Dipole barrier

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... 

Fig. 1.15. Actual energy level diagram of the Au/pentacene interface. The position of the HOMO and the dipole barrier A are estimated from photoelectron spectroscopy [50].

The reference in this expression is implicitly the vacuum zero if the sample had no surface "dipole barrier potential fa (Fig. 3a). In order to relate a measured photoelectron energy shift, which is referred to individual Fermi levels, to such a Abu we must include an estimate of any Fermi level shift with respect to this crystal vacuum zero. An experimental value for the work function, 0, is, un-... 

Fig. 3. The scheme of a bound level and p for a free uncharged sample a) with its surface dipole barrier term, 0b, and b) with the dipole barrier set to zero. Equations for the chemical shift in a bound level of a solid, as given in this review and in most of the published literature, do not include 0b- Behavior of 0b must be taken into account if experimental work functions, 0, are utilized to estimate the chemical shift of ep...

Instead, the interface exhibits an additional dipole barrier A that shifts the vacuum level upward by more than 1 eV, hence increasing the barrier height by the same amount. The rather large interface dipole is explained by the fact that the electron density at a metal surface presents a tail that extends from the metal free surface into vacuum, thus forming a dipole pointing at the metal bulk. Molecules deposited on the metal tend to push back this tail, thus reducing the surface dipole and decreasing the work function of the metal. 

For N oo the first term tends to the electrostatic dipole barrier iies(+oo) — Ves(—oo), i.e. the difference between the electrostatic potentials far outside and deeply inside the metal, and the second term equals the chemical potential. In this limit Aip is the work function of the solid [2]. 

Up to this point, we have discussed an unrealistic semi-infinite solid with a laterally infinitely extended surface, so the actual dimension of the surface did not enter into our considerations. However, the finiteness of any real surface has important consequences for the surface dipole barrier. Consider a finite circular surface of radius R. Owing to the dipolar charge distribution, the surface can be modeled by a circular parallel-plate capacitor with charge density dipole moment S = [Pg.110]

Several remarks are in place. First we note that for the infinite surface /t-ech depends on the surface dipole barrier, but not so for the finite one. For the infinite surface u (oo) is a well-defined quantity, it can be chosen as the zero of the potential energy and in that case 4> is identical with —/Uech- For the finite surface, in contrast, electron removal from the bulk is an activated process and the potential drops off toward macroscopic distances. In fact, as we shall see below, every surface orientation of a metal crystal has a different dipole barrier and hence a different... 

---