J. Phys. Chem. B 2001, 105, 4535-4537
4535
Inhibition of the [2+2] Cycloaddition: Maleic Anhydride on Si(100)-2×1 T. Bitzer,* T. Rada,† and N. V. Richardson School of Chemistry, St. Andrews UniVersity, St. Andrews KY16 9ST, U.K. ReceiVed: August 10, 2000; In Final Form: March 12, 2001
The chemisorption of maleic anhydride on Si(100)-2×1 at room temperature has been studied with scanning tunneling microscopy (STM). We observe that, at low coverages below 0.12 monolayer, the unsaturated organic compound adsorbs almost exclusively on inter-dimer sites and not through a direct [2+2] cycloaddition. The STM images provide evidence that the adsorption of a second maleic anhydride molecule on an adjacent inter-dimer site results in the cleavage of the silicon dimer bond located between the two molecules. The chemisorption of the organic molecules induces a local rearrangement of unsaturated silicon dimers.
Introduction
Experimental Details
The potential application of hybrid silicon/organic film devices has motivated a series of investigations.1 The most technically important silicon surface is Si(100), which reconstructs to a dimer row structure upon high-temperature preparation inside ultrahigh vacuum (UHV). Recently, it has been shown that, at room temperature, the five-membered cyclic hydrocarbon cyclopentene (C4H8) adsorbs on Si(100)-2×1 through a [2+2] cycloaddition, in which the translational and rotational ordering of the Si dimers is extended to the adsorbed molecule.2 However, in the case of the closely related organic compound pyrroline (C4H7N) an additional reaction scheme has been observed, in which a Si-N bond is formed.3 Functionalization of the surface with anhydride groups can be achieved by the chemisorption of the five-membered, cyclic organic compound maleic anhydride on Si(100)-2×1.4 A combined near edge X-ray absorption fine structure (NEXAFS) and high-resolution electron energy loss spectroscopy (HREELS) study of maleic anhydride on Si(100)-2×1 has shown that, at low dosages, molecules adsorb through a rehybridization of the carbon atoms in the olefin bond from sp2 to sp3.5 The observed rehybridization points to reactions I or II (Scheme 1), in which maleic anhydride adsorbs above a silicon dimer through a direct [2+2] cycloaddition or in an inter-row adsorption site between adjacent dimers in a [1+2+1] addition, respectively. At higher dosages, an additional reaction path arises, in which the unsaturated molecule bonds to one side of the silicon dimer with the olefin bond preserved and formation of an Si-H bond (reaction III), which has been attributed to the presence of surface defects.5 However, the HREELS and NEXAFS studies unambiguously rule out chemisorption through cleavage of the anhydride group.4,5 In this paper, we present STM studies of the initial adsorption of maleic anhydride on Si(100)-2×1 at room temperature, to identify the adsorption site of the unsaturated molecule. In particular, we were interested in finding evidence for a weakening of silicon dimer bonds which has been suggested in the case of a direct [2+2] addition reaction.6,7
The experiments were carried out in an UHV system (base pressure < 10-10 mbar) equipped with STM (Omicron). For the STM study, tungsten tips were used which were prepared by an electrochemical etching technique. The filled state STM images were taken at a tunneling current of 0.2 nA. Samples were cut from on-axis, one-sided polished, wafers (P doped) with a resistivity of 1-3 Ωcm. Clean Si(100)-2×1 was prepared by heating the silicon crystal to 1500 K while maintaining a pressure of
