Activation energy semiconduction

Table 3 summarizes some of the present state-of-the-art parameters obtained for undoped and doped i -SiH(F) material thus produced. The device-quahty material exhibits semiconductivity because In G vs 10 /Texhibits a straight line with a conductivity activation energy of eV, which is... 

Semiconducting Properties. Sihcon carbide is a semiconductor it has a conductivity between that of metals and insulators or dielectrics (4,13,46,47). Because of the thermal stabiUty of its electronic stmcture, sihcon carbide has been studied for uses at high (>500° C) temperature. The Hall mobihty in sihcon carbide is a function of polytype (48,49), temperature (41,42,45—50), impurity, and concentration (49). In n-ty e crystals, activation energy for ioniza tion of nitrogen impurity varies with polytype (50,51). 

This may explain the observed semiconductive behavior with activation energy of 0.13 eV. The room temperature EPR line width of 30 G decreases to 20 G at 77 K. 

Another semiconducting fulleride salt, [Ru(bpy)3](C5o)2 with bpy = 2,2 -bipyridine, crystallizes on the Pt electrode surface out of dichloromethane solutions saturated with [Ru(bpy)3]PF5 within a few minutes [79]. The NIR spectra of benzonitrile solutions of this salt demonstrate that the only fulleride anion present is 55 . The temperature dependence of the conductivity is typical for a semiconductor, with the room temperature conductivity being 0.01 S cm and the activation energy 0.1 kj mol (0.15 eV). It was postulated that there is an electronic overlap between the two ions of this salt leading to a donation of electron density from the 55 to the ligand orbitals in the [Ru(bpy)3] " AI 0.7) [79]. 

The first 10 powder diffraction lines for each phase are given in Table 1. Bulk magnetic susceptibility measurements (77-300°K) show that both platinum disulfide and platinum ditelluride are diamagnetic, with susceptibilities of -31(2) and -12(1) emu/mole respectively, as expected for low-spin d6 octahedral ions. Platinum disulfide shows semiconducting behavior between 77 and 300°K, with an activation energy of 0.10(1) eV, whereas platinum ditelluride is metallic. 

The main features of this investigation are the following. First, a "deactivation process similar to that observed on the pure nickel oxide was found on the modified catalysis as well, with the same logarithmic law to represent its evolution with time. Second, the kinetic equations which were found to fit the data on pure nickel oxide also apply to the modified catalysts. Thus there is a low-temperature mechanism operative between 100° and 180°C. For all the samples assembled in Table II, the activation energies were practically the same, about 2 kcal./mole and essentially equal to the value for pure nickel oxide. This indicates that, for this particular mechanism of the reaction, the added ions and the semiconductivity changes do not affect directly the catalytic process. 

If further work confirms our explanations which connect catalytic inversion with the inversion of physical properties of the modified nickel oxide catalysts, the correlation between semiconductivity and oxidation catalysis found in the Princeton work and in Schwab s studies will appear quite convincing. To sum up, the activation energy of the carbon monoxide oxidation has been found to decrease with increasing semiconductivity on both sides of the inversion point of physical properties of nickel oxide catalysts. 

Schwab and co-workers (5-7) found a parallel between the electron concentration of different phases of certain alloys and the activation energies observed for the decomposition of formic acid into H2 and CO2, with these alloys as catalysts. Suhrmann and Sachtler (8,9,58) found a relation between the work function of gold and platinum and the energy of activation necessary for the decomposition of nitrous oxide on these metals. C. Wagner (10) found a relation between the electrical conductivity of semiconducting oxide catalysts and their activity in the decomposition of N2O. 

The oxidation of carbon monoxide on nickel oxide has often been investigated (4, 6, 8, 9, II, 16, 17, 21, 22, 26, 27, 29, 32, 33, 36) with attempts to correlate the changes in the apparent activation energy with the modification of the electronic structure of the catalyst. Published results are not in agreement (6,11,21,22,26,27,32,33). Some discrepancies would be caused by the different temperature ranges used (27). However, the preparation and the pretreatments of nickel oxide were, in many cases, different, and consequently the surface structure of the catalysts—i.e., their composition and the nature and concentration of surface defects— were probably different. Therefore, an explanation of the disagreement may be that the surface structure of the semiconducting catalyst (and not only its surface or bulk electronic properties) influences its activity. 

Table IV contains some comparative data regarding the electrical conductivity of some polychelates based on Fe3+ and Mn2+. The data dealing with electrical conductivity of polychelates, the starting polymers (for polyethylene terephthalate,

Interesting results have been obtained in studies of the catalytic activity for oxidation by phthalocyanine polymers, containing different metal ions in the same molecule 87-90>. If Fe was mixed with a series of other transition metal ions, differences in activity were found to be dependent on the metal ion, and correlations between the catalytic activity and the thermal activation energy of semiconductivity were found. With copper as the second metal ion, maximum activities were found at a ratio Fe/Cu = 1. Many other chelate polymers have been tested for their oxidation activity, and a dependence of the catalytic activity on the donor properties of the ligand was found 91>92). 

In the case of intrinsic band conduction the experimental activation energy SA is identified with half the band gap (Eq. (2.37)) in the case of extrinsic or impurity semiconductivity, SA is either half the gap between the donor level and the bottom of the conduction band or half the gap between the acceptor level and the top of the valence band, depending upon whether the material is n or p type. In such cases the temperature dependence is determined by the concentration of electronic carriers in the appropriate band, and not by electron or hole mobility. 

The resistivities we find at room temperature are more than a factor of 10 lower than those reported by Ozerov for a single crystal at room temperature (26). Furthermore, our activation energies, 0.02 e.v., are considerably lower than the 0.12 e.v. derived from the data of Ozerov, although his thermal studies were all carried out on powders. Nevertheless, our conclusion is that the sodium-vanadium bronze is normally semiconducting and in this respect differs essentially from the equivalent sodium-tungsten bronzes. 

In this expression, A, the activation energy (half the semiconducting energy gap Ec of the material), and Rx, the preexponential factor, are assumed to be independent of T (Rx is the lower limit of R when T — °°) is the Boltzmann constant. 

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