For investigating materials at a nanoscale range, Atomic Force Microscopy is a widely used and powerful technique. Nonetheless, the technique is not easy to use, which generates various results between researchers with different experience levels.
Inspired by neural networks to make dynamic amplitude modulation AFM (AM-AFM) easy for researchers of every competence level, NT-MDT Spectrum Instruments (formerly NT-MDT) has created the intelligent software, Scantronic™.
Also known as “tapping” or “semi-contact”, the AFM amplitude modulation (AM) mode, is based on dependency of cantilever amplitude oscillation on distance between sample surface and tip. This mode is regularly chosen over other AFM modes because it minimizes impact on the sample and the tip, which can help to maintain both.
Although, in practice a broad quantity of AFM images bear artifacts of distinct kinds, comprising uncontrollable commuting of cantilever oscillation regime, detachment of the probe from the surface (parachuting effect) (Fig. 1) or extra noise of measured signals (ringing). These distortions can influence the results’ interpretation following analysis and can therefore result in misguided conclusions based on those results.
Figure 1. Topography images of the glass surface on the left and polymer PS-b-PMMA on the right when scanning parameters are not optimal. Images of large particles have defects looking as coma or “tails” and appeared because of probe detachment from the surface. The image of the polymer has a different type of defects, which manifests itself in the form of areas with a higher relief (light areas on the image), and are caused by switchs of the probe oscillation from the repulsion mode to the attraction mode.
To obtain a high image quality the user has to simultaneously and manually manage significant parameters throughout the scan to avoid artifacts comprising tip oscillation amplitude, set point, scanning speed and feedback gain.
This is important specifically for rough surface, objects weekly connected to the surface (nanoparticles, nanotubes, single molecules), soft samples (gels, soft polymers, biological objects), or samples where the topography isn’t known before the scan.
Not only does this demand good understanding of the microscope, the fundamentals of its functioning and how different parameters will impact the image quality in different scenarios, but also experience with these different types of samples to reliably create acceptable images. All this does not influence well to the efficiency of the AFM.
This shows another obstacle as gaining experience necessitates access to samples and the time required to learn the equipment and the scanning process. As a response, automation of the parameter tuning required for image acquisition in AM-AFM is highly tactical for users to avoid wasting significant time and efforts on overcoming the instrument before they can receive them.
The intelligent ScanTronic™ software quickly provides the user with high-quality and solid results on their samples in AM ACM.
Intelligent AFM Software – ScanTronic from NT-MDT Spectrum Instruments
The ScanTronic application has been created to provide auto-tuning of scanning parameters for imaging in AM-AFM with the help of neural networks. This software is useful not only for beginners but also for experienced researchers, as minimal knowledge of sample properties and minimal involvement from the end user is needed, particularly where the topography of the sample is unknown beforehand.
The ScanTronic software keeps noise to a minimum, significantly reducing image defects of the topography such as feedback excitation, parachuting and switching between attraction and repulsion tip-surface interaction regimes. The ScanTronic provides high quality imaging of samples despite different roughness and material nature.
The Scantronic algorithm singles out defects throughout the tuning procedure and eradicates or reduces them through feedback gain optimization, including adjusting the set point, tip oscillation amplitude and scanning speed.
Figure 2. Images of the surface relief of fluoroalkane on silicon, obtained with insufficient (left), excess (middle) and optimal (right) feedback gain factors. In case of insufficient feedback gain, the small relief is reproduced well, but “tail” defects (“parachuting”) appear on the large objects. When it is too high, the image becomes noisy due to the excitation of feedback. At optimal feedback gain, small and large relief features are reproduced without defects and with minimal noise.
Figure 2 displays an example of non-optimal feedback setting (left and center) in comparison with the image, when feedback gains are optimally selected.
One the most common defects in AM-AFM is caused by choosing the wrong set point and probe oscillation amplitude that stimulate in turn switching the tip oscillation from an attractive to a repulsive regime.
The topography of these images looks “torn” and in many cases, together with low quality results can guide a researcher to the wrong conclusions. ScanTronic removes or majorly minimizes these types of defects with confident force control that automatically tunes the set point and amplitude to fit the sample and allows scanning only in attractive or repulsive regimes.
Figure 3 shows examples of images captured in the attraction regime and occurring jumps between attraction and repulsion.
Figure 3. On top: topography (on the left) and phase contrast (on the right) images of PS-b-PMMA, obtained by scanning in the attraction mode. Below: topography (left) and phase contrast (right) images with defects caused by switching between attraction and repulsion regimes. The bright areas on the topography and the corresponding dark ones in the phase contrast image are due to changes of the probe oscillation regimes.
An advantage of scanning in an attractive regime is the preservation of tip sharpness, as a result this increases the life time of the probe considerably. However, the presence of the topography with steep slopes on the surface, like large particles, risks crushing the probe during the scan. ScanTronic’s algorithm monitors the feedback error signal and does not allow the tip to be crushed during tuning and scanning.
The ScanTronic software can also control the low sample-tip interaction force in an attraction regime. ScanTronic makes the imaging of difficult and soft samples straightforward (fig.4).
Figure 4. Topography images of NC membrane with a rough surface (top) and fluoroalkane particles loosely bound to the surface (bottom).
The module has a user-friendly interface, this allows for the straight forward launching of the imaging procedure (Fig. 5). To run a measurement, an attractive or repulsive scanning regime is chosen and the scan is started.
Figure 5. The interface of ScanTronic™.
Eliminating Artifacts in AM-AFM
If the surface has features with more or less steep borders, defects on their images can occur as a result of tip detachment from the surface (parachuting effect). It occurs quite often when the image of almost entire scanned surface is of high quality and only a few small areas, usually associated with individual particles, have such defects. As a result, the quality of this image becomes unacceptable and re-scanning of the area is necessary.
The GTransform™ algorithm, which is built into the program, allows for the elimination of the “parachuting” defects in the topography image. As a result, an acceptable result can be obtained from the moderate quality image (see Fig. 6).
Figure 6. Topography image before and after correction of the "parachuting" effect by using the GTransform™. The GTransform™ eliminates the "tails" arising due probe detachment from the object surface with a steep slope.
Furthermore, the GTransform™ algorithm enables extracting substantial information from a scanned image. Indeed, the images of phase contrast, spreading current or lateral forces also contain, in addition to information related to the heterogeneity of the surface properties, the contrast produced by the topography.
This can remarkably mask the map of measured surface property heterogeneity in some cases. Figure 7 shows an example of using the GTransform™ algorithm for phase contrast images to extract information only related to material property inhomogeneity.
GTransform™ eliminates or considerably minimizes the distortions of phase contrast images induced by surface topography.
Figure 7. On top: Image of phase contrast of dye molecule aggregates before (left) and after (right) removing the effect of topography using GTransform™. Below: correction of the phase contrast image of fluoroalkane molecule aggregates using GTransform™.
The ScanTronic software provides an easy gateway into producing high quality AFM images for those who are beginners. The ScanTronic has one-click optimization and in-depth understanding and experience are not necessary to produce reliable and trustworthy images.
The ScanTronic software provides assistance for experts, this frees up time that would otherwise be spent producing rudimentary images, without sacrificing control over the procedure. It allows researchers to widen the range of materials that they routinely scan into tricky, delicate or soft materials, thus increasing the potential scope of their research.
This information has been sourced, reviewed and adapted from materials provided by Spectrum Instruments.
For more information on this source, please visit Spectrum Instruments.