Experimental evidence and computational analysis of the electronic densitymodulation induced by gaseous molecules at Si(001) surfaces upon self-assembling organic monolayer

In this paper we will report about the development of an innovative way to self-assemble aromatic molecules as monolayers on a Si(100) surface through the formation of a direct Si-C bond, these systems acting as chemical sens ors for gases. Specifically, we will present an investigation on how donor/ acceptor groups substituted on the benzene ring tailor the gas-surface inte raction. The new sensors were obtained by wet chemistry nucleophilic attack onto halogenated silicon surfaces. Infrared multiple internal reflection ( MTR) experiments and high-resolution electron energy loss spectroscopy anal yses confirmed the formation of Si-C bonds. Gases were detected by measurin g the electrical conductivity changes at 308 K in the presence of trace amo unts of oxidants (SOx, CO and NOx, 0.5-5.0 ppm) in Ar. An analysis of the s ensor response showed that gas-ring interactions actually modulate the pi - system of the aromatic ring. A correlation between gas composition and surf ace conductance was found. In order to provide a consistent interpretation of these resistivity changes, computer simulations have been carried out. W e successfully modeled the supramolecular interactions between the organic fragments and the gas, finding that this interaction unbalances the charge distribution in the aromatic fragment, creating in turn a depletion/accumul ation layer at the Si surface which can be held responsible for the conduct ivity modulation experimentally observed. (C) 2001 Elsevier Science B.V. Al l rights reserved.