Calibrate PeakForce QNM

Three parameters are needed to fully calibrate PeakForce QNM:

1. Deflection Sensitivity. See Calibrate the Deflection Sensitivity for a procedure to measure this.
2. Spring Constant. See Calibrate the Spring Constant Using Thermal Tuning for a procedure to measure this.
3. Tip Radius. See Measure the Tip Radius for a procedure to measure this.

A fourth parameter, the Sample’s Poisson’s Ratio, is needed to convert the measured reduced modulus, E*, to the sample modulus, Es. The reduced modulus is related to the sample modulus by the following equation:

where νt and Et are the Poisson’s ratio and Young’s modulus of the tip and νs and Es are the Poisson’s ratio and Young’s modulus of the sample. We assume that the tip modulus, Et, is much larger than the sample modulus, Es, and can be approximated as infinite and calculate the sample modulus using the sample Poisson's Ratio

Poisson's ratio generally ranges between about 0.2 and 0.5 (perfectly incompressible) giving a difference between the reduced modulus and the sample modulus between 4% and 25%. Because the sample’s Poisson's ratio is not generally known, many publications report only the reduced modulus. Entering zero for this parameter will cause the system to return the reduced modulus.

Cantilever Parameters

After you have measured the cantilever Spring Constant and the Tip Radius, enter them into the Cantilever Parameters panel in the Scan Parameters window of the NanoScope software window, shown in Figure 1. If available, enter the Sample Poisson’s Ratio.

Figure 1: The Cantilever Parameters panel

Feedback Parameters

Peak Force Setpoint

A Peak Force Setpoint that is too high can either damage the sample or wear the tip. It is generally desirable to reduce the Peak Force Setpoint to as small a value as is possible. However, in order to achieve accurate Elastic modulus measurement, sufficient sample deformation is needed. If the deformation is less than 2 nm, increase the Peak Force Setpoint to achieve sufficient sample deformation.

For the relative method, you should adjust the Peak Force Setpoint to keep the Deformation the same for both the reference and measurement samples.

Feedback Gain

Reduce the Feedback Gain to lower the noise in the property channels.

If ScanAsyst Auto Gain is On, set the ScanAsyst Noise Threshold to 0.5 nm or less.

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