Carbon encapsulated metal nanoparticles are an increasingly important class of materials due to the wide range of electronic, magnetic and mechanical properties they display. However, traditional deposition techniques are often complex or lead to a poor quality film. Ionised magnetron sputter deposition is a promising development to traditional magnetron sputtering which combines film deposition with ion bombardment. By adding an RF powered, inductively coupled plasma positioned between the deposition targets and the substrate, the ionisation fraction of the depositing flux is greatly increased. This additional ion flux can then be controlled through the use of an electrical substrate biasing. This controls the energy flux to the surface and therefore the resulting microstructure. Carbon-nickel thin films were grown by ionised magnetron sputter deposition. The films themselves were characterised using a wide variety of techniques to measure not only their structure but their properties. Additionally, the inductively coupled plasma itself was characterised using a Langmuir probe. It was determined that upon application of a negative substrate biasing, the ion flux to the growing film remained constant, however the energy of the species increased. This resulted in a columnar structure of nickel carbide which coarsened as the bias (and therefore the energy of the ions) was increased. Conversely the application of a positive bias gives a large flux of low energy bombardment. This led to the formation of metallic nickel nanoparticles (? 30 nm diameter) which were surrounded by several layers of ordered graphitic shells forming a so-called "nano-onion" structure. The transition between these phases is a result of an increase in adatom mobility when there is a high flux, low energy ion bombardment which allows the nickel and carbon to phase separate. Upon separating, the nickel templates graphite growth due to their similar bond lengths leading to the formation of the graphitic cages. The transition between these two structures is measured through X-ray diffraction which shows a transition from the hexagonal carbide phase to the cubic nickel phase. This is accompanied by an increase in ordering of the carbon as the bias is increased as measured by Raman spectroscopy. Additionally, it is observed that there is an increase in carbon ordering when a negative bias is applied, due to the additional energy from ion bombardment leading to graphite formation. Magnetic measurements showed a transition from a non-magnetic state when the structures were largely carbide, to a magnetic state when metallic. However at room temperature the structures display superparamagnetic behaviour due to the small size of the particles. Measurements of electronic conductivity showed a negative temperature coeffcient of resistivity for all samples demonstrating no metallic conduction path was present. A large drop in resistivity as the temperature increases was assigned to thermally activated conduction. At low temperatures the conductivity is dependent on tunnelling across small regions of amorphous carbon, while at higher temperatures it is possible to excite the electrons into a conduction band allowing them to conduct more easily.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:608067 |
| Date | January 2013 |
| Creators | Bosworth, David |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.repository.cam.ac.uk/handle/1810/253562 |
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