Ab Initio Thermodynamics of Hydrocarbons Relevant to Graphene Growth at Solid and Liquid Cu Surfaces

Using ab initio thermodynamics,
the stability of a wide range of
hydrocarbon adsorbates under various chemical vapor deposition (CVD)
conditions (temperature, methane and hydrogen pressures) used in experimental
graphene growth protocols at solid and liquid Cu surfaces has been
explored. At the employed high growth temperatures around the melting
point of Cu, we find that commonly used thermodynamic models such
as the harmonic oscillator model may no longer be accurate. Instead,
we account for the translational and rotational mobility of adsorbates
using a recently developed hindered translator and rotator model or
a two-dimensional ideal gas model. The thermodynamic considerations
turn out to be crucial for explaining experimental results and allow
us to improve and extend the findings of earlier theoretical studies
regarding the role of hydrogen and hydrocarbon species in CVD. In
particular, we find that smaller hydrocarbons will completely dehydrogenate
under most CVD conditions. For larger clusters, our results show that
metal-terminated and hydrogen-terminated edges have very similar stabilities.
While both cluster types might thus form during the experiment, we
show that the low binding strength of clusters with hydrogen-terminated
edges could result in instability toward desorption.