Seminars Archive

ATMOSPHERIC CORROSION AND PHOTO-EDGE STUDIES OF GRAPHENE-LIKE TWO DIMENSIONAL ARSENIC AND GERMANIUM SULFIDE CRYSTALS AND GLASSES BY ENERGY DEPENDENT PHOTOLUMINESCENCE, SRPS, XPS AND RAMAN SPECTROSCOPY.

Abstract
Arsenic sulfide minerals are found naturally and have been used as artists' pigments since prehistoric times. Orpiment As2S3 gives a yellow pigment and realgar As4S4 usually gives an orange-red. Recently by macro FT-Raman and energy-dependent micro-Raman spectroscopy we found the light-induced structural changes in glassy As-S system with realgar inclusion [1]. Newly observed features in the Raman spectra of As-S glass are related to transformations of As4S4 molecules. The initial structure of glassy closed units, and units connected in a glassy network by weak van der Waals forces, the α (β)-As4S4 molecules are transformed into the pararealgar p-As4S4 form during laser illumination. The effectiveness of transformation depends mainly on the photon energy used for irradiation but transformation was observed for all photon energies in the range from 1.65 to 2.54 eV. Our findings are multidisciplinary and together with the additional related investigations are helpful to describe the photo-degradation processes in pigment used since antiquity. The red color of the pigment based on realgar α-As4S4 on exposure to light transformed to pararealgar p-As4S4 that exhibits a yellow color. So light, necessary for viewing a work of art can damage the artwork. Process of light induced polymorph transformation in air is accompanied by formation of arsenolite, As2O3. However, the mechanism is not completely clarified so far. Based on SRPS and surface enhance Raman spectroscopy studies we found new photo-aging processes occurring on the surface of amorphous As2S3 film useful for chalcogenide photonics application. In addition to arsenolite formation, the new band in photon energy dependent photoluminescence (PL) of orpiment, realgar and glassy As-S with realgar inclusions typical for As2O3xH2O substance have been found. We report for the first time the results of experimental studies of room temperature PL on surface of two dimensional (2D) graphene-like crystalline c-GeS with three-coordinated Ge on sulfur and β-GeS2 with four-coordinated Ge on sulfur after long-term ageing. PL of c-GeS exhibits excitation-dependent photoluminescence with increasing intensity of the PL bands up to Eex=4.6 eV. At Eex=4.6 eV, the PL of c-GeS exhibits a strong broad violet band centered at 3.15 eV. An analogous intense violet PL band at UV excitation is known for rutile-like GeO2. Excitation-dependent PL of 2D β-GeS2 have IR, visible and UV peaks (shoulders) at 1.6, 1.85, 1.95, 2.12, 2.25, 2.37, 2.55, 2.7, 2.8, 2.95, 3.0, 3.20, 3.4 eV. In contrast with PL spectra of c-GeS, the excitation-dependent PL spectra of β-GeS2 exhibit a broad green band centered at 2.37 eV whose intensity increases with increasing excitation photon energy up to Eex=2.75 eV. Such behavior of PL spectra of β-GeS2 is typical for GeO2 with quartz-like structure. Peaks observed at different positions and with different intensities in the energy dependent PL spectra could be related to the presence of different types of defects in natural oxides formed on the sample surface. The positions of the PL bands in excitation-dependent photoluminescence of aged and freshly fractured g-GeS2 (TiVj) prepared using different quenching rates (Vj) and melt temperatures (Ti) are compared and analyzed together with the PL, XPS spectra of GeS, β-GeS2 and crystalline GeO2. 1. R. Holomb, V. Mitsa et al. .Chalcogenide Letters, Vol. 2, No. 7, July 2005, pp. 63 – 69.