Photograph of a Rubrene molecular crystal (right) and an FET device based on this material (left).	a) Resistance vs. temperature data demonstrating superconductivity in our ion-implanted plastic material. b) SEM image of the cross-section of the device showing the buried Sn-C conducting layer, c) and d) Photographs of two of our measured sample.

Plastics are generally considered to be poor conductors of electricity. However, polymers can be made conductive by manipulating their chemical structure so that they contain long chains of alternating single and double carbon bonds. This has lead to the development of ‘plastic electronics’, with a number of associated advantages including mechanical flexibility, robustness, chemical versatility, low weight, and most significantly low cost and ease of large scale production. Organic electronics is also a growing area of basic research as much of the electronic properties of organic semiconductors are still poorly understood.

An important factor in developing field-effect transistors (FETs) using organic materials is how to improve the electrical mobility (i.e., how easily current can flow through the device) to a level sufficient for practical applications. To do this you just need to reduce the disorder in your conducting system by moving from an amorphous structure (currently most conducting polymer materials consist of a tangled spaghetti of very long molecules) to more ordered structures, for example, crystals formed from small conducting molecules. The most interesting and potentially useful molecular crystals for FET applications are polyacene crystals, which consist of molecules that contain up to 5 benzene rings fused together into a linear chain.

We spent much of 2004 working on developing our first research-quality organic crystal FETs for this new project. This included building and optimizing furnaces for physical vapour growth of various organic molecular crystals, and developing a capability to produce elastomer transistor stamps for making the FETs. In late 2004, we produced our first high-quality crystals and made our first working FET device and in the year ahead we will begin exploring the electronic properties of these molecular crystal FETs in more detail.

We have also continued our measurements on the electronic properties of ion-implanted plastics as part of an ongoing collaboration with Paul Meredith’s group at the University of Queensland. In late 2003 we discovered both metallic conductivity and superconductivity at very low temperatures in this new material. This discovery has resulted in the filing of an Australian Preliminary Patent Application in June.

Adam Micolich, Alex Hamilton,
Jack Cochrane and Ali Rashid