Figure 1. Organisation of undecane (organic molecular) layer, silicon regions and electrolyte interface and bulk.

Surface modification of silicon by covalent attachment of organic molecules is currently an area of intensive research because of its applications for bio-functionalising semiconductor devices

We have a method for covalently attaching organic molecular monolayers directly to the atomically-flat Si(111) surface of silicon (Figure 1). An electrical impedance spectroscopy methodology we have developed allows the characterisation of the dielectric and conductance properties of various substructural layers in this system, including details of the depletion region established in the silicon close to the surface. Variations in these dielectric substructure with applied DC potential allowed us to determine the potential dependence of the thickness of this depletion region in the silicon and also monitor electrostrictive effects in the organic layers chemically bonded to the silicon surface.

Figure 2. Thickening of one of the substructural layers of the silicon depletion regions with increasing V.

The dielectric measurements were supplemented with low angle X-ray reflectometry and X-ray photoelectric spectroscopy (XPS) measurements of the monolayer to obtain further structural information, chemical composition and molecular density data.

Figure 3. Thicknesses of the alkane (hydrocarbon) region consistent with alkane electro-mechanical properties

It is hoped to extend the experiments to include bimolecular lipid bilayer membranes “tethered” to silicon substrates via covalently attached molecular tethers. Such systems would provide a new research tool for studies in biophysics and would have applications in drug screening.

This work is a collaboration between the School of Physics (Biophysics), School of Biotechnology and Biomolecular Sciences (UNSW) and Neutron Scattering Group (ANSTO).

Till Böcking, Terry Chilcott, Hans Coster, Kevin Barrow and Michael James