Unique lack of chemical reactivity for 2,3-dimethyl-2-butene on a Si(100)-2×1 surface

Abstract

Adsorption of 2,3-dimethyl-2-butene on Si(100)-2×1 has been studied by a combination of multiple internal reflection Fourier transform infrared spectroscopy and computational investigations implementing density functional theory (DFT). Since the previously studied olefins have been shown to form a di-σ product on this surface following [2+2] cycloaddition, it was also initially expected for 2,3-dimethyl-2-butene. Infrared spectra taken at 100K show that 2,3-dimethyl-2-butene adsorbs on the surface molecularly at this temperature. Heating the surface to room temperature left no indication of a chemisorbed product. Large doses at room temperature did not produce any observable absorption bands in the infrared spectrum, indicating that [2+2] cycloaddition of 2,3-dimethyl-2-butene does not occur. This assessment was verified by the Auger electron spectroscopy studies confirming that neither room temperature exposure nor annealing to 800K produced any carbon remaining on this surface. These experimental observations of the absence of a chemical reaction between an olefin and a very reactive silicon surface were substantiated by DFT investigation of the adsorption kinetics. The formation of two possible π-bonded precursors was considered, and the energies required to form the di-σ-bonded product from either one of these precursors were predicted to be substantially higher than the desorption barrier. Thus, 2,3-dimethyl-2-butene is a unique olefin that is very inert with respect to the Si(100)-2×1 surface, making it a desirable carrier gas or a ligand in the precursor molecules in a number of deposition processes involving silicon substrates.