Novel DNA-Templated Nanotechnology Devices

Aims
The main aim of this project is the preparation of devices such as transistors and sensors using novel semiconducting materials prepared by coating a long thin molecular template (DNA) in order to form wires with a diameter on the scale of a few nanometres - nanowires. A nanometer is a 10-millionth of a centimetre and the DNA double helix is about 2 nm in diameter. DNA is also a relatively stiff molecule, which is essential if the material coating the DNA is to take the form of a long, thin wire. Other advantages to DNA-templating methods derive from the fact that DNA is water soluble and the chemistry required to coat these molecules is mild and takes place at ambient temperature in aqueous solution. Indeed, nanowires prepared in aqueous solution may be readily converted into inks that can be used to create electronics by inkjet printing technology. The project aims are:
1. To prepare a range of semiconducting nanowires on DNA templates.
2. To prepare aqueous nanowire inks and use these to print electronic devices.
3. To assess the electrical performance of transistors prepared from DNA-templated nanowires.

Methodology and Approach
The project will be laboratory based with various preparation and characterisation methods being utilized. DNA-templated nanowires can be prepared in various ways using both metal sulfides (MS) or conducting polymers (CP). Cadmium sulphide (CdS) will be the main semiconductor studied used during this project; it is prepared by precipitation of an aqueous solution of a cadmium salt using sulfide-in the presence of DNA. In the absence of DNA, the CdS forms as particles, but under appropriate conditions, the CdS will form on the DNA molecules as a wire.

Conductive polymers, such as polypyrrole, can be formed by oxidation of the monomer (repeat unit) in aqueous solution by ferric chloride.
The nanowires will be characterised using methods such as UV-Vis absorption and atomic force microscopy (AFM) to confirm the size and composition of the nanowires. To-date only simple measurements of current at fixed applied voltage using two contacts have been recorded for this type of nanowire and the project aims to develop the methods required to produce transistors that require three contacts to be applied to each nanowire. The third contact will act as a gate to regulate the flow of current between the other two.

Training will take place in the necessary chemical, inkjet printing and clean-room techniques to produce 3 terminal devices from the nanowires using photolithography and metal deposition. These devices will then be tested using a probe station to evaluate their performance as transistors.

In addition to cadmium sulfide (CdS), there are other promising semiconductor materials that can be prepared on DNA in aqueous solutions. Many metal sufides with semiconductive behaviour are known and these may be produced by precipitating the metal sulfide from an aqueous metal salt in the presence of DNA. Other interesting and promising semiconducting materials include bismuth telluride (Bi2Te3) due to its narrow band gap of 0.21 eV. Templating this onto DNA is possible using galvanic displacement of DNA templated copper nanowires, however no electrical measurements have yet been reported. Characterisation of the prepared Bi2Te3 nanowires will involve AFM as well as energy-dispersive X-ray spectroscopy (EDX) and photoemission spectroscopy to confirm their chemical composition. Bismuth compounds are of particular interest for manufacturing technology because they are often non-toxic and are environmentally benign.
After sufficient electrical testing has taken place the CdS and Bi2Te3 NW's will then be adapted to form an ink using a mixture of water and ethylene glycol. Inks must be able to pass through the jets in an inkjet printer meaning that certain properties must be optimised such as viscosity.