Semi-conducting behavior

Conjugated polymers have been the subject of great interest, both theoretically and experimentally, since the discovery of conductivity in doped polyacetylene in the seventies [1]. Many works have been devoted to their synthesis, characterizations and properties [2]. They have found many apphcations, particularly in the field of optoelectronics, as light-emitting diodes (LEDs), field-effect transistors (FETs), solar cells, etc., due to the semi-conducting behavior of the conjugated backbone [3]. Among them, polythiophenes (PTh) and polypyrroles (PPy) have been extensively studied because of their synthesis versatility and environmental stability [4-6]. Their functionalization permits the combination of their... 

Depending upon the bridging ligands, e.g. pyrazine or diiso-cyanobenzene whole the variety of bridged macrocyclic metal complexes e.g. [PcFe(pyz)] and [PcFe(dib)] [1>2] have been synthesized and characterized. The e compounds show a semi-conducting behavior to an Upper limit of 10 S/cm without oxidative doping. 

The rather low semi-conducting behavior of [PcFeL] compounds (L = pyz, dib) can be increased not only by oxidative doping with iodine as reported earlier [5] systematic investigations on electrochemical doping processes are carried o t. As an example electrochemical doping of [PcFe(pyz)] 2 10 ) with a variety of counterions BF, ... 

Many liquid alloys, in particular, the alkali-group IV alloys, exhibit (Zintl) anion clustering and show strong effects of compound formation. A typical example of such Zintl systems are sodium-tin alloys. In the solid NaSn crystal the Zintl anions Sn appear [1]. An interesting question is the stability of these anions in the liquid. Furthermore, the electrical conductivity of these alloys shows a strong dependence on composition [2] For the limiting (sodium-rich or tin-rich) cases a metallic (small) conductivity appears, but for the nearly equimolar compositions a semi-metallic behavior - with a considerably smaller conductivity - is observed. 

A comprehensive report which focussed on the La2 xSrxCu O4-x/2+S ser es was published (139) in 1983 by this research group. In this broad review they reported the magnetic and electrical transport properties of these mixed-valent copper oxides in the temperature range 120-650 K. They concluded that the original semiconducting behavior in La2Cu04 transformed to semi-metallic behavior as the Cu3+ content increased with Sr-substitution. No experiments were conducted below 50 K, and therefore superconductivity was not observed. Three series of compounds, with 0.00 < x < 1.20 were... 

Besides the possibility to understand chemical composition and structural features, the balances above are connected to the electronic density of states, DOS (Fig. 2.7). In the case of a completely balanced situation, the Fermi level is located in a gap or pseudo gap of the calculated electronic DOS indicating a semi-conducting or bad-metal behavior in electronic transport. If excess electrons are present, the Fermi level is located in the conduction band, suggesting n-type metallic or metal-like electronic transport. 

Nano-confinement of metal and semi-conductor materials can lead to marked changes in their electronic behaviour. Their unique properties resulted in an increased interest in using these nanoparticles (NPs) in materials science. Furthermore, with the discovery of the symbiotic nature of metal/semi-conductor heterostructures, the use of NPs in applications such as photocatalysis and opto-electric devices, like photovoltaic cells, has increased. The exceptional properties of carbon nanotubes (CNTs), as well as their unique structure, have led to increased investigation into their behavior in such hetero-structured complexes. Large surface-to-volume ratios, chemical inertness, and lack of porosity make CNTs prime candidates as catalyst supports. In more complex systems, the electrical properties of the CNTs increase the yield of catalyzed reactions due to the electronic interactions of certain NPs and CNTs. Based on the fact that charge transfer between quantum dots and CNTs has been reported, certain semi-conducting NPs have been covalently linked to CNTs to make hetero-junction electronic devices. ... 

Semi-conducting GaAs of n- and p-type is produced by adding shallow acceptors (mainly Zn, Be, C) and shallow donors (mainly Si, Te) with concentrations of 0(10 cm ) directly into the melt. Then, the Fermi level is pinned near the valence and conduction band, respectively. Silicon, which is most frequently used as a n-type dopant, shows amphoteric behavior in GaAs, i.e. it can be substituted for an As, as well as a Ga atom with roughly [Sica] 0.9 [Si]. 

The development of low-pressure synthesis methods for diamond, such as the chemical vapor deposition (CVD) technique, has generated enormous and increasing interest and has extended the scope of diamond applications. Highly efficient methods have been developed for the economical growth of polycrystalline diamond films on non diamond substrates. Moreover, these methods allow the controlled incorporation of an impurity such as boron into diamond, which in this case forms a ptype semiconductor. By doping the diamond with a high concentration of boron (B/C = O.Ol), conductivity can be increased, and semi-metallic behavior can be obtained, resulting in a new type of electrode material with all of the unique properties of diamond, such as hardness, optical transparency, thermal conductivity and chemical inertness [1,2]. 

The effective diffusivity depends on the statistical distribution of the pore transport coefficients W j. The derivation shows that the semi-empirical volume-averaging method can only be regarded as an approximation to a more complex dynamic behavior which depends non-locally on the history of the system. Under certain circumstances the long-time (t —> oo) diffusivity will not depend on t (for further details, see [191]). In such a case, the usual Pick diffusion scenario applies. The derivation presented above can, with minor revisions, be applied to the problem of flow in porous media. When considering the heat conduction problem, however, some new aspects have to be taken into accoimt, as heat is transported not only inside the pore space, but also inside the solid phase. 

A key extension of this work is to incorporate additional types of related recycled items or conduct a more complicated network such that the materials may or may not move through all tiers sequentially. Another extension of the research in this chapter is to examine the individual or the system behavior of the semi-centralized or semi-decentralized network, which may contain several independent recycling organizations or firms and several municipal collection sites or recyclers. 

Among the unit operations, adsorption may be considered a prototype for all fluid-solid separation operations. When it is conducted under countercurrent conditions, the calculation methods required are entirely analogous to those for countercurrent absorption or extraction (H3). Often, however, it is most economical to conduct adsorption in a semi continuous arrangement, in which the solid phase is present as a fixed bed of granular particles. The fluid phase passes through the interstices of this bed at a constant flow rate and for an extended period of time. The concentration gradients in the fluid and solid phases display a transient or unsteady-state behavior, and their evolution depends upon the pertinent material balances, rates, and equilibria. 

The use of secondary modifiers, e. g. quinoline, and the choice of solvent also play important roles in directing semi-hydrogenation selectivity. For example, in the hydrogenation of 1-octyne over a series of Pd/Nylon-66 catalysts metal loading had no effect on selectivity when the reaction was performed in n-heptane as solvent. When the same experiment was conducted in n-propanol, however, an inverse relationship between selectivity and catalyst metal loading was observed [56], This effect has been interpreted as a polar solvent-induced modification of the Pd active sites, which alters the relative adsorption behavior of the alkyne and alkene species [57], Modification by addition of quinoline is reported to benefit the selective production of a cij-vitamin D precursor from the related disubstituted alkyne [58] ... 

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