Scanning Tunneling Microscope Apparatus

Transcription

1 CHM 424/426 BURLEIGH SCANNING TUNNELING MICROSCOPE Fall 04 Scanning Probe Microscopy (SPM) gathers information based upon localized interaction with a point source rather than a wave source. In this form of microscopy only the surface topology is being sampled. The probe does not sample features below the surface, as would be possible for a wave-source microscope. The tip of the probe surveys the topology through a number of possible surface-tip interactions, including attractive or repulsive atom-atom interactions. The resolution of the various SPM s depends on the distance-dependence of the surface-tip interaction and the radius of the tip. In the case of the Scanning Tunneling Microscope (STM), the image contrast depends on electron tunneling to produce a current which falls off exponentially with distance. This distance sensitivity is the same as the atomic dimensions and is ultimately responsible for the atomic resolution of the STM. The key breakthrough in achieving atomic resolution in scanning probe microscopy was the pioneering work of Gerd Binnig and Heinrich Rohrer. They realized that if a metallic tip could be maintained 10 angstroms from the surface, tunneling currents in the nanoamp range could be easily detected. The main obstacle was how to keep a tip this close to a sample without crashing the tip into the sample. They discovered that a mechanical probe could be made atomically sharp, which would lead to only one atom being closer to the sample during a scan than all the other atoms. Since the tunneling current is highly distance-dependent, the single atom provides an image contrast mechanism that has the right dimensions to give atomic resolution. This laid the foundation for the field of scanning probe microscopy. ELECTRONIC SCANNING PROBE MICROSCOPE (SPM) CONTROLLER ARIS 400 STM HEAD GateWay 2000 COMPUTER FIGURE 1: Scanning Tunneling Microscope Apparatus Figure 1 shows the apparatus that makes up the scanning tunneling microscope. The Electronic SPM Controller is the device where all electronic parameters are adjusted. This unit controls the ARIS-400 STM Head, which houses the sample and the probe tip. The computer takes the data output of the controller and creates an image of the topography of the samples surface. 1

2 The STM Head houses the sample and tip, both of which are easy to install and remove. The sample is attached to a tubular piezoelectronic (PZT) for X and Y rastering (scanning) over the sample surface and fine Z movement perpendicular to the surface. The head also contains an amplifier circuit that converts the small tunneling current to a voltage and then amplifies it to a usable level before sending it to the control electronics. The control electronics uses the concept of negative feedback to keep the sample and tip within the tunneling distance as the sample is rastered under the tip. The control electronics accomplishes this by increasing the voltage to the Z element of the PZT if the tunneling current falls below the desired value or decreases the voltage if the tunneling current rises above the desired value. As the control electronics applies corrections to keep the tunneling current constant, an error signal is generated. This error signal and the amplified tunneling current are sent to the computer. In real time the computer reads this data, sends X and Y raster signals to the control electronics, and, synchronous with the raster, displays the data on its monitor, forming the image. A Quick History of the Microscope The word microscope refers to an instrument capable of magnifying the dimensions of an object so that it can be seen by the human eye. The eye itself is an extraordinary device with a resolution of approximately 100 microns (0.1 millimeters). Thus, the word microscope is confined to resolving structures of less than 100 microns. The basic optical microscope uses light in the visible part of the spectrum ( nanometer wavelengths) to magnify objects. The diffraction of light limits the resolution to around 0.1 microns, which is a factor of 1000x magnification over the eye. This level of magnification is sufficient to observe microorganisms, bacteria, some viruses, and single cells. The electron microscope was created in an attempt to increase the resolution by using X- rays instead of visible light. Since X-rays have much shorter wavelengths than visible light, the diffraction is also much smaller. This allows for a much more powerful microscope, and led to obtaining our first glimpses of the details of structure at the atomic level. Phosphor screens are used to convert the electron density output into a visual format. The Scanning Electron Microscope (SEM) images the surface of materials by analyzing the intensity of a reflected electron beam and any secondary electrons produced by an incident electron beam. The electron beam is scanned across the surface in an X-Y fashion to obtain the surface topology in a point-by-point manner. The resolution of the SEM is around 100 angstroms (0.01 microns), and is limited primarily by the focus of the electron beam. The uses of SEM include the study of biological materials, crystal growth, optical components and semiconductor nanotechnology. However, the sample must be housed in a high vacuum chamber and non-conductive samples must be coated by a thin film of gold. The Acoustical Microscope relies on acoustic waves for imaging. The distinct advantage of sound waves as a probe is that they can penetrate optically opaque materials and reveal hidden structures. One of the most important uses of ultrasonics has been in biomedical applications for examining internal organs (such as MRI s) and fetal examinations (ultrasounds). Acoustic 2

3 microscopy, however, requires a coupling fluid to transmit the sound wave to the object under study. If water is used as the coupling fluid, the maximum resolution is around 0.2 microns. The above are all examples of microscopy which rely on the interaction of a wave source with an object that affects either the index of refraction (wave speed) or the absorption. Scanning Probe Microscopy (SPM) gathers information based upon localized interaction with a point source rather than a wave source. In this form of microscopy only the surface topology is being sampled. The probe does not sample features below the surface, as would be possible for a wave-source microscope. The tip of the probe surveys the topology through a number of possible surface-tip interactions, including attractive or repulsive atom-atom interactions. The resolution of the various SPM s depends on the distance-dependence of the surface-tip interaction and the radius of the tip. In the case of the STM, the image contrast depends on electron tunneling to produce a current which falls off exponentially with distance. This distance sensitivity is the same as the atomic dimensions and is ultimately responsible for the atomic resolution of the STM. The first scanning probe microscope was the scanning profilometer. This instrument acted like a sophisticated record player, where a stylus probe was brought into contact with a surface and mechanically scanned across it. The image is constructed as variations in the deflection of the stylus. This microscope worked fairly well, but because it physically touched the surface, atomic resolution could not be achieved because of limitations to the sharpness of the stylus probe. The key breakthrough in achieving atomic resolution in scanning probe microscopy was the pioneering work of Gerd Binnig and Heinrich Rohrer. They realized that if a metallic tip could be maintained 10 angstroms from the surface, tunneling currents in the nanoamp range could be easily detected. The main obstacle was how to keep a tip this close to a sample without crashing the tip into the sample. They discovered that a mechanical probe could be made atomically sharp, which would lead to only one atom being closer to the sample during a scan than all the other atoms. Since the tunneling current is highly distance-dependent, the single atom provides an image contrast mechanism that has the right dimensions to give atomic resolution. This laid the foundation for the field of scanning probe microscopy. HOW THE STM WORKS There are five scientific and technical processes or ideas that the STM integrates to make atomic resolution images of a surface possible. Each of these processes was used in other areas of science before the invention of the STM. They are as follows: --- The principle of quantum mechanical tunneling. --- Achievement of controlled motion over small distances using piezoelectronics. --- The principle of negative feedback. --- Electronic data collection. --- Vibration isolation. 3

4 Quantum Mechanical Tunneling Quantum mechanical tunneling is not some obscure process that only occurs under extreme conditions in a crowded basement laboratory of a research university. Quantum mechanical tunneling explains some of the most basic phenomena we observe in nature. One example is the radioactive decay of plutonium. If quantum mechanical tunneling did not occur, plutonium would remain plutonium instead of changing into elements lower on the periodic chart. Plutonium converts to other elements when 2 neutrons and 2 protons are ejected from the nucleus because of tunneling. Even the fundamental force that binds atoms into molecules can be thought of as a manifestation of quantum mechanical tunneling. In this lab, we will look at how tunneling manifests itself in another way. We will attempt to understand how a single electron starts out in one metal and then reappears in another metal, even though they are not touching. To begin, let s examine what electron tunneling means in the real world. Consider two pieces of metal. Metals are good conductors of electricity, i.e. electrons can move very easily and quickly from one end of the metal to the other. Imagine connecting one of the pieces of metal to the negative terminal of a battery and the other piece of metal to the positive terminal, as shown in Figure 2. If the metals are not touching, no current will flow through the battery. The electrons are free to move around the metal but cannot leave it. The electrons are analogous to water in a reservoir that is blocked by a dam. The water can move about the reservoir but have no access to the river below. If the metals are brought together so that they touch, current will flow freely through the contacting area. The electrons have a free path from the negative terminal to the positive terminal of the battery. This current flow is analogous to opening up the gates of the dam and allowing the water to flow down the river into the ocean. + Battery - Metal 1 Metal 2 FIGURE 2: Two pieces of metal, each connected to a battery terminal. While the metals are well separated no current flows through the battery. The unusual experimental feature of tunneling is this: when the metals are brought close together, but are not quite touching, a small electric current can be measured. The current gets larger the closer the metals are brought together, until it reaches its maximum value when the metals are touching. The unusual phenomena about the current flow is that the electrons do not move in the space between the metals, but just suddenly appear in the other side. The metals must be only 10 angstroms apart to produce detectable tunneling current. Figure 3 shows current as a function of the separation between metals. The distances involved are so small that special tools are needed to adjust the distances or the small electric currents will not be detected. This requires the use of piezoelectrics. 4

5 20 15 CURRENT (na) SEPARATION (angstroms) FIGURE 3: Current flowing through a battery as a function of the separation of the metals. It shows the exponential increase i n current as the metals get closer. One of the basic principles of quantum mechanics is that electrons have both a particle and a wave nature. So we should picture the electron not as a hard ball impinging on the barrier, but as a cloud with a size of a few angstroms. When the cloud collides with the barrier, part of the cloud may penetrate it. For thick barriers (>10 angstroms), the cloud will be reflected like a hard particle. For thin barriers, however, part of the cloud may penetrate the barriers and appear on the other side. This process is called tunneling because the electron does not have enough kinetic energy to travel over the barrier, but is able to exist on the other side. It is as if the electron found a way to dig a tunnel through the barrier. In the scanning tunneling microscope, one of the metals is the sample being imaged and the other metal is the probe tip. The sample is usually flatter than the probe. If the probe is sharpened into a tip it will most likely have one atom at the end. Mathematically, for each angstrom change in separation, the probability that an electron tunnels decreases by an order of magnitude. Therefore, all of the tunneling electrons will pass through this atom at the tip. It is this feature that leads to the atomic resolution capabilities of the microscope. Piezoelectricity and PZT Scanning Tubes During a scan, the sample and tip separation is maintained within a few angstroms as the sample is moved under the PtIr tip. This motion is accomplished using piezoelectric ceramics (referred to as PZT). An electric field applied across a piezoelectric ceramic causes expansion in one direction and contraction in another. This process is illustrated in Figure 4. The sensitivity of a piezoelectric ceramic depends on the particular arrangement of the atoms in the crystal and its thickness. The sensitivity of the piezoelectric materials used in the 7 mm scanner of the Personal STM is about 150 angstroms/volt. The computer running the STM can control voltages with millivolt accuracy, thus the motion can be controlled with angstrom sensitivity. 5

6 Open Circuit Closed Circuit Contract Expand FIGURE 4: Piezoelectric (PZT) response to applied electric fields changes dimensions. Three-dimensional motion is achieved by shaping a PZT ceramic into a hollow cylinder, as is shown in Figure 5. The sample is attached to the top of the cylinder, while the bottom of the piezoelectric and the tip are fixed. Four electrodes are formed on the outside of the cylinder and one electrode is formed on the inside of the cylinder. By independently controlling the voltages applied across these electrodes, the piezoelectric cylinder can bend in any direction and be extended and retracted. Two examples of this are shown in Figure 6. In Figure 6, part (A), a negative voltage is applied to the four outside electrodes and a positive voltage is applied to the inside electrode. The electric field, therefore, points from the inside to the outside of the cylinder. This is opposite the poled direction of the tube. The cylinder will shrink between the electrodes (the wall will become slightly thinner) and, to keep the volume of material constant, the tube will become longer. If an opposite polarity is applied to the electrodes, the reverse will happen and the tube will shrink. Applying voltages this way gives control of the motion parallel to the tube s centerline. In Figure 6, part (B), a positive voltage is applied to the left electrode, a negative voltage to the right electrode, and the other electrodes are held at ground. In this example, the left side of the tube will shrink and the right side of the tube will lengthen, with the front and back of the tube remaining the same length. The tube can accommodate this shrinking and growing by bending to the left (X-direction). If the opposite polarity is put on the left and right electrodes, the tube will bend to the right. If voltages are applied in the same way across the front and back electrodes, the sample bends to the front and to the back (Y-direction). The voltages to the left and right electrodes and to the front and back electrodes can be applied simultaneously to move the tip at any direction in the X-Y plane. A voltage applied to the inside electrode will cause the end of the tube to move up or down (Z-direction) at the same time moving in X and Y. By applying voltages in this way, the end of the PZT scanning tube can be positioned anywhere within a three-dimensional region with subangstrom resolution. The X and Y raster is shown in Figure 7. 6

7 PZT Assembly Electrode contact Top View Side View FIGURE 5: A PZT ceramic made into a hollow cylinder with electrode quadrants produces fine motion control o f the t i p. (A) (B) FIGURE 6 : ( A) Applying a potential between the inner electrode o f the PZT cylinder and i t s outer electrode quadrants, causes the PZT t o become thinner and t o elongate. (B) Applying a potential between opposite outer electrodes o f the PZT cylinder with respect to the inner electrode will bend the PZT. Y X FIGURE 7: X-Y Raster: the sample moves under the tip from left to right (X) and up a little (Y) at the end of each X-raster. 7

8 Negative Feedback Measuring tunneling current provides a way to sense the location of the tip relative to the sample. If there is no tunneling current, the tip is too far from the sample. If there is a small tunneling current, the tip is near the sample. Negative feedback turns this position sensor into a microscope. The device that measures the tunneling current is connected to electronic feedback circuitry that obeys the following rules: if it senses a decrease in the current, it moves the sample closer to the tip of the probe, and if it senses an increase in current, it moves the sample away from the tip. Imagine a sample with some bumps on it, as shown in Figure 8. The microscope is operated by positioning the tip somewhere over the left-hand side of the sample and lowering it towards the sample until a tunneling current is detected. The tip is then moved to the right. When the tip approaches a bump, the tunneling current increases. The feedback loop senses this increase and raises the tip to maintain a constant tunneling current. The tip will continue to rise until it is over the top of the bump. As the tip moves farther to the right, the current decreases and the tip must be lowered to maintain a constant tunneling current. By the time the tip has reached the far right, it has traced out a cross section of the topography of the sample. NOTE: The sample is actually raised or lowered on our STM, not the probe tip. Path traced by tip TIP Sample surface FIGURE 8 : The movement o f the tip as i t scans the surface o f a sample. When the tip gets close t o a bump, the tunneling current increases. The feedback l o o p, i n response, then moves the sample downward. Electronic Data Collection The final result of running the STM must be a picture. Unlike the optical microscope, you cannot look directly at the image that will be produced by the Scanning Tunneling Microscope. With an STM, the image is a collection of voltages, therefore the STM must be interfaced to a computer with graphic capabilities to look at the image. The STM image is built up one point at a time. At each data point during a raster scan, the computer generates the voltages required to move the tip in the X-Y pattern and sends this signal to the control electronics which amplify the voltage to a large enough value to move the PZT scanner. The control electronics use negative feedback to apply a voltage to the PZT scanner to move the sample away from and toward the tip (in the Z-direction) so that the tip traces a contour of the surface. The control electronics simultaneously sends the Z axis voltage, required to maintain a constant tunneling current, back to the computer. 8

9 Vibration Isolation One of the problems that delayed the development of Scanning Probe Microscopes was the intuitive feel that it would be impossible to hold two objects only a few angstroms apart without having them crash into each other. Even though it can not be seen by the naked eye, objects are always in motion relative to each other. Walking around a room causes desks and chairs to vibrate with an amplitude of around 1 micrometer, which equals 10,000 angstroms! To have the feedback loop work as mentioned, these vibrations must be eliminated. So how does one isolate the microscope from these vibrations? The key issue in the design of a mechanical system, sample, and probe is having a rigid (high resonant frequency) and kinematically correct assembly. The rigid body will vibrate as a whole and the sample-probe separation is not affected. Try to setup the scanning probe microscope on the lower level of your building and away from foot or vehicle traffic. Outside walls generally are the most stable areas of a building. Avoid placing near heavy equipment, fans, air conditioning devices and high-power electrical devices such as motors or generators. Place the STM Head on the heavy plate that lies on the partially inflated inner tube, as this works well as a vibration isolation table. EXPERIMENTAL PROCEDURE Front Panel Settings on the Electronic SPM Controller 1) Turn the Electronic SPM Controller ON. The power switch is located on the right back of unit. 2) Set MAGNIFICATION to X2. This setting will provide a 25,000 A scan range for the scanning module. 3) Adjust the X and Y OFFSET CONTROLS so that the segmented LED displays are set in the middle position (12 o clock position). 4) Set the STATUS MONITOR to REFERENCE FORCE/CURRENT selection. Set the REFERENCE CURRENT so that the LED display of the SPM MONITOR reads 9 na. 5) Set the STATUS MONITOR to BIAS VOLTAGE selection. Set the BIAS VOLTAGE to V. 6) Set the PROPORTIONAL GAIN to the 12 o clock position. 7) Set the INTEGRATOR GAIN to the 12 o clock position. 8) Set the DIFFERENTIATOR GAIN to the 8 o clock position. 9) Set the LOWPASS FILTER (cutoff frequency) to the 8 o clock position. 9

10 Tip Preparation In the Accessory Kit you find two types of wire. We will use only the Platinum-Iridium (PtIr) wire, as this wire can be cut and used directly as a tip for STM. Since having a sharp tip is essential for the image to be clear, the tip will be supplied and already mounted on the tip holder. NOTE: You must take extra caution when handling the tip so the fine tip at the end of the wire does not touch anything. Make sure that the STM Controller is turned OFF. Use the tip mount tweezers to hold and slide the tip mount into the tip mount holder that is in the STM Head Assembly (see Figure 11). Make sure the tip mount snaps into place properly. TIP MOUNT HOLDER SAMPLE MOUNT HOLDER FIGURE 11: Front panel of ARIS-400 STM Head Test Sample Preparation (Holographic Gold Grating) In the Accessory Kit, you will find a Holographic Gold Grating mounted sample (single period hologram with a sinusoidal spacing of around 3000 Angstroms, Item #2). To prepare the test sample, proceed with the following steps: 1) Remove the black protective cap. 2) Hold the APPROACH RETRACT switch on the rear panel of the ARIS-400 STM Head downwards to retract the Sample Scanning Module away from the tip. The scanning module should be at its lowest position to avoid damaging the tip when you place the sample in position. 3) Insert the sample with the grating lines parallel to the Y-axis. To do this, line the bead of glue that holds the sample onto the sample mount so that it runs left to right nearest to you as you place the sample mount holder on the STM Head Assembly. 10

11 4) Use the sample mount tweezers to hold the sample mount and to slide it into the sample mount holder. Make sure the sample mount snaps in place properly. The ball plunger on the sample mount holder should engage in the middle of the sample mount. Approach: Sample and Tip Engagement The final set up, before scanning and acquiring images, includes resetting the True Image SPM software and activating the Electronic SPM Controller AUTO APPROACH. Before scanning, follow these steps: 1) True Image SPM Software: With the computer ON, start the True Image software by double-clicking on the PSPM icon. 2) Select New Window option from the Window menu. 3) Select the Scanner Maximums... option from the Calibrate menu. Set the Max scanner range in X- and Y- directions to 50,000 A. Set the Max scanner range in Z-direction to 15,000 A. These maximum values are approximate values. 4) Select the Configuration option from the Collect menu. Set the Configuration menu parameters on the computer screen to those shown in Figure 12 on the next page. The configuration options are defined on the next page as well. 5) Check to make sure the REFERENCE CURRENT is set to about 9.0 na on the electronic controller, as well as the other controller settings. Notice that this is shown at the bottom of the configuration screen. 6) Use the APPROACH toggle switch at the rear panel of the STM Head to move the Sample Scanning Module upwards and bring the top surface of the sample close to the tip. Try to bring the tip within half of a millimeter of the sample top surface. NOTE: The tip must be close but must not touch the top surface of the sample! As you bring the tip close to the sample, use the magnifying glass by placing it directly flush upon the front of the STM Head. It should focus nicely at this distance, and increase visibility greatly. 7) Check the FEEDBACK ACTIVE light. If it is ON, then press the COARSE RETRACT toggle switch down about one-half of a second to reset the electronics. 8) Press the AUTO APPROACH toggle switch upwards for about one second. The AUTO APPROACH indicator must go on and the Sample Scanning Module starts slowly moving upward. If indicator does not turn on, then repeat step 5) and then step 6). 9) When the ACTUAL CURRENT equals the REFERENCE CURRENT, the AUTO APPROACH indicator goes off and the FEEDBACK indicator goes on. 11

12 Data points in X -direction: Samples. Scan Mode Data points in Y -direction: Samples. Single Number of substeps: 16 Substeps. Continuous Scan range in X -direction: ( ) Angstroms. Data Type Scan range in Y -direction: ( ) Angstroms. Current Scan range in Z -direction: Angstroms. Topographic Scan Range Scale Pre Amp Mode Force Automatic Manual Linear Log Friction Scan Delays Tilt Removal Sample Delay (msec/sample) 0.15 Plane Removal Retrace Delay (msec/step) 0.15 X Line Removal Scanline Delay (msec/line) 0.00 Frame Delay (msec/frame) Linearity Approximate total scan time (MM:SS): 5:42 System Status OK Bias voltage: Volts. Zoom Factor: 2 Cancel Tunneling Current: 9.00 Nanoamps. Z Gain Factor: 1 Default FIGURE 12: The parameters of the Configuration option for the gold grating, with definitions below. Data points in X and Y-directions: Number of substeps: Scan range in X and Y-directions: Scan Range Scale: Sample Delay: Retrace Delay: Scanline Delay: Frame Delay: Scan Mode: Topographic: Force: Friction: Tilt Removal: Linearity: Data points in X and Y. These numbers must be powers of 2, and must be identical for both. Number of substeps you may wish to take between each of the X and Y data points. This # must be in multiples of 2. The range given in parenthesis shows the available X and Y scan range for a particular Zoom Factor selected. The numbers in the boxes are the actual scan sizes. Maximum Z range of the acquired data. This depends on the module scan maxima and the Z Gain Factor. Delay after each step in X prior to data taking. Delay after each step during X retrace. Delay after completion of each scan line. Delay at the first point of the image (at the top left). Scanning one frame at a time or continuously. Constant force imaging. Constant height imaging. Lateral force of friction (not available). Dynamic background removal. Activates the X-Y linearization circuits. Use this option only for large scan ranges. This removes the inherent PZT creep and non-linear behavior. 12

13 Acquiring Images Now you are ready to acquire the first image with the ARIS-3400 STM system. This section will also provide you with information on image optimization and filtering. 1) On the True Image SPM Software, select Scan Control from the Collect menu and click on Scan to begin scanning. 2) During the scan you have the following options: Press C to capture the current scan upon completion. Press H to halt the current scan and allow for changes. Press A to abort the current scan and exit. 3) You may optimize the image quality by adjusting the feedback parameters and the speed of the scan. -- If you scan too fast the feedback might not have enough time to adjust, resulting in smeared images. -- You can adjust the PROPORTIONAL GAIN and use the LOWPASS FILTER to remove high frequency noise. -- The DIFFERENTIATOR GAIN would allow you to eliminate the imaging artifacts which appear as shadows in the image. You can also use the DIFFERENTIATOR GAIN to improve the quality of the image when taking fast images (images taken within 1-2 minutes acquisition time. Do not hesitate adjusting feedback parameters or the scan speed to learn their effect on the image quality. 4) Press C to capture a large image of the sample. At the end of the scan the raw data of the image is re-displayed on the screen, complete with length and height scales. 5) Select the Tilt Removal option from the Filter menu. This will remove the background tilt which may be due to how the sample is mounted. 6) Select the Contrast View Data option from the Display menu. Contrast the image in the Standard Deviation mode. 7) You may apply other filtrations to optimize the image quality at this point as well. Note that you can select Undo Last option from the Filter menu to eliminate the last filtering step which you perform on the image. 8) Save the image after optimization in the 424 Folder tues_730, wed_130 or thur_730 group# as AU_2X.IMG, short for gold grating at X2 magnification. 13

14 9) To PRINT the image, select Print option from the File menu. Select WINDOW when printing. It should take less than a minute for the printout to be completed by the HP Color LaserJet. Other Features of the True Image Software 1) Three Dimensional Viewing: Once the above image is optimized, you can actually see the surface in 3-D mode. To do this, go to the Display menu and pull down to 3-Dimensional. By changing the parameters in this screen, one can rotate the image both sideways and updown. The Z-axis can also be magnified, thus enlarging the surface detail. 2) Cross Section Analysis: The cross section of the surface can also be observed. Go to the Analysis menu and pull down to the Cross Section option. In this mode you will notice the top-view image at the upper-left part of the screen. With the mouse, move the cursor to where you want to start your cross section line. Click on the mouse and drag until you are at the end of the line for cross sectional analysis. Click on the mouse, and at the lower portion of the screen will be the cross section of the line that you just drew. When this is done repeatedly, the line color changes. Also on this screen is the Section Analysis option, which allows for the length and height of the sample to be easily determined. Click on section analysis. On the lower section of the screen where the cross section is shown, place the mouse over the left vertical line. When a double-sided arrow appears, click with the mouse and place the vertical line at the initial point of interest. Now, use the mouse similarly to move the primed vertical line to the ending point of interest. After the two lines have been placed, click in the small colored box to have the desired numerical data appear in the top right region of the screen. Use this feature to determine the length and height of the two best individual gratings. EXPERIMENTAL for gold coated grating After acquiring a good image of the gold grating at the preset operating parameters, see the effect of changing the following conditions: Reference Current: The higher the reference current, the closer the STM tip must be to the sample during the scan. Try running scans with the reference current at 3 na and 6 na, and compare results to the 9 na trial. Bias Voltage: Run a scan with the bias voltage (voltage difference between the sample and the tip) at 0.20 V, and compare with the 0.10 V trial. Magnification: Run scans with the magnification set at x5 and x10. 14

15 Data points in X -direction: Samples. Scan Mode Data points in Y -direction: Samples. Single Number of substeps: 16 Substeps. Continuous Scan range in X -direction: ( ) 250 Angstroms. Data Type Scan range in Y -direction: ( ) 250 Angstroms. Current Scan range in Z -direction: 25 Angstroms. Topographic Scan Range Scale Pre Amp Mode Force Automatic Manual Linear Log Friction Scan Delays Tilt Removal Sample Delay (msec/sample) 0.10 Plane Removal Retrace Delay (msec/step) 0.00 Line Removal Scanline Delay (msec/line) 0.00 Frame Delay (msec/frame) Linearity Approximate total scan time (MM:SS): 2:12 System Status Bias voltage: Volts. Zoom Factor: 200 Tunneling Current: 5.00 Nanoamps. Z Gain Factor: 1 OK Cancel Default FIGURE 13: The parameters of the Configuration option for HOPG sample. HOPG Sample (Highly-Oriented-Pyrolitic-Graphite) Set the configuration menu as shown in Figure 13 above. Since this sample is essentially flat, once we have the STM tip very close to the sample with the auto approach in constant current mode, we will change to the constant height mode, and the instrument will measure the current at each data point. The larger the measured current, then the higher the surface of the sample. This is because if the surface was high, then it would be closer to the STM tip, resulting in a larger tunneling current. Front Panel Settings on the Electronic SPM Controller 1) Set MAGNIFICATION to X200. This setting will provide a 250 A scan range for the scanning module. 2) Set the STATUS MONITOR to REFERENCE FORCE/CURRENT selection. Set the REFERENCE CURRENT so that the LED display of the SPM MONITOR reads 5 na. 3) Set the STATUS MONITOR to BIAS VOLTAGE selection. Set the BIAS VOLTAGE to V. 4) Set the PROPORTIONAL GAIN to the 3 o clock position. 5) Set the INTEGRATOR GAIN to the 9 o clock position. 6) Set the DIFFERENTIATOR GAIN to the 8 o clock position. 7) Set the LOWPASS FILTER (cutoff frequency) to the 3 o clock position. 15

16 True Image SPM Software Settings 1) Select New Window option from the Window menu. 2) Select the Configuration option from the Collect menu. Set the configuration menu as shown on the previous page for the HOPG sample. Make sure the Reference Tunneling Current is at around 5.0 na, and the Bias Voltage is at around V. 3) Use the APPROACH toggle switch to manually bring the sample within a half millimeter of the tip. 4) Check the FEEDBACK ACTIVE light. If it is ON, then press the COARSE RETRACT toggle switch down about one-half of a second to reset the electronics. 5) Press the AUTO APPROACH toggle switch upwards for about one second. The AUTO APPROACH indicator must go on and the Sample Scanning Module starts slowly moving upward. If indicator does not turn on, then repeat step 3) and then step 4). 6) When the ACTUAL CURRENT equals the REFERENCE CURRENT, the AUTO APPROACH indicator goes off and the FEEDBACK indicator goes on. At this point, select the Configuration option from the Collect menu. Change the Data Type from topographic to Current. This will set the instrument up for constant height mode. Run a scan of the HOPG sample as you did with the holographic gold sample (page 13). Use Tilt Removal and Contrast View Data as before. Save as HOPGx200.img. Run a new trial with a different bias voltage and compare to the first trial. Run a new scan with the magnification at x2000, which will produce a 25 angstrom x 25 angstrom scan. Copper Samples: Here we are going to see if the surface of copper is affected by various conditions. Take one of the copper samples, place it on a watch glass, and put one drop of 6M NaOH on it. Take another copper sample, place it on another watch glass, and put one drop of 1M HCl on it. Place both into the oven for one hour, which should be at around 160 o C. Set up the STM with the same conditions that worked best with the gold-coated grating. Notice that the copper metal sample has both a shiny side and a dull oxidized side. Run a trial for each side of the copper sample at a magnification of x10 and compare the two images. Then, gently dry the NaOH off of the copper sample with a chem wipe, and run a trial of this copper sample. Repeat for the sample that had HCl on it. The HCl should evaporate while in the oven and will not need to be dried with the chem wipe. 16