Atomic Force Microscopy

To book time on the AFM, please use the AFM scheduler.

. Basic rules about booking time:

• People who have scheduled time get priority.
• Don't block off an entire 9am-5pm workday - leave at least an hour for someone to do a quick characterization.

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

• Date
• Name (write it in before you start imaging)
• What tip you used (TM or EFM)
• Frequency (measured using the “tune” button)
• General comments, e.g. raised back legs for larger sample, etc.

You should not attempt to use the AFM on your own until you have had training by a more experienced group member.

When getting to know the AFM you must spend some time watching an experienced person use it. Several aspects of the AFM require experience and dexterity:

• lifting the heavy head off the stage and flipping it over
• using a pair of tweezer to change the AFM tip.

The only way to learn this dexterity is careful instruction from an experienced user.

Below is a summary of the general procedure for TM imaging. For more detailed instructions, see Sam's AFM guide.

TM imaging

1. Sign in.
2. Determine which tip is loaded. You can do this by clicking the “tune” button (it looks like a tuning fork), then “autotune”; an EFM tip will be ~50 kHz, whereas a TM tip will be somewhere around 300kHz. Often you will be able to tell which tip is loaded simply by looking at the boxes surrounding the AFM, since we only regularly use two tips; the empty case is usually the tip that's mounted.
3. If changing the mount or tip:
   • Unlock the tip head by tightening the dial on the right side. Carefully raise the head, rotating it until the head is up. Slide the mount back into the groove, and loosen the dial to lock the mount in place. Wearing gloves, remove the old tip mount. If you plan to replace the tip, place the mount on the round holder; replace the tip, and then remount the tip on the head and flip it back over, following the procedure above.
4. Roughly align laser:
   • Using the dials on the top of the head, roughly align the laser to the tip. The “Sum” listed on the screen should be between 2 and 3.
5. Mount your sample on the rotating silver base, which should be pulled out towards you. Roughly align the chip, and remember to turn on the vacuum switch. To move the sample, clock on the “Move/Focus Stage” button (looks like a magnifying glass, with red lines behind it). Hold down the “lock” button on the trackball, and move the stage using the trackball until your sample is approximately under the light/laser point.
6. Focus on the tip:
   • This process uses the optics only, so don't worry about crashing the tip.
   • Click on the “Tip Focus” button (magnifying glass with yellow triangle in the background).
   • Move the camera's central point by turning the dials on the side of the camera labelled X and Y, until the tip is centered on the cross given. Focus on the tip by holding down the “Focus” button on the trackball and zooming in and out using the trackball.
7. Tune the tip by clicking on the “Tune” button, then “Autotune.” Double-check that your tip is the one you thought by checking the resonance frequency. Note that the autotune function will not work if your autotune range is set incorrectly; for EFM it should be 20-100kHz, TM from 100-500kHz.
8. Focus on the surface:
   • You can crash the tip doing this, so be careful.
   • Click the “Move/Focus Stage” button again.
   • Move to either an edge or another macroscopic feature, holding down lock and using the trackball.
   • Hold down the focus button, and use the trackball to zoom in.
   • When you're properly focused, move to your target device or area.
9. Set your presets. Some suggestions:
   • Scan size: 10um
   • x and y offset: 0
   • Scan rate: 1 Hz
   • Tip velocity: 20 um/s
   • Samples/line: 256 (increase later for better resolution)
   • Lines: 32
   • Data scale: 10um
   • Integral gain: 0.3
   • Proportional gain: 3.0
   • Interleave: off (this is for EFM imaging)
10. Engage the tip using the leftmost button (looks like a tip, and there's a green color). A scan will begin.
    • Lower the amplitude setpoint if your tip is not engaging the surface. This will reset each time you disengage and reengage the surface.
    • You may move in X or Y up to 20um using the offsets. If you wish to move further than that, you will need to disengage the surface (the button has a tip with a red color) and move using the “Move/Focus Stage” button and the trackball.
11. To save a file:
    • Click on the “Capture” menu, and select “Capture Filename.” Give the file a brief name, and end it with a number which will iterate for future files (.001, for example).
    • Click on the “Capture button (looks like a camera). The program will now save the first complete scan in which you haven't changed any of the scan settings. If you have changed settings in the course of a run but still want it to save the file, clock the camera button again and the program will force the capture.
12. To access your saved data:
    • Click on the right-most button (it looks like a spectrum). Select your image file, and go to the “Image” menu; click “Select Left Image,” and then click the “Flatten” button (looks like a rolling pin), click “execute” and then “quit.”
    • To save the image, go to the “Utility” menu and choose “TIFF Export.” Choose the “Reverse” radio button so that your image will be easy to print, and give your file a recognizable name. Note the directory where it's going.
    • To measure a nanotube's diameter, click the button that looks sort of like a razor blade (?). You'll see a small version of your image on the right-hand screen. Click this once, then move your cursor perpendicular to the nanotube you want to measure. You'll notice a white line following your cursor; this is the line cut you're taking. Click again, on the other side of the tube, to set it. Now, click on one of the red arrows on the top-most line-cut picture, and move the mouse until the red arrow is at the top of a peak. Do the same with the second arrow, setting it at the base of the same peak. We measure nanotube diameter by height.
13. When finished imaging:
    • Withdraw the tip using the disengage button. Remove your sample and leave everything on.

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.

Default settings

• Scan size 1 micron
• Scan rate 1 Hz
• Integral gain 10
• Proportional gain 0
• Free amplitude 1 V (~ 40 nm)
• Set point amplitude 0.6 V

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.

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

• Set Point - photodetector measures an amplitude. By adjusting the z-voltage (press tip harder on sample) the feedback tries to reach this amplitude.
• Integral Gain - reflects the strength of the feedback.
  • low number → will not track the surface faithfully
  • high number → will oscillate
    • best setting: right below oscillation
• Proportin Gain - don't know. help says: “doesn't really matter”. so “0” is a good choice.
• Drive Amplitude - this is the amplitude the DAC puts on the piezo that drives the cantilever.
• Drive Frequency - this number will be set by auto-tuning.

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.

Minot Group page of advanced AFM techniques

Program won't let you analyze data:

• restart the program

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

• If you're imaging a sample with large particles try reducing your setpoint/increasing your drive amplitude and lowering your scan rate.

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

• Obtain a small disk of Mica (9.9mm Diameter found in the drawer to the right of the south fume hood)
• Attach the disk to the center of a microslide using silver paint, super glue, or epoxy (all found in the same drawer)
• Allow to dry. (The silver paint takes about 24 hours to fully set up)
• Now take a small piece of scotch tape and stick it to the surface of the mica.
• Using the edge of a pair of tweezers push firmly down on mica surface and “smooth” out any bubbles. The goal of this step is to get the tape evenly stuck to the entire Mica surface.
• Now slowly pull back the tape removing the top layer of the Mica and look to see if you removed a perfectly circular layer. If not repeat this step until you do. (In my trials at this it took 3-5 attempts)
• If you do this correctly you will be left with a surface that only varies on the picometer scale.

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

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:

• Budget Sensors BS-Tap300AL tapping tips (40 N/m), and Tap150AL (5 N/m) www.budgetsensors.com/. Gold coated tips (more expensive) are also available. These tips were originally recommended to us by Scott MacLaren at the Univerisity of Illinois AFM center. “very inexpensive but good”. We have also been very impressed with the consistent good quality. Prices are:
  • $3900/380 = $10.26 each
  • $890/50 = $17.80 each
  • $210/10 = $21.00 each

• Olympus AC 160 TS (40 Newtons/meter) & AC 240 TS (2 Newtons/meter), www.asylumresearch.com. Recommended by Jason Li at Asylum Research. Prices are
  • $6000/375 = $16.00 each
  • $1850/70 = $26.43 each
  • $1000/35 = 28.57 each