Theoretical study of the surface modification of indium tin oxide with trifluorophenyl phosphonic acid molecules: Impact of coverage density and binding geometry

The interface composed of 3,4,5-trifluorophenyl phosphonic acid (PA) molecules chemisorbed on the indium-tin oxide (ITO) surface was studied at the density functional theory level. The impact of coverage density and binding geometry of the PA molecules on the ITO surface characteristics has been investigated systematically by increasing the PA surface coverage density from one to four PA molecules per surface unit cell. The calculations point to several bonding schemes where the phosphonate (PO₃) moieties are directly bonded to the surface In/Sn atoms and/or form hydrogen bonds with the hydroxyl groups present on the ITO surface. Both PA coverage density and binding geometry strongly influence the work-function modifications of the ITO surface upon chemisorption. The oxygen core-level binding energy shifts have been calculated and compare favorably with the experimental X-ray photoelectron spectroscopy data. Substantial depolarization effects are observed within the PA monolayer at high coverage densities.