Functionalization of single-crystal silicon and silicon carbide surfaces via amine and alcohol groups
AuthorAdhikari, Lekh Nath
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Reactions of simple amines, alcohols, ethers, and bi-functional molecules on the Si(100)-(2x1) surface were studied theoretically and experimentally. Additionally, theoretical calculations were performed for the interaction of several of these types of molecules on the 3C-SiC(100), 4H-SiC(1000), and 6H-SiC(1000) surfaces. The adsorption and desorption experiments on the Si(100)-(2x1) surface were performed in an ultra-high vacuum (UHV) chamber using Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS). Theoretical studies were performed for the adsorption of these molecules onto clusters that are representative of the Si(100)-(2x1), 3C-SiC(100)-(3x2), 4H-SiC(1000)-(2x2), and 6H-SiC(1000)-(3x3) surfaces using the Gaussian suite of programs.AES studies showed a rapid increase in surface carbon and nitrogen signals for all amines studied and a similar increase in carbon and oxygen signals for all alcohols studied. The signals appear to level off with increasing dose in all cases, indicating surface saturation after room temperature adsorption of these molecules onto the Si(100)-(2x1) surface. Primary amines and alcohols give a saturation coverage of roughly one half of a monolayer, indicating one molecule reacting per silicon surface dimer, while mixed alcohol/amines (3-amino-propanol) and ethers, such as diethyl ether, dihydrofuran, and tetrahydrofuran, appear to have lower saturation coverages that depend on the identity of the adsorbing species.All saturated primary amines (methylamine, ethylamine, and propylamine) studied showed a parent ion desorption peak in TDS, along with an imine formation channel. These are interpreted as arising from surface-bound alkyl amidogen radicals resulting from N-H bond cleavage upon adsorption on the Si(100)-(2x1) surface. Hydrogen elimination from a surface bound alkyl containing radical appears to be a common decomposition reaction, yielding imines in the case of adsorbed saturated primary amines, aldehydes in the case of adsorbed saturated alcohols, and alkenes in the case of adsorbed saturated ethers. Unsaturated amines and alcohols, however, appear to react with the surface to produce alkene products upon TDS, which must involve hydrogen addition instead of elimination. The energetics for these reactions, as revealed through TDS, are sensitive to the exact nature of the adsorbing radical species.Theoretical studies of the interaction of amines and alcohols on the cubic SiC(100)-(3x2) surface showed similar results to those observed for these molecules on the Si(100)-(2x1) surface. However, the adsorbed products on the cubic SiC(100) surface were predicted to be slightly more stable than those on the Si(100) surface in all cases. Hexagonal SiC surfaces (4H- and 6H-) appear to be less reactive with water and ammonia than that of cubic surface based on these computational results.