The electron work function is a measure of the minimum energy required to extract an electron from the surface of a solid. Hölzl, J., and Schulte, F. K., Work Functions of Metals, in Solid Surface Physics, Höhler, G., Editor, Springer-Verlag, Berlin, Electronic work function, energy (or work) required to withdraw an electron completely from a metal surface. This energy is a measure of how tightly a particular. Work function in metals: Figure 1 shows a schematic energy diagram of a metal. The valence bands are filled with electrons up to the Fermi energy (EF).
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Figure 2 shows the energetics of a photoelectric-effect-based measurement of the work function. Electrons with sufficient kinetic energy to overcome the barrier at the interface are able to escape the metal — these are represented work function of metals the blue box in Figure 2.
Electronic work function
These photoelectrons then travel away from the metal surface experiencing the work function of metals depicted with the thick black line in Figure 2. As the electrons move further away from the surface their kinetic energy increases, according to Equation 1. The generated photocurrent is then measured as a function of the photoelectron kinetic energy.
Two important features are present in a typical plot of photocurrent — kinetic energy. First, a sharp onset at low photoelectron kinetic energy, Emin is present.
Electronic work function | physics |
As already mentioned, this onset defines the lowest energy electrons able to overcome the work function of the surface. Energetics of electrons in a photoelectric-effect-based measurement of the work function.
After ejection from the metal the electrons experience the potential shown with the thick black line. The kinetic energies of two photoelectrons originating from different energy levels in the metal are shown with thick green lines.
In a real experiment the photoelectron-kinetic-energy analyzer causes an additional drop in the work function of metals, which adds a rigid offset to the kinetic energy of the photoelectrons. More on this can be found in Nieuwenhuys  The second feature is the high kinetic energy onset of the photocurrent, Emax, and it is a manifestation of the electron population around the Fermi level of the metal; i.
In practice the photoelectrons are additionally accelerated by an external electric field, which has to be taken account in the calculations.
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One may distinguish between two groups of experimental methods for work function measurements: Absolute methods employ electron emission from the sample induced by photon absorption photoemissionby high temperature thermionic emissiondue to an electric field field electron emissionor using electron work function of metals.
Relative methods make use of the contact potential difference between the sample and a reference electrode.
Work Functions for Photoelectric Effect
Experimentally, either an anode current of a diode is used or the displacement current between the sample and reference, created by an artificial change in the capacitance between the two, is measured the Kelvin Probe method, Kelvin probe force microscope. Methods based on thermionic emission[ edit ] The work function is important in the theory of thermionic emissionwhere thermal fluctuations provide enough energy to "evaporate" electrons out of a hot material called work function of metals 'emitter' into the vacuum.
If these electrons are absorbed by work function of metals, cooler material called the collector then a measurable electric current will be observed.
Thermionic emission can be used to measure the work function of both the hot emitter and cold collector.
Generally, these measurements involve fitting to Richardson's lawand so they must be carried out in a low temperature and low current regime where space work function of metals effects are absent.
Energy level diagrams for thermionic diode in forward bias configuration, used to extract all hot electrons coming out from the emitter's surface.
Typical values for metals range work function of metals two to five electron volts. When metals of different work functions are joined, electrons tend to leave the metal with the lower work function where they are less tightly bound and travel to the metal of higher work function.
This effect must be considered whenever connections are made between dissimilar metals in certain electronic circuits.