Novel DNA-templated nanotechnology devices

Abstract

This thesis describes the preparation and characterisation of DNA-templated nanowires followed by their incorporation into field effect transistors. DNA has been used as a template in order to grow nanowires consisting of vanadium dioxide (VO2), cadmium sulphide (CdS) and a cadmium sulphide - carbon nanotube mix- ture (CNT-CdS) via a precipitation reaction. The growth of these nanowires is a two stage process in which the materials first coordinate with DNA before nucleating and then grow- ing to form long and smooth wire like structures. The novel VO2 nanowires formed have been characterised using a mixture of Fourier Transform Infrared Spectroscopy (FTIR), UV-Visible Spectroscopy (UV-Vis), Powder X-ray Diffraction (pXRD) and X-ray Photoelectron Spectroscopy (XPS) in which it was found that the vanadium interacted with the DNA and formed a product with a structure simi- lar to VO2 nanorods. Further investigations were carried out by using Atomic Force and Scanned Conductance Microscopy (AFM and SCM respectively) which showed long wire- like structures with heights upwards of 8 nm that showed weakly conducting nature when probed with SCM. AFM studies also showed that without DNA being present, no wire like structures were formed. Basic current-voltage (IV) curves were recorded over a range of temperatures using platinum interdigitated electrodes (IDEs) which confirmed the weakly conducting nature of the nanowires and that they showed Arrhenius behaviour. The con- ductance of the prepared nanowires was noticeably higher than any other DNA-templated nanowires prepared in past studies. Finally the samples were incorporated into field effect transistors (FET) and the source-drain current was measured as the gate voltage was al- tered. The FETs showed basic transistor functionality in which at larger gate voltages the current between the source and drain also increased. The effect of temperature was also examined for the VO2-DNA nanowires where it was discovered that heating the samples to 150 ◦C caused an irreversible rise in conductance. 2 The CdS nanowires have been characterised in the past however and AFM was used to confirm the formation of nanowires, which showed a diameter of around 6 - 8 nm. IV curves were again recorded over a range of temperatures using IDEs and the samples showed thermally-activated conductivity. The CdS-DNA nanowires were then incorporated into FETs but showed no reliable transistor like behaviour. This is thought to be due to the low conductance of the material as well as the high contact resistance. The final material investigated was CdS-CNT-DNA in which the DNA was wrapped around multi-walled carbon nanotubes (MWCNTs) before CdS was added. FTIR of this ma- terial showed distinct differences in wavenumbers for bare DNA and the templated DNA, showing signs that an interaction is occurring. This was confirmed by using AFM in which wire like structures with heights of 4 nm. In the absence of DNA no wire like structures form, only small particles with heights of 8 - 12 nm. IV curves showed good conductance and by measuring over a temperature range, the materials semiconducting properties were confirmed. The samples were then incorporated into FETs and they showed basic transistor behaviour with a low transconductance.