The Measurement of Work-Function Changes

The change in work function which accompanies adsorption on a metal surface may be determined directly by thermionic, field-emission, and photoelectric methods. Indirect methods rely on the measurement of a C.P.D. between a reference electrode and the original and covered surfaces, respectively. 

For thermionic emission from a uniform surface, the maximum electron flux j across the uniform surface at a temperature T in the absence of an applied field is given by 

The emission equation is valid for each of the patches on a nonuniform surface, but the total measured current from the various patches depends on the relative magnitudes of the collecting field and patch fields, as well as on the work function. Hence, the effect of some patches may be out of proportion to their area and the average work function of a polycrystalline surface measured thermionically may differ somewhat from the true average work function 

In practice, unless p is small, measurable thermionic currents are obtained only at high temperatures. Thus, when the work function is increased by an adsorption process, the electronegative film may be partly evaporated before the requisite temperature for thermionic emission has been reached. An exceptional case is the adsorption of O2 on W. However, the thermionic method has proved very useful for stud5ring the electropositive films produced by alkali metals (39). Cs, for example, reduces the work function to such an extent that thermionic measurements may be made at temperatures as low as 150°. 

It comprises an axially mounted W filament and three cylindrical collectors Ci,Ci, and Ci. All collectors are maintained at the same positive potential with respect to the W filament, but only the emission current collected from Ci is recorded. In the region of C2 the temperature and the surface concentration of adsorbate on the emitter are assumed to be uniform. 

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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. 

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. 

As pointed out above the comparison between both sets of results relies upon the physical equivalence of the two measureable quantities work function (surface science) and electrode potential (electrochemistry) 111. This equivalence has been realized in electrochemistry some time ago and has been exploited to analyze measured values of the potential of zero charge 111 and of work function changes upon emersion of electrodes at fixed potential 181. In the simulation experiments the approach is quite similar in that one prepares a well-defined composition of the synthetic electrochemical adsorbate layer and then obtains the electrostatic potential drop across it by a work function measurement. 

The discriminator is the balance within the furnace. The balance allows the measurement of any temperature changes between the sample and reference. For a DSC experiment, the individual heaters under both pans work to keep the temperatures the same (or the differential temperature equal to zero) and the power drawn to maintain this is measured. For a DTA experiment, the difference in temperature between the sample and the reference is recorded as a function of furnace temperature. The environment in the furnace is controlled. 

The following tables summarize the results of experimental work function data. Tjub is the substrate temperature in K during deposition. RT denotes room temperature. The methods of work function measurement are VC Vibrating capacitor method RP Retarding potential method SEC(E) and SEC(P) Electron- and photon-induced secondary electron method respectively FE Field emission method. A( )min is the work function change at the minimum, A( )ML is the work function change at one saturated monolayer, fig is the initial dipole moment. Where appears in the table, a minimum was not observed. The tables are adapted and updated from [89A1],... 

XM measures the changes occurring at the surfaces of a metal and water as the two phases are brought in contact to create an interface. In surface science concepts, Xm corresponds to the decrease in work function... 

Figure 2.16. Work function changes versus CO exposure for clean and K-covered Pt(l 11) at 300 K measured from the onset of the electron emission of He I UPS spectra.42 Reprinted with permission from Elsevier Science.

There is further emphasis on adsorption isotherms, the nature of the adsorption process, with measurements of heats of adsorption providing evidence for different adsorption processes - physical adsorption and activated adsorption -and surface mobility. We see the emergence of physics-based experimental methods for the study of adsorption, with Becker at Bell Telephone Laboratories applying thermionic emission methods and work function changes for alkali metal adsorption on tungsten. 

In surface science, work function measurements are considered to be rather sensitive towards changes of the sample surface. Work function measurements are used to follow adsorption processes and to determine the dipole established at the surface. During oxygen adsorption and oxide formation the sign of the work function change allows one to distinguish between oxygen atom adsorbed on the surface or sub-surface [30]. 

Despite these arguments and the conceptual attractiveness of the procedure which is sketched in Fig. 1 convincing evidence for the relevance of a particular gas phase adsorption experiment can only be obtained by direct comparison to electrochemical data The electrode potential and the work function change are two measurable quantities which are particularly useful for such a comparison. In both measurements the variation of the electrostatic potential across the interface can be obtained and compared by properly referencing these two values 171. Together with the ionic excess charge in the double layer, which in the UHV experiment would be expressed in terms of coverage of the ionic species, the macroscopic electrical properties of the interracial capacitor can thus be characterized in both environments. 

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