Photoconductive crystal

Since the Institute was among the few research establishments in Berlin to survive relatively undamaged the physical destruction and social unrest that accompanied the end of the war and the fall of the Third Reich, alheit thoroughly looted, it became a refuge for scientists who lacked other institutional ties and needed to retool professionally. This included, in addition to Havemann and Kallmann, Rudolph Frerichs, who worked in the AEG research laboratory until the end of the war on the use of cadmium sulfide crystals in photo-detectors for military purposes. During his sojourn at the Institute, Frerichs continued his research into photoconductivity and the synthesis of photoconductive crystals. In 1947, he departed for the U.S.A. and resumed his academic career at Northwestern University in Evanston, Illinois. The electrochemist Friedrich Todt and chemists Willy Lautsch and Richard Asmus were similarly orphaned and led research groups at the Institute that focused upon topics such as issues in corrosion research, pharmaceuticals and the production of scientific chemicals, respectively. 

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). 

Electrical and Photoconductive Properties of Orthorhombic Sulfur Crystals... 

Bube RH, McCarroll WH (1959) Photoconductivity in indium sulfide powders and crystals. J Phys Chem Solids 10 333-335... 

Another type of absorption is also possible, i.e., exciton absorption which enriches the crystal in free excitons if the latter annihilate then on the lattice defects, causing a change in the charged state of the defects and leading to the appearance of free carriers in the crystal. In this case photoconduction arises as a secondary effect. 

W.F.H. Micklethwaite, The Crystal Growth of Cadmium Mercury Telluride Paul E. Petersen, Auger Recombination in Mercury Cadmium Telluride R.M. Broudy and V.J. Mazurczyck, (HgCd)Te Photoconductive Detectors M.B. Reine, A.K. Sood, and T.J. Tredwell, Photovoltaic Infrared Detectors M.A. Kinch, Metal-Insulator-Semiconductor Infrared Detectors... 

Materials that exhibit photoconductivity and/or electrooptical response can be found in large numbers among molecular glasses (Fig. 3.40). Dihydropyridines with Tg 25 C and low tendency of crystallization have been used (e.g., 2BNCM, 73), adding only a small fraction of a binding polymer (<10%), and 2,4,7-trinitro-9-fluorenone (TNF) as a sensitizer [310]. A common strategy is... 

Adam D, Schuhmacher P, Simmerer J, Haussling L, Siemensmeyer K, Etzbach KH, Ringsdorf H, Haarer D (1994) Fast photoconduction in the highly ordered columnar phase of a discotic liquid-crystal. Nature 371 141... 

Another study found no appreciable difference in either dark or light resistivity between acetate and chloride baths [5]. Interestingly, there was apparently a large difference in crystal size between the two baths (see Sec. 4.1.2 on crystal size), which implies that the crystal size is not an important factor in determining the resistivity or photoconductivity, at least for this bath. 

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. 

Ammonium hydroxide has also been used in place of NaOH or KOH [20,24,26]. In reported contrast to films deposited from alkali metal hydroxide, these films, prepared at or slightly above room temperature, were photoconductive (photosensitivity ca. 10) as deposited without need for air-annealing [26]. The crystal size of films deposited at different temperatures was measured (XRD line broadening) to be 10-15 nm (30°C), 17 nm (40°C), and 39 mn (50°C) [24,26]. The presence of strain in the crystals was inferred from the same XRD measurements [24]. 

The changes in resistivity with annealing of films deposited from selenourea and selenosulphate baths, as well as evaporated films, were compared [71,72]. Although there were small differences between the various films, no major difference was found. Additionally, the resistivity of as-deposited films, deposited from both selenourea and selenosulphate baths, does not change with time over a period of months in air. However, after annealing in air at 350°C when the resistivity increases, there is a gradual decrease in room-temperature resistivity (and also in photoconductivity response) with time [73], These variations were related to formation of PbSeOs and adsorbed oxygen on the surface of the annealed crystals. 

In Refs. 10 and 11, aqueous NaiSiOs was added to SbCls in glacial acetic acid (SbCls hydrolyzes in water unless complexed or the solution is moderately acidic or strongly alkaline). A pH of ca. 3 was optimum below 2.5, adhesion was poor above 4, basic antimony salts precipitated. The solution was kept below room temperature to prevent rapid bulk precipitation. No XRD pattern was found for the as-deposited film, which was presumed to be amorphous. Annealing at 170°C crystallized the film, at least partly. The bandgap of the as-deposited film was reported to be 2.48 eV and that of the annealed film 1.76 eV. Photoconductivity was exhibited by the annealed film but not by the as-deposited one. 

The resistivity of the films increased from 10 O-cm for very low Cd content to a maximum of ca. 10 U-cm at 6% Cd and then slowly dropped again with increasing Cd. This maximum correlates with the minimum crystal size, suggesting a dominant role of grain boundaries in the conduction mechanism. The spectral response of the photoconductivity blue-shifted with increase Cd content up to a peak response at 1.35 p,m for 8.4% Cd. 

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. 

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