Kelvin probe force microscopy (KPFM), also known as surface potential microscopy, is a noncontact variant of
atomic force microscopy (AFM).
By
raster scanning
A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image s ...
in the x,y plane the work function of the sample can be locally mapped for correlation with sample features. When there is little or no magnification, this approach can be described as using a scanning Kelvin probe (SKP). These techniques are predominantly used to measure
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
and
coatings.
With KPFM, the
work function
In solid-state physics, the work function (sometimes spelt workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here "immediately" ...
of surfaces can be observed at
atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, ...
ic or
molecular scales. The work function relates to many surface phenomena, including
catalytic activity, reconstruction of surfaces, doping and band-bending of
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
s, charge trapping in
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
s and
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
. 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.
History
The SKP technique is based on
parallel plate capacitor experiments performed by
Lord Kelvin
William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British mathematician, mathematical physicist and engineer born in Belfast. Professor of Natural Philosophy at the University of Glasgow for 53 years, he did important ...
in 1898. In the 1930s
William Zisman
Dr. William Albert Zisman (1905–1986)Dr. William A. Zisman: A Retrospective, Labstracts, July 6, 2009, p3 was an American chemist and geophysicist.
Life and career
He was born in Albany, NY and spent his youth in Providence, RI up to the age of ...
built upon Lord Kelvin's experiments to develop a technique to measure contact potential differences of dissimilar
metals
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typical ...
.
Working principle
In SKP the probe and sample are held parallel to each other and electrically connected to form a parallel plate capacitor. The probe is selected to be of a different material to the sample, therefore each component initially has a distinct
Fermi level. When electrical connection is made between the probe and the sample
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
flow can occur between the probe and the sample in the direction of the lower to the higher Fermi level. This
electron flow causes the equilibration of the probe and sample Fermi levels. Furthermore, a
surface charge develops on the probe and the sample, with a related potential difference known as the contact potential (V
c). In SKP the probe is vibrated along a perpendicular to the plane of the sample. 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. The resulting ac sine wave is demodulated to a dc signal through the use of a
lock-in amplifier
A lock-in amplifier is a type of amplifier that can extract a signal with a known carrier wave from an extremely noisy environment. Depending on the dynamic reserve of the instrument, signals up to a million times smaller than noise components, p ...
. Typically the user must select the correct reference phase value used by the lock-in amplifier. Once the dc potential has been determined, an external potential, known as the backing potential (V
b) can be applied to null the charge between the probe and the sample. When the charge is nulled the Fermi level of the sample returns to its original position. This means that V
b is equal to -V
c, which is the work function difference between the SKP probe and the sample measured.
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 cantilever is not piezoelectrically driven at its mechanical
resonance
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied Periodic function, periodic force (or a Fourier analysis, Fourier component of it) is equal or close to a natural frequency of the system ...
frequency ω
0 as in normal AFM although an alternating current (AC) voltage is applied at this frequency.
When there is a direct-current (DC) potential difference between the tip and the surface, the AC+DC voltage offset will cause the cantilever to vibrate. The origin of the force can be understood by considering that the energy of the capacitor formed by the cantilever and the surface is
: