STM of HOPG - Nanoscience Instruments

STM of HOPG

I. Introduction
The purpose of this experiment is to operate the scanning tunneling microscope (STM) in

both low and high-resolution modes, with the ultimate goal of imaging the surface of graphite at
the atomic level. Important aspects of this experiment include image collection, processing,
analysis and interpretation.

Graphite is thermodynamically the most stable of the two common allotropic forms of
carbon (diamond is the other form). The structure of graphite (left) and diamond (right) are
shown in Figure 1 below.

Figure 1. The structure of graphite and diamond. The C-C bond distances are also shown.
Graphite is black and has a layered structure, with each graphene layer composed of a planar
arrangement of fused hexagonal benzene rings. Therefore, graphite is highly anisotropic and
hydrophobic. The trigonal bonding in each graphene sheet involves overlap of carbon sp2 hybrid
orbitals in the plane, whereas the overlap of carbon 2pz orbitals produces delocalized rings of π
electrons lying above and below each benzene ring, which makes graphite a good electrical
conductor. The graphene layers are bonded to each other by weak van der Waals (London
dispersion) forces, so that they can easily slide over one another, which is why graphite is soft and
slippery. In contrast, diamond is composed of a tetrahedral network of covalently-bonded carbon

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atoms, where the single covalent bonds are formed by the overlap of sp3 hybrid orbitals on each
carbon atom.

Graphite is an important material that has many uses. Natural graphite is used to make
pencil “leads”, and synthetic graphite is used to make battery electrodes, brushes for motors and
generators, and solid lubricants. Carbon black and soot are composed of extremely small crystals
of imperfect (amorphous) graphite. Carbon black is produced by burning hydrocarbons in the
absence of air to “crack” the hydrocarbon. Coal is graphitic in the sense that it contains fused
benzene rings, with the eventual metamorphosis product being graphite. Coke is amorphous
graphite prepared by heating coal in the absence of air, and it is used to make iron. Carbon fibers
and cloth are prepared by heating textiles like rayon. At low temperatures, the textile fiber
pyrolyzes, i.e., it decomposes into carbon and gaseous products. At higher temperatures, the
carbon becomes graphitized. The resulting carbon fibers have great strength and are also used in
composites such as carbon-fiber-reinforced epoxy plastic used in aircraft parts, golf clubs, fishing
rods, etc. Carbon cloth is used in spacecraft to dissipate atmospheric heat.

This experiment focuses on a form of graphite called highly oriented pyrolytic graphite
(HOPG). HOPG is highly oriented with respect to the layer-stacking direction. HOPG has been
chosen for this experiment because it can be easily cleaved to expose a fresh conductive surface
and can also serve as a substrate for other materials that can be imaged by STM. In addition,
atomic-level images of HOPG can be used to calibrate the STM for high-resolution imaging.

From the images obtained in this experiment, you can measure and interpret the surface
roughness, microscopic surface features, and the arrangement and interatomic distances of the
carbon atoms on the HOPG surface.

NOTE: Useful information on graphite and other forms of carbon can be found in the INVSEE
Allotropes of Carbon Module.

II. Sample Preparation

Obtain a sample of HOPG from your instructor. Expose a fresh surface for the SPM
experiment by pressing a piece of double-stick tape to the HOPG surface and then gently peeling
it off the surface. The resulting HOPG surface should appear flat to the naked eye. Repeat the
above surface-peeling procedure until the HOPG surface appears to be flat. Mount the HOPG
sample onto the STM base and proceed to set up and operate the STM, as described in the STM
Operating Instructions.

III. Image Acquisition

The following low and high-resolution images should be collected in this experiment and
included with your laboratory report:

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1. 10 µm x 10 µm low-resolution image.
2. 5 µm x 5 µm low-resolution image.
3. 1 µm x 1 µm low-resolution image.
4. 100 nm x 100 nm high-resolution image.
5. 25 nm x 25 nm high-resolution image.
6. 10 nm x 10 nm high-resolution image.
7. A high-resolution image of your choice to show the best atomic-level resolution
possible.
NOTES: 1. Each image should be optimized prior to image capture, as described in the STM
Operating Instructions.

2. If you encounter difficulty achieving atomic resolution, then try changing the X and
Y offsets to scan another region. If you are still not successful, then try minimizing the gain
values and increasing the scan rate. If neither of the above adjustments result in atomic
resolution, then consult your instructor.

To serve as a guide, an example of a high-resolution STM image of graphite is shown in
Figure 2.

Figure 2. 5 nm x 5 nm STM image of graphite.

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IV. Image Processing

For the first three low-resolution images, it may be necessary to use the “Flatten”
function available in the “Modify” pull-down menu to correct for any microscopic sample tilt,
which is evidenced by a continuous change in contrast across the image in one direction.
However, terraces and step edges may be more clearly defined in height profiles if the “Flatten”
function is not used.

For the last four high-resolution images, it may be necessary to use the “Spectrum 2D”
fast Fourier transform function, as described in the STM Operating Instructions.

Consult your instructor if additional image processing is necessary.

V. Image Analysis

1. Measure the average surface roughness of each image using the “roughness”
function (rms value) available under the “Analysis” pull-down menu. Prepare a table to
summarize your data as well as a graph of the surface roughness as a function of scan
size.
2. Use the “Section” function available under the “Analysis” pull-down menu to
measure horizontal and vertical distances for your images. For the best low-resolution
image, draw a line across the image to plot the depth profile along that line. Measure
the length of any horizontal features (terraces) as well as the height of any vertical
features (steps). Summarize your results in a table.

3. For the best high-resolution image, draw a line through a row of the primary features
(atoms) in the image to plot the depth profile along that line. Measure the horizontal
distances for five different adjacent carbon atoms in the image to determine the average
distance between the carbon atoms. Also measure the height of five different carbon
atoms in the image to determine the average carbon-atom height. Summarize your
results in a table.

4. For the best high-resolution image, measure all the angles defined by a particular
carbon atom and two of its nearest neighbors. To measures angles, access the "Top
View" function available under the "View" pull-down menu. On the second monitor,
click the "Angle" function. Using the left mouse button (left click), click a point on the
image to represent the center of the angle. Left click a second position with the mouse
to define one side of the angle. Left click a third point to define the second side of the
angle. The measured angle will appear on a white strip at the bottom of the monitor.
Click the right mouse button to terminate the angle-measuring function. Repeat the

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measurement two more times using different carbon atoms to determine the average
angles. Summarize your results in a table.

VI. Image Interpretation

1. (a) What is the origin of the surface roughness for the best low-resolution image?

(b) If the distance between adjacent graphene sheets is 3.35 Å, how many graphene
layers have been removed for each of the step heights measured, if any, for this image?

2. (a) What is the origin of the surface roughness for the best high-resolution image?

(b) Discuss the plot of surface roughness versus scan size in both the low and high-
resolution regions.

3. (a) Is the symmetry of the atomic arrangements in the best high-resolution image the
same as that expected for the hexagonal rings in graphite?

(b) How do they differ?

4. (a) If the distance between nearest-neighbor carbon atoms in a hexagonal graphene
ring is 1.42 Å, construct the graphene ring and superimpose it on the best high-
resolution image. Are the carbon-atom positions coincident?

(b) Which carbon atoms in the hexagonal graphene ring appear in the best high-
resolution image?

(c) On the basis of the three-dimensional structure of graphite, try to explain your
answers to (a) and (b). (Hint: You may find the Avogadro’s Number section of the
INVSEE Allotropes of Carbon Module helpful to answer this question.)

5. (a) Based in your average carbon-carbon distance and angles and assuming that the
C-C distance in a hexagonal graphene sheet is 1.42 Å, draw a figure showing the
positions of the carbon atoms imaged in this experiment as open circles.

(b) Show the positions of any carbon atoms not imaged in this experiment as shaded
circles. How would you propose to image these carbon atoms?

6. Compare your average carbon-atom height with the radius of the carbon atom in the
graphene rings. What are the implications of this comparison?

NOTE: The INVSEE Allotropes of Carbon Module may be helpful to interpret your
images.

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VII. Questions
1. (a) What are the sample requirements for a STM experiment?
(b) What property of the sample is measured in a STM experiment?
2. What are the basic operating principles of a scanning tunneling microscope operating
in (a) the constant-current mode and (b) the constant-height mode? (c) Identify one
advantage and one disadvantage of each mode of operation.
3. Draw a basic functional diagram of a scanning tunneling microscope operating in the
constant-current mode and explain how an image is obtained.
4. (a) Compare the structures, bonding, and electrical and mechanical properties of
graphite and diamond.
(b) Could STM be used to image both materials? Why?
5. Answer the following questions from the image provided in Figure 2:
(a) What is the symmetry of the nearest-neighbor carbon atoms surrounding a
particular carbon atom?
(b) What are all the angles defined by a particular carbon atom and two of its nearest
neighbors?
(c) What is the distance (in Å) between two adjacent carbon atoms? Is this distance
in good agreement with that shown in Figure 1?

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