Measurement of work function

The electron work function of a solid, efj , is defined as the work necessary to remove an electron from the highest occupied level inside the solid to a point in vacuo just outside the surface. (Further details of the theory of the work function can be obtained by reference to works on solid state physics - .) When adsorption takes place, dipoles are created, altering the potential barrier which the electron must penetrate to escape the surface. j is increased if the dipole produced on adsorption is directed away from the smface (electronegative layer), or decreased if the dipole is directed towards the surface (electropositive layer). By considering the adsorbed layer as a parallel plate condenser, it can be shown that the potential drop (AF) across the layer is given by 

The kinetics of adsorption can be studied from the rate of change of 0 provided H is constant. However, this is not necessarily the case except over a limited range. It must also be remembered that each crystal plane will have a different work function in the clean state. If more than one crystal plane is exposed, an average value will be obtained. Added to this is the fact that many gases adsorb differently on different crystal planes which makes the interpretation of results rather complicated. 

Methods of work function measurement are of two types, electron emission methods and condenser methods. In the former method heating, irradiation with light of a suitable wavelength or application of sufficiently strong electrical fields is used to cause electrons to tunnel through the surface potential barrier. The latter method consists of measuring the contact potential difference between the surface under study and a reference electrode. 

For thermionic emission from a uniform surface, the saturation current density j extrapolated to zero applied field is given by 

When a surface is irradiated by light of variable frequency v, electrons will be emitted if the incident photons impart sufficient energy to enable them to overcome the potential barrier at the surface. At the threshold frequency vq, 

---

Payne, M. C., and Inkson, J. C. (1985). Measurement of work functions by tunneling and the effect of the image potential. Surface Science 159, 485-495. 

Equation (6.62) indicates that (abs) can be obtained from measurements of work function and an estimation of the value of % (Bockris and Argade, 1968) and from... 

A further problem arising with semiconductivity measurements is that the surface conductivity may be of a different type to the bulk conductivity. Measurements of work function or field effect mobility are then necessary to fully define the process. 

The absolute value of Ep depends on the choice made for the reference state. Frequently this is taken as zero for a free electron in a vacuum, and Ep levels in metals and semiconductors can be determined from measurements of work functions or electron affinities (Figure... 

Figure 14.19 Measurement of work function changes (b) and surface potentials (c) in APXPS measurements. For details, see text. The sketch in the upper right-hand part of the figure shows the principal arrangement of X-ray beam, sample surface, and acceptance volume of the electrostatic lens.

The pzc of solid electrodes depends on the ciystallogiaphic orientation of the surface. This is a consequence of the surface dipole dependence on the crystal orientation as in the measurements of work functions. One sees that measurements of the pzc and of work functions are closely related to each other. The main ddference, apart from the different reference state, is the additional presence of a dipole layer in the electrolyte which modifies the overall dipole moment and may also change the dipole on the metal surface. 

The experimental approaches in studies of interface properties are analyzed. Several electrical methods based on measurements of work function, thermopower, and electrical conductivity and their applications in studies of defect-related properties are described. Several applied aspects of interface electrical phenomena are also briefly considered. 

In the present paper, we describe a scanning Kelvin nanoprobe (SKN) as an alternative label-free detection approach for work with protein microarrays (7). Based on the measurement of work function change, the SKN can measure inherent electrical properties of proteins on a solid surface, which depend on protein dimension, orientation, polarization and molecular interactions. 

It is desirable to build the thermionic emission diode designed by Eckstrein and Forman [30] in the mass spectrometer, since it enables a quantitative measurement of work function variation with the adsorbed O2 on the Re surface and the discovery of an even higher work function surface (see section 2.2). 

For the measurement of work-function differences as they occur, for instance, during adsorption, several methods are in use. The most common ones are the Kelvin method and the diode method. 

---