Energy Loss Function Spectra Research Articles

Phonon modes controlled by primary chemical structure of partially fluorinated dimyristoylphosphatidylcholine (DMPC) revealed by multiple-angle incidence resolution spectrometry (MAIRS).

Partially fluorinated dimyristoylphosphatidylcholines (DMPCs) involving double alkyl chains are employed to control the phonon generation in thin films, which is examined by infrared (IR) spectroscopy coupled with multiple-angle incidence resolution spectrometry (MAIRS). technique. Compounds having perfluoroalkyl (Rf) chains are known to exhibit phonon bands in IR spectra because of the strong dipole-dipole interactions. Since the phonon bands of an organic matter have a similar shape to the normal absorption bands, however, recognition of the phonon modes is difficult and confusing for IR spectroscopists. Here, we show that MAIRS works out for finding phonon modes in monolayers: the Berreman shift is readily captured by the MAIRS in-plane and out-of-plane (OP) spectra. By measuring the longitudinal-optic (LO) energy-loss function spectrum of a bulk sample, the degree of molecular aggregation in the monolayer is also revealed by comparing the OP spectrum of the monolayer to the LO one. In addition, partially fluorinated DMPC compounds having both hydrocarbon and Rf chains are prepared, and they are used to obstruct the self-aggregation of the Rf groups in the film. As a result, the phonon characteristics are mostly lost in the MAIRS spectra as expected.

Electronic and optical properties of polar intermetallic compound BaSn3: First principles study

The electronic and optical properties of intermetallic compound BaSn3 are investigated at high pressure from the first principles density functional theory calculations using the plane-wave pseudo-potential method within generalized gradient approximations in the forms of PBEsol (PBEsol-GGA). Furthermore, the electronic properties such as band structure, the total and partial densities of states have been computed and assessed in comparison with the existing studies. Meanwhile, the calculated band structures demonstrate that this compound has a metallic character. In addition to that, the electronic structures were discussed to reveal the bonding characteristics, and the electronic charge density plots show the presence of covalent with partial ionic bonding in this compound.On the other hand, an accurate calculation of frequency-dependent polarized optical functions (including the dielectric function, refractive index, birefringence extinction coefficient, absorption coefficient, also reflectivity and energy-loss function spectra) were performed and presented in the energy range 0–20 eV. The results obtained are discussed within the frame of available information on the electronic band structure of this compound. We can conclude that the optical spectra exhibited a relative optical anisotropy in different polarizations of the incident light along [100], [111] and [001] crystallographic directions. On the top of that, the optical conductivity of BaSn3 was also determined.

Quantum confinement effect on the electronic and optical features of InGaN-based solar cells with InGaN/GaN superlattices as the absorption layers

The short period group III-V nitrides superlattices (SLs), have turned out contemporary in the technology of optoelectronics and solar cell applications. Our theoretical simulation is carried out by means of first-principles full potential linearized augmented plane waves (FP-LAPW) methodology within generalized gradient approximations (GGA) in conjunction with the modified Becke–Johnson (mBJ) potential. In this respect, a comprehensive study for the electronic structures and optical aspects of (InxGa1−xN)n/(GaN)n (001) zinc-blende superlattices (SLs) (x=0.5 and n=3–4), is carried out. Specifically, the electronic band structure calculations and their related features, like the absorption coefficient, reflectivity, refractive index and electron energy loss function spectra of these systems are computed over a wide photon energy scale up to 25eV. The effect of periodicity layer numbers and In composition (x=50%) on the band gaps of (InxGa1−xN)n/(GaN)n SLs is examined. The tailoring of the underlying energy band gap relies on the In content and the periodicity of the superlattices. All these ultrathin superlattices (n=3–4) possess a direct energy band gap. The InGaN layers have immense prominence in ascertaining the underlying energy gap of these superlattice because of the distinctive quantum confinement impact. Furthermore, the flexible energy gap of these ultrathin-period SLs leads to the alteration of the absorption coefficients and static refractive indices. It is viable to attain InGaN solar cells possessing high efficiencies since the energy gap covers all the spectrum optical regime. Predominantly, it is feasible to tune the optical characteristics of these short-period SLs and provide plausible results for the optoelectronic devices applications.

Elastic, Electronic, Optical and Thermal Properties of Na2Po: An Ab Initio Study

The structural, elastic, electronic, optical and thermodynamic properties of the sodium polonide Na2Po compound have been studied through the full potential linearized augmented plane wave plus local orbitals (FP-LAPW + lo) and tight-binding linear muffin-tin orbital (TB-LMTO) methods. The exchange–correlation potential was treated within the local density approximation for the TB-LMTO calculations and within the generalized gradient approximation for the FP-LAPW + lo calculations. In addition, Tran and Blaha-modified Becke–Johnson (TB-mBJ) potential and Engel–Vosko generalized gradient approximation were used for the electronic and optical properties. Ground state properties such as the equilibrium lattice constant, bulk modulus and its pressure derivative were calculated and compared with available data. The single-crystal and polycrystalline elastic constants of the considered compound were calculated via the total energy versus strain in the framework of the FP-LAPW + lo approach. The calculated electronic structure reveals that Na2Po is a direct band gap semiconductor. The frequency-dependent dielectric function, refractive index, extinction coefficient, reflectivity coefficient and electron energy loss function spectra are calculated for a wide energy range. The variations of the lattice constant, bulk modulus, heat capacity, volume expansion coefficient and Debye temperature with temperature and pressure were calculated successfully using the FP-LAPW + lo method in combination with the quasi-harmonic Debye model.

Electronic and optical properties of h-BN nanosheet: A first principles calculation

Based on ab-initio calculations, we investigate the electronic and linear optical properties of hexagonal boron nitride (h-BN) nanosheet using the full potential linearized augmented plane wave method in the framework of the density functional theory (DFT). The frequency dependent optical properties like dielectric tensor, absorption coefficient, optical conductivity, refraction and extinction coefficients and energy loss function spectrum of the h-BN nanosheet are calculated for both parallel (E||x) and perpendicular (E||z) electric field polarizations. The dielectric function is derived within the random phase approximation (RPA) for these calculations. The results show that the h-BN nanosheet has a semiconductor character with a wide band gap of about 4.96eV. The optical conductivity in E||x direction and E||z direction starts with a gap of about 2.92eV and 6.73eV, respectively, confirms that this nanosheet has semiconductor properties. The calculated results are in good agreement with available experimental data. These results propose potential application for the development of the BN nano-structures in electronic and optoelectronic devices.

Structural, elastic, electronic and optical properties of the quaternary nitridogallate LiCaGaN2: First-principles study

First-principles density functional theory calculations were performed to predict some of the not yet explored physical properties of the monoclinic quaternary nitridogallate LiCaGaN2. The calculated lattice parameters, including the lattice constants, angle β and internal atomic coordinates, are in excellent agreement with the corresponding measured ones, proving the reliability of the chosen theoretical approach. The equation of state, pressure dependence of the lattice constants, unit cell volume, angle β and bond-lengths were explored in detail. The single-crystal and polycrystalline elastic constants and their pressure dependence were numerically estimated. The mechanical stability, ductility/brittleness, average elastic wave velocity, Debye temperature and elastic anisotropy were also assessed. The electronic structure and its evolution with external applied hydrostatic pressure were explored. The bonding character was demonstrated by calculating the site-projected density of states, charge density and effective Mulliken charges of all ions. The effective masses of the charge-carriers were numerically estimated. The complex dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity and electron energy-loss function spectra were calculated for different polarizations of the incident light. Pressure dependence of the static dielectric constant, static refractive index and static reflectivity are also reported. To the best of our knowledge, this is the first attempt to explore the aforementioned physical properties for the title material.

An ab initio study of the structural, elastic, electronic and optical properties of the newly synthesized nitridoaluminate LiCaAlN2

The structural parameters, elastic constants, electronic structure and optical properties of the recently reported monoclinic quaternary nitridoaluminate LiCaAlN2 are investigated in detail using the ab initio plane-wave pseudopotential method within the generalized gradient approximation. The calculated equilibrium structural parameters are in excellent agreement with the experimental data, which validate the reliability of the applied theoretical method. The chemical and structural stabilities of LiCaAlN2 are confirmed by calculating the cohesion energy and enthalpy of formation. Chemical band stiffness is calculated to explain the pressure dependence of the lattice parameters. Through the band structure calculation, LiCaAlN2 is predicted to be an indirect band gap of 2.725 eV. The charge-carrier effective masses are estimated from the band structure dispersions. The frequency-dependent dielectric function, absorption coefficient, refractive index, extinction coefficient, reflectivity coefficient and electron energy loss function spectra are calculated for polarized incident light in a wide energy range. Optical spectra exhibit a noticeable anisotropy. Single-crystal and polycrystalline elastic constants and related properties, including isotropic sound velocities and Debye temperatures, are numerically estimated. The calculated elastic constants and elastic compliances are used to analyse and visualize the elastic anisotropy of LiCaAlN2. The calculated elastic constants demonstrate the mechanical stability and brittle behaviour of the considered material.

A Theoretical Study of Bulk Tungsten (W) Based on Momentum Transfer (q-Dependent)

The ground state electronic properties of bulk (W) were studied within the density functional theory (DFT). We have also analyzed the momentum- (q-) dependent loss function, dielectric constant, and optical conductivity (OC) within TD-DFT random-phase approximation (RPA). The loss function is plotted in the energy range 0–55 eV. The energy loss function spectrum shows four prominent peaks, two lower peaks below along with two sharp peaks above 30 eV. The different nature of peaks depends on the momentum transfer q. The peak caused by interband transition showed a less pronounced dispersion. From the dielectric function curve we have predicted the plasmon excitation at around 1.75 eV and calculated the corresponding plasma frequency (ωp)=26.585×1014 s−1.

Structural phase transition, mechanical and optoelectronic properties of the tetragonal NaZnP: Ab-initio study

Ab-initio full potential augmented plane wave plus local orbitals method has been used to investigate the structural phase transition, mechanical and optoelectronic properties of the Nowotny–Juza filled-tetrahedral compound NaZnP. The exchange-correlation potential was treated within the generalized gradient approximation of Perdew–Burke and Ernzerhof (GGA-PBE) and the modified Becke–Johnson potential (TB-mBJ) to improve the accuracy of the electronic band structure. Total-energy and geometry optimizations have been carried out for all structural phases of NaZnP. The following sequence of pressure-driven structural transitions has been found: Cu2Sb-type→β-phase→α-phase. The single-crystal elastic constants of NaZnP in the Cu2Sb-type structure have been calculated using total-energy versus strain method and their corresponding elastic moduli of polycrystalline aggregate, including Young’s modulus, shear modulus and Poisson’s ratio, have been derived. From the elastic parameters, it is inferred that this compound is brittle in nature. The elastic anisotropy was studied in detail using three different indexes; especially the 3D direction dependence of the Young’s modulus was visually described. Furthermore, calculated electronic band structure shows that NaZnP in the Cu2Sb-type phase has a direct energy band gap (Γ–Γ). The TB-mBJ approximation yields larger fundamental band gaps compared to those of PBE-GGA. The examined charge density distributions for the Cu2Sb-type structure show a covalent character for Zn–P bond and ionic nature for Na–P bond. Additionally, real and imaginary parts of the dielectric function, reflectivity and energy loss function spectra have been calculated for radiation up to 30.0eV with an incident radiation polarized parallel to both [100] and [001] crystalline directions.

Common electronic band gaps and similar optical properties of ZnO nanotubes

Electronic and optical properties of single-walled zinc oxide (ZnO) nanotubes are investigated from the first-principles calculations. Electronic structure calculations show that ZnO nanotubes are all direct band gap semiconducting nanotubes and the band gaps are relatively insensitive to the diameter and chirality of tubes. The origin of the common electronic band gaps of ZnO nanotubes is explained in terms of band-folding from the two-dimensional band structure of graphite-like sheet. Moreover, the optical properties such as dielectric function and energy loss function spectra of different ZnO nanotubes are very similar, relatively independent of diameter and chirality of tubes. The calculated dielectric function and loss function spectra show a moderate optical anisotropy with respect to light polarization.

Linear optical response of (6,0) boron nitride nanotubes adsorbed with molecular hydrogen

AbstractWe performed a study of the molecular adsorption of hydrogen on BN nanotubes. We present ab initio calculations for the linear optical response of single wall zigzag BN(6,0) nanotubes as a function of the hydrogen adsorption on the exterior surface of the nanotube. The calculation of the linear optical response is performed by using density functional theorywith the use of plane waves and pseudopotentials. We consider four different nanotube‐structures adsorbed with different coverage of molecular hydrogen.We find optimized atomic coordinates for such structures and calculate binding energies for the molecule of hydrogen on the nanotube.After having the linear response of the considered structures, we have calculated their energy loss function spectra. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dispersion analysis of polarized IR reflectance spectra of Tutton salts: The ν3(SO 42−) frequency region

Polarized IR reflectance spectra of eight Tutton salts single crystals: M′ 2M″(SO 4) 2·6H 2O; M′: K + or NH 4 +; M″: Ni 2+, Co 2+, Zn 2+ or Mg 2+) were recorded at near-normal incidence. Dispersion analysis was performed for the whole mid-IR region. The results on ν 3(SO 4 2−) frequency region are reported in this paper. Reflectance spectra were recorded using radiation with polarization along the b crystallographic axis and in the ac crystal plane. Results of the fit for the transversal frequency, oscillator strength, attenuation constant and the orientation of the transition moments in the ac crystal plane for the ν 3(SO 4 2−) frequency region are presented. The longitudinal frequencies for the A u vibrations were obtained from the energy-loss function spectra, while for the B u vibrations a particular approximation was used. In addition to frequency values, the directions of the longitudinal transition dipole moments in the ac crystal plane were also found. The dependence of the principal axes of the real and the imaginary parts of the dielectric tensor, on the radiation frequency, was established. An attempt was made to estimate the transition moment directions of the ν 3(SO 4 2−) mode components by correlation of the spectroscopic and structural findings.

Theoretical Assignments of the Optical Conductivity and Energy-loss Function Spectra of EuB 6

On the basis of the calculated band structure of EuB6, with a higher accuracy, the optical conductivity and energy-loss function spectra were calculated, and the results were in good agreement with the experiments. All possible interband electronic transitions, ▪ the points in the first Brillouin zone where the transitions took place, and the transition probability were figured out through specific calculations. The wave functions of the initial and final states related to individual transitions were obtained as well. The calculated results were used to analyze, in detail, the correlation between the spectral peaks and the interband electronic transitions. On the basis of this, the experimental spectra were assigned, confirming the reasonable part of the empirical assignments given by Kimura et al. and pointing out the inaccurate part, and also supplying some electronic transitions, which made an important contribution to the spectral peaks, but had not been considered by Kimura et al.

Far-infrared and millimeter wave spectroscopy of superionic copper conductors

Far-infrared and millimeter wave spectra of copper ion conducting crystal RbCu4Cl3+xI2−x, which has the same structure as the room temperature silver ion conductor RbAg4I5, were investigated. Broad absorption peaks observed around 40, 80, and 110–200cm−1 at room temperature show doublet structures at low temperature; this may be attributed to the difference of local structure by chlorine and iodine ion. The 110–200cm−1 bands seem to be symmetric breathing modes of CuX4 (X=Cl or I) tetrahedron and the frequency shift coincides with the square root of the mass ratio of conduction ions. The 80cm−1 band seems to be Rb-X vibration in RbX6 octahedron. The 40cm−1 band seems to be the attempt mode which is an outward motion of the mobile ion in halogen cage. The increase of the absorption intensity at the low energy side with temperature corresponds to an increase of the DC conductivity. Plasmon fitting in energy loss function spectra was attempted.

Study on Band Structure of YbB 6 and Analysis of Its Optical Conductivity Spectrum

The electronic structure of YbB 6 crystal was studied by means of density functional (GGA + U) method. The calculations were performed by FLAPW method. The high accurate band structure was achieved. The correlation between the feature of the band structure and the Yb-B6 bonding in YbB 6 was analyzed. On this basis, some optical constants of YbB 6 such as reflectivity, dielectric function, optical conductivity, and energy-loss function were calculated. The results are in good agreement with the experiments. The real part of the optical conductivity spectrum and the energy-loss function spectrum were analyzed in detail. The assignments of the spectra were carried out to correlate the spectral peaks with the interband electronic transitions, which justify the reasonable part of previous empirical assignments and renew the missed or incorrect ones.

Optical Properties and London Dispersion Forces of Amorphous Silica Determined by Vacuum Ultraviolet Spectroscopy and Spectroscopic Ellipsometry

Precise and accurate knowledge of the optical properties of amorphous silica is important because of the increasing application of SiO2 in optical and electrooptical devices, including photolithography masks for semiconductor fabrication, recently as a potential 157 nm mask substrate. The optical properties in the vacuum ultraviolet (VUV) region have been investigated, because they convey detailed information on the electronic structure and interatomic bonding of the material. In this work, we have combined spectroscopic ellipsometry and VUV spectroscopy to directly determine the optical functions of SiO2 in this range, thereby reducing the uncertainty in the low‐energy extrapolation of the data, essential for Kramers–Kronig analysis of VUV reflectance. We report the complex optical properties of SiO2, over the range of 1.5 to 42 eV, showing improved agreement with theory when contrasted with earlier results. In addition to the features previously reported at 10.4, 11.6, 14.03, and 17.10 eV, new interband transitions have been observed at 21.3 eV along with O 2s transitions at 32 eV. We found the bulk plasma peak to be 23.7 eV in the energy loss function spectrum. Based on the magnitude of these new results, the Hamaker constant for SiO2|Vacuum|SiO2 is 71.6 zJ, which is larger than the previously reported value of 66 zJ.

Ab Initio Self-Consistent Calculations of the Polarizability and Related Functions of Cubic SiC

Crystalline orbitals obtained by the linear combination of atomic orbitals−self consistent field (LCAO−SCF) method at the Hartree−Fock (HF) and Kohn−Sham (KS) levels, are used to calculate the polarizability and related functions such as dielectric constant, reflectance, and energy-loss function (ELF). Comparison of our calculations with experiment leads to a satisfactory agreement. The ELF spectra show clearly the energy ranges corresponding to the electronic excitations of the different orbitals. The carbon K-edge range is accurately analyzed in connection with the density of states.

Millimeter wave spectroscopy and color centers of MAg 4I 5 (M=Rb, K and NH 4) family

Reflectivity spectra of superionic conductors of MAg 4I 5 (M=Rb, K and NH 4) and Rb 4Cu 16Cl 13I 7 were measured in a spectral region between 3.5 and 200 cm −1 at a temperature range from 15 K to 470 K. A structure of “ionic plasmon” was observed near 10 cm −1 (MAg 4I 5) or 20 cm −1 (Rb 4Cu 16Cl 13I 7) in energy loss function spectra. Coloration of MAg 4I 5 by X-ray irradiation was observed. The absorption band were intense in NH 4Ag 4I 5 only by hole centers. The coloration rate of NH 4Ag 4I 5 increases drastically at the temperature around the superionic phase transition as observed in RbAg 4I 5 and KAg 4I 5.

The energy loss spectrum in oxygen-octahedron ferroelectrics

Abstract The characteristic energy loss function spectra in oxygen-octahedron ferroelectrics up to 35 eV have been investigated by VUV spectroscopy methods. Information about the energy spectrum of electrons in these crystals has been received.