First principle calculation of physical properties of barium based chalcogenides BaM4S7 (M = Ga,Al); a DFT,DFT-D and hybrid functional HSE06 study

The electronic structure, elastic and optical properties have been calculated for the novel nonlinear optical (NLO) crystals BaQ₄S₇ (Q = Ga, Al) using plane wave pseudo-potential density functional theory (DFT) method as implemented in CASTEP and ABINIT codes. In this study we used both hybrid HSE06 and DFT-D functionals with GGA approximation. These NLO compounds, which belong to the mm2 point group, are particularly interesting because of their transparency in the mid-infrared region and wide energy band gap. We present results for electronic structure, elastic tensor coefficients, refractive indices and second order nonlinear optical susceptibilities. The calculated energy band gap and frequency dependent refractive indices as well as the NLO coefficients of BaGa₄S₇ are in good agreement with the experimental values. With no reported theoretical or experimental energy band gap and optical properties of BaAl₄S₇, we present for the first time its electronic structure and above mentioned optical coefficients. This compound has higher direct band gap with 3.74 eV, better optical birefringence and second-order NLO coefficients than most NLO compounds. The second-order NLO coefficients for BaAl₄S₇ have been calculated as d₃₁ = 3.15 pm/V, d₃₁ = 2.20 pm/V, d₃₃ = −6.31 pm/V.     

Transparent with wide band gap InZnO nano thin film: Preparation and characterizations

Novel indium zinc oxide (InZnO) thin film of 100 nm thickness was prepared onto pre-cleaned glass plate by thermal evaporation technique from InZnO nanoparticles. The metal oxide (In–O and Zn–O) bond and In, Zn and O elements present in the films were confirmed by Fourier transform infrared spectroscopy and energy dispersive X-ray spectroscopy. The X-ray diffraction patterns revealed the mixed phase of cubic In₂O₃ and wurzite-hexagonal ZnO structure. SEM images showed smooth surface with uniform distribution of grains (201–240 nm) over the entire film surface. High transparency and low absorption obtained from optical study. The band gap energy was evaluated to be about 3.46–3.55 eV by Tauc’s plot. The structure, smooth surface and high transparency with wide band gap energy lead the thermally evaporated InZnO nano thin film to be used for transparent layer in optoelectronic devices in the future.     

FP-LAPW investigation of structural,electronic, linear and nonlinear optical properties of ZnIn2Te4 defect-chalcopyrite

A theoretical study of structural, electronic, linear and nonlinear optical properties of ZnIn₂Te₄ defect-chalcopyrite is presented using the full-potential linearized augmented plane-wave (FP-LAPW) method. The exchange and correlation potential is treated by the generalized-gradient approximation (GGA). Moreover, the Engel and Vosko GGA formalism (EV-GGA) is also used to improve the band gap results. The lattice parameters (a, c) and the atomic positions (x, y and z) are optimized and found in good agreements with the available experimental data. Our calculations performed for band structure and density of state show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ resulting in a direct energy gap of about 0.89 eV for GGA and 1.20 eV for EV-GGA. The linear optical properties namely, the real and imaginary parts of the dielectric function and the reflectivity spectrum are calculated. This compound possesses a considerable negative birefringence. Based on the density functional theory the nonlinear optical properties are calculated and their spectra are analyzed.     

Electronic structure of alkali-metal/alkaline-earth-metal fluorine beryllium borate NaSr3Be3B3O9F4 single crystal: DFT approach

The electronic band structure, total and angular momentum resolved projected density of states for NaSr₃Be₃B₃O₉F₄ are calculated using the all-electron full potential linearized augmented plane wave plus local orbitals (FP-LAPW + lo) method. The calculations are performed within four exchange correlations namely; local density approximation (LDA), general gradient approximation (PBE-GGA), Engel–Vosko generalized gradient approximation (EVGGA) and the recently modified Becke–Johnson potential (mBJ). Calculations suggest that NaSr₃Be₃B₃O₉F₄ is a direct wide band gap semiconductor. The exchange correlations potentials exhibit significant influence on the value of the energy gap being about 4.82 eV (LDA), 5.16 eV (GGA), 6.20 (EVGGA) and 7.20 eV (mBJ). The mBJ approach succeed by large amount in bringing the calculated energy gap closer to the experimental one (7.28 eV). The angular momentum resolved projected density of states shows the existence of a strong hybridization between the various orbitals. In additional we have calculated the electronic charge density distribution in two crystallographic planes namely (1 0 1) and (0 0 −1) to visualized the chemical bonding characters.     

A simple method to prepare indium oxide nanoparticles: Structural,microstructural and magnetic properties

Indium oxide nanoparticles of ～12 nm were synthesized by a simple chemical route using indium(III) nitrate. Nanoparticles are formed after calcining the dried precursor in air at 400 °C for 10 h. TEM analysis showed that the morphology and size of the In₂O₃ samples were affected by ultrasonication. FTIR and Raman studies reveal that the nanoparticles are single-phase cubic structure of In₂O₃. NEXAFS study was used to quantify the Indium and oxygen valence state. Magnetic behavior of indium oxide nanoparticles was found to be diamagnetic. UV spectra show a weak band at ～308 nm corresponds to optical band gap energy of 4.03 eV.     

Optical constants of thermally evaporated Se–Sb–Te films using only their transmission spectra

Amorphous Se_82 ? xTe₁₈Sb_x thin films with different compositions (x = 0, 3, 6 and 9 at.%) were deposited onto glass substrates by thermal evaporation. The transmission spectra, T(λ), of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. Based on the use of the maxima and minima of the interference fringes, a straightforward analysis proposed by Swanepoel has been applied to derive the optical constants and the film thickness. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. Tauc relation for the allowed non-direct transition describes the optical transition in the studied films. With increasing antimony content the refractive index increases while the optical band gap decreases. The optical band gap decreases from 1.62 to 1.26 eV with increasing antimony content from 0 to 9 at.%. The chemical-bond approach has been applied successfully to interpret the decrease of the optical gap with increasing antimony content.     

Single-crystal oxoborate (Pb3O)2(BO3)2WO4: Growth and characterization

An oxoborate, (Pb₃O)₂(BO₃)₂WO₄, has been prepared by solid-state reaction methods below 620 °C. Single-crystal XRD analysis shows that it crystallizes in the orthorhombic group Cmcm with a = 18.480(4) Å, b = 6.3567(13) Å, c = 11.672(2) Å, Z = 4. The crystal structure is composed of one-dimensional 1/∞ [Pb₃O]⁴⁺ chains formed by corner-sharing OPb₄ tetrahedra. BO₃ and WO₄ groups are located around the chains to hold them together via PbO bonds. The IR spectra further confirmed the presence of BO₃ groups. Furthermore we have performed theoretical calculations by employing the all-electron full potential linearized augmented plane wave (FP-LAPW) method to solve the Kohn Sham equations. Starting from our XRD data we have optimized the atomic positions by minimizing the forces. These are used to calculate the electronic band structure, the atomic site-decomposed density of states, electron charge density and the chemical bonding features. The calculated electronic band structure and densities of states suggest that this oxoborate possesses a wide energy band gap. The valence band maxima and the conduction band minima are located at Y point in the Brillouin zone resulting in a direct energy band gap of 2.3 eV using the local density approximation and 2.6 eV for the Engel–Vosko generalized gradient approximation. This compares well with our experimentally measured energy band gap of 2.9 eV. From our calculated electron charge density distribution, we obtain an image of the electron clouds that surround the molecules in the unit cell of the crystal. The chemical bonding features were analyzed and the substantial covalent interactions are observed between Pb and O, B and O and W and O atoms.     

First-principles study of the electronic,optical and bonding properties in dolomite

The structural, electronic, optical properties and chemical bonding of dolomite CaMg(CO₃)₂ (rhombohedral calcite-type structure) are investigated using plane wave pseudopotential density-functional theory (DFT) method taking the local density approximation (LDA) and the generalized gradient approximation (GGA) as the exchange–correlation energy functional. The structural properties are consistent with the early experimental and theoretical results. The indirect electronic band gap is estimated to be ～5.0 eV, which is less than the optical band gap measured from the fundamental absorption edge of ～6.0 eV. The optical band gap is also consistent with the experimental band gap of similar calcite-type structure. A noticeable difference for the LDA and GGA derived transition peaks and a significant optical anisotropy are observed in the optical spectra. The analysis of electronic density of states, Mulliken charge and bonding population shows the coexistence of covalent and ionic bonding in the dolomite structure and the results are consistent with previous theoretical calculations.     

Properties and thermal stability of solution processed ultrathin,high-k bismuth titanate (Bi2Ti2O7) films

Ultrathin bismuth titanate films (Bi₂Ti₂O₇, 5–25 nm) are deposited onto SiO₂/Si substrates by aqueous chemical solution deposition and their evolution during annealing is studied. The films crystallize into a preferentially oriented, pure pyrochlore phase between 500 and 700 °C, depending on the film thickness and the total thermal budget. Crystallization causes a strong increase of surface roughness compared to amorphous films. An increase of the interfacial layer thickness is observed after anneal at 600 °C, together with intermixing of bismuth with the substrate as shown by TEM-EDX. The band gap was determined to be ～3 eV from photoconductivity measurements and high dielectric constants between 30 and 130 were determined from capacitance voltage measurements, depending on the processing conditions.     

Temperature dependence of electronic band transition in Mn-doped SnO2 nanocrystalline films determined by ultraviolet-near-infrared transmittance spectra

Mn-doped SnO₂ (SMO) nanocrystalline films with the composition from 2.5 to 12.5% have been prepared on quartz substrates by pulsed laser deposition. The temperature dependence of electronic structures and optical constants in the SMO films have been investigated by transmittance spectra from 5.3 to 300 K. Optical response functions have been extracted by fitting the transmittance spectra in the photon energy range of 0.5–6.5 eV with the Adachi's model. It was found that the absorption edge presents a red-shift trend with increasing Mn composition, and the optical band gap (OBG) is varied between 4.22 and 3.44 eV. Moreover, as the Mn composition increases, the temperature dependence of OBG becomes weaker. The band gap narrowing value [(5.3 K)–(300 K)] has been reduced from 98 to 3 meV and linearly decreases with the Mn composition. The phenomena could be attributed to the transition from low doping level SnO₂ band-like states to Mn-related localized states. Moreover, the Urbach energy shows the degree of the structural disorder, which could be explained by an empirical formulas in different temperature regimes.     

Effect of thickness on structural,optical, electrical and morphological properties of nanocrystalline CdSe thin films for optoelectronic applications

This paper presents effect of thickness on the physical properties of thermally evaporated cadmium selenide thin films. The films of thickness 445 nm, 631 nm and 810 nm were deposited employing thermal evaporation technique on glass and ITO coated glass substrates followed by thermal annealing in air atmosphere at temperature 300 °C. The as-deposited and annealed films were subjected to the XRD, UV–Vis spectrophotometer, source meter, SEM and EDS to find the structural, optical, electrical, morphological and compositional analysis respectively. The structural analysis shows that the films have cubic phase with preferred orientation (1 1 1) and nanocrystalline nature. The structural parameters like inter-planner spacing, lattice constant, grain size, number of crystallites per unit area, internal strain, dislocation density and texture coefficient are calculated. The optical band gap is found in the range 1.69–1.84 eV and observed to decrease with thickness. The electrical resistivity is found to increase with thickness for as-deposited films and decrease for annealed films. The morphological studies show that the as-deposited and annealed films are homogeneous, smooth, fully covered and free from crystal defects like pin holes and voids. The grains in the as-deposited films are densely packed, well defined and found to be increased with thickness.     

Structural,optical and dielectric studies of novel non-linear Bisglycine Lithium Nitrate piezoelectric single crystal

The novel non-linear semiorganic Bisglycine Lithium Nitrate (BGLiN) single crystals were grown by slow evaporation technique. The structural analysis revealed that it belongs to non-centrosymmetric orthorhombic structure. The presence of various functional groups in the grown crystal was confirmed by FTIR and Raman analysis. Surface morphology of the grown crystal was studied by scanning electron microscopy. The optical studies show that crystal has good transmittance (more than 80%) in the entire visible region and a wide band gap (5.17 eV). The optical constants such as extinction coefficient (K), the reflectance (R) and refractive index (n) as a function of photon energy were calculated from the optical measurements. With the help of these optical constants the electric susceptibility (χ_c) and both the real (ε_r) and imaginary (ε_i) parts of the dielectric constants were also calculated which are required to develop optoelectronic devices. In photoluminescence studies, a broad emission band centered at 404 nm was found in addition to a small band at 352 nm. A broad transition (from 29 to 33 °C) was observed with low dielectric constant value. A high piezoelectric charge coefficient (d₃₃) of 14 pC/N was measured at room temperature which implies its usefulness for various sensor applications. The second harmonic generation efficiency of crystal was found to be 1.5 times to that of KDP. From thermo gravimetric analysis and differential thermal analysis, thermal stability and melting point (246 °C) were investigated. The dielectric behavior, optical characterization, piezoelectric behavior and the non-linear optical properties of the Bisglycine Lithium Nitrate single crystals were reported for the first time which established the usefulness of these crystals for various piezo- and opto-electronics applications.     

Preparation of calcium-doped boron nitride by pulsed laser deposition

Calcium-doped BN thin films Ca_xBN_y (x = 0.05–0.1, y = 0.7–0.9) were grown on α-Al₂O₃(0 0 1) substrates by pulsed laser deposition (PLD) using h-BN and Ca₃N₂ disks as the targets under nitrogen radical irradiation. Infrared ATR spectra demonstrated the formation of short range ordered structure of BN hexagonal sheets, while X-ray diffraction gave no peak indicating the absence of long-range order structure in the films. It was notable that Ca-doped film had 5.45–5.55 eV of optical band gap, while the band gap of Ca-free films was 5.80–5.85 eV. This change in the band gap is ascribed to interaction of Ca with the BN sheets; first principle calculations on h-BN structure indicated that variation of inter-plane distance between the BN layers did not affect the band gap. This study highlights that PLD could prepare BN having short-range structure of h-BN sheets and being doped with electropositive cation which varies the optical band gap of the films.     

Combined theoretical studies of the optical characteristics of II-IV-V2 semiconductor thin films

The optical absorbance of four ternary thin films, i.e. MgSiP₂, MgGeP₂, MgSiAs₂, MgGeAs₂ have been theoretically examined over a wide range of wavelength from 300 nm to 800 nm. The combination of first-principle electronic structure calculations and the optical matrix approach for modeling the multilayer assembly have been employed for theoretical studies. The analysis of the calculated absorbance spectra at room temperature with unpolarized light and normal incidence, revealed that MgGeAs₂ with a direct energy band gap of 1.6 eV exhibit a considerable high optical absorption, where a thickness of 3.2 μm of this thin film is sufficient to absorb 90% of the incident light and generates a maximum photocurrent of ∼23 mA/cm².     

Recombination luminescence and trap levels in undoped and Al-doped ZnO thin films on quartz and GaSe (0 0 0 1) substrates

Photoluminescence spectra of ZnO and ZnO:Al (1.00, 2.00 and 5.00 at.%) films on GaSe (0 0 0 1) lamellas and amorphous quartz substrates, obtained by annealing, at 700 K, of undoped and Al-doped metal films, are investigated. For all samples, the nonequilibrium charge carriers recombine by radiative band-to-band transitions with energy of 3.27 eV, via recombination levels created by the monoionized oxygen atoms, forming the impurity band laying in the region 2.00 ? 2.70 eV. Al doping induces an additional recombination level at 1.13 eV above the top of the valence band of ZnO films on GaSe substrates. As a result of thermal diffusion of Zn and Al into the GaSe interface layer from ZnO:Al/GaSe heterojunction, electron trap levels located at 0.22 eV and 0.26 eV below the conduction band edge of GaSe, as well as a deep recombination level, responsible for the luminescent emission in the region 1.10 ? 1.40 eV, are created.     

Structural and optical properties of In doped Se–Te phase-change thin films: A material for optical data storage

Se_75−xTe₂₅In_x (x = 0, 3, 6, & 9) bulk glasses were obtained by melt quench technique. Thin films of thickness 400 nm were prepared by thermal evaporation technique at a base pressure of 10⁻⁶ Torr onto well cleaned glass substrate. a-Se_75−xTe₂₅In_x thin films were annealed at different temperatures for 2 h. As prepared and annealed films were characterized by X-ray diffraction and UV–Vis spectroscopy. The X-ray diffraction results show that the as-prepared films are of amorphous nature while it shows some poly-crystalline structure in amorphous phases after annealing. The optical absorption spectra of these films were measured in the wavelength range 400–1100 nm in order to derive the extinction and absorption coefficient of these films. It was found that the mechanism of optical absorption follows the rule of allowed non-direct transition. The optical band gap of as prepared and annealed films as a function of photon energy has been studied. The optical band gap is found to decrease with increase in annealing temperature in the present glassy system. It happens due to crystallization of amorphous films. The decrease in optical band gap due to annealing is an interesting behavior for a material to be used in optical storage. The optical band gap has been observed to decrease with the increase of In content in Se–Te glassy system.     

Chemical synthesis of Cu2Se nanoparticles at room temperature

A simple and rapid method has been developed to synthesize cuprous selenide (Cu₂Se) nanoparticles by the reaction of selenium nanoparticles sol with copper sulfate solution containing ascorbic acid at room temperature. Cu₂Se nanoparticles were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and energy dispersive X-ray analysis (EDX). The results indicated that Cu₂Se nanoparticles were cubic crystal structure and spherical with the diameter about 75 nm. The ultraviolet–visible absorption spectrum (UV–vis) and cyclic voltammetry of Cu₂Se nanoparticles were also investigated. The optical band gap energy of Cu₂Se nanoparticles was 1.94 eV. On the basis of a series of experiments and characterizations, the formation mechanism of Cu₂Se nanoparticles was discussed.     

Formation of F-doped ZnO transparent conductive films by sputtering of ZnF2

Fluorine-doped ZnO transparent conductive thin films were successfully deposited on glass substrate by radio frequency magnetron sputtering of ZnF₂. The effects of rapid thermal annealing in vacuum on the optical and electrical properties of fluorine-doped ZnO thin films have been investigated. X-ray diffraction spectra indicate that no fluorine compounds, such as ZnF₂, except ZnO were observed. The specimen annealed at 500 °C has the lowest resistivity of 6.65 × 10^? 4 Ω cm, the highest carrier concentration of 1.95 × 10²¹ cm^? 3, and the highest energy band gap of 3.46 eV. The average transmittance in the visible region of the F-doped ZnO thin films as-deposited and annealed is over 90%.     

Phase change and optical band gap behavior of Se0.8S0.2 chalcogenide glass films

Se_0.8S_0.2 chalcogenide glass films have been prepared by thermal vacuum evaporation technique with thickness 583 nm. Annealing process at T ≥ 333 K crystallizes the films and nanostructured films are formed. The crystallite size was increased to 24 nm as the annealing temperature increased to 373 K. Orthorhombic crystalline system was identified for the annealed films. SEM micrographs show that films consist of two parallel surfaces and the thickness was determined by cross section imaging. The optical transmittance is characterized by interference patterns as a result of these two parallel surfaces, besides their average value at longer wavelength decreases as a result of annealing process. The band gap, E_g is red shifted due to crystallization by annealing. As the phase of the films changes from amorphous to crystalline in the annealing temperature range 333–363 K, a non sharp change of the band gap (E_g) is observed. This change was explained by Brus’s model of the energy gap confinement behavior of the nanostructured films. The optical refractive index increases suddenly when the system starts to be crystallized by annealing.     

Density functional theories study on the properties of Ga1 − xCrxAs

The structural, electronic, magnetic and optical properties of Ga_1 − xCr_xAs (x = 0, 6.25%, 12.5%) have been studied by first-principles calculations based on the HSE hybrid density functional theories. The optical properties, including the complex dielectric function, optical refractive index, extinction coefficient and absorption coefficient are discussed for radiation up to 15 eV. The results predicate that the system of Ga_1 − xCr_xAs exhibits typical half-metallic properties, in which Cr forms deep levels in the forbidden band and reduces the energy gap, increases static dielectric constant and obviously red-shifts the absorption edge. With the increase of the fraction of Cr, the material gradually exhibits noticeable anisotropy in the photon energy range of 0–5 eV. In addition, the p-d hybridization reduces the magnetic moment of Cr from its free space charge value of 3 μ_B and a smaller atomic magnetic moments of As and Ga atoms are generated.