M.M. El-Nahass

Structural and optical studies of thermally evaporated CoPc thin films

Abstract The structure of the thermally evaporated cobalt phthalocyanine (CoPc) thin film in the β-form is investigated, and shows a single strong peak indicating preferential orientation in the (1 0 0) direction. Some structural parameters such as crystallite grain size, dislocation density and the number of crystallites per unit surface area are determined. The spectral parameters are determined by applying the electronic orbital transitions. But the optical parameters are deduced using band-model consideration for thin films of Pc. The spectral and optical parameters have also been investigated by using the spectrophotometric measurements of transmittance and reflectance in the wavelength range 200– 2500 nm . The absorption spectra recorded in the UV–VIS region show two absorption bands of phthalocyanine (Pc) molecule, namely the Soret band (B) and the Q-band. The Q-band shows its characteristic splitting (Davydov splitting) with Δ Q=0.23 eV . Some of the important spectral parameters, namely optical absorption coefficient (α), molar extinction coefficient (emolar), oscillator strength (f), electric dipole strength (q2) and absorption half bandwidth (Δλ) of the principle optical transitions have been evaluated. The fundamental and the onset indirect energy gaps could be estimated as 2.90 + or − 0.05 and 1.51 eV , respectively. The refractive index showed an anomalous dispersion in the absorption region as well as normal dispersion in the transparent region. From analysis of dispersion curves, the dielectric constants, the dispersion parameters and the molar polarizability were obtained. All the above parameters were obtained for films as deposited and as annealed. No remarkable annealing effect on many parameters was observed.

Optical properties of thermally evaporated SnS thin films

Abstract Thermally evaporated SnS amorphous chalcogenide films undergo structural transformation upon annealing in the temperature range between 432–573 K. The optical properties of amorphous and annealed films were investigated using spectrophotometric measurements of the transmittance and reflectance at normal incidence in the wavelength range 250–2500 nm. The films are transparent for a wavelength >1250 nm. The refractive index (n) and the absorption index (k) are independent of film thickness in the measured film thickness range (55–365 nm). The dispersion energy, Ed, of amorphous films increased from 20.2 to 23.85 eV for crystalline films. The types of optical transition responsible for optical absorption are indirect allowed and direct forbidden transitions with energy gaps of 1.4 and 2.18 eV for the amorphous films and of 1.38 and 2.33 eV for the crystalline films, respectively.

Optical properties of tin diselenide films

SnSe2 films were deposited on substrates at 300 K by a conventional thermal evaporation technique. The as-deposited films were amorphous and transformed to the crystalline phase on post-deposition annealing above 573 K in an inert atmosphere. The optical properties of the films were investigated, using spectrophotometric measurements of the transmittance and reflectance at normal incidence in the wavelength range 400–2000 nm. The refractive index data fit a single oscillator model with a dispersion parameter 5.149×10−14 and 5.773×10−14 eVm2 for the amorphous and crystalline films, respectively. The high-frequency dielectric constant of the amorphous films decreased from 9.871 to 7.475 for the crystalline films. The analysis of the spectral behaviour of the absorption coefficient in the intrinsic absorption region revealed an indirect forbidden and a direct allowed transition with energy gaps 0.99 and 2.05 eV for the amorphous films and 0.96 and 2.02 eV for the crystalline films, respectively.

Fabrication and electrical characterization of p-NiPc/n-Si heterojunction

Nickel-phthalocyanine (NiPc) thin film was prepared by thermal evaporation method on n-Si single-crystal substrate to fabricate p-NiPc/n-Si heterojunction. The electrical transport properties of the p-NiPc/n-Si heterojunctions were investigated by temperature-dependent current-voltage (I-V) measurements and room temperature capacitance-voltage (C-V) measurements. The temperature-dependent I-V characteristics revealed that the forward conduction was determined by thermionic-emission and space-charge-limited current (SCLC) mechanisms at low and high voltage, respectively. On the other hand, the reverse current is limited by the carrier generation process. The 1/C2-V plot indicated the junction was abrupt and the junction built-in potential was 0.61V at room temperature.

Optical properties of AsSe1.5−xTex glassy system

Abstract Thin AsSe1.5−xTex films with 0⩽x (∼300 K ) . The optical constants, the refractive index, n, and the absorption index, k, of the films were determined for the investigated compositions of different thickness values (100– 300 nm ) using spectrophotometric measurements of the transmittance, T, and the reflectance, R, at normal incidence in the spectral range 400– 2500 nm . The obtained values of both n and k were found to be independent of the film thickness within the above mentioned thickness range. The estimated indirect and direct optical energy gap decreased as tellurium content increased in the parent sample AsSe1.5. The values of dispersion energy, Ed, and lattice dielectric constant, eL, of the system have been determined and correlated with the type and amount of chemical bonds and the relative proportion of the constituent elements in the examined compositions.

Dispersive optical constants of glassy AsSe1-xTex system

Abstract The compositional dependence of the optical constants, the refractive index n, and the absorption index k, of the AsSe1−xTex thin films with 0

Electrical conduction mechanisms in thermally evaporated tungsten trioxide (WO3) thin films

Thin films of amorphous tungsten trioxide, a-WO3, have been thermally evaporated onto glass substrate held at 350K. Annealing at 723K caused the formation of polycrystalline tungsten trioxide, c-WO3, with a monoclinic structure. The dark DC electrical conductivity of both a-WO 3 and c-WO3 was studied over a temperature range from 298 to 625K in two environmental conditions (air and vacuum). A simple Arrhenius law, a polaron model and a variable range hopping model have been used to explain the conduction mechanism for a-WO3 films. Using the variable range hopping model, the density of localized states at the Fermi level, N(EF), was found to be 1.08 × 1019eV -1cm-3. The mechanism of electrical conduction in c-WO3 films is explained by means of the Seto model. The Seto model parameters were determined as the energy barrier (Eb ≤ 0.15eV), the energy of trapping states with respect to the Fermi level (Et ≤ 0.9eV) and the impurity concentration (ND ≤ 4.05 × 1015eV-1cm-3). The thickness dependence of resistivity of c-WO3 films has been found to decrease markedly with increasing film thickness, which is explained on the basis of the effective mean free path model. Using this model, the mean free path of electrons in c-WO 3 films was evaluated. The temperature dependence of the thermoelectric power for a-WO3 films reveals that our samples are n-type semiconductors.

Ab initio HF, DFT and experimental (FT-IR) investigation of vibrational spectroscopy of P-N,N-dimethylaminobenzylidenemalononitrile (DBM)

The optimized geometry and vibrational frequencies of P-N,N-dimethylaminobenzylidenemalononitrile (DBM) were obtained by ab initio HF and DFT/B3LYP levels with complete relaxation in the potential energy surface using 6-31++G(d,p) and 6-311++G(d,p) basis sets. The Fourier-transform infrared (FT-IR) spectrum of DBM has been recorded in the region 4000-400 cm(-1). The harmonic vibrational frequencies were calculated and the scaled values have been compared with experimental FT-IR spectrum. The calculated frequencies are in good agreement with the experimental frequencies.

Optical properties of ternary ZnS x Se1−x polycrystalline thin films

Ternary ZnSxSe1−x polycrystalline thin films were prepared by evaporation in vacuum of 10−5 Torr. The molecular fractionx varied in the region o≦x≦1. The optical constants (the refractive indexn, the absorption indexk, and the absorption coefficientα) were determined in the wavelength range 300–1600nm. A plot representingα2=f(hv) shows that the ZnSxSe1−x polycrystalline thin films of different compositions have two direct transitions corresponding to the energy gapsE andE+Δ. The variation in eitherE orE+Δ withx indicates that this system belongs to the amalgamation type. Such variation follows a quadratic equation. The bowing parameter was found to be 0.456 eV, roughly equal to the calculated value 0.60 eV using the empirical pseudopotential method based on the virtual-crystal approximation, in which the disorder effect has not been taken into account.

Structural and optical properties of nano-spin coated sol–gel porous TiO2 films

Abstract Three thicknesses of TiO2 films, 174, 195, and 229 nm, were deposited onto quartz substrates by sol–gel spin coating method. The as-deposited thin films were characterized by nano-crystallite with different sizes (19–46 nm) and relatively high porous structure. Optical constants were determined and showed the lowest refractive index of 1.66 for the as-prepared films that ever reported till now. Obtained results were discussed through current theoretical ideas.