Multi-asperity Nanotribology of Self-Assembled Monolayers Grafted on Silicon Wafers Displaying Various Crystallographic Orientations and Nanostructures

Abstract

Micro/nanotribological study of self-assembled monolayers (SAMs) derived from n-alkyltrichlorosilanes deposited on silicon wafers displaying various crystallographic orientations—Si (100), Si (111) and Si (110)—has been conducted using a ball-on-disc nanotribometer. The parameters that have been varied are (i) the alkyl chain length, (ii) the tribological parameters (i.e. the normal load, the sliding velocity, the sliding distance and the relative humidity level) and (iii) some surface characteristics of the silicon substrates (i.e. crystallographic orientation, roughness, and thickness of the amorphous native oxide). Experimental results show that the key parameter controlling the tribological behaviour is not the alkyl chain length as generally reported for adsorbed and grafted monolayers, but rather the film’s homogeneity—i.e. the degree of packing and the surface coverage of the monolayer—in connection with the crystallographic orientation of the substrate.