A. Lio et al., COMPARATIVE ATOMIC-FORCE MICROSCOPY STUDY OF THE CHAIN-LENGTH DEPENDENCE OF FRICTIONAL-PROPERTIES OF ALKANETHIOLS ON GOLD AND ALKYLSILANES ON MICA, JOURNAL OF PHYSICAL CHEMISTRY B, 101(19), 1997, pp. 3800-3805
A comparative study of the frictional properties of alkanethiols and a
lkylsilanes as a function of chain length is presented. The monolayers
were produced by self-assembly on Au(lll) and mica, respectively. The
same tip was used for all the experiments, and freshly cleaved mica w
as used as a reference. For both types of films, the frictional forces
depend strongly on the number of carbon atoms in the alkane chain (CH
3-(CH2)(n-1)-R). Thiols and silanes give rise to similar frictional fo
rce for the same It when n > 11, while for n < 11 the behavior is mark
edly different; the silanes exhibit higher friction, larger than that
for the thiols by a factor of similar to 3 for n = 6. The increase in
friction is attributed to the increased disorder that occurs when goin
g 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-energ
y modes (kinks, bending, distortions) that are available for excitatio
n and energy dissipation.