Raman Phonon Research Articles

Nontrivial metallic state of MoS2

The electrical conductivity and Raman spectroscopy measurements have been performed on MoS$_2$ at high pressures up to 90 GPa and variable temperatures down to 5 K. We find that the temperature dependence of the resistance in a metallic 2H$_a$ phase has an anomaly (a hump) which shifts with pressure to higher temperature. Concomitantly, a new Raman phonon mode appears in the metallic state suggesting that the electrical resistance anomaly may be related to a structural transformation. We suggest that this anomalous behavior is due to a charge density wave state, the presence of which is indicative for a possibility for an emergence of superconductivity at higher pressures.

Impact of annealing on the structural and optical properties of ZnO nanoparticles and tracing the formation of clusters via DFT calculation

In this report, nanoparticles (NPs) of zinc oxide (ZnO) were synthesized through auto-combustion route and annealed in air at different temperatures: 200, 400, 600 and 800 °C. The single phase nature has been confirmed via X-ray diffraction (XRD) and selected area electron diffraction (SAED) analysis. The average of crystallite sizes increased progressively as the thermal annealing increase and ranging between 13.8 and 39.7 nm. Raman spectra resembled to that of hexagonal ZnO wurtzite structure, the narrowing in E2H (438 cm−1) Raman phonon mode in sequence with further annealing indicates improvement in the crystallinity and reduction in the local atomic defects of oxygen vacancy (Vo2+). The defects create energy deep bands within the band gap region and diminish the UV emission efficiency that has been assessed through photoluminescence (PL) spectroscopic. The donor band of oxygen vacancy (Vo2+) was ∼1 eV above valence band (VB), whereas the acceptor band of zinc vacancy (VZn2−) was at ∼0.85 eV above the VB. The ZnO NPs that was annealed at lowest temperature of 200 °C exhibited different behavior trend in which the UV-PL band was diminished clearly, blue shifted to lower wave length and appeared as small plateau at the range of 380–270 nm corresponding to high band gap energy (3.8–4.6 eV), which is indicative to low crystal quality and presence of clusters. We used density function theory (DFT) calculation for computing the HOMO-LUMO band gap of supposed clusters ZnnOn (n = 2, 3, 5, 10). The high band gap energy of the simulated clusters was agreed with that of ZnO NPs annealed at 200 °C that gives indirect evidence on the formation of clusters.

Influence of Cr-doping on structural, morphological, optical, dielectric and magnetic properties of KNbO3 ceramics

In this work, we report on the structural, morphological, optical, dielectric and magnetic properties of undoped and Cr doped KNbO3 (KNb1-xCrxO3; x = 0.0, 0.01, 0.03 and 0.05 mol.%) synthesized at 900 °C by solid state reaction method. X-ray diffraction analysis showed the formation of single phase KNbO3 with the orthorhombic structure. Further, the Rietveld refinement analysis revealed that no impurity or secondary phase formation related to the Cr doping. The crystallite size of KNb1-xCrxO3 varies between 553 nm and 948 nm. All the characteristic Raman phonon modes of KNb1-xCrxO3 confirmed the formation of compound with the orthorhombic structure. The presence of K, Nb, O and Cr elements and a shift in binding energy of Nb and O was observed by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses, respectively. Surface morphology of KNb1-xCrxO3 samples exhibit different morphologies, namely rods, cubes and micro edges. Diffuse reflectance spectra revealed that band gap of KNb1-xCrxO3 ceramics decreases with increasing Cr doping level. The photoluminescence emission peaks at ∼410–492 nm (blue) and 520 nm (green) indicate the oxygen vacancies present in the sample. A high coercivity (593 Oe) and saturation magnetization (0.021 emu g−1) is obtained for 0.03 mol.% of Cr doping. The dielectric permittivity was enhanced with the increase of Cr doping concentration in KNbO3 matrix.

Ultrafast optically induced ferromagnetic/anti-ferromagnetic phase transition in GdTiO3 from first principles

Epitaxial strain and chemical substitution have been the workhorses of functional materials design. These static techniques have shown immense success in controlling properties in complex oxides through the tuning of subtle structural distortions. Recently, an approach based on the excitation of an infrared active phonon with intense midinfrared light has created an opportunity for dynamical control of structure through special nonlinear coupling to Raman phonons. We use first-principles techniques to show that this approach can dynamically induce a magnetic phase transition from the ferromagnetic ground state to a hidden antiferromagnetic phase in the rare earth titanate GdTiO3 for realistic experimental parameters. We show that a combination of a Jahn–Teller distortion, Gd displacement, and infrared phonon motion dominate this phase transition with little effect from the octahedral rotations, contrary to conventional wisdom.

Isotope effect on superconductivity and Raman phonons of Pyrochlore Cd2Re2O7

Cd2Re2O7 is the only α-Pyrochlore exhibiting superconductivity with a transition temperature (Tc) of ∼ 1 K. In this study, we present the effect of oxygen isotope (18O) as well as combined 18O and cadmium isotope (116Cd) substitution on the superconductivity and Raman scattering spectrum of Cd2Re2O7. The change of Tc and the energy gap Δ(T) are reported using various techniques including point contact spectroscopy. The shift in Raman phonon frequencies upon isotope substitution will be compared with measurement of the isotope effect on the superconducting transition temperature.

Low-temperature heat capacities of Y2GaSbO7 and R2FeSbO7 (R = Y, Dy) pyrochlore compounds: Quasi-harmonic phonons calculation

Abstract Heat capacities at constant pressure, C°p,m, of three polycrystalline pyrochlore compounds, Y2GaSbO7, Y2FeSbO7 and Dy2FeSbO7, have been measured in the temperature range up to 300 K. Temperature variation of the Debye temperatures, θD(T), for Y2GaSbO7 is determined using a quasi-harmonic expression of Debye lattice heat capacity, which has been employed to estimate the phonon heat capacity, Cph, for the other two compounds, e.g., Y2FeSbO7 and Dy2FeSbO7. Theoretical analysis using phonon model shows that the acoustic and optic phonons contribute significantly to the thermodynamic properties. The effect of anharmonicity in the lattice vibrations at higher temperature (above 220 K) was also considered for Y2GaSbO7. The Cph of Y2FeSbO7, calculated using the experimental infra-red and Raman phonon spectra, are in good agreement with the measured heat capacity data and thus provides a good taste of understanding to the molecular dynamics in the pyrochlore structure. The Schottky heat capacity has been deduced from the crystal-field level scheme of Dy3+ ions and added to the Cph of Dy2FeSbO7 to estimate the total heat capacity, which is in reasonable agreement with the experimental heat capacity, C°p,m, of Dy2FeSbO7. From the integrated heat capacity data, the entropy and enthalpy relative to absolute zero have been generated for Y2GaSbO7 and Y2FeSbO7. The room-temperature thermal conductivity was estimated to be ∼2 W⋅m−1⋅K−1 for Y2GaSbO7 implying that Y2GaSbO7 could be a potential candidate for thermal barrier coating material.

Effect of high Fe doping on Raman modes and optical properties of hydrothermally prepared SnO2 nanoparticles

Fe-doped tin dioxide nanoparticles SnO2:Fe (x%) with x ranging from 0 to 20 were elaborated by a performed hydrothermal method. A deep structural study on the obtained nanoparticles was carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The XRD measurements revealed that the rutile structure of the SnO2 was kept even for high iron concentrations. It is also observed that when iron amount increases the grain size decreases and the band gap is obviously red shifted. HRTEM imaging shows a good crystallinity of the elaborated NPs and confirm the grain size magnitude deduced from XRD measurements. The Raman modes associated to ferric phases are absent in the Fe-doped spectra which is in agreement with the XRD and HRTEM results proving the formation of SnO2:Fe solid phase. The Raman A1g mode seems to be strongly affected by the morphology transformation undergone by SnO2 NPs and by the induced local symmetry break when introducing iron. Forbidden Raman phonon modes were detected and an enhancement of the (110) surface oxygen bridging seems to follow the iron doping. This latter structural feature seems to present a major contribution to the notable enhancement of hydroxyl groups’ adsorption on the Fe-doped SnO2 surface. The variation of the Urbach energy Eu with Fe amount is discussed and is found to be in accordance with the different interpretations. The PL spectra reveals that the UV-violet band of the SnO2 NPs was affected by iron presence and the induced non-systematic variation of this emission was discussed. A decrease in visible emission was noted when introducing iron revealing that Fe doping affects the density of singly charged oxygen vacancies.

The properties of spray-deposited zinc sulfide thin films using trisodium citrate complexant

Zinc sulfide thin films were deposited on soda-lime glass substrates by chemical spray pyrolysis technique using trisodium citrate complexant. The films were deposited at substrate temperatures of 300 to 400 °C using 0.1 M zinc chloride, 0.1 M thiourea, and 0.01 M trisodium citrate. The thin films’ transmittance is between 40.83 and 50.86% at 550 nm, and energy band gap varied between 3.52 and 3.72 eV. The films have very high refractive indices in the range of 3.07 to 4.02 at 550 nm. The X-ray diffraction (XRD) analyses of the films revealed hexagonal structure. The crystallite size changed between 7.030 and 45.486 nm. The surface morphology is flower-like, dense, and contains rectangular nanorod-like particles and cracks. Raman spectroscopy revealed the presence of first-order (transverse optic (TO) and longitudinal acoustic), second-order (2TO and 2LO), and third-order (3LO) Raman phonons. The Fourier transform infrared spectra indicate the presence of chemical bonds in the thin films, while photoluminescence indicates near band edge emissions. The contact angle revealed hydrophilic nature of the deposited ZnS thin films.

Nuclear quadrupolar spin-lattice relaxation with realistic phonon density of states in alkali halides

The nuclear quadrupolar spin-lattice relaxation times T1 have been calculated in alkali halides, whereby the phonon densities of states have been extracted from the shell model lattice dynamics. The calculations at room temperature were performed on 79Br in KBr, NaBr, and RbBr; 23Na in NaI, NaCl, and NaBr; 35Cl in NaCl and KCl; and 127I in NaI and KI. The obtained values of relaxation time T1 are an order of magnitude smaller than those calculated with the Debye-model phonon density of states and agree well with experiments. The temperature dependence of acoustic and optical phonon contributions to the nuclear relaxation rate (first-order Raman phonon process) are carried out in the temperature range 20 to 300 K for 23Na and 127I in NaI. The role of optical phonons becomes important from 77 K on; the temperature dependence of T1 agrees with experimental data throughout most of the temperature range studied (77–300 K), while differing quantitatively from the predictions obtained on the basis of the Debye model.

NaX solvation bonding dynamics:hydrogen bond and surface stress transition (X = HSO4, NO3, ClO4, SCN)

Raman phonon differential spectrometrics (DPS) and contact angle measurements resolved that solvation of the NaX (X=HSO4, NO3, ClO4, SCN) complex salts stiffens the HO stretching phonon from 3200 to ~3500cm−1 and softens the O:H nonbond phonon from 180 to ~70cm−1 with rising of solution surface stress. The solute capability of bond transition in terms of the fraction coefficient, follows the relation, fNaX(C)∝1-exp(−C/C0) towards saturation, with C being the solute concentration and C0 the decay constant. Observations evidence that: (i) the solute ionic field electrification aligns, stretches, and polarizes its neighboring H2O molecules, which shortens the HO bond but lengthens the O:H nonbond via OO Coulomb repulsion; (ii) the effect of X− electrification on the O:HO bond relaxation varies with solute type and solute concentration. Exercises not only verify the essentiality of solvent O:HO bond cooperative relaxation and polarization but also demonstrate the power of DPS that resolves processes occurred upon solvation.

Effect of pressure on optical properties of the transition metal dichalcogenide MoSe2

Transition Metal Dichalcogenides TMDs MoX2 (X=S, Se, Te) are an emergent class of layered materials displaying exciting optoelectronic properties which can be modified by varying the number of layers, by intercalation, or by applying an external strain/compression. In particular, these semiconducting materials can get a band gap closure under pressure and enter in a metallic phase. Here, we investigate the optical properties of MoSe2 under high pressure by means of Raman spectroscopy over a wide pressure range (0-30 GPa). No anomaly has been observed in the pressure dependence of the frequencies of the vibrational modes A1g, E12g (Raman first order) and E1g - E22g (Raman second-order), in agreement with the absence previously reported of any pressure induced structural transition. Interestingly, our detailed line-shape analysis show a clear anomaly in the pressure behavior of the linewidth of the A1g and E12g phonons at the insurgent metallization process which was observed in previous infrared and transport experiments. Our results indicate that the linewidths of Raman phonons peaks can be sensitive to even subtle pressure-induced electronic rearrangements and can thus be used to monitor the insurgence of a pressure-induced semiconductor-metal transition.

Thermal Conductivity and Raman Spectra of Carbon Fibers

Due to its unique physical properties, carbon fiber (CF) has been widely studied for extensive application in aerospace and machinery. In this study, the thermal diffusivity of three kinds of CF sample is characterized by the transient electrothermal technique at room temperature. By subtracting the effect of radiative losses, the effective thermal diffusivity of CFs can be calculated as $$6.46\times 10^{-6}\,\hbox {m}^{2}\cdot \hbox {s}^{-1}$$ , $$6.58\times 10^{-6}\,\hbox {m}^{2}\cdot \hbox {s}^{-1}$$ and $$2.01\times 10^{-4}\,\hbox {m}^{2}\cdot \hbox {s}^{-1}$$ , respectively. For the first time, the emissivity coefficient of carbon fiber is calibrated as 0.78. Combined with Raman spectra and phonon scattering, we found that the better crystalline structure and low defect in CF have an obvious impact on its thermal diffusivity.

Effect of Nitrogen Impurities on the Raman Line Width in Diamond, Revisited

The results of a high-resolution Raman scattering study of a diamond crystal with a high content of single substitutional nitrogen impurities (550 ppm) in the temperature range from 50 to 673 K are presented and compared with the data for defect-free diamond. It is established that the increase of the nitrogen concentration in diamond leads to the temperature-independent increase of the Raman line width. Analysis of the experimental data allows us to conclude that this broadening should be attributed to the defect-induced shortening of the Raman phonon lifetime. We believe that this mechanism is responsible for the increase of the Raman line width caused by most point-like defects in diamond. No pronounced effects of the nitrogen defects on the Raman line position and phonon anharmonicity are observed.

3R and 2H polytypes of MoS2: DFT and DFPT calculations of structural, optoelectronic, vibrational and thermodynamic properties

We report the results of a theoretical study on the behavior of the structural, optoelectronic, vibrational, including infrared and Raman theoretical spectra, phonon spectrum, and thermodynamic properties of 3R- and 2H- polytypes of molybdenum disulfide (MoS2) using density functional theory (DFT) considering both the local density and generalized gradient approximation, LDA and GGA, respectively. Calculated lattice parameters are close to the experimental measurements, and an indirect band gap E(A→KΓ)=1.33 eV (0.68 eV) was obtained within the GGA (LDA) level of calculation, considering the 3R-polytype, and for the 2H- polytype an indirect band gap E(Γ→KΓ)=1.30 eV (0.70 eV) was obtained within the GGA (LDA) approximation. The complex dielectric function and absorption of 3R-MoS2 and 2H-MoS2 polytypes were shown to be sensitive to the plane of polarization of the incident light. The phonon dispersion relation together with density of states (DOS) as well as theoretical peaks of the infrared (IR) and Raman spectra in the frequency range of 0–800 cm−1 was analyzed and assigned, considering the norm-conserved pseudopotentials. The thermodynamic potentials, the specific heat at constant volume and Debye temperature of the 3R-MoS2 and 2H-MoS2 polytypes are also calculated, whose dependence on the temperature are discussed.

Thermal behavior of mullite between 4 K and 1320 K

AbstractThe evolution of metric parameters of 2:1 and 3:2 mullites have been measured between 4 K and 1320 K using neutron and X‐ray powder diffraction. Negative thermal expansion was observed at low temperature for the a‐cell parameter and consequently for the cell‐volume, which is more pronounced for 2:1 mullite than those for 3:2 mullite. Each parameter is simulated using Grüneisen first‐order approximation for the zero pressure equation of state at 0 K, where the vibrational energy was calculated using microscopic approach. While the b‐ and c‐cell parameters require only one Debye term, a second Debye spectrum with negative Grüneisen parameter was required to fit the a‐cell parameter as well as the cell volume. At 4 K, 300 K and 1320 K the model, respectively, calculates the volume thermal expansion coefficients of 0.09x10−6 K−1, 9x10−6 K−1, and 17.3x10−6 K−1 for 2:1 mullite, and 0.09x10−6 K−1, 8.7x10−6 K−1, and 17.3x10−6 K−1 for 3:2 mullite. Temperature‐dependent Raman spectra and phonon density of states hint for the possible microscopic sources of the cell contraction at low temperature. A simple polynomial approach is presented to calculate the elastic stiffness coefficients of the 3:2 mullite, which are not available from experiments.

Chemical spray pyrolysis deposition of zinc sulphide thin films using ethylenediaminetetraacetic acid disodium salt complexant

Zinc sulphide thin films were deposited by chemical spray pyrolysis technique. The films were deposited at substrate temperatures of 300, 350 and 400 °C using 0.1 M zinc chloride, 0.1 M thiourea and 0.01 M ethylenediaminetetraacetic acid disodium salt (EDTA). The films have moderate transmittance between 56.00 and 77.73% over visible range, and the band gap energy varies between 3.87 and 4.12 eV. The films have high values of refractive index in the range 1.67 to 2.71 at 550 nm. The X-ray diffraction analysis of the films shows the presence of hexagonal and cubic structures. The crystallite size ranges between 6.122 and 43.023 nm. The surface morphology is flower-like, sheet-like and island-shaped nanoclusters. Raman spectroscopy reveals the presence of first-order, second-order and third-order Raman phonons. The Fourier transform infrared spectra show the presence of chemical bonds in the thin films while photoluminescence shows near band edge emission. The contact angle indicated hydrophilic nature of the deposited ZnS thin films.

On the morphology of the interlayer surface and micro-Raman spectra of layered films in topological insulators based on bismuth telluride

Resonant micro-Raman spectra and the morphology of the interlayer Van der Waals surface are studied for layered thin films of n-Bi2Te3 and solid solutions based on Bi2Te3. It is found that the composition, thickness, surface morphology, and the method of obtaining films affect the relative intensity of Raman phonons, which are sensitive to the topological surface states of Dirac fermions.

Surface chemical structure and doping characteristics of boron-doped Si nanowires fabricated by plasma doping

We investigated the conduction characteristics of plasma-doped Si nanowires (NWs) after various rapid thermal annealing (RTA) times. The plasma doping (PD) process developed a highly-deposited B layer at the NW surface. RTA process controls electrical conductivity by mediating the dopant diffusion from the surface layer. The surface chemical and substitutional states of the B plasma-doped Si NWs were analyzed by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. To elucidate the detailed structure of the NWs, we analyzed the change in the optical phonon mode caused by the incorporated B atoms. For this purpose, we examined Fano resonance by the investigation of the asymmetry, line-width, and phonon wavenumber in Raman spectra. The changes in symmetry level of the Raman peak, phonon lifetime, and internal strain were closely related to the number of electrically activated borons, which was drastically increased with RTA time. The change in electrical and optical characterizations related to the doping characteristics of the NWs was investigated using a 4-point probe and terahertz time-domain spectroscopy (THz–TDS). The resistivity of the NWs was 3000 times lower after the annealing process compared to that before the annealing process, which is well consistent with the optical conductivity data. The data provide the potential utility of PD in conformal doping for three-dimensional nanodevices.

Atom–atom force field for simulation of zirconia bulk, nanosheets and nanotubes

The set of three pair atom–atom potentials for zirconia crystals and nano objects was developed. Besides Buckingham potential and Coulomb interaction, these force fields employ the additional functional forms: Fermi-Dirac, Inverse Gaussian or Morse potential. The developed force fields are capable to reproduce the structural, mechanical and thermodynamic properties of five zirconia phases: monoclinic, tetragonal, cubic, brookite and cotunnite with acceptable accuracy. The proposed force fields give monoclinic phase as a ground state structure and predict the correct energy ordering of the phases. The Raman and IR phonon frequencies, temperature dependencies of the thermodynamic properties of monoclinic and tetragonal zirconia are predicted in good and moderate agreement with the experimentally observed data and results of first-principles calculations. To test the transferability of proposed potentials, the zirconia-based nanosheets and nanotubes were simulated. Results of structure optimisation and calculation of the nanotubes’ strain energies are well compared with the corresponding data of the first-principles calculations.

Raman Spectroscopic Observation of Gradual Polymorphic Transition and Phonon Modes in CuPc Nanorod

A Raman scattering investigation of a single α-CuPc nanorod is demonstrated here within the temperature range 300–770 K. From the typical thermal dispersion of Raman shift and phonon line width of in-plane B1g mode at 1526 cm–1, the metastable α polymorph is observed to undergo unambiguous phase transition to thermally stable β phase. The phase transition temperature (456 K) is observed to be significantly lower than that reported for bulk samples. Extensive computation of normal modes associated with both α- and β-CuPc reveal that the “fingerprint” domain across 1300–1600 cm–1 is particularly sensitive to this polymorphic transition where a silent B2g mode appears at 1303 cm–1 in the spectral vicinity of molecular edge vibration sensitive Ag–Bg Davydov doublet.