Growth of Graphene Films on Pt(111) by Thermal Decomposition of Propylene
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Graphene is a single atomic layer of graphite. Several methods for producing graphene have been found, the first being the mechanical exfoliation from a graphite crystal. In this study we attempt to produce single-layer graphene by decomposition of propylene on the surface of Pt(111). Platinum has a face centered crystal structure, and the (111) plane forms a closepacked layer of atoms, which has a hexagonal symmetry. Graphene crystallizes in the honeycomb structure, which also has a hexagonal symmetry. Therefore, the Pt(111) surface has the proper symmetry for the growth of epitaxial graphene layers. Furthermore, platinum is a natural catalyst for the decomposition of organic molecules. The goal of this project was to form graphene films by catalytic decomposition of propylene at a low enough temperature to prevent the solvation of carbon into the bulk of the platinum, which would cause multilayer graphene formation upon cooling of the crystal.
Two methods of growing graphene films were attempted: a) deposition of a propylene layer at low temperature (room temperature or at -175 °C) followed by annealing and b) deposition of propylene on the platinum crystal that was held at high temperature (475 °C). The deposition of propylene on Pt(111) at room temperature was found to form a p(2x2) overlayer from the low energy electron diffraction measurements. Upon annealing, the p(2x2) overlayer dissapeared and a ring structure associated with graphene was found to form at 700°C. Deposition of propylene at -175 °C resulted in an increase in difuse background with no p(2x2) diffraction pattern being observed. Annealing to 700 °C also produced a ring structure associated with graphene. Dosing propylene at 475 °C resulted in an almost total loss of the Pt(111) diffraction pattern for doses beyond a few Langmuir. Since carbon has a solubility of 0.5 atomic percent at 500 °C, the carbon atoms from the dissociated propylene are most likely diffusing into the surface region of the platinum. Because a temperature of 700 °C is needed to order the graphene overlayer, it was determined that the relatively high solubility of carbon in platinum below 700 °C prevents the formation of self-terminated single-layer graphene.