Experimental evidence and computational analysis of the electronic density modulation induced by gaseous molecules at Si001) surfaces upon self-assembling organic monolayer Monica Bollani a,b , Rossella Piagge c , Ahmed Charaõ È d , Dario Narducci e,* a Consorzio Milano Ricerche, v. Cicognara 7, 20129 Milano, Italy b Edi®s UMR 6518, 13397 Marseille, France c Siemens, Cascina Castelletto, 20019 Settimo M.se, Italy d TECSN UMR 6122, 13397 Marseille, France e Istituto Nazionale per la Fisica della Materia and Department of Materials Science, v. Cozzi 53, 20125 Milano, Italy Accepted 11 October 2000 Abstract Inthispaperwewillreportaboutthedevelopmentofaninnovativewaytoself-assemblearomaticmoleculesasmonolayers on a Si100) surface through the formation of a direct Si±C bond, these systems acting as chemical sensors for gases. Speci®cally, 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 re¯ection MIR) experiments and high-resolution electron energy loss spectroscopy analyses con®rmed 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 SO x ,COandNO x ,0.5±5.0ppm)inAr.Ananalysisofthesensorresponseshowedthat gas-ring interactions actually modulate the p-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 beencarriedout.Wesuccessfullymodeledthesupramolecularinteractionsbetweentheorganicfragmentsandthegas,®nding 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. # 2001 Elsevier Science B.V. All rights reserved. PACS: 68.35.j; 68.55.j; 73.26; 79.20.R Keywords: Surface conductivity; Self-assembling; Supramolecular interactions; Sensors 1. Introduction Over the last decade quite a large body of interest has focused on the possibility of modulating the sur- face properties of standard semiconductors. For Applied Surface Science 175±176 2001) 379±385 * Corresponding author. Tel.: 39-02-6448-5137; fax: 39-02- 6448-5400. E-mail address: dario.narducci@unimib.it D. Narducci). 0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0169-433201)00129-5