Paper SS-TuP18
Surface Chemistry of Carboxylic Acids on Si(100)2×1

Tuesday, October 21, 2008, 6:30 pm, Room Hall D

Organic functionalization of Si(100)2x1 has attracted a lot of recent attention due to its potential applications in organic-inorganic hybrid semiconductor devices, molecular electronics, chemical and biological sensors, and optical materials. Understanding of the behaviour of organic molecules on Si(100)2×1 surface is fundamentally important, because the electronic and chemical properties and selectivity of the inorganic substrate are modified by the organic adsorbate. Among the wide range of organic functional groups, the carboxyl group is one of the key constituents of amino acids, the building blocks of peptides and proteins. Under appropriate cleaning preparation, the Si(100) surface reconstructs to the 2×1 surface, on which the dangling bonds of adjacent Si atoms pair to create surface Si=Si dimer rows. The interaction of the carboxyl group with the Si=Si dimers, composed of a full σ bond and a partial π bond, is therefore of special interest to understanding the chemical reactivity and selectivity of biomolecules in general with this widely industrially used semiconductor. The present work investigates the dissociative adsorption of acetic acid and acrylic acid on the UHV prepared Si(100)2×1 surface at room temperature using X-ray photoelectron spectroscopy (XPS), temperature programming desorption (TPD), and density-functional theory (DFT) calculation. Selectivity of the surface towards acrylic acid, bifunctional molecule, has been investigated as well. Our DFT calculation by using B3LYP/6-31++G(d,p) shows several possible adsorbate-substrate configurations (ASCs) for the adsorption of these carboxylic acids on Si(100)2×1 surface. Acetate and acrylate resulting from the O-H dissociation of the related carboxylic acid on the Si(100) bind to the surface through oxygen atom. Our DFT and XPS results enable the identification of the adstructures arising from bonding involving either one oxygen atom (the unidentate ASC) and two oxygen atoms (the inter-dimer and intra-dimer bidentate ASCs). Our C 1s features prove the formation of bidentate adstructure at the early stage and of both unidentate and bidentate adtructures at the saturation level of exposure. The thermal evolution of the adsorbates studied by TPD and XPS of the flash-annealed surface confirm desorption of some species detected by Q-mass spectroscopy and formation of SiC on Si.