STM Imaging of Liquid Crystals
It has been found that if a
liquid organic compound is placed on a very smooth, conducting surface
(graphite is most commonly used) the organic compound will in some cases
self-organize into a 2-D crystalline monolayer at the surface. When the
sample is then scanned an STM, the tip penetrates the bulk material (which
is insulating) until it gets close enough to the surface for a tunneling
current to flow. When the surface monolayer is thin enough to fit between
the tip and surface during scanning, so that the organic molecules modulate
the tunneling current in some (as yet poorly understood) way, an image
of the monolayer is obtained.
Liquid crystals seem to be
well-suited for STM study. While many other organic molecules have been
imaged by STM, liquid crystals, particularly the cyanobiphenyls, predominate
in the literature. It is tempting to speculate that the greater order
inherent in liquid crystals (relative to isotropic fluids) promotes the
formation of stable 2-D monolayers that can be imaged by STM.
Early work with 4-octyl-4'-cyanobiphenyl
(8CB) suggested that the tunneling current was greatest when the tip was
over aromatic regions of the molecules, and least when the tip was over
aliphatic regions of the molecules. This causes aromatic regions to image
bright by STM and aliphatic regions to image dark. All reported STM results
have supported this.
We have obtained near-atomic
resolution images of two-dimensional heteroepitaxial crystals composed
of relatively "functionally rich" chiral mesogens on graphite.
In the typical example shown above, each diagonal column comprises a pair
of molecules (only the aromatic cores are imaged bright).
This work is aimed at developing
an improved understanding of the commercially crucial phenomenon of liquid
crystal alignment by studying well-characterized surfaces.
For more information on this project, contact David Walba.
Atomic Force Microscopy
Atomic Force Microscopy (AFM)
was developed by Binnig, Gerber, and Quate in 1986. After the huge success
of STM, they realized it was quite possible to construct a spring that
would be sensitive enough to respond to inter-atomic forces, thus enabling
one to create a different species of SPM. Instead of tunneling current
between a tip and a sample, atomic force between this spring and a sample
is used as the feedback signal to control the microscope. All the remaining
part essentially remains the same as STM.
The atomic force is measured by the deflection of the tip and in turn, by the amount of displacement of the reflected laser beam off the tip. The picture below shows the schematic of the operation of an AFM. The reflected beam enters an array of photodiodes, which converts the amount of the displacement into an electric signal.
AFM nowadays has several operation
modes. Among them, the "contact" mode is most widely used. A
sample is placed upon a piezotube. In the contact mode, the feedback loop
maintains the atomic force between the tip and the sample constant by
adjusting the height of the piezotube, so that the piezotube follows the
contour of the surface of the sample exactly. The adjustment is recorded
in the computer controlling the microscope and is processed to produce
the 3-dimensional topography of the surface of the sample. Since AFM exploits
interatomic forces instead of tunneling current, it's not limited by the
conductivity of the sample. There is virtually no limitation on the species
of the sample it can image. It can operate even in fluid as well as in
gas. Consequently, the AFM emerged as one of the most important research
tools in biology where most of the samples are not conductive and require
a natural environment such as water.
Here are some examples of AFM images. Click on an image in view in larger size.
UV-patterned Self-Assembled Monolayer(SAM) on glass. The SAM is about 2.0nm high.
A molecular resolution image of Crystalline 8SI, a Liquid Crystal, on graphite. Note the hexagonal packing. Below is the image viewed from the top.
For more information on this project, contact Noel Clark.