Kelvin probe force microscope
The map of the work function produced by KPFM gives information about the composition and electronic state of the local structures on the surface of a solid.
The SKP technique is based on parallel plate capacitor experiments performed by Lord Kelvin in 1898.
[4] In the 1930s William Zisman built upon Lord Kelvin's experiments to develop a technique to measure contact potential differences of dissimilar metals.
[6] This vibration causes a change in probe to sample distance, which in turn results in the flow of current, taking the form of an ac sine wave.
This means that Vb is equal to -Vc, which is the work function difference between the SKP probe and the sample measured.
[8] The cantilever in the AFM is a reference electrode that forms a capacitor with the surface, over which it is scanned laterally at a constant separation.
The origin of the force can be understood by considering that the energy of the capacitor formed by the cantilever and the surface is plus terms at DC.
The resulting vibration of the cantilever is detected using usual scanned-probe microscopy methods (typically involving a diode laser and a four-quadrant detector).
A map of this nulling DC potential versus the lateral position coordinate therefore produces an image of the work function of the surface.
EFM operates much like magnetic force microscopy in that the frequency shift or amplitude change of the cantilever oscillation is used to detect the electric field.
If both elements were brought in contact, a net electric current would flow between them until the Fermi levels were aligned.
During the scan VDC will be adjusted so that the electrostatic forces between the tip and the sample become zero and thus the response at the frequency ω becomes zero.
This can be done in (at least) two different ways: 1) The topography is captured in AC mode which means that the cantilever is driven by a piezo at its resonant frequency.
Then, this line is scanned again, while the cantilever remains on a defined distance to the sample without a mechanically driven oscillation but the AC voltage of the KPFM measurement is applied and the contact potential is captured as explained above.
It is important to note that the cantilever tip must not be too close to the sample in order to allow good oscillation with applied AC voltage.
The Volta potential decreased over subsequent measurements, and eventually the peak over the scratch completely disappeared implying the coating has healed.
[24] SKP is useful to study this sort of corrosion because it usually occurs locally, therefore global techniques are poorly suited.
Surface potential changes related to increased localized corrosion were shown by SKP measurements.
SKP has been used to investigate the surface potential of materials used in solar cells, with the advantage that it is a non-contact, and therefore a non-destructive technique.
[29] As a non-contact, non-destructive technique SKP has been used to investigate latent fingerprints on materials of interest for forensic studies.
