You are here: Experiment Guide > Imaging Modes > Interleave Scanning and LiftMode > PeakForce KPFM Imaging

PeakForce KPFM Imaging

PeakForce KPFM (PeakForce Frequency Modulated-Kelvin Probe Force Microscopy) is a combination of Peak Force Tapping Mode and frequency modulated KPFM (FM-KPFM) mode.

PeakForce KPFM measures surface potential or work function using a Lift Mode Surface Potential Imaging (AM-KPFM) variation of the FM-KPFM Imaging mode while simultaneously providing correlated nanomechanical property information using the Bruker's proprietary Peak Force Tapping Mode.

PeakForce KPFM leverages the advantages of PeakForce Tapping:

Because of PeakForce's direct force control, softer probes may be used in this mode, thereby increasing sensitivity.

PeakForce KPFM Probe Requirements

Bruker recommends PFQNE-AL probes for PF FM-KPFM measurements.

Sample Preparation

Prepare the sample as described in Electrical Sample Preparation.

PeakForce KPFM Procedure

 
  1. Mount a sample onto the sample holder.
  2. Mount an appropriate probe into the standard probe holder (see Prepare and Load the Probe Holder for details).
  1. Click the Select Experiment icon to open the Select Experiment window.

(Hover over the image to view larger)

Figure 1: The PeakForce KPFM Select Experiment window

  1. Select the following:
    • Experiment Category: Electrical & Magnetic
    • Experiment Group: Electrical & Magnetic Lift Modes
    • Select Experiment: PeakForce KPFM
 
  1. Click Load Experiment.
  1. Click the Setup icon to open the Setup window.
 
  1. Align the laser on the cantilever and place the crosshair there. See Figure 2.

Figure 2: Align the laser on the cantilever and place the crosshair there.

 
  1. Fast Thermal tuning, used to find the cantilever resonance, is performed when you exit the Setup view. This is made visible by the HSDC window, shown in Figure 3.

Figure 3: The HSDC window, indicating that a cantilever tune operation is underway.

  1. Engage the probe onto the sample
  1. Scan the sample
 
  1. Set the Lift Scan Height as low as possible without hitting the sample, typically 75 - 100 nm.
  2. Entering a value in the Potential Offset field adds that value to the measured value in the Potential channel. This can be particularly useful when measuring work functions as this function measures the difference in potential between the probe and the sample - entering the value of the work function of the probe will then provide a direct measurement of work function of the sample.
NOTE: The stored data is unaffected by the Potential Offset. I.e. offline measurements do not see this input.
  1. Refer to the PeakForce QNM Operation sections for details regarding PeakForce mode imaging.

A sample result showing the surface potentials (work functions) of three different materials is shown in Figure 4.

Figure 4: PeakForce KPFM scan of a Au-Si-Al (left to right) sample.

Advanced PeakForce KPFM Imaging

Automatic frequency selection is the default for PeakForce KPFM imaging. You may wish, however, to manually select an operating frequency. To do this:

  1. Enter the Expanded mode.
 
  1. Set Freq. Control in the Potential (Interleave), shown in Figure 5, panel to User-defined.

Figure 5: Selecting User-defined Frequency Control

You can manually select the operating frequency of the LiftMode. For details, refer to Advanced PeakForce KPFM-AM Operation.

PeakForce KPFM Parameters

You may wish to manually adjust the parameters shown in Table 1.

Parameter Description
Lock-In1 BW Needs to be smaller than twice the Drive 3 Frequency which is fm in Figure 8 of FM-KPFM Imaging. This lets the Lock-In respond to f0 while filtering f0 ± fm. If the Lock-In BW is too low, the tracking ability will be reduced. Automatic Freq. Control is thus easier to use than User-defined Freq. Control.
Lock-In2 BW Needs to be larger than four times the Drive 3 Frequency which is fm in Figure 8 of FM-KPFM Imaging. This is used to include the first and second harmonics, f0 ± 2fm. Extra bandwidth of Lock-In2 does not degrade image quality as Lock-In3 follows.
Lock-In3 BW Lock-In3, cascaded with Lock-In2, is used for the surface potential feedback.
Drive3 Amplitude Higher Drive3 Amplitudes will result in higher signal-to-noise ratios.

Table 1: Adjustable FM-KPFM parameters

 

www.bruker.com Bruker Corporation
www.brukerafmprobes.com 112 Robin Hill Rd.
nanoscaleworld.bruker-axs.com/nanoscaleworld/ Santa Barbara, CA 93117
   
  Customer Support: (800) 873-9750
  Copyright 2010, 2011. All Rights Reserved.