162 Bull. Korean Chem. Soc. 2010, Vol. 31, No. 1 Seung Chul Oh et al. DOI 10.5012/bkcs.2010.31.01.162 Role of Coverage and Vacancy Defect in Adsorption and Desorption of Benzene on Si(001)-2×n Surface Seung Chul Oh, Ki Wan Kim, Abdulla H. Mamun, Ha-Jin Lee, † and Jae Rayng Hahn * Department of Chemistry and Institute of Photonics and Information Technology, Chonbuk National University, Jeonju 561-756, Korea. * E-mail: jrhahn@jbnu.ac.kr † Jeonju Center, Korea Basic Science Institute, Jeonju 561-756, Korea Received September 29, 2009, Accepted December 3, 2009 We investigated the adsorption and desorption characteristics of benzene molecules on Si(001)-2×n surfaces using a variable-low temperature scanning tunneling microscopy. When benzene was adsorbed on a Si(001)-2×n surface at a low coverage, five distinct adsorption configurations were found: tight-binding (TB), standard-butterfly (SB), twisted- bridge, diagonal-bridge, and pedestal. The TB and SB configurations were the most dominant ones and could be rever- sibly interconverted, diffused, and desorbed by applying an electric field between the tip and the surface. The population ratios of the TB and SB configurations were affected by the benzene coverage: at high coverage, the population ratio of SB increased over that of TB, which was favored at low coverage. The desorption yield decreased with increasing ben- zene coverage and/or density of vacancy defect. These results suggest that the interaction between the benzene molecules is important at a high coverage, and that the vacancy defects modify the adsorption and desorption energies of the benzene molecules on Si(001) surface. Key Words: Scanning tunneling microscopy, Silicon surfaces, Benzene, Intermolecular interaction, Vacancy defect Introduction Research into the attachment of organic molecules to Si surfaces 1 has intensified because of the potential applications of these systems in the construction of organic-silicon hybrid na- nostructures for use in advanced molecular electronics, biosen- sors, and optical devices. For this reason, the chemisorption of organic molecules on Si surfaces has been examined in numer- ous theoretical and experimental investigations. In the construc- tion of organic-silicon hybrids, molecular coverage on the Si surface is one of the crucial factors determining electronic de- vice functions. Electronic property of organic molecular film devices varies with molecular packing pattern and the electric carrier mobility. In addition, vacancy defects on the Si surface should play important roles in determining the quality and func- tion of the organic molecular devices, because they can be more reactive than clean Si or may determine the whole surface mor- phology of the Si surface. 2 For more realistic applications, knowledge of the binding structures of organic molecules on Si surfaces as a function of coverage and defect density is therefore important to develop organic hybrid nanostructures. The adsorption of benzene on Si substrates is of great interest as a model system for studying the molecular adsorption of hydrocarbons on semiconductor surfaces and because this sys- tem is considered as a promising precursor for technologically relevant processes, such as the chemical vapor deposition of diamond films on Si surfaces. Many experimental and theore- tical studies have been reported on the adsorption mechanism of benzene on Si(001), 3-17 Si(111), 18 and Si(5 5 12) 19 surfaces. While no complete experimental determinations have been made of the local chemisorption structure of benzene on Si(001), a number of different geometries have been proposed by a variety of techniques including thermal desorption spectroscopy (TDS), 4,5 high-resolution electron energy loss spectroscopy, 4 ultraviolet photoelectron spectroscopy (UPS), 5 near-edge x-ray absorption fine structure (NEXAFS), 6 high resolution photoe- mission spectroscopy, 7 and scanning tunneling microscopy (STM). 8,9,13-17 Several studies using STM and theoretical cal- culations 8-10 claim that benzene on Si(001)-2×1 is initially ad- sorbed in the standard-butterfly (SB) configuration and then quickly adopts the tight-binding (TB) configuration, which is more energetically stable. Taking into account the reported acti- vation energies for this conversion-and the relative stabilities of the two configurations-complete conversion to the TB con- figuration would be expected relatively quickly. However, other studies using NEXAFS and UPS concluded that only the SB configuration is present on the surface as a major and stable species. 11 A different, first-principles calculations based on clu- ster models supported this conclusion. 12 Many aspects of the local structure of adsorbed benzene on Si(001)-2×1 at high coverage also remain unclear. Such discrepancies can be due to two possibilities. (i) The STM and conventional ensemble-averaged spectroscopy studies focused on the very initial and the saturated coverage, respec- tively. (ii) The STM studies must be conducted at very low de- fect density while the defect density is hardly defined on the en- semble-averaged spectroscopic studies. Therefore, the cover- age- and/or defect-dependent change of the adsorption charac- teristics observed here may reconcile the apparent discrepancies between the previous studies. On the Si(001)-2×n surface, we found five types of adsorption configurations of benzene mole- cules. Their relative population ratios, as well as the intermole- cular-conversion and desorption processes, were studied using STM. These characteristics were found to differ from those of