Role of Silicon Nanowire Diameter for Alkyl (Chain Lengths C1–C18) Passivation Efficiency through Si–C Bonds

Abstract

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 (C1-9)-Si NW > (C14-18)-Si NW, i.e., the shorter the alkyl chain, the greater the Si-C binding energy. However, the oxidation resistance of the (C2-18)-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.