Room temperature conducting films

As is the case for the dark resistivity, the dependence of the sensitivity of the photoconductivity (defmed here as the ratio between light and dark conductivity) on the deposition parameters is far from clear-cut. Some observations can be made, however. The first (obvious) one is that for a high sensitivity, the dark resistivity must be high. Apart from this, there does seem to be a general trend (clear-cut in the triethanolamine and citrate baths and seen also by the lack of appreciable photoconductivity in the one low- (room-) temperature-deposited film reported [40]) of an increase in photosensitivity (due to decrease in light resistivity) with increasing deposition temperature. 

Advantage has been taken of organometallie complexes with long-chain alkyl-ammonium or pyridinium salts to increase the dimensionality and, hence, the lateral conductance [746, 747]. Indeed, the highest reported room temperature conductivity to date (25 Scm ) was measured in bromine-doped LB films prepared from tridecylmethyl-ammonium Au-(dmit)2 [741, 742],... 

Several attempts to induce orientation by mechanical treatment have been reviewed 6). Trans-polyacetylene is not easily drawn but the m-rich material can be drawn to a draw ratio of above 3, with an increase in density to about 70% of the close-packed value. More recently Lugli et al. 377) reported a version of Shirakawa polyacetylene which can be drawn to a draw ratio of up to 8. The initial polymer is a m-rich material produced on a Ti-based catalyst of undisclosed composition and having an initial density of 0.9 g cm-3. On stretching, the density rises to 1.1 g cm-3 and optical and ir measurements show very high levels of dichroism. The (110) X-ray diffraction peak showed an azimuthal width of 11°. The unoriented material yields at 50 MPa while the oriented film breaks at a stress of 150 MPa. The oriented material, when iodine-doped, was 10 times as conductive (2000 S cm-1) as the unstretched film. By drawing polyacetylene as polymerized from solution in silicone oil, Basescu et al.15,16) were able to induce very high levels of orientation and a room temperature conductivity, after doping with iodine, of up to 1.5 x 10s S cm-1. 

Chemical and phase purity are not always desirable. For example, H- and N-doped silicon carbide films behave as high temperature semiconductors, while silicon carbonitride glasses offer properties akin to glassy carbon with room temperature conductivities of 103 2 cm-118. Additional reasons for targeting materials that are not chemically or phase pure stem from the desire to control microstructural properties. 

Both single crystals (6x 10-5 2 1 cm-1) and sublimed thin films (10 5 n-1cm-Thigh vacuum) of LuPc2 show room temperature conductivity which are more than six orders of magnitude higher than for MPc derived from divalent Cu, Ni and Zn. 

Moreover, the room temperature conductivity of the films shows values as high as 135 S cm, that is, very close to the single crystal value of 400 S The high conductivity value and metallic... 

Figure 9. Increase in room temperature conductivity of HMDT film with time at higher temperature.

A wide variety of transition-metal complexes have been incorporated into PPy [1,21,24-34], ranging from macrocyclic complexes, cluster compounds and simple species such as Pt(CN)4. A summary of the preparation conditions and room-temperature conductivities reported for various PPy films containing transition-metal complexes appears in Table 12.1. Unfortunately, most of these materials have not been investigated thoroughly. It has been frequently assumed (and rarely proven) that the counter-anions maintained their chemical integrity upon incorporation. The conductivities listed in Table 12.1 show considerable variation and appear dependent on the nature of the counteranion incorporated as well as the preparation conditions employed. 

The room-temperature conductivities obtained for a number of PPy films appear in Table 12.3. We have applied both the linear four-probe and van der Pauw methods to the measurement of conductivity. The linear four-probe method has certain advantages the samples (narrow strips of film) are easily obtained and any anisotropy of the conductivity (within the film plane) can be investigated. The major source of error in conductivity measurements is the thickness measurement. The latter is conveniently done using a micrometer when the film thickness is greater than about 20 /zm. (Accurate thickness measurements of very thin films require the use of other techniques, such as electron microscopy.) It is advisable to make a number of conductivity measurements using specimens from different regions of a given film. 

Figure 12.20. Room-temperature conductivity (logarithmic scale) versus wavelength of the absorption in the 300-500 nm region of the UV-visible spectrum for various PPy films A = PPyFeEDTA (film 12) B = PPyCrEDTA (film 11) C = PPy(Cr(NCS)t ) (ref [22]) D = PPyCoCDTA(film 13) E = PPyCoEDTA (film 10) F = PPyCoPDTA (ref [22]) G == PPy Ci(oxX/ (ref [22]) and H = PPyPTS (film IB).

The experimental results presented so far refer to a-Si H films prepared at substrate temperatures T, > 200°C. The influence of T, on the room temperature conductivity of doped and undoped a-Si H films is displayed in Fig. 13. The results show a reduction of the doping effect with decreasing T in particular for phosphorus-doped material. This decay is accompanied by an increase in the dangling-bond spin density (Stuke, 1977) as well as in the hydrogen concentration. Hydrogen evolution experiments suggest that a void-rich structure is formed that differs for boron-doped and undoped or phosphorus-doped material (Beyer and Wagner, 1981). 

Meyerson and Smith were the first to grow and investigate a-C H films doped with boron and phosphorus, by additions of diborane or phosphine to the hydrocarbon gas used for deposition [66]. With a dopant concentration of 10%, the room temperature conductivity increased by about five orders of magnitude to 10 cm. However, as shown by Thiele et al., this effect ist not a true p- or... 

In contrast to Au- and Cu-C H, for example, showing a percolation-type sudden increase of room temperature conductivity at a medium metal content of several 10 vol-%, the a(RT) values in Ta-C H films tend to increase continuously by about six orders of magnitude right from the lowest concentrations in nearly metal free films up to less than 10 at-% Ta. It is only at low temperatures, that an insulator/metal transition is observable as a four orders of magnitude jump in a(3 K) between 11.5 and 14.3 at-%. A detailed discussion of the electrical properties of MeC H films is given elsewhere [70]. 

It was later shown by MacDiarmid et al. [166] that these doped polyaniline-poly(ethylene oxide) electrospim fibers could be deposited onto an oxidized silicon wafer. They demonstrated that it was possible to measure the conductivity of individual electrospun fibers by depositing gold electrical contacts onto the wafer using a shadow mask evaporation process. It was found that the room temperature conductivity for a 1.3 pm diameter CSA-doped polyaniline-polyfethylene oxide) electrospun fiber was 33 S/cm, which is two orders of magnitude higher than the conductivity of the corresponding thermally annealed film prepared from the same solution. This result indicates that... 

As well as optical properties, the method of thin film preparation also influences the electrical properties. As-deposited WO3 films are n-type semiconductors. The room temperature conductivity usually lies in the range (Q.crn) , as shown in Table 38.1. Moreover, it... 

In the past, the four-point probe method (36) has been used to measure the conductivity of the non-woven fiber mat and crosschecked with measurements of the conductivity of cast films produced from the same solution. Figure 3 shows the effect of the weight percentage content of HCSA-PANI on the room temperature conductivity obtained from the HCSA-PANl/PEO electrospun fibers and the film (34).. 

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