Selectivity of the Chemisorption of Vinylacetic Acid on the Si(001)2×1 Surface Han-Na Hwang, Jae Y. Baik, Ki-Seok An, Sun S. Lee, and Yunsoo Kim* Thin Film Materials Laboratory, AdVanced Materials DiVision, Korea Research Institute of Chemical Technology, Daejeon 305-600, Korea Chan C. Hwang and Bongsoo Kim Beamline Research DiVision, Pohang Accelerator Laboratory, Pohang UniVersity of Science and Technology, Pohang 790-784, Korea ReceiVed: January 10, 2004; In Final Form: April 7, 2004 The adsorption of vinylacetic acid (VAA) on the Si(001)2×1 surface has been investigated using low-energy electron diffraction, X-ray photoelectron spectroscopy, and synchrotron radiation photoemission spectroscopy. VAA adsorbs on the Si(001)2×1 surface at room temperature, not through the reaction of the vinyl group (CH 2 dCH-) but through the dissociative adsorption of RCOOH (R ) CH 2 dCHCH 2 ) into RCOO(ad) as a unidentate adsorbate and H(ad) without breaking the Si dimers. Curve-fitting of the Si 2p core level spectra taken after VAA exposures shows the development of two new surface components, S O and S H , shifted by +0.93 and +0.40 eV, respectively, from the bulk component. The binding energy shifts indicate that the S O and S H components are due to the Si-OOC-R and Si-H species formed by the dissociative adsorption of VAA. The adsorption behavior was confirmed by the analysis of the C 1s and O 1s core level spectra. The C1s core level spectrum for the VAA-saturated surface clearly shows three components contributed by carbon atoms in the OdC-O, -C-C-, and -CdC- groups, while the O 1s core level spectrum consists of two components due to oxygen atoms in the Si-O- and -CdO groups. There is no evidence for the bonding between the Si dimer and the -CdC- bond of VAA through the [2+2] cycloaddition reaction, which is expected in the adsorption of unsaturated hydrocarbon compounds on the Si(001)2×1 surface, nor for the hetero-[2+2] reaction of the -CdO group with the Si dimer. This selectivity in the reacting group is discussed on the basis of the differences in reaction barriers between the Si dimers and the functional groups of the VAA molecule. I. Introduction Covalent attachment chemistry of organic molecules on silicon surfaces has been of great interest in surface science. Apart from the fundamental interest, the attachment of organic molecules to the Si surfaces has been one of promising subjects for integrating the wide range of available chemical function- alities of organic molecules with micro- and nano-electronic technology. 1 The Si(001)2×1 reconstruction is induced by dimerization of the top layer Si atoms in order to reduce the number of surface dangling bonds of the ideal bulk-terminated Si(001) surface, which results in a strong σ and a weak π bonds on the surface because of the strained geometry. 2-4 The weak π bond is very reactive in molecule adsorption. 5,6 Several examples of molecule attachment chemistry on the Si(001)2×1 surface have been reported for organic molecules having various single- and multi- functional groups, such as alkenyl, 7-12 hydroxyl, 13-15 mer- capto, 16 carboxyl, 17-21 and amino 18,22,23 groups, etc., which can be employed as important building blocks for attaching organic molecules to the Si surface. In this study, the adsorption of vinylacetic acid on the Si- (001)2×1 surface has been investigated. Vinylacetic acid (VAA), as a typical monomer having both a vinyl (H 2 CdCH-) and a carboxyl (-COOH) groups, is a bifunctional organic molecule having usable single and double bonds. Thus, VAA may be chosen to demonstrate the chemical selectivity and reactivity of the functional groups with the Si dimers on the Si(001)2×1 surface. Dissociative chemical interactions of carboxyl and hydroxyl groups with the dangling bonds of the Si(001)2×1 surface at room temperature have been reported from high-resolution electron energy loss spectroscopy (HREELS), near-edge X-ray absorption fine structure (NEXAFS), low-energy electron dif- fraction (LEED), and/or photoemission spectroscopy measure- ments for the adsorption of formic acid (HCOOH), 17,20 methanol (CH 3 OH), 14 ethanol (CH 3 CH 2 OH), 13,15 2-propanol ( i PrOH), 15 tert-butyl alcohol ( t BuOH), 15 benzoic acid (C 6 H 5 COOH), 18,19 and 4-aminobenzoic acid (H 2 NC 6 H 4 COOH) 21 on the Si(001)2×1 surface. It was confirmed that chemical interactions induce the HCOO-Si (or CH 3 O-Si, CH 3 CH 2 O-Si, R-C 6 H 4 -COO-Si, etc.) and the H-Si bonds on the surface. The adsorption of an unsaturated hydrocarbon on the Si(001)2×1 surface was identi- fied as the [2+2] cycloaddition reaction (or Diels-Alder reaction) between the CdC bond and the Si-Si dimer. 7-12 In addition, the hetero [2+2] cycloaddition reaction of simple carbonyl-containing compounds on the Si(001)2×1 surface has been observed. 24-26 Recently, experimental and theoretical studies of the unsaturated ketones including ethyl vinyl ketone on the Si(001)2×1 surface showed that the ene and the hetero Diels-Alder reactions are favored. 27 On the other hand, maleic anhydride adsorbed preferentially on inter-dimer sites of the * Author to whom correspondence should be addressed. Tel: 82-42- 860-7350. Fax: 82-42-861-4245. E-mail: yunsukim@krict.re.kr. 8379 J. Phys. Chem. B 2004, 108, 8379-8384 10.1021/jp0498769 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/25/2004