Zone-center Optical Phonon Modes Research Articles

Lattice vibration and dielectric properties of CaNb2O6

The lattice vibration and dielectric properties of CaNb2O6 have been studied using density functional perturbation theory. The complete set of the zone center optical phonon modes and frequency dependent dielectric function of the CaNb2O6 have been determined. The assignment of mode vibrations to all of the optical phonon frequencies was made by analyzing the eigen displacements. We find that the highest frequency mode of all the Raman active symmetry species is due to NbO6 stretching vibration. Our calculated dielectric functions and quality factor are in very good agreement with the corresponding experimental results.

Electronic and phononic origins of BaSO4 as an ultra-efficient radiative cooling paint pigment

Radiative cooling has recently revived because of its significant potential for saving energy and combating climate change. Several ultra-efficient particle-matrix cooling nanocomposites such as BaSO4-acrylic paints have been demonstrated via trial-and-error approaches, but the atomistic characteristics of their pigments remain elusive. In this work, we use first-principles calculations to predict the full-spectrum optical constants of BaSO4, and successfully explain the ultra-high reflectance in the solar spectrum (0.28–2.5 μm) and simultaneously high normal emittance in the sky window (8–13 μm) observed in previous experiments. Efficient radiative cooling pigments require high refractive index n and low extinction coefficient κ in the solar spectrum. However, our results show that they cannot be tuned independently, but are both tied to the electronic band gap. Eliminating κ would require a high band gap, which would yield low n, creating a dilemma to address for radiative cooling. By systematic comparison, we show that BaSO4 outperforms the commonly used α-quartz (α-SiO2), and we identify two pertinent characters of BaSO4: i) Although the band gap of BaSO4 is high enough to eliminate solar absorption, it is also moderate enough to enable reasonably high refractive index for strong scattering, and ii) BaSO4 has complex crystal structure and appropriate bond strength that yield a high number of infrared-active zone-center optical phonon modes in the Reststrahlen bands, and these modes show strong four-phonon scattering which is a previously unknown mechanism that contributes to the high emissivity in the sky window. Our first-principles approach and physical insights pave the way for further search of efficient radiative cooling materials.

Anharmonic organic cation vibrations in the hybrid lead halide perovskite CH3NH3PbI3

Ultrafast vibrational dynamics of inorganic-organic hybrid lead halide perovskite CH3NH3PbI$ in the room-temperature tetragonal phase is investigated under moderate photoexcitation slightly below the band gap. Time-resolved optical Kerr effect signal exhibits periodic modulations due to the libration and twisting of the methylammonium molecule at 4 and 8 THz, in addition to the inorganic sublattice deformation at 1.2 THz. These oscillations are heavily damped, and their frequencies exhibit blueshifts with increasing pump density, contrary to the expectation from conventional laser heating. Our ab initio lattice dynamics simulations reveal strongly anharmonic vibrational potentials for the zone-center optical phonon modes, and thereby confirm the anharmonicity to be the origin of the experimentally observed frequency blueshifts.

High-pressure Raman scattering in bulk HfS2: comparison of density functional theory methods in layered MS2 compounds (M\u2009=\u2009Hf, Mo) under compression

We report high-pressure Raman-scattering measurements on the transition-metal dichalcogenide (TMDC) compound HfS2. The aim of this work is twofold: (i) to investigate the high-pressure behavior of the zone-center optical phonon modes of HfS2 and experimentally determine the linear pressure coefficients and mode Grüneisen parameters of this material; (ii) to test the validity of different density functional theory (DFT) approaches in order to predict the lattice-dynamical properties of HfS2 under pressure. For this purpose, the experimental results are compared with the results of DFT calculations performed with different functionals, with and without Van der Waals (vdW) interaction. We find that DFT calculations within the generalized gradient approximation (GGA) properly describe the high-pressure lattice dynamics of HfS2 when vdW interactions are taken into account. In contrast, we show that DFT within the local density approximation (LDA), which is widely used to predict structural and vibrational properties at ambient conditions in 2D compounds, fails to reproduce the behavior of HfS2 under compression. Similar conclusions are reached in the case of MoS2. This suggests that large errors may be introduced if the compressibility and Grüneisen parameters of bulk TMDCs are calculated with bare DFT-LDA. Therefore, the validity of different approaches to calculate the structural and vibrational properties of bulk and few-layered vdW materials under compression should be carefully assessed.

Phonon mode softening and elastic properties of hafnium under pressure

The effect of pressure on the zone-center optical phonon modes and the elastic properties of hafnium has been studied by both experimental (Raman scattering) and theoretical (DFT) approaches. It was found an anomaly dependence of the phonon frequency of E$_{2g}$ mode in the pressure range from 0 to 67 GPa. The calculated electronic structure of hafnium shows significant changes under pressure, which have a pronounced spatial anisotropy similar to the anisotropy of the observed phonon properties. The dependencies of the elastic properties on pressure including the components of the elasticity tensor, bulk and shear modulus, Poisson ratio and Universal anisotropy index obtained during the calculations has characteristic features indicating the occurrence of the structural phase transition.

Structural and ferroelectric phase evolution in [KNbO3]1−x[BaNi1/2Nb1/2O3−δ]x (x=0,0.1)

The phase transition evolution for ${[{\mathrm{KNbO}}_{3}]}_{1\ensuremath{-}x}{[{\mathrm{BaNi}}_{1/2}{\mathrm{Nb}}_{1/2}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}]}_{x}\phantom{\rule{0.16em}{0ex}}(x=0,0.1)$ is determined via complementary dielectric permittivity and Raman-scattering measurements. Raman scattering by optical phonons over the range of 100--$1000{\mathrm{cm}}^{\ensuremath{-}1}$ for 83 K $lTl873$ K reveals six discernible zone-center optical phonon modes. Mode behaviors are observed through structural and ferroelectric phases in the solid solution $x=0.1$ and compared with those for end member $x=0$ and with the results of temperature-dependent dielectric permittivity. Rigorous peak fitting analyses of spectra collected from the solid solution and end member indicate structural and ferroelectric phase transition temperatures that are close to those for the ${\mathrm{KNbO}}_{3}$ end member despite the inclusion of 5 atomic % of ferroelectrically inactive Ni cations. Density functional theory calculations were performed in the solid solution and end member using both cation displacement and Berry phase-based methods. Differences in the electronic and polar properties between the solid solution and the end member highlights local and nonlocal characteristics, which are discussed in relation to the experimental data.

Phonon pressure coefficients and deformation potentials of wurtzite AlN determined by uniaxial pressure-dependent Raman measurements

We studied bulk crystals of wurtzite AlN by means of uniaxial pressure-dependent Raman measurements. As a result, we derive the phonon pressure coefficients and deformation potentials for all zone center optical phonon modes. For the ${A}_{1}$ and ${E}_{1}$ modes, we further experimentally determined the uniaxial pressure dependence of their longitudinal optical--transverse optical (LO-TO) splittings. Our experimental approach delivers new insight into the large variance among previously reported phonon deformation potentials, which are predominantly based on heteroepitaxial growth of AlN and the ball-on-ring technique. Additionally, the measured phonon pressure coefficients are compared to their theoretical counterparts obtained by density functional theory implemented in the siesta package. Generally, we observe a good agreement between the calculated and measured phonon pressure coefficients but some particular Raman modes exhibit significant discrepancies similar to the case of wurtzite GaN and ZnO, clearly motivating the presented uniaxial pressure-dependent Raman measurements on bulk AlN crystals.

Vibrational and dielectric properties of α-Si3N4 from density functional theory

Electronic, elastic, vibrational, thermodynamical, and optical properties of α-Si3N4 are studied by density functional theory. An assignment of the symmetry for each particular zone-center optical phonon mode in the α phase of Si3N4 has been performed for the first time. The reflectivity and energy-loss spectra are determined from the calculated dielectric function. The contributions to the static dielectric tensor due to purely electronic screening and infrared-active phonon modes are analyzed. Based on the calculated electronic structure the major contributions to optical transitions are identified on the elemental and orbital basis. Vibrational an optical properties of α-Si3N4 are compared to the respective properties of β-Si3N4 and available experimental data. We extend description of the Si3N4 system by investigating the structural stability of its α polymorph as a function of temperature and pressure. The phase diagram, involving the alpha and γ polymorphic structures of Si3N4 is predicted within the approach of the quasiharmonic theory. Results of our studies are analyzed and discussed with respect to the available experimental and theoretical data. Most of the investigated quantities remains in close correspondence with the data determined experimentally.

Effect of Nitrogen on the Growth of Boron Doped Single Crystal Diamond

Boron-doped single crystal diamond films were grown homoepitaxially on synthetic (100) Type Ib diamond substrates using microwave plasma assisted chemical vapor deposition. A modification in surface morphology of the film with increasing boron concentration in the plasma has been observed using atomic force microscopy. Use of nitrogen during boron doping has been found to improve the surface morphology and the growth rate of films but it lowers the electrical conductivity of the film. The Raman spectra indicated a zone center optical phonon mode along with a few additional bands at the lower wavenumber regions. The change in the peak profile of the zone center optical phonon mode and its downshift were observed with the increasing boron content in the film. However, sharpening and upshift of Raman line was observed in the film that was grown in presence of nitrogen along with diborane in process gas.

Composition-dependent elastic modulus, vibration frequency and polaron properties of ZnSexTe1−x system

Using first-principles pseudopotential approach based on the density functional perturbation theory under the virtual crystal approximation, the compositional dependence of the elastic modulus, zone-center optical phonon modes, and polaron properties of the alloy system ZnSexTe1−x over the whole compositional range from pure ZnTe to pure ZnSe is presented and discussed. Generally, a reasonably good agreement is obtained between our results and the available experimental data for ZnTe and ZnSe parent compounds. Other case, our results are predictions and may serve for a reference for future work.

Synthesis and Characterization of Boron-Doped Single Crystal Diamond

ABSTRACTThe boron-doped single crystal diamond films were grown homoepitaxially on synthetic (100) oriented Type Ib diamond substrates using a Microwave Plasma Chemical Vapor Deposition (MPCVD) technique. Raman spectrum showed a few additional bands at the lower wavenumber regions along with the zone center optical phonon mode for diamond. The change in the peak profile of the zone center optical phonon mode and its downshift were observed with the increasing boron content in the film. A modification in surface morphology of the film with increasing boron content had been observed by atomic force microscopy. Four point probe electrical measurement indicated that different conduction mechanisms are operating in various temperature regions for these semiconducting films.

A first-principles study of the structural, elastic, electronic and phonon properties of LiMgP and LiMgAs in the α, β and γ phases

We have studied the structural, elastic, electronic and phonon properties of LiMgP and LiMgAs in the α, β and γ phases by employing the plane wave pseudopotential method based on density functional theory, within the local density approximation. Lattice parameters, bulk modulus and its pressure derivative, and second order elastic constants were found in good agreement with the literature data. We found that the γ phase of these compounds are in metallic nature as opposed to α and β phases. A linear-response approach to density functional theory was used to derive the phonon dispersion and the phonon density of states. The calculated zone-center optical phonon modes for these materials were compared with available experimental and theoretical results. Atomic displacement patterns for LiMgP in α phase at Γ and X symmetry points were presented.

Elastic and phonon properties of FeSi and CoSi in the B2 structure

First principles calculations of structural, electronic, elastic, and phonon properties of the intermetallic compounds FeSi and CoSi in the B2 (CsCl) structure are presented, using the pseudopotential plane-wave approach based on density functional theory, within the local density approximation. The optimized lattice constants, independent elastic constants, bulk modulus, and first-order pressure derivative of the bulk modulus are reported for the B2 structure and compared with earlier experimental and theoretical calculations. A linear-response approach to density functional theory is used to derive the phonon dispersion curves, and the vibrational partial and total density of states. Atomic displacement patterns for FeSi at the Γ, X, and R symmetry points are presented. The calculated zone-center optical phonon mode for FeSi is in good agreement with experimental and theoretical data.

High pressure Raman study of LiBC

The effects of pressure on the zone-center optical phonon modes of LiBC, which is structurally and electronically similar to the superconductor MgB 2 , have been investigated by Raman spectroscopy. At ambient pressure, four main lattice modes have been revealed with frequencies at 1173, 831, 552, and 174 cm ―1 , respectively, while only lines at 1173 and 174cm ―1 correspond to the expected Raman active E 2g modes. The longitudinal-optical phonon modes involving the Li sheets which can provide a coupling between neighboring B―C planes are expected to couple with the plasmon mode and are suggested to be related with the appearance of the forbidden lines. With increasing pressures, it is found that LiBC remains stable up to 15.5 GPa. All the observed modes harden and exhibit broadening effects under high pressure, especially for the B―C bond stretching mode (at 1173 cm ―1 ) which shows a nonlinear pressure dependence. It indicates that the broadening may be caused by complex coupling interactions, and the pressure-enhanced coupling should give rise to a strongly anharmonic behavior of these modes.

Phonon deformation potentials in wurtzite GaN and ZnO determined by uniaxial pressure dependent Raman measurements

We report the phonon deformation potentials of wurtzite GaN and ZnO for all zone center optical phonon modes determined by Raman measurements as a function of uniaxial pressure. Despite all the structural and optical similarities between these two material systems, the pressure dependency of their vibrational spectra exhibits fundamental distinctions, which is attributed to their different bond ionicities. In addition, the LO-TO splitting of the A1 and E1 phonon modes is analyzed which yields insight into the uniaxial pressure dependency of Born’s transverse effective charge eT∗.

Pressure-induced metallization and resonant Raman scattering in Zn1−xMnxTe

Pressure-induced resonant Raman scattering is adopted to analyze the zone-center optical phonon modes and crystal characteristics of Zn1−xMnxTe (0≦x≦0.26) thin films. The pressure (Pt) at which the semiconducting undergoes a transition to the metallic phase declines as a function of Mn concentration (x) according to the formula Pt(x)=15.7−25.4x+19.0x2 (GPa). Pressure-dependent longitudinal and transverse optical phonon frequencies and the calculated mode Grüneisen parameters were adopted to investigate the influence of Mn2+ ions on the iconicity. The experimental results indicate that the manganese ions tend to increase the iconicity of ZnTe under ambient conditions, whereas an external hydrostatic pressure tends to reduce the iconicity and the bond length of Zn1−xMnxTe.

Polarized infrared reflectance study of wurtzite GaN thin film: The effects of angle of incidence on the optical phonon modes

We report on the polarized IR reflectance study of wurtzite GaN thin film grown on 6H-SiC substrate measured at various angles of incidence, namely, from 15° to 75°. Attention is focused on the effects of incident angles on the Brillouin zone center optical phonon modes of the GaN thin film. The reflection spectra are compared to the calculated spectra generated with a damped single harmonic oscillator model. Good agreement between the measured and calculated spectra has been obtained. Overall, the results revealed that the optical phonon modes of the GaN thin film and 6H-SiC substrate are independent of the angle of the incidence beam.

Temperature dependence of the A1(LO) and E2 (high) phonons in hexagonal InN nanowires

The frequencies and dampings of the zone-center optical phonon modes of A1(LO) (longitudinal-optical) and E2 (high) in wurtzite InN nanowires have been investigated by micro-Raman scattering in the temperature range from 80 to 300 K. Our results reveal that the phonon frequencies decrease and the linewidths broaden with increasing temperature. The obtained experimental data of the frequencies and linewidths at various temperatures can be well described by an empirical model which takes into account the contribution of the thermal expansion of lattice and symmetric decay of phonons into two and three identical phonons with lower energy. The results show that decay into two phonons is the probable channel for the A1(LO) mode and three-phonon decay dominates the E2 (high) mode.

Effect of pressure on the Raman modes of antimony

The effect of pressure on the zone-center optical phonon modes of antimony in the $A7$ structure has been investigated by Raman spectroscopy. The ${A}_{g}$ and ${E}_{g}$ frequencies exhibit a pronounced softening with increasing pressure, the effect being related to a gradual suppression of the Peierls-like distortion of the $A7$ phase relative to a cubic primitive lattice. Also, both Raman modes broaden significantly under pressure. Spectra taken at low temperature indicate that the broadening is at least partly caused by phonon-phonon interactions. We also report results of ab initio frozen-phonon calculations of the ${A}_{g}$ and ${E}_{g}$ mode frequencies. The presence of strong anharmonicity is clearly apparent in calculated total energy versus atom displacement relations. Pronounced nonlinearities in the force versus displacement relations are observed. Structural instabilities of the Sb $A7$ phase are briefly addressed in the Appendix.

Polarized ultraviolet Raman spectroscopy of β-Si3N4

We have studied single crystals of β-Si3N4 with polarization-resolved Raman spectroscopy in order to unambiguously obtain the symmetry character of zone center optical phonon modes. By applying ultraviolet excitation radiation, approaching resonance condition, we have overcome the usual difficulties with overwhelming fluorescence obscuring Raman spectra of β-Si3N4 in the visible. Superior extinction ratios have been achieved and the application of group theory allows us to assign ten out of eleven Raman-active modes in excellent correlation with recent theoretical results. The probable nature of the eleventh mode is discussed.