Comparative Atomic Force Microscopy Study of the Chain Length Dependence of Frictional Properties of Alkanethiols on Gold and Alkylsilanes on Mica

A comparative study of the frictional properties of alkanethiols and alkylsilanes as a function of chain length is presented. The monolayers were produced by self-assembly on Au(111) and mica, respectively. The same tip was used for all the experiments, and freshly cleaved mica was used as a reference. For both types of films, the frictional forces depend strongly on the number of carbon atoms in the alkane chain (CH3−(CH2)n-1−R). Thiols and silanes give rise to similar frictional force for the same n when n > 11, while for n < 11 the behavior is markedly different; the silanes exhibit higher friction, larger than that for the thiols by a factor of ∼3 for n = 6. The increase in friction is attributed to the increased disorder that occurs when going from a thiol to a silane anchor or when decreasing n. It is proposed that disorder favors the increase of the number and type of low-energy modes (kinks, bending, distortions) that are available for excitation and energy dissipation.