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Class:
QUANTUM TUNNELING
Date:
Visual Quantum Mechanics
exploring the very small
ACTIVITY 2
Seeing the Very Small – The Scanning Tunneling Microscope In the previous activity, you observed several images of nanostructures and learned how nanostructures may be used in the future to create useful tools, such as enhanced computer technology. One of the methods used to fabricate nanostructures uses an instrument called the scanning tunneling microscope (STM) to move individual atoms on a surface. The scanning tunneling microscope is also used to produce the images of nanostructures similar to the ones you observed using the World Wide Web in Activity 1. Objectives After completing this activity, you should be able to: • describe how a scanning tunneling microscope works, and • understand the potential energy diagram for an electron in a scanning tunneling microscope. Before you study the scanning tunneling microscope, we need to determine why this special instrument is needed to create an image of a nanostructure.
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What is the wavelength range of visible light?
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How does this range compare with the typical size of a nanostructure observed in Activity 1?
Kansas State University @1996, Physics Education Research Group, Kansas State University. Visual Quantum Mechanics is supported by the National Science Foundation under grant ESI 945782. Opinions expressed are those of the authors and not necessarily of the Foundation.
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Based on your answers to the previous questions, can an ordinary optical microscope (where light reflects from the surface of a sample) be used to create an image of a nanostructure? Explain.
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Consider an electron moving at a speed of 104 m/s (which is the typical speed for an electron in a TV picture tube). What is the electron’s de Broglie wavelength? (electrons mass = 9.11x10-31 Kg, Planck’s Constant = 6.63x10-34 Js)?
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Based on your answer to the previous question, can an electron microscope be used to create an image of a nanostructure? Explain.
Kansas State University
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The scanning tunneling microscope is an instrument that can be used to observe nanostructures or other extremely small objects (1 nanometer or smaller) for which light or electron beams cannot be used. A photograph of a commercial scanning tunneling microscope is shown in Figure 2-1.
Figure 2-1: A scanning tunneling microscope used in a research laboratory. The tower in the center houses the probe tip and the sample.
The scanning tunneling microscope (STM) consists of an extremely fine metal tip called a “probe” which is placed a few tenths of a nanometer above the surface of the sample (Figure 2-2). By moving the tip over the surface, the scanning tunneling microscope “maps” the surface of the sample and generates an image. In this activity you will learn how the scanning tunneling microscope measures the surface.
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Probe Tip
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Sample Surface
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Figure 2-2: A diagram of the nanometer scale features on the surface of a sample and the probe tip of a scanning tunneling microscope suspended a few tenths of a nanometer above the surface.
A typical image taken by a scanning tunneling microscope is shown in Figure 2-3 below. The dark and light areas represent regions of high and low elevation respectively.
Figure 2-3: An STM image of carbon atoms on a graphite surface.
Other images taken by the scanning tunneling microscope can be seen on the World Wide Web at the sites given below. The colored images do not show the true color of the sample. The color scheme is chosen to represent the high and low regions on the sample surface. STM Image Gallery (http://www-i.almaden.ibm.com/vis/stm/gallery.html) [File Name: www2-1.htm] Tour the entire site but be sure to visit the “Lobby.” ♦ Describe the process by which the scanning tunneling microscope images were produced.
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Describe your favorite image. Why was it your favorite?
Harry’s STM Images (http://www.chem.wisc.edu/~harry/data.html) [File Name: www2-2.htm] ♦ Based on these images what kind of physical phenomena and effects can we observe with a scanning tunneling microscope?
Scanning Tunneling Microscope (http://www.lassp.cornell.edu/~ardlouis/dissipative/ stm.html) [File Name: www2-3.htm] ♦ Based on what you saw, explain the general working principle of a scanning tunneling microscope.
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Manipulating Atoms with an STM (http://www.lassp.cornell.edu/~ardlouis/dissipative/ atom_manip.html) [File Name: www2-4.htm] ♦ How can a scanning tunneling microscope be used to create the logo shown in Figure 2-4?
Figure 2-4: Logo of the Cornell University National Nanofabrication Facility made by manipulating atoms.
Sakurai Lab (STM Group) (http://fistm.imr.tohoku.ac.jp/research.html) [File Name: www2-5.htm] (The abbreviation UHV stands for Ultra High Vacuum) ♦ Describe some of the research that uses scanning tunneling microscopes.
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Under what conditions are samples for scanning electron microscopy prepared? Why?
Some other WWW sites displaying images taken using scanning tunneling microscopes are listed below. • UM-StL Scanned Tip and Electron Image Lab (http://newton.umsl.edu/stei_lab/ stei_lab.html) [File Name: www2-6.htm] Several color images, all on the main page. It takes some time for them all to load. Weiss Group Picture Gallery (http://stm1.chem.psu.edu/Pictures.html) [File Name: www2-7.htm] Several good images from the chemistry department at Pennsylvania State University.
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Based on your observations of the above images, what do all the scanning tunneling microscope images at the above WWW sites have in common?
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How do the images that you have seen differ from one another?
Now we will look more closely at how the scanning tunneling microscope works. Remember that the probe tip of the scanning tunneling microscope is typically a few tenths of a nanometer away from the surface. The tip of the scanning tunneling microscope measures the surface without ever touching the surface.
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How might the scanning tunneling microscope’s probe tip measure a sample’s surface?
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In the next activity, you will learn how information about the sample surface is obtained by the probe tip. We will now explore whether it is possible for electrons to flow from the surface to the probe tip if there is no contact between the two. In the past you have dealt with the flow of electrons --- for example through a light emitting diode (LED) --- you tried to understand the phenomenon using potential energy diagrams because potential energy diagrams enable us to predict the motion of an object, such as an electron through a material. We know that electrons, like everything else in nature seek the lowest energies available to them. Thus if we observe a region where there are more electrons than in an adjoining region, we can conclude that the region populated by more electrons must have a lower potential energy, while the region with fewer electrons has a higher potential energy. This information can be used to construct a potential energy diagram of the system with two such regions, indicating where the potential energy is higher and where it is lower. For example, in the case of two regions shown in Figure 2-5, the region on the left has more electrons than the region on the right. The related potential energy diagrams are shown below. Number of electrons
REGION 1
REGION 2
REGION 1
REGION 2
Electron Potential Energy
Figure 2-5 : The potential energy diagram for an electron in two regions, which is determined based on information about the relative number of electrons in each region. Region 1 with more electrons has lower potential energy.
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Going back to our scanning tunneling microscope — the surfaces of the sample and probe tip are conductive; hence they both contain relatively more electrons than the gap between them. Based on this fact, sketch the potential energy diagram for an electron moving along the line A-B in the three regions specified in the diagram below.
A
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Probe Tip Sample Surface
Potential Energy
Region 1 (Inside the SAMPLE)
Region 2 (Between the sample and the probe tip: VACUUM)
Region 3 (Inside the PROBE TIP)
Distance Figure 2-6: Potential energy diagram for an electron in a scanning tunneling microscope.
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Explain your reasoning for the potential energy diagram that you sketched above.
Test the accuracy of the above potential energy diagram by using the Scanning Tunneling Microscope Simulator program. Start the program by clicking the STM Simulator icon. Use File/Open from the pull-down menu. A dialog box will open prompting you for the file name. Select the only file available, “Image1.stm.”, and click OK.
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Position the cursor on the image and, while keeping the left mouse button pressed, move the cursor across the image. A dashed line will appear. This line indicates the path followed by the probe tip moving across the surface of a sample in a real scanning tunneling microscope. Use View/Cross Section from the pull-down menu to open the Cross Section window. The probe tip appears as a red triangle near the left edge of the Cross-Section window above the surface of the sample. Click on the probe tip and with the shift key pressed, drag the probe tip close to the middle of the Cross Section window and release it there, a few centimeters vertically above the sample surface (in a real scanning tunneling microscope the distance would be a few tenths of a nanometer). Now use View/Potential Energy from the pull-down menu to open the Potential Energy window. The potential energy diagram for an electron appears in the Potential Energy window. Again, use File/Save from the pull-down menu to save all the open windows and settings in the file Image1.stm. No prompt for file name will appear.
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How does the potential energy diagram in the program compare with your prediction made in Figure 2-6? Be as specific as possible.
For the probe tip to receive any information from the surface, electrons will need to flow from Region 1 (Sample) to Region 3 (Probe Tip) as indicated by the potential energy diagram in Figure 2-6. ♦ Suppose there were no electrons in Region 1, with energies greater than the potential energy in Region 2. Based on the potential energy diagram that you sketched above, would you expect any information (electrons) to flow from the sample surface to the probe tip? Explain.
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Indicate on the potential energy diagram in Figure 2-6 the total energies needed by an electron to move from the sample to the probe tip. Explain.
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If the energy of an electron were lower than the potential energy difference between the sample (Region 1) and the vacuum (Region 2), would you expect the electron to pass from the sample through the vacuum to enter the probe tip (Region 3)? Explain.
We find that, based on our previous experience with potential energy diagrams, an electron with less total energy than the potential energy of Region 2 should not be able to move from Region 1 to Region 3. In a scanning tunneling microscope, however, electrons from the sample do appear at the probe tip; apparently some mechanism allows these electrons to get from the sample to the probe tip. In the next activity, we will discover the mechanism by which electrons from the sample surface appear at the probe tip, in spite of a lack of physical contact between the probe tip and the sample.
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