Role of silicon nanowire diameter for alkyl (chain lengths C₁-C₁₈) passivation efficiency through Si-C bonds

The effect of silicon nanowire (Si NW) diameter on the functionalization efficiency as given by covalent Si-C bond formation is studied for two distinct examples of 25 ± 5 and 50 ± 5 nm diameters (Si NW₂₅ and Si NW₅₀, respectively). A two-step chlorination/alkylation process is used to connect alkyl chains of various lengths (C₁-C₁₈) to thinner and thicker Si NWs. The shorter the alkyl chain lengths, the larger the surface coverage of the two studied Si NWs. Increasing the alkyl chain length (C₂-C₉) changes the coverage on the NWs: while for Si NW₂₅ 90 ± 10% of all surface sites are covered with Si-C bonds, only 50 ± 10% of all surface sites are covered with Si-C bonds for the Si NW₅₀ wires. Increasing the chain length further to C₁₄-C₁₈ decreases the alkyl coverage to 36 ± 6% in thin Si NW₂₅ and to 20 ± 5% in thick Si NW₅₀. These findings can be interpreted as being a result of increased steric hindrance of Si-C bond formation for longer chain lengths and higher surface energy for the thinner Si NWs. As a direct consequence of these findings, Si NW surfaces have different stabilities against oxidation: they are more stable at higher Si-C bond coverage, and the surface stability was found to be dependent on the Si-C binding energy itself. The Si-C binding energy differs according to (C_1-9)-Si NW > (C_14-18)-Si NW, i.e., the shorter the alkyl chain, the greater the Si-C binding energy. However, the oxidation resistance of the (C_2-18)-Si NW₂₅ is lower than for equivalent Si NW₅₀ surfaces as explained and experimentally substantiated based on electronic (XPS and KP) and structure (TEM and HAADF) measurements.