Scanning Tunneling Microscopy of Graphite
S.M. Theberge, S.F. Nelson, and T.W. Shattuck
Colby College, Waterville, Maine
Further Reading (besides the Introduction below): Section 7.5 in Zumdahl (pp. 302-305).
INTRODUCTION
In this laboratory, you will determine the bond length between carbon atoms in graphite using
scanning tunneling microscopy (STM). This type of microscope is very different from light
microscopes that are used for objects (i.e. cells) in the micrometer (10-6 m) range. With an STM,
it is possible to detect single atoms. Atoms are only one or two angstroms (Å) across. An
angstrom is 10-10 meters or 0.1 nanometers (10-9 m). Scanning tunneling microscopy uses the
phenomenon of electron tunneling as a very sensitive measure of sample height and surface
electron density. The STM tip is placed in close proximity to the surface of the sample (i.e. 10
Å). A small voltage is placed across the STM tip and the sample. At such short distances a very
small current can be measured. This tunneling current is very sensitive to the thickness of the
dielectric layer between the tip and the surface. The dielectric layer is either just air or a vacuum.
Our instrument is open to the atmosphere. Small variations in sample height cause a change in
the thickness of the air layer between the tip and the surface, which causes a large change in the
tunneling current. In STM, the tip is made from platinum/iridium (Pt/Ir) wire. This soft metal
alloy can be drawn out to a very fine tip. The tip is moved across the surface of the sample and
the current at the different locations is measured. Figure 1 shows the interaction of the tip with
the surface being imaged.
Figure 1. Representation of the tip-surface interaction.
The large spheres represent atoms in both sample and tip. The small dots represent current
flowing through the system. Strictly speaking these small dots are not electrons flowing on the
outside of both surfaces; rather, the dots represent the electron density as a result of the overlap
of the atomic wavefunctions of the tip and the surface. To record an image the tip is scanned
across the surface in a raster pattern, Figure 2.
Figure 2. The tip is moved across the surface in a raster pattern, just as in a CRT style TV or
computer monitor.
With STM, we are not magnifying as in light microscopy, we are creating images from the
interactions of the electron cloud around the atoms in the sample with the probe tip of the STM,
Figure 3. STM can be done in two different modes, constant height and constant current modes.
In constant height mode, the tip is moved across the surface at a constant height and the variation
of the tunneling current is recorded as a function of tip position. As the surface height or electron
density increases, the tunneling current increases giving rise to bright spots on the recorded
image. In constant current mode, the tip height is changed to maintain a constant tunneling
current. In this mode, as the surface height or electron density increases, the tip must be retracted
to maintain a constant current. The constant current is maintained by a feedback loop which
controls a piezoelectric actuator that can move the tip closer or farther away as necessary. By
measuring a known reference surface, the relationship between the control voltage applied to the
actuator and the tip height can be determined. In constant current mode, a bright area on the
image represents a higher tip position.
In STM, the tip must be positioned accurately in the x and y directions to form the raster
pattern to move the tip over the sample. The tip height, in the z-direction, must also be positioned
accurately. These positioning tasks are done using the piezoelectric effect. Some metal oxides,
for example BaTiO4, exhibit the piezoelectric effect. When pressure is applied to such a ceramic,
the dimensions change and the result is a small voltage difference across the solid. This effect is
used in microphone elements, acoustic guitar pickups, and in load cells for electronic balances.
The converse is also true; when a voltage is applied across a piezoelectric solid, the dimensions
change. The change in dimension with applied voltage is quite small; however, the effect is quite
precise. For STM, the scanning dimensions and the tip height are in the angstrom to nanometer
range, so the piezoelectric effect is particularly suited for careful computer controllable
positioning. In STM, the z-direction is actually more sensitive than the x and y positioning. So
the height or the sample, or the corresponding changes in electron density can be determined to
fractions of an angstrom. An STM tip can also be used to move atoms around on a surface. This
ability has suggested the possibility of molecular synthesis on an atom-by-atom basis. The mass
yield of such a process is a little disappointing, however.
In an ideal experiment, the probe tip would be drawn out to a single atom as in Figure 3.
Electrons in orbitals on the surface of the tip interact with the electrons in orbitals on the surface
of the sample. With a single atom on the tip, very accurate sharp images would be obtained.
Because forming a tip with only a single atom is not possible, the images produced by STM are
often quite fuzzy (as in Figure 5). Mathematically, the ideal image is convoluted by the atom
distribution on the tip. Post-acquisition treatment of the data is necessary to increase the
resolution of the images.
Figure 3. Schematic representation of electron cloud interaction between sample and tip in ideal
imaging situation.
Graphite has a layered structure. The individual layers are made from hexagonal arrays of sp2
hybridized carbon atoms, Figure 4. In pyrolytic graphite, large crystallites are laid down parallel
to the surface of the sample. A fresh, atomically flat surface can be prepared by placing a piece
of adhesive tape on the surface and then simply pealing off the tape.
Figure 4. The layers in graphite are hexagonal arrays of sp2 hybridized carbon atoms
.The raw STM image of graphite is given in Figure 5. The lighter regions represent high points
on the surface or areas of larger electron density and the darker regions represent lower points on
the surface or lower electron density.
Figure 5. Raw STM image of graphite. Total area represented is 100 Å2.
The image is sensitive to both sample height and electron density. However, since pyrolytic
graphite has an atomically flat surface, the sample height is caused only by the profile of
individual atoms in the flat hexagonal rings. Individual atoms can not be resolved however. The
bright and dark areas on the image correspond to alternate atoms. The electron density on the
surface is caused by the variations of the electron density in the delocalized π orbitals. This
variation in π electron density alternates for every other atom across the surface of the crystallite,
Figure 6.
Figure 6. The π electron density is alternatively larger and smaller on alternate atoms.
PROCEDURE
Prepare a fresh surface for the pyrolytic graphite sample using some adhesive tape. Your
instructor will show you how to prepare a new tip using Pt/Ir wire. Note that in our instrument,
the sample is moved instead of the tip. The tip remains stationary. However, the resulting image
is the same whether the tip or the sample is moved.
1.
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8.
Under the WINDOW menu, create a new window. Under the COLLECT menu select
SCAN CONTROL. Click the CONFIGURE button to configure instrument. Make sure
all DELAYS are set to 0.00. The DATA TYPE should be set to CURRENT. PLANE
REMOVAL should also be selected. SCAN MODE should be continuous. Click OK and
set the Z GAIN to 25.
On the instrument control box, set the magnification to 1000X. Adjust BIAS VOLTAGE
to 0.03 V. Set the REFERENCE CURRENT to 5nA. Adjust the proportional gain to the
9 o’clock setting. Set the INTEGRATOR GAIN to 3 o’ clock, and set the
DIFFERENTIATOR GAIN to 7 o’ clock.
Using the camera attached to the TV screen move the sample close to the tip using the
COURSE APROACH switch. When you are close to the surface push the AUTO
APPROACH switch up and allow the instrument to move into position. When the
FEEDBACK ACTIVE light appears. Continue with analysis.
Begin acquisition by pressing the SCAN button.
Adjust the LOWPASS FILTER (right-most knob on instrument box) until the image
produced is of the best quality. When satisfied, type C to capture the image.
Under the FILE menu SAVE your file (remember the name!). Using the FILTER
menu, choose filtering technique(s) to enhance your image. When you are satisfied,
under the FILE menu OPEN file 500xgr.img and compare it to your file. Did you
optimize resolution? Use other techniques to optimize your file if necessary.
Repeat steps 3-5 for 2000X, and 5000X. The data files to compare are 1000xgr.img,
2000xgr.img, and 5000xgr.img respectively. Under the FILE menu print the
2000X and 5000X images.
Under the ANALYSIS menu, choose CROSS SECTION and obtain information about
the distances between high points in your 2000X and 5000X images. The lab instructor
will assist with this process.
Keep track of the different filters performed to optimize resolution of your images.
REPORT
Part 1.
Include a copy of the graphite images you produced at 2000X and 5000X magnification.
Indicate the filtering techniques that you found the most useful in improving the resolution of the
images. Explain why you feel they worked the best. Assuming a 120° angle for a C-C-C bond on
the surface of graphite (as in Figure 5) calculate the bond distance. What is the average bond
distance between carbon atoms in graphite from your measurements? Compare your results with
the literature value. Calculate the % error. Speculate on the sources of the error in your
experiment.
Answer the following questions
1. What anti-vibrational precautions did you observe in Keyes 108D to ensure isolation for the
instrument? Why is vibration isolation important?
2. In terms of electronegativity, how would the surface of silicon appear compared to graphite?
Figure 7. Two possible STM tips.
3. Which tip in Figure 7 would give better results? What problems would the other tip cause?
Part 2.
Create a chunk of graphite using Spartan. Use the AM1 level of theory so that you can work
with many atoms. Plot an electron density surface. Do you observe the alternating pattern of
electron density for pairs of carbon atoms as in the STM image? If not suggest a reason why this
pattern might not be observed in the calculations.
Literature Cited:
1. Giancarlo, Leanna C.; Fang, Hongbin; Avila, Luis; Fine, Leonard W.; Flynn, George W.
Molecular Photography in the Undergraduate Laboratory: Identification of Functional Groups
Using Scanning Tunneling Microscopy J. Chem. Educ. 2000 77 66. (January 2000)
2. Rapp, Carl Steven Getting Close with the Instructional Scanning Tunneling Microscope J.
Chem. Educ. 1997 74 1087. (September 1997)
3. Coury, Jr., Louis A.; Johnson, Mario; Murphy, Tammy J. Surface Analysis by Scanning
Tunneling Microscopy J. Chem. Educ. 1995 72 1088. (December 1995)
4. Braun, Robert D. Scanning tunneling microscopy of silicon and carbon (MODLAB). J.
Chem. Educ. 1992 69 A90