Electrically Conducting Thin Films by Ion Implantation of Pyrolyzed Polyacrylonitrile
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ELECTRICALLY CONDUCTING THIN FILMS BY ION
IMPLANTATION OF PYROLYZED POLYACRYLONITRILE R. A. BASHEER General Motors Research Laboratories Warren, MI 48090-9055 ABSTRACT
Heat treatment of polyacrylonitrile leads to products with semiconductor to metal like conductivities. The electrical properties of these materials
are fyrther modified by ion implantation.
It is noted that the conductivity
(-10-" ( cm)-1) of heat treated Polyacrylonitrile at 435°C (PAN435) increases upon ion jmplantat on with As', Kr', Cl or F+ -eaching a maximum value of 8.9 x 10 (0 cm) a a dose of 5 x 101 ion/cmE r and an energy of 200 KeY for the case of F+ implantation. On the other hand, ion implantation of the more conducting heat treated PAN at 750°C (PAN750) leads to a decrease in the electrical conductivity of the material. It is proposed that the conductivity modifications are primarily due to structural rearrangements induced by the energetic ions. Specific chemical doping contribution to conductivity is noted for halogen implantation in PAN435. The temperature dependence of conductivity of PAN heat treated at 750"C suggests a two path conduction, namely, a three dimensional variable range hopping conduction and a metallic conduction. After ion implantation, the conductivity-temperature dependence is interpreted in terms of a variable range hopping conduction mechanism.
INTRODUCTION Interest in organic conducting polymers has been growing steadily ever since the discovery of high electrical conductivity in doped polyacetylene. A large number of experimental and theoretical investigations have been
devoted to the elucidation of the electronic properties of doped polyacetylene [1]. However, the chemical instability of this material under ambient conditions has prompted interest in air stable conductive polymers as an alternative to polyacetylene. In this regard, I have been investigating thin film materials obtained by pyrolysis of polyacrylonitrile. The products of pyrolysis are stable in air and show a wide range of conductivity depending on the degree of carbonization or 'graphitization' which is controlled by the applied pyrolytic temperature. Furthermore, it has been shown that such materials may be chemically doped by electron acceptors or donors to produce higher conductivities (2]. In this study, ion implantation is employed as an alternative method in the modification of the electrical conductivity of pyrolyzed Polyacrylonitrile products. This was motivated by recent studies showing dramatic increases in the electrical conductivity of certain insulating and semiconducting polymers [3]. Experiments involving implantation with ions of varied chemical reactivities and ionic masses were conducted in order to ascertain the nature of ion implantation induced conductivity modifications in these materials. Experimental Polyacrylonitrile thin films (0.3 - l#) were deposited from dimethylformamide solution onto a thermally grown silicon dioxide on a silicon wafer. After complete removal of the solvent at 200"C under vacuum in a tube f
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