Semiconductor behaviour

Thus, despite all the work carried out on starch-iodine, the exact nature of the guest iodine atoms is still not totally resolved. Teitelbaum, Ruby and Marks 156) have examined the compound using Raman and 129I Mossbauer spectroscopy and concluded that the pentaiodide ion 1 was the major chromophore present. However work based on iodine compounds of cyclodextrins 3-134 135) has shown that a variety of polyiodide species is possible, and the starch-iodine inclusion compound could conceivably involve I2 If, I2 I- I2, or I5 species 157). The anhydrous amylose-iodine compound has recently been found to exhibit semiconductor behaviour 158). 

Low levels of structural Ge" have also been observed in natural hematite from the Apex mine, Utah (Bernstein Waychunas, 1987) and to achieve charge balance, incorporation of two Fe for one Ge", i.e. similar to the two Fe" for one in ilme-nite, has been suggested. Synthetic, single crystals of Ge substituted hematite have also been grown by a chemical vapour transport method (Sieber et al. 1985). A range of elements including Zr, Ge, Hf, V, Nb, Ta, W and Pb has been used as low level dopants (2 10 - 0.2 g kg ) to improve the semiconductor behaviour of hematite anodes (Anderman Kermedy, 1988). The increase in unit cell c from 1.3760 to 1.3791 nm and in a from 0.50378 to 0.50433 nm indicated that Nd (as an inactive model for trivalent actinides of similar ionic size (Am r = 0.0983 nm Nd " r = 0.098 nm)) was incorporated in the structure (Nagano et al. 1999). 

This process of creating electronic defects is called valence induction, and it increases the conductivity range of NiO tremendously. Indeed, at high Li concentrations, the conductivity approaches that of a metal (although it still exhibits semiconductor behaviour in that its conductivity increases with temperature). 

The electrical conductivity of CoOP as a function of temperature is shown in Figure 6. Above room temperature the compound exhibits metallic behaviour but coincidental with the development of the superstructure the conductivity falls rapidly with decreasing temperature. Below 250 K CoOp behaves as a semiconductor with an activation energy of meV.74 The conduction has been shown to be frequency dependent below 250 K.75 Thermopower studies have also clearly demonstrated the changeover from metallic behaviour above 300 K. to semiconductor behaviour below 250 K.72 The behaviour of ZnOP is very similar to that of CoOp, with the phase transition from the Cccm to Pccn space group occurring at 278 K. Superstructure formation is complete by about 260 K.77... 

The electrical conductivity of several of these compounds has been reported.88 Compressed pellet four-probe measurements at room temperature are usually in the range 10 2-5 fl I cm-1 although a value of 150-500 has been reported for orientated polycrystals. Most of the compounds show semiconductor behaviour with activation energies of about 35 meV. Ko.6o[Ir(CO)2Cl2]-0.5H20 shows evidence of a transition to more metal-like behaviour near room temperature,88 It seems very likely that if good quality single crystals of these compounds could be obtained then they would exhibit conduction properties similar to those of the cation-deficient tetracyanoplatinates or bis(oxalato)platinates. 

The conductivity in the stack direction is 5 x 10-z ll1 cm 1 and the compound displays semiconductor behaviour from room temperature down to 110 K with a transition between 220-180 K.116... 

The conductivity in the stack direction is 5 x 10 cm and the compound displays semiconductor behaviour from room temperature down to 110 K with a transition between 220-180 The structure of Rb[Pt S2C2(CN)2 2] 2H20 contains the planar anions arranged in a dimeric eclipsed configuration to form a columnar stacked structure. The conductivity in the metal atom chain direction is 2.5 x 10 cm . Polycrystalline. samples of compounds of the type... 

Furthermore from the measured bond lengths we can deduce that inside each tetrad the two central TCNQ molecules (A and A on the fig. 5) bear less charge than the two others. This distribution gives an explanation for the paramagnetic and semiconductor behaviour. 

As a final point, which illustrates the importance of surface processes to semiconductor behaviour, Neave et al. [346] and Kiinzel and Ploog [347] have shown that deep level incorporation during the growth of GaAs films from beams of Ga and arsenic is dependent on the arsenic species used. The deep levels are believed to be associated with intrinsic defects and films prepared from As4, in which a pairwise interaction is involved, contain a higher concentration of three specific deep centres than those prepared from As2 where only simple dissociative chemisorption occurs. 

Fig. 2 presents a stack of IPE spectra taken at different Fe coverage, along with the reference spectra corresponding to the substrate (continuous line) and to a clean and well ordered Fe(OOl) surface (top spectra). The features A and D in the spectrum from clean ZnSe can be assigned to transitions between bulk states, as they display a sizable angular dispersion, typical of band-like states. The semiconductor behaviour is clearly evident from the delayed onset of the spectrum with respect to the Fermi level, Ep. The onset corresponds to the semiconductor conduction band minimum (CB), which, as estimated from the graphic extrapolation shown in Fig. 2,... 

Several different types of metal isocyanide complex exhibit solid state optical properties indicative of metal-metal interactions, but most exhibit low electrical conductivities. One series of salts [Rh(CNR)4]X , where R = phenyl, vinyl, ethyl or methyl and X = Cl, PFg, BF4 or CIO4, exhibit much hi er conductivities and Rh—Rh separations as short as 2.94 A. Single crystals of [Rh(CNCHCH2)4]C104 have a room temperature conductivity of 2 Q cm with an activation energy of 100 meV. An indication of a metallic state was observed below the decomposition temperature of 330 K but it was not possible to conclude whether the high conductivity was the result of partial oxidation, small band gap intrinsic semiconductor behaviour or extrinsic semiconductor behaviour due to Rh impurities . More recent studies indicate the existence of polynuclear species but a full structural study of an infinite metal atom chain compound wth isocyanide ligands has not been made, ... 

There has been considerable interest in recent years in nanoparticles based on the cadmium sulfides and sulfoselenides. These materials, referred to as quantum dots, display unique optical and electrical properties that are quite different from the properties exhibited at pigmentary particle size, and are attributed to their semiconductor behaviour. The most apparent of these properties is their intense fluorescence, the emission wavelength of which may be tuned based on particle size. Quantum dots have potential for applications in medicine, displays, lasers and solar energy conversion. 

A number of rmsaturated polyazamacrocyclic complexes and their derivatives exhibit unusual properties, such as high metal-like conductivities or, conversely, semiconductor behaviour [90-96], Metallophthalocyanines and subphthalocyanines are promising non-linear optical materials for application in optoelectronic devices [97, 98] Porphyrins have been used in organic light-emitting diodes [99],... 

Fig. 34. Normalized resistivity p (T) = p ry p 295K) as a function of reciprocal temperature for three CeHj specimens, showing M-S transitions and pure semiconductor behaviour The actual H content of the CeHiji sample was probably somewhat higher than indicated (Shinar et al. 1989).

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