Aluminum surface, work function

Modification of the top electrode may also be achieved. This was done by adding a small amount of surfactant, such as an ether phosphate or an ether sulfate, to the spin-coal solution of the luminescent polymer [89[. The lipophobic ether chains segregate at the surface of the (predominantly) hydrocarbon polymer, becoming available for complexation with the aluminum cathode which is deposited on top. Thus, the dipole in the surfactant points away from the electrode and lowers its work function to improve the injection of electrons. 

The work function of the rubbing surfaces and the electron affinity of additives are interconnected on the molecular level. This mechanism has been discussed in terms of tribopolymerization models as a general approach to boundary lubrication (Kajdas 1994, 2001). To evaluate the validity of the anion-radical mechanism, two metal systems were investigated, a hard steel ball on a softer steel plate and a hard ball on an aluminum plate. Both metal plates emit electrons under friction, but aluminum produced more exoelectrons than steel. With aluminum, the addition of 1% styrene to the hexadecane lubricating fluid reduced the wear volume of the plate by over 65%. This effect considerably predominates that of steel on steel. Friction initiates polymerization of styrene, and this polymer formation was proven. It was also found that lauryl methacrylate, diallyl phthalate, and vinyl acetate reduced wear in an aluminum pin-on-disc test by 60-80% (Kajdas 1994). 

An example of an Al(lll) surface is shown in Fig. 8.4. From the measured work function, cj)=3.5 eV, we find k = 0.96A", The atomic distance is a = 2.88 A, which also equals the parameter m in the Morse formula. An estimation of the parameter Uq in the Morse formula, or the binding energy per pair of A1 atoms, can be made as follows The evaporation heat of aluminum is 293 kJ/mol, which is 3.0 eV per atom. Aluminum is an fee crystal, where each atom has 12 nearest neighbors. Therefore, the binding energy per pair of A1 atoms is about 0.5 eV. Substitute these numbers into Eq. (8.16) and Eq. (8.17), we find the forces at the T site and the H sites which reproduce the result of first-principle calculations by Ciraci et al. (1990a). 

The electronic interaction is small if the metal used as an adsorbent has a work function low in relation to the polarizability of the adsorbed atoms. On adsorption of sodium atoms on an aluminum surface of = 4.08 volts, for instance, Brady and Jacobsmeyer (56) obtained a noticeable increase of the photoelectric emission only after five atom layers of sodium had been condensed. In this case the alkali layer itself and not the metal of the sublayer emitted the electrons. 

Recently Franken and Ponec (44) have published photoelectric work functions of Ni-Al alloy films. Since the phase diagram contains many inter-metallic compounds, the surface composition will now be determined by that compound which yields the lower surface energy, in this case the compound having the largest amount of aluminum. Indeed after an initial steep decrease in work function over a limited concentration range, a plateau of constant work function is found. 

The correlation between the heat offormation AHf and the work function WF. Table 5.11 is showing the heat of formation of AHf of the oxides, sulfides, chlorides and phosphorus compounds and the average work function (WF) of the elements. The work function is a complicated physical property related to the crystal orientation, surface composition, and chemisorption (Shpenkov, 1995). Taking an aluminum surface as an example (Huber and Kirk, 1966), the adsorption of dry oxygen to one monolayer coverage will lower the work function by 0.05... 

Why are the electron energies of Okll and Ckll in Figure 7.3 different from those in Figure 7.2 Estimate the work function of an aluminum oxide surface from Figures 7.2 and 7.3. 

Also important is the fact that the aluminum film used always oxidized, even before the organic monolayer was put on it. Thus the fact that the work function of the surface covered with aluminum does not vary between OTS and Me-Si coverage, may result from the existence of this oxide layer. 

A similar use of the technique of vacuum evaporation, is put to work in the manufacture of aluminized TV picture tubes. In this case, the picture tube is used as the vacuum chamber and aluminum is flashed from a heated tungsten filament projecting into the neck of the tube. The aluminum deposits on the inside surface of the tube and covers the phosphor screen. When in use, the aluminum film serves to reflect the light emitted from the phosphor screen which would otherwise be lost inside the tube. In this respect, the aluminum film fulfills the same function as the mirror in back of a kerosene lamp. 

In previous work (1), we examined the impact of surface impurities on electrochemical reactions beneath polysiloxane encapsulant coatings. Test specimens consisted of aluminum combs prepared on Si02 by photolithography. Leakage currents were measured as functions of temperature, surface impurity levels, and relative humidity. In all cases, increasing relative humidity, RH, produced a monotonic rise in leakage current, corresponding to increased water... 

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