Graduation Year

2013

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Physics

Major Professor

Ivan I. Oleynik

Co-Major Professor

Vasily V. Zhakhovsky

Keywords

Carbon Chains, Carbon Vapor, Liquid Carbon, Molecular Dynamics, REBO, Stone Wales

Abstract

The mechanisms of the sublimation of graphene at zero pressure and the condensation of carbon vapor is investigated by molecular dynamics (MD) simulations. The interatomic interactions are described by the Reactive Empirical Bond Order potential (REBO). It is found that graphene sublimates at a temperature of 5,200 K. At the onset of sublimation, defects that contain several pentagons and heptagons are formed, that are shown to evolve from double vacancies and stone wales defects. These defects consisting of pentagons and heptagons act as nucleation sites for the gaseous phase. The influence of the interatomic interactions on the sublimation process are also investigated by comparing the results using the REBO potential with the Screened Environment Dependent (SED)-REBO potential. Two-dimensional MD simulations are also performed, and it is found that graphene melts at a much higher temperature and forms many more point defects than in three dimensions. It is also observed that carbon chains make up the two-dimensional molten state.

The isothermal equation of state of gaseous and liquid carbon, as well as the coexistence of the two phases is calculated at 6,000 K and up to a few GPa. The analysis shows that the material that forms immediately following the phase transformation in graphene is actually a coexistence of liquid and gaseous phases, but it is primarily two-fold coordinated, so it is mostly a gas, hence the identification of the phase transformation as sublimation. The coexistence pressure for liquid and gaseous carbon is found using the Maxwell Construction to be 0.0365 GPa at 6,000 K. It was previously believed that carbon vapor consists exclusively of carbon chains. We find that under compression, at a pressure lower than the coexistence pressure, carbon vapor develops a small amount (6 %) of sp2 bonds indicating a slight non-chain bonding character. The diffusion coefficient of this dense gas is calculated to be in between that of the liquid and gaseous phases.

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