half-metallic, metallic and semiconductor behaviors upon surface functionalized type. Our results reveal that (i) phonon band dispersion diagram of Janus pristine M′MC MXenes are dynamically stable (ii) ab initio molecular dynamic simulations (AIMD) clearly proved that they kept thermal stabilities at high temperature (900 K) and (iii) Raman-active vibrational modes are predicted. Janus surface-functionalized MXenes suggest that they are promising candidates for spintronic applications, which should stimulate interest in their synthesis.

Raman spectra of sol-gel auto-combustion synthesized Mg-Ag-Mn and Ba-Nd-Cd-In ferrite based nanomaterials

In this present work, we have done the comparison of raman spectra of hard ferrite i.e. Ba0.7Nd0.3Cd0.5xIn0.5xFe12-xO19 (x = 0.0, 0.2, 0.3) and soft ferrite i.e. Mg1-xMnxAgyFe2-yO4 (x = 0.1, 0.2, 0.3, 0.4, 0.5, y = 0.0, 0.1, 0.2, 0.3, 0.4) which defines the novality and achievement of this work. These ferrite based nanomaterials were synthesized with the aid of sol-gel auto-combustion method in order to analyze Raman spectra carried with 514.5 nm laser of argon ion (50 mW) by using Renishaw inVia Reflex micro-Raman spectrometer. The Raman spectra of synthesized Mg1-xMnxAgyFe2-yO4 (x = 0.1, 0.2, 0.3, 0.4, 0.5, y = 0.0, 0.1, 0.2, 0.3, 0.4) and Ba0.7Nd0.3Cd0.5xIn0.5xFe12-xO19 (x = 0.0, 0.2, 0.3) ferrites confirmed the formation of five active Raman vibrational modes (A1g + Eg+3T2g). Out of this, Raman spectral analysis of Mg1-xMnxAgyFe2-yO4 (x = 0.1, 0.2, 0.3, 0.4, 0.5, y = 0.0, 0.1, 0.2, 0.3, 0.4) nanomaterials confined that Eg mode is found in the range of 194.74–212.77 cm−1, T2g vibrational mode is found to be in the range of 326.17–339.58 cm−1, 482.27–587 cm−1 and 608.84–614.02 cm−1 whereas A1g configuration mode is found to be in the range of 685.47–701.52 cm−1. In addition to this, Raman spectral analysis of Ba0.7NdO.3Cd0.5xIn0.5xFe12-xO19 (x = 0.0, 0.2, 0.3) nanohexaferrites consists of five Raman active vibrational modes in the range as ν1 = 329.02–334.70 cm−1, ν2 = 410.33–410.90 cm−1, ν3 = 522.06–524.65 cm−1, ν4 = 612.52–616.66 cm−1 and ν5 = 679.97–685.16 cm−1. The Raman spectra of both the compositions continuously indicates the symmetric and anti-symmetric behaviour of the band position of spinel and hexagonal magnetic nanomaterials.

Laser-induced structure transition of diamond-like carbon coated on cemented carbide and formation of reduced graphene oxide

We report on the structural evolution of diamond-like carbon (DLC) films by the nanosecond pulsed laser annealing process. DLC film is coated on cemented carbide (WC-Co) by cathodic arc ion plating, which is then annealed by ArF laser (193 nm, 20 ns) at different laser fluences (0.9–1.7 J/cm2). Upon laser annealing, Raman spectra divulge higher sp3 fractions accompanied by a blue shift in the G-peak position, which indicates the changes of sp2 sites from rings to chains. At higher fluence (>1.2 J/cm2), the film converts into reduced graphene oxide confirmed by its Raman-active vibrational modes: D, G, and 2D.

Conventional synthesis and characterization of cubically ordered La2FeMnO6 double perovskite compound

The cubically ordered double perovskite compound, La2FeMnO6, has been synthesized using high temperature solid-state reaction method. Rietveld refinement of powder X-ray diffraction data indicates the formation of La2FeMnO6 compound, having cubic structure with space group Pm-3m. It is also inferred that the LaFeO3 perovskite structure accommodates the Mn3+ ions to stabilize the double perovskite La2FeMnO6 structure. The scanning electron micrograph reveals the uniform distribution of crystallites without any segregation. Energy dispersive spectra confirms the presence of La, Fe, Mn and O elements in the prepared compound. The existence of higher order Raman active vibrational modes confirms the phase purity of the prepared sample, where higher-order phonon modes may be related to the strong spin-lattice coupling arising from the interaction between the adjacent magnetic sublattices. The magnetic exchange interactions between transition metal ions influences the anti-ferromagnetic ordering (AFM) existing in the La2FeMnO6 compound. The magnetization measurement confirms the dominant exchange interactions between Fe3+ and Mn3+ ions in the material which provides a complex environment for the competition between ferri-magnetism (FIM) and AFM interactions. Hence, a phase pure double perovskite La2FeMnO6 compound prepared at high temperature is characterized for its structural, optical and magnetic properties.

Density functional theory studies on the structures and vibrational spectroscopic characteristics of nickel, copper and zinc naphthalocyanines.

Molecular geometries and electronic structure calculations and spectroscopic assignments for metallonaphthalocyanines NiNc, CuNc and ZnNc are performed on the optimized geometries at B3LYP/6-31G* level. The order of the bond lengths of the NM bonds is computed to be NiNc < CuNc < ZnNc. The Mulliken atomic charges of the central M vary in the same order as the bond lengths. The metal dependent frequencies of IR-active and Raman-active vibrational modes decrease in the order of NiNc > CuNc > ZnNc, which is in reverse sequence of the N-M bond length. The strongest Raman lines predicted at 1570 (NiNc), 1546 (CuNc) and 1525 (ZnNc) cm-1 are highly sensitive to the metal ion. Comparison of the calculated properties of MNcs and MPcs (metallphthalocyanines) compounds reveals some interesting and meaningful results due to extension of the conjugated systems.

Raman detection of hidden phonons assisted by atomic point defects in a two-dimensional semimetal

Defects usually have an important role in tailoring various properties of two-dimensional (2D) materials. However, optical detection of defects, especially single-atom point defects, is very challenging in 2D layers. Here, we report our systematic studies on the Raman-activated defect vibrational modes in 2D semimetallic material by combining Raman spectroscopy, density functional theory (DFT) calculation and scanning tunneling microscopy (STM). We observed three common Raman-active vibrational modes located at 95 (A_{1g}^2), 228 (A_{1g}^1), and 304 cm−1 (B_{1g}^1) in ZrSiTe few-layers, consistent with our theoretical calculations. Moreover, a pronounced mode sitting at 131.7 cm−1 was found in the ZrSiTe monolayer. This mode fades out quickly in the bilayer (2L) and eventually disappears in 4L. The high-resolution STM images and DFT calculations suggest this mode to be an intralayer shear mode at the Brillouin zone boundary which is activated by atomic point defects, and STM-based inelastic tunneling spectrum further confirms the existence of such a defect mode. The appearance of such ‘forbidden’ modes in Raman spectra may pave an avenue for the optical characterization of single-atom point defects in metallic 2D layers.

Distinguishing Nonlinear Terahertz Excitation Pathways with Two-Dimensional Spectroscopy.

High-field terahertz (THz) spectroscopy is enabling the ultrafast study and control of matter in new and exciting ways. However, when intense electromagnetic pulses are used in any kind of pump-probe spectroscopy, several nonlinear excitation pathways can result, leading to scenarios that required the development of multidimensional spectroscopies to illuminate the observed dynamics. Here we demonstrate a clear example where two-dimensional (2D) THz vibrational spectroscopy is needed to distinguish between nonlinear-excitation pathways in CdWO_{4}. We nonlinearly excite a set of Raman-active vibrational modes in CdWO_{4} with broadband THz pulses, and 2D spectroscopy allows us to determine the dominant excitation pathway. We provide a general framework for 2D THz and multi-THz nonlinear phonon spectroscopy in solid systems, which has important implications in contributing needed clarity to the nascent field of nonlinear phononics.

Characterizing the properties of anticancer silibinin and silybin B complexes with UV–Vis, FT-IR, and Raman spectroscopies: A combined experimental and theoretical study

We present a combined experimental and theoretical study dedicated to analyze the structure, optical properties, and vibrational behavior of the anticancer silibinin complex and one of its molecular constituents, namely; silybin B. A comparison of the UV–Vis and FT-IR characterization of these samples reveals the presence of almost identical features. In both cases, the existence of four absorption maxima located at 204, 230, 287, and 330 nm are observed, as well as vibrational bands that show a complex distribution in the 300–1700 cm−1 frequency domain together with three infrared excitations placed at ∼3180, 3450, and 3600 cm−1 typically associated with different CH and OH bond vibrations. For the first time, we present the Raman spectra of these samples. Interestingly, the Raman response reveals more notable differences between the two molecular complexes, mainly in the 300–900 cm−1 frequency range. Furthermore, we obtain that the wavelength of the excitation laser plays a fundamental role in revealing precise features of the spectra. Based on density functional theory (DFT) calculations we also simulate the UV–Vis, circular dichroism, infrared, and Raman spectra of model silybin A and silybin B (C25H22O10) molecules in both isolated and aggregated forms. An excellent agreement is found when contrasting experimental and theoretical data. Based on this comparison, we identify the main infrared- and Raman-active vibrational modes of silybin B, and infer those frequencies that seems to be unique to silybin A. Wavelength dependent calculations of the Raman spectra also reveal the crucial role played by the energy of the incident laser in determining the intensities of the Raman active modes, allowing for a better comparison between theory and experiment. Finally, simulations of the circular dichroism spectra for isolated silybin A and B species reveal clear differences between the two molecules, being thus an appropriate tool to distinguish the presence of specific isomers in a sample.

The effects of non-hydrostatic stress on the structure and properties of alpha-quartz

The study of the effects of non-hydrostatic stresses on rock-forming minerals is fundamental for understanding how minerals respond to tectonic stresses in the Earth’s interior. Larger deviatoric stresses of the order of GPa can arise from physical interaction between minerals (e.g. host–inclusion systems, compositional gradients) at the micrometric scale, even if at the large-scale large stresses are annealed by plastic flows. Thus, experimental data under hydrostatic pressure do not provide us access to all the possible modifications and structural changes experienced by minerals subjected to deviatoric stresses. Therefore, we carried out ab initio hybrid Hartree–Fock/Density Functional Theory simulations to determine the properties of alpha-quartz, since it is one of the most abundant minerals in the Earth’s crust with a very simple chemistry and structure. We calculated its structure, elastic parameters and Raman-active vibrational modes as a function of different applied strains, which allow us to show how phonon frequencies and structural parameters (bond lengths and angles) are affected by deviatoric stress conditions rather than hydrostatic pressure.

Elucidating the structural properties of gold selenide nanostructures

Noble transition metal chalcogenide gold selenide is a relatively unexplored layered material.

Using nanotubes to study the phonon spectrum of two-dimensional materials.

We propose a convenient method to characterize the acoustic phonon branches of 2D monolayer materials using measurements of the infrared- and Raman-active vibrational modes of nanotubes. The relations we employ are derived from a symmetry analysis based directly on the line groups of nanotubes. We perform extensive ab initio calculations for the MoS2 monolayer and nanotubes to evaluate the method and illustrate all our results. Specifically, we show how the low-energy phonon transmission, a determining factor in thermal transport, can be easily and successfully reconstructed by this procedure.

Formation and characterization of nano- and microstructures of twinned cubic boron nitride.

Nano- and microstructures of phase-pure cubic boron nitride (c-BN) are synthesized by employing nanosecond pulsed-laser annealing techniques at room temperature and atmospheric pressure. In a highly non-equilibrium synthesis process, nanocrystalline h-BN is directly converted into phase-pure twinned c-BN from a highly undercooled melt state of BN. By changing nucleation and growth rates, we have synthesized a wide range of sizes (90 nm to 25 μm) of c-BN. The electron diffraction patterns show the formation of twinned c-BN with [11[combining macron]1] as the twin axis. The twinning density in c-BN can be controlled by the degree of undercooling and quenching rates. The formation of twins predominantly occurs prior to the formation of amorphous quenched BN (Q-BN). Therefore, the defect density in nano c-BN formed under higher undercooling conditions is considerably larger than that in micro c-BN, which is formed under lower undercooling conditions. The temperature-dependent Raman studies show a considerable blue-shift of ∼6 cm-1 with a decrease in temperature from 300 to 78 K in nano c-BN as compared to micro c-BN. The size-effects of c-BN crystals in Raman spectra are modeled using spatial correlation theory, which can be used to calculate the correlation length and twin density in c-BN. It has also been found that the Raman blue-shift in nano c-BN is caused by anharmonic effects, and the decrease in Raman linewidth with decreasing temperature (300 to 78 K) is caused by three- and four-phonon decay processes. The bonding characteristics and crystalline nature of the synthesized c-BN are also demonstrated by using electron energy-loss spectroscopy and electron backscatter diffraction, respectively. We envisage that the controlled growth of phase-pure nano and microstructures of twinned c-BN and their temperature-dependent Raman-active vibrational mode studies will have a tremendous impact on low-temperature solid-state electrical and mechanical devices.

Mineralogy and spectroscopy (VIS near infrared and micro-Raman) of chromite from Nidar ophiolite complex, SE Ladakh, India: Implications for future planetary exploration

To determine the chemical composition of minerals on planetary surfaces from spectroscopic data, it is required to establish the link between mineralogy, spectroscopy and chemical composition of rock forming minerals on Earth. To meet this requirement, we performed integrated mineralogical and spectroscopic (Visible-Near Infrared and micro Raman) characterisation of chromite from the chromitite layer in the mantle section of the Nidar ophiolite complex, SE Ladakh, J&K, India. Chromite commonly occurs as monominerallic chromitite layers (up to 1 meter wide) and disseminated grains within the dunites and harzburgites. The Cr2O3 content is generally high (>60 wt%) and chemically homogeneous across the grains. The X-ray Diffraction (XRD) analyses show highest peak intensity at ∼36° 2θ, followed by other diagnostic peaks for chromite. Micro-Raman of chromite yields very strong diagnostic Raman active vibrational modes at 685 cm−1 (A1g), followed by 520 cm−1 (F2g(2)). The other peaks, viz. 446 cm−1 and 610 cm−1 are rather weak and correspond to Eg and F2g, respectively. VNIR (0.4μ to 2.4μ) reflectance spectroscopy of chromite suggested strong absorption near 2 μm and could be useful to quantify the chemical composition (especially Cr2O3, Al2O3 and Cr#). We found a good correlation of VNIR absorption and chemical composition of chromite (e.g. band positions versus Al2O3 and Cr# are well within 95% correlation interval). The intense Raman peak at 685 cm−1 corresponds to higher Cr content. This study, therefore, implies a significant understanding in determination of mineral chemistry (especially Chromites) by VNIR and micro Raman spectroscopy, which could be a useful tool for the future planetary exploration.