AFM is a well-known technique. However, there are also many different variations of AFM, some of which are well-known and some which aren’t. In this article, we look at one of the lesser-known AFM variations, known as electrostatic force microscopy (EFM). This is a variation of AFM that is similar to magnetic force microscopy (MFM), another AFM technique, and can be used to determine the electrostatic forces between a sample and the AFM tip. Here, we look at how this is done.
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What is Electrostatic Force Microscopy (EFM)?
Electrostatic force microscopy (EFM) is perhaps one of the lesser known variations of AFM. Many variations use AFM principles alongside those of another technique. However, EFM is one of the few methods where it is an adaptation of just AFM principles for deducing specific properties of materials.
In the case of EFM, it measures the electrostatic force between the sample and the tip as a function of the electric field gradient distribution above the sample surface. The main differences between standard AFM and EFM are that EFM operates in an imaging mode not used in conventional AFM approaches and that it uses a conductive tip to scan the surface.
How EFM Works
EFM works on a similar principle to magnetic force microscopy (MFM), in that it uses a dynamic non-contact lift mode to measure the forces between the tip and the sample. The differing factor being the types of forces measured. In the case of EFM, it measures the electrostatic forces that arise from the attraction and/or repulsion of charges between the tip and the surface. EFM is operated in a non-contact mode because these forces are long-range forces. Because they are long-range forces, the distance between the tip and surface can be as much as 100 nm (or more).
EFM generally performs two scans when imaging a surface. This is the principle of the AFM lift mode (not seen with conventional AFM imaging methods), which means that after one scan, the cantilever and tip are lifted to a greater height before scanning again. The first scan is done as per the usual non-contact mode of AFM and is scanned close to the surface. The tip is then lifted to a greater distance away from the sample for the second scan to measure the electrostatic interactions between the tip and the surface.
The first scan measures the van der Waals forces that are always present between the tip and the sample, and this enables the basic topography of the surface to be mapped. When there is an interaction, the change in the position of the cantilever is recorded by a laser beam deflecting off the cantilever onto a position-sensitive photodiode (PSPD). This position location mechanism is also present throughout the second scan (to locate the different interactions) to enable the topography of the surface and areas of intermolecular attraction/repulsion to be mapped and correlated.
In the second scan, as the tip is set to scan the surface, an electrical current is passed through the tip. This can be as either an alternating current (AC) or a direct current (DC). As the electrical current is passed, the cantilever beam oscillates at its resonant frequency (or close to it). As the tip passes over the sample, it is at a distance where electrostatic forces come into play. When there is a long-range connection between the tip and the surface, the resonant frequency changes due to a change in the force gradient.
For attractive forces, the resonant frequency lowers, and this makes the cantilever “softer”, whereby the tip is pulled closer to the sample. For repulsive forces, the process reverses, and the cantilever becomes “stiffer”. This is manifested through a higher resonant frequency and through the tip moving away from the sample. The changes in the resonant frequency are deduced using either phase detection, frequency modulation or amplitude detection methods.
EFM doesn’t have as much widespread use as other AFM techniques. However, it is most commonly used to map the electronic characteristics of sub-micron electrical materials (often for those with complex structures and/or molecular assemblies). On an overall application front, EFM is used for detecting electrical failures, detecting trapped charges, mapping electrical polarizations and for performing electrical read/write processes.