Abstract
(July 15, 2015) Large-area
synthesis of high-quality graphene by chemical vapour deposition on metallic
substrates requires polishing or substrate grain enlargement followed by a
lengthy growth period. Here we demonstrate a novel substrate processing method
for facile synthesis of mm-sized, single-crystal graphene by coating
polycrystalline platinum foils with a silicon-containing film. The film reacts
with platinum on heating, resulting in the formation of a liquid platinum
silicide layer that screens the platinum lattice and fills topographic defects.
This reduces the dependence on the surface properties of the catalytic
substrate, improving the crystallinity, uniformity and size of graphene domains.
At elevated temperatures growth rates of more than an order of magnitude higher
(120 μm min−1) than typically reported are achieved, allowing savings in costs
for consumable materials, energy and time. This generic technique paves the way
for using a whole new range of eutectic substrates for the large-area synthesis
of 2D materials.
Graphene is a two-dimensional material that has great
potential for use in a wide variety of applications such as transparent
flexible electrodes, photonic and electronic devices, energy storage, sensors
and coatings to name a few1. Exceptional electrical, optical and mechanical
properties of the material greatly depend on the quality of the synthesized
graphene that can inherently depend on the underlying substrate. Notably, the
production of large area, single crystal graphene with a controlled number of
layers is a challenge that has yet to be resolved.
Introduction
Atmospheric pressure chemical vapour deposition (APCVD) has
received much attention due to its simplicity, improved safety and lower cost
over alternative procedures. When a solid substrate is used with APCVD,
graphene growth is strongly influenced by the crystallographic lattice of the
substrate, its defects, roughness and grain boundaries2, 3, 4, 5.
Millimetre-sized single-crystal graphene flakes have been synthesized recently
on solid copper6 and platinum7. Time- and energy-consuming processes such as
long annealing, mechanical, chemical and electrochemical polishing,
re-solidification or their combinations were necessary to artificially increase
the substrate grain size, eliminate the grain boundaries and decrease the
roughness of the surfaces. Moreover, to produce large area, single crystal,
monolayer graphene with CVD, the kinetics of the synthesis must be finely
balanced. Typically this is achieved with low growth rates of a few μm per
minute7, 8. The synthesis time is thus lengthy (hours or days) and significant
quantities of source materials are required. With unmodified commercially
available polycrystalline Cu the achievable flake size is limited to only tens
of micrometres9. It is also very difficult to achieve uniformity in the size
and shape of graphene flakes within a sample on such substrates. Consequently,
graphene films grown on solid polycrystalline metals often consist of small,
irregular-sized domains with many high-angle domain boundaries, which degrade
the quality of graphene10. CVD on liquid Cu gives aligned hexagonal graphene
flakes of irregular size distribution of up to 120 μm or regular size
distribution of 20–30 μm11. When these flakes coalesce they often have good
crystallinity and low-angle grain boundaries. However, there is little control
of the nucleation and the size of such flakes. The method is also difficult to
upscale due to the liquidity or droplet formation.