Methyl monolayers improve the fracture strength and durability of silicon nanobeams

Monolayer-thick coatings have a significant effect on the fracture strength and durability of 210-nm-thick, smooth (∼0.4nm rms roughness), single crystal silicon nanobeams. The initial Weibull fracture strength of beams terminated with a methyl (CH3) monolayer was 18.2GPa. This strength did not degrade after a 23-day exposure to air. In contrast, beams terminated with a monolayer of hydrogen atoms were initially weaker than methyl-terminated beams, and their strength degraded rapidly in air. After a 23-day air exposure, the strength of H-terminated beams was reduced by at least 30%. Since strength durability is correlated with the oxidation resistance of the monolayers, the degradation of H-terminated beams is attributed to the formation of oxide nuclei that act as local stress concentrators.