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The mechanism and intermediates of a Diels–Alder-type cycloaddition reaction between dienes and the silicon dimers of a Si(100)-2×1 surface, which was theoretically predicted by Doren and Konecny, have been investigated. The reactions of 1,3-butadiene and 2,3-dimethyl-1,3-butadiene were studied using multiple internal reflection infrared spectroscopy, thermal desorption spectrometry, and near edge x-ray absorption fine structure (NEXAFS) measurements. The results show that the compounds physisorb on Si(100)-2×1 at cryogenic temperature. Infrared studies of the room temperature adsorption of both dienes indicate that reaction leads to the formation of stable, chemisorbed Diels–Alder adducts. By NEXAFS measurements on 2,3-dimethyl-1,3-butadiene, we determine that the angle between the π orbitals of the reaction product and the Si(100)-2×1 surface is near 40°. Upon heating, the chemisorbed butadienes primarily decompose to form adsorbed carbon and hydrogen at the surface. Hydrogenation of chemisorbed butadienes by atomic hydrogen was also investigated, and conversion from the Diels–Alder geometry to [2+2] bonding is observed. This effect is attributed to cleavage of the Si–Si dimer bond upon atomic hydrogen exposure. Temperature dependent studies suggest the presence of an activation barrier for chemisorption at low temperature. The parallels between our experimental results and the theoretical studies of these reactions are discussed.

For over a quarter of a century the hydrogen-terminated Si(111) single-crystalline surface has been the gold standard as a starting point for silicon surface modification chemistry. However, creating a well-defined and stable interface based on Si-N bonds has remained elusive. Despite the fact that azides, nitro compounds, and amines do lead to the formation of surface Si-N, each of these modification schemes produces additional carbon- or oxygen-containing functional groups that in turn react with the surface itself, leaving contaminants that affect the interface properties for any further modification protocols. We describe the preparation of a Si(111) surface functionalized predominantly with Si-NH-Si species based on chlorination followed by the room temperature ammonia treatment utilizing NH(3)-saturated tetrahydrofuran (THF). The obtained surface has been characterized by infrared spectroscopy and X-ray photoelectron spectroscopy. This analysis was supplemented with DFT calculations. This newly characterized surface will join the previously established H-Si(111) and Cl-Si(111) surfaces as a general starting point for the preparation of oxygen- and carbon-free interfaces, with numerous potential applications.