- Title
- The growth of graphene/graphite thin film allotropes on copper substrates
- Creator
- Kasman,
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2014
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Graphene, a monolayer of graphite, has drawn a great deal of attention for organic electronic applications. Excellent optical transmittance combined with its extraordinary electrical properties makes it an attractive material for using as a transparent, conductive electrode in the fabrication of organic solar cells (OSCs). To bring graphene to mass production, synthesis methods are required for its growth as a large-area film. Chemical vapour deposition (CVD) is a novel technique proposed to produce high quality, large-area graphene films. Recently, the use of copper as a catalyst for graphene growth has been more popular compared with other metals since the carbon solubility in copper is much lower, allowing better control over the number of graphene layers. Here, we report optimisation of graphene growth on two different copper substrates - copper foil and thin copper film - at low temperatures (below 500 °C) using poly(methyl methacrylate) (PMMA) as a carbon source. It was found that the 140 °C heated precursor promotes the growth of a mono-layer graphene, while the 400 °C heated precursor results in multi-layer graphene. For optimisation, three different temperature parameters, including the annealing temperature of the copper substrates (Tanneal), precursor temperature (Tprecursor) and growth temperature (Tgrowth), have been identified and independently studied. Firstly, the optimal Tanneal of the copper substrates prior to graphene deposition was required to be sufficiently high (> 900 °C) to cause the recrystallisation of large Cu grains in the substrate surface. Secondly, Tprecursor needs to be high enough to produce volatile precursor fragments which subsequently decompose on the catalyst surface. In the case of PMMA, this requires Tprecursor > 140 °C. Finally, Tgrowth has to be sufficiently high to activate carbon diffusion and rearrangement on the catalyst surface. Particularly, this temperature needs to be at least 450 °C for a Cu foil catalyst. By working at the Tanneal of 900 °C, it was also found that the Cu foil substrate annealed for 10 minutes under hydrogen flow rate of 100 sccm and the thin Cu film substrate annealed for 1 hour under hydrogen flow rate of 50 sccm were the optimal annealing conditions. A pre-thermal annealing study of the copper substrates was undertaken to examine the change of structural and morphological properties. In this study, it was found that the substrates have a good nanocrystalline cubic structure, dominated by (100) with the crystallize size of ~ 200 nm and (111) planes with the crystallite size of ~ 100 nm for the Cu foil and the thin Cu film, respectively. This treatment also showed that the hydrogen reducing gas effectively removes the copper oxide impurities. Moreover, the samples annealed beyond 900 °C have a smooth surface morphology with uniform coverage and grain sizes ~ 1.3 and ~ 88 µm for thin Cu film and Cu foil, respectively. The annealed copper substrates were further used as catalysts for growing graphene. By working at a growth temperature of 450 °C, the graphene growth time on the Cu foil catalyst and the thickness of thin Cu film catalyst were also optimised, and were found to be 1 minute and 500 nm, respectively. The transmittance of graphene films resulting from the growth on both of the substrates was above 85%. However, their electrical properties were significantly different; the lowest sheet resistances obtained were 1.2 and 14.0 kΩ/ for the graphene growth on the Cu foil and thin Cu film catalysts, respectively. These results are much higher compared to the best reported Rsheet for single-layer graphene (ca.350 kΩ/) [1-3]. For the processed graphene/graphite films used as a transparent, conductive electrode for fabricating OSC devices, they demonstrated working devices with PCEs of between 0.07 - 0.36 %. The low PCEs of the fabricated devices were due to the high sheet resistance (a few kΩ/sq) of these device electrodes.
- Subject
- CVD; low temperature; PMMA precursor; copper catalysts; pGraphene/graphite thin film
- Identifier
- http://hdl.handle.net/1959.13/1047598
- Identifier
- uon:14806
- Rights
- Copyright 2014 Kasman
- Language
- eng
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