Adsorption of Phenyl Isothiocyanate on Si(001): A 1,2-Dipolar Surface Addition Reaction Mark D. Ellison* and Robert J. Hamers Department of Chemistry, UniVersity of Wisconsin-Madison, 1101 UniVersity AVenue, Madison, Wisconsin 53706 ReceiVed: January 4, 1999 The adsorption of phenyl isothiocyanate (PITC) on silicon (001) surfaces has been investigated using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, scanning tunneling microscopy (STM), and ab initio calculations. The experimental evidence shows that adsorption onto the Si surface occurs in a highly selective manner, through the NdCdS group and not through the benzene ring. Both experimental and theoretical evidence suggests that the adsorption involves the interaction of the N and C atoms of the NdCdS group with the SidSi dimer, forming a four-member Si 2 NC ring at the interface. This process leaves the aromatic ring nearly unperturbed and is analogous to a 1,2-dipolar addition reaction. Theoretical calculations confirm that this is the most stable product. I. Introduction The adsorption of organic molecules on silicon surfaces is of great technological importance. 1 The ability to form an ordered organic adlayer on silicon presents an advance that opens opportunities for several fields, such as microelectronics and biosensing. Of particular interest in this respect is the Si- (001) surface, which undergoes a reconstruction involving a pairing of surface atoms into dimers. 2 These dimers are formally held together with a double bond, suggesting that the chemistry of Si(001) may be similar to the chemistry of organic alkenes. The adsorption of the two simplest unsaturated hydrocarbons, ethylene and acetylene, on Si(001) has been extensively studied. 3-10 Ethylene adsorbs onto Si(001) so that the CdC π bond and the SidSi π bond break, forming two new Si-C σ bonds. Initially, acetylene was also thought to bond in a fashion similar to that of ethylene, but recent evidence suggests that acetlyene’s adsorption behavior is more complex. 11 Recently, the adsorption behavior of more complex alkenes has also received a great deal of attention. Several examples of [2 + 2] cycloaddition reactions, in which the double bond of the organic molecule reacts with the double bond between the Si atoms to form a four-member Si 2 C 2 ring, have been reported. 12-16 In addition, molecules that contain at least two conjugated double bonds have been shown to adsorb via a [4 + 2] (“Diels-Alder”) reaction. 17-20 Moreover, certain molecules adsorb via both cycloaddition processes, and some studies have investigated the competition between the [2 + 2] and [4 + 2] pathways in the adsorption process. 17 Finally, recent investigations have shown that these cycloaddition reactions on the Si(001) surface are not limited solely to alkenes. We have demonstrated the possibility of a cycloaddition reaction involving a NdN bond, 21 while White et al. have shown that a CdO bond may also undergo a [2 + 2] addition on Si(001). 22 The high reactivity of alkenes toward the dimers on the Si(001) surface is unexpected, because the [2 + 2] reactions in solution-phase chemistry are formally symmetry forbidden. 23 The reactions of alkenes with the Si surface dimers are thought to be so facile because they proceed through a low-symmetry intermediate. 7 To improve our understanding of the reactivity of unsaturated compounds with the Si(001) surface, we have investigated the adsorption of phenyl isothiocyanate, C 6 H 5 -NdCdS (PITC). This molecule has the potential to adsorb on the surface in several different configurations. Benzene adsorbs quite readily on Si(001), although the exact adsorption geometry is the subject of debate. 24-31 By analogy, then, PITC may adsorb to Si(001) via the phenyl ring. Additionally, the NdC and CdS double bonds each present an opportunity for a [2 + 2] cycloaddition reaction with the Si dimers. Solution-phase organic reactions have shown that PITC will undergo a cycloaddition reaction with an alkene via the NdC group, 32,33 while sulfonyl-PITC will react with an alkene via the CdS moiety. 33 The reactions reported in the literature also demonstrate that PITC will undergo a cycloaddition reaction via the NdCdS group. 32 Finally, PITC may adsorb onto Si- (001) by forming a five-member ring that involves the formation of two new σ bonds, one Si-N bond and one Si-S bond. This 1,3-dipolar addition has been predicted theoretically, 34 but we were unable to find any reports of this type of reaction in the literature. Thus, the NdCdS group also presents several possibilities for reaction of PITC with the Si dimers on Si(001). Clearly, the PITC molecules have several possible adsorption pathways. Here, we present strong evidence that a 1,2-dipolar addition is the preferred adsorption geometry. II. Experimental Section The adsorption of phenyl isothiocyanate (PITC) on Si(001) at 300 K was investigated using several methods: X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), scanning tunneling microscopy (STM), and ab initio calculations. Individually, each of these methods reveals a specific subset of information about the adsorbate- surface system. When taken together, the results from the four techniques provide a comprehensive picture of the adsorption process. All of the experiments described here were performed in ultrahigh vacuum (UHV) chambers having base pressures of < 1 × 10 -10 Torr. The Si(001) samples were cut from n-type wafers (Wacker Chemitronic), degreased in methanol, and * Author to whom correspondence should be addressed. 6243 J. Phys. Chem. B 1999, 103, 6243-6251 10.1021/jp990010q CCC: $18.00 © 1999 American Chemical Society Published on Web 07/09/1999