It is widely believed that nano-engineered conducting polymer composites could provide a high quality electronic material, where carbon nanotubes, which have a high aspect ratio and high conductivity, would enhance the thermal, mechanical, and electrical stability of the composite. Composites of chemically synthesized, high molecular weight polyaniline (PANI) and pristine single walled carbon nanotubes (SWNTs) are investigated for electronic device applications. Various PANI/SWNT compositions were used to fabricate metal / semiconductor contact devices. Measured IV characteristics indicate that the current level of the neat PANI increases with the addition of SWNTs. Furthermore, it is observed that as the SWNT concentration in the composite increases, the current level in the sample increases. IV characteristics of these composite materials were also studied at different temperatures. The current was observed to increase with temperature. This trend was consistent with all PANI/SWNT compositions.
The dominant transport mechanisms operating in these devices were investigated by plotting the forward IV data on a log-log scale, which reveals two power-law regions with different exponents. The power-law relationship can be expressed as, , where K is a constant and m is the exponent, which is determined from the slope of the curve. At lower voltages, the exponent m1 is ~1, implying that the charge transport mechanism is governed by Ohm's law. The charge transport mechanism at higher voltages, where exponent m2 is ~1.3, lies between the limits of Ohms law and trap-free square law, which is consistent with space-charge-limited (SCL) emission in the presence of shallow traps. Furthermore, m2 was observed to decrease with increase in temperature. Such non-ideal diode behavior is strongly influenced by localized defect states. Space-charge-limited current injection cannot dominate until some critical field is reached at which the injected excess free electron concentration becomes comparable to the thermally generated, and consequently neutralized, carrier concentration. The critical voltage (Vcrit), which characterizes the onset of SCL conduction, increases with an increase in temperature for all the composite compositions studied. These initial results indicate that with further improvements in material consistency and reduction in defect states, the polyaniline/single walled carbon nanotube composite material can be used to fabricate organic electronic devices leading to many useful applications in microelectronics.
This work was supported in part by the ERC program of NSF No. EEC-9731680 and NASA Ames research center.
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