Band structure and optical functions of ternary chain TlInSe2

The band structure of ternary chain TlInSe₂ is calculated by a pseudo-potential method with allowance for non-locality of ionic pseudo-potentials. In the obtained band structure the symmetry and forbidden character of the direct transitions at band gap are ascertained to be the same as reported earlier from the results of the empirical pseudo-potential calculations. The imaginary part of the components parallel and perpendicular to the c-axis of the dielectric function tensor of TlInSe₂ is calculated at photon energies up to 10 eV. The real part of these components is obtained by extrapolation of the imaginary part to higher energies and subsequent Kramers–Kronig transformation. The obtained dielectric function is compared with the one obtained ellipsometrically in the range 0.85–6.5 eV. The results of comparison are rather favorable.     

Molybdenum carbide nano-sheet as a high capacity anode material for monovalent alkali metal-ion batteries—Theoretical investigation

This contribution presents a theoretical investigation of monovalent metal-ion adsorption and diffusion on two-dimensional (2D) buckled nanostructure of molybdenum carbide (MoC) by using the first principle method. We find that buckled MoC nanostructure exhibits great stability, semiconducting electronic property, and high performance as electrode material. Interestingly, Crystal Orbital Hamilton population (COHP) method results show that buckled MoC is chemically stable in a wide range of temperatures, and various Li, Na, ions adsorbed configurations, which is beneficial for anode materials. Especially, single-layer MoC exhibits a superior theoretical capacity of 993.16 mA h g⁻¹ for Li-ions and 496.58 mA h g⁻¹ for Na/K-ions. The storage capacity of 1200 mA h g⁻¹ is found for the adsorption of ions on multilayer bulk MoC. Moreover, migration energy barriers are predicted as 0.38 eV for Li, 0.32 eV for Na, and 0.24 eV for K; these remarkable results determine the applicability of buckled MoC as ideal anode material for metal-ion battery applications.     

Structural and AC conductivity study of CdTe nanomaterials

Cadmium telluride (CdTe) nanomaterials have been synthesized by soft chemical route using mercapto ethanol as a capping agent. Crystallization temperature of the sample is investigated using differential scanning calorimeter. X-ray diffraction and transmission electron microscope measurements show that the prepared sample belongs to cubic structure with the average particle size of 20 nm. Impedance spectroscopy is applied to investigate the dielectric relaxation of the sample in a temperature range from 313 to 593 K and in a frequency range from 42 Hz to 1.1 MHz. The complex impedance plane plot has been analyzed by an equivalent circuit consisting of two serially connected R-CPE units, each containing a resistance (R) and a constant phase element (CPE). Dielectric relaxation peaks are observed in the imaginary parts of the spectra. The frequency dependence of real and imaginary parts of dielectric permittivity is analyzed using modified Cole–Cole equation. The temperature dependence relaxation time is found to obey the Arrhenius law having activation energy ~0.704 eV. The frequency dependent conductivity spectra are found to follow the power law. The frequency dependence ac conductivity is analyzed by power law.     

Study of electronic and optical properties of two-layered hydrogenated aluminum nitrate nanosheet

First principle calculations based on density functional theory using GW approximation and two particle Bethe–Salpeter equation with electron-hole effect were performed to investigate electronic structure and optical properties of two-layered hydrogenated AlN. According to many body green function due to decrease in dimension and considering electron-electron effect, direct (indirect) band gap change from 2 (1.01) eV to 4.83 (3.62) eV. The first peak in imaginary part of dielectric function was observed in parallel direction to a plane obtaining 3.4 was achieved by bound exciton states possess 1.39 eV. The first absorption peak was seen in two parallel and perpendicular directions to a plane which are in UV region.     

Magnetic and dielectric study of Bi2CuO4

Magnetic and dielectric properties have been investigated for Bi₂CuO₄, which has the same chemical formula as that of the parent materials of cuprate superconductors R₂CuO₄ (R: rare earths). Magnetization measurements show the antiferromagnetic transition of the Cu²⁺ spins at ～42 K, as reported previously. Dielectric measurements for the frequencies of 1 kHz to 1 MHz show that the dielectric constants are 100–500 at room temperature. The dielectric dispersion reveals that the dielectric response lacks spatial coherence, a property which indicates the possible existence of phase separation as suggested for La₂CuO₄. The imaginary part of dielectric response gives the activation energy of 0.22 eV, suggesting that the dielectric response is governed by the electron hopping between the Cu ions.     

The role of torsional modes in the electronic absorption spectrum of acetone

The electronic absorption spectrum of acetone is revisited to evaluate the role of hot bands due to low lying torsional modes in the assignment of vibronic transitions. The UV–VUV photoabsorption spectrum of acetone is recorded in the energy region 3.5–11.8 eV at a resolution of ～4 meV at 4 eV and ～10 meV at 10 eV using synchrotron radiation. The absorption spectrum is dominated by richly structured Rydberg series (ns, np and nd) converging to the first ionization potential of acetone at 9.708 eV. Careful consideration of hot band contributions from torsional modes and symmetry selection rules have resulted in an improved set of vibronic assignments as compared to earlier room temperature work. Revised quantum defect values for some of the Rydberg transitions and a few new assignments in the nd series are also reported in this paper.     

Near-infrared photoluminescence and thermally stimulated current in Cu3Ga5Se9 layered crystals: A comparative study

Near-infrared photoluminescence (PL) and thermally stimulated current (TSC) spectra of Cu₃Ga₅Se₉ layered crystals grown by Bridgman method have been studied in the photon energy region of 1.35–1.46 eV and the temperature range of 15–115 K (PL) and 10–170 K (TSC). An infrared PL band centered at 1.42 eV was revealed at T = 15 K. Radiative transitions from shallow donor level placed at 20 meV to moderately deep acceptor level at 310 meV were suggested to be the reason of the observed PL band. TSC curve of Cu₃Ga₅Se₉ crystal exhibited one broad peak at nearly 88 K. The thermal activation energy of traps was found to be 22 meV. An energy level diagram demonstrating the transitions in the crystal band gap was plotted taking account of results of PL and TSC experiments conducted below room temperature.     

Dielectric relaxation of ZnO nanostructure synthesized by soft chemical method

Thioglycerol capped nanoparticles of ZnO have been prepared in methanol through chemical technique. Nanostructures of the prepared ZnO particles have been confirmed through X-ray diffraction measurement. The Debye–Scherrer formula is used to obtain the particle size. The average size of the prepared ZnO nanoparticles is found to be 50 nm. The frequency-dependent dielectric dispersion of the sample is investigated in the temperature range from 293 to 383 K and in a frequency range from 100 Hz to 1 MHz by impedance spectroscopy. An analysis of the complex permittivity (ε′ and ε′′) and loss tangent (tan δ) with frequency is performed assuming a distribution of relaxation times. The frequency-dependent maxima of the imaginary part of impedance are found to obey Arrhenius law with activation energy ∼1 eV. The scaling behavior of dielectric loss spectra suggests that the relaxation describes the same mechanism at various temperatures. The frequency-dependent electrical data are analyzed in the framework of conductivity and modulus formalisms. The frequency-dependent conductivity spectra obey the power law.     

STM light emission from Si(1 1 1)-(7 × 7) surface using a silver tip

Scanning tunneling microscope-light emission (STM-LE) from the Si(1 1 1)-(7×7) surface has been measured using silver tips. For silver tips photon emission was enhanced by more than 100 times as compared with that for tungsten or platinum–iridium alloy tips. A broad spectrum with a single peak at ∼2.25 eV was observed. The spectrum obtained can be reproduced by a theory based on the macroscopic dielectric response of the tip-sample system, indicating that the observed emission arises from the localized plasmons on the silver tip excited by tunneling electrons. Spatial variations in the emission intensity at the atomic scale was observed even under low bias voltage (2 V) and low tunneling current (1 nA) conditions.     

Optical and dielectric studies of solution grown glycinium maleate single crystal

Bulk single crystals of glycinium maleate have been grown from aqueous solution by slow evaporation method by optimizing the growth parameters within a period of 15 days. From X-ray diffraction analysis, the crystal was found to crystallize in monoclinic structure (space group C_2/c) with a = 17.866 Å, b = 5.684 Å, c = 17.408 Å and β = 112.65°. Presence of characteristic functional groups was confirmed in FTIR analysis. UV–Vis spectral analysis has revealed the absence of any high absorbance region between the wavelengths ranging from 300 to 900 nm. The optical band gap was calculated and found to be 3.91 eV. The activation energy for conduction at different frequencies was calculated and found to decrease from 0.54 eV to 0.41 eV as frequency increased from 100 Hz to 2 MHz. The dielectric behavior, conduction mechanism and the optical characterization of the glycinium maleate single crystals are being reported for the first time.     

Intrinsic excitons in 12CaO·7Al2O3

Unusual crystal structure of 12CaO·7Al₂O₃ is composed by a framework of positively charged nanocages, which enable accommodation of various negative ions (and even electrons) inside these cages. Different filling of cages leads to significant changes in electronic structure and as the result in luminescence properties, as well. Luminescence was studied using time-resolved spectroscopy in VUV in the temperature range from 6 to 300 K. Electron loaded samples exhibit UV luminescence band peaked at ～5 eV. The excitation spectrum of this emission has the onset at the energy gap value of 6.8 eV, and its decay is well described with the sum of two exponential functions with life-times of τ₁ = 3.7 ns and τ₂ = 29 ns, respectively. Its thermal quenching is well approximated by the sum of two Mott-Seitz type curves with the activation energies of 34 meV and 70 meV. Experimental results indicate that this luminescence is possibly due to radiative decay of two singlet self-trapped exciton states, which hole components are localized on two non-equivalent framework oxygens.     

Structural and dielectric properties of Ba3V2O8 ceramics

Polycrystalline sample of Ba₃V₂O₈ was prepared by a high-temperature solid-state reaction technique. Preliminary X-ray diffraction (XRD) analysis confirms the formation of single-phase compound of hexagonal (rhombohedral) crystal structure at room temperature. Microstructural analysis by scanning electron microscope (SEM) shows that the compound has well defined grains, which are distributed uniformly throughout the surface of the sample. The dielectric properties of the compound studied in a wide frequency range (10²–10⁶ Hz) at different temperatures (25–400 °C), exhibits that they are temperature dependent. Detailed analysis of impedance spectra showed that the electric properties of the material are strongly dependent on frequency and temperature. The activation energy, calculated from the temperature dependence of ac conductivity (dielectric data), was found to be 0.23 eV at 50 kHz in the higher temperature region.     

Tuning the electronic properties of single-walled SiC nanotubes by external electric field

The electronic properties of SiC nanotubes (SiCNTs) under external transverse electric field were investigated using density functional theory. The pristine SiCNTs were semiconductors with band-gaps of 2.03, 2.17 and 2.25 eV for (6,6), (8,8) and (10,10) SiCNTs, respectively. It was found the band gaps was reduced with the external transverse electric filed applied. The (8,8) and (10,10) SiCNTs changed from semiconductor to metals as the intensity of electric field reached 0.7 and 0.5 V/Å. The results indicate that the electronic properties of SiCNTs can be tuned by the transvers electric field with integrality of the nanotubes.     

Studies of Li intercalation of hydrogenated graphene on SiC(0001)

The effects of Li deposition on hydrogenated bilayer graphene on SiC(0001) samples, i.e. on quasi-freestanding bilayer graphene samples, are studied using low energy electron microscopy, micro-low-energy electron diffraction and photoelectron spectroscopy. After deposition, some Li atoms form islands on the surface creating defects that are observed to disappear after annealing. Some other Li atoms are found to penetrate through the bilayer graphene sample and into the interface where H already resides. This is revealed by the existence of shifted components, related to H–SiC and Li–SiC bonding, in recorded core level spectra. The Dirac point is found to exhibit a rigid shift to about 1.25 eV below the Fermi level, indicating strong electron doping of the graphene by the deposited Li. After annealing the sample at 300–400 °C formation of LiH at the interface is suggested from the observed change of the dipole layer at the interface. Annealing at 600 °C or higher removes both Li and H from the sample and a monolayer graphene sample is re-established. The Li thus promotes the removal of H from the interface at a considerably lower temperature than after pure H intercalation.     

First-principles study of electronic and optical properties of the optical non linear LiMoO3(IO3)

The electro-optical properties, including: energy density of states function, dielectric function, refraction coefficient, extinction coefficient, band structure, energy gap and optical conductance of LiMoO₃(IO₃) structure with single-crystal data are computed and discussed in this paper. The LDA + U and generalized gradient approximations (GGA) with the full-potential linearized augmented plane wave method (FP-LAPW) in the framework of density functional theory (DFT) are used to compute the energy gap and other properties of this structure by considering the orbital dependent potential for coupled d orbital brought from experimental results and the U is applied to the Mo d-manifold. The results of energy gap are 2.1 eV and 1.5 eV for LDA + U and for GGA methods, respectively, which LDA + U method result is very close to experimental results.     

Evolution of interface properties of the Pentacene/Bi(0001) system

Using angle-resolved photoemission spectroscopy we measured the evolution of the electronic properties of the Pentacene (Pn)/Bi(0001) interface. From thickness dependent photoemission spectra of the substrate and Pn film we conclude that Pn growth is epitaxial. Pentacene highest occupied molecular orbital (HOMO) valence band features are identical for sub-monolayer (ML) as well as for thick films which suggests a thickness independent film morphology. The Pn/Bi interaction is weak and results in a lowering of the HOMO binding energy by 180 ± 5 meV and 80 ± 5 meV for the first and second MLs respectively. The interface dipole (ID) is fully developed over the first ～ 1.2 ML of Pn coverage and has a value of ID = 310 ± 10 meV. The hole injection barrier across the interface is Φ_h = 1.03 ± 0.01 eV.     

Thermal,dielectric and conductivity studies on PVA/Ionic liquid [EMIM][EtSO4] based polymer electrolytes

Polymer electrolyte films of (PVA+15 wt% LiClO₄)+x wt% Ionic liquid (IL) 1-ethyl-3-methylimidazolium ethylsulfate [EMIM][EtSO₄] (x=0, 5, 10, 15) were prepared by solution cast technique. These films were characterized using TGA, DSC, XRD and ac impedance spectroscopic techniques. XRD result shows that amorphosity increases as the amount of the IL in PVA+salt (LiClO₄) is increased. DSC results confirm the same (except (PVA+15 wt% LiClO₄)+10 wt% IL). The dielectric and conductivity measurements were carried out on these films as a function of frequency and temperature. The addition of IL significantly improved the ionic conductivity of polymer electrolytes. Relaxation frequency vs. temperature plot for (PVA+15 wt% LiClO₄)+x wt% IL were found to follow an Arrhenius nature. The dielectric behavior was analyzed using real and imaginary parts of dielectric constant, dielectric loss tangent (tan δ) and electric modulus (M′ and M″).     

HREELS study of C70 molecules adsorbed on a Si(1 1 1)-(7×7) surface

We have measured the coverage dependence of vibrational excitation spectra of C₇₀ molecules adsorbed on a Si(1 1 1)-(7×7) surface using high-resolution electron-energy-loss spectroscopy. At the monolayer coverage, the intensity of the 57 meV peak increases, and those of the 83 and 178 meV peaks decrease. Taking into account the dipole selection rule, the change in intensity of the 57 meV peak indicates that the average angle between the long axes of C₇₀ molecules and surface normal is about 40°. The decreases in intensities of the 83 and 178 meV peaks suggest that the rotational motion of molecules is quenched upon adsorption. We will discuss the Coriolis interaction between the accidentally degenerate A₂″ and E₁′ modes.     

Ab-initio calculation of optical properties of AA-stacked bilayer graphene with tunable layer separation

Density functional theory based calculations revealed that optical properties of AA-stacked bilayer graphene are anisotropic and highly sensitive to the interlayer separation. In the long wave length limit of electromagnetic radiation, the frequency dependent response of complex dielectric function becomes vanishingly small beyond the optical frequency of 25.0 eV. Besides, static dielectric constant shows a saturation behaviour for parallel polarization of electric field vector when interlayer spacing is greater than 2.75 Å. As a consequence, an appropriate modification of effective fine structure constant is observed as a function of layer separation. Moreover, the bilayer systems are highly transparent in the optical frequency range of 7.0–10.0 eV. The electron energy loss function exhibits two different in-plane collective (plasmon) excitations and a single out of plane plasmon excitation. The spectral nature of different frequency dependent optical properties is observed to be very similar to that of the monolayer pristine graphene apart from their exact numerical values.     

Rare earth free exchange spring magnet FeCo/FePt(001): Giant magnetic anisotropy and energy product

Using the full potential linearized augmented plane wave (FLAPW) method, we have investigated the thickness dependent magnetic properties of rare earth free exchange spring magnet FeCo/FePt(001). The FeCo adlayer thickness is increased from one monolayer (ML) to four ML coverage. It is observed that the FeCo adlayers and Fe atoms in FePt substrate show almost half metallic behavior, while an ordinary metallic feature is found in Pt atoms. The average magnetization increases with FeCo thickness and the estimated maximum energy product reaches 66 MGOe in FeCo(4 ML)/FePt(001). A giant perpendicular magnetocrystalline anisotropy (MCA) energy of 18.20 meV/cell is found in pure FePt(001) and it becomes 17.35 meV/cell even in FeCo(4 ML)/FePt(001). In addition, we find very large coercivity field in FeCo/FePt(001) systems. For instance, the calculated maximum coercivity field in FeCo(4 ML)/FePt(001) is about 188 kOe. Both energy product and coercivity field calculations may imply that the FeCo/FePt can be utilized for potential rare earth free exchange spring magnet material.