1

CHM 424/426 BURLEIGH SCANNING TUNNELING MICROSCOPE

Fall ‘04

Scanning Probe Microscopy (SPM) gathers information based upon localized interactionwith a point source rather than a wave source. In this form of microscopy only the surfacetopology is being sampled. The probe does not sample features below the surface, as would bepossible for a wave-source microscope. The tip of the probe surveys the topology through anumber of possible surface-tip interactions, including attractive or repulsive atom-atominteractions. The resolution of the various SPM’s depends on the distance-dependence of thesurface-tip interaction and the radius of the tip. In the case of the Scanning TunnelingMicroscope (STM), the image contrast depends on electron tunneling to produce a current whichfalls off exponentially with distance. This distance sensitivity is the same as the atomicdimensions and is ultimately responsible for the atomic resolution of the STM. The key breakthrough in achieving atomic resolution in scanning probe microscopy was thepioneering work of Gerd Binnig and Heinrich Rohrer. They realized that if a metallic tip could bemaintained 10 angstroms from the surface, tunneling currents in the nanoamp range could beeasily detected. The main obstacle was how to keep a tip this close to a sample without crashingthe tip into the sample. They discovered that a mechanical probe could be made atomicallysharp, which would lead to only one atom being closer to the sample during a scan than all theother atoms. Since the tunneling current is highly distance-dependent, the single atom providesan image contrast mechanism that has the right dimensions to give atomic resolution. This laidthe foundation for the field of scanning probe microscopy.

Figure 1 shows the apparatus that makes up the scanning tunneling microscope. TheElectronic SPM Controller is the device where all electronic parameters are adjusted. This unitcontrols the ARIS-400 STM Head, which houses the sample and the probe tip. The computertakes the data output of the controller and creates an image of the topography of the samplessurface.

ARIS 400 STM HEAD ELECTRONIC SCANNING PROBEMICROSCOPE (SPM) CONTROLLER

FIGURE 1: Scanning Tunneling Microscope Apparatus

GateWay 2000 COMPUTER

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) overthe sample surface and “fine” Z movement perpendicular to the surface. The head also containsan amplifier circuit that converts the small tunneling current to a voltage and then amplifies it to ausable level before sending it to the control electronics. The control electronics uses the concept of “negative feedback” to keep the sample and tipwithin the tunneling distance as the sample is rastered under the tip. The control electronicsaccomplishes this by increasing the voltage to the Z element of the PZT if the tunneling currentfalls below the desired value or decreases the voltage if the tunneling current rises above thedesired value. As the control electronics applies corrections to keep the tunneling current constant, anerror signal is generated. This error signal and the amplified tunneling current are sent to thecomputer. In real time the computer reads this data, sends X and Y raster signals to the controlelectronics, 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 anobject so that it can be seen by the human eye. The eye itself is an extraordinary device with aresolution of approximately 100 microns (0.1 millimeters). Thus, the word microscope isconfined to resolving structures of less than 100 microns. The basic optical microscope uses light in the visible part of the spectrum (400-700nanometer wavelengths) to magnify objects. The diffraction of light limits the resolution toaround 0.1 microns, which is a factor of 1000x magnification over the eye. This level ofmagnification 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, thediffraction is also much smaller. This allows for a much more powerful microscope, and led toobtaining our first glimpses of the details of structure at the atomic level. Phosphor screens areused to convert the electron density output into a visual format. The Scanning Electron Microscope (SEM) images the surface of materials by analyzing theintensity of a reflected electron beam and any secondary electrons produced by an incidentelectron beam. The electron beam is scanned across the surface in an X-Y fashion to obtain thesurface topology in a point-by-point manner. The resolution of the SEM is around 100angstroms (0.01 microns), and is limited primarily by the focus of the electron beam. The uses ofSEM include the study of biological materials, crystal growth, optical components andsemiconductor nanotechnology. However, the sample must be housed in a high vacuum chamberand non-conductive samples must be coated by a thin film of gold. The Acoustical Microscope relies on acoustic waves for imaging. The distinct advantage ofsound waves as a probe is that they can penetrate optically opaque materials and reveal hiddenstructures. One of the most important uses of ultrasonics has been in biomedical applications forexamining internal organs (such as MRI’s) and fetal examinations (ultrasounds). Acoustic

3

microscopy, however, requires a coupling fluid to transmit the sound wave to the object understudy. 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 sourcewith 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 apoint source rather than a wave source. In this form of microscopy only the surface topology isbeing sampled. The probe does not sample features below the surface, as would be possible for awave-source microscope. The tip of the probe surveys the topology through a number ofpossible surface-tip interactions, including attractive or repulsive atom-atom interactions. Theresolution of the various SPM’s depends on the distance-dependence of the surface-tipinteraction and the radius of the tip. In the case of the STM, the image contrast depends onelectron tunneling to produce a current which falls off exponentially with distance. This distancesensitivity is the same as the atomic dimensions and is ultimately responsible for the atomicresolution of the STM. The first scanning probe microscope was the scanning profilometer. This instrument actedlike a sophisticated record player, where a stylus probe was brought into contact with a surfaceand mechanically scanned across it. The image is constructed as variations in the deflection of thestylus. 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 stylusprobe. The key breakthrough in achieving atomic resolution in scanning probe microscopy was thepioneering work of Gerd Binnig and Heinrich Rohrer. They realized that if a metallic tip could bemaintained 10 angstroms from the surface, tunneling currents in the nanoamp range could beeasily detected. The main obstacle was how to keep a tip this close to a sample without crashingthe tip into the sample. They discovered that a mechanical probe could be made atomicallysharp, which would lead to only one atom being closer to the sample during a scan than all theother atoms. Since the tunneling current is highly distance-dependent, the single atom providesan image contrast mechanism that has the right dimensions to give atomic resolution. This laidthe 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 makeatomic resolution images of a surface possible. Each of these processes was used in other areasof 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.

4

Quantum Mechanical Tunneling

Quantum mechanical tunneling is not some obscure process that only occurs under extremeconditions in a crowded basement laboratory of a research university. Quantum mechanicaltunneling explains some of the most basic phenomena we observe in nature. One example is theradioactive decay of plutonium. If quantum mechanical tunneling did not occur, plutonium wouldremain plutonium instead of changing into elements lower on the periodic chart. Plutoniumconverts to other elements when 2 neutrons and 2 protons are ejected from the nucleus becauseof tunneling. Even the fundamental force that binds atoms into molecules can be thought of as amanifestation of quantum mechanical tunneling. In this lab, we will look at how tunnelingmanifests itself in another way. We will attempt to understand how a single electron starts out inone 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 twopieces of metal. Metals are good conductors of electricity, i.e. electrons can move very easily andquickly from one end of the metal to the other. Imagine connecting one of the pieces of metal tothe negative terminal of a battery and the other piece of metal to the positive terminal, as shownin Figure 2. If the metals are not touching, no current will flow through the battery. Theelectrons are free to move around the metal but cannot leave it. The electrons are analogous towater in a reservoir that is blocked by a dam. The water can move about the reservoir but haveno access to the river below. If the metals are brought together so that they touch, current willflow freely through the contacting area. The electrons have a free path from the negative terminalto the positive terminal of the battery. This current flow is analogous to opening up the gates ofthe dam and allowing the water to flow down the river into the ocean.

The unusual experimental feature of tunneling is this: when the metals are brought closetogether, but are not quite touching, a small electric current can be measured. The current getslarger the closer the metals are brought together, until it reaches its maximum value when themetals are touching. The unusual phenomena about the current flow is that the electrons do notmove in the space between the metals, but just suddenly appear in the other side. The metalsmust be only 10 angstroms apart to produce detectable tunneling current. Figure 3 showscurrent as a function of the separation between metals. The distances involved are so small thatspecial tools are needed to adjust the distances or the small electric currents will not be detected.This requires the use of piezoelectrics.

Metal 1 Metal 2

FIGURE 2: Two pieces of metal, each connected to a battery terminal. While themetals are well separated no current flows through the battery.

+

-

Battery

5

One of the basic principles of quantum mechanics is that electrons have both a particle and awave nature. So we should picture the electron not as a hard ball impinging on the barrier, but asa cloud with a size of a few angstroms. When the cloud collides with the barrier, part of thecloud may penetrate it. For thick barriers (>10 angstroms), the cloud will be reflected like a hardparticle. For thin barriers, however, part of the cloud may penetrate the barriers and appear onthe other side. This process is called tunneling because the electron does not have enough kineticenergy to travel over the barrier, but is able to exist on the other side. It is as if the electron founda way to dig a tunnel through the barrier. In the scanning tunneling microscope, one of the metals is the sample being imaged and theother metal is the probe tip. The sample is usually flatter than the probe. If the probe issharpened into a tip it will most likely have one atom at the end. Mathematically, for eachangstrom change in separation, the probability that an electron tunnels decreases by an order ofmagnitude. Therefore, all of the tunneling electrons will pass through this atom at the tip. It isthis 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 thesample 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 inone direction and contraction in another. This process is illustrated in Figure 4. The sensitivityof a piezoelectric ceramic depends on the particular arrangement of the atoms in the crystal andits thickness. The sensitivity of the piezoelectric materials used in the 7 mm scanner of thePersonal STM is about 150 angstroms/volt. The computer running the STM can control voltageswith millivolt accuracy, thus the motion can be controlled with angstrom sensitivity.

0SEPARATION (angstroms)

CURRENT (nA)

20

15

10

5

0

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.

2 4 6 8 10

6

Three-dimensional motion is achieved by shaping a PZT ceramic into a hollow cylinder, asis shown in Figure 5. The sample is attached to the top of the cylinder, while the bottom of thepiezoelectric and the tip are fixed. Four electrodes are formed on the outside of the cylinder andone electrode is formed on the inside of the cylinder. By independently controlling the voltagesapplied across these electrodes, the piezoelectric cylinder can bend in any direction and beextended 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 apositive voltage is applied to the inside electrode. The electric field, therefore, points from theinside to the outside of the cylinder. This is opposite the poled direction of the tube. Thecylinder will shrink between the electrodes (the wall will become slightly thinner) and, to keep thevolume of material constant, the tube will become longer. If an opposite polarity is applied tothe electrodes, the reverse will happen and the tube will shrink. Applying voltages this waygives 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 theright electrode, and the other electrodes are held at ground. In this example, the left side of thetube will shrink and the right side of the tube will lengthen, with the front and back of the tuberemaining the same length. The tube can accommodate this shrinking and growing by bending tothe left (X-direction). If the opposite polarity is put on the left and right electrodes, the tube willbend 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 rightelectrodes and to the front and back electrodes can be applied simultaneously to move the tip atany direction in the X-Y plane. A voltage applied to the inside electrode will cause the end of thetube to move up or down (Z-direction) at the same time moving in X and Y. By applyingvoltages in this way, the end of the PZT scanning tube can be positioned anywhere within athree-dimensional region with subangstrom resolution. The “X and Y raster” is shown in Figure7.

Open Circuit Closed Circuit

Contract

Expand

FIGURE 4: Piezoelectric (PZT) response to applied electric fields changes dimensions.

7

Electrode contact

PZT Assembly Top View

+

-

-+

-+

(A)

(B)

Side View

+

+

+-

-

-

FIGURE 5: A PZT ceramic made into a hollow cylinder with electrodequadrants produces “fine” motion control o f the t i p .

FIGURE 6 : (A) Applying a potential between the inner electrode o f the PZTcylinder and i t s outer electrode quadrants, causes the PZT t o become thinner and t oelongate. (B) Applying a potential between opposite outer electrodes o f the PZTcylinder with respect to the inner electrode will bend the PZT.

Y

X

FIGURE 7: “X-Y Raster: the sample moves under the tip fromleft to right (X) and up a little (Y) at the end of each X-raster.

8

Negative Feedback

Measuring tunneling current provides a way to sense the location of the tip relative to thesample. If there is no tunneling current, the tip is too far from the sample. If there is a smalltunneling current, the tip is near the sample. Negative feedback turns this position sensor into amicroscope. The device that measures the tunneling current is connected to electronic feedback circuitrythat obeys the following rules: if it senses a decrease in the current, it moves the sample closer tothe 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 isoperated by positioning the tip somewhere over the left-hand side of the sample and lowering ittowards 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 thisincrease and raises the tip to maintain a constant tunneling current. The tip will continue to riseuntil it is over the top of the bump. As the tip moves farther to the right, the current decreasesand the tip must be lowered to maintain a constant tunneling current. By the time the tip hasreached 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.

Electronic Data Collection

The final result of running the STM must be a picture. Unlike the optical microscope, youcannot 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 acomputer with graphic capabilities to look at the image. The STM image is built up one point ata time. At each data point during a raster scan, the computer generates the voltages required tomove the tip in the X-Y pattern and sends this signal to the control electronics which amplify thevoltage to a large enough value to move the PZT scanner. The control electronics use negativefeedback to apply a voltage to the PZT scanner to move the sample away from and toward thetip (in the Z-direction) so that the tip traces a contour of the surface. The control electronicssimultaneously sends the Z axis voltage, required to maintain a constant tunneling current, backto the computer.

Path traced by tipTIP

FIGURE 8 : The movement o f the t ip as i t scans the surface o f a sample.When the t ip gets c lose t o a bump, the tunneling current increases.The feedback l o o p , i n response, then moves the sample downward.

Sample surface

9

Vibration Isolation

One of the problems that delayed the development of Scanning Probe Microscopes was theintuitive feel that it would be impossible to hold two objects only a few angstroms apart withouthaving them crash into each other. Even though it can not be seen by the naked eye, objects arealways in motion relative to each other. Walking around a room causes desks and chairs tovibrate with an amplitude of around 1 micrometer, which equals 10,000 angstroms! To have thefeedback loop work as mentioned, these vibrations must be eliminated. So how does one isolate the microscope from these vibrations? The key issue in the designof a mechanical system, sample, and probe is having a rigid (high resonant frequency) andkinematically correct assembly. The rigid body will vibrate as a whole and the sample-probeseparation is not affected. Try to setup the scanning probe microscope on the lower level of your building and awayfrom 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 electricaldevices such as motors or generators. Place the STM Head on the heavy plate that lies on thepartially 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 ofunit.

2) Set MAGNIFICATION to X2. This setting will provide a 25,000 A scan range for thescanning module.

3) Adjust the X and Y OFFSET CONTROLS so that the segmented LED displays are set in themiddle position (12 o’clock position).

4) Set the STATUS MONITOR to REFERENCE FORCE/CURRENT selection. Set theREFERENCE 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+0.10 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.

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 isessential 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 ofthe wire does not touch anything.

Make sure that the STM Controller is turned OFF. Use the tip mount tweezers to hold andslide the tip mount into the tip mount holder that is in the STM Head Assembly (see Figure11). Make sure the tip mount snaps into place properly.

Test Sample Preparation (Holographic Gold Grating)

In the Accessory Kit, you will find a Holographic Gold Grating mounted sample (single periodhologram with a sinusoidal spacing of around 3000 Angstroms, Item #2). To prepare the testsample, 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 Headdownwards to retract the Sample Scanning Module away from the tip. The scanning moduleshould be at its lowest position to avoid damaging the tip when you place the sample inposition.

3) Insert the sample with the grating lines parallel to the Y-axis. To do this, line the bead ofglue that holds the sample onto the sample mount so that it runs left to right nearest to youas you place the sample mount holder on the STM Head Assembly.

TIP MOUNT HOLDER

SAMPLE MOUNT HOLDER

FIGURE 11: Front panel of ARIS-400 STM Head

11

4) Use the sample mount tweezers to hold the sample mount and to slide it into the samplemount holder. Make sure the sample mount snaps in place properly. The ball plunger onthe 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 SPMsoftware 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 bydouble-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 scannerrange in X- and Y- directions to 50,000 A. Set the Max scanner range in Z-direction to15,000 A. These maximum values are approximate values.

4) Select the Configuration option from the Collect menu. Set the Configuration menuparameters on the computer screen to those shown in Figure 12 on the next page. Theconfiguration 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 electroniccontroller, as well as the other controller settings. Notice that this is shown at the bottomof the configuration screen.

6) Use the APPROACH toggle switch at the rear panel of the STM Head to move the SampleScanning Module upwards and bring the top surface of the sample close to the tip. Try tobring 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 youbring the tip close to the sample, use the magnifying glass by placing it directly flushupon the front of the STM Head. It should focus nicely at this distance, and increasevisibility greatly.

7) Check the FEEDBACK ACTIVE light. If it is ON, then press the COARSE RETRACTtoggle 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 AUTOAPPROACH indicator must go on and the Sample Scanning Module starts slowly movingupward. If indicator does not turn on, then repeat step 5) and then step 6).

9) When the ACTUAL CURRENT equals the REFERENCE CURRENT, the AUTOAPPROACH indicator goes off and the FEEDBACK indicator goes on.

12

FIGURE 12: The parameters of the Configuration option for the gold grating, with definitions below.

Data points in X and Data points in X and Y. These numbers must be powers of 2, andY-directions: must be identical for both.

Number of substeps: Number of substeps you may wish to take between each of the Xand Y data points. This # must be in multiples of 2.

Scan range in X and The range given in parenthesis shows the available X and Y scanY-directions: range for a particular Zoom Factor selected. The numbers in the

boxes are the actual scan sizes.

Scan Range Scale: Maximum Z range of the acquired data. This depends on the modulescan maxima and the Z Gain Factor.

Sample Delay: Delay after each step in X prior to data taking.

Retrace Delay: Delay after each step during X retrace.

Scanline Delay: Delay after completion of each scan line.

Frame Delay: Delay at the first point of the image (at the top left).

Scan Mode: Scanning one frame at a time or continuously.

Topographic: Constant force imaging.

Force: Constant height imaging.

Friction: Lateral force of friction (not available).

Tilt Removal: Dynamic background removal.

Linearity: Activates the X-Y linearization circuits. Use this option only forlarge scan ranges. This removes the inherent PZT creep andnon-linear behavior.

OK

Cance l

Default

Data points in X -direction: 32 64 128 256 Samples. Scan Mode

Data points in Y -direction: 32 64 128 256 Samples. SingleNumber of substeps: 16 Substeps. Continuous

Scan range in X -direction: ( ) 25000 Angstroms. Data Type

Scan range in Y -direction: ( ) 25000 Angstroms. CurrentScan range in Z -direction: 15000 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) 150.00 Linearity

Approximate total scan time (MM:SS): 5:42

System Status

Bias voltage: +0.10 Volts. Zoom Factor: 2 Tunneling Current: 9.00 Nanoamps. Z Gain Factor: 1

13

Acquiring Images

Now you are ready to acquire the first image with the ARIS-3400 STM system. This sectionwill 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 onScan 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 ofthe 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 removehigh frequency noise.

-- The DIFFERENTIATOR GAIN would allow you to eliminate the imaging artifacts whichappear as shadows in the image. You can also use the DIFFERENTIATOR GAIN toimprove the quality of the image when taking “fast images” (images taken within 1-2minutes acquisition time. Do not hesitate adjusting feedback parameters or the scan speed tolearn 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 theimage 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 tiltwhich may be due to how the sample is mounted.

6) Select the Contrast View Data option from the Display menu. Contrast the image in theStandard Deviation mode.

7) You may apply other filtrations to optimize the image quality at this point as well. Note thatyou can select Undo Last option from the Filter menu to eliminate the last filtering stepwhich 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.

14

9) To PRINT the image, select Print option from the File menu. Select WINDOW whenprinting. It should take less than a minute for the printout to be completed by the HP ColorLaserJet.

Other Features of the True Image Software

1) Three Dimensional Viewing: Once the above image is optimized, you can actually see thesurface 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 up-down. 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 theAnalysis menu and pull down to the Cross Section option. In this mode you will notice thetop-view image at the upper-left part of the screen. With the mouse, move the cursor towhere you want to start your cross section line. Click on the mouse and drag until you areat the end of the line for cross sectional analysis. Click on the mouse, and at the lowerportion of the screen will be the cross section of the line that you just drew. When this isdone 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 onsection 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 withthe mouse and place the vertical line at the initial point of interest. Now, use the mousesimilarly to move the primed vertical line to the ending point of interest. After the twolines have been placed, click in the small colored box to have the desired numerical dataappear 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 effectof changing the following conditions:

Reference Current: The higher the reference current, the closer the STM tip must be to thesample 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 andthe tip) at 0.20 V, and compare with the 0.10 V trial.

Magnification: Run scans with the magnification set at x5 and x10.

15

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 currentmode, we will change to the constant height mode, and the instrument will measure the currentat 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 alarger 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 thescanning module.

2) Set the STATUS MONITOR to REFERENCE FORCE/CURRENT selection. Set theREFERENCE 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-0.02 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.

OK

Cance l

Default

Data points in X -direction: 32 64 128 256 Samples. Scan Mode

Data points in Y -direction: 32 64 128 256 Samples. SingleNumber 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 RemovalSample Delay (mSec/Sample) 0.10 Plane Removal

Retrace Delay (mSec/Step) 0.00 Line Removal

Scanline Delay (mSec/Line) 0.00Frame Delay (mSec/Frame) 150.00 Linearity

Approximate total scan time (MM:SS): 2:12

System Status

Bias voltage: -0.02 Volts. Zoom Factor: 200 Tunneling Current: 5.00 Nanoamps. Z Gain Factor: 1

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 shownon the previous page for the HOPG sample. Make sure the Reference Tunneling Current isat around 5.0 nA, and the Bias Voltage is at around -0.02 V.

3) Use the APPROACH toggle switch to manually bring the sample within a half millimeter ofthe tip.

4) Check the FEEDBACK ACTIVE light. If it is ON, then press the COARSE RETRACTtoggle 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 AUTOAPPROACH indicator must go on and the Sample Scanning Module starts slowly movingupward. If indicator does not turn on, then repeat step 3) and then step 4).

6) When the ACTUAL CURRENT equals the REFERENCE CURRENT, the AUTOAPPROACH indicator goes off and the FEEDBACK indicator goes on.

At this point, select the Configuration option from the Collect menu. Change the Data Typefrom 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). UseTilt 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 angstromscan.

Copper Samples:

Here we are going to see if the surface of copper is affected by various conditions. Take one ofthe copper samples, place it on a watch glass, and put one drop of 6M NaOH on it. Takeanother copper sample, place it on another watch glass, and put one drop of 1M HCl on it. Placeboth into the oven for one hour, which should be at around 160oC.

Set up the STM with the same conditions that worked best with the gold-coated grating. Noticethat the copper metal sample has both a shiny side and a dull oxidized side. Run a trial for eachside 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 thiscopper sample. Repeat for the sample that had HCl on it. The HCl should evaporate while inthe oven and will not need to be dried with the chem wipe.