Photoconductivity and nonlinear

PROPERTIES OF SPECIAL INTEREST Electrical Conductivity, photoconductivity, and nonlinear optical activity. 

As mentioned above, photorefractivity is a multifunctional property which is produced by the combined action of photoconductivity and nonlinear optical properties of a material (24). It is a result of the functions of photogeneration, transport, charge trapping, and the electro-optic effect. 

Multifunctional monomers have been synthesized whose polymers are exhibiting both photoconductivity and nonlinear optical (NLO) properties [5], Second-order optical nonlinearity requires the total system does not possess a center of symmetry. Therefore, the monomers have a rather complicated structure. The functionalities can be introduced by standard reactions in organic chemistry. 

It was reported [256] that a polymer formed by condensation of iV-(3-isopentyl)-3,6-diformyl-carbazole and 4-14-bis[(-aminoethyl)aminophenyl-azo]nitrobenzene yields a new polyazomethine, carbazole-azo polymer. The product is soluble in organic solvents. The polymer possesses carbazole moieties and azo type nonlinear optical chromophores in the backbone. It shows high photoconductivity and nonlinear optical properties. 

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

Polysilanes can be regarded as one-dimensional analogues to elemental silicon, on which nearly all of modern electronics is based. They have enormous potential for technological uses [1-3]. Nonlinear optical and semiconductive properties, such as high hole mobility, photoconductivity, and electrical conductivity, have been investigated in some detail. However, their most important commercial use, at present, is as precursors to silicon carbide ceramics, an application which takes no advantage of their electronic properties. 

Nowadays, polymeric photoconductors may be used in electrophotography, microfilms, photothermoplastic recording, spatial light modulators, and nonlinear elements. The combination of photosensitivity with high quality electrical and mechanical properties permits the use of such materials in optoelectronics, holography, laser recording and information processes. The applications of the various types of polymers were reported in the final parts of the relevant items in the earlier sections. Here, we will briefly analyze the common features of photoconductive polymer applications. The separate questions of each type have been dealt with in some books and papers [3, 11, 14, 329]. 

The carrier generation mechanism in PPV has been addressed by studying the transient photoconductivity and the photoluminescence as a function of the external electric field, E, in samples oriented by tensile drawing [166]. The transient photocurrent is proportional to E at low fields, but increases nonlinearly for E> 10 V/cm. The field at which the photoconductivity becomes nonlinear (the onset field, EP ) depends on the degree of alignment the higher the draw... 

Material science interest focuses on potential applications of its specific electronic and band structure. Metal-like conductivities, semiconductor properties, photoconductivity and the nonlinear optical features of URPAC samples suggest devices for electromagnetic shielding, energy storage, microelectronics, optoelectronic and optooptical communication or optical computing. 

The discussion so far has concentrated on the fundamental spectroscopic and photophysical properties of semiconductor nanoclusters. These nanoclusters represent a new class of novel materials and many potential applications are being evaluated. In the next several chapters I discuss several topics that are of interest in the photoscience area nonlinear optical properties, photoconductivity, and photochemical conversion. 

A separate section is devoted to phthalocyanines and porphyrins as discotic liquid crystals. The nonlinear optical effects as well as photoconductivity and the possibility of the formation of Langmuir-Blodgett-films of phthalocyanines and derivatives is discussed at some length. Several practical applications of these unconventional materials are mentioned. 

Polysilanes (Fig. 1) [1] have attracted considerable attention due to their usefulness as precursors for thermally stable ceramics [2, 3] or a material for microlithography [4, 5] and also due to their potentiality in preparation of new types of material showing semiconducting, photoconducting, or nonlinear optical property [6-8]. 

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