Organic thin films: a comparison of their electrical and gas sensitivity

Abstract

This thesis presents a study of the electrical properties and gas sensitivity of thin films of four different organic materials. These are: (i) Langmuir-Blodgett (LB) films of tetrabutylammonium Ni(dmit)(_2) complex (Bu(_4)-Ni(dmit)(_2) complex) mixed with tricosanoic acid (TA); (ii) thin films of polypyrrole (PPy) mixed with palmitic acid (PA) obtained using the LB technique followed by two solid state reactions; (iii) cast films of polycyanopropylmethylsiloxane (PCMS); and (iv) LB films of a co-ordination polymer?5,5' methylenebis (N-hexadecylsalicydeneamine) (MBSH) (poly(CuMBSH)). LB films of (Bu(_4)-Ni(dmit)(_2) complex)/TA were characterised by electrical measurements?(AC in the frequency range 10(^2)-10(^6) Hz) at room temperature. For the other three types of films, characterisation of the structure and electrical behaviour (DC and AC in the frequency range 10(^2)-10(^6) Hz) on varying the temperature (in the range 90 - 298 K) and during the exposure to benzene, ethanol, acetonitrile and water (concentrations in the range 10(^2) -10(^5) ppm) was undertaken. During exposure to vapours, reversible changes in the electrical properties of the films were observed. The electrical behaviour and the changes during exposure to vapours were interpreted in terms of models in the literature, assuming a bulk dissolution of the vapours in the organic films. In all cases an 'anomalous' response to water was observed. For poly(CuMBSH), this effect was interpreted in terras of a Low Frequency Dispersion. The device characteristics for gas sensing applications are also discussed. Estimated minimum detectable concentrations were between 1-100 ppm for the three organic solvent vapours. Moreover, it is shown that the fi-equency behaviour for the admittance changes of the PCMS and poly(CuMBSH) devices could be exploited for the improvement of the sensitivity of a single device. The unique response of all the films to water vapour could be useful for its discrimination.