Taking advantage of optical and electrical properties of organic moleculesfor gas sensing applications

The discipline of molecular electronics has grown rapidly over the last 10 years and is driven by the promise of the enhanced applied physical propert ies of functionalised organic materials compared to their inorganic partner s. The subject can be divided generally into two broad themes, namely activ e molecular-scale electronics (or photonics), in which the control or gener ation of charge (or photons) at the nanoscale is attempted, and passive sup ra-molecular electronics (or photonics), in which the specific functionalit y of the molecules is modified by some interaction or process. In this pape r, an example of the latter approach to molecular electronics will be given and this will describe the gas sensing properties of a tetra-substituted p orphyrin molecule. The optical absorbance spectrum of LB film assemblies of 5,10,15,20-tetrakis(3,4-bis[2-ethylhexyloxy]phenyl)-21H,23H-porphine(EHO) is highly sensitive to low concentrations of NO2. LB films prepared at much faster than conventional deposition rates (similar to 1000 min min(-1)) yi eld t(50) response and recovery times of 25 and 33 s, respectively, and sho w a sensitivity of 60% relative absorbance change (at 430 nm) for 4.4 ppm N O2. The morphology of these films is revealed using atomic force microscopy to contain isolated micron-size domains which are composed of grains of se veral nm in diameter. This unconventional structure leads to a useful sensi ng material as a result of the molecular functionality of the porphyrin cou pled to the enhanced surface area of the porous film assembly. The EHO film shows a gradually diminishing optical response as its temperature is incre ased, resulting from the shift in the adsorption- desorption equilibrium to wards desorption. The spectrum recovers fully after exposure to NO2. The ra te of recovery is slow at room temperature but can be accelerated dramatica lly with gentle heating (similar to 350 K) for a few seconds. The kinetics of the gas sensing process have been modelled and found to fit Elovichian s urface adsorption for an initial fast surface adsorption process. This is f ollowed by a much slower diffusive process in which the NO2 molecules diffu se through the bulk of the assembly. The concentration dependence of the op tical response over the range 0.8-4.4 ppm follows a Langmuir model. (C) 200 1 Elsevier Science B.V. All rights reserved.