Towards a low temperature synthesis of graphene with small organic molecule precursors

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Vargas Morales, Juan Manuel
Tolbert, Laren M.
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Graphene, a 2D honeycomb lattice of sp² hybridized carbons, has attracted the attention of the scientific community not only for its interesting theoretical properties but also for its myriad of possible applications. The discovery of graphene led to the Nobel Prize in physics for 2010 to be awarded to Andrei Geim and Konstantin Novoselov. Since its discovery, many methods have been developed for the synthesis of this material. Two of those methods stand out for the growth of high quality and large area graphene sheets, namely, epitaxial growth from silicon carbide (SiC) and chemical vapor deposition (CVD). As it stands today, both methods make use of high concentrations of hydrogen (10-20%) in N₂ or Ar, high temperatures, and a vacuum system. Epitaxial growth from SiC in addition requires very expensive single crystal SiC wafers. In the case of CVD, organic molecules are used as the carbon source to grow graphene on a metal substrate. Although graphene has been grown on many metal substrates, the experiments highlighted here make use of copper as the metal substrate of choice since it offers the advantage of availability, low price, and, most importantly, because this substrate is self-limiting in other words, it mostly grows single layer graphene. Because the CVD method provides with a choice as for the carbon source to use, the following question arises: can a molecule, either commercially available or synthesized, be used as a carbon source that would allow for the synthesis of graphene under low temperatures, low concentrations of hydrogen and at atmospheric pressure? This dissertation focuses on the synthesis of graphene at lower temperatures by using carbon sources with characteristics that might make this possible. It also focuses on the use of forming gas (3% H₂ and 97% N₂ or Ar) in order to make the overall process a lot safer and cost effective. This dissertation contains two chapters on the synthesis of organic molecules of interest, and observations about their reactivity are included. CVD experiments were performed at atmospheric pressure, and under vacuum. In both instances forming gas was used as the annealing and carrier gas. Results from CVD at atmospheric pressure (CVDAP), using organic solvents as carbon sources, show that at 1000℃, low quality graphene was obtained. On the other hand, CVD experiments using a vacuum in the range of 25 mTorr to 1 Torr successfully produced good quality graphene. For graphene growth under vacuum conditions, commercially available and synthesized compounds were used. Attempts at growing graphene at 600℃ from the same carbon sources only formed amorphous carbon. These results point to the fact that good quality graphene can basically be grown from any carbonaceous material as long as the growth temperature is 1000℃ and the system is under vacuum. In addition to the synthesis of graphene at low temperatures, there is a great amount of interest on the synthesis of graphene nanoribbons (GNR’s) and, as with graphene, several approaches to their synthesis have been developed. One such method is the synthesis of GNRs encapsulated in carbon nanotubes. Experiments were conducted in which aluminosilicate nanotubes were used. These nanotubes provided for an easier interpretation of the Raman spectrum since the signals from the nanotubes do not interfere with those of the GNR’s as in the case when carbon nanotubes are used. The use of aluminosilicate nanotubes also allowed for the successful synthesis of GNR’s at temperatures as low as 200℃ when perylene was used as the carbon source.
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