Zdeněk Černošek

Raman spectra and photostructural changes in the short-range order of amorphous As-S chalcogenides

Photoinduced changes of optical transmission, diffuse reflectance, refractive index and Raman spectra of As x S 100-x stoichiometric (x = 40) and non-stoichiometric (x = 38, 42) films, bulk glasses and glassy powders were measured. The photoinduced changes of the absorption edge, diffuse reflectance and refractive index were accompanied by changes in Raman band intensities and positions and, therefore, by changes of structure. The Raman spectra of freshly evaporated films, and spectra of As-rich, contain some bands of As 4 S 4 molecules whose content (e.g. ≥ 20 mol% in As 42 S 58 freshly evaporated films, ∼10 mol% for As 42 S 58 glasses) depends on the stoichiometry and state of the films (freshly evaporated, exposed or annealed films) or upon the state of samples (bulk, powdered glass, exposed or annealed powder). The As 4 S 4 content decreased by light exposure and also by annealing, while irradiation increases the content of As-As bonds. A model is proposed in which the photoinduced changes of the optical properties in As-S glasses are connected with photostructural changes, which change the short-range order, namely the densities of individual chemical bonds.

Photoinduced changes of the structure and physical properties of amorphous chalcogenides

Abstract The recent results of the study of photoinduced effects in chalcogenide glasses and layers were reviewed and discussed. The main attention was paid to changes of structure, optical transmissivity and reflectivity and to photoinduced optical anisotropy.

Raman scattering in GeS2 glass and its crystalline polymorphs compared

Abstract Raman spectrum of glassy GeS 2 and low-resolution ones of polycrystalline α- and β-GeS 2 were studied. It was shown that the medium range structure of glassy GeS 2 is similar to the three-dimensional structure of β-GeS 2 . Our conclusion of similarity of medium range order of glassy GeS 2 and β-GeS 2 was also confirmed by detection of β-GeS 2 microcrystals grown from glassy GeS 2 at annealing temperature sufficiently below glass transition temperature.

Structural relaxation near the glass transition temperature

Abstract Enthalpy relaxation processes on a model bulk glass, As 2 Se 3 , were studied below the glass transition temperature, T g , as well as at temperatures close to the glass transition overshoot. Extents of relaxation processes below T g were measured by enthalpy of endothermic relaxation peak. Relaxation processes above T g were determined by melting enthalpies of partly crystallized glass during relaxation. We found an exponential temperature dependence of physical aging. Completely annealed glasses reach the equilibrium having excess of enthalpy compared to the equilibrium supercooled liquid at the same temperature. Using the Tool–Narayanaswamy–Moynihan (TNM) model, we found that the fictive temperature, T f , decreased during isothermal annealling toward aging temperature, T a , and at equilibrium is equal to annealing temperature, T a . When the glass comes to equilibrium the distribution of relaxation times decreases. The description of relaxation processes at the temperatures slightly above T g is more complicated. The nuclei are created in the first step. In the second one these nuclei can create thermodynamically non-equilibrium agglomerates, which consequently can decrease their energy by becoming amorphous.

Non-isothermal structural relaxation and the glass transition temperature

Abstract The glass transition was studied by a new stepwise differential scanning calorimetry (DSC) technique on the model glasses As 2 Se 3 and As 2 S 3 . The glass transition was separated to the two components: a reversible (thermodynamic) one, reflecting temperature changing of the vibrational amplitude, and an irreversible (kinetic) one, bears on structural relaxation. The value of the glass transition temperature, T g , determined from the thermodynamic part of the glass transition was found to be independent both on the heating/cooling rate and the thermal history of glass. The value of T g depends only on the chemical composition of the glass and thus it could be regarded as a material constant. The heating/cooling rate dependence of T g , known from DSC or DTA measurements, is due to kinetic part of the glass transition.

Photoinduced changes of the reactivity of amorphous chalcogenide layers

Abstract The optically induced changes of dissolution rates of As x S 100- x ( x = 38, 40, 42) amorphous films produced by thermalor flash-evaporation were measured. The fresh-evaporated (as-evaporated) films dissolved more slowly than films which were exposed or annealed. We suggest that the slower dissolution is caused by the poorly soluble ‘molecular structures’ (As 4 S 4 , As 4 , S n , S 8 ), in the films. The dissolution rates of these films can be increased, when cetyltrimethylamonium bromide (CTAB) or other alkyl- or aryl-ammonium compounds are sorbed onto the chalcogenide surface. The Raman spectra revealed that the sorption takes place on As-rich parts of the film. The interaction of sorbed ions with a chalcogenide surface is probably of the Coulomb type.

Photoinduced Effects in Amorphous Chalcogenides

New results of the study of photoinduced effects in amorphous chalcogenide layers and glasses are reviewed. The main accent is given to the changes of short range order, to the mechanisms of optically induced effects, to the optically induced changes of reactivity of chalcogenide layers. Both irreversible and reversible effects are discussed. Attention is paid also to different materials in which the photoinduced effects were studied recently.

Kinetic Analysis of Non-isothermal DSC Data. Computer-aided test of its applicability

The applicability of the kinetic analysis of data obtained by non-isothermal differential scanning calorimetry (DSC) is discussed. The Johnson-Mehl-Avrami (JMA) model was used for the computer simulation of DSC traces subsequently analysed by common methods of kinetic analysis of non-isothermal data. For the temperature-independent kinetic exponent n of the JMA equation, the kinetic analysis was shown to provide correct results, e.g. a correct kinetic model and apparent activation energy. On the other hand, for the temperature-dependent kinetic exponent, there is a great possibility of erroneous determination of the correct kinetic model and apparent activation energy, especially at higher heating rates. Since the temperature dependence of n cannot be determined on the basis of non-isothermal DSC experiments, conclusions must be drawn with appropriate caution.

Crystallization Kinetics of Glassy As2Se3

Isothermal crystallization of bulk As2Se3 glass was studied in temperature range 270-360°C. Johnson-Mehl-Avrami (JMA) equation describes the crystallization process in the whole temperature range. By means of analysis of JMA equation the temperature dependence of kinetic exponent n was found, its value changes from 3.8 to 1.9 with increasing temperature. The relationship between the value of n and crystal morphology was briefly discussed. Furthermore the value of apparent activation energy E was determined as well as melting enthalpy. Temperature dependence of crystal growth rate was also determined.

Crystalline arsenic trisulfide: preparation, differential scanning calorimetry and Raman scattering measurements

Abstract Fully crystalline samples of high-purity arsenic trisulfide have been prepared for the first time. Comparison of X-ray diffraction spectrum (XRD) of prepared crystals with XRD spectrum of natural orpiment, as well as Raman scattering experiments, confirmed the crystallinity of the prepared sample. Using differential scanning calorimetry, the melting enthalpy (78 J/g) and apparent activation energy of the melting process (306 kJ/mol) were found. A kinetic analysis of the peaks, as the melting process took place, showed that melting can be described well by the Johnson–Mehl–Avrami equation with a kinetic exponent slightly more than 1. A detailed computer fit of Raman spectrum is described.