Raman Active A1g Research Articles

Investigation of pressure-driven superconductivity in TlInTe2: an ab initio study

The Zintl compound TlInTe2 is an intriguing material because of its outstanding thermoelectric properties at ambient pressure. Interestingly, it has recently been found that TlInTe2 exhibits a V-shape dependence of the superconducting critical temperature (T c) under increasing pressure, which has been linked to the reversed behavior of the Raman active Ag phonon mode and anharmonic effects. In this study, we have performed first-principles calculations of the electron-phonon interactions and the superconducting properties of TlInTe2 in order to understand this unusual pressure-induced response. In contrast to experiment, we find a dome-shaped pressure-induced dependence of T c with a maximum value of 0.23 K at 18 GPa, significantly lower than the experimental results. Electron doping has the potential to adjust the T c to fall within the experimental range, but it necessitates considerably high levels of doping. Furthermore, our analysis of the phonon spectra and phonon lifetimes, including anharmonic effects, show that anharmonicity is unlikely to influence the superconducting properties of TlInTe2. It remains an open question whether there is indeed an unusual V-shape T c dependence with pressure or whether the phonon-mediated theory of superconductivity used here breaks down in this system.

Highly photoactive rGO-MnO2/CuO nanocomposite photocatalyst for the removal of metanil yellow dye and bacterial resistance against Pseudomonas Aeruginosa

The superior photocatalytic, biological, and electrochemical properties of metal oxide nanocomposites have made them an important part of contemporary nanotechnology research. Nanocomposite involving MnO2 and CuO has been widely utilized for catalytic and electrochemical applications. However, the different oxidation states of MnO2 and lower band gap of CuO limits the efficiency of the devices involving the composite of these two semiconductors. As a result, reduced graphene oxide (rGO) is integrated into the MnO2/CuO matrix. rGO-MnO2/CuO and MnO2/CuO nanocomposites (NCs) were synthesized using one-pot green synthesis and chemical precipitation respectively. rGO decorated MnO2/CuO NC was green synthesized from graphene oxide using Alternanthera sessilis leaf extract. XRD detected peaks related to orthorhombic structured MnO2 and monoclinic structured CuO for both the composites. Star shaped nanostructures are observed for rGO incorporated MnO2/CuO nanocomposite. MC and rMC composites have band gaps of 2.16 and 2.04 eV, respectively. FTIR spectrum showed the characteristic peaks for MnO2 and CuO in the rGO-MnO2/ CuO composite. Raman active Ag and Bg CuO modes occur at 270 and 450 cm−1 and Mn-O symmetric vibrations at 590 and 540 cm−1. The incorporation of rGO into the MnO2/CuO composite increased its photocatalytic activity from 87 % to 96 % against the degradation of metanil yellow dye by increasing its electron conductivity, adsorption capacity, and light absorption capacity. The MnO2/CuO nanocomposite with rGO demonstrated enhanced antibacterial activity, with a zone of inhibition of 24 mm compared to 13 mm for the control and 18 mm for the MnO2/CuO composite.

First-principles calculations on ferroelectricity and lattice dynamics of Type-II multiferroic SmMn2O5

In this work, we report on the structural, electronic, and ferroelectric properties of SmMn2O5 by using first-principles density functional theory plus on-site Coulomb interaction (DFT + U) calculations. A thorough analysis was preformed to reveal the competing characteristics of different high-temperature (T) phases and the polarization mechanism in the low-T multiferroic phase. We show that the structural characteristics of the high-T phases have a strong influence on the low-T multiferroicity. In addition to the spin-induced lattice distortion that reduces substantially the purely electronic ferroelectricity, the dominant polarization mechanism in low-T SmMn2O5 still originates from the electronic polarization. By performing mode decomposition of the Hellmann–Feynman forces and the lattice distortion induced by the q = (0.5, 0, 0) magnetic order, we find that the Raman-active Ag mode characterized by the Mn4+O6 octahedron distortion and synergistic displacement of Mn3+ and Sm ions is of primary importance, while the infrared (IR)-active B2u mode plays a secondary role. These findings provide a theoretical foundation for future studies concerning the enhanced magnetoelectric effects of SmMn2O5 due to its pure exchange–striction mechanism.

The influence of processing methods on the dielectric properties of BaTi1-xGdxO3-x/2 - Based materials

In this study, the intermediate rare-earth oxide Gd2O3 (Gd) was substituted in different amounts (x = 0.2–2 mol%) for the formulation of BaTi1-xGdxO3-x/2 (BTGx) dielectric materials. The effect of B-site substitution was confirmed by the additional Raman active A1g octahedral peak at ~835cm-1 strengthened at x ≥ 0.4 mol%. Additionally, properties of 0.9BTG0.007-0.1BA dielectric ceramics were analysed based on the influence of various processing methods as a function of sintering temperature. The focal samples were labelled Method-A (direct-mix) and Method-B (indirect-mix). As the sintering temperature (1075–1200 °C) increased, the 1 kHz response of the ε–T curves of Method-A samples transformed from a single peak to broad-narrow double peaks of high dielectric loss tangent (tan δ). Nonetheless, samples of Method-B possessed a clearly defined transmission electron microscopy (TEM) core-shell structure, flattened double-peak ε-T curves, optimised dielectric properties (ε = ~1563–1851 and tan δ < 1.5% at room temperature), and a wide-ranging temperature behaviour that meets the X8R dielectric standards (ΔC/C25°C < ±15%). The maximum dielectric breakdown strength of Method-B samples reached ~131 kVcm, while the energy storage density was ~0.726 J/cm3 at a maximum efficiency of ~80% at 1100 °C. Thus, exhibiting good potentials for balancing temperature stability with energy storage applications.

Chlorine doping of MoSe2 flakes by ion implantation.

The efficient integration of transition metal dichalcogenides (TMDs) into the current electronic device technology requires mastering the techniques of effective tuning of their optoelectronic properties. Specifically, controllable doping is essential. For conventional bulk semiconductors, ion implantation is the most developed method offering stable and tunable doping. In this work, we demonstrate n-type doping in MoSe2 flakes realized by low-energy ion implantation of Cl+ ions followed by millisecond-range flash lamp annealing (FLA). We further show that FLA for 3 ms with a peak temperature of about 1000 °C is enough to recrystallize implanted MoSe2. The Cl distribution in few-layer-thick MoSe2 is measured by secondary ion mass spectrometry. An increase in the electron concentration with increasing Cl fluence is determined from the softening and red shift of the Raman-active A1g phonon mode due to the Fano effect. The electrical measurements confirm the n-type doping of Cl-implanted MoSe2. A comparison of the results of our density functional theory calculations and experimental temperature-dependent micro-Raman spectroscopy data indicates that Cl atoms are incorporated into the atomic network of MoSe2 as substitutional donor impurities.

Cubic shaped hematite (α-Fe2O3) micro-structures composed of stacked nanosheets for rapid ethanol sensor application

Herein, the synthesis, characterization, and ethanol gas sensing properties of hematite iron oxide(α-Fe2O3) microstructures were investigated. Detailed examinations using different techniques confirmed the high-density growth and rhombohedral crystal structure of the synthesized Fe2O3 micro-structures with an average crystal size of 38.29 nm. The appearance of several Raman-active A1g(1), A1g(2), and Eg modes in Raman-scattering spectrum further revealed the formation of the hematite phase. The sensor device was fabricated using Fe2O3 micro-structures and tested for various gases such as ethanol, carbon monoxide (CO), and hydrogen (H2). It was observed that the fabricated sensor demonstrated a high response of 13.1for 100 ppm ethanol gas concentration at a temperature of 400 °C. Including the response and recovery times for the fabricated sensors, the transient response for various gases was also recorded and analyzed. The fabricated sensor exhibited a rapid gas response at 400 °C for ethanol gas (13.1) compared to CO (1.95) and H2 (1.71) gases for 100 ppm gas concentrations. A plausible ethanol sensing mechanism was also proposed and presented.

Raman scattering from the bulk inactive out\u2013of\u2013plane {{bf{B}}}_{{bf{2}}{bf{g}}}^{{bf{1}}} mode in few\u2013layer MoTe2

We report a study of Raman scattering in few-layer MoTe2 focused on high-frequency out-of-plane vibrational modes near 291 cm−1 which are associated with the bulk-inactive {{rm{B}}}_{2{rm{g}}}^{1} mode. Our temperature-dependent measurements reveal a double peak structure of the feature related to these modes in the Raman scattering spectra of 4- and 5-layer MoTe2. In accordance with literature data, the doublet’s lower- and higher-energy components are ascribed to the Raman-active A1g/{{bf{A}}{boldsymbol{^{prime} }}}_{{bf{1}}} vibrations involving, respectively, only the inner and surface layers. We demonstrate a strong enhancement of the inner mode’s intensity at low temperature for 1.91 eV and 1.96 eV laser light excitation which suggests a resonant character of the Raman scattering processes probed under such conditions. A resonance of the laser light with a singularity of the electronic density of states at the M point of the MoTe2 Brillouin zone is proposed to be responsible for the observed effects.

Phonon anomalies in FeS

We present results from light scattering experiments on tetragonal FeS with the focus placed on lattice dynamics. We identify the Raman active A1g and B1g phonon modes, a second order scattering process involving two acoustic phonons, and contributions from potentially defect-induced scattering. The temperature dependence between 300 and 20K of all observed phonon energies is governed by the lattice contraction. Below 20K the phonon energies increase by 0.5-1 cm$^{-1}$ thus indicating putative short range magnetic order. Along with the experiments we performed lattice-dynamical simulations and a symmetry analysis for the phonons and potential overtones and find good agreement with the experiments. In particular, we argue that the two-phonon excitation observed in a gap between the optical branches becomes observable due to significant electron-phonon interaction.

Synthesis and spectroscopic characterization of alkali-metal intercalated ZrSe2.

We report on the synthesis and spectroscopic characterization of alkali metal intercalated ZrSe2 single crystals. ZrSe2 is produced by chemical vapour transport and then Li is intercalated. Intercalation is performed from the liquid phase (via butyllithium) and from the vapour phase. Raman spectroscopy of intercalated ZrSe2 reveals phonon energy shifts of the Raman active A1g and Eg phonon modes, the disappearance of two-phonon modes and new low wavenumber Raman modes. Angle-resolved photoemission spectroscopy is used to perform a mapping of the Fermi surface revealing an electron concentration of 4.7 × 1014 cm-2. We also perform vapour phase intercalation of K and Cs into ZrSe2 and observe similar changes in the Raman modes as for the Li case.

Layer-dependent properties ofSnS2andSnSe2two-dimensional materials

The layer dependent structural, electronic and vibrational properties of SnS2 and SnSe2 are investigated using first-principles density functional theory (DFT). The in-plane lattice constants, interlayer distances and binding energies are found to be layer-independent. Bulk SnS2 and SnSe2 are both indirect band gap semiconductors with Eg = 2.18 eV and 1.07 eV, respectively. Few-layer and monolayer 2D systems also possess an indirect band gap, which is increased to 2.41 eV and 1.69 eV for single layers of SnS2 and SnSe2. The effective mass theory of 2D excitons, which takes into account the combined effect of the anisotropy, non-local 2D screening and layer-dependent 3D screening, predicts strong excitonic effects. The binding energy of indirect excitons in monolayer samples, Ex~0.9 eV, is substantially reduced to Ex = 0.14 eV in bulk SnS2 and Ex = 0.09 eV in bulk SnSe2. The layer-dependent Raman spectra display a strong decrease of intensities of the Raman active A1g mode upon decreasing the number of layers down to a monolayer, by a factor of 7 in the case of SnS2 and a factor of 20 in the case of SnSe2 which can be used to identify number of layers in a 2D sample.

Femtosecond dynamics of decoupled superlattice zones in 4Hb–TaSe2 single crystals

Strongly correlated electronic crystal systems marked by temperature-dependent phase transitions often feature an emergence of new structural periodicities, arising from an interplay between modulated electron density and periodic lattice distortion. We investigate the structural evolution of superlattice clusters in 4Hb–TaSe2 single crystals on a subpicosecond and atomic resolution via femtosecond electron diffraction (FED). The superlattice clusters comprise of two six-member rings each, related by a rotational symmetry and held in position by Coulombic mode repulsion. The two six-member rings are decoupled into two possible zones, which are further decoupled into respective site-specific satellite reflections in the reciprocal space. Governed by the interlayer interactions and the electron diffraction simulation analysis, FED measurements are used to demonstrate the suppression and dynamics of the decoupled superlattice zones. The simultaneous temporal evolution of the two CDW reflection orders are assigned to the decoupled Raman-active Ag and soft phonon modes previously reported on an isomorphic crystal structure (4Hb–TaS2). It is found that the charge density wave amplitude mode dominates over the corresponding phase mode.

High Pressure Vibrational Properties of WS2 Nanotubes.

We bring together synchrotron-based infrared and Raman spectroscopies, diamond anvil cell techniques, and an analysis of frequency shifts and lattice dynamics to unveil the vibrational properties of multiwall WS2 nanotubes under compression. While most of the vibrational modes display similar hardening trends, the Raman-active A1g breathing mode is almost twice as responsive, suggesting that the nanotube breakdown pathway under strain proceeds through this displacement. At the same time, the previously unexplored high pressure infrared response provides unexpected insight into the electronic properties of the multiwall WS2 tubes. The development of the localized absorption is fit to a percolation model, indicating that the nanotubes display a modest macroscopic conductivity due to hopping from tube to tube.

Unusual features of charge carrier traps energy spectra in silicon organic polymers revealed by advanced TSL

The peculiarities of charge carrier traps’ energy spectra in poly (di-n-hexylsilane) films have been studied by the enhanced fractional thermally stimulated luminescence (TSL) in the temperature range of 5–200K. For the first time, we have shown that the majority of fractional energy values (>80%) is distributed between a set of horizontal energy levels suggesting a discontinuity of the traps’ energy spectrum. These data distinctly differ from the results of earlier studies where a quasilinear dependence of the activation energy on temperature was found. It is shown that the significant width of TSL bands originates from the dispersion of the frequency factor. It is also established that the values obtained for the activation energy correlate well with the frequencies of the symmetric Raman active Ag modes at 268 and 373cm−1 of the silicon chain, which confirms the suggestion about the hole location on the segments of the silicon organic polymers backbone.

The dynamical process of the phase transition from VO2(M) to VO2(R)

The dynamical process of the metal-insulator transition, from VO2(M) to VO2(R), is studied in the framework of the dynamics theory. It is found that the thermal exciting of the Raman-active Ag mode with frequency of 212.7 cm-1 in the VO2(M) lattice drives the compound to be the VO2(R) lattice. The intermediate structures during the phase transition are revealed, from which we find that when the distortion of the atomic network away from its initial network in the M phase exceeds 60%, the system becomes metallic. At the moment, the monoclinic symmetry of the crystal remains still, but more V ions are dimerized. This strongly suggests that the dimerization of the V ions in the compound plays a critical role in the transition from the M phase to the R phase.

Sol–gel preparation and optical characterization of TbxY3−xAl5O12

Tb3+ doped Yttrium Aluminum Garnet (Tb: YAG) is a very promising optical material. This article reports synthesis of Tb doped YAG polycrystalline powder by sol–gel method. The sample was heat treated at different temperatures for characterization purpose. The powder was characterized by X-ray diffraction analysis (XRD), Scanning electron microscopy (SEM), Photoluminescence (PL) studies and Raman Spectroscopy. 5D4→7Fj transitions were observed due to presence of Tb3+ in the YAG structure. Phonon band and few inter manifold transitions in 7F6 and 7F5 have been observed. Raman active A1g, Eg, T2g modes have been successfully identified in the phonon band at room temperature Raman measurements.

Raman study of electric-field-induced first-order metal-insulator transition in VO2-based devices

An abrupt first-order metal-insulator transition (MIT) as a current jump has been observed by applying a dc electric field to Mott insulator VO2-based two-terminal devices. The size of the jumps was measured to be asymmetrical depending on the direction of the applied voltage due to heating effects. The structure of VO2 is investigated by micro-Raman scattering experiments. An analysis of the Raman-active Ag modes at 195 and 222cm−1, explained by pairing and tilting of V cations, and 622cm−1, shows that the modes below a low compliance (restricted) current do not change when the MIT occurs, whereas a structural phase transition above the low compliance current is found to occur secondarily, due to heating effects in the device induced by the MIT. The MIT has applications in the development of high-speed and high-gain switching devices.

First-principles studies of solid molecular halogens under pressure: metallization pressures, Raman-active Ag modes and scaling relations

We conducted theoretical studies of the properties of solid halogens (iodine, bromine and chlorine) under pressure, employing the full potential linear muffin–tin orbital method with the local density approximation. From the electronic dispersion curves, we estimated the metallization pressures as 10, 35 and 100 GPa for iodine, bromine and chlorine, respectively. We also examined the pressure dependence of frequencies of Raman-active A g modes using the frozen-phonon method, and found that there was good agreement with experimental data. We found frequency softening in iodine and bromine, and consider this as not only a precursor of molecular dissociation, but also a consequence of metallization. We examined the scaling rules, and found that these agreed with the band-theoretical results.

Diameter distribution of single wall carbon nanotubes in nanobundles

The frequency of the Raman active A1g radial breathing mode has been widely used as a tool to estimate the distribution of diameters of single wall carbon nanotubes (SWNT). However, the relation between frequency and diameter is not straightforward and results are model-dependent. Because most of the experiments are performed on bundles and not on isolated tubes, the model should especially take into account the van der Waals intertube interactions. Here, we use a pair-potential approach to account for such interactions and we derive a nonlinear relation between the SWNT diameter and the frequency of the A1g radial breathing modes. We demonstrate a good agreement between calculations and the diameters derived from diffraction experiments on the same samples.

Ordering and stabilization of C60 films on the ([formula omitted])R30° Sn/Pt(111) surface alloy

The deposition and growth of C60 on the (3×3)R30° Sn/Pt(111) surface alloy is compared with that on the Pt(111) surface at submonolayer, monolayer and multilayer coverages. We find that alloying Pt(111) with Sn arrests the charge transfer from Pt to adsorbed C60, as seen from the absence of a shift in the T1u(1) vibrational levels of C60 probed by high resolution electron energy loss spectroscopy (HREELS). From low energy electron diffraction (LEED) observations it is determined that whereas ordering and graphitization of C60 on Pt(111) take place at 900K, graphitization of C60 is inhibited by Sn alloyed into Pt(111). The rotated hexagonal LEED pattern of the ordered C60 monolayer is stabilized by the presence of Sn on Pt(111) until 1100K, which is close to the fragmentation temperature of solid C60. Upon heating C60 films on the (3×3)R30° Sn/Pt(111) surface alloy, Sn is dealloyed at about 500K, and this dealloyed Sn reacts with C60 at 450–700K, possibly resulting in polymerization. Auger electron spectroscopy annealing studies and the rise in intensity of the unpolarized Raman-active Ag(2) mode at 1467cm−1 support this conclusion. High temperature fragmentation of C60 in the presence of Sn leads to HREELS peaks at 250 and 740cm−1, prior to the formation of graphite.

Far-Infrared, Combination Band, and Raman Spectra of the Ring-Puckering Vibration of 1,4-Disilacyclohexa-2,5-diene

The far-infrared spectrum of 1,4-disilacyclohexa-2,5-diene shows a series of four closely spaced bands originating at 70.5 cm-1 resulting from the ring-puckering vibration. Four mid-infrared bands near 2214 cm-1 result from combinations of the puckering with the Raman active B2g SiH2 antisymmetric stretch at 2143.2 cm-1, and seven difference bands near 2083 cm-1 result from combinations with the Raman active Ag SiH2 symmetric stretch at 2154.0 cm-1. The ring-puckering transitions result from a single-minimum potential energy function, V(cm-1) = 1.23 × 104x2 + 2.40 × 104x,4 where x is the puckering coordinate. The molecule is planar and less rigid than 1,4-cyclohexadiene, thus apparently showing no unusual interactions between the silicon atoms and the CC π systems.