Interfacial Mechanical Properties of $n$ -Alkylsilane Monolayers on Silicon Substrates

The interfacial properties of
$n$
-alkylsilane monolayers on silicon were investigated by normal force spectroscopy, lateral force measurements, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Monolayers of
$(\hbox{CH}_{3}{( \hbox{CH}_{2})}_{n - 1}\hbox{SiCl}_{3})$
with chain lengths
$n = 5$
, 8, 12, and 18 were prepared and NEXAFS spectra were used to compute the dichroic ratio,
$R_{\rm I}$
. As
$n$
decreased from 18 to 5, the film structures change from ordered
$(R_{ \rm I} = 0.41)$
to disordered
$(R_{\rm I} = 0.12)$
states. Normal force spectroscopy data were analyzed with a modified elastic adhesive contact model to extract Young's modulus,
$E_{\rm film}$
, and the work of adhesion,
$w$
, of the film;
$E_{\rm film}$
decreased from 1.2 to 0.67 GPa, and
$w$
increased from 48.6 to 60.1
$\hbox{mJ}\cdot\hbox{m}^{-2}$
as
$n$
decreased from 18 to 5. Lateral force measurements quantified the reduction in friction via an interfacial shear strength,
$\tau$
, and a lateral deformation analog,
$\eta$
. Monolayer adsorption reduced
$\tau$
from 3500 MPa for
$\hbox{SiO}_{2}$
to less than 50 MPa for
$n = 12$
and 18 alkylsilanes and was dependent on contact pressure. Conversely,
$\eta$
was pressure invariant, with values of
$\approx$
3500 MPa for
$n = 5$
and 8 and
$\approx$
1000 MPa for
$n = 12$
and 18.
$\hfill$
[2012-0165]