Graphene overgrowth at step edges

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Graphene overgrowth at step edges
Profound reshaping of Ir steps
→ Ir steps locally align C zigzag rows
→ maximizing number of C-Ir σ bonds
Moiré orientation preserved across the steps
→ graphene climbs up the steps wo defects
Graphene/Ir(111), growth at 1120 K, 170×170 nm2
Coraux et al., New J. Phys. (2009)
Graphene/Pt(111), growth by segregation, LEEM
Sutter et al., Phys. Rev. B (2009)
Johann Coraux
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Metal step edge retraction
Carbon nanotubes + Ni particles, in situ TEM
Helveg et al., Nature (2004)
5 nm
Graphene/Ir(111), STM, 170×170 nm2
- Metal transport along step edges
- Metal atoms expelled towards the metallic terrace under graphene
- Metal step-bunching
Coraux et al., New J. Phys. (2009)
STM
Starodub et al., Phys. Rev. B (2010)
Graphene/Ru(0001)
LEEM, 3.5×3.5 µm2
40 % increase of the vacancy island area
Reconstructed subgraphene Ru layer
Johann Coraux
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First stages of the growth
For Pt, Ir (and other metals with a weak interaction
with graphene ?):
uphill growth delayed (energetically costly
transient state ?)
Graphene/Ru(0001): growth exclusively at ascending step edge (strong C-Ru bond !)
Sutter et al., Nature Mater. (2008)
Johann Coraux
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Graphene sheets from nanoislands
PEEM: work function contrast (Ir > 5.3 eV, dark; graphene ~ 4.7 eV, bright)
Ir(111), 10-7 mbar C2H4, 1220 °C, time ×10, field of view 20 µm
- full coverage
- self-limited to a single layer
- large area
STM, 70×73 nm2
Across the boundary between coalesced islands,
moiré lines are continuous → continuity of the carbon lattice !
Coraux et al., New J. Phys. (2009)
Johann Coraux
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Growth conditions and order
Graphene/Ir(111), STM, 125×125 nm2
870 K
1120 K
1320 K
Higher order at higher temperatures (single orientation at 1320 K)
STM, 455×455 nm2
1120 K
870 K
Better oriented
domains are
favoured
Lower island density and
larger island size at high T
and low P
→ Trick: slowly grow large, sparse, nicely oriented graphene islands (high T + low P).
Johann Coraux
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C2H4 C2H4
Self-limited growth
Graphene/Ir(111)
0%
Coraux et al., New J. Phys. (2009)
2H2
graphene
C
20 %
50 %
desorption
Ir(111)
Graphene coverage increase vs time:
adsorption site area
dΘ/dt = φ×S×(1 – Θ)×Ω
T = 1120 K
desorption from graphene
sticking coefficient (desorption ?)
molecular flux = P/√(2πMkBT)
→ Θ = 1 – exp[-P×t×S×Ω/√(2πMkBT]
- Graphene growth can only proceed in the presence of bare metal.
- Graphene coverage asymptotically tends to 100 %.
- No noticeable desorption (S ~ 1) for ethylene up to 1500 K.
H2C=CH2
H3C-CH2-CH2-CH3
Desorption in the case of butane, methane.
CH4
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Carbon diffusion / attachment to graphene
Graphene/Ir(111), growth at 870 K
STM, 110×110 nm2
Coraux et al., New J. Phys. (2009)
- Graphene coverage independent of terrace size, though carbon
is everywhere
→ efficient C transport (> 1 µm @ 870 K)
limiting step = carbon incorporation at graphene edges !
(metal/metal: no barrier for adatom attachment to metal edges)
Uniform carbon coverage
LEEM
Graphene/Ru(0001)
Loginova et al., New J. Phys. (2008)
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Graphene growth by C cluster attachment
LEEM, in situ:
Electron reflectivity as a probe for C adatom concentration.
At 3.7 eV, on Ru(0001), Θadatom = 0.223×[I0-I(t)]/I0
(I0 = intensity before C deposit)
940 K
A certain C concentration is needed
for graphene growth to proceed (cnucl).
Graphene is in equilibrium with a high
carbon concentration
(ceq, 0.016 ML @ 940 K).
Growth happens very far from
equilibrium.
Loginova et al., New J. Phys. (2008)
Johann Coraux
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Graphene growth by C cluster attachment
graphene island @ t
@ t+dt
Growth rate of a graphene island: v = dr/dt = d(πr2/2)/dt×1/(πr) = 1/P×dA/dt
r
Ru(0001)
v is not proportional to c, while in many systems, v = nattach – ndetach
with ndetach independent of c and nattach proportional to c
Interpretation: instead of monomers, n-mers are relevant
cn = exp[(nµ – En)/kBT]
(energy of n isolated C atoms – formation energy for a n-mer)
µ = kBT×ln(c/ceq) for an ideal gas monomer sea
→ cn = (c/ceq)n exp(-En/kBT)
Growth rate: proportional to how far the n-mer concentration is far from equilibrium
→ v ∝ cn – cneq = exp(-En/kBT)×[(c/ceq)n – 1]
Fits to experimental data yield n = 4.8 ± 0.5:
pentamers are the building blocks during growth !
on Ru(0001): Loginova et al., New J. Phys. (2008), also see Amara et al., Phys. Rev. B (2006) on Ni(111)
on Ir(111); Loginova et al., New J. Phys. (2009)
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Formation & stability of rotational variants
Different situations depending on the support:
- on Pt(111), either a large spread of orientations (CVD), or several variants (at least 6) for growth close to
equilibrium (segregation),
- on Ir(111), 4 variants, one of which ([1120]C // [110]Ir) is preferred,
- on Ni(111), 2 variants where identified so far, one of which ([1120]C // [110]Ir) is prominent,
- on Ru(0001), only 1 variant is known ([1120]C // [110]Ir).
60° ϕ
0°
Pt(111)
ϕ
[1120]C // [110]Ir
Ir(111)
Ru(0001)
Ni(111)
Binding energy
Johann Coraux
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Formation & stability of rotational variants
[1120]C // [110]Ir
Graphene/Ir(111), LEEM, 24×18 µm2
Loginova et al., New J. Phys. (2009)
- The preferred variant always grows first.
- Other variants nucleate at its edges.
→ heterogeneous nucleation, at edge defects.
Graphene/Ir(111)
PEEM/LEEM
van Gastel et al., Appl. Phys. Lett. (2009)
30°
0°
- Once formed, non-preferred variants grow faster.
Interpretation:
the kinetics for cluster attachment to the graphene edges
(limiting step) are different: larger energy barrier in the case of the
preferred orientation.
Remark: high temperature favours the formation of the variants
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Getting rid of rotational variants
- The different variants have different electronic structure (interaction with the substrate):
work function contrast as seen by PEEM.
Graphene/Ir(111)
PEEM, field of view 50 µm
- The preferred variant is the most chemically inert.
0°
after growth...
… after O2 etching...
30°
PEEM, field of view 100 µm
Growth at 1130 K,
O2 etching (5×10-8 mbar, 1130 K)
→ > 99 % selectivity to preferred variant, with the remaining
of the other variants
LEEM
STM, 250×250 nm2 field of view 4 µm
...etc
Alternative method (100 % selectivity):
1- grow graphene islands with well-defined orientation
and edges (see next part of this lecture),
2- continue the growth of the plain sheet
van Gastel et al., Appl. Phys. Lett. (2009)
Johann Coraux
µ-LEED