Characterisation and gas sensing properties of tert-butyl silicon-[bis ethyloxy]-phthalocyanine LB films

An ultrathin Langmuir-Blodgett film of tert-butyl silicon-[bis ethyloxy]-phthalocyanine (tbPcSi(OC2H5)2) has been investigated as an interesting candidate for the detection of nitrogen dioxide gas at a concentration of 5 ppm (by volume). Surface-pressure area isotherms reveal an area per molecule of around 80 Å2 and that a compact 'solid' phase occurs in the region 10-30 mN m−1. Five layer LB films were deposited onto gold interdigitated electrodes on glass in order to facilitate I-V measurements and assess the conductometric sensing properties in response to 5 ppm NO2. The I-V characteristic is Ohmic and yields a conductivity in the range 3.4 × 10−6 to 5.9 × 10−6 (Ω cm)−1 which is comparable to that obtained for other phthalocyanine monomers. Exposing the LB films to NO2 results in an increase in current (for a 5 V bias) by a factor of 8 and a response time (t50) of approximately 200 s. Preliminary UV-vis spectroscopic investigations indicate that the film also responds optically.