Unsaturated cyclic-hydrocarbon molecules on a Si001) surface A ®rst-principles approach Kazuto Akagi * , Shinji Tsuneyuki Institute for Solid State Physics, University of Tokyo, Kashiwanoha 5-1-5, Kashiwa-shi, Chiba 277-8581, Japan Received 18 October 2000; accepted for publication 5 February 2001 Abstract Chemisorption of unsaturated cyclic-hydrocarbon molecules C 6 H 10 ,C 6 H 8 and C 5 H 8 on Si0 0 1) clean surface was investigated based on ®rst-principles calculations, and the adsorption structures were discussed in comparison with the experimental data. As for C 6 H 10 , two kinds of stable conformation were determined, which are consistent with two types of characteristic STM images and spectroscopic data. On the other hand, we also found two kinds of adsorption structures for C 6 H 8 molecules, but the structure supported by STM and UPS observations is slightly unstable compared with the other one, and the situation is still mysterious. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Density functional calculations; Chemisorption; Silicon; Carbon; Self-assembly 1. Introduction Chemisorption of hydrocarbon molecules on Si0 0 1) clean surface has been well studied espe- cially for the species such as acetylene and ethyl- ene. Most of these studies seem to be performed from a view point of the contamination of silicon substrates by carbon atoms or the growth of SiC crystals [1]. On the other hand, a view point to- ward the creation of novel materials by inten- tionally modifying a surface also can be taken. Chemisorption of unsaturated cyclic-hydrocarbon molecules is one of such interesting subjects. For examples, Liu and Hamers reported an experiment which indicates possibility of the well ordered and saturated surface for the cyclopenteneC 5 H 8 )/ Si001) system [2]. Recently, Yoshinobu et al. investigated chemi- sorption of cyclohexeneC 6 H 10 ) and 1,4-cyclohexa- dieneC 6 H 8 ) molecules on Si001) clean surface in detail under ultra-high vacuum condition at 90 K or room temperature by LEED, XPS, UPS, EELS and STM. They aimed a self-assembled and highly ordered surface which allows a molecular level operation or a controlled two-dimensional chemical reaction ®eld [3]. Their results are sum- marized as follows: · The 21) structure from Si001) surface is kept even after the saturated adsorption of these molecules.LEED) Surface Science 493 2001) 131±137 www.elsevier.com/locate/susc * Corresponding author: Fax: +81-471-36-3291. E-mail address: akagi@issp.u-tokyo.ac.jp K. Akagi). 0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0039-602801)01200-6