How to Acquire the Best AFM Resolution in Air

An important question in the field of atomic force microscopy (AFM) is “What is the best possible AFM resolution that can be achieved in air?” However, just as important as this question is the need to define what the question actually means.

Frequently researchers hear the phrase ‘atomic imaging’ and take this for its literal meaning, expecting to obtain details of atomic structures in their AFM images. One reason for this is researchers have seen atomic scale images before, such as the example given in Figure 1.

HOPG image taken with XE-100 (5 nm x 5 nm scan size). This image shows atomic lattice, not individual atoms.

Figure 1. HOPG image taken with XE-100 (5 nm x 5 nm scan size). This image shows atomic lattice, not individual atoms.

A common misunderstanding is that the capacity for an AFM to take atomic scale images indicates its performance. It is well established that in images of mica and graphite (HOPG), as in Figure 1, what is actually shown is the spacing between the atoms rather than the actual atoms.1,2 This means the image is actually of the sample’s atomic lattice, meaning it is not an accurate image of the atomic details of mica and graphite (HOPG).

Despite this fact many researchers still think that AFM systems should be able to image a sample’s atomic structure. Whilst UHV-AFMs can be used to take images of samples at atomic resolution, for ambient AFMs the lateral resolution is limited by the curvature and radius of the tip of the AFM cantilever.

The AFM cantilever tip’s radius tends to be a few nanometers in size and the best method of achieving high resolution images is ensuring that this tip remains in good condition.

Preserving the AFM Tip is the Most Important Factor

One could argue that the stability of an AFM can be measured by its ability to take images of a samples atomic lattice, however this is not an accurate assessment for every sample. This is because each AFM system is designed to work with a specific sample set over a small scan range, for example the scanning of a mica or graphite sample over a 1 μm2 area.

It should also be noted that to take atomic lattice images researchers must use small range scanners with open loop feedback and these systems are not appropriate for some samples.

As introduced earlier, an AFM’s ultimate resolution is significantly dependent on the radius of the AFM cantilever tip, which can lie between 2 – 5 nm. The sharp point of the cantilever tip is very brittle, meaning it is blunted when it comes into contact with a sample; which results in a loss of resolution and image quality for subsequent analyses.

Ultimate Resolution of Ambient AFM with Non-Contact Mode AFM

Another common misunderstanding is that using a non-contact AFM method (NC-AFM)3 results in reduced resolution as there is no contact between the surface and the tip, and that a large tip is required for sample spacing.  The majority of AFM manufacturers choose to run NC-AFM far away from the surface because of slow Z-servo feedback, though this results in low resolution.

If Z-servo feedback is slow and is restricted by the Z-scanner’s bandwidth the tip can sometimes come into contact with the sample surface when it is slowly scanned over the sample.

If the scan is taking place under ambient conditions the sample is often coated in a layer of liquid. If the AFM tip comes into contact with this layer it can be trapped there by meniscus forces; stopping the cantilever from scanning the sample.

For this reason, if NC-AFM is run with a small distance between the tip and the sample even tiny fluctuations in the force acting between the sample and tip can ruin an experiment. This can only be avoided using a NC-AFM system that has enough control to stay in the attractive force region.

It is difficult to execute NC-AFM imaging with a small tip-sample distance. For this reason tapping imaging was developed as this method avoids the challenge of maintaining the correct sample-tip spacing.4

The crosstalk eliminated (XE) AFM from Park Systems overcomes the challenge of maintaining this distance with the use of a high force Z scanner which is actuated by several, novel stacked piezos.

The Z scanner in the XE-AFM has a resonance frequency of 10 kHz and is decoupled from the XY scanner, which provides the system with the ability to use a high force actuator and improve its Z-scan bandwidth. This means that the AFM can work at a very small functional distance of just a few nanometers allowing stable, high-resolution ADM images to be taken in ambient conditions.

The XE-AFM’s True Non-Contact Mode makes it simple and straightforward to take Non-Contact AFM images of different sample types at the highest possible resolution of an ambient AFM (Figure 2).

High resolution (100 nm x 100 nm) image of a gold surface, acquired by True Non-Contact Mode of the XE-100 AFM.

Figure 2. High resolution (100 nm x 100 nm) image of a gold surface, acquired by True Non-Contact Mode of the XE-100 AFM.

References

  1. H.A. Mizes, Sang-il Park, and W.A. Harrison, Phys. Rev. B 36, 4491 (1987).
  2. T.R. Albrecht, H.A. Mizes, J. Nogami, Sang-il Park, and C.F. Quate, Appl. Phys. Lett. 52, 362 (1988).
  3. Y. Martin, C.C. Williams, H.K. Wickramasinghe, J. Appl. Phys. 61, 4723 (1987).
  4. Q. Zhong, D. Innis, K. Kjoller, V.B. Elings, Surf. Sci. Lett. 290, L688 (1993).

This information has been sourced, reviewed and adapted from materials provided by Park Systems Inc.

For more information on this source, please visit Park Systems Inc.

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