Electrical conductivity and thermal diffusivity of zinc naphthalocyanine

Abstract

Naphthalocyanine and its metal complexes are candidate materials for molecular electronic devices such as optical storage media, electrochromic displays and sensors [1,2]. Yanagi et al. [3] have investigated the molecular orientation of vacuum deposited zinc naphthalocyanine (ZnNc) by infrared (IR) spectroscopy, electronic spectroscopy and X-ray diffraction (XRD). Here we report the results of our study on the electrical conductivity and thermal diffusivity of ZnNc and iodine doped ZnNc. ZnNc was synthesized and purified by the method reported by Kaplan et al. [4]. The compound was characterized by elemental analysis, IR and electronic spectra. The data were in agreement with those reported in the literature [3]. ZnNc samples were doped with iodine in the solution phase by stirring 100 mg of powdered ZnNc with a saturated solution of iodine in carbon tetrachloride. The doped material was filtered and washed with carbon tetrachloride to remove excess iodine. This sample was dried at 360 K in vacuo as reported in the case of cobalt phthalocyanine [5]. The samples for measurement were formed into pellets of diameter 5.0 mm and thickness 1.1 mm at a pressure of 2000 kg cm -2. The electrical conductivity was measured in a shielded cell under dynamic vacuum in the temperature range 100-450 K using a Keithley model 617 electrometer. Thermal diffusivity of the pellets was measured by a photoacoustic technique, using 488 nm Ar ÷ laser beam at 26 mW power level. The laser beam was chopped using a Stanford Research System model SR 540 chopper, and front surface illumination was used for finding out the characteristic frequency f~. The schematic diagram of the experimental set up used for determination of the thermal diffusivity by the photoacoustic technique is shown in Fig. 1. The electrical conductivity of pristine ZnNc shows an increase in conductivity with temperature, the Arrhenius plots showing two linear regions of different activation energies, as shown in Fig. 2. The plot does not give any clear indication of a sharp transition. The activation energies in different temperature ranges are given in Table I. Doping with iodine enhances the electrical conductivity. For the iodine doped samples the Arrhenius plot gives two distinct regions with activation energies 0.05 eV and 0.14eV The enhanced electrical conductivity may be due to charge