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.

Parts of this research have been published in:
Parks, D. C., et al., Physical Review Letters, 70, 607-610 (1993)
Walba, D. M., et al., Science, 267, 1144-1147 (1995)
F. Stevens, D. J. Dyer, D. M. Walba, Langmuir, 12, 436-440 (1996)
F. Stevens, D. J. Dyer, D. M. Walba, J. Vac. Sci. Technol. B, 14, 38-41 (1996)

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.


The length of the white line in the image is 0.254nm.

For more information on this project, contact Noel Clark.

 

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