Electrical Properties and Electronic Applications

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. 

The chemical inertness of diamond, even in hot acidic or caustic oxidizing media [81], and the possibility of making conductive CVD diamond films with resistivities down to less than 0.01 Hem [82] on a wide variety of substrates and over large areas, has in the recent years stimulated an increased interest in applications for analytical and preparative electrochemistry. In a comparative study, Swain 

The evolution of halogen is promoted by the absorption of halogen atoms on the electrode surface (16) however, owing to a small affinity of the hydrogenated diamond surface towards absorption of atomic species, the subsequent reaction (17) is kinetically strongly hindered  

It is inferred that generally electrochemical reactions involving surface-absorbed species will proceed comparably slowly on pure diamond surfaces. On the other hand, outer sphere electron transfer reactions not requiring a strong surface interaction of the species involved, will in principle not be affected by the inert nature of the diamond surface, as to be demonstrated, for example, by the reversible behavior of the Ru(III)/Ru(II) redox couple [85]. 

The actual technical potential of diamond electrodes for analytical and preparative electrochemical applications can so far not be fully assessed. From various prospective applications mentioned in the literature, making use of the exceptional stability, wide potential window and low background currents of diamond electrodes, only one example shall be mentioned here, the reduction of nitrate to ammonium according to the net Eq. (18) 

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