Specific Process Knowledge/Characterization/AFM: Atomic Force Microscopy/KPFM

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By Berit Herstrøm @nanolab January 2018

KPFM (Kelvin Probe Force Microscopy) measurements can be done with this AFM Icon. It is best for mapping the surface potential on a sample with nanometer resolution but it can also be calibrated to give quantitative values.

To get the potential/work function: set the Interleave mode to "Lift" while scanning with the KPFM workspace

Here I shortly explain how to calibrate to get work function values

To get a quantitative value for the work function of a sample material you need to calibrate the AFM tip you are using. This is done by measuring a material with a known work function. We do not have a certified sample for this but we are using a sample that came with the system. It has some lines of Au-Si-Al right next to each other. The below image is from the Bruker application note: File:PeakForce-Kelvin-Probe-Force-Microscopy-App-Note-BRUKER.pdf - used with permission

For reference I prefer using the gold as the two other materials form oxides and I think that using the values for Si and Al are then less accurate. But even for gold you can find several different values for the work function. Take a look at this reference: [11]. The work function value range for gold seems to be from 5.10eV to 5.47eV. 5.10eV for poly crystaline gold and the higher values for single crystalline gold. I believe I do not have single crystalline gold, so I will use the 5.10eV as the reference value for the sample.

To get the work function of the your sample of interest, you need to find the work function of the tip as what you measure with KPFM is the work function difference between the tip and the sample. 

Measured surface potential = Work function (Tip) - Work function (sample)

So 

Work function (Tip) = Measured surface potential*e + Work function (sample)

When measuring on the gold to find the work function of the tip: 

Work function (Tip) = Measured surface potential*e + 5.10eV

for example: Once we measured the potential -0.514 V on the gold. There the tip work function is: 

Work function (Tip) = -0.514 V * e + 5.10 eV = 4.586 eV

The value can be typed into the workspace and after that the potential window will show you the work function.

The accuracy of your results are limited by

  • the accuracy of the value you use for calibration.
  • It is also limited by the state of the tip. If the tip radius change after the calibration this will affect your results and the tip should be calibrated again.
  • Other things to consider is that the sample surface must be grounded to the stage. Here you can use Al-tape from the surface of the your sample (be very careful not to hit the tape with the AFM probe.
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