Differences

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--- afm [2025/09/03 22:26] – [Step-by-step walk through for AC mode imaging] ethanminot
+++ afm [2025/09/03 22:44] (current) – [Imaging rules of thumb] ethanminot
@@ Line 121: / Line 121: @@
 **Beginner settings**
-  *Scan size 2 micron (look at a random small feature on a flat background to verify the sharpness of the tip)
+  *Scan size 4 micron (look at a random small feature on a flat background to verify the sharpness of the tip)
   *Scan rate < 15 micron/s
   *Integral gain 10
@@ Line 127: / Line 127: @@
   *Set-point amplitude 0.75 V
-Sometimes the image is improved by lowering the set-point amplitude a few clicks. For example, this might fix parachuting.
+**Adjustments to these settings**
-Sometimes the image is improved by withdrawing and re-running the autotune procedure (the resonant frequency might have changed).
+When you withdraw from the surface, check the free-air amplitude is the same as you when you first tuned the tip. If **free-air amplitude has drifted**, you can manually change the drive amplitude.
-When you withdraw from the surface, check the free air amplitude is the same as you when you first tuned the tip.
+Sometimes the image is improved by withdrawing and re-running the autotune procedure (**resonant frequency might have changed**). The autotune procedure will make changes to both the drive amplitude and drive frequency.
-Another thing to try is a slower scan 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.
+Sometimes the image is improved by lowering the set-point amplitude a few clicks. For example, this might fix **parachuting**. Minor changes to set-point amplitude can be made in real time, during imaging.
+Another thing to try is a slower **scan 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.
+If you notice **phase jumping** (a jump from below 90 degree to above 90 degrees), you should try adjusting the drive frequency. Make test images with different values of drive frequency such that the free-air phase is 70 degrees, 80 degrees, 100 degree and 110 degrees. To make these test images, you'll need to maintain a constant free-air amplitude by simultaneously adjusting drive amplitude. By doing this, you are searching for imaging parameters for which the cantilever oscillations are most stable.
+It’s hard to predict a priori whether the best images will be acquired with phase below 90, or above 90. The best imaging regime for a given day depends on tip sharpness, cantilever stiffness, the sample’s mechanical/adhesive properties, the material and coating of the tip, and the humidity in the room. These factors modify the functional form of long-range van der Waals forces, the electrostatic forces, the way the tip indents the sample, and the capillary forces related to the water meniscus. A nonlinear tip–sample force with respect of tip-sample separation leads to bistabilities, and these bistabilities cause the phase jumps which mess up the AFM image.
 **Using nanotubes as a diagnostic tool**