Frequencies Of Raman-active Modes Research Articles

Pt-and Pd-doped carbon nanotubes with different diameters as CO sensors: A comparative DFT study

The density functional theory (DFT) is employed to assess the electronic characteristics and vibrational frequencies of CO adsorbed by several Zigzag carbon nanotubes with different diameters that are substitutionally doped using Pt and Pd. Computations were based on the Quantum Espresso (PBE) and Gaussian 16/6-31G(d,p) or LanL2DZ basis sets. The formation energy, adsorption energy, and interaction distances between CO and carbon nanotube as a function of the tube diameter were evaluated for Pt- and Pd- doped carbon nanotubes. Such doped nanotubes have relatively higher adsorption energy at smaller diameters, amounting to −2.784 eV and −2.635 eV for Pd-doped and Pt-doped carbon nanotubes, respectively. The adsorption energies of CO gas on Larger tubes Pt and Pd doped carbon nanotube are −2.207 eV, and −2.583 eV eV, respectively. Following CO adsorption, the smaller diameter Pt-doped carbon nanotube has a relatively greater change in electrical conductivity and chemical reactivity than other Pd- or Pt-doped nanotubes with relatively higher diameters. These changes are attributed to energy gaps concerning the alpha and beta HOMO and LUMO molecular orbitals. The computed Raman active mode frequencies extensively depend on diameter pre- and post-adsorption on the Pd- and Pt -nanotubes. This work offers several scientific suggestions concerning the use of Pt and Pd doped carbon nanotubes with different diameters for CO molecule sensor use cases. Such sensor materials having charge movement-based sensing might be employed as acceptors or donors.

Raman scattering and Cr3+ luminescence study on the structural behavior of δ-AlOOH at high pressures

Abstract δ-AlOOH is regarded as a potential water carrier that is stable in the Earth’s lower mantle down to the core-mantle boundary along the cold slab geotherm; thus, knowledge of its structural evolution under high pressure is very important for understanding water transport in the Earth’s interior. In this work, we conducted Raman scattering and luminescence spectroscopic experiments on δ-AlOOH at pressures up to 34.6 and 22.1 GPa, respectively. From the collected Raman spectra, significant changes in the pressure dependence of the frequencies of Raman-active modes were observed at ~8 GPa, with several modes displaying softening behavior. In particular, the soft A1 mode, which corresponds to a lattice vibration of the AlO6 octahedron correlated to OH stretching vibrations, decreases rapidly with increasing pressure and shows a trend of approaching 0 cm−1 at ~9 GPa according to a quadratic polynomial extrapolation. These results provide clear Raman-scattering spectroscopic evidence for the P21nm-to-Pnnm structural transition. Similarly, the phase transition was also observed in the luminescence spectra of Cr3+ in both powder and single-crystal δ-AlOOH samples, characterized by abrupt changes in the pressure dependences of the wavelength of the R-lines and sidebands across the P21nm-to-Pnnm transition. The continuous decrease in R2-R1 splitting with pressure indicated that the distortion of the AlO6 octahedron was suppressed under compression. No abnormal features were clearly observed in our Raman or luminescence spectra at ~18 GPa, where the ordered symmetrization or fully centered state with hydrogen located at the midpoint of the hydrogen bond was observed by a previous neutron diffraction study. However, some subtle changes in Raman and luminescence spectra indicated that the ordered symmetrization state might form at around 16 GPa.

High-Pressure Raman Study of Fe(IO3)3: Soft-Mode Behavior Driven by Coordination Changes of Iodine Atoms

We report high-pressure Raman spectroscopy studies of Fe(IO3)3 up to nearly 21 GPa that have been interpreted with the help of density-functional theory calculations, that include the calculation of phonon dispersion curves and elastic constants at different pressures. Zero-pressure Raman-active mode frequencies and their pressure dependences have been determined. Modes have been assigned and correlated to atomic movements with the help of calculations. Interestingly, in the high-frequency region there are several modes that soften under compression. These modes have been identified as internal vibrations of the IO3 coordination polyhedron. Their unusual behavior is a consequence of the changes induced by pressure in the coordination sphere of iodine, which gradually change from a three-fold to almost six-fold coordination under compression. The coordination change is favored by the decrease of the stereoactivity of the iodine lone electron pair, so that likely real six-fold coordination is attained after a first-order phase transition previously reported to occur above 21 GPa. The strong non-linear behavior found in Raman-active modes as well as in theoretically calculated elastic constants has been discovered to be related to the occurrence of two previously unreported isostructural phase transitions at 1.5-2.0 GPa and 5.7-6.0 GPa as shown by dynamic instabilities close to the Brillouin zone center.

Structural and Lattice-Dynamical Properties of Tb2O3 under Compression: A Comparative Study with Rare Earth and Related Sesquioxides.

We report a joint experimental and theoretical investigation of the high pressure structural and vibrational properties of terbium sesquioxide (Tb2O3). Powder X-ray diffraction and Raman scattering measurements show that cubic Ia3̅ (C-type) Tb2O3 undergoes two phase transitions up to 25 GPa. We observe a first irreversible reconstructive transition to the monoclinic C2/m (B-type) phase at ∼7 GPa and a subsequent reversible displacive transition from the monoclinic to the trigonal P3̅m1 (A-type) phase at ∼12 GPa. Thus, Tb2O3 is found to follow the well-known C → B → A phase transition sequence found in other cubic rare earth sesquioxides with cations of larger atomic mass than Tb. Our ab initio theoretical calculations predict phase transition pressures and bulk moduli for the three phases in rather good agreement with experimental results. Moreover, Raman-active modes of the three phases have been monitored as a function of pressure, while lattice-dynamics calculations have allowed us to confirm the assignment of the experimental phonon modes in the C- and A-type phases as well as to make a tentative assignment of the symmetry of most vibrational modes in the B-type phase. Finally, we extract the bulk moduli and the Raman-active mode frequencies together with their pressure coefficients for the three phases of Tb2O3. These results are thoroughly compared and discussed in relation to those reported for rare earth and other related sesquioxides as well as with new calculations for selected sesquioxides. It is concluded that the evolution of the volume and bulk modulus of all the three phases of these technologically relevant compounds exhibit a nearly linear trend with respect to the third power of the ionic radii of the cations and that the values of the bulk moduli for the three phases depend on the filling of the f orbitals.

Raman Spectra and Strain Effects in Bismuth Oxychalcogenides

A new type of two-dimensional layered semiconductor with weak electrostatic but not van der Waals interlayer interactions, Bi2O2Se, has been recently synthesized, which shown excellent air stability and ultrahigh carrier mobility. Herein, we combined theoretical and experimental approaches to study the Raman spectra of Bi2O2Se and related bismuth oxychalcogenides (Bi2O2Te and Bi2O2S). The experimental peaks were fully consistent with the calculated results, and were successfully assigned. Bi2O2S was predicted to have more Raman-active modes due to its lower symmetry. The shift of the predicted frequencies of Raman active modes was also found to get softened as the interlayer interaction decreases from bulk to monolayer Bi2O2Se and Bi2O2Te. To reveal the strain effects on the Raman shifts, a universal theoretical equation was established based on the symmetry of Bi2O2Se and Bi2O2Te. It was predicted that the doubly degenerate modes split under in-plane uniaxial/shear strains. Under a rotated uniaxial strain, the changes of Raman shifts are anisotropic for degenerate modes although Bi2O2Se and Bi2O2Te were usually regarded as isotropic systems similar to graphene. This implies a novel method to identify the crystallographic orientation from Raman spectra under strain. These results have important consequences for the incorporation of 2D Bismuth oxychalcogenides into nanoelectronic devices.

High-pressure structural and vibrational properties of monazite-type BiPO4, LaPO4, CePO4, and PrPO4

Monazite-type BiPO4, LaPO4, CePO4, and PrPO4 have been studied under high pressure by ab initio simulations and Raman spectroscopy measurements in the pressure range of stability of the monazite structure. A good agreement between experimental and theoretical Raman-active mode frequencies and pressure coefficients has been found which has allowed us to discuss the nature of the Raman-active modes. Besides, calculations have provided us with information on how the crystal structure is modified by pressure. This information has allowed us to determine the equation of state and the isothermal compressibility tensor of the four studied compounds. In addition, the information obtained on the polyhedral compressibility has been used to explain the anisotropic axial compressibility and the bulk compressibility of monazite phosphates. Finally, we have carried out a systematic discussion on the high-pressure behavior of the four studied phosphates in comparison to results of previous studies.

Structural and vibrational properties of (Bi1−xLax)FeO3 and (Bi1−yBay)(Fe1−yTiy)O3 multiferroic ceramics investigated by Raman scattering

Abstract La-modified Bi 1−x La x FeO 3 (x = 0.15–0.40) and BaTiO 3 -modified (Bi 1−y Ba y )(Fe 1−y Ti y )O 3 (y = 0.10−0.33) multiferroic ceramics were synthesized via solid-state reactions, and their structural and vibrational properties were investigated by X-ray diffraction (XRD) and Raman spectroscopy. The XRD patterns reveal that the La-modified Bi 1−x La x FeO 3 system exhibits an rhombohedral – orthorhombic structural transition at x = 0.40. The degree of rhombohedral distortion in the Bi 1−x La x FeO 3 ceramics was decreased with increasing the La content, resulting in the disappearance of some Raman active modes (e.g. A 1 -4, E (TO1), E (TO2), E (TO6)). Similarly, in the BaTiO 3 -modified (Bi 1−y Ba y )(Fe 1−y Ti y )O 3 system, a composition-driven structural transition from rhombohedral ( R 3c) phase to cubic phase appeared at y = 0.30, indicating a morphotropic phase boundary between the rhombohedral and cubic phases. It is found that the Raman mode frequencies of the A 1 -1, A 1 -2, A 1 -3 and E (TO3) modes are slightly dependence of either the La content in Bi 1−x La x FeO 3 system or the BaTiO 3 content in (Bi 1−y Ba y )(Fe 1−y Ti y )O 3 system, whereas the frequencies of the E (TO8) and E (TO9) Raman active modes are significantly dependent upon the modified compositions. Such compositional dependence of the Raman active mode frequencies are discussed and interpreted based on a spring oscillator model in combination with the structural phase transition, the degree of rhombohedral distortion, and the B–O bond length in the BO 6 octahedron. The present results are useful for understanding the correlation between the structural and physical properties of the La-modified Bi 1−x La x FeO 3 and the BaTiO 3 -modified (Bi 1−y Ba y )(Fe 1−y Ti y )O 3 ceramics.

Pressure effects on crystal and electronic structure of bismuth tellurohalides

We study the possibility of pressure-induced transitions from a normal semiconductor to a topological insulator (TI) in bismuth tellurohalides using density functional theory and tight-binding method. In BiTeI this transition is realized through the formation of an intermediate phase, a Weyl semimetal, that leads to modification of surface state dispersions. In the topologically trivial phase, the surface states exhibit a Bychkov–Rashba type dispersion. The Weyl semimetal phase exists in a narrow pressure interval of 0.2 GPa. After the Weyl semimetal–TI transition occurs, the surface electronic structure is characterized by gapless states with linear dispersion. The peculiarities of the surface states modification under pressure depend on the band-bending effect. We have also calculated the frequencies of Raman active modes for BiTeI in the proposed high-pressure crystal phases in order to compare them with available experimental data. Unlike BiTeI, in BiTeBr and BiTeCl the topological phase transition does not occur. In BiTeBr, the crystal structure changes with pressure but the phase remains a trivial one. However, the transition appears to be possible if the low-pressure crystal structure is retained. In BiTeCl under pressure, the topological phase does not appear up to 18 GPa due to a relatively large band gap width in this compound.

Theoretical investigations on vibrational properties and thermal conductivities of ternary antimonides TiXSb, ZrXSb and HfXSb (X = Si, Ge)

We have performed density functional calculations of the vibrational and thermodynamic properties of the ternary antimonides TiXSb, ZrXSb and HfXSb (X = Si, Ge). The direct method is used to calculate the phonon dispersion relation and phonon density of states for these compounds as well as their infrared and Raman active mode frequencies for the first time. Their dynamical stability is confirmed by phonon spectra. The lattice thermal conductivities of these compounds have been calculated from third-order force constants and plotted as a function of temperature. We have also evaluated the high temperature thermal conductivity by means of the Clarke’s model and Cahill’s model. Some selected thermodynamical properties, e.g. Gibbs free energy, entropy and heat capacity at constant volume are predicted theoretically and discussed. We have showed the relationships between thermodynamical properties and temperature.

Phonon Properties of Few-Layer Crystals of Quasi-One-Dimensional ZrS3 and ZrSe3

The frequencies of Raman active modes of few-layer ZrS3 and ZrSe3 and their dependence on the number of layers have been investigated by Raman scattering spectroscopy and first-principles calculations. We find a considerable downshift of the frequency of the Ag3 mode with decreasing the number of layers, and a slight downshift of the Ag5 mode. The same frequency shifts are reproduced by the calculations with remarkable accuracy. We explain the difference in the frequency shifts of the Ag3 and Ag5 modes in terms of intrachain, interchain, and interlayer force constants in simple one-dimensional lattice models. We thus show that the large shift of the Ag3 mode reflects the quasi-one-dimensional structure of ZrS3 and ZrSe3, which does not exist in the more widely studied two-dimensional materials.

First-principles calculations of phonon and thermodynamic properties of OsSi2

The structure, lattice dynamics, and some thermodynamic properties of orthorhombic OsSi2 were investigated using a first-principles density functional theory (DFT). Linear response theory was used to calculate the phonon dispersion relation and phonon density of states for OsSi2 as well as its infrared and Raman active mode frequencies. In this study, the thermodynamic properties, including vibrational entropy (Svib), constant-volume specific heat (Cv), and Debye temperature (ΘD), were predicted theoretically and discussed.

Investigation of structural, elastic, and lattice-dynamical properties of Ca2Si, Ca2Ge, and Ca2Sn based on first-principles density functional theory

The structural, elastic, phonon and thermodynamic properties of orthorhombic Ca2X (X=Si, Ge, and Sn) were systematically investigated using first-principles density functional theory (DFT). The elastic constants (cij), elastic modulus (B, G, E), Poisson’s ratio (ν), and elastic Debye temperature (ΘD) were determined and compared with the results of previous calculations. For the first time, linear response theory is used to calculate the phonon dispersion relation and phonon density of states for these compounds as well as their infrared and Raman active mode frequencies. In this study, the thermodynamic properties such as vibrational entropy (Svib) and constant-volume specific heat (Cv) were predicted theoretically and discussed.

Structural and Vibrational Properties of CdAl2S4 under High Pressure: Experimental and Theoretical Approach

The behavior of defect chalcopyrite CdAl2S4 at high pressures and ambient temperature has been investigated in a joint experimental and theoretical study. High-pressure X-ray diffraction and Raman scattering measurements were complemented with theoretical ab initio calculations. The equation of state and pressure dependences of the structural parameters of CdAl2S4 were determined and compared to those of other AB2X4 ordered-vacancy compounds. The pressure dependence of the Raman-active mode frequencies is reported, as well as the theoretical phonon dispersion curves and phonon density of states at 1 atm. Our measurements suggest that defect chalcopyrite CdAl2S4 undergoes a phase transition above 15 GPa to a disordered-rocksalt structure, whose equation of state was also obtained up to 25 GPa. In a downstroke from 25 GPa to 1 atm, our measurements indicate that CdAl2S4 does not return to the defect chalcopyrite phase; it partially retains the disordered-rocksalt phase and partially transforms into the spin...

Compositional dependence of Raman-active mode frequencies and line widths in TlInS2xSe2(1−x) mixed crystals

The Raman spectra of mixed crystals TlInS2xSe2(1−x) have been investigated in the composition range of 0.25≤x≤0.75 and in the high-frequency region of 250–350cm−1 at room temperature. It was observed that Raman-active mode frequencies decrease as the selenium atoms content increases in the mixed crystals. The effect of crystal disorder on the line broadening of three high-frequency modes is reported.

Magnetic and micro-Raman studies of hexagonal-DyMnO3

Dc-susceptibility measurements and Raman active phonon frequencies of hexagonal DyMnO3 retrace the Mn3+ ions antiferromagnetic transition at TN ∼ 70 K and their spin reorientation at TSR ∼ 48 K. The temperature evolution of Raman active mode frequencies and their over-hardening are associated with Dy3+ and Mn3+ ion displacements below TN and with a spin–phonon coupling that involves apical oxygen.

Lattice dynamics of bismuth tellurohalides

Lattice dynamics of two bismuth tellurohalides, BiTeI and BiTeCl, and the influence of spin-orbit coupling on their vibrational properties are investigated using first-principles calculations in the density functional perturbation formalism. We also report on the Raman study of these bismuth tellurohalides as well as BiTeBr. It is shown that the inclusion of spin-orbit interaction results in a sizable softening of the phonon spectra of BiTeI and BiTeCl. For both compounds a strong anisotropy in the dielectric tensor and in the Born effective charges is found. Predicted Raman active mode frequencies in BiTeI and BiTeCl are in good agreement with the experimental Raman data.

Effects of static charging and exfoliation of layered crystals

Using a first-principle plane-wave method we investigate the effects of static charging on the structural, electronic, and magnetic properties of suspended, single-layer graphene, graphane, fluorographene, BN, and MoS${}_{2}$ in a honeycomb structure. The limitations of periodic boundary conditions in the treatment of negatively charged layers are clarified. Upon positive charging, the band gaps between the conduction and valence bands increase, but the single-layer nanostructures become metallic owing to the Fermi level dipping below the maximum of valence band. Moreover, their bond lengths increase, leading to phonon softening. As a result, the frequencies of Raman active modes are lowered. A high level of positive charging leads to structural instabilities in single-layer nanostructures, since their specific phonon modes attain imaginary frequencies. Similarly, excess positive charge is accumulated at the outermost layers of metallized BN and MoS${}_{2}$ sheets comprising a few layers. Once the charging exceeds a threshold value, the outermost layers are exfoliated. Charge relocation and repulsive force generation are in compliance with classical theories.

Effects of static charging and exfoliation of layered crystals

Using a first-principle plane-wave method we investigate the effects of static charging on the structural, electronic, and magnetic properties of suspended, single-layer graphene, graphane, fluorographene, BN, and MoS${}_{2}$ in a honeycomb structure. The limitations of periodic boundary conditions in the treatment of negatively charged layers are clarified. Upon positive charging, the band gaps between the conduction and valence bands increase, but the single-layer nanostructures become metallic owing to the Fermi level dipping below the maximum of valence band. Moreover, their bond lengths increase, leading to phonon softening. As a result, the frequencies of Raman active modes are lowered. A high level of positive charging leads to structural instabilities in single-layer nanostructures, since their specific phonon modes attain imaginary frequencies. Similarly, excess positive charge is accumulated at the outermost layers of metallized BN and MoS${}_{2}$ sheets comprising a few layers. Once the charging exceeds a threshold value, the outermost layers are exfoliated. Charge relocation and repulsive force generation are in compliance with classical theories.

Apparent phonon side-band modes in π-conjugated systems: Polymers, oligomers and crystals

The emission spectra of many π-conjugated polymers and oligomers contain side-band replicas with apparent frequencies that do not match the Raman active mode frequencies. Using a time dependent model we show that in such many mode systems, the increased damping of the time dependent transition dipole moment correlation function results in an effective elimination of the vibrational modes from the emission spectrum; subsequently causing the appearance of a regularly spaced progression at a new apparent frequency. We use this damping dependent vibrational reshaping to quantitatively account for the vibronic structure in the emission spectra of π-conjugated systems in the form of films, dilute solutions and single crystals. In particular, we show that by using the experimentally measured Raman spectrum we can account in detail for the apparent progression frequencies and their relative intensities in the emission spectrum.

Raman scattering studies on mercuric iodide at high pressures and at low temperatures

Raman scattering measurements have been performed on single crystal red mercuric iodide under high-pressures up to ∼5.5 GPa and at low-temperatures down to 40 K. Our low-temperature measurements help clarify the issues related to the accidental degeneracy of the E g 1 and B 1 g 2 modes in the tetragonal phase. Raman modes stiffen and their widths reduce with the lowering of the temperature. All the Raman active mode frequencies increase with pressure up to 1.3 GPa, beyond which the modes of the tetragonal phase disappear and new modes appear supporting the earlier reported red → high-pressure yellow structural transition at this pressure. Knowledge of both temperature and pressure dependences has allowed us to separate, for all the Raman active modes, the explicit phonon-excitation contribution and the implicit volume-driven contribution to the observed temperature coefficient d ω/d T for both the phases.