Table of Contents

Atomic Force Microscopy

New users on the AFM are expected to get AFM training and then pass the AFM quiz. To find answers to some of the quiz questions, the “AFM Manualette” is a useful resource.

Note that OSU also has an AFM facility run by Brady Gibbons in Materials Engineering.

Scheduling time

To book time on the AFM, please use the group calendar (contact Ethan for access to this google calendar). Basic rules about booking time:

AFM Log Book

Please record your usage of the microscope in the AFM Log. Include:

If the Log Book is full, the word document of the form can be found in the Minot Group/Shared folder on the T drive.

Learning The AFM

For an overview, you should first, spend 5 minutes reading the Wikipedia article on AFM.

A useful YouTube video (I made a giant atomic force microscope) shows a giant AFM to explains how the tip-cantilever-surface interact with each other, and how the image is acquired.

Training involves

  1. A session where you watch an experienced user (and ask questions)
  2. Reading the “AFM Manualette”.
  3. A session where you “fly the AFM” with an experienced co-pilot talking you through each step.
  4. Take a written test to check that you understand the important concepts: AFM quiz.
  5. A session where you “fly the AFM” with an experienced co-pilot watching you. You can follow your own written notes, or use the “step-by-step walk through” that is provided below. This final supervised session should include changing a tip. (Use an old tip first for practice).

The “AFM Manualette” is located in two places:

With the AFM Manualette, there are some useful concept videos. I recommend watching The movies “Amplitude Feedback” and “Cantilever SEM”. There is also a comprehensive manual (Ver_04_08). I recommend reading chapter 5.

You must understand basic questions like

We use ac-mode imaging. So, it’s important to understand the basic idea of shaking a cantilever at its base to excite the first vibrational mode (the system achieves 100-nanometer-amplitude motion at the free-end of the cantilever, by shaking the base of the cantilever by a fraction of a nanometer) https://www.youtube.com/watch?v=AA6gWHu7GRs&ab_channel=SLURocketry

Step-by-step walk through for AC mode imaging

  1. Sign into the black notebook (on the table next to the AFM)
  2. Open version 16 of the AFM software (not version 14). It is critical that you use the correct version (switching between older/newer versions can corrupt the x-y stage calibration, which causes violent shaking of the x-y stage when the user starts scanning). A copy of a V16 AFM control Igor experiment is in the local Public documents folder.
  3. Click the first option, “Template”
  4. Once software loads, set AC mode in master panel
  5. Place the sample to be imaged on the tray. Make sure that the stage x-y control thumbscrews are centered.
  6. Make sure the vibration isolation stage is on and isolation is enabled.
  7. Raise the legs on the MFP-3D tripod by ~5 turns to ensure the tip does not smash into the sample.
  8. Set the MFP-3D over sample
  9. Align Laser:
    • Turn on the camera - Click the lower left icon with a picture of a camera on it
    • Turn on the camera light - Switch on the box which sits on top of the AFM
    • Align camera on cantilever - Two knobs sticking up at the very rear of the MFP-3D
    • Turn on the laser - Key switch on the AFM computer
    • Focus camera on the tip - Use the the focus ring toward the rear of the MFP-3D
    • Move laser toward the tip of the cantilever - Use the thumbscrews on the back & right side of the MFP-3D to maximize the 'Sum' signal
    • Adjust the photodetector (PD) - Use the thumbscrews on the left of the MFP-3D
      • Set the 'Deflection' in the S&D meter to zero (tapping mode) or a sliver negative (contact mode)
  10. X Set AC Mode - In main tab of the master panel select 'AC Mode' in the 'Imaging Mode' pull down menu
  11. Tune the AFM
    • Open 'Tune' tab in the master panel
    • Set 'Target %' to -5.0 % (this setting favors repulsive mode imaging, often recommended for beginners)
    • Click the 'Auto Tune' button and wait for tuning to finish. Software will set drive frequency.
  12. Engage the tip
    • Set the I gain to 10
    • Make the 'Set Point Voltage' about 95% of the target amplitude (1 V by default)
    • Click 'engage' in the S&D meter and
    • Lower the tip (tripod thumbwheel — left = up, right = down) while watching the amplitude. The amplitude will drop as you near the surface. The computer will beep when feedback kicks in to stop the amplitude from dropping below the setpoint. Continue lowering until the Z voltage is in the middle of its range.
    • If the Z-voltage goes down instead of up, re-tune. If that doesn't fix it, you might have junk stuck to the tip.
    • Lower the 'Set Point Voltage' by about 10%.
    • Lower the tip until the 'Z voltage' approximately in the middle of its range.
    • Lower the 'Set Point Voltage' by about 10%. Watch for “hard stop” on the Z-voltage.
    • Disengage the tip by clicking 'withdraw' in the S&D meter
  13. Close AFM Hood
  14. Set image details in the main tab
  15. Scan the sample, clicking 'frame up' or 'frame down' will start a scan

When done imaging

  1. Click 'Stop!!!' button to stop the current scan and withdraw the tip
  2. Open AFM Hood
  3. Manually retract tip from sample - Give the front thumbwheel a few clockwise twists
  4. Turn off laser - Key on the AFM computer
  5. Turn off camera light - Switch on the box sitting on top of the AFM
  6. Place MFP-3D onto its shelf holder
  7. Remove sample
  8. Close software and log out
  9. Leave controller and PC running unless expecting a power outage

Imaging rules of thumb

It is easiest to get a good image on a small scan area (~ 1 micron). Starting from the default settings you can fine tune the image and then start increasing the scan size. Good settings will minimize ringing and reduce shadows while keeping the scan rate reasonably fast.

Beginner settings

Rule of thumb: “One high quality slow scan is worth ~5 low quality fast scans.”

It is tempting to be impatient and try to 'tune' the imaging parameters to get the data you want from a 2 - 5 minute scan. This strategy often backfires though, if you need to return to old AFM images and find they are junk aside from the information you 'tuned' the parameters for. Also, 'tuning' these parameters in the first place probably takes ~10 minutes so you're not really saving time anyway. The best fix for many imaging problems is simply to lower the 'rate'. The price you pay is scan time. However, I've found that you actually save time (and headache) by taking a single high quality slow scan rather than a bunch of quick ones with little parameter adjustments in between. I find that adjusting the rate so that the scan speed is <10 micron/sec works well in nearly all cases.

Using nanotubes as a diagnostic tool

When imaging nanotubes they should not be blurry in xy. Rather, they should be sharp lines about 20-40 nm thick and a few nm tall. If tubes are blurry in the xy, verify you are using enough scan lines (try 256 or 512). If they are still blurry reduce the 'rate' and/or the 'set point'. If the tubes are still blurry, do a force curve to verify that the tip is intact. If the tubes are still blurry after following these steps then you've likely got a thin layer of crud on your nanotubes.

Ringing On a perfectly flat surface, the amplitude should stabilize at the set point. If the amplitude cannot stabilize (often called “ringing”), the integral gain may need to be reduced.

Shadows Tall objects cast shadows because it takes time for the AFM tip to relocate the surface after it steps off a cliff. The rate at which the tip finds the surface is proportional to

Gain x (Measured Amplitude - Set Point Amplitude)

Therefore, shadows are minimized by high gain and high amplitude difference (at the cost of more ringing and higher hammering force respectively). Shadows can also be minimized by slow scan velocity (at the cost of small scan area or lots of time).

Attractive vs repulsive When finding nanotube diameters, or looking at soft biological samples, it is useful to work in attractive mode imaging. The cantilever should be tuned above resonance (+5 %) and the amplitude should be small (for example 0.2 V). For more info see the Asylum phase imaging posterphase imaging poster.

If the PhaseTrace switches from values below 90 to values above 90 then you're switching between repulsive and attractive mode respectively (see graph below). This wears out the tip and leads to weird artifacts in your height image. Black dots show up in the phase picture. To leave this unstable imaging condition, either de-/increase the Set Point (at the cost of more force/more shadows) or increase/reduce the Drive Amplitude. When changing Drive Amplitude, it is best to stop imaging and watch the measured amplitude and then choose an appropriate Set Point. In the following graph you can see the phase jumping from attractive to repulsive mode.

(pictures from Asylum Research poster)

Understanding the AC feedback In the right picture above, you see a sketch of the AFM.

Replacing AFM Tips

When to change an AFM tip

AFM tips are like shaving razors. You use them until they are blunt (or until they get tenticals stuck to the tip which then stick to the surface). There are tricks to prolonging the life of an AFM tip, and ways to know if it should be thrown out.

  1. Watch the phase image. If the phase is jumping from <90 to >90 degrees, you will quickly ware out the tip. Change imaging parameter to favor either repulse or attractive imaging.
  2. If the tip is blunt, sharp objects will look fat in your images. For example, a 1 nm tall nanotube which usually look 20 nm wide will start looking 100 nm wide.
  3. If something is stuck to your AFM tip, it will show up in a Force curve.

To replace a tip

  1. Invert the AFM head onto the stabilization table. Remove the tip holder by pressing the black rubber button and secure it in the temporary mount by the laser.
  2. Gently unscrew the clamp holding the tip down with a Phillips screwdriver. You don't want to completely unscrew it, just loosen it enough to remove the tip.
  3. Grab the old tip by its sides with a pair of tweezers and pull it away from the clamp. It should slide out freely.
  4. Remove a new cantilever from the box with a pair of plastic tweezers using a twisting motion - be careful not to touch the end with the tip! Grab the new cantilever with your tweezers so the end you want to insert into the tip holder is pointing away from you and the tip is pointing upwards.
  5. Slide the tip between the tip holder and the clamp (probably just as the old one was placed). The cantilever should be placed so it isn't inserted too far or too short. Too far, and the tip will not be well situated in the holder and zeroing the deflection will be impossible (see the figure below).
  6. Once the tip is inserted properly, tighten the clamp and replace the holder in the AFM head. Make sure to rotate the AFM head in the opposite direction you used to invert the head, to avoid twisting the head cable.
  7. Test the tip insertion two ways: i) Make sure the laser spot can be placed on the tip and the deflection can be zeroed without placing the wheels at their maximum range. ii) Auto tune the AFM, and make sure the amplitude and phase plots are free of any irregularities.

Trouble shooting

AFM doesn't do what you want it to do (e.g. laser doesn't turn on or z-voltage behaves strange) it is always a good idea to

The computer takes a long time to log in.

You keep getting streaky images(Streaky images can have multiple sources)

If you see an amplitude and phase image that look reasonable, but the height and z-sensor have no texture (like imaging air would look) be sure that the computer doesn't need to update. In the past, a pending windows update has caused this issue.

Sample preparation

Static When working with insulating substrates, charge build up can be a problem. You will have trouble engaging with the sample if it is charged. Try waving the “static master” plunomium alpha particle source over the sample for 30 seconds.

Mica Mica is a silicate mineral that has a tendancy to split nearly perfectly between layers leaving a very very smooth surface. This quality allows us to use Mica as a testing ground to find the distribution of small particles.

Mica Preparation

Flat gold surfaces Atomically flat gold is the standard substrate for STM (scanning tunneling microscopy). It is tricky to get atomically flat gold. One technique is flame annealing. There is info at this website: http://www.arrandee.com/Products/Gold_on_Glass/body_gold_on_glass.html

Image analysis

The MFP-3D software in Igor offers many convenient analysis features. Such as roughness calculations, cross sections, etc. The detailed information on the image processing can be found below.

AFM Image Processing

Representation Panel:
Analyze Panel:
Modify Panel:

FFT icon can be used to fill mask and exclude the undesired data points. (ask Landon) The processed image can be found in the representation panel, named as HtT* 0 which can be saved.

For more specialized image analysis try ImageJ. ImageJ is a free software from the National Institutes of Health (NIH). It is a useful tool for doing complex image analysis tasks, such as measuring the length of wiggly CNTs or DNA, or measuring particle sizes and outputting size distributions.

Gwyddion is another free software designed for analyzing scanning microscope probe images. It is intuitive and has many features to improve image quality. Carly has found it useful for looking at images of graphene.

Supplies

Silicon AFM tips can be bought in boxes of 10 or 50 units, or as an entire wafer (380 units). There is a big discount for buying a full wafer.

A spring constant of 40 Newton/meter is very common because it can be used for very small tapping amplitudes without snapping to the surface. It is widely used and therefore reasonably cheap. However, it is not great for pushing into the surface because the tip breaks off easily.

A spring constant of 2-5 Newton/meter is very versatile. It can be used for tapping mode imaging, nanolithography, electric force imaging, contact mode imaging in liquids and force distance curves.

Recommended AFM tips:

Data export for visitors

The easiest way for visitors to take home their data is the IgorPro format (.ibw files). IgorPro is the platform on which the Asylum Research AFM has been built. A trial copy of IgorPro software is available from the IgorPro website. The trial version of the software will get you a long way. The academic copy costs $400. The MFP3D software (written by Asylum Research to run on the Igor plaform) is open source. We can supply you with a copy.

Contacting Asylum Research

Asylum Research has a very friendly and helpful support department. Using the long distance access number (ask Ethan for this number) you can reach AR support at 888-472-2795 or email them at support@asylumresearch.com. When emailing about a specific AFM image related using and you need to send them files, you need to upload the files to their FTP server using a Java interface at www.asylumresearch.com/upload. You should put all the files together in a zip file if your sending more than one file. If you're having a technical issue with the AFM software, the AR support guys can remotely control the MFP-3D software and try to figure out whats going on. For the quickest results calling them is the best method.