AVS 56th International Symposium & Exhibition
    Surface Science Monday Sessions
       Session SS1-MoM

Paper SS1-MoM2
Electronic and Steric Effects in the Reactions of Isocyanates and Isothiocyanates at the Ge(100)-2x1 Surface

Monday, November 9, 2009, 8:40 am, Room M

Session: Vibrational Spectroscopy and Surface Reactions
Presenter: P.W. Loscutoff, Stanford University
Authors: P.W. Loscutoff, Stanford University
K.T. Wong, Stanford University
S.F. Bent, Stanford University
Correspondent: Click to Email
Organic functionalization of semiconductor surfaces has seen increased interest in recent years, due to the ever-decreasing feature sizes of microelectronics, and the advance of organic electronics. Advances in both of these fields require precise interface control at atomic dimensions, and modification of surfaces with organic molecules provides the flexibility necessary for use in a broad spectrum of applications. In this study, we examine the surface products formed by the reactions of various isocyanates and isothiocyanates with the Ge(100)-2x1 surface. Although these two moieties differ only by substitution of an oxygen atom with a sulfur atom, they display markedly different reactivity at the reconstructed germanium surface. Using a combination of infrared spectroscopy, x-ray photoelectron spectroscopy and density functional theory, we examine the reactions of phenyl, tert-butyl, and ethyl isocyanate and the corresponding isothiocyanates. Both phenyl isocyanate and phenyl isothiocyanate form multiple adsorption products at the surface. The isocyanate reacts with a Ge surface dimer to form a [2+2] cycloaddition product across the C=N bond to produce a surface-bound carbonyl in addition to other products. In contrast, phenyl isothiocyanate reacts across the C=S bond to form a [2+2] cycloaddition product, in addition to reaction across the C=N bond. When the tert-butyl group is substituted for the phenyl ring, the reactivity changes such that dative-bonded products with an intact, surface-bound isocyanate or isothiocyanate group are observed. Upon reaction of ethyl isocyanate and ethyl isothiocyanate at the Ge(100)-2x1 surface, yet another product distribution is observed, which lacks dative-bonded products but demonstrates time-dependence. The wide array of products observed for this set of isocyanate and isothiocyanate molecules demonstrates the influence of the molecular chain on the surface reactivity of these groups, and displays a versatility that could prove useful for tuning reactivity to achieve a desired surface product.