Chapter 1: Magnetic and Atomic Force Microscopy (MFM and AFM)
• Section 1: A Brief History and Theory of Atomic and Magnetic Force Microscopy
The first prototype atomic force microscope (AFM) was tested in February 1986 at IBM’s Almaden
campus near Stanford University by Gerd Binnig, Christoph Gerber, and Calvin Quate [1]. The first
atomic force microscopes used contact measurement to determine surface topography. In this method,
a cantilever is moved across a sample and the cantilever displacement is measured to image the
sample’s surface topography (Figure 1.1). The cantilever displacement is continually monitored by a
position‐sensitive detector through the reflection of a diode laser beam from the end of the cantilever
as it drags across the sample suface. This method of imaging is generally considered a destructive
measurement in that measurement of the surface may also fundamentally alter the surface.
Figure 1.1 – Contact measurement [2]
One solution to the problem of destructive microscopy is non‐contact atomic force microscopy (NC‐
AFM). The first successful NC‐AFM scan was completed by Giessibl, Kitamura, and Iwatsuki in 1995 [2].
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The basic premise of NC‐AFM is that a cantilever oscillates over the sample with a certain frequency.
The cantilever experiences a weak repulsive force which results in a shift in the cantilever frequency.
Since 1995, many advances have been made in NC‐AFM and such achievements include true atomic
resolution, three‐dimensional measurements of atomic forces (also known as atomic force
spectroscopy), mechanical manipulation of individual atoms, and mechanical assembly atom by atom
[2]. One new technique that may improve resolution even further is the so called dual‐AC NC‐AFM,
which involves driving the cantilever at both the fundamental frequency and at a higher harmonic
simultaneously [3].
Figure 1.2 – Non‐contact measurement [2]
One form of NC‐AFM is magnetic force microscopy (MFM). Magnetic force microscopy can be used
to detect the magnetic field due to some sample. For MFM, the cantilever is magnetized and driven by a
piezoelectric bimorph. The cantilever’s displacement as controlled by the bimorph takes the form
( )tωαμμ sin0 += (1.1)
where μ is the distance above the sample, 0μ is the equilibrium position of the cantilever, α is the
amplitude, and ω is the cantilever frequency. In the presence of a magnetic material, the cantilever will
experience a force like that of a dipole in a magnetic field
2
( )BmFrrrr
⋅∇= (1.2)
where is the dipole moment of the cantilever and mr Br is the magnetic field due to the sample. Only
the z‐component of the force on the cantilever is measured
dz
dBmF zzz = (1.3)
The final equation of motion then takes the form of a driven damped harmonic oscillator
( ) ( tsFkdtds
dtsdm ωαγ sin02
2
=−+− ) (1.4)
where m is no longer the dipole moment, m is now the mass of the cantilever, is the cantilever’s
displacement from equilibrium, γ is the damping constant, k is the spring constant of the cantilever, Fo is
the initial force on the probe, α is the amplitude of the driving force, and ω is the cantilever’s driven
frequency.
s
A sample of a typical MFM scan of a zip disk, figure 1.3, can be seen on the next page. It should be
mentioned that this scan represents a map of the force that the cantilever experiences, not the field
that it encounters. Therefore, because the force is proportional to the gradient of the field, the areas of
higher intensity represent the greatest change in the field. In other words, the regions of higher
intensity denote the individual domain walls where the greatest changes in field take place. The picture
of the zip disk below shows the domains shifting from in‐plane to out‐of‐plane which is a good
illustration of the changes in magnetic field that are essential to storing binary data, that is, 0’s and 1’s.
A more in‐depth discussion of the specific means of operation of our magnetic force microscope in
both contact and non‐contact modes can be found in Chapter 1, Section 2, along with procedural
details.
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Figure 1.3 – Three‐dimensional MFM scan of a zip disk
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• Section 2: Operation Guide to the Q‐Scope 250 AFM/MFM (Also Includes Instructions for Use of the MFM/AFM ScanAtomic 5.0 Software)
How to use the MFM/AFM ScanAtomic 5.0 software
To begin using the MFM/AFM select the “Shortcut to Scan Atomic” button by double clicking the
icon on the Desktop. Once done the Ambios Technology startup screen will
appear. Figure 1.4 shows the startup screen has many functions that can be accessed by single clicking
the icons.
Shortcut to ScanAtomic.lnk
Figure 1.4 – ScanAtomic Startup Screen
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• Open: Opens a previously saved scan.
• Browse: Allows the user to view 12 scans at once and then select scan for further viewing or
manipulation.
• Save: Saves the current completed scan
• Configure: Allows the user to adjust the microscope settings.
• Laser Align: Allows the user to see laser signal and detector position, needed during laser
alignment.
• Wavemode: Allows the user to lock in a driving frequency for the cantilever.
• Scan: Will bring up scan screen to begin scanning or to view previous scans.
• Laser On/Off: Turns the laser on or off
• Probe Position: Allows the user to view camera, adjust probe height and camera brightness
• Engage: Allows the user to automatically lower the cantilever into scanning position.
• Withdraw: Withdraws the probe one step.
• Image Graph: Allows user to view a 3‐D model of scan.
• Section 3: Overview of the Microscope
The operational part of the AFM, the part that actually
does the measuring, is the cantilever. The cantilever consists of a
cross and a die as shown in Figure 1.5. The approximate
measurements for each die are 1.5 by 3.6 mm. Because of their
small size, the cantilevers are extremely fragile (and expensive)
and great care must be taken when replacing or positioning each Figure 1.5 – Cantilever
Diagram [4]*
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cantilever. Figure 1.6 shows a cantilever that has been mounted to the microscope.
• Section 4: AFM/MFM Experimental Procedures
Experiment 1 – Contact Mode Scan of a Diffraction Grating
The first experiment is to complete a simple contact mode scan of a diffraction grating with 1.1
micron spacing. This will allow you become more familiar with the Scan Atomic software as well as the
microscope itself.
Section 1.1 – Installing the Probe
(WARNING: BEFORE INSTALLING A
CANTILEVER, MAKE SURE THAT THE LASER IS
TURNED OFF.) Cantilever installation is relatively simple.
First remove the scan tube from the microscope base. Next,
choose a contact probe. Using a pair of tweezers, lift the
cantilever out of the cantilever box. Use another pair of tweezers or a small lever to lift up on the tab of
the probe holder (Figure 1.6). Making sure that the die is facing out, gently place the cantilever in the
probe holder as shown in Figure 1.6. Once the cantilever is in place, release the tab. The final step is to
adjust the cantilever so that the cross is centered in the probe holder. During this entire procedure, it is
very important to make sure that the cantilever tip does not bump against anything because even a
slight bump will break the cantilever off of the cross. Finally, replace the scan tube into the microscope
base and tighten the secure screw into place. CAUTION: Take great care not to damage the scan head
Figure 1.6 – Mounted cantilever [4]*
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or bump it as it is also extremely delicate. If you improperly handle the scan head the tilt lever or
probe holder could potentially come unglued, resulting in thousands of dollars in repair costs. Also,
to raise the scan head up and down select the button, do not manually raise the scan head by
the dial on top of the machine, this could strip the gears and ruin the device.
A video detailing the installation of an AFM probe can be made available by Dr. Boley upon
request.
Section 1.2 – Aligning the Laser
The AFM uses a diode laser to detect the position of the cantilever by measuring the angle of
reflection of the laser beam off the cantilever. Figure 1.7 shows the basic laser setup. Because of
manufacturing variations between different cantilevers and variations on how the cross is inserted in
the probe holder, the laser must be realigned every time the cantilever is replaced. Occasionally, it is
necessary to make minor adjustments between scans as well, but this does not happen often. A semi
conductor diode laser is used within the scan head to provide the feedback for the sample surface
imaging. Its wavelength is 670 nm, in the red region of the spectrum.
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Begin by pressing the toggle laser button to turn on the laser and then press the camera
button . Adjust the camera from side to side
until the cantilever is visible. You should see a
picture similar to Figure 1.8. If the cantilever is not
visible, adjust the tilt lever until you can see it. If
the cantilever is still not visible, this means that the
probe is broken and you must replace it. There is
also another means of viewing the cantilever probe
which consists of connecting the video feed from
the scan head into the video input of a television.
From there the cantilever should definitely be
visible unless it is broken.
Adjust the x and y laser position knobs for the laser until the laser spot is visible on the cross. If
the laser spot disappears under the cross, this means that the spot was only a reflection and not the
laser spot. Once the laser spot is on the cross, adjust the x position until the laser spot is just to the left
of the cantilever but still on the cross (Figure 1.9). Turn the laser y position knob approximately a
quarter of a turn counter‐clockwise. The laser
spot should slide down the end of the
cantilever. With VERY small x and y laser
position adjustments, place the laser spot between three‐quarters and the end of the cantilever (Figure
1.10). Do NOT move the laser position adjustments more than an eighth of a turn in either direction
because it is easy to lose track of the laser spot if larger adjustments are made at this point.
Figure 1.7 – AFM laser optics [4]*
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cross laser spot
spot on camera cantilever
Figure 1.8 – Cantilever as seen by the camera
Figure 1.9 – Laser spot on the cross
Figure 1.10 – Laser spot properly positioned
One problem with positioning the laser like this is that the
angle of reflection to the detector is not equal to the angle of
reflection to the camera. To remedy this problem, we use the
alignment window. Turn the mirror position knob fully counter‐
clockwise. This positions a mirror in the path of the laser beam and
diverts the beam from the detector to the alignment window (Figure
1.11). If the beam is properly aligned, the laser spot will appear as a concise point near the center as
shown. For our AFM, the point is generally fairly diffuse and closer to the top of the window. Normal
operation is still possible with the beam that we generally use, although sometimes MFM cantilever
amplitude suffers as a result, as will be shown later. To improve the laser spot in the beam alignment
window, either VERY small adjustments can be made to the laser position or small adjustments to the
cantilever tilt lever can be made. CAUTION: Don’t turn the knob too far! Once the desired laser spot
has been observed, turn the mirror knob completely clockwise. If the mirror is left in place, the beam
will be blocked from reaching the detector and make aligning the detector impossible.
Figure 1.11 – Beam alignment window [4]*
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Section 1.3 – Configuring the AFM Settings
Access the configuration menu by pressing the button. This action will display the
configuration menu as seen in Figure 1.12. Options can be changed either by clicking and dragging the
red dials or by typing a numerical value in the white boxes. The blue boxes do not allow for numerical
entry. Here is a list of options that are commonly accessed from this menu.
Figure 1.12 – Configuration menu
• Scan size: Allows the user to specify the size of the area to be scanned. For example a Scan size
setting of 40 µm will scan an area 40 µm by 40 µm.
• Scan Rate: Sets the number of image lines scanned per second.
• Setpoint: This functions differently depending upon the Scan Type. See below for a further
discussion.
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• ScanDirection: Designates an angle of rotation about the z‐axis at which the region will be scanned.
This will be counterclockwise from x towards y when viewed from down the positive z‐axis.
• Integral, Proportional, and Derivative Gain: These three controls determine the response of the z‐
axis feedback circuitry, the circuitry responsible for maintaining the probe deflection
(contact mode) or vibration damping (non‐contact mode) when engaged with the surface.
• Scan Resolution: The number of images points in a horizontal line of the scan. Higher scan
resolutions reveal greater surface detail, but require more time to acquire the image.
• Scan Type: Allows the user to choose what type of scan to take. There are about ten choices, but
only two will be described below.
• Delta Z: Allows the user to choose how far above the sample the cantilever will oscillate in non‐
contact modes.
• Bias Voltage: This control is used to set the voltage of the probe holder when the electric field
gradient is measured in an “ME” mode.
• Center X and Y (µm): This allows the user to adjust the start position of the scanner. 0 is the center,
to move the scan left and down the user will need to select a negative number for x and y,
to move up and to the right the user will need to select a positive number for x and y and so
on within the limits of 41 µm.
• Xy signal Mode: This allows the user to select between different methods of rastering the probe
across the surface. This setting needs to be on Standard mode for our device.
• Z signal Mode: This allows the user to select between different methods of measuring the Z
position of the probe tip. This setting needs to be on Standard mode for our device.
• XY disabled: This setting allows the user to disable the X, Y raster during a scan. The control box for
this application needs to be unchecked for normal operation.
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Section 1.4 ‐ Configuring Contact Mode
To run the AFM in contact mode, choose the “Z Height” option from the Scan Type drop down
menu. This mode is a contact‐mode topology scan based on maintaining a constant force between the
probe tip and the surface. See Table 1.1 for a list of appropriate settings for Z‐Height mode. In general,
unless you have a mastery of AFM operation, the Integral, Proportional, and Derivative Gain should
NEVER be changed because improper settings have the potential to damage the cantilever and the scan
tube itself. The settings shown in Table 1.1 have been meticulously chosen to optimize scans for our
microscope per consultation with Ambios Staff and former graduate students.
For this experiment, choose a scan size of 40 µm, a
scan rate of 2 Hz, and a scan resolution of 1024. The faster
scan rate has been chosen to make a rough image of the
scan. A second scan will be taken later to get a better
image. Once these have been set, press the “Ok” button to
send the settings to the microscope and close the window.
Option Range of SettingsScan Size 1 - 40 µmScan Rate 0.5 - 2 HzSetpoint 0.00 VScan Direction 0o
Integral Gain 300Proportional Gain 250Derivative Gain 0Scan Resolution 100-1024
Table 1.1 – Z‐Height settings Section 1.5 – Aligning the Detector
The next step is to align the detector. Begin by pressing the align laser button on the
ScanAtomic toolbar. The beam align window (Figure 1.13) will appear. The scale on the right indicates
the intensity of laser light that is being detected. The target indicates the position of the laser on the
detector. Adjusting the x and y detector position knobs will move the red cursor around the target. The
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required position of the cursor depends upon the type of microscopy. Figures 1.13 shows the desired
cursor location for the contact mode. Adjust the x and y detector position knobs until the cursor is in its
desired location. CAUTION: Be careful not to turn the knobs too far or you will strip the detector
position screws.
Figure 1.13 – Desired cursor position for contact mode
Note that if the cursor is in the correct position, the arrow of the intensity scale should be in the
green region. If it is not, minor adjustments may need to be made to the laser position, or the cantilever
tilt. In either case, make sure that the camera is on before adjusting the laser. Usually, moving the laser
spot either further down the cantilever or closer to the center will remedy this problem.
Section 1.6 – Engaging the Probe
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Begin by placing the diffraction grating slide on the sample stage and securing it to the stage by
any practical means (scotch tape works well). Close the lid of the microscope enclosure and lock it in
place. Access the scan head control screen by pressing the button. The scan head control screen
is shown in Figure 1.14.
Figure 1.14 – Scan head control screen
While watching the sample AT ALL TIMES, press the ”fast speed down” button to lower the
cantilever towards the sample. As long as you hold the down button, the cantilever will lower towards
the sample. Don’t worry about being too far from the sample. The computer will lower the cantilever
into its final position. Once the cantilever is close to the sample, close the scan head control screen and
access the laser alignment window to confirm that the detector is still aligned. Sometimes the tilt lever
will get bumped when lowering the cantilever, so the alignment must be checked after lowering the
cantilever into position.
15
The next step is to access the engage menu by pressing the button. The engage screen for
AFM is shown in Figure 1.15. The only difference for contact mode is that the “Wavemode” box is not
there. Once this is done, press the “engage” button. The computer moves the cantilever closer in single
step increments (Figure 1.16). Depending on how far the user is from the sample surface, this
procedure could take a really long time so be patient. Once the cantilever is in range, a screen like
Figure 1.17 will appear.
Figure 1.15 – Basic engage screen
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Figure 1.16 – Engage screen while the cantilever is being lowered
17
Figure 1.17 – Cantilever in range
Now that the cantilever is in range, it is time to begin a scan.
Section 1.7 – Scanning the Diffraction Grating
To access the scan screen, you can either press the scan option at the top of the engage screen,
or by pressing the button on the ScanAtomic toolbar. The scan screen is shown in Figure 1.18. To
begin scanning, press the “scan” button in the upper left‐hand corner of the scan screen. The scan can
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be aborted at anytime by pressing this button again. This final image should look like that in Figure
1.19. If the scan does not remotely resemble that in Figure 1.19, consult the Quesant Operator’s
Manual [4], or the trouble‐shooting guide toward the end of this chapter. Once the scan is completed,
you can save it by clicking the “save as” option on the menu bar at the top of the window. Upon clicking
“save as,” a window will appear. Click “OK” and then save the scan in an appropriate directory. The
appropriate directory is “C:/ProgramFiles/SPM/Images” and then in the folder that fits the particular
scan to be saved.
Figure 1.18 – The scan screen
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Figure 1.19 – Initial diffraction grating scan
Now that the scan is saved, press the withdraw button at least three times to withdraw the
probe from the sample. The sample can now be safely moved using the micrometer dials on the sample
stage. One‐eighth of a turn of the micrometer corresponds to a displacement of 40 µm. To re‐engage
the probe, you can either press the “engage” option located next to the “withdraw” option on the menu
bar, or by closing the scan screen and pressing the button on the Scan Atomic toolbar. From
there you can re‐engage the probe, and take another scan.
Now, in the configuration menu, change the scan rate to 0.5 Hz and press “Ok”. Engage the
probe again and to obtain a clearer image (Figure 1.20). Save this image as well.
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Figure 1.20 – Final diffraction grating scan 3‐D
Continue to make other scans by withdrawing the probe three times, adjusting the settings in
the configuration menu, and engaging the probe to get a better understanding about how each setting
will affect the scan. As you save scans, make a note about the settings used for each scan.
Notice that on the left side of the scan screen there is a box called “Feedback Settings.” This box
allows you to change to a different setting in mid‐scan. Changes will not take effect until the “download
parameters” button is pressed. Adjusting these parameters in mid‐scan is not recommended until you
have an advanced understanding of the AFM’s operations.
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Once a scan has been completed, there are several ways to
manipulate each scan. One option is the “Tilt Removal” option as
shown in Figure 1.21. When the AFM scans a particular region, the
scan head may not be perfectly parallel to the scanning surface, this allows you to removal the angle of
tilt of the scan head. Bear in mind that in most cases the scan head can never be made exactly normal
to the sample surface. When a scan is saved, only the tilt removal that has been selected is saved. It
does not save all of the data in one file. To save more than one tilt removal, the data must be saved in
multiple files. It is important to save your original scan data before removing any tilt in case the tilt
removal process actually worsens the ability to identify features of the scanned surface. Two other
adjustments can be made to the scans and that is line and spot removal. These allow you to take any
high or low point and average them into their surroundings. If there is an extremely large peak in your
scan then it could potentially over power the whole scan and would need to be fixed. To access these
features just click on either and then drag the cursor over the area that needs to be fixed.
Once that is done simply select the “Rescale” option in the retouch file so that the final image scale
reflects the data you have kept.
Figure 1.21 – Tilt Removal
Once all scans have been completed, access the scan head control screen and withdraw the
probe entirely before removing your sample to prevent accidental damage to the cantilever.
‐
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• Section 5: A Trouble‐shooting Guide to Atomic Force Microscopy
This section is to act as a trouble‐shooting guide for problems encountered when operating
Quesant’s Q‐Scope 250 and the ScanAtomic software. It has been arranged so that each section
contains problems encountered during each process in aligning, configuring, and running the AFM. The
general format for this appendix is as follows:
Problem.
Solution.
Of course, this section cannot possibly cover every error message or problem you may encounter. If you
get stuck and no one can figure out your problem, you can always contact former students1,3 or Ambios
Technology2 at their toll‐free number, 877‐429‐4200.
Section 5.1 – Aligning the Laser
The cantilever and cross appear blurry.
This indicates that the camera is positioned either too high or too low. Unscrew the camera and adjust
it up and down until the image comes into focus.
1 Matt Beckner ([email protected]) e-mail generally answered in a timely fashion. 2 At the time of publication, Ambios Technology owned all the rights to Quesant’s AFMs. 3 Christopher Milby ([email protected]) email generally answered in a timely fashion.
23
The cross is in focus, but the cantilever is not visible.
Adjust the tilt lever until the cantilever is visible. If adjusting the tilt lever has no effect, this means that
the cantilever has broken off and must be replaced.
When I try to move the laser spot onto the cross, it disappears.
This means that the laser spot is not the laser spot at all but rather a reflection. Continue adjusting the x
and y laser position knobs until the spot is visible on the cross.
Section 5.2 – Configuring the AFM Settings
I can’t change the Delta Z, Bias Voltage, or set the cantilever frequency.
This means that the scan type is set to something other than “ME Tphase.”
Section 5.3 – Aligning the Detector
The cursor does not move no matter how much I move the detector position knobs.
This could be caused by one of two problems:
(1) The beam alignment mirror is still in the path of the beam, preventing the beam from reaching
the detector, or
(2) The settings in the configuration menu have not been downloaded to the control unit.
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The blue cursor is in its desired location, but the laser signal is still not in the green.
Adjust the laser spot either further down, or closer to the middle of the cantilever. This should improve
the signal. Adjusting the tilt lever may work also.
Section 5.4 – Setting the Cantilever Frequency
The cantilever amplitude is either very low, or non‐existent.
Adjust the cantilever tilt lever until the peak reaches a maximum. Return to the detector alignment
screen and re‐align the detector. Repeat this procedure until an adequate peak is obtained. Adjustment
of the laser spot on the cantilever may also be necessary.
The button is locked so that I cannot lock‐in a frequency.
You must zoom‐in on the peak, by clicking on the peak, before you can lock‐in a frequency.
Section 5.5 – Engaging the Probe
I have a message saying that the approach was aborted due to loss of laser signal.
Check to make sure that the tilt lever is not in contact with a portion of your sample such as a Petri dish
edge. This will cause the laser to come out of alignment upon engaging. This also happens if the blue
25
cursor on the detector alignment screen is in the wrong position. Remember there is a different desired
cursor position depending on which type of microscopy is being used.
There is an error message regarding the Setpoint Voltage.
This could be caused by many things. Try these solutions in this order.
(1) Restart the program.
(2) Restart the computer.
(3) Check the alignment screen and make sure the red cursor is in the correct location.
(4) Realign the laser optics.
(5) Replace the probe.
(6) Try a probe from a different box.
(7) Contact a former student.
(8) Call Ambios Technology (877‐429‐4200).
The probe was in range and feedback was on, but it quickly went out of range.
Adjust the setpoint voltage until it is back in range. This is only a temporary solution for the current
scan. The laser optics will need to be re‐aligned.
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Section 5.6 – Scanning a Sample
The scan was good, but then the image turned to black suddenly.
The scan found a peak (probably an anomaly) that was much higher than the background. Use the spot
remover tool after the scan is completed to remove the spot.
The beginning of the scan was good, but the rest was black.
This indicates that the probe probably broke during scanning. Check the camera to verify this. If it did,
replace the probe. If the scan was in contact mode, run the next scan at a lower scan rate. If the scan
was in a non‐contact mode, raise the z‐height for the next scan. If the probe is not broken, then the
laser came out of alignment in mid‐scan.
I’ve opened a scan that was taken in Z‐Height mode, but when I go to the view screen, the z scale is in
arbitrary units instead of nanometers or micrometers.
The scan was saved while viewing a buffer other than z‐height. The original z‐height data is now lost.
Therefore, it is important to save your original data from the Z‐height mode as soon as you collect it.
27
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References:
1. Giessibl, Franz J. and Quate, Calvin F. “Exploring the nanoworld with Atomic Force Microscopy.”
Physics Today Dec. 2006: 44‐50.
2. Morita, S., Wiesendanger, R., and Meyer, E. Noncontact Atomic Force Microscopy. Berlin: Springer,
2002.
3. Proksch, Roger. Multifrequency, repulsive‐mode amplitude‐modulated atomic force microscopy.
App. Phys. Lett. 89, 113121 (2006).
4. Operator’s Manual: Q‐Scope 250. Agoura Hills, CA: Quesant, 2003.
5. Matthew, Beckner. “Size Effects on Magnetic Hysteresis, Torque Load Sensitivity, and Domain Wall
Profiles in ESR‐420 Steel.” B.S. Honors Thesis. Western Illinois University, May 2006.
All figures marked with an “ * ” are:
[Modelled after a figure from Operator’s Manual Q‐Scope 250, by Ambios Technology Corporation,
2007]