Substrate Effects on the Oxidation Process of Self-Assembled Monolayers

Electrochemical oxidation lithography is a scanning force lithography mode which demonstrated to be a useful tool to fabricate nanostructures with nanometer resolution. Applications of this technique include the fabrication of split-ring resonator arrays for plasmonic applications, as well as the fabrication of nanoelectronic device features, i.e., nanometric gap structures, carbon nanotube assemblies, single particle placement among others. In particular for optical applications there is a strong desire to utilize substrates others than silicon wafers for the fabrication of nanostructures. Up to now these possibilities were not at hand. Here we report on the extension of the range of suitable substrates for the electrochemical oxidation and functionalization process. Electrochemical oxidation lithography is based on the local electrochemical oxidation of a self-assembled monolayer consisting of n-octadecyltrichlorosilane (OTS) by means of a negatively biased conductive SFM tip. The self-assembly of OTS is however not limited only to the functionalization of silicon but can be applied also to a number of other technologically relevant materials, i.e., indium tin oxide, titanium dioxide and metal oxides like aluminum oxide. We investigated the peculiarities of the self-assembly of OTS on these substrates and studied in detail the characteristics of the electro-oxidation process. Essential parameters for the self-assembly as well as for the oxidation process are presented in a comparative study. In depth Scanning Kelvin Probe investigations were performed with the aim to elucidate the chemical nature of the formed species in the oxidation process. As a main result the strong shielding performance of the OTS monolayer could be demonstrated which effectively screens the surface potential of the underlying substrates; a result which has significant impact in the design and manipulation of electronic properties of nanostructured surfaces. These results open on the one hand the spectrum of applicable substrates for the fabrication of functional nanostructures and pave on the other hand also the way towards new applications, in particular for optical and plasmonic research, since for the first time also transparent substrates could be used to combine the advantages of electro-oxidative SFM lithography with applications which essentially require transparent substrate materials.