Vv. Tsukruk et Vn. Bliznyuk, ADHESIVE AND FRICTION FORCES BETWEEN CHEMICALLY-MODIFIED SILICON AND SILICON-NITRIDE SURFACES, Langmuir, 14(2), 1998, pp. 446-455
We report the results of probing adhesion and friction forces between
surfaces with functional terminal groups with chemically modified scan
ning probe microscopy (SPM) tips. Surfaces with terminal groups of CH3
, NH2, and SO3H were obtained by direct chemisorption of silane-based
compounds on silicon/silicon nitride surfaces. We studied surface prop
erties of the resulting self-assembled monolayers (SAMs) in air and aq
ueous solutions with different pHs. Work of adhesion, ''residual force
s'', and friction coefficients was obtained for four different types o
f modified tips and surfaces. Absolute values of the work of adhesion
between various surfaces, W-ad, were in the range 0.5 - 8 mJ/m(2). The
work of adhesion for different modified surfaces correlated with chan
ges of solid-liquid surface energy estimated from macroscopic contact-
angle measurements. Friction properties varied with pH in a register w
ith adhesive forces showing a broad maximum at intermediate pH values
for a silicon nitride/silicon nitride mating pair. Similar broad maxim
a were observed in the acid range for a NH2-terminated SAM and in the
basic range for a SO3H-terminated SAM. This behavior can be understood
considering the changes of the surface charge state determined by the
zwitterionic nature of silicon nitride surfaces with multiple isoelec
tric points.