J. Phys. Chem. B 2003, 107, 11987-11995
11987
Adsorption Structures of Phenylacetylene and 1-Phenyl-1-propyne on a Si(100)-(2 × 1) Surface Ki-Yeo Kim,† Byoung-Kook Song,‡ Sukmin Jeong,*,§ and Heon Kang*,‡ Department of Chemistry, Pohang UniVersity of Science and Technology, Pohang 790-784, South Korea, School of Chemistry, Seoul National UniVersity, Kwanak-ku, Seoul 151-742, South Korea, and Department of Physics, Chonbuk National UniVersity, Chonju 561-756, South Korea ReceiVed: June 20, 2003; In Final Form: August 1, 2003
In an attempt to study the reaction of multifunctional organic molecules with a Si surface, we examined the adsorption of phenylacetylene (HCtC-C6H5) and 1-phenyl-1-propyne (H3C-CtC-C6H5) on Si(100)-(2 × 1) by using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The extra methyl group in 1-phenyl-1-propyne was used as a marker for determining the molecular orientation in the STM image. Two distinct adsorption structures were revealed for these molecules. One is analogous to the so-called di-σ adsorption structure of acetylene, involving the binding of a CtC group of molecules on top of a Si dimer. In the other structure, which is unique, a molecule adsorbs across the valley between two Si dimer rows (the “valley-bridge” structure) such that a phenyl ring binds to two Si atoms in one dimer row and a CtC group binds to the next dimer row. DFT calculations predicted the adsorption energy of phenylacetylene to be 2.44-2.67 eV in the di-σ structure and 0.45-1.59 eV in the valley-bridge structures.
I. Introduction There has been growing research effort in recent years to attach organic molecules to a Si surface,1,2 owing to the potential application of this approach to the construction of organic silicon hybrid structures related to advanced microelectronics, biosensors, and optical devices. Important in such applications is a knowledge of the binding structure of organic functional groups to the surface. For this reason, the chemisorption of organic molecules on Si has been the subject of active investigations involving both theory and experiment.3-23 In the organic adsorption studies, the most popular substrate surface has been the (100) face of a Si crystal because on this surface many silicon-based devices have been fabricated. A Si(100) surface has a specific structure referred to as a Si dimer (SidSi) formed via (2 × 1) surface reconstruction. The Si dimer can be described, at least formally, in terms of a σ bond and a weak π bond.24-26 The high reactivity of the Si-Si π bond offers the major driving force for the chemisorption of unsaturated hydrocarbons; the π bond of an unsaturated hydrocarbon reacts with the π bond of a Si dimer in an analogous way to the cycloaddition reaction between two unsaturated organic molecules. Alkenes and dienes are well known1,9-12 to adsorb covalently on top of a Si dimer in this fashion through [2 + 2] and [4 + 2] cycloaddition, respectively. Unlike alkenes, acetylene with a CtC triple bond exhibits more than one binding structures on Si(100)-(2 × 1) at room temperature, according to recent STM studies.13,14 Figure 1a shows the adsorption structures of acetylene proposed in the STM studies. In structure A, C2H2 adsorbs on top of a Si dimer, forming an sp2-hybridized, di-σ species. In end-bridge structure B, C2H2 binds to the side of two adjacent Si dimers. C2H2 can * Corresponding authors. E-mail:
[email protected]. Fax: +82 2 889 8156 (Heon Kang). † Pohang University of Science and Technology. ‡ Seoul National University. § Chonbuk National University.
Figure 1. Various adsorption structures proposed for C2H2 (a) and C6H6 (b) on Si(100)-(2 × 1). (A) di-σ: C2H2 is adsorbed on top of a Si dimer; (B) end-bridge: C2H2 bonds to the side of two adjacent Si dimers; (C) r-bridge and (D) p-bridge: C2H2 bonds to two adjacent dimers, with its orientation parallel and perpendicular, respectively, to the dimer row; (E) (end-bridge)2: two end-bridge structures formed across two adjacent dimers. (F) 1,4 single dimer: C6H6 sits on top of a dimer via the bonding of the C1 and C4 atoms of benzene; (G) tight bridge and (H) twisted bridge: four sequential C atoms bond to Si. H' indicates another form of the twisted bridge structure that can be formed next to a C-type defect.
also be tetracoordinated to two adjacent dimers, resulting in the r-bridge (C) or p-bridge structures (D) depending on whether the molecular orientation is parallel or perpendicular, respectively, to the dimer row. The (end-bridge)2 structure (E) consists of two end-bridge structures across two adjacent dimers. The di-σ and end-bridge structures (A and B) have acquired
10.1021/jp035760l CCC: $25.00 © 2003 American Chemical Society Published on Web 10/04/2003
11988 J. Phys. Chem. B, Vol. 107, No. 43, 2003 convincing evidence for their presence.13,14 The third binding structure, however, seems somewhat controversial as to whether it is the r-bridge (C),13 (end-bridge)2 (E),14 or p-bridge structure.15 The adsorption of benzene on a Si(100) surface has also been well studied.16,17 Benzene readily adsorbs on Si(100)-(2 × 1) at room temperature, losing its ring aromaticity in three different configurations, as shown in Figure 1b. In the 1,4 singledimer structure (F), C6H6 sits on top of a Si dimer through the bonding of C1 and C4 atoms of benzene to the dimer. In the tight-bridge (G) or twisted-bridge (H) structure, four C atoms bond to Si, leaving one double bond that is perpendicular or parallel, respectively, to the dimer row. The twisted-bridge structure was found only in conjunction with a C-type defect present on Si(100) in an STM study.18 In this case, this structure may take the form of H' shown in Figure 1b. When a molecule contains more than one reactive group, its adsorption behavior can be more complicated. The different reactive groups may compete for adsorption sites and may interact with the surface in a cooperative manner. An adsorbed molecule may further react with another incoming molecule by utilizing its reactive parts. Such diverse possibilities can hamper the experimental determination of the adsorption structures of organic molecules with multifunctional groups. Yet, understanding the binding characteristics of multifunctional molecules is essential to the development of organic semiconductor devices. In the present work, we have chosen phenylacetylene (HCt C-C6H5) and 1-phenyl-1-propyne (H3C-CtC-C6H5) as examples of multifunctional molecules to study the reaction on Si(100). Both molecules have a CtC bond and a phenyl ring as reactive groups, and the only difference is an extra methyl group in 1-phenyl-1-propyne. A methyl group can be used as a marker for resolving molecular orientation in STM images.12,22 The adsorption structures of these molecules are examined by scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations. Several multifunctional organic molecules containing a phenyl ring have been examined for adsorption on Si(100); these include phenyl isothiocyanate,19 styrene,20 benzonitrile,21 and, very recently, phenylacetylene.23 These studies report that the molecules adsorb on Si(100) only through the interaction of the external groups (NdCdS, CdC, CtN, and CtC) with the surface, leaving the phenyl ring nearly unperturbed. II. Methods II.A. Experimental Method. STM experiments were conducted in an ultrahigh vacuum (UHV) chamber with a base pressure of 5 × 10-11 Torr, which allowed the sample surface to be kept free of contaminants over a large area. The Si(100) samples were cut from an n-type Si wafer with 5-Ω cm resistivity. They were rinsed with isopropyl alcohol and then placed in the UHV chamber to be annealed at 1000 K overnight. A clean Si(100)-(2 × 1) surface was prepared by several cycles of Ar+ sputtering at 2 keV and 30 µA cm-2 for 5 min and annealing at 1300 K for 3 min. Phenylacetylene and 1-phenyl1-propyne (Aldrich Chemical) were purified by freeze-pumpthaw cycles before use. The sample vapor was introduced into the UHV chamber through a leak valve and guided by a stainless steel tube doser to 1 cm above the substrate surface. The sample purity was checked by a quadrupole mass spectrometer attached to the chamber. The surface coverage of adsorbed molecules was controlled to be 0.01-0.1 monolayer (ML), where 1 ML corresponds to two molecules per Si dimer, by exposing the sample vapor at a partial pressure of
