Investigation of the novel charge transfer complex Cd- TCNQ
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Investigation of the novel charge transfer complex Cd-TCNQ A. Arena, A. M. Mezzasalma, S. Patan`e, and G. Saitta Dipartimento di Fisica della Materia e Technologie Fisiche Avanzate, Istituto Nazionale per la Fisica della Materia, Universit`a di Messina, Salita Sperone 31, 98166 S. Agata, Messina, Italy (Received 25 July 1995; accepted 11 December 1996)
A novel metal salt of TCNQ (tetracyanoquinodimethane) has been synthesized via an electrochemical process using cadmium as donor. Thin films of the same material have been obtained by evaporating under high vacuum the metal and the organic molecule. The electronic properties of a pellet of the powdered material have been investigated by means of standard UV-VIS-IR spectroscopy, x-ray photoelectron spectroscopy, and Auger electron spectroscopy. The complex shows two absorption structures at about 3.4 eV and 1.7 eV due to the in-plane intramolecular transitions and a shoulder at about 1.4 eV probably due to the bond with the metal. The infrared spectrum is dominated by a deep absorption structure distinctive of the charge transfer. The electrical conductivity of the sample shows a semiconductor behavior in the investigated temperature range; the best fit of the transport data provides an activation energy of about 0.12 eV and shows that at low temperature the electrical conductivity is mainly due to a hopping process among differently charged sites.
I. INTRODUCTION
In the last few years many compounds based on the p acceptor molecule tetracyanoquinodimethane (TCNQ) and various partners have been subject to considerable investigation.1 Some of these complexes are obtained by means of a direct redox process involving the electronpoor molecule and a metal or an organic electron-rich partner; in other cases the reaction is more complicated and involves the use of other ligands.2,3 The ability of TCNQ to form conducting or semiconducting charge transfer (CT) crystals is well known. These CT compounds are usually constituted by quasi-one-dimensional columns of TCNQ molecules and stacked columns of electron acceptors. Most of these materials display high electrical conductivity, and some of them show structural modification induced by the electric and electromagnetic field. These unusual features, truly fascinating and promising for use in electric and electronic engineering, optics and optoelectronics, are largely dependent on the degree of charge transfer between the donor sites and the acceptor ones. Recent studies4 pointed out the potential to synthesize thin films of charge transfer compounds, evaporating under high vacuum the acceptor species and the donor ones; this technique allows one to obtain homogeneous thin films suitable for the realization of simple devices, and for this reason is very interesting. In this work, a novel salt of TCNQ has been synthesized using cadmium metal as donor by means of an electrochemical process; thin films of the same materials have been obtained by means of the co-evaporation of cadmium
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