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--- afm [2025/10/07 11:39] – [Step-by-step walk through for AC mode imaging] ethanminot
+++ afm [2025/10/07 12:02] (current) – [Step-by-step walk through for AC mode imaging] ethanminot
@@ Line 82: / Line 82: @@
     * Move laser toward the tip of the cantilever - the control wheels are labeled LDX and LDY (laser deflection x and y). They are located on the back & right side of the MFP-3D tripod. Use these controls to maximize the 'Sum' signal. If the laser is badly misaligned, it can be hard to locate. If you "loss" the laser spot, ask an experienced user for help.
     * Adjust the photodetector (PD) position. The control wheel for the photodetector is located on the left side of the MFP-3D tripod and is labeled PD. Set the PD 'Deflection' meter to near zero.
-  - X Set AC Mode - In main tab of the master panel select 'AC Mode' in the 'Imaging Mode' pull down menu
   - Tune the AFM
     *  Open 'Tune' tab in the master panel
     *  Set 'Target %' to -5.0 %. This setting is a first guess at the ideal drive frequency (setting it slightly less than the resonant frequency). You may have to test different drive frequencies later if you notice "phase jumping" during imaging.
-    * Click the 'Auto Tune' button and wait for tuning to finish. Software will set drive frequency.
+    * Click the 'Auto Tune' button and wait for tuning to finish. Software will set drive frequency and drive amplitude. The graph of amplitude and phase should look like the textbook curves for a driven damped harmonic oscillator.
+    * In the read-out window of the software, you will now see a 1-volt amplitude signal underneath the sum signal. Maximize this amplitude signal by moving the laser along the length of the AFM cantilever. When the amplitude is maximized, the sum signal will be slightly less than its max value.
+    * Click 'Auto Tune' one more time (or manually adjust the drive amplitude).
   - Engage the tip
     * Set the I gain to 10
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   - Set imaging parameters in the main tab. For example, you might start with a scan size of 2 microns. See the imaging rules of thumb listed below.
   - Do your first test scan the sample. Clicking 'frame up' or 'frame down' will start a scan. You should do a test scan before you spend much time searching for a specific location on your sample.
-  - Observe the quality of the scan. Look for phase jumps (remedies are described below). Look for parachuting. Look for the tip loosing contact with the surface. If needed, you can change the set-point amplitude during an image. The Set-point should be low enough that the tip stays in contact with the surface. However, don't make the set-point too low. The ideal set-point amplitude is typically about two clicks away from loosing contact with the surface.
+  - Observe the quality of the scan. See the section below called "Tuning the AFM to get a good image". Look for phase jumps. Look for parachuting. Look for the tip loosing contact with the surface. Remedies are described below.
   - If you got a good image, you are ready for the rest of your session. If you need the microscope to search a large area, raise the tip by one "swipe" of the thumbwheel so you can safely make coarse motion in the x-y plane.
@@ Line 119: / Line 120: @@
   - Leave controller and PC running unless expecting a power outage
-===== Imaging rules of thumb =====
+===== Tuning the AFM to get a good image =====
-It is easiest to get a good image on a small scan area (~ 2 micron). A small scan also facilitates verification of tip sharpness. 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**
+Check the **scan rate**. Is it less than 15 microns/second? Slower scans are more stable and reproducible. You might save time in the long run (and headache) by keeping scan rate less than 15 microns/second. Use an even slower scan rate if the features on the sample are very tall.
-  *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
-  *Free amplitude 1 V (corresponds to a cantilever motion of ~ 100 nm)
-  *Set-point amplitude 0.75 V
-**Adjustments to these settings**
 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.
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 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.
-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.
+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. The set-point should be low enough that the tip stays in contact with the surface. However, don't make the set-point too low. The ideal set-point amplitude is typically about two clicks lower than loosing contact with the surface.
-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 need to make adjustments to the drive frequency. Phase jumping will wear out the AFM tip (making it blunt). Phase jumping also introduces artifacts in the height data. 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.
-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.
+===== Other imaging rules of thumb =====
+It is easiest to get a good image on a small scan area (~ 2 micron). A small scan also facilitates verification of tip sharpness. 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**
+  *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
+  *Free amplitude 1 V (corresponds to a cantilever motion of ~ 100 nm)
+  *Set-point amplitude 0.75 V
-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**