Absolute vs. Relative Calibration Methods

There are two methods of obtaining calibrated, quantitative results from PeakForce QNM. The first method (the relative method) avoids accumulated errors that can cause errors in modulus measurements, but has the downside in that it requires a reference sample that can be measured by the same probe as the unknown sample.The second method (the absolute method) does not require a reference sample, but requires accurate measurement of the tip end radius (typically by scanning an artifact sample like TipCheck) and spring constant (typically with thermal tune for soft cantilevers). Both methods require measurement of the deflection sensitivity on a hard sample.

For both methods, it is important to choose a probe that can cause enough deformation in the sample and still retain high force sensitivity. Figure 1 shows the recommended probes and the modulus range over which they work best.

Figure 1: Modulus ranges covered by various probes. The modulus of the reference sample for each range is indicated as well.

Table 1: Legend

The Relative Method

The relative method of calibration uses a sample of known modulus to obtain the ratio of spring constant to the square root of tip end radius. It is still important to accurately calibrate the deflection sensitivity in order to obtain modulus results. An outline of the procedure follows:

1. Calibrate the Deflection Sensitivity on a clean, hard sample (Sapphire or Silicon, which can be used for samples with modulus less than 10 GPa). See Calibrate the Deflection Sensitivity for a procedure to measure this.
2. If quantitative Adhesion or Dissipation data is required, use the NanoScope Thermal Tune function to obtain the spring constant, otherwise enter the nominal value from the manufacturer. See Calibrate the Spring Constant Using Thermal Tuning for a procedure to measure this.
3. Image the reference sample using PeakForce QNM and adjust the Tip Radius parameter to make the measured Modulus equal the known value of the reference sample.
4. Image the unknown sample adjusting the Peak Force Setpoint to match the deformation depth used during imaging of the reference sample.

The Absolute Method

The absolute procedure is very similar to the relative procedure except for two important differences:

1. The spring constant calibration (Step 2) is not optional.
2. Instead of using the reference sample, the tip end radius is measured by scanning a tip calibration artifact (such as TipCheck Bruker part #RS) and analyzing the resulting image. See Measure the Tip Radius for a procedure to measure this.

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