DC conduction

Chemical and biological sensors (qv) are important appHcations of LB films. In field-effect devices, the tunneling current is a function of the dielectric constant of the organic film (85—90). For example, NO2, an electron acceptor, has been detected by a phthalocyanine (or a porphyrin) LB film. The mechanism of the reaction is a partial oxidation that introduces charge carriers into the film, thus changing its band gap and as a result, its dc-conductivity. Field-effect devices are very sensitive, but not selective. 

If the potential applied across the cathode and the anode is constant (dc), conductive materials can be analyzed if the potential is varying at radio frequency (rf), both conductive and non-conductive materials can be analyzed. 

The extrapolation w — 0 gives the DC conductivity. A detailed description of this extrapolation procedure will be given elsewhere [15]. 

The resisitivites p corresponding to the DC conductivities results reproduce the measured strong dependence of p on the composition correctly. The CP results agree with the experiments very well. The only exception is the composition with 20% sodium where the relative deviation is about 100% however, the absolute deviation is small. 

Several mixtures of hexanethiol capped gold nanopartides and triphenylene based discotic LCs have been studied. These mixtures display liquid crystal behavior (columnar mesophases) and an enhancement in the DC conductivity, due to the inclusion of gold nanoparticies into the matrix of the organic LC [70]. Other studies of mixtures of gold nanoparticies with mesogens indude a series of cholesteryl phenoxy alkanoates. The inclusion of the nanopartides does not change the inherent liquid crystal properties of the cholesteryl derivative but the mesophases are thermally stabilized [71]. 

Figure 4. Plot of poly-I conductivity as a function of potential. A series of potential step of 20mV were employed on a sandwich electrode. Each potential was held until Faradaic current ceased, where upon a DC conductivity measurement, AE = 60MV, was taken, before proceding with the next potential. The results are for 0.05 M II electrolyte in acetonitrile vs. Ag+/Ag.

The dynamic viscoelasticity of particulate gels of silicone gel and lightly doped poly-p-phenylene (PPP) particles has been studied under ac excitation [55]. The influence of the dielectric constant of the PPP particles has been investigated in detail. It is well known that the dielectric constant varies with the frequency of the applied field, the content of doping, or the measured temperature. In Fig. 11 is displayed the relationship between an increase in shear modulus induced by ac excitation of 0.4kV/mm and the dielectric constant of PPP particles, which was varied by changing the frequency of the applied field. AG increases with s2 and then reaches a constant value. Although the composite gel of PPP particles has dc conductivity, the viscoelastic behavior of the gel in an electric field is qualitatively explained by the model in Sect. 4.2.1, in which the effect of dc conductivity is neglected. 

Sowada and Warman (1982) have described a dc conductivity method for Ar gas at 295 K and 45 atm. Following a 20-ns pulse of irradiation, the conductivity rises to a peak at -50 ns, due to the Ramsauer effect, before settling to a plateau, which is ascribed to thermal conductivity since the collecting field is very low. Since there is little electron loss, the conductivity profile is proportional to the mobility profile this in turn can be considered a kind of image of collision frequency as a function of electron energy. The time to reach the conductivity plateau, -150 ns, is the measure of thermalization time in the present case. At a density of -9 X 1021 cm-3, the conductivity maximum vanishes, indicating the disappearance of the Ramsauer minimum according to Sowada and Warman. 

The critical role of the latter process was clearly shown in the extremely elegant work of Wegner and Riihe (1989) who measured the temperature dependence of the DC conductivity of a range of polythiophene and polypyrrole derivatives as a function of the interchain separation. The derivatives... 

Figure 29. For the latter case, it is considered that the semiconducting re-electron systems are separated by insulating hydrocarbon spacers, resulting in alternate thin layers of organic semiconductor and insulator in these monolayer assemblies. The direct current - voltage (I - V) characteristics were mea- sured for the multilayers H2Pc(R)8 and Cu-Pc(R)8 in directions perpendicular and parallel to the film plane. In both cases, the linear I - V relationships of Ohm s law were observed at low electric field and obtained DC conductivities are summarized in Table 3. The normal conductivity (ajJ were ca. 10 13 S cm-1, while the lateral ones p//) were 3.4 x 10-7 and 9.9 x 10 7 S cm 1 for films of the metal-free and copper Pc derivatives, respectively. The former (ojJ tended to decrease slightly with increase of Figure 29. Schematical illustration of the substituent alkyl chain length,...

Table 3. DC conductivities of octa-alkyl Pc derivatives in the films.

Carrique F, Arroyo FJ, Jimenez ML, Delgado Av. Influence of double-layer overlap on the electrophoretic mobility and DC conductivity of a concentrated suspension of spherical particles. J. Phys. Chem. B 2003 107 3199-3206. 

Glass-Forming Liquids II. Detailed Comparison of Dielectric Relaxation, DC-Conductivity and Viscosity Data. 

Kilbride et al. (2002 Andriotis et al., 2003) measured the alternating current (ac) and direct current (dc) conductivities of polymer-SWCNT composite thin films such as PMPV and polyvinylalcohol (PVA), the result showed that the ac conductivity... 

Fig. 4.14 (a) Optical transmittance of graphene on a polyethylene terephthalate (PET) flexible substrate [19]. Optical and electrical data for PEDOT PSS-based composites with SWCNT (b) transmittance at 550 nm, (c) sheet resistance, (d) DC conductivity and (e) ratio of DC to optical conductivity. 

The dc conductivity, a, of a homogeneous polymer electrolyte, at temperature T, and pressure P, can be expressed in general terms as... 

Some important dielectric behavior properties are dielectric loss, loss factor, dielectric constant, direct current (DC) conductivity, alternating current (AC) conductivity, and electric breakdown strength. The term dielectric behavior usually refers to the variation of these properties as a function of frequency, composition, voltage, pressure, and temperature. 

A difference of two orders of magnitude has been measured in the DC conductivity of AU55 and Aufj at 100 K [29,30]. From this, and the MES observation [46] that the total electron count in the metal core of Au s may be about 2 less than in AU55, it would appear that there is a very nonuniform distribution of levels, with a much larger energy splitting for the HOMO level of Au j than for AU55. Furthermore, the density of states in these levels must be very low. 

DC Conduction. Cross-sectional and top views of the test structures for the dc conduction measurements are shown in Figure 1. Fabrication begins with p-type silicon wafers 3 inches in diameter, which are first doped with boron on the front... 

The effective bulk polyimide resistivity can be extracted from comparison of the measured curves in Figure 6 with this model, and values at 100°C and 158°C are shown in Figure 7, which is a reproduction of the Arrhenius plots of Figure 4. It is seen that both the values of the resistivity and the temperature dependence are in good agreement with those obtained from the dc conduction measurements. In addition, the saturated value of the measured threshold instability is well predicted by the model. 

Figure 7. Temperature dependence comparison of threshold voltage drift and dc conduction data. Conditions area = 0.01 cm AH = 0.7 eV I —A> PI-2 and , PI-3 I/pp—AVrjpjc data.

Although two peaks of comparable amplitude are presented (see Fig. 2.1), only the first, denoted as Mi, is actually related to the carriers release from trap, the second, denoted as M2, is connected with dark conductivity variation with temperature (DC conductivity-determined relaxation peak related to the movement of equilibrium carriers). 

Such behavior of the TSDC peak is typical. On the other hand, the DC conductivity-determined relaxation peak shifts to lower temperatures when the band gap and/or preexponential factor decreases (Figs. 2.2 and 2.3). 

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