Preparation and characterization of self-assembled monolayers on indium tin oxide

Self-assembly of amphiphilic molecules, alkanethiols CH3(CH2)(n)SH (n = 15, 17), carboxylic acids CH3-(CH2)(15)COOH (n = 16, 18) and bifunctional SH(CH2)(15)COOH on indium tin oxide (ITO) surfaces was investigated by X-ray photoelectron and near edge X-ray absorption fine structure spectroscopies, contact angle measurements, and atomic force microscopy. These molecules were found to form homogeneous, well-oriented monolayers on ITO. XPS indicates that at low coverages, thiol molecules adsorb as thiolates, whereas at saturation coverage similar to 30% of the molecules adsorb as unbound thiols. From NEXAFS, we determined average alkyl chain tilts of 33 degrees for thiol from the neat liquid, 44 degrees for thiols prepared via CVD, 38 degrees for carboxylic acids from hexadecane, and 43 degrees for COOH(CH2)(15)SH from ethanol. Adsorption from a mixed hexadecane solution results in a preferred adsorption of carboxylic acids over thiol. Thus, COOH(CH2)(15)SH is found to bind to ITO via the carboxylic acids, forming a thiol-terminated SAM.