Dr. Paul Beecher

Ph.D. Thesis: "Charge Transport in Nanocrystal-based Electronic Devices"

Abstract

This thesis explores the electronic transport properties of assemblies of metal nanocrystals. As it is envisaged that metal nanocrystals will begin to provide solutions to commercial electronics problems within the next decade, it is an imperative that the physics of these materials are comprehensively understood. It has been well established that chemical methods for nanocrystal synthesis can hugely increase the level of achievable nanocrystal functionality, and the ability of nanocrystals to self-assemble to form “artificial atom” solids. This work is devoted to studying the properties of metal nanocrystals, particularly investigating their collective properties, and exploring methods whereby those collective properties can be modified.

In Chapters 2 and 3, charge transport through laterally contacted assemblies of CoPt₃ nanocrystals is investigated. The transport properties of weakly coupled nanocrystal arrays are first studied with respect to nanocrystal size. Mild thermal processing is then exploited to tune inter-nanocrystal coupling progressively in situ from Mott insulating through to metallic regimes. Variable temperature dc electrical characterisation demonstrates that the devices annealed at lower temperatures behave as Mott insulators with transport characteristics governed by single-electron charging energies of the electrically isolated nanocrystals. Nanocrystals remain structurally and chemically intact following higher temperature anneals. Thermal analysis suggests that the observed insulator-to-metal transition is due to melting and re-organisation of the nanocrystal ligand shells causing a reduction in mean inter-nanocrystal distance. It is believed that this constitutes the first report of an in situ anneal-induced insulator-to-metal transition in metal nanocrystal arrays. 

Chapter 4 describes a novel method for forming 1D nanocrystal structures based on the use of magnetic fields to direct the growth of nanocrystal wires from CoPt₃ colloidal solutions. Electrical characterisation data indicate that the activated charge transport properties of the wires are determined by the nanocrystal charging energy, which is governed by the size and capacitance of the individual nanocrystals of which the wires are composed. FIB-based deposition of Pt metal at the wire-electrode junctions is employed to optimise the wire-electrode contacts, improving device fabrication yield. Finally, in Chapter 5, the templated assembly of gold nanowires using thiol-modified carbon nanotubes and double-stranded calf-thymus DNA as templates is also demonstrated. The gold nanowires are ohmic conductors with resistivities several decades larger than that of polycrystalline bulk gold. The relative contributions of the grain boundary and surface scattering resistances and the contact resistance to the measured resistivities are considered.