FT-IR/ATR Analysis of the Silicon/Aqueous Solution Interface Using Sputtered Silicon Thin Films to Access the 1550–1100 cm −1 Spectral Region

Abstract

An FT-IR attenuated total reflection (ATR) method is described for quantitative in situ analysis of the adsorption and rinsing-removal of surfactants from silicon surfaces. Spectral bands at wavenumbers below 1550 cm⁻¹ are nearly inaccessible when single-crystal silicon ATR internal reflection elements (IREs) are used. A new ATR technique was attempted in order to overcome this limitation. The silicon was sputtered as a thin film onto a thin Al₂O₃ buffer layer, which had been previously sputtered onto a ZnSe IRE to improve adhesion of the silicon layer. The method allowed observation of species at the silicon/aqueous solution interface below 1550 cm⁻¹, to 1100 cm⁻¹. Absorption bands due to adsorbed octylphenol polyethylene oxide (OPEO) and dodecyl trimethyl ammonium bromide (DTAB) surfactants were observed in the 1550–1100 cm⁻¹ spectral region, which were assigned to benzene-ring modes and the aliphatic stretching vibrations for OPEO and to the aliphatic stretching vibrations for DTAB. A mathematical method to calculate adsorption density for stratified ATR IRE systems having more than three phases (i.e., ZnSe/Al₂O₃/Si/aqueous solution) was developed and applied to the determination of the adsorption density of DTAB and OPEO surfactants on silicon, in situ. The method was confirmed through spectra obtained with a single-crystal Si IRE and the previous three-phase calculation method. The agreement indicates that the two surfaces have very similar physisorption chemistry. In addition, this method allows direct, in situ observations of the oxidation-induced growth of a Si-O-Si band near 1150 cm⁻¹ and its removal by dilute HF solutions.