Surface dipole energy

The small (0.5 eV) variation of the Fermi levels and the similarity of the gross band structures suggest that the work functions of the lanthanide metals have no strong dependence on Z. Indeed, available measured work functions for polycrystalline Ce, Nd, Sm, Eu, Gd, Er, and Yb are all 3.0 + 0.5 eV (Holzl and Schulte 1979). This experimental fact in conjunction with our Sp values implies that the surface dipole energy also varies slowly with Z and that D 1 eV (relative to the crystal zero). 

HRELS = high-resolution, electron-energy-loss spectroscopy. " Surf. Sci. (in press). Ref. (123). Ref (101). Softened pCHj surface-mode. Weak band observed around 1500 cm could be a surface-dipole-forbidden, Pfc mode. Hidden under intense SCHj mode of free C2H4 in the matrix. " One of these bands belongs to Ni2(C2H4)2. 

The energy needed to surmount the surface dipole layer is the surface contribution to the work function. It depends very much on the structure of the surface For fee metals the (111) surface is the most densely packed surface, and has the largest work function because the dipole barrier is high. A more open surface such as fee (110) has a smaller work function. Also, when a surface contains many defects, the... 

This correlation has been explained in terms of two effects (1) the surface energies of the two metals involved and (2) the formation of a surface dipole potential. 

Polymer films were produced by surface catalysis on clean Ni(100) and Ni(lll) single crystals in a standard UHV vacuum system H2.131. The surfaces were atomically clean as determined from low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Monomer was adsorbed on the nickel surfaces circa 150 K and reaction was induced by raising the temperature. Surface species were characterized by temperature programmed reaction (TPR), reflection infrared spectroscopy, and AES. Molecular orientations were inferred from the surface dipole selection rule of reflection infrared spectroscopy. The selection rule indicates that only molecular vibrations with a dynamic dipole normal to the surface will be infrared active [14.], thus for aromatic molecules the absence of a C=C stretch or a ring vibration mode indicates the ring must be parallel the surface. 

When working with metal electrodes, the energy of the electrons in the metal is lower than the vacuum level by the work function of the metal, which tends to be 3-5 eV. Work functions of some materials relevant to LED devices are collected in Table 10.2 [11]. The work function can vary depending upon the crystal facet from which emission is measured (or if the metal is amorphous), and sample preparation details. The photoelectric (PE) effect is exploited in XPS (ESCA) or UPS to measure the work function. It is very critical to realize that, in these experiments, what is measured is the energy required to remove an electron to a point just outside the surface of the solid, not to infinity. At this range, the dipolar forces at the surface are still active, and one can learn about surface dipoles in the material. 

The second important difference is that the interface potential is present at the (outer) Helmholtz layer of the semiconductor/soiution interface. The interface potential is produced by surface dipoles of surface bonds as well as surface charges due to ionic adsorption equilibria between the semiconductor surface and the solution. If the interface potential can be regulated by a change in the chemical structure of the semiconductor surface, then the semiconductor band energies can be shifted to match the energy levels of the solution species (oxidant or reductant). This is another advantage of the semiconductor system because this enables improvement of the electron transfer rate at the semiconductor/soiution interface and the energy conversion efficiency. 

Besides the effects of different surface dipoles, the concentration and energy position of surface states depend also largely on surface orientation with the result that the electric excess charge in surface states can be very different on different surfaces. This is indicated in Figure 3 by a comparison between the flat band situation and the situation at equal electrode potential for different surfaces. Case (a) is a surface free of surface... 

Fig. 21. Total potential energy of glyoyl-L-alanine at intervals of 1 kcal mole-1 using the same values of the parameters as in Fig. 20, but ignoring the dipole-dipole energy (i.e., D= oo). This potential surface yields o/ j,l = 3-95 (Brant and Flory, 1965c).

The free energy of the surface layer formed by surface dipoles and the water molecules between them is assumed to be independent of the distance 2d. 

We will assume, as before,23 that the free energy of the surface layer formed by the surface dipoles and the water molecules between them is independent of the distance between the two plates. Consequently, the free interaction energy, per unit area, of two planar plates separated by a distance D = 2d is composed of a chemical term Frh, an entropic term due to the mobile counterions, and a term due to the electrostatic fields 23... 

---