Optical spectroscopy of hybrid sol-gel coatings doped with noble metals

Sol-gel coatings in the xM.(100 - x)SiO2 system, (M = Cu, Ag and Au) (x = 0.1-10 mol%), are deposited on soda lime glass slides by using silicon tetramethoxide Si(OCH3)(4)) and methyltriethoxysilane (SiCH3[OCH2CH3](3)) as silica precursors. Anhydrous CuCl, CuCl2. 2H(2)O, Cu(NO3)(2). 3H(2)O, CuSO4. 5H(2)O, AgNO3 and HAuCl4. 3H(2)O are used as copper, silver and gold sources. Coatings with thicknesses ranging from 100 to 900 nm are deposited on the substrates by dip-coating and subsequently densified at 500 degrees C for 1 h in air. Spectroscopic studies of the coatings as a function of the thicknesses and the metal concentration are carried out by photoluminescence (PL) and optical absorption (OA). In addition, direct observations of some gold coatings were performed by transmission electron microscopy (TEM). Results indicate that for silver and copper containing coatings the excitation and emission spectra arise from electronic transitions in Ag+ and Cu+ ions and no significant absorption bands due to colloidal precipitation are observed. Gold containing coatings show purple coloration due to an absorption peaking in the 520-560 nm range, which is characteristic of gold colloids. The presence of these colloids is confirmed by TEM observations.