Semiconductor metallic behaviour

An alternative approach to stabilizing the metallic state involves p-type doping. For example, partial oxidation of neutral dithiadiazolyl radicals with iodine or bromine will remove some electrons from the half-filled level. Consistently, doping of biradical systems with halogens can lead to remarkable increases in conductivity and several iodine charge transfer salts exhibiting metallic behaviour at room temperature have been reported. However, these doped materials become semiconductors or even insulators at low temperatures. 

The present author (Mott 1949,1956,1961) first proposed that a crystalline array of one-electron atoms at the absolute zero of temperature should show a sharp transition from metallic to non-metallic behaviour as the distance between the atoms was varied. The method used, described in the Introduction, is now only of historical interest. Nearer to present ideas was the prediction (Knox 1963) that when a conduction and valence band in a semiconductor are caused to overlap by a change in composition or specific volume, a discontinuous change in the number of current carriers is to be expected a very small number of free electrons and holes is not possible, because they would form exdtons. 

Complexes can conveniently be divided into those that show only semiconductor-type behaviour and those that show metallic behaviour, possibly restricted to a certain temperature range. 

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... 

Since their early studies Eley et al. (1959) (see Eley, 1967) have largely confined their attentions to the study of the electronic and structural properties of bipyridinium2+ (TCNQ)2" and related complexes (Ashwell et al., 1975a, b, c Ashwell et al., 1977a, b, c Eley et al., 1977). Most complexes, such as 4,4-bipyridyl (TCNQ)2, five (N,N-dialkyl-4,4 -bipyridylium)2+ (TCNQ)J+ salts (alkyl = methyl, ethyl, propyl, isopropyl and benzyl ), and l,2-di(N-ethyl-4-pyridinium) ethylene2+ (TCNQ), are low gap semiconductors except one form of the last compound which exhibited metallic behaviour. The asterisked complexes comprise planar sheets of TCNQ molecules grouped in tetrads. 

Fig. 1.5. Transition from metallic behaviour with half-filled 7r-band to a bandgap semiconductor due to Peierls distortion...

The behaviour of 2 is quite different. On the fig. 3, the resistance vs. T curves exhibit a small decrease typical of metallic behaviour followed by a sharp metal-semiconductor transition at 145 K. On figs. 3 and 4, we can see that the metal-semiconductor transition which is sharp at the first cooling cycle, broadens when repeating heating and cooling cycles. 

To demonstrate the usefulness of the MD/DF approach, we now discuss applications to structure determination in clusters of elements of groups 13, IS, and 16. Clusters of the last two are typically covalently bonded systems. The bulk systems are generally semiconductors or insulators, and there is a substantial energy gap between the highest occupied and lowest unoccupied molecular orbitals. The first, typified by aluminium, show aspects of metallic behaviour. One of the advantages of the DF method is that it can be applied with comparable ease to elements of all atomic numters. 

The family of organic conductors perylene2[M(mnt)2] (mnt = maleonitriledithiolate) has been studied for more than fifteen years, since the Cu and Ni compounds were reported to be semiconductors by Alcacer and Maid [1,2]. Subsequent work paid special attention to the Au, Pt and Pd compounds [3-9], that exhibit quasi one-dimensiond metallic behaviour down to a temperature Tj j, where a metal-insulator transition occurs. These compounds crystallize in space group P2i/c [3,8], and the b axis is the high-conductivity direction (G / 1(P). 

Carbon nanotubes (CNTs) are members of the Fullerene structural family. Their name is derived from their long, hollow structure with the walls formed by one-atom-thick sheets of carbon, called graphene (Fig. 3.8). The manner in which these sheets are connected to form tubes at specific and discrete ( chiral ) angles and the combination of the angle and radius decides the nanotube properties for example, whether the individual nanotube exhibits metal or semiconductor like behaviour. The values of n and m are used not only for the chirality or twist but also to draw a line between metallic and semiconducting nanotubes. In other words, chirality in turn affects the conductance, density, lattice structure, and certain other properties of the nanotube. A nanotube is considered metallic if the value m is divisible by three. Otherwise, the nanotube is semiconducting. 

As outlined above, electron transfer through the passive film can also be cmcial for passivation and thus for the corrosion behaviour of a metal. Therefore, interest has grown in studies of the electronic properties of passive films. Many passive films are of a semiconductive nature [92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102 and 1031 and therefore can be investigated with teclmiques borrowed from semiconductor electrochemistry—most typically photoelectrochemistry and capacitance measurements of the Mott-Schottky type [104]. Generally it is found that many passive films cannot be described as ideal but rather as amorjDhous or highly defective semiconductors which often exlribit doping levels close to degeneracy [105]. 

For tire purjDoses of tliis review, a nanocrystal is defined as a crystalline solid, witli feature sizes less tlian 50 nm, recovered as a purified powder from a chemical syntliesis and subsequently dissolved as isolated particles in an appropriate solvent. In many ways, tliis definition shares many features witli tliat of colloids , defined broadly as a particle tliat has some linear dimension between 1 and 1000 nm [1] tire study of nanocrystals may be drought of as a new kind of colloid science [2]. Much of die early work on colloidal metal and semiconductor particles stemmed from die photophysics and applications to electrochemistry. (See, for example, die excellent review by Henglein [3].) However, the definition of a colloid does not include any specification of die internal stmcture of die particle. Therein lies die cmcial distinction in nanocrystals, die interior crystalline stmcture is of overwhelming importance. Nanocrystals must tmly be little solids (figure C2.17.1), widi internal stmctures equivalent (or nearly equivalent) to drat of bulk materials. This is a necessary condition if size-dependent studies of nanometre-sized objects are to offer any insight into die behaviour of bulk solids. 

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