Effective negative electron affinity

Figure 10.19 Schematic band diagrams for a semiconductor with (a) positive eiectron affinity (PEA), (b) negative eiectron affinity (NEA), and (c) effective negative electron affinity.

One may expect generally that even in the case of molecules with negative electron affinities or with high threshold electron energies for attachment, some environmental effects or the effect of the vdW molecule formation bring about the large enhancement in the cross sections or the rate constants for the lower-energy electron attachment to these molecules. Based on the above discussions, the reasons for this expectation are summarized as follows [10,11] ... 

It is empirically known that as grown diamond films do not exhibit a charge-up under SEM observation, while it is not so for single erystal diamonds. This is most likely because of the effect of surface conduction by H-termination. It is also known that H-terminated undoped and B-doped diamonds exhibit negative electron affinity (NEA), where the vacuum level is energetically lower than the conduction band. It is thus expected that onee electrons are excited to the conduction band of diamond, they are spontaneously emitted to the vacuum presumably across a small barrier at the surface. 

The simple alkenes have negative electron affinities and therefore will not capture electrons. At the pressure of water vapor used, Reaction 2 is fast enough to preclude other reactions of H20+ (22). Charge or proton transfer to propylene may also be excluded on energetic grounds. The effect of propylene on G( H2) w from water vapor containing small amounts of isopropyl alcohol will therefore be owing to the capture of H atoms in competition with their abstraction reaction from isopropyl alcohol... 

Apart from transistors, several other solid state devices have been discussed [78], like junctions, photon and electron beam switches and various kinds of sensors. One property of diamond which has stimulated considerable interest in the recent years is the negative electron affinity (NEA) of suitably prepared surfaces [78,80]. The electron affinity, of a material is defined as the difference between the energy of a free electron in vacuum and the bottom of the conduction band Fyac - E. In Fig. 8 the electronic bands of p-doped clean and H-terminated (111) diamond surfaces near the surface are depicted, based on the results of UV-photoemission measurements. For the H-terminated surface, the electron affinity becomes negative once an electron is injected into the conduction band from a suitable contact or by UV excitation, it will easily leave the crystal and be emitted into vacuum. This effect, which is also observed on monohydride terminated (100) surfaces, is not unique to diamond but was also observed in a few other semieonductors with high band gaps [80]. Apart from a scientific interest, the NEA of diamond makes it an attractive eandidate for the replacement of thermionic emitters as electron beam sourees and as a miniature electron emitter for field emission displays. 

Electron affinities are intrinsically much more difficult to measure than ionization potentials. In fact, all determinations before 1970 were indirect and unreliable. Today, the principal experimental technique uses the photoelectric effect. A beam of anions is crossed with a light (laser) beam and the frequency is recorded at which the anion dissociates and scattered electrons occur [38]. The most accurate experimental values of A are listed in Table 1.3. At present, there is no practical way of measuring negative electron affinities, which are only available from theoretical calculations. Thus, a universal method to calculate A2 of atoms and molecules has been suggested by von Szentpdly [39],... 

Van Der Weide 1, Zhang Z, Baumann PK et al (1994) Negative-electron-affinity effects on the diamond (100) surface. Phys Rev B50 5803-5806... 

Of chlorine, oxygen, fluorine, and neon, which has the highest (that is, the most negative) electron affinity Briefly but carefully explain your answer in terms of two of the components of the interconnected network of ideas to make sense of the periodic table. As part of your answer, calculate the effective nuclear charges that operate on electrons being added to fluorine, oxygen, and neon. 

Deuterium substitution rednces the electron affinity of organic snbstrates. The reduced electron affinity of deuterinm-snbstitnted analogs is due to the larger electron-donating effect of deuterium when compared to hydrogen. Its inductive constant is small bnt negative (-0.0011). The deuterium electron donor effect develops in organic ion-radicals also. 

It is clear that deuterium as a substituent has the electron-donating effect. In other words, it can decrease electron affinity of the whole molecule. Potentials of reversible one-electron reduction for naphthalene, anthracene, pyrene, perylene, and their perdeuteriated counterparts indicate that the counterparts exhibit slightly more negative potentials (Goodnow and Kaifer 1990, Morris and Smith 1991). For example, the measurable differences in the reduction potentials are equal to -13 mV for the pair of naphthalene-naphthalene-dj or -12 mV for the pair of anthracene-anthracene-djo. The possible experimental error does not exceed 2 mV (Morris and Smith 1991). In another example, in DMF with 0.1 M n-Bu4NPFg, the deuterated pyrenes were invariably found to be more difficult to reduce than pyrene itself. The largest difference observed, 12.4 mV, was between perdeuteriated pyrene and pyrene bearing no deuterium at all with standard deviations between 0.2 and 0.4 mV (Hammerich et al. 1996). 

---