In SKPM, a conducting cantilever is scanned over the surface area of the sample being analysed at a constant height in order to map its ‘work function’. The determination of the work function is based on the measurement of the electrostatic forces between the AFM tip and the sample being analysed. This application was derived from the AFM set up.
SKPM is similar to Electrostatic Force Microscopy (or EFM), which is often used in laboratories to measure the special resolution of a surface potential map. EFM images can be created by measuring the cantilever oscillation, phase and frequency shift in response to the electrostatic force gradients, which is similar to the techniques adopted by SKPM.
Both methods prove similar in their approach to analysing samples, i.e. they both use a non-contact cantilever, however there are crucial differences between the two applications.
The cantilever of an AFM is a reference electrode – i.e. it is stable and has a well-known electrode potential. It forms a capacitor with the surface of the sample, over which it is scanned laterally at a constant separation. An alternating current (AC) voltage is generally applied at this frequency, as the cantilever is not driven at its mechanical resonance frequency.
The work surface itself relates to many surface phenomena, and can be observed at atomic or molecular scales. The microscope used by the researcher would analyse the sample, measuring its catalytic activity; surface reconstruction; doping and band-bending of semiconductors; charge trapping in dielectrics and corrosion.
The information obtained by the scope would then provide the researcher with data about the composition and electronic state of the local structures on the surface as a solid sample. When there is a direct-current (DC) potential difference between the tip and the surface of the sample, the AC+DC voltage offset will cause the cantilever to vibrate.
This is typically detected using scanned-probe microscopy methods, usually involving a diode laser and a four-quadrant detector. A null circuit is then used to drive the DC potential of the tip to a value that minimises the vibration. A map of the DC potential versus the position coordinate and produce an image of the work function of the surface being analysed.
The principles of SKPM are similar to those of Enhanced EFM in that they both work from a DC bias feedback loop and require the use of metal cantilevers in order to conduct electricity. The DC bias that zeros the force provides the researcher with a measure of a sample’s surface potential.
EFM directly measures the force produced on a charged tip by the electric field from the surface. This application operates similarly to magnetic force microscopy, in that the frequency or amplitude of the cantilever oscillation is measured in order to detect the electric field of the sample. The main difference between them is that the MFM technique measures magnetic force gradients instead of electrostatic force gradients.
In EFM, the force arises due to the attraction of repulsion of the separated charges using long-range application. The cantilever oscillates and does not make contact with the surface area of the sample.
The main difference between EFM and SKPM methods is the way the signal is obtained from a lock-in amplifier. This signal is used to measure a sample’s surface potential. EFM is also far more sensitive to topographic artefacts than SKPM.