Ab initio prediction of structural,electronic, magnetic and optical properties of Ba2GdSbO6

A first-principles approach is used to study the structural, electronic, optic and magnetic properties of Ba₂GdSbO₆, using full-potential linearized augmented plane wave (FP-LAPW) scheme within GGA+U approach. Features such as the lattice constant, bulk modulus and its pressure derivative are reported. The calculated band structure and density of states show that the material under load has an indirect energy band gap L→X for majority-spin direction and Г→X for the minority spin channel. The analysis charge densities show that bonding character as a mixture of covalent and ionic nature. The optical properties are analyzed and the origin of some peaks in the spectra is described. Besides, the dielectric function, refractive index and extinction coefficient for radiation up to 14 eV have also been reported.     

Electronic band structure and optoelectronic properties of double perovskite Sr2MgMoO6 through modified Becke-Johnson potential

The crystal structure, electronic and optical properties of double perovskite Sr₂MgMoO₆ have been calculated by using the full-potential linear augmented plane wave (FP-LAPW) method. The band structure and density of states (DOS) were carried out by the modified Becke–Johnson (mBJ) exchange potential approximation based on the density functional theory (DFT). The calculated band structure shows a direct band gap (Γ–Γ) of 2.663 eV for Sr₂MgMoO₆. The compound Sr₂MgMoO₆ has a triclinic structure with the space group I-1, the lattice parameters a=5.5666 Å, b=5.5661 Å and c=7.9191 Å, which are used in our calculations. The optical parameters, like dielectric constant, refractive index, reflectivity and energy loss function were also calculated and analyzed. This work provides the first quantitative theoretical prediction of the optical properties and electronic structure for the triclinic phase of Sr₂MgMoO₆.     

First principles calculations of structural,electronic and optical properties of zinc aluminum oxide

A first principles study of structural, electronic and optical properties of zinc aluminum oxide (ZnAl₂O₄) by means of the full potential linear augmented plane wave method is presented. The local density approximation is used for the exchange-correlation potential. A direct band gap of 4.19 eV, in agreement with experiment (E_g=3.9 eV), was determined. ZnAl₂O₄ is transparent in the visible spectral region; the excitonic transition associated with the fundamental band gap is 4.17 eV. The refractive index value is 1.74 in the ultraviolet spectral region.     

Phase transformation kinetics and optical properties of Ga–Se–Sb phase-change thin films

Phase transformation kinetics in Ga₂₅Se_75?xSb_x glasses have been determined by non-isothermal differential scanning calorimetric measurements at heating rates of 5, 10, 15, 20 and 25 K/min. The values of glass transition (T_g) and crystallization temperature (T_c) are found to be composition and heating rate dependent. The activation energy of crystallization and glass transition have been determined from the dependence of T_c and T_g on the heating rate. Thin films of Ga₂₅Se_75?xSb_x glasses have been prepared by vacuum evaporation technique with thickness 400 nm. These thin films were crystallized by thermal annealing and laser-irradiation. The phase change phenomena have been studied by measuring optical absorption of as-prepared and crystallized thin films in the wave length region 400–900 nm. The optical absorption data indicate that the absorption mechanism is non-direct transition. Optical band gap values decrease with increase in Sb contents in Ga–Se as well as with increase in annealing temperature and laser-irradiation time. The optical band gap is shifted due to crystallization by annealing/laser-irradiation. As the phase of the films changes from amorphous to crystalline, a non sharp change of the optical band gap is observed. This gradual decrease in optical band gap was explained to be a result of an amorphous–crystalline phase transformation.     

Structural,optical and electrical properties of Cu2FeSnSe4 and Cu(In,Al)Se2 thin films

Cu-based semiconductors Cu₂FeSnSe₄ (CFTSe) and Cu(In, Al)Se₂ (CIAS) have been fabricated using radio-frequency magnetron sputtering combined with rapid thermal selenization processing. For CFTSe, the heating rate ranging from 60 to 150 °C/min results in a difference in structure, morphology and optical properties. Thin film exhibits a pure phase structure, smooth surface and a band gap of 1.19 eV as the heating rate elevated to 90 °C/min. Furthermore, the CFTSe thin film selenized at 90 °C/min own the smallest value of cell volume compared with the others samples, which represents a more stable structure. In terms of the other Cu-based material CIAS, three different selenization pressures, i.e., 1, 5 and 10 Torr, have been employed for CIAS preparation. Thin film transforms into single phase with dense morphology along with the pressure of 1 Torr. The diverse band gap of CIAS thin films from 1.34 to 2.18 eV attribute to two reasons: (i) the various Al content will affect the hybridization degree of Al–Se, and finally tunes the band structure, (ii) amounts of CuSe has a certain degree of effect on the band gap of the CIAS. In addition, the electrical properties of CFTSe and CIAS are also researched with the open circuit voltage (V_oc) of 94 and 365 mV, respectively, signifying potential applications of CFTSe and CIAS for the thin film solar cells.     

Characterization of ternary MgxZn1−xO thin films deposited by electron beam evaporation

Mg_xZn_1−xO (0≤x≤1) thin films were deposited on glass and quartz substrates by electron beam evaporation and effect of the Mg content of the film on its structural, optical and electrical properties were investigated. The structure, surface morphology, optical transmittance, band gap, refractive index and electrical resistivity were found to depend on the Mg content of the film. XRD data revealed that films were polycrystalline in nature. The structure of the films having Mg content in the range of 1–0.74 was cubic, mixed cubic-hexagonal phases for x=0.47 and hexagonal phase for x=0. The composition analysis showed that Mg content in Mg_xZn_1−xO film is high as compared to the corresponding target alloy. It was observed that the optical band gap increases from 3.3 to 6.09 eV, refractive index at 550 nm decreases from 1.99 to 1.75, transmittance increases from about 70% to 90% and electrical resistivity increases from 0.5 to 1.48×10⁶ Ω cm with the increase of Mg concentration in the film from 0 to 1. The results reported in this work are useful for window layer of solar cells and other optoelectronic devices.     

Ab initio studies of structural,electronic, optical,elastic and thermal properties of Ag-chalcopyrites (AgAlX2: X=S,Se)

The ab-initio calculations for the structural, electronic, optical, elastic and thermal properties of Ag-chalcopyrites (AgAlX₂: X=S and Se) have been reported using the full potential linearized augmented plane wave (FP-LAPW) method. In this paper, the recently developed Tran–Blaha modified Becke–Johnson potential is used along with the Wu-Cohen generalized gradient approximation (WC-GGA) for the exchange-correlation potential. Results are presented for lattice constants, bulk modulus and its pressure derivative, band structures, dielectric constants and refractive indices. We have also computed the six elastic constants (C₁₁, C₁₂, C₁₃, C₃₃, C₄₄, C₆₆). The thermodynamical properties such as thermal expansion, heat capacity, Debye temperature, entropy, bulk modulus are calculated employing the quasi-harmonic Debye model at different temperatures (0–900 K) and pressures (0–8 GPa) and the silent results are interpreted. Hardness of the materials is calculated for the first time at different temperatures and pressures.     

First-principles calculations of structural,elastic, electronic and optical properties of XO (X=Ca,Sr and Ba) compounds under pressure effect

The structural, elastic, electronic and optical properties of XO (X= Ca, Sr and Ba) compounds were investigated by the density functional theory. A good agreement was found between our calculated results and the available theoretical and experimental data of the lattice constants. Young's modulus, Poisson ratio, bulk modulus, elastic constants and their pressure derivatives are also calculated. SrO and BaO compounds present a transition phase at 39.72 and 27.28 GPa. The SrO compound shows a change from direct band gap (Γ–Γ) to indirect band gap (Γ–X) at about 15 GPa. The top of the valence bands reflects the s electronic character for all structures. We investigate the effective mass of electrons as function of pressure at the Γ point for CaO, SrO and BaO compounds. Calculations of the optical spectra have been performed for the energy range 0–60 eV. The origin of the spectral peaks was interpreted based on the electronic structures. The enhancement of pressure increases the static dielectric function and refractive index of CaO, SrO and BaO.     

The significant effect of film thickness on the properties of chalcopyrite thin absorbing films deposited by RF magnetron sputtering

In this paper, thickness dependent structural, surface morphological, optical and electrical properties of RF magnetron sputtered CuIn_0.8Ga_0.2Se₂ (CIGS) thin films were studied using X-ray diffraction (XRD), Transmission electron microscopy (TEM), Field emission scanning electron microscopy (FE-SEM), Atomic force microscopy (AFM), UV–vis–NIR spectrophotometer and Keithley electrical measurement unit. The peak intensity along (112) plane as well as crystallite size was found to increase with thickness. However, for higher film thickness >1.16 μm, crystallinity reduced due to higher % of Cu content. TEM analysis confirmed pollycrysallinity as well as chalcopyrite phase of deposited films. The band gap was found to decrease with increase in thickness yielding a minimum value of 1.12 eV for film thickness 1.70 μm. The I–V characteristics showed the ohmic behavior of metal semiconductor contact with higher conductivity for film thickness 1.16 μm.     

Determination of structural and optical properties of gold phthalocyanine thin films using density functional theory

We determined some optical and electrical properties of thin gold phthalocyanine films. Calculations were performed in the framework of density functional theory using the full potential linear augmented plane wave method. Studies on the density of states and band structure yielded a bandgap energy (E_g) of approximately 2 eV. Two trap energy levels were observed at 0.9 and 1.3 eV. Analysis of the dielectric function and electric loss function revealed a plasmon oscillation at 1.8 eV. In addition, we determined static refractive index values in the x, y and z directions of n_0xx = 2.16, n_0yy = 1.66 and n_0zz = 2.07. The optical bandgap of gold phthalocyanine was estimated to be 0.97 eV. Calculations revealed strong absorption at 400–700 nm, which compares favorably with experimental results.     

Structural and optical characterizations of spin coated cobalt-doped cadmium oxide nanostructured thin films

Cd_1−xCo_xO thin films (with molar ratios x=0.0–8.0%) were grown onto glass substrates via the sol–gel spin coating technique. XRD results indicate that a CdO single phase with a cubic polycrystalline structure is formed. The crystallinity of CdO thin films is gradually deteriorated with increasing the Co ratio. AFM images of the films confirm the decrease of the grain size of the CdO films with increasing Co content. The direct optical band gap is red shifted from 2.580 eV to 2.378 eV with the increase of Co content. The refractive index, the dispersion parameters, and the optical conductivity of CdO thin films showed an enhancement with increasing cobalt dopant ratio. The correlation between the structural modifications and the resultant optical properties are reported.     

Investigation of the properties of ferromagnetic ZnO:Cr2O3 nanocomposites

Present work focuses on the structural, optical and magnetic properties of ZnO:Cr₂O₃ nanocomposites. ZnO nanoparticles were synthesized and the structure was confirmed using powder x-ray diffraction. ZnO nanoparticles was grown in the hexagonal wurtzite structure with the preferential orientation along (101) plane. ZnO:Cr₂O₃ composites have been synthesized by doping different concentration of Cr₂O₃ (1, 3 and 5 wt%) into ZnO. The incorporation of Cr₂O₃ was confirmed using Fourier transformed infrared spectroscopy. UV–visible absorption spectra have been observed and interpreted for the determination of optical constants of ZnO:Cr₂O₃ composites. The optical constants like optical band gap, refractive index were determined and the effect of Cr₂O₃ on these constants was investigated. Relation between optical band gap and the refractive index were obtained. Magnetic studies using vibrating sample magnetometer reveal the ferromagnetism at 150 K in the composites with 3 and 5 wt% of Cr₂O₃.     

First principles study of electronic and optical properties of the ternary SnSb4S7 using modified Becke-Johnson potential

The electronic and optical properties of SnSb₄S₇ compound are calculated by the full-potential linearized augmented plane-wave (FP-LAPW) method. The density of states (DOS) is carried out by the modified Becke-Johnson (mBJ) exchange potential approximation based on density functional theory (DFT). The compound SnSb₄S₇ has a monoclinic structure with the space group P2₁/m with lattice parameters of a=11.331 Å, b=3.865 Å and c=13.940 Å. The band gap is calculated to be 0.8 eV. The optical parameters, like dielectric constant, refractive index, reflectivity and energy loss function were also calculated and analyzed. The present work provides information about variation of the electronic and optical properties which reveals that SnSb₄S₇ is suitable for optoelectronic devices.     

Growth of highly (1 1 1) oriented CuIn0.75Al0.25Se2 thin films

CuIn_0.75Al_0.25Se₂ thin films prepared onto glass substrates at T_S=573 K were single phase, nearly stoichiometric and polycrystalline with a strong (1 1 1) preferred orientation showing sphalerite structure. The results of X-ray diffraction and electron diffraction studies are compared, interpreted and correlated with micro-Raman spectra. The optical absorption studies indicated a direct band gap of 1.16 eV with high absorption coefficient (>10⁴ cm^?1) near the fundamental absorption edge.     

Behavior of chemically deposited PbS thin films subjected to two different routes of post deposition annealing

Lead sulfide (PbS) thin films were prepared on soda lime glass substrates at room temperature by Chemical Bath Deposition (CBD) technique. This paper reports a comparative study of characteristic properties of as-prepared PbS thin films after thermal treatment through two different routes. Studies were carried out for as-prepared as well as rapidly and gradually annealed samples at 100, 200 and 300 °C. The characterizations of the films were carried out using X-ray diffraction, scanning electron microscopy and optical measurement techniques. The structural studies confirmed the polycrystalline nature and the cubic structure of the films. As-deposited films partly transformed to Pb₂O₃ when gradually annealed to 300 °C. The presence of nano crystallites was revealed by structural and optical absorption measurements. The values of average crystallite size were found to be in the range 18–20 nm. The variation in the microstructure, thickness, grain size, micro strain and optical band gap on two types of annealing were compared and analyzed. Data showed that post deposition parameters and thermal treatment strongly influence the optical properties of PbS films. Optical band gap of the film gets modified remarkably on annealing. Direct band gap energy values for rapidly and gradually annealed samples varied in the range of 1.68–2.01 eV and 1.68–2.12 eV respectively. Thus we were succeeded in tailoring direct band gap energies by post deposition annealing method.     

Structure,luminescence and photocatalytic activity of Mg-doped ZnO nanoparticles prepared by auto combustion method

A series of Zn_1−xMg_xO nanoparticles with x=0 to 0.15 were prepared by auto combustion method using citric acid as the fuel and chelating agent. Structure, luminescence and photocatalytic properties were systematically investigated by means of X-ray diffraction, scanning electron microscopy, photoluminescence spectra, ultraviolet–visible absorbance measurement and photochemical reactions etc. The samples retained hexagonal wurtzite structure of ZnO and single phase below x=0.13, and the sizes of the nanoparticles were 60–70 nm. The photoluminescence spectroscopy demonstrated blue shift of ultraviolet emission with increasing Mg doping concentration. Both optical measurements of the as grown and Mg doped ZnO nanoparticles showed that the optical band gap could be modified from ~3.28 eV to 3.56 eV as the Mg content x increased from 0 to 0.13. The photocatalytic activities of the samples were evaluated by photocatalytic degradation of methyl orange, and the results showed that the doping of Mg into ZnO nanoparticles could enhance photocatalytic activity compared to the undoped ZnO nanoparticles, which was attributed to increased band gap and superior textural properties. In addition, according to the PL and photocatalytic studies, the critical doping content of effective Mg in ZnO is up to 0.09.     

Fabrication of polymer blend composites based on [PVA-PVP](1−x):(Ag2S)x (0.01 ≤ x ≤ 0.03) with small optical band gaps: Structural and optical properties

In this paper composite materials, based on polymer blends of polyvinyl alcohol (PVA): polyvinyl pyrrolidone (PVP) with small optical band gap, has been studied. Silver sulfide (Ag₂S) semiconductor particles have been synthesized in PVA:PVP blend host polymer, using in situ method. X-ray diffraction (XRD) analyses and Fourier transform infrared (FTIR) spectroscopy for the composite samples were carried out. From the XRD pattern, distinguishable crystalline peaks caused by the Ag₂S semiconductor particles were observed. From the result of FTIR spectroscopy, the intensity of the FTIR bands were shifted and increased, revealing the occurrence of interactions between the PVA:PVP blend system and Ag₂S particles. The composite samples were found to exhibit absorption spectra that cover UV–visible to near infrared regions. The absorption edge was found to be 5 eV for pure PVA:PVP system and shifted to 1.15 eV for incorporated PVA:PVP with 3 M of Ag₂S. The refractive index was also evaluated for the samples and observed to be increased from 1.15 to 1.52 as doping increased to the highest. A linear relationship between the refractive index and the filler fraction has been reported. Theoretical discussion of optical dielectric loss, which is a crucial parameter for the band gap estimation, was given. The achieved results reveal that spectra of the optical dielectric loss (ɛ_i) can be used to study the band gap structure and Tauc's model can be important in determining the types of electronic transition. The optical band gap was found to decrease from 5.2 eV for the pure PVA:PVP to 1.1 eV for doped PVA:PVP with 3 M of Ag₂S. Such reduction can be associated with the increase of optical dielectric constant. Finally, the correlation between optical dielectric constant and density of states was discussed.     

Optoelectronic properties of CdTe/Si heterojunction prepared by pulsed Nd:YAG-laser deposition technique

The present study is on the optoelectronic properties of isotype CdTe/c-Si heterojunction photodetector made by deposition of CdTe by pulsed laser deposition (PLD) technique on clean monocrystalline Si. Optical, electrical and structural properties of grown CdTe film were investigated. The optical data show that the optical band gap of CdTe was around 1.45 eV at 300 K. The CdTe/Si junction exhibits fair diode rectification and the soft breakdown occurred at V_B>9 V. Dark and illuminated I–V characteristics of the CdTe/Si photodetector are examined at room temperature. The photodetector showed good photosensitivity in the visible and near-infrared regions with a value as high as 0.5A/W at 950 nm.     

The effects of thickness on properties of B and Ga co-doped ZnO films grown by magnetron sputtering

We investigated the effects of thickness on the electrical, optical, structural and morphological properties of B and Ga co-doped ZnO (BGZO) films grown by radio frequency (RF) magnetron sputtering. All the prepared BGZO films showed preferentially c-axis orientation and structure of hexagonal wurtzite. The results also indicated that with an increase in film thickness, the crystallite sizes of the films were increased and the optical band gap (E_g) was decreased. Below a critical thickness of about 210 nm, the thickness of the BGZO films significantly affected the electrical properties of the films. The average transmittance for all the grown films did not change obviously with the thickness.     

Effect of cobalt substitution on the optical properties of bismuth ferrite thin films

Effect of cobalt substitution on the band gap and absorption coefficient of the BiFeO₃ thin films formed on quartz substrate by low cost spin coating method have been investigated. BiFe_1−xCo_xO₃ (x=0, 0.03, 0.06 and 0.10) thin films are polycrystalline and it retains the rhombohedral distorted perovskite structure up to 10 mole % of Co substitution. Smooth and compact surface morphology with uniform size particles are observed in SEM micrographs. Narrowing and broadening of band gap is observed as a function of Co content. Two strong emission peaks at ~2.51 eV and ~2.38 eV are recorded for all films with noticeable change in intensity. Results obtained from the optical absorption and photoluminescence spectroscopy experiments have shown that there exists an inverse correlation between the variation in the band gap and the concentration of oxygen vacancies. Band gap decreased by ~100 meV and absorption coefficient increased by 28% at the wavelength of 375 nm in 6 mole % Co substituted thin film and these observations are necessary requirements to improve the efficiency of photovoltaic devices.