Photoconductive properties

Lead Telluride. Lead teUuride [1314-91 -6] PbTe, forms white cubic crystals, mol wt 334.79, sp gr 8.16, and has a hardness of 3 on the Mohs scale. It is very slightly soluble in water, melts at 917°C, and is prepared by melting lead and tellurium together. Lead teUuride has semiconductive and photoconductive properties. It is used in pyrometry, in heat-sensing instmments such as bolometers and infrared spectroscopes (see Infrared technology AND RAMAN SPECTROSCOPY), and in thermoelectric elements to convert heat directly to electricity (33,34,83). Lead teUuride is also used in catalysts for oxygen reduction in fuel ceUs (qv) (84), as cathodes in primary batteries with lithium anodes (85), in electrical contacts for vacuum switches (86), in lead-ion selective electrodes (87), in tunable lasers (qv) (88), and in thermistors (89). 

The substance appears to be a semiconductor with pronounced photoconductive properties (117). 

Another interesting applications area for fullerenes is based on materials that can be fabricated using fullerene-doped polymers. Polyvinylcarbazole (PVK) and other selected polymers, such as poly(paraphcnylene-vinylene) (PPV) and phenylmethylpolysilane (PMPS), doped with a mixture of Cgo and C70 have been reported to exhibit exceptionally good photoconductive properties [206, 207, 208] which may lead to the development of future polymeric photoconductive materials. Small concentrations of fullerenes (e.g., by weight) lead to charge transfer of the photo-excited electrons in the polymer to the fullerenes, thereby promoting the conduction of mobile holes in the polymer [209]. Fullerene-doped polymers also have significant potential for use in applications, such as photo-diodes, photo-voltaic devices and as photo-refractive materials. 

Electrical and Photoconductive Properties of Orthorhombic Sulfur Crystals... 

Metalloid that exists as two allotropes, one glasslike and the other metallike photoconducting properties useful in photocopy machines included in the diet, it has health benefits. 

Our investigation of sNPS showed that the samples prepared by the chemical etching method described above have consistent photoluminescence, conductivity and photoconductivity properties, which have remained unchanged over 5 years. sNPS structure was investigated by scanning electronic microscopy (Fig. 9.1). 

As noted at the beginning of this chapter, most of the early studies in CD focused on PbS, followed by PbSe, driven by their photoconducting properties. For optimum use as photoconductors, the deposited films were annealed in an oxygen-containing atmosphere. Most of this section will focus on nonannealed films, and annealed films will be treated only very briefly. More details on the annealed as well as as-deposited fihns will be given in Chapter 5. 

We can conclude this section with the insight, gained from this overview of the electrical and photoconductivity properties of these films, that, in spite of the many studies already carreid out, a comprehensive and systematic study of these properties and their correlation with a wide range of deposition parameters is still needed in order to understand what determines these properties. These studies should also include postdeposition treatments— not so much annealing, which has been carried out, but surface treatments (e.g., immersion in triethanolamine), which could show the importance (or lack of it) of the crystal surface condition. 

Specific photoconductive properties of PbS films have already been treated. This section deals with more general aspects of CD PbS (for the most part, also relevant for PbSe) films. 

PbS deposition, the early work used mostly selenourea (or selenourea derivatives), while the more recent studies used the selenosulphate baths. As for PbS, these two baths will be treated separately. Optical and photoconducting properties will not be treated separately as for PbS, for which there is a larger body of results, but are included with the description of the various deposition baths. 

Organic solids have received much attention in the last 10 to 15 years especially because of possible technological applications. Typically important aspects of these solids are superconductivity (of quasi one-dimensional materials), photoconducting properties in relation to commercial photocopying processes and photochemical transformations in the solid state. In organic solids formed by nonpolar molecules, cohesion in the solid state is mainly due to van der Waals forces. Because of the relatively weak nature of the cohesive forces, organic crystals as a class are soft and low melting. Nonpolar aliphatic hydrocarbons tend to crystallize in approximately close-packed structures because of the nondirectional character of van der Waals forces. Methane above 22 K, for example, crystallizes in a cubic close-packed structure where the molecules exhibit considerable rotation. The intermolecular C—C distance is 4.1 A, similar to the van der Waals bonds present in krypton (3.82 A) and xenon (4.0 A). Such close-packed structures are not found in molecular crystals of polar molecules. 

Chalcogenide glasses As2Sej Xerography based on photoconducting property... 

Traps and recombination centers which depend on purity, crystal defects and preparation, can exert an influence, and electrode contacts, carrier injections, and other factors can interfere with measurements. Yet there is no doubt that the photoconductive gain (quantum yield) G can be reproduced by different methods. As in the case of dark conductivity, the photoconductivity properties are related to the electronic and structural behavior of pure and doped organic compounds, also those in the polycrystalline state. 

Now the research in the photoconductive properties of the polyconjugated materials is growing fast. Heterocycle or heteroatom-containing polymers are involved in this process due to their excellent mechanical and electric properties. The sensitized photoeffect in polyconjugated materials was first observed in 1964 [19,20] and the high significance of the increase in the photosensitivity of these compounds became apparent... 

Photoconductivity of Kapton, (polypyromellitimide where R is oxygen) was investigated in detail [224-234]. Frenkel, Onsager, hopping and other models were used by different authors for explanation of the photoconductive properties. The photoconductivity spectra of Kapton film for various directions of the electric field are presented in Fig. 29 [230]. The high anisotropy depending on... 

Aftertreatment. Thermal treatment at temperatures up to 1000°C improves the pigment properties of the ZnO and is mainly applied to oxide produced by the direct method. Controlled atmospheric calcination also improves the photoconducting properties of the high-purity oxide used in photocopying. 

The highest purity material is calcined with additives such as Bi203 and used in the manufacture of varistors [2.80]. The photoconducting properties of ZnO are used in photoreproduction processes. Doping with alumina causes a reduction in electrical resistance hence, it can be used in the coatings on the master papers for offset reproduction [2.81]. 

Interest in the photoconductive properties of the carbazole nucleus has also prompted studies concerned with its incorporation into condensation polymers. Examples of polymers prepared include polyamides (34), polyesters (35) and polyurethanes (36) (80MI11105). Thorough studies on the CT interactions of these polymers with the monomeric acceptor 2,4,7-trinitrofluorenone have been done. In all cases, the formation constant for the CT complex was higher with polymers than for monomeric models. At least two polymer... 

Chl-coated semiconductor (n-type) electrodes and metal electrodes can act as efficient photoanodes and photocathoes, respectively, for visible light conversion. The former system functions as a dye-sensitized semiconductor electrode, while the latter is presumably driven by the photoconductive properties of a Chi solid layer and/or charge separation involving the Chl-metal contact barrier. 

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... 

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