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afm_advanced_techniques [2019/09/09 20:28] – created ethanminotafm_advanced_techniques [2023/06/29 12:34] (current) – dublin
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 =====Static Force Curves and Measuring the Spring Constant===== =====Static Force Curves and Measuring the Spring Constant=====
 Static force curves allow you make graphs of deflection versus tip height for single pushes onto a sample.  There is a calibration process that is necessary to make these measurements accurate. Static force curves allow you make graphs of deflection versus tip height for single pushes onto a sample.  There is a calibration process that is necessary to make these measurements accurate.
-  *In the //Main// tab of the //Master Panel// choose //Contact Mode// in the //Imaging Mode// drop down menu + 
-    * Now the force curve will happen in contact mode. Be sure to switch back to //AC Mode// before you begin imaging again! +**Start with the tip far away from the surface to measure virtual deflection.** If you don't, the tip will crash into the surface because the trigger channel is set to "none"
-  *Find a clean area in your image for the tip to push against the substrate. To get the AFM to preform the force curve exactly where you want go the //Go There// tab in the Force tab of the Master Panel. Under //Go There// will be a button, //pick point//, which will put a marker on your last active AFM window (ht, amp, phase..). Once you've put that marker where you want click the //pick point// button once more, it should now say //that's it//. To actually move the tip to that spot you need to push the //Go There// button, and if you're the skeptical type you can always check the show tip option to see where the tip is hanging out. +
-  *With the force curve location chosen, switch to the //Cal. tab// and click the //Relative radio// button above the //Trigger Channel// drop down list.+
 Calibrating the measured deflection is a two part process: Calibrating the measured deflection is a two part process:
   *First, with the Trigger Channel left at none and all other options left at their default values, press the //Single Force// button near the bottom of the //Master Panel//.   *First, with the Trigger Channel left at none and all other options left at their default values, press the //Single Force// button near the bottom of the //Master Panel//.
-  *A new window should open with a noisy red and blue graph that looks linear. +  *A new window should open with a noisy red and blue graph that looks linear. This line is the virtual deflection. 
-  *In the //Cal. tab// open the //Set Sens.// drop down list and click on //Virtual Defl Line//.  A black line will appear fitted on your newly opened graph.  If it looks like a good fit, right click the line and select //Remove fit_DeflVolts//. You should perform another //Single Force// if you wish to verify how well the virtual deflection was removed. *Note* Sometimes the virtual deflection won't go away, the only method right now that fixes this is restarting the MFP-3D software.+  *In the //Cal. tab// open the //Set Sens.// drop down list and click on //Virtual Defl Line//.  A black line will appear fitted on your newly opened graph. The program automatically records these fit parameters in the //virtual deflection// box in the //Cal// tab. Virtual deflection is now calibrated.
     * Terminology and physics: 'Virtual deflection' is a change in deflection as a function of height which is due to tiny changes in the optical path of the laser throughout the Z range. Virtual deflection should not be confused with 'squeeze-film damping', which is a reduction in free-air amplitude as the tip is lowered toward a surface. Squeeze-film damping arises due to air that becomes 'trapped' between the cantilever and the sample, thereby applying extra forces to the cantilever as it oscillates. Squeeze-film damping is observed only in the close vicinity of a sample, whereas virtual deflection can be observed in the absence of a sample.     * Terminology and physics: 'Virtual deflection' is a change in deflection as a function of height which is due to tiny changes in the optical path of the laser throughout the Z range. Virtual deflection should not be confused with 'squeeze-film damping', which is a reduction in free-air amplitude as the tip is lowered toward a surface. Squeeze-film damping arises due to air that becomes 'trapped' between the cantilever and the sample, thereby applying extra forces to the cantilever as it oscillates. Squeeze-film damping is observed only in the close vicinity of a sample, whereas virtual deflection can be observed in the absence of a sample.
  
-For the second part of the calibration you will engage the surface with the tip.+For the second part of the calibration you will engage the surface with the tip. In this step, you will calibrate the //Deflection InvOLS//, or Inverse Optical Lever Sensitivity. It is the sensitivity of the detector-cantilever combination, which lets you relate deflection to an actual force value. (It's the inverse because the bigger the number, the less sensitive the detector is.)
   *In the //Trigger Channel// drop down list choose //Deflection// and set the //trigger point// to 20 nm.   *In the //Trigger Channel// drop down list choose //Deflection// and set the //trigger point// to 20 nm.
   *Press //Single Force//.  The first graph you see will have a lot of extraneous data as the tip finds the surface.  After the tip has reached the set amount of the deflection (trigger point value) it will disengage and retract a small distance from the surface. The tip can CRASH into the surface if you leave it in this extended position for very long so its best to withdraw after you've taken your last force curve.   *Press //Single Force//.  The first graph you see will have a lot of extraneous data as the tip finds the surface.  After the tip has reached the set amount of the deflection (trigger point value) it will disengage and retract a small distance from the surface. The tip can CRASH into the surface if you leave it in this extended position for very long so its best to withdraw after you've taken your last force curve.
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   *With the graph window selected press CTRL+I to bring up the cursors. Place the cursors (circle and square objects that pop up at the bottom of the window) on the left part of the graph where the tip is pressing against the surface, selecting a representative portion of the line. (The line of interest is the far left linear line with a negative slope)   *With the graph window selected press CTRL+I to bring up the cursors. Place the cursors (circle and square objects that pop up at the bottom of the window) on the left part of the graph where the tip is pressing against the surface, selecting a representative portion of the line. (The line of interest is the far left linear line with a negative slope)
-  *Make sure the cursors are on the same line, either engage or retract. Do this by pressing the left/right arrow keys. If the the cursors are on the same line they will move in the same direction. +  *Make sure the cursors are on the same line. Do this by pressing the left/right arrow keys. If the the cursors are on the same line they will move in the same direction. 
-  *Select //Deflection// from the //Set Sens.// drop down list. Remove the fit the same way as before and withdraw the tip from the surface by hitting either //Withdraw// or //STOP!!//. You are now done calibrating the Deflection. +  *Select //Deflection InvOLS// from the //Set Sens.// drop down list. The program automatically records the offset in the "Defl InvOLS" box.  //Withdraw// or //STOP!!//. You are now done calibrating the Deflection. 
-  *These force curves are a good way to see if you have a bad tip. A dirty tip that has picked up alot of junk during imaging will typically have engage and retract curves that aren't even close to one another.  While sometimes these tips can still image decently it will be impossible to get accurate force measurements with them.+  *These force curves are a good way to see if you have a bad tip. A dirty tip that has picked up a lot of junk during imaging will typically have engage and retract curves that aren't even close to one another.  Sometimes these tips can still image decently it will be impossible to get accurate force measurements with them.
  
 {{:dirty_tip.jpg?1000|force curve from dirty tip}} {{:dirty_tip.jpg?1000|force curve from dirty tip}}
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   *First make sure you've hit //withdraw// and the tip is no longer hovering above the surface for taking force curves.   *First make sure you've hit //withdraw// and the tip is no longer hovering above the surface for taking force curves.
   *Set the deflection to zero using the the left side thumb wheel on the AFM head.   *Set the deflection to zero using the the left side thumb wheel on the AFM head.
-  *Go into the //Thermal// tab of the //Master Panel// and click the //Do Thermal// button at the bottom. This will bring up the thermal graph.  Allow it to collect a number of samples, you'll see the Current Samples bar increasing in number.  One hundred samples would be a bare minimum. +  *Click the //Thermal// button on the right side of the //Master Panel// and click the //Capture// button at the top left of the //Thermal Graph// window. This will bring up the thermal graph.  Allow it to collect a number of samples.  One hundred samples would be a bare minimum. 
-  *There should be a spike in the graph that corresponds to your driving frequency.  Use the mouse to drag a box around this peak and expand it through the right click menu. When the peak is well defined press Stop Thermal. Using the cursors (CTRL+I) find the frequency corresponding to the center of the peak and enter this value into the Zoom Center bar. +  *There should be a spike in the graph that corresponds to your driving frequency. Ensure that this makes sense from your tuning parameters — lower-frequency peaks can appear as well.  Use the mouse to drag a box around this peak and expand it through the right click menu. When the peak is well defined press Stop Thermal.  
-  *Check the show fit box and then press Fit Guess. A blue line will now fit to the peak.  If the fit looks good press Try Fit.+  *Click the //Fit// button. A blue line will now fit to the peak.  If the fit looks good, continue.
   *The Spring Constant of the cantilever is now measured and updated in both the Thermal and Main tabs.   *The Spring Constant of the cantilever is now measured and updated in both the Thermal and Main tabs.
          
afm_advanced_techniques.1568086107.txt.gz · Last modified: 2019/09/09 20:28 by ethanminot