THESIS
2013
xxi, 93 pages : illustrations (chiefly color) ; 30 cm
Abstract
The growth and defect structure of graphene (g) on Ru(0001) and Ir(111) surface
is investigated using low energy electron microscopy (LEEM) and micro-low
energy electron diffraction (μLEED). The aim of this work is to gain a deeper
understanding of the origin of small angle lattice rotations that were recently discovered
in g/Ru(0001) by similar techniques and to explore if they are present
on other metal substrate surfaces. Since small angle rotation domains may alter
the electron properties of g/Ru(0001) from first principle calculation, it is also
the goal to find growth conditions that produce more uniform graphene layer
with high orientation uniformity. μLEED measurements from small a 250nm
size areas and shadow dark-field (SDF) imaging, are used to characterize local
rot...[
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The growth and defect structure of graphene (g) on Ru(0001) and Ir(111) surface
is investigated using low energy electron microscopy (LEEM) and micro-low
energy electron diffraction (μLEED). The aim of this work is to gain a deeper
understanding of the origin of small angle lattice rotations that were recently discovered
in g/Ru(0001) by similar techniques and to explore if they are present
on other metal substrate surfaces. Since small angle rotation domains may alter
the electron properties of g/Ru(0001) from first principle calculation, it is also
the goal to find growth conditions that produce more uniform graphene layer
with high orientation uniformity. μLEED measurements from small a 250nm
size areas and shadow dark-field (SDF) imaging, are used to characterize local
rotation features in graphene grown by chemical vapor deposition with ethylene.
In LEED mode, superstructure spots are is related to the moiré superlattice of
graphene on metal surface, which has high sensitivity to the lattice rotation angle
and lateral periodicity change in graphene.
We find that the small angle rotation domain defect structure in g/Ru(0001)
is diminished but cannot be completely eliminated using higher CVD growth
temperature. However, uniformly oriented graphene can be grown by CVD if
the crystal is first pre-loaded with C atoms by bulk segregation. Although the
uniform rotation domain can be extended to 10 μm size, it is also found that the
lateral periodicity within these uniformly oriented graphene varies considerably.
A strain energy calculation using the Keating's model predicts a correlation
between the corrugation and periodicity of graphene.
In studies of g/Ir(111) using similar techniques, significant rotational disorder is
observed in graphene that is nominally aligned with the Ir(111) substrate. On
the other hand, graphene exhibits significantly better rotational uniformity with
it is misaligned from the substrate by larger angles. μLEED measurements are
used to record the relationship between the size of moiré superlattice and moiré
rotation angle. Fitting this data with the known relationship is used to determine
the graphene lattice constant. Interestingly, the data points are bounded by two
sharp boundaries corresponding to two different lattice constants of graphene,
which suggested that there may exist a spatial variation of lateral periodicity of
graphene in this system as well.
Compared to the small rotation domains in g/Ru(0001), which has fixed lateral
periodicity of graphene, uniform rotation domain has variable lateral periodicities
of graphene. This suggests that there is a correlation between small rotation
angles and the periodicities of graphene. These finding may help to understand
the controversies in corrugation and lateral periodicities of g/Ru(0001) system.
The change oflateral periodicities found in g/Ir(111) can act as a further evidence
that the graphene in graphene on metal substrates system in general has a certain
range of lateral periodicities.
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