What Is a Kelvin Probe?
A Kelvin Probe is a technique for measuring work function and is one of the measurement methods of microscopy.
A metal probe is placed close to the sample surface and the contact potential difference resulting from the difference in work function between the sample and the probe is measured. By using a probe with a known work function, it is possible to determine the work function of the sample surface.
Applications of Kelvin Probe
1. Work Function of Thin Films
The work function of a sample surface varies depending on the quality of the thin film and the presence of impurities. In solar cells and sensors, thin film quality and higher-order structure can affect device characteristics.
By measuring thin films deposited under various conditions using the Kelvin Probe method, it is possible to analyze the correlation between film structure and properties.
2. Solar Cells and EL Devices
Kelvin Probe measurements are often used for metallic and semiconductor materials. Kelvin Probe measurements are used in silicon solar cells, organic thin film solar cells, organic EL, and electrode surface analysis.
The Kelvin Probe makes it possible to map the work function of the surface, which is then analyzed in conjunction with images of the surface measured with a microscope.
3. Identifying the Location of Metal Corrosion
Mapping the work function using the Kelvin Probe method also makes it possible to identify corroded areas of metals. Since the work function changes at locations where chemical changes such as corrosion have occurred, it is possible to map where unexpected reactions have occurred in the sample.
Principle of Kelvin Probe
Kelvin Probe is a measurement technique based on Atomic Force Microscopy (AFM) and has a spatial resolution on the order of micrometers. Kelvin Probe is a technique in which a metal probe is brought into contact with the sample surface.
When the probe contacts the sample surface, electron transfer occurs and the Fermi level changes, resulting in a change in the potential of the sample surface. Since the amount of this potential change depends on the work function of the probe and the sample, the work function of the sample surface can be determined by using a probe with a known work function.
By bringing the probe into contact with the entire sample, it is possible to map the work function within the thin film, and the change in work function can be used to identify areas where corrosion or film quality changes are occurring.
Other Information About Kelvin Probe
1. Measurable Materials
The Kelvin Probe method allows non-destructive measurement of samples. Both organic and inorganic thin films can be measured.
Therefore, it is sometimes used to measure the work function of each layer in the cross section of a multilayer film, or to measure the work function during the thin film growth process. In addition, it is often used for basic physical chemistry research, such as analyzing chemical reactions that occur on catalyst surfaces based on changes in surface work functions.
2. Work Function
The work function is the minimum energy required to extract electrons out of a solid. Electrons are numerous in a solid and range from the vacuum level, which has the lowest kinetic energy, to successively higher energies. The uppermost level corresponds to the Fermi level, and the difference between this vacuum level and the Fermi level is the work function.
Devices such as discharge electrodes and plasma display panels need to extract electrons from the inside of solids, and the work function plays an important role. Work function is a key word for electronic devices.
3. Fermi Level
Electrons in matter have various energies depending on their temperature. The Fermi-Dirac distribution function represents the probability of occupying an energy level that an electron has at a given temperature. The energy level with an occupancy probability of 0.5 is called the Fermi level.
The energy levels of electrons in a single atom are discrete, whereas in an object consisting of multiple atoms, there is a wide range of energy levels that electrons can occupy. Because of this limitation on the energy levels an electron can have, the structure of the Fermi level and the energy levels is what distinguishes conductors, semiconductors, and insulators.