Experimental evidence and computational analysis of the electronic density modulation 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 sensors for gases. Specifically, we will present an investigation on how donor/acceptor groups substituted on the benzene ring tailor the gas-surface interaction. 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 analyses confirmed the formation of Si-C bonds. Gases were detected by measuring the electrical conductivity changes at 308 K in the presence of trace amounts of oxidants (SOx, CO and NOx, 0.5-5.0 ppm) in Ar. An analysis of the sensor response showed that gas-ring interactions actually modulate the pi -system of the aromatic ring. A correlation between gas composition and surface conductance was found. In order to provide a consistent interpretation of these resistivity changes, computer simulations have been carried out. We 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/accumulation layer at the Si surface which can be held responsible for the conductivity modulation experimentally observed. (C) 2001 Elsevier Science B.V. All rights reserved.