Phthalocyanine Langmuir-Blodgett films and their associated devices
Interest in the Langmuir-Blodgett (LB) technique has led to a number of investigations into different types of materials that can be deposited in the form of monolayers. For example, as well as the classic long Cham fatty acids and alcohols, materials such as polymerisable molecules, aromatic hydrocarbons and dye substances can now all be produced in monomolecular form. Unfortunately, few of these materials yet fulfil the requirements of mechanical and thermal stability that will be necessary if LB films are to be used commercially. This work has dealt with the use of phthalocyanine, a substance well known for its thermal and chemical stability, in the production of LB films. Initially two compounds were investigated, dilithlum phthalocyanine and tetra-tert-butyl phthalocyanine. Although it was found that both materials produced layers of reproducible quality which adhered tenaciously to various substrates and to each other, single monolayers were not obtained. More success has been achieved using an asymmetrically substituted phthalocyanine molecule. Electron microscopy studies have shown that the majority of films are polycrystalline. However, a substitute CuPc proved to be a valuable exception. Multilayer films of this molecule were found to have domains of the order of 3 mm in size showing a preferred orientation. Even so, it has to be accepted that the phthalocyanine films produced to date are not as structurally perfect as for example, multilayers of lo-tricosenoic acid. Our ability to produce monomolecular layers of phthalocyanine now extends the range of possible applications for this material. For instance it is known that the fine control of insulator thickness is crucial in the optimisation of photovoltaic and electroluminescent metal-lnsulator-semlconductor devices. Examples of both types of device have been demonstrated using our phthalocyaine films. For the bistable switch, a gallium arsenide substrate was used; both gallium phosphide and zinc selenlde have been utilized in the electroluminescent structures. Moreover, In the case of phthalocyanine another possibility presents Itself. It has long been known that the conductivities of this material and its derivatives are very sensitive to the presence of certain gases, particularly the oxides of nitrogen. The increased conductivity of such materials has been demonstrated to be confined to the surface of the crystal. Hence many phthalocyanine gas detector systems have been based on thin films. Unfortunately because phthalocyanine exhibits polymorphism, the exact structure of such films can be complicated. making interpretation of results and subsequent device optimisation difficult. Also the response and recovery times of these thin film devices can also be excessively long. It is possible that monomolecular LB films of phthalocyanine could well overcome some or all of these problems. Our experiments have concentrated on asymmetrically substituted copper phthalocyanine and its usefulness to detect nitrogen dioxide. Preliminary results show the response and recovery times for the simple gas structures to be faster than those previously reported for other thin film phthalocyanine devices. It is suggested that this is due to the more ordered structure of the LB film, which enables the gas to adsorb on, or desorb from the molecular sites more readily.