Electronic transport

The effects of disorder in the unoriented Durham polyacetylene are also manifested in the transport properties, and we review here the picture that has been obtained for the transport mechanisms in the semiconducting (low carrier concentration) regime. Polyacetylene prepared by all routes shows a conductivity as prepared (i.e. not intentionally doped) that is due to extrinsic carriers which are p-type. There are several possible sources for the doping which creates these carriers, and we identify doping with 

We show the temperature dependence of the conductivity of both unoriented and oriented Durham polyacetylene in figure 6. We note that the temperature dependence of the conductivity is similar for both samples, showing an activated behaviour with an energy of activation of about 0.4 eV near room temperature. The anisotropy for the orient sample is about 40, but that the conductivity for the oriented sample is very much higher than for the unoriented sample. Thus, the room temperature conductivity of the oriented sample along the stretch direction is 3 x lO S/cm, but the corresponding value for the unoriented sample is a factor of 1000 lower, at 3 x 10 S/cm. 

S(T) is linear with T for T T T0. At T0 there is a bending over, again giving S T) linear with 7for another 120 K.6 We distinguish between S (T), the low-temperature thermopower below T0, and Sh( T), the high-temperature value, For Sn-rich alloys, Se T) is close to the free-electron value 

we will discuss concentration dependences and the scaling behaviour of these properties with Z. Later, we will discuss temperature dependencies taking into account effects due to the inelastic excitation of phonon-rotons. 

6The liny hump near T0 is artificial due to the reference material (Pb) [5.80, 82]. 

Concentration as well as temperature dependences of p in both the amorphous [5.86-89] and the liquid state [5.90-92] are understood in the framework of the diffraction or extended Faber-Ziman model [5.90, 92, 93]. Experimentally determined total as well as model partial-structure functions have been successfully applied. Disregarding inelastic-scattering effects and extensions of 

The decrease of S(2kr) with increasing Sn-content (Fig. 5.18) explains the decrease of p with x. We will not go into further details but refer instead to the literature mentioned above. 

The conductivity is the product of the carrier density and the carrier mobility, 

Fermi energy in a-Si H cannot be brought closer than about 0.1 eV to Ef., so that the conductivity can hardly be measured below about 100 K and there is only limited information about the sharpness of the mobility edge. Most of the detailed tests of the mobility edge theories are made on disordered crystals and in metals in which the Fermi energy can be made to cross the mobility edge, giving measurable conductivity at low temperatures (Thomas 1985). 

The central problem in studying the conductivity is to find the energy and temperature dependence of ct( ) and the related p( ) and to understand the physical processes involved in the transport. The motion of the carriers at non-zero temperatures may be either in extended states or by hopping in localized states and the magnitude of the conductivity is determined by the elastic and inelastic scattering mechanisms. In addition, when there is any local inhomogeneity of the 

Illustration of the density of states at the band edge, together with the electron distribution the conductivity o( ) and mobiUty 

The Al-containing MAX phases and Ti3SiC2 have another useful attribute, namely that their elastic properties are not a strong function of temperature. For example, at 1273 K the shear and Young s moduli of Ti3AlC2 are about 88% of their room-temperature values [50, 67]. In that respect, their resemblance to the MX binaries is notable. 

For the Al-containing MAX phases, for example, in some cases, the M—A1 bonds are stronger - at least in the c-direction - than the M—X bonds (see Section 7.5.2). 

In order to understand the electronic transport of a solid, it is necessary to know its charge carrier densities and mobilities. For most solids, the Hall coefficient (Rh) is used to determine the concentration and sign of the majority charge carriers. Once known, the mobility is determined from the conductivity values, a. The MAX phases, however, are unlike most other metallic conductors in that their Hall and Seebeck coefficients are quite small - in some cases vanishingly small - and a weak function of temperature [52, 84—87]. Furthermore, the magnetoresistance (MR) (Aq/q = q(B) — q(B = 0)/q(B = 0)]), where B, the applied magnetic field intensity, is positive, parabolic, and nonsaturating. Said otherwise, the MAX phases are compensated conductors, and a two-band conduction model is needed to understand their electronic transport. In the low-field, B, limit of the two-band model, the 

There are four unknowns the concentration of electrons and holes, n, p, and their mobilities, pn and ip, respectively. Given the small Th and Seebeck coefficient values. 

The transport parameters for select MAX phases are summarized in Table 7.3. Based on these results, it is apparent that  

The carrier concentration in an intrinsic semiconductor is related to the energy gap. According to Ashcroft and Mermin (1976), 

The plot shown in Fig. 5.8 compares the temperature dependence of the carrier concentration predicted by Eq. 5.1 with the DC conductivity along the 350 bar isobar (Baker, 1968 Fischer and Schmutzler, 1979). The calculated conductivity is represented by the expression 

The dominant role of the temperature-dependent carrier concentration implied by Fig. 5.8 permits us to draw some semi-quantitative conclusions about electronic transport in semiconducting liquid selenium. Using the simple semiconductor model, we can estimate the carrier mobility from Eq. 5.2 from the fitted prefactor in Eq. 5.1. For this purpose we assume that the carrier masses are not too different from the free electron mass, that is, nig — nif, mo- This yields an estimated carrier concentration at 400 °C, 2 3 x 10 cm. The corresponding 

50 (ft cm ). Addition of 1% S shifts the required pressure to 550 bar while a similar amount of tellurium reduces the 50 (fl cm ) pressure to 380 bar. These experiments demonstrate vividly the intermediate status of selenium between insulating sulfur and metallic tellurium. 

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The term vitamin K2 was applied to 2-methyl-3-difarnesyl-l,4-naphthoquinone, m.p. 54 C, isolated from putrefied fish meal. It now includes a group of related natural compounds ( menaquinones ), differing in the number of isoprene units in the side chain and in their degree of unsaturation. These quinones also appear to be involved in the electron transport chain and oxidative phosphorylation. 

Hwang K C and Mauzerall D C 1993 Photoinduced electron transport across a lipid bilayer mediated by Nature 361 138-40... 

Electron transfer reactions are conceptually simple. The coupled stmctural changes may be modest, as in tire case of outer-sphere electron transport processes. Otlier electron transfer processes result in bond fonnation or... 

In solid state materials, single-step electron transport between dopant species is well known. For example, electron-hole recombination accounts for luminescence in some materials [H]. Multistep hopping is also well known. Models for single and multistep transport are enjoying renewed interest in tlie context of DNA electron transfer [12, 13, 14 and 15]. Indeed, tliere are strong links between tire ET literature and tire literature of hopping conductivity in polymers [16]. 

This section presents tire basic tlieoretical principles of condensed phase electron transport in chemical and biochemical reactions. 

Bockrath M, Cobden D H, McEuen P L, Chopra N G, Zettl A, Thess A and Smalley R E 1997 Single-electron transport in ropes of nanotubes Science 275 1922-5. 

Datta S 1997 Electronic Transport in Mesoscopic Systems (Cambridge Cambridge University Press)... 

An important enzyme in bio logical electron transport called cytochrome P450 gets Its name from its UV absorp tion The P stands for pig ment because it is colored and the 450 corresponds to the 450 nm absorption of one of Its derivatives... 

The ready reversibility of this reaction is essential to the role that qumones play in cellular respiration the process by which an organism uses molecular oxygen to convert Its food to carbon dioxide water and energy Electrons are not transferred directly from the substrate molecule to oxygen but instead are transferred by way of an electron trans port chain involving a succession of oxidation-reduction reactions A key component of this electron transport chain is the substance known as ubiquinone or coenzyme Q... 

Traditionally, the electron and proton transport pathways of photosynthetic membranes (33) have been represented as a "Z" rotated 90° to the left with noncycHc electron flow from left to right and PSII on the left-most and PSI on the right-most vertical in that orientation (25,34). Other orientations and more complex graphical representations have been used to depict electron transport (29) or the sequence and redox midpoint potentials of the electron carriers. As elucidation of photosynthetic membrane architecture and electron pathways has progressed, PSI has come to be placed on the left as the "Z" convention is being abandoned. Figure 1 describes the orientation in the thylakoid membrane of the components of PSI and PSII with noncycHc electron flow from right to left. 

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

Light and photosynthetic electron transport convert DPEs into free radicals of undetermined stmcture. The radicals produced in the presence of the bipyridinium and DPE herbicides decrease leaf chlorophyll and carotenoid content and initiate general destmction of chloroplasts with concomitant formation of short-chain hydrocarbons from polyunsaturated fatty acids (37,97). 

A. Trebst and M. Avron, eds.. Photosynthesis P. Photosynthetic Electron Transport andPhotophosphorylation, Tnyclopedia of Plant Physiolog i, NS., Springer-Vedag, Berlin, 1977. 

Insects poisoned with rotenone exhibit a steady decline ia oxygen consumption and the iasecticide has been shown to have a specific action ia interfering with the electron transport iavolved ia the oxidation of reduced nicotinamide adenine dinucleotide (NADH) to nicotinamide adenine dinucleotide (NAD) by cytochrome b. Poisoning, therefore, inhibits the mitochondrial oxidation of Krebs-cycle iatermediates which is catalysed by NAD. 

Hydramethylnon [67485-29-4] is tetrabydro-5,5-dimetbyl-2-(1 H)-pyrimidinone [bis-l,5-(4-trifluoromethylphenyl)-3-penta-l,4-dienylidene] hydrazone (152) (mp 189°C). It is a slow-acting stomach poison used in baits and traps to control ants and cockroaches. Its mode of action is inhibition of mitochondrial electron transport. 

A compound which is a good choice for an artificial electron relay is one which can reach the reduced FADH2 active site, undergo fast electron transfer, and then transport the electrons to the electrodes as rapidly as possible. Electron-transport rate studies have been done for an enzyme electrode for glucose (G) using interdigitated array electrodes (41). The following mechanism for redox reactions in osmium polymer—GOD biosensor films has... 

The abihty of iron to exist in two stable oxidation states, ie, the ferrous, Fe ", and ferric, Fe ", states in aqueous solutions, is important to the role of iron as a biocatalyst (79) (see Iron compounds). Although the cytochromes of the electron-transport chain contain porphyrins like hemoglobin and myoglobin, the iron ions therein are involved in oxidation—reduction reactions (78). Catalase is a tetramer containing four atoms of iron peroxidase is a monomer having one atom of iron. The iron in these enzymes also undergoes oxidation and reduction (80). 

The decline in immune function may pardy depend on a deficiency of coenzyme Q, a group of closely related quinone compounds (ubiquinones) that participate in the mitochondrial electron transport chain (49). Concentrations of coenzyme Q (specifically coenzyme Q q) appear to decline with age in several organs, most notably the thymus. 

N. E. Mott and E. A. Davis, Electronic Transport in Non-Cystalline Materials Clarendon Press, Oxford, U.K., 1979. 

The main advantages that compound semiconductor electronic devices hold over their siUcon counterparts He in the properties of electron transport, excellent heterojunction capabiUties, and semi-insulating substrates, which can help minimise parasitic capacitances that can negatively impact device performance. The abiUty to integrate materials with different band gaps and electronic properties by epitaxy has made it possible to develop advanced devices in compound semiconductors. The hole transport in compound semiconductors is poorer and more similar to siUcon. Eor this reason the majority of products and research has been in n-ty e or electron-based devices. 

The physical properties of tellurium are generally anistropic. This is so for compressibility, thermal expansion, reflectivity, infrared absorption, and electronic transport. Owing to its weak lateral atomic bonds, crystal imperfections readily occur in single crystals as dislocations and point defects. 

Conducting Polymer Blends, Composites, and Colloids. Incorporation of conducting polymers into multicomponent systems allows the preparation of materials that are electroactive and also possess specific properties contributed by the other components. Dispersion of a conducting polymer into an insulating matrix can be accompHshed as either a miscible or phase-separated blend, a heterogeneous composite, or a coUoidaHy dispersed latex. When the conductor is present in sufftcientiy high composition, electron transport is possible. 

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