Scanning probe electro-oxidative lithography of OTS monolayers on ITO films: A Scanning Kelvin Probe Microscopy investigation

Transparent conductive oxides like indium tin oxide (ITO) play a pivotal role in a wide range of innovative applications, such as new generation solar cells. In many of these applications the tailoring of surface properties on the nanometer scale represents a highly desirable target. The local oxidation of self-assembled monolayers (SAMs) using a scanning probe is a promising technique to achieve surface modifications on the nanometer scale.[1] So far, electro-oxidative lithography of SAMs has been reported mainly on Si wafers while there are no previous reports on transparent oxides. Here, we demonstrated for the first time the oxidative lithography of n-octadecyltrichlorosilane (OTS) SAM deposited onto an ITO layer. The electro-oxidative lithography of OTS-ITO results in a local overoxidation of the substrate, as confirmed by tests onto bare ITO. In addition, the monolayer oxidation causes the conversion of –CH3 groups into hydrophilic functionalities. The latter phenomenon was proven by the site-selective deposition of Ag nanoparticles onto electro-oxidized areas. The lithography process was compared to that on OTS-Si substrates and systematically investigated by means of Scanning Kelvin Probe Microscopy (SKPM). Surface potential images of OTS-Si and OTS-ITO showed remarkable similarities, while the bare substrates exhibit completely different behavior. Such significant differences, which can be attributed to the presence of the oxidized monolayer, are not observable in the case of topography or lateral force images. Thus, SKPM analysis represents a powerful and unique tool to investigate the mechanism of the monolayer oxidation.