Effect of acidic and basic surface dipoles on the depletion layer of indium tin oxide as measured by in-plane conductance

Four-point probe measurements of indium tin oxide (ITO) films demonstrate the ability to decrease or increase in-plane resistance by surface treatment with acids or bases, respectively. Treatment of ITO surfaces with solutions of varying pH indicates that the degree of protonation or hydroxylation of basic or acidic surface sites, respectively, alters the $n$-type depletion layer depth and consequently affects the in-plane conductance. Modifying the ITO surface with a series of nearly constant-pH tetraalkylammonium hydroxide species reveals that the in-plane conductance is also modulated by the size of the adsorbed surface dipole layer. The overall change in in-plane conductance upon acid or base treatment is attributed to the net effect of surface charge and adsorbed dipole layer magnitude. Due to the intrinsically high signal–to–noise ratio of the in-plane resistance, these observations have implications for the use of ITO in condensed phase sensing applications.