Showing papers in "Physica B-condensed Matter in 2011"

First principles study of structural, vibrational and electronic properties of graphene-like MX2 (M=Mo, Nb, W, Ta; X=S, Se, Te) monolayers

Abstract: Using first principles calculations, we investigate the structural, vibrational and electronic structures of the monolayer graphene-like transition-metal dichalcogenide (MX2) sheets. We find the lattice parameters and stabilities of the MX2 sheets are mainly determined by the chalcogen atoms, while the electronic properties depend on the metal atoms. The NbS2 and TaS2 sheets have comparable energetic stabilities to the synthesized MoS2 and WS2 ones. The molybdenum and tungsten dichalcogenide (MoX2 and WX2) sheets have similar lattice parameters, vibrational modes, and electronic structures. These analogies also exist between the niobium and tantalum dichalcogenide (NbX2 and TaX2) sheets. However, the NbX2 and TaX2 sheets are metals, while the MoX2 and WX2 ones are semiconductors with direct-band gaps. When the Nb and Ta atoms are doped into the MoS2 and WS2 sheets, a semiconductor-to-metal transition occurs. Comparing to the bulk compounds, these monolayer sheets have similar structural parameters and properties, but their vibrational and electronic properties are varied and have special characteristics. Our results suggest that the graphene-like MX2 sheets have potential applications in nano-electronics and nano-devices.

First principle study of the structural and optoelectronic properties of cubic perovskites CsPbM3 (M=Cl, Br, I)

Abstract: The highly accurate all electrons full potential linearized augmented plane wave method is used to calculate structural, electronic, and optical properties of cubic perovskites CsPbM3 (M=Cl, Br, I). The theoretically calculated lattice constants are found to be in good agreement with the experimentally measured values. It is found that all of these compounds are wide and direct bandgap semiconductors with bandgap located at R-symmetry point, while the bandgap decreases from Cl to I. The electron densities reveal strong ionic bonding between Cs and halides but strong covalent bonding between Pb and halides. Optical properties of these compounds like real and imaginary parts of dielectric functions, refractive indices, extinction coefficients, reflectivities, optical conductivities, and absorption coefficients are also calculated. The direct bandgap nature and high absorption power of these compounds in the visible–ultraviolet energy range imply that these perovskites can be used in optical and optoelectronic devices working in this range of the spectrum.

Optical, physical and structural studies of boro-zinc tellurite glasses

Abstract: To investigate the modification effect of the modifier ZnO on boro-tellurite glass, a series of glasses with compositions 50B 2 O 3 –(50− x )ZnO– x TeO 2 have been prepared by conventional melt quenching technique. Amorphous nature of the samples was confirmed through X-ray diffraction technique. Optical absorption and IR structural studies are carried out on the glass system. The optical absorption studies revealed that the cutoff wavelength increases while optical band gap ( E opt ) and Urbach energy decreases with an increase of ZnO content. Refractive index evaluated from E opt was found to increase with an increase of ZnO content. The compositional dependence of different physical parameters such as density, molar volume, oxygen packing density, optical basicity, have been analyzed and discussed. The IR studies showed that the structure of glass consists of TeO 4 , TeO 3 /TeO 3+1 , BO 3 , BO 4 and ZnO 4 units.

DSC, TGA and dielectric properties of carboxymethyl cellulose/polyvinyl alcohol blends

Abstract: Films with different compositions of polyvinyl alcohol (PVA) and carboxymethyl cellulose (CMC) blends have been prepared using the casting method. Differential scanning calorimetery (DSC), thermogravimetric analysis (TGA) and dielectric spectroscopy of all compositions have been investigated. It was found that PVA and CMC are compatible in the studied range of composition. With increasing CMC content, the thermal stability of PVA increases. Based on DSC and TGA data, the activation energies of all the investigated samples were calculated. The absorption edge ( E a ) was also determined from Ultraviolet–visible (UV–vis) spectra. Dielectric permittivity, loss tangent and ac conductivity of all samples were studied as functions of temperature and frequency. The results show that the dielectric dispersion consists of both dipolar and interfacial polarization. The frequency dependence of the ac conductivity indicates that the correlated barrier hopping (CBH) is the most suitable mechanism for conduction. The polaron binding energy ( U M ) was determined. Results of the present system are compared with those of similar materials.

Investigations on the effect of complexation of NaF salt with polymer blend (PEO/PVP) electrolytes on ionic conductivity and optical energy band gaps

Abstract: Sodium ion conducting polymer blend electrolyte films, based on polyethylene oxide (PEO) and polyvinyl pyrrolidone (PVP) complexed with NaF salt, were prepared using solution casting technique. The complexation of the salt with the polymer blend was confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and UV–vis spectroscopy. Electrical conductivity of the films was measured with impedance analyzer in the frequency range of 1 Hz to 1 MHz and in the temperature range of 303–348 K. It was observed that the magnitude of conductivity increased with the increase in the salt concentration as well as the temperature. UV–vis absorption spectra in wavelength region of 200–800 nm were used to evaluate the optical properties like direct and indirect optical energy band gaps, optical absorption edge. The optical band gaps decreased with the increase in Na+ ion concentration. This suggests that NaF, as a dopant, is a good choice to improve the electrical properties of PEO/PVP polymer blend electrolytes.

Flow and heat transfer over a rotating porous disk in a nanofluid

Abstract: The steady flow of an incompressible viscous fluid due to a rotating disk in a nanofluid is studied. The transformed boundary layer equations are solved numerically by a finite difference scheme, namely the Keller-box method. Numerical results for the flow and heat transfer characteristics are obtained for various values of the nanoparticle volume fraction parameter φ and suction/injection parameter h0. Two models for the effective thermal conductivity of the nanofluid, namely the Maxwell–Garnett model and the Patel model, are considered. It is found that for the Patel model, the heat transfer rate at the surface increases for both suction and injection, whereas different behaviors are observed for the Maxwell–Garnett model, i.e. increasing the values of φ leads to a decrease in the heat transfer rate at the surface for suction, but increases for injection. The results of this study can be used in the design of an effective cooling system for electronic components to help ensure effective and safe operational conditions.

Magneto-thermoelasticity with thermoelectric properties and fractional derivative heat transfer

Abstract: In this work, a new model of the magneto-thermoelasticity theory has been constructed in the context of a new consideration of heat conduction with fractional derivative. A one-dimensional application for a conducting half-space of thermoelectric elastic material, which is thermally shocked in the presence of a magnetic field, has been solved using Laplace transform and state-space techniques (Ezzat, 2008 [1]). According to the numerical results and its graphs, a conclusion about the new theory of magneto-thermoelasticity has been constructed. The theories of coupled magneto-thermoelasticity and of generalized magneto-thermoelasticity with one relaxation time follow as limited cases. The result provides a motivation to investigate conducting thermoelectric materials as a new class of applicable materials.

Attractive microwave absorption and the impedance match effect in zinc oxide and carbonyl iron composite

Abstract: The flower-like ZnO and ZnO/carbonyl-iron composite have been prepared by a sonochemical route and ball-milling process, respectively. For ZnO/carbonyl-iron composite, a reflection loss ( RL ) exceeding −20 dB was obtained in a broad frequency range of 8.4–17.9 GHz with a thin thickness of 1.2–2.3 mm. An optimal RL of −61 dB was found at 11.7 GHz for an absorber thickness of 1.91 mm. It is demonstrated that the attractive microwave-absorption properties are a consequence of a proper electro-magnetic impedance match and geometrical cancellation at the air–material interface. In addition, an impedance mismatch function was proposed, which provides an effective method to determine the microwave absorbing properties from the intrinsic materials constants. The calculated value of matching frequency and thickness is well consistent with the experimental data. The method also provides a simple theoretical graphic aid for determining the absorption characteristics and the location of the matching conditions in the frequency domain.

Study of dielectric and impedance properties of Mn ferrites

Abstract: The paper reports on the effect of Al substitution on the structural and electrical properties of bulk ferrite series of basic composition MnFe2 � 2xAl2xO4 (0.0r xr 0.5) synthesized using solid state reaction method. XRD analysis confirms that all the samples exhibit single phase cubic spinel structure excluding presence of any secondary phase. The dielectric constant shows a normal behaviour with frequency, whereas the loss tangent exhibits an anomalous behaviour with frequency for all compositions. Variation of dielectric properties and ac conductivity with frequency reveals that the dispersion is due to Maxwell–Wagner type of interfacial polarization in general and hopping of charge between Fe +2 and Fe +3 as well as between Mn +2 and Mn +3 ions at octahedral sites. The complex impedance plane spectra shows the presence of two semicircles up to x¼ 0.2, and only one semicircle for the higher values of x. The analysis of the data shows that the resistive and capacitive properties of the Mn ferrite are mainly due to processes associated with grain and grain boundaries.

Cerium (III) doping effects on optical and thermal properties of PVA films

Abstract: Cerium chloride (CeCl 3 ) doped polyvinyl alcohol (PVA) films were prepared by casting technique. The effect of CeCl 3 concentrations on the structural, optical and thermal properties of the PVA films was studied by X-ray diffraction (XRD), FT-IR, UV-visible, transmittance ( T ), reflectance ( R ), differential scanning calorimetry (DSC) and thermogravimetry (TG). Both of the XRD and the DSC results affirm the increase in amorphousity. Absorption spectra of the doped films have shown an absorption band at 260 nm assigned to the trivalent state of cerium ions. Absorption, transmittance and reflectance spectra were used for the determination of the optical constants. The results indicate that the optical band gap ( E g ) was derived from Tauc's extrapolation and decreases with the cerium content. The refractive index increases with monotonic behavior as the cerium content increases. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomernico model for obtaining the dispersion parameters. The obtained optical parameters were found to be strongly affected by CeCl 3 dopant. Thermal analysis showed that the thermal parameters of PVA are enhanced by CeCl 3 . The dependence of the activation energy of the decomposition temperature on doping level was estimated.

CO gas sensing of CuO nanostructures, synthesized by an assisted solvothermal wet chemical route

Abstract: CuO nanostructures with different morphologies and sizes were grown in a controlled manner using a simple low-temperature hydrothermal technique. By controlling the pH of reaction mixture, spherical nanoparticles and cloudlike CuO structures were synthesized at 100–150 °C with excellent efficiency. These CuO nanostructures have been tested for CO gas monitoring by depositing them as thick films on an interdigitated alumina substrate and evaluated the surface resistance of the deposited layer as a function of operating temperature and CO concentrations. The gas sensitivity tests have demonstrated that the CuO nanostructures, especially cloudlike morphology, exhibit high sensitivity to CO proving their applicability in gas sensors. The role of the nanostructure on the sensing properties of CuO is also discussed.

Structural and magnetic behaviour of aluminium doped barium hexaferrite nanoparticles synthesized by solution combustion technique

Abstract: Nanocrystalline M-type Al 3+ substituted barium hexaferrite samples having generic formula BaFe 12− x Al x O 19 (where x =0.00, 0.25, 0.50, 0.75, 1.00) were synthesized by the solution combustion technique. The precursors were prepared using stoichiometric amounts of Ba 2+ , Fe 3+ and Al 3+ nitrate solutions with citric acid as a chelating agent. The barium nitrate to citric acid ratio was taken as 1:2 and pH of the solution was kept at 8. The sintered samples were characterized by XRD, EDAX, SEM, TEM and VSM techniques. Pure barium hexaferrite shows only single phase hexagonal structure while samples at 0.25≤ x ≥1.00 show α-Fe 2 O 3 peaks with M-phase of barium hexaferrite in the X-ray diffraction pattern. The lattice parameters ( a and c ) obtained from XRD data decreases with increase in aluminium content x . The particle size obtained from X-ray diffraction data is in the nanometer range. The magnetic behaviour of the samples was studied using vibrating sample magnetometer technique. The saturation magnetization ( M s ) and magneton number ( n B ) decrease from 38.567 to 21.732 emu/g and from 7.6752 to 4.2126μ B, respectively, with increase in Al 3+ substitution x from x =0.0 to 1.0.

Microstructure and optical properties of ZnO nanoparticles prepared by a simple method

Abstract: In this investigation, ZnO nanoparticles were prepared by a simple and rapid method. This method is based on the short time solid state milling and calcinations of zinc acetate and citric acid powders. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, photoluminescence and UV–vis spectroscopy. It was shown that the calcination temperature significantly affected the particle size and optical properties of the synthesized ZnO nanoparticles. Calculation based on the XRD data shows that the average sizes of ZnO particles are in agreement with those from TEM images and the size of the particles increases on increasing the calcination temperature. Also the band gap of samples decreased from 3.29 to 3.23 eV on increasing the calcination temperature from 350 to 600 °C. Photoluminescence analyses show that many defects such as interstitial zinc, zinc vacancy and oxygen vacancy are responsible for the observed optical properties.

Effect of La substitution on conductivity and dielectric properties of Bi1−xLaxFeO3 ceramics: An impedance spectroscopy analysis

Abstract: Bismuth ferrite (BFO) and La-substituted BFO with composition Bi 1− x La x FeO 3 ( x =0.05, 0.1 and 0.15) (BLFO x =0.05–0.15 ) ceramics were prepared using the solid state reaction route. A structural phase transition from rhombohedral phase to triclinic phase was observed for BLFO x =0.05–0.15 ceramics. Modulus spectroscopy reveals the deviation of dielectric behavior from ideal Debye characteristics and the dependence of conductivity on ion hopping in BFO and BLFO x =0.05–0.15 ceramics. The conductivity of the BFO ceramics decreases for La content of 5 mol%, followed by a subsequent increase with 10 and 15 mol% of lanthanum doping. The typical values of the activation energies at high temperature reveal the contribution of short range movement of doubly ionized oxygen vacancies to the conduction process in BFO and BLFO x =0.05 ceramics. Both short range and long range motion of oxygen vacancies are responsible for large conductivity in BLFO x =0.1 and 0.15 ceramics.

UV–visible and infrared absorption spectra of gamma irradiated CuO-doped lithium phosphate, lead phosphate and zinc phosphate glasses: A comparative study

Abstract: Undoped and CuO-doped lithium phosphate, lead phosphate and zinc phosphate glasses were prepared. UV–visible and infrared absorption spectra of the prepared samples were measured before and after successive gamma irradiation. Experimental optical spectra of the undoped samples reveal strong UV absorption bands, which are attributed to the presence of trace iron impurities in both the lithium and zinc phosphate glasses while the lead phosphate glass exhibits broad UV bands due to combined absorption of trace iron impurities and divalent lead ions. The CuO-doped glasses reveal an extra broad visible band due to Cu 2+ ions in octahedral coordination. The effects of gamma irradiation have been analyzed for both the sharing of all constituent components including trace iron impurities. Infrared absorption spectra of the prepared samples were investigated by the KBr disk technique. The FTIR spectra reveal main characteristic absorption bands due to different phosphate groups. The IR spectra are observed to be slightly affected by the increase of CuO in the doping level (0.2–3%) indicating the stability of the main network units.

Electrical and switching properties of NiAlxFe2−xO4 ferrites synthesized by chemical method

Abstract: Nickel-aluminum ferrite system NiAlxFe2−xO4 has been synthesized by wet chemical co-precipitation method. The samples were studied by means of X-ray diffraction, d.c. electrical resistivity, a.c. electrical resistivity, a.c. conductivity and switching properties. The XRD patterns confirm the cubic spinel structure for all the synthesized samples. The crystallite size calculated from XRD data which confirm the nano-size dimension of the prepared samples. Electrical properties such as a.c. and d.c. resistivities as function of temperature were studied for various Al substitution in nickel ferrite. The dielectric constant and dielectric loss tangent were also studied as a function of frequency. The dielectric constant follows the Maxwell–Wagner interfacial polarization. A.C. conductivity increases with increase in applied frequency. The d.c. resistivity decreases as temperature increases, which indicate that the sample have semi-conducting nature. Verwey hoping mechanism explains the observed variation in resistivity. The activation energy is derived from the temperature variation of resistivity. Electrical switching properties were studied as I–V measurements. The current controlled negative resistance type switching is observed in all the samples. The Al substitution in nickel ferrite decreases the required switching field.

Nanostructure and optical properties of M doped ZnO (M=Ni, Mn) thin films prepared by sol–gel process

Abstract: The transparent thin films of undoped, Mn-doped, and Ni-doped zinc oxide (ZnO) have been deposited on glass substrates via sol–gel technique using zinc acetate dehydrate, nickel chloride, and manganese chloride as precursors. The structural properties and morphologies of the deposited undoped and doped ZnO thin films have been investigated. X-ray diffraction (XRD) spectra, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to examine the morphology and microstructure of the thin films. Optical properties of the thin films were determined by photoluminescence (PL) and UV/vis spectroscopy. The analyzed results indicate that the obtained films are of good crystal quality and have smooth surfaces, which have a pure hexagonal wurtzite ZnO structure without any Mn or Ni related phases. The band gap energy was estimated by Tauc's method and found to be 3.28, 3.26, and 3.34 eV for ZnO, Ni-doped ZnO, and Mn-doped ZnO thin films at room temperature, respectively. Room temperature photoluminescence is observed for the ZnO, Ni-doped ZnO, and Mn-doped ZnO thin films.

Effects of external electric and magnetic fields on the linear and nonlinear intersubband optical properties of finite semi-parabolic quantum dots

Abstract: In this work, influences of external electric and magnetic fields on the optical rectification coefficient, the linear and the third-order nonlinear optical absorption coefficients as well as refractive index changes of finite semi-parabolic quantum dots are investigated. In this regard, energy eigenvalues and eigenfunctions of the system are calculated numerically, and optical properties are obtained using the compact density matrix approach. The results show that external electric and magnetic fields have a great influence on these optical quantities.

Optical and electrical properties of SnS semiconductor crystals grown by physical vapor deposition technique

Abstract: Tin sulfide (SnS) is a material of interest for use as an absorber in low cost solar cells. Single crystals of SnS were grown by the physical vapor deposition technique. The grown crystals were characterized to evaluate the composition, structure, morphology, electrical and optical properties using appropriate techniques. The composition analysis indicated that the crystals were nearly stoichiometric with Sn-to-S atomic percent ratio of 1.02. Study of their morphology revealed the layered type growth mechanism with low surface roughness. The grown crystals had orthorhombic structure with (0 4 0) orientation. They exhibited an indirect optical band gap of 1.06 eV and direct band gap of 1.21 eV with high absorption coefficient (up to 10(3) cm(-1)) above the fundamental absorption edge. The grown crystals were of p-type with an electrical resistivity of 120 Omega cm and carrier concentration 1.52 x 10(15) cm(-3). Analysis of optical absorption and diffuse reflectance spectra showed the presence of a wide absorption band in the wavelength range 300-1200 nm, which closely matches with a significant part of solar radiation spectrum. The obtained results were discussed to assess the suitability of the SnS crystal for the fabrication of optoelectronic devices. (C) 2011 Elsevier B.V. All rights reserved.

Structure and properties of soda lime silicate glass doped with rare earth

Abstract: Soda-lime-silicate glasses doped with different rare-earth oxides (La 2 O 3 , CeO 2 , Nd 2 O 3 , Gd 2 O 3 and Y 2 O 3 ) of 1 mol% content were prepared with the traditional melting–quenching methods. In order to reveal the effects of rare-earth elements on the behavior of soda-lime-silicate glass, the structure of soda-lime-silicate glasses doped with different rare-earth oxides were determined with Fourier transform infrared spectrometer using the KBr method, and viscosity of glass melts were measured by the rotating crucible viscometer, the melting temperature of the studied glasses were derived on the basis of Arrhenius Equation, moreover the density, bending strength and molar volume were measured and calculated. The effect of rare-earth dopants on the structure of soda-lime-silicate was analyzed by a shift of peak position and variation in the full-width at half-maximum. The effect of doping rare-earth oxides into glass on the viscosity, density and bending strength was interpreted by changing in structure of soda-lime-silicate glasses doped with rare-earth oxides.

Effects of the hydrostatic pressure and temperature on optical properties of a hydrogenic impurity in the disc-shaped quantum dot

Abstract: The combined effects of hydrostatic pressure and temperature on the optical absorption coefficient and refractive index changes of a hydrogenic impurity in a disc-shaped quantum dot with parabolic confinement in the presence of an external electric field have been investigated by using the perturbation method within the effective-mass approximation. Analytical expressions for the linear and third-order nonlinear absorption coefficients and refractive index changes have been obtained by using the compact-density matrix formalism. We discussed the linear, third-order nonlinear, total absorption coefficients and refractive index changes as functions of photon energy, relaxation time, pressure, and temperature with I=1.5×104 W/cm2, F=50 kV/cm, and ℏ ω 0 = 50 meV . Our results show that the pressure and temperature play an important role in the optical absorption coefficients and refractive index changes in a disc-shaped quantum dot.

Effect of aluminum substitution on the structural and magnetic properties of cobalt ferrite synthesized by sol–gel auto combustion process

Abstract: Aluminum substituted cobalt ferrite powders (CoFe2−xAlxO4) with varying composition from 0.0 to 1.0 in the step of 0.2 have been obtained by sol–gel auto combustion technique using citric acid as a fuel. The metal nitrate to fuel ratio was maintained 1:4 throughout the synthesis of CoFe2−xAlxO4. The thermal analysis of as prepared samples is done by TGA technique. The compositional stoichiometry of the prepared samples is confirmed by Energy dispersive X-ray analysis technique. Single phase cubic spinel structure and nano phase structure of the synthesized powders were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The crystallite size of 16–26 nm was obtained using Scherrer formula. SEM analysis shows the formation of uniform grain growth. The grain size obtained from SEM results is of the order of 30 nm. Maximum specific surface area was observed to be of the order of 52 m2/gm. The highest value of saturation magnetization and coercivity was observed for pure cobalt ferrite sample and it decreases as the aluminum content x increases. A strong co-relation between the saturation magnetization and aluminum content was observed. The decrease in magnetic properties is due to the substitution of aluminum ions in place of Fe3+.

Raman spectra of soda–lime–silicate glass doped with rare earth

Abstract: Soda–lime–silicate glasses doped with different rare earth oxides (La 2 O 3 , CeO 2 , Nd 2 O 3 , Gd 2 O 3 and Y 2 O 3 ) of 1 mol% content were prepared by the traditional melting–quenching methods. In order to reveal the effects of rare earth elements on the behavior of soda–lime–silicate glass the structures of soda–lime–silicate glasses doped with different rare earth oxides were determined using an INVIA confocal microRaman spectrometer equipped with a CCD detector, and viscosities of glass melts were measured using a rotating crucible viscometer; the melting temperature of the studied glasses was derived on the basis of the Arrhenius equation. Three expressions of the fraction of non-bridging oxygen (NBO/NBO+BO), average number of non-bridging oxygen (NBO) per tetrahedron (NBO/tetrahedron) and average number of bridging corners per tetrahedron (bridges/tetrahedron) for investigated soda–lime–silicate glasses were given, and the effect of rare earth dopants on the structure of soda–lime–silicate was characterized by the Raman shift, variation of the [SiO 4 ] tetrahedron structural unit Q n ( n =1,2,3,4), fraction of non-bridging oxygen and the average number of bridging corners per tetrahedron. The effect of doping rare earth oxides into glass on the viscosity and melting temperature was interpreted by changes in structure of soda–lime–silicate glasses doped with rare earth oxides.

Structural and luminescence investigations on Sm3+ doped sodium fluoroborate glasses containing alkali/alkaline earth metal oxides

Abstract: Structural and luminescence behavior of the Sm3+ doped 49B2O3+25xCO3+25NaF+1Sm2O3, (where x=Li2, Na2, K2, Ca and Mg) glasses have been studied and reported. Structural analysis of the prepared glasses was made through FTIR spectra and their optical properties were studied using optical absorption, luminescence and lifetime measurements. The bonding parameters and the oscillator strengths were determined from the absorption spectra. These parameters have been used to obtain the Judd−Ofelt parameters. Using the Judd−Ofelt parameters, radiative properties such as transition probability (A), stimulated emission cross section ( σ P E ) and branching ratio's (βR) for the trivalent samarium ions corresponding to their 4G5/2→6HJ (J=5/2, 7/2, 9/2 and 11/2) excited state transitions have been calculated and reported. The decay curve of the 4G5/2→6H7/2 transition exhibits non−exponential behavior. The non-exponential behavior has been attributed to the energy transfer through cross-relaxation between adjacent Sm3+ ions. The lifetime of the 4G5/2 level is found to be comparatively less than that of the reported Sm3+ glasses and the same is due to the presence of OH groups, which is confirmed through the FTIR spectral studies.

Dielectric behavior and transport properties of ZnO nanorods

Abstract: Highly optical, good crystalline and randomly aligned ZnO nanorods were synthesized by the hydrothermal method. The dielectric properties of ZnO nanorods were attributed to the interfacial polarization at low frequencies (below 10 kHz) and orientational polarization at higher frequencies. The observed ω ( n −1) dependence of dielectric loss was discussed on the basis of the Universal model of dielectric response. Dielectric loss peak was composed of the Debye like loss peak at higher frequencies and interfacial loss peak at lower frequencies. Charge transport through the grain and grain boundary region was investigated by impedance spectroscopy. At higher temperatures the conductivity of the nanorod was mainly through the grain interior and the overall impedance was contributed by the grain boundary region. The activation energy of nanorod was calculated as 0.078 eV, which is slightly higher than the reported bulk value.

Annealing temperature effect on the structural, optical and electrical properties of ZnS thin films

Abstract: Zinc sulfide thin films were prepared on glass substrates at room temperature using a chemical bath deposition method. The obtained films were annealed at temperatures ranging from 100 to 500 °C in steps of 100 °C for 1 h. The films were characterized by X-ray diffraction (XRD), Raman spectroscopy, energy dispersive X-ray analysis (EDX), optical absorption spectra, and electrical measurements. X-ray diffraction analysis indicates that the deposited films have an amorphous structure, but after being annealed at 500 °C, they change to slightly polycrystalline. The optical constants such as the refractive index (nr), the extinction coefficient (k), and the real (e1) and imaginary (e2) parts of the dielectric constant are calculated depending on the annealing temperature. Aside from the ohmic characteristics of the I–V curve, a nonlinear I–V curve owing to the Schottky contact is also found, and the barrier heights (ϕbn) for Au/n-ZnS and In/n-ZnS heterojunctions are calculated. The conductivity type was identified by the hot-probe technique.

Synthesis, structural, magnetic and optical properties of nanocrystalline ZnFe2O4

Abstract: Nanocrystalline zinc ferrite (ZnFe2O4) is synthesized by high-energy ball-milling after 12 h from a powders mixture of zinc oxide (ZnO) and hematite (α-Fe2O3) with balls to powders mass ratio of 20:1. X-ray diffraction, vibrating sample magnetometer (VSM), the Mossbauer spectrometry and photoluminescence (PL) are used to characterize the samples. Rietveld analysis and VSM measurements show that the powder has an average crystallites size of 10 nm and a ferrimagnetic behavior with a saturation magnetization of 30 emu/g. After annealing at 700 °C, the lattice parameter reduces from 8.448 to 8.427 A and the sample transforms into a superparamagnetic behavior, which was confirmed as well by the room temperature Mossbauer spectrometry. Different mechanisms to explain the obtained results and the correlation between magnetism and structure are discussed. Finally, the broadband visible emission band is observed in the entire PL spectrum and the estimated energy band gap is about 2.13 eV.

Electronic structure and optical properties of Sb2S3 crystal

Abstract: The electronic and optical properties of Sb2S3 are studied using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in Wien2k. In this approach, the alternative form of the generalized gradient approximation (GGA) proposed by Engel and Vosko (EV-GGA) was used for the exchange correlation potential. The calculated band structure shows a direct band gap. The contribution of different bands was analyzed from total and partial density of states curves. Moreover, the optical properties, including the dielectric function, absorption spectrum, refractive index, extinction coefficient, reflectivity and energy-loss spectrum are all obtained and analyzed in detail.

Towards efficient upconversion and downconversion of NaYF4:Ho3+,Yb3+ phosphors

Abstract: NaYF4 microcrystals co-doped with Ho3+ and Yb3+ were prepared by a facile hydrothermal synthesis. The products were characterized by X-ray diffractometer, scanning electron microscopy, and photoluminescence spectroscopy. Upon excitation with a 980 nm laser diode, the sample shows an intense green upconversion emission centered at 540 nm corresponding to the 5S2→5I8 transition of Ho3+. The quadratic dependence of the green emission intensity on the excitation power reveals a two-phonon upconversion process. On the contrary, upon excitation with 448 nm, both visible and near-infrared emissions peaked at 483, 540, 644, 749, and 978 nm are simultaneously observed, which could be assigned to the electronic transitions of Ho3+: 5F3→5I8, 5S2→5I8, 5F5→5I8, 5S2→5I7, and Yb3+: 2F5/2→2F7/2, respectively. The energy transfer processes between Ho3+ and Yb3+ ions and the involved mechanisms have been investigated and discussed.

Synthesis, electrical and dielectric properties of FeVO4 nanoparticles

Abstract: The electrical and dielectric properties of FeVO 4 nanoparticles were studied at different temperatures from ambient to 200 °C. The samples were prepared by simple co-precipitation method using ferric nitrate and ammonium metavanadate as the starting precursors. The powder X-ray diffraction pattern inferred the single phase formation and triclinic structure of FeVO 4 . The morphology of the particles was elucidated from SEM studies. Detailed studies on the electrical and dielectric properties of the compound were carried out by using solid state impedance spectroscopy. A maximum dc conductivity of 4.65×10 −5 S cm −1 was observed at the measuring temperature of 200 °C. The calculated activation energy from dc conductivity was found to be 0.28 eV. It was evident that the electrical transport process in the system was due to the hopping mechanism. The detailed dielectric studies were also carried out.