Orientation of 5CB Molecules on Aligning
Substrates Studied by ARXPS
Here we
report studies of alignment of LC molecules adsorbed on several
aligning surfaces by Angle
Resolved X-ray Photoelectron Spectroscopy (ARXPS) which
allowed us to obtain direct information about orientation of the
adsorbed molecules on the aligning surface by elemental and chemical
analysis with a nanometer depth resolution
We studied the orientation of
pentyl-cyano-biphenyl (5CB) on four different aligning substrates: bare
glass, polyvinylalcohol (PVA), NaCl plates and fused quartz.
 |
In
ARXPS, the effective sampling depth is varied by changing the angle of
the sample with respect to the detector. The actual depth sampled, d,
is given by the equation:
d=3λ sin θ;
where λ is the effective attenuation
length of the photoelectron and, θ
is
the take-off
angle between the sample surface and the analyzer. Comparing relative intensities at low and high take-off angles indicates whether a species is enriched or depleted in the surface region. This allows for determination of the molecular orientation to the surface. For 5CB, comparison of total C and N elemental signals at different take-off angles will indicate whether the 5CB molecule is attached to the substrate by cyano group or alkyl tail, or it has no preferential orientation. |
The
orientation of 5CB molecule on different substrates was elucidated from
the N/C ratio plotted as a function of take-off angle.
 |
A small change in N=C ratio with a change of take-off angle for NaCl and glass substrates indicates a low tilt of 5CB molecules on these substrates and a tendency to be oriented with cyano-groups towards the substrate. For the PVA substrate, XPS reveals evident enrichment of N-atoms near the substrate surface. This indicates that 5CB molecules are oriented with the polar cyano-groups down the substrate. This result is consistent with expectations, as the C-OH groups of PVA are very attractive for the polar cyano- fragment of 5CB. The highest slope of the change of N=C ratio with decreasing H is observed for the quartz sample. This is reasonable due to strong interactions between cyano- groups and the polar quartz surface. |
It was found that different
surfaces (fused quartz, bare glass, NaCl, PVA) orient the adsorbed 5CB
molecules with different tilt angles while providing qualitatively the
same type of LC alignment in the bulk. Previous studies showed that LC
bulk does not strongly affect the orientation of adsorbed molecules on
the substrates. Therefore, these results demonstrate that the
orientation of LC molecules in the bulk can strongly differ from the
orientation of the first adsorbed monolayer and it is determined
by
integral anisotropic action of the surface and interface over interface
region.
For complete report: Artyushkova, K; Fulghum, JE; Reznikov, Y Mol.
Cryst. Liquid Cryst. ; 2005; v.438, p.1769-1777