A.A. Othman

Thermal annealing and UV irradiation effects on structure, morphology, photoluminescence and optical absorption spectra of EDTA-capped ZnS nanoparticles

Monodispersed ZnS nanoparticles (NPs) were prepared by the chemical precipitation method. Thermally induced structural, morphological and optical changes have been investigated using x-ray diffraction, high-resolution transmission electron microscopy, optical absorption, photoluminescence (PL), and Fourier transform infrared and Raman spectroscopy. It was found that D increases with increasing annealing temperature (T a). The onset of the ZnS phase transition from cubic to hexagonal structure takes place at 400 °C, while cubic ZnS transforms into hexagonal ZnO via thermal oxidation in air at 600 °C. It is also noted that increasing T a results in the red shift of the optical band gap () and the thermal bleaching of exciton absorption. The PL spectrum of as-prepared ZnS nanopowder shows UV emission bands at 363 and 395 nm and blue and green emission at 438 and 515 nm, respectively. With increasing T a up to 500 °C, these bands were quenched and red-shifted. In addition, the UV irradiation effects on colloidal ZnS NPs were investigated. UV irradiation at a dose <13 J cm−2 leads to a decrease in D, the blue shift of and the enhancement of PL intensity. This behavior was explained in terms of surface modification by photopolymerization, the formation of a ZnSO4 passivation layer, as well as the reduction of D by photocorrosion. At a UV irradiation dose <13 J cm−2 both and D did not change and PL intensity was quenched, which were caused by the creation of nonradiative surface states by the photodegradation of the capping agent and photopassivated layer. The mechanism of the PL emission process in ZnS NPs was discussed and an energy band diagram was proposed.

Influence of sodium intercalation and temperature on the exciton absorption in GaSe

Abstract Analysis of the experimental absorption data in GaSe showed that, at low temperature (110 K), intercalation process of sodium atoms, as well as the presence of silicon impurities and native defects such as Ga vacancies and interstitials in a high concentration, result effectively in the broadening of the ground state of the direct exciton and the decrease of exciton peak intensity. On the other hand, increasing temperature leads to the increase of the broadening parameter Λ and a shift of both the position of exciton peaks Eex, and the band gap Eg toward longer wavelength. These effects can be ascribed to exciton-phonon and exciton-imperfection interactions. Furthermore, acceptable and reasonable values of the temperature coefficient β of the above-mentioned parameters were determined and are equal to +8.64×10−4, −3.52 × 10−4 and −3.38 × 10−4 eVK−1, respectively.

Arylidene polymers: 15. Optical band gap and structural relationship of new polyester films of dibenzylidenecyclohexanone

Abstract The optical properties of poly[oxyisophthaloyloxy-(2-methoxy-p-phenylene methylidene-(2-oxo-1,3-cyclohexanediylidene) methylidyne-(3-methoxy-p-phenylene)] (I) and poly[oxysebacoyloxy-(2-methoxy-p-phenylene)methylidene-(2-oxo-1, 3-cyclohexanediylidene)methylidene-(3-methoxy-p-phenylene)] (II) polymer films were studied. The transmission and absorption measurements were carried out in the energy range 1.9–3.2 eV at 300 K. The data on absorption coefficient versus photon energy were analysed and interpreted in terms of the Tauc model, (α h ω) 1 2 = B 1 2 ( h ω − E opt g ) . Analysis of experimental results shows that the absorption characteristics give a good fit to the Tauc model, and indicates that both polymers I and II have an indirect band gap. The values of the optical gap Eoptg and the slope of the Tauc plots B 1 2 were determined. The study shows that the indirect allowed optical transitions are the predominant transitions in the optical absorption process. From the data a correlation between optical band gap and polymer structure was observed and discussed.

Gamma irradiation effects on the electrical conductivity behaviour and IR absorption spectra of some new synthesized poly(arylidene-cycloalkanones)

Abstract γ irradiation effects on the electrical conductivity behaviour of some new synthesized poly(2,5-arylidenecyclopentanone) (sample 1), poly(2,6-arylidenecyclohexanone) (sample 2) and a copolymer (sample 3) have been investigated. Thermal studies over the temperature range 300–500 K showed that the conductivity behaviour follows a two-term Arrhenius-type equation. The first term has a small activation energy which predominates at a lower temperature (extrinsic region) and the second term with a higher activation energy predominates at higher temperatures (intrinsic region). Changes in the activation parameters with the change in the γ-ray dose and the nature of the polymers were discussed. The IR absorption spectra showed a decrease in the intensity of the γ-irradiated sample bands. A mechanism based on the formation of trapped free radicals and their recombination has been suggested. Differential scanning calorimetry measurements indicated that the crystallization peak changes considerably in magnitude with the increase of the γ-ray dose. This peak almost disappears at higher doses. However the onset of temperature of crystallization ( T c ) decreases with increasing γ-ray dose.

Reversible photodarkening in amorphous Ga20S75Sb5 and Ga20S40Sb40 thin films

Amorphous Ga20S75Sb5 and Ga20S40Sb40 thin films were prepared by thermal evaporation onto clean glass substrates. Transmittance and reflectance measurements were carried out at 300 K, in the wavelength range 400–800 nm, for the virgin and UV illuminated films. Reversible photodarkening is reported for the first time in those films. The photo-induced shift in the non-direct optical energy gap is determined and discussed in the light of current models of photodarkening. The effect of Sb content on the optical band gap and the shift of the absorption edge is also discussed in terms of the coordination number.

Photoinduced effects on the optical constants of a-Ge–Se–Bi chalcogenide glassy thin films

In addition to the conversion from p-type to n-type conductivity that occurs in Ge–Se–Bi thin films when Bi is incorporated in a certain concentration. We found that, when these films were illuminated to UV light, after being annealed at glass transition temperature T g, the photobleaching is dominant for Ge20Se80−x Bi x (x=0, 2.5, and 5 at.%), while for Ge20Se72.5Bi7.5 photodarkening is dominant. The photoinduced changes in the optical constants were studied. The refractive index (n) has been analysed according to the Wwmple–DiDominico single oscillator model and the values of E o and E d for exposed and unexposed films were determined, respectively. The photostructural effects were discussed in the light of single–double well model proposed by Tanaka and chemical bond approach.

Arylidene polymers. XV. Optical band gap and structural relationship of new polyester films of dibenzylidenecyclohexanone

Optical properties of poly(oxyisophthaloyloxy(2-methoxy-p-phenylene)methylid- ine(2-oxo-1,3-cyclohexanediylidene)methylidene(2-methoxy-p-phenylene)) I and poly(oxysebacoyloxy(2-methoxy-p-phenylene)methylidene(2-oxo-1,3-cyclohex- andeiylidene)methylidene(3-methoxy-p-phenylene)) II polymer films were studied. Transmission and absorption measurements were carried out in the energy range 1.9 to 3.2 eV at 300 K. Absorption coefficient data were analysed and interpreted in terms of the Tauc model ( alpha h(cross)w)12/= square root B(h(cross)w-Egopt). Analysis of experimental results shows that the absorption characteristics give a good fit to the Tauc model. This indicates that both polymers (I and II) have an indirect band gap. The values of the optical gap Egopt and the slope of Tauc plots square root B were determined. The study shows that the indirect allowed optical transitions are predominant in the absorption process. Correlation between the optical band gap and polymer structure was observed and discussed.

Temperature Dependence of Absorption Spectra and Optical Parameters for Polyester Films of Dibenzylidene Cyclohexanone

Effect of temperature on the optical absorption spectra and optical parameters is investigated in new synthesized polyester films of poly[oxyiso-phthaloyloxy(2-methoxy-p-phenylene)methylidyne(2-oxo-1, 3-cyclohexanediylidyne)methylidyne(3-methoxy-p-phenylene)] (I) and poly[oxysebacoyloxy-(2-methoxy-p-phenylene) methylidene (2-oxo-1, 3-cyclohexanedi-ylidene)-methy-lidene-(3-methoxy-p-phenylene)] (II). Absorption measurements are carried out in the temperature range 80–370 K and in the photon energy range 1.9–4 eV. The optical absorption spectra of polymers I and II reveal excitonic absorption bands at 80 and 120 K respectively. These bands are associated with n →π* optical transitions along polymer backbone chains. Analysis of optical absorption data indicates that the Tauc equation for indirect optical transitions successfully explains the optical absorption process in polymeric film I, whereas Urbach’s law is more suitable for describing the optical absorption in polymeric film II. Temperature dependence of the optical bandgap, E opt g, the Tauc slope √B, and the width of Urbach’s tail states (E e) exhibits anomalous parabolic behaviour which can be explained in terms of various current theories. Moreover, DSC, x-ray diffraction and morphological studies by TEM are used in the explanation of the experimental results. The values of optical parameters, as well as the temperature coefficient of the optical bandgap for these polymers, are calculated and reported.