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

Abstract

Self-assembly of amphiphilic molecules, alkanethiols CH3(CH2)nSH (n = 15, 17), carboxylic acids CH3(CH2)nCOOH (n = 16, 18) and bifunctional SH(CH2)15COOH 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 ∼30% of the molecules adsorb as unbound thiols. From NEXAFS, we determined average alkyl chain tilts of 33° for thiol from the neat liquid, 44° for thiols prepared via CVD, 38° for carboxylic acids from hexadecane, and 43° for COOH(CH2)15SH from ethanol. Adsorption from a mixed hexadecane solution results in a preferred adsorption of carboxylic acids over thiol. Thus, COOH(CH2)15SH is found to bind to ITO via the carboxylic acids, forming a thiol-terminated SAM