Zinc photoconductivity

This work will attempt to demonstrate the importance of adsorbed oxygen on many properties of zinc oxide, namely the conductance, fluorescence, photoconductance, and the adsorption of hydrogen. The contribution, potential, and current of each of these studies to a more complete understanding of the adsorption process will be discussed. 

Properties dependent on adsorption are not confined to conductivity. Luminescence of materials may be affected, as Ewles and Heap (7) have shown for the case of silica, for which the luminescent peak at 4000 A. was shown to be associated with the adsorption of the OID radical. Many workers have demonstrated the dependence of the contact potential on the adsorption of gases. For example, Brattain and Bardeen (8) have shown that the contact potential of germanium varies with the adsorption of water vapor. Photoconductivity may be dependent on the adsorption. For example, Bube has shown (9) that the adsorption of water vapor has a marked effect on the photoconductivity of cadmium sulphide. He concluded (10) that the effect was indirect surface changes affect the lifetime of the excess carriers, thus affecting the photoconductivity. Melnick (11), however, working with zinc oxide, has produced evidence that part of the photoconductivity in this case is directly associated with excitation from adsorption levels. 

From the various concepts developed in this and the preceding section, it would appear that an energy-level structure as shown in Fig. 1 may explain the gross effects observed for the conductivity, fluorescence, and photoconductivity of zinc oxide. 

Photoconductivity in zinc oxide, on the other hand, appears to be influenced by the surface through a different effect. Absorption of light effectively excites the electrons trapped in surface levels into the conduction band. This chapter will be primarily devoted to a consideration of this concept, proposed by Melnick (11), that photoconducting electrons are produced through the ionization of surface levels, specifically the adsorbed oxygen levels on zinc oxide. The decay of the photoconductivity. 

Photoconductive response (the rate of creation, or the rise, of the photocurrent, and the rate of decay of the photocurrent) appears to be divided into fast and slow responses. The fast responses, with time constants for rise and decay of the order of a second or less, have been adequately interpreted by Mollwo, et al. (53-55), Weiss (56), and Heiland (47,57) as bulk processes. These authors have concluded that the fast response processes are associated with the double ionization of interstitial zinc, and have proposed that the photon excites electrons from the valence band, and that the hole immediately recombines with the electron from an interstitial Zn+, producing double-ionized zinc ions. 

Melnick (11) has made the most extensive study of the slow photoconductive response. Using zinc oxide sintered in air, he has studied responses in the time range from 0.3 to 10 sec. 

The evidence that adsorption has an influence on the luminescence of zinc oxide is not as clear-cut as it is in the case of photoconductivity and conductivity. No experiments have been carried out, to the author s knowledge, directly correlating adsorption to luminescence, such as the work by Ewles and Heap (7) on silica, which showed correlations between its fluorescence and the adsorption of the hydroxyl radical. 

At present the studies of the luminescence of zinc oxide give little precise information on the adsorption properties of this substance, in contrast to the more fruitful studies of its photoconductivity and conductivity. But there exists a possible correlation of the 3950 A. fluorescence with the adsorption of oxygen. Further study of the fluorescent properties may clarify this correlation. 

When hydrogen sulfide is passed into cadmium salt solutions, cadmium sulfide is formed as a yellow precipitate with a zinc blende structure (cubic, (i-form). The P-form can be converted into the a-form (e.g., by heating). a-Cadmium sulfide shows photoconductivity due to defects in the crystal lattice (usage in photovoltaic cells) [3.106]. The solubility in water at 25°C is 1.46 x 10 10 mol/L [3.107], Cadmium sulfide forms the basis for all cadmium pigments. 

Zinc sulfate has been used as a mordant in calico printing for preserving wood and skins for the manufacture of other zinc salts for clarifying glue and in electrodeposition processes. The sulfide is used as a pigment and as a source of other pigments ZnS is used in X-ray and television screens and, with a trace of a radium or mesothorium salt, in luminous dials of watches. The telluride has been used as a photoconducting semiconductor. 

Metal chelates of 8-hydroxyquinoline such as (111) with photoconductive properties are reported to be useful in electrophotographic systems.233 The incorporation of a tin complex into a photo-conductive zinc oxide layer is stated to reduce dark decay . In other words, the electrostatic charge applied to the photoconductor has a longer lifetime. Two of the complexes disclosed for this application are (112) and (113). These compounds are prepared from dibutyltin oxide by reaction with 2-mercaptopropionic add and thioglycolic acid, respectively 234... 

Bauer, W. (1977). Experimental studies on the spectral sensitization of photoconductivity on zinc oxide crystals. J. Phys. Chem. Solids 38 463-467. 

The existence of photodesorption may be understood in the following way. Photoconductivity studies 104) show that irradiation within the fundamental absorption band of zinc oxide leads to the production of electrons and holes. Other things being equal, these are free to participate in adsorption and desorption, and will react with oxygen as follows ... 

Titanium dioxide, with its high refractive index, provides brightness and opacity, which is why it is used in paper manufacturing. Titanium dioxide also contributes to surface smoothness. The rutile form is more opaque than the anatase. Zinc oxide is popular in document duplication papers because of its photoconductivity. Recently, a hollow-sphere polymer pigment has become a useful new material in the paper industry to decrease paper weight. 

Zinc is also a primary metal in the penny coin, which used to be made of copper. In addition, zinc can be mixed with copper to form brass, which is a very useful alloy because of its durability and hardness. Another useful compound is zinc oxide. Made by burning zinc vapor in air, zinc oxide is used to make white paints. Used as an ointment, zinc oxide makes a good sunscreen because it blocks the Suns harmful ultraviolet rays that damage the skin. Zinc oxide also has another important property it is photoconductive, which means that it conducts electricity better when exposed to light. For example, a photocopier contains a photoconductive plate that is sometimes made with zinc oxide. When this plate is electrically... 

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