Thermionic work function contact potential

Thermionic work function contact potential. Representing two different metals a and jS, initially separate and each at zero electrostatic potential, by the diagram in Fig. 48 a, in which the ordinates are the energy levels of electrons, we see that the energy level of the electrons in oc is higher than that in / by the difference between the thermionic work functions, On connecting the two metals, as in Fig. 48 6, the 

There is a small correction to equation (8), for the Peltier electromotive force between the metals,1 which arises from differences in the concentrations of electrons in the upper levels of the two metals, when not at absolute zero its value is of the order 10 2 volts, and may be neglected for most purposes. 

The existence of a contact potential between two different metals was recognized over a century ago by Volta, who ascribed the origin of the electromotive force of galvanic cells to it. This point of view receded somewhat into the background in the later decades of last century, but is now re-established, as will be seen in 5. It is not very easy to demonstrate the existence of this contact potential and its actual value depends very much on the cleanliness of the surface indeed without very careful cleaning of the surface, and removal of surface films, which requires a high standard of vacuum technique, the true value for the clean metal can scarcely be obtained at all. 

There are three principal methods for measuring the contact potential, i.e. the difference of electrostatic potential between two points in space, each just outside one of the metals. Volta,2 and later Kelvin,2 used a condenser method, depending on the fact that if the distance between the plates of a condenser is altered, when they are at different potentials, current flows from one and to the other. The two metals are made the plates of an adjustable condenser, and are put into contact they then assume a potential difference equal to the Volta potential. After breaking the contact, the distance between the plates is altered, when current 

A very sensitive modification of this method has been described by Zisman 1 one of the plates is made to vibrate rapidly parallel to itself, thus periodically altering the distance between the plates. The oscillating current thus caused to flow to and from the plate is amplified and operates a loud speaker. A potentiometer is used to vary the potential applied to the plates, until the sound vanishes, and the value of the applied potential is then the contact potential. 

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Table XVI gives recent values of the thermionic work functions for several clean metals and also (for discussion later) the accepted values of the standard electrode potential of the metal in contact with an activity molar aqueous solution of one of its salts, where the concentration is such that the activity coefficient multiplied by the molarity is unity.

Langmuir,2 in considering the importance of contact potentials for electrolytic cells, pointed out in 1916 that there is a general parallelism between the thermionic work function and the standard electrode potentials. This is shown in Table XVI, where the last column gives the difference between the electrode potential, on the normal hydrogen scale, and the work function. This difference varies much less than the values of either the work function, or the electrode potentials, separately. 

Readers of Gurney s paper in 1931 have sometimes considered, from the frequent appearance of the thermionic work function in this paper, that the values of over-potential should, on the theory that the block lies at (9), depend on g. This does not appear to be justified although electrons have to be extracted from the metal, they do also in a reversible electrode, and in either case the x 8 cancel out, as described at the end of 4, through a second contact potential elsewhere in the circuit. A glance at the figures for overpotential on p. 324 shows no correlation with x> from Table XVI. 1 Z. phyrikal. Chem., 113, 213 (1924). 

Bosworth and Rideal (67,68), too found with the contact-potential method an increase of the work function by 1.38 volts when nitrogen was adsorbed on tungsten at 90°K. Since the method described in section III,Id, was used, where a second tungsten filament is heated for the emission of thermionic electrons, it is to be expected that in this case, too, N atoms were formed, which reached the cold wire near the hot one and were adsorbed. 

Neglecting the No(dCp/dNy, the Vs are the latent heats. Thus we have the important statement that the contact difference of potential between two metals equals the difference of their latent heats, or approximately of their work functions. This relation is found to be verified experimentally. The contact difference of potential can be found by purely electrostatic experiments, and the work functions by thermionic emission the results obtained in these two quite different types of experiment are in agreement. The small correction terms arising from the No(dCp/dN) s lie almost within the errors of the experiments, so that we hardly need consider them in our statement of the general theorem. 

However, experimental ]V curves often deviate from the ideal /scl- In these cases, the measured current /inj is injection limited caused by a nonohmic contact or poor surface morphology. When the MO interface is nonohmic, carrier injection can be described by the Richardson-Schottky model of thermionic emission the carriers are injected into organic solid only when they acquire sufficient thermal energy to overcome the Schottky barrier ((()), which is related to the organic ionization potential (/p), the electron affinity (AJ, the metal work function (O, ), and the vacuum level shift (A) [34,35]. Thus, the carrier injection efficiency (rj) can be calculated by the following equation ... 

When atoms of barium, caesium, potassium, thorium or similar metals are deposited on a surface of metallic tungsten, stable films may be built up varying in nominal thickness frpm a fraction of a monolayer to many monolayers. The new composite surface has contact potentials and thermionic or photoelectric work functions different from those of the clean metal. The movements of atoms in these films can therefore be followed by the variation in the thermionic or photoelectric currents, i, which alter as the fraction d of the surface covered alters. It is important to find how the current i depends on 0. 

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