Lecture 13 graphene properties

Report
Graphene
Castro-Neto, et al. Rev.
Mod. Phys. 81 (2009) 109
Single atomic layer of graphite
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I. Graphene Electronic Properties (isolated graphene
sheets)
II. Graphene Formation—Growth on SiC
III.Graphene Growth on BN, Co3O4, etc.
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Castro-Neto, et al. Rev.
Mod. Phys. 81 (2009) 109
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Graphene’s band structure yields
unusual properties
Castro Neto
EF
The velocity of an electron
at the Fermi level (vF)
Is inversely related to meff
Effective mass (m*) ~ [dE2/dk2]-1
Most semiconductors, 0.1 m0 < m* < 1 me
Graphene, m* < 0.01 m0 (depending on number of carriers)
Therefore, expect VERY high mobility in graphene
Both holes and electrons can be carriers
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Effective mass for graphene does get very
small as n~ 1012
Castro-Neto, et al. Rev. Mod. Phys. 81
(2009) 109
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The Big Problem with graphene: an imagined conversation:
A. OK: Graphene is great, lots of interesting properties for devices!
B. How do you make a device?
A. You need a sheet of graphene!
B. OK, how do you get a sheet of graphene?
A. HOPG, scotch tape, and tweezers!
B. !@#$%%
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How do you “grow” graphene?
You can evaporate Si from SiC(0001) (either face)
Popularized by the de Heer group at Georgia Tech.
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Can grow multilayer films of graphene on SiC (azimuthally
rotated from each other—electronically decoupled!)
Anneal at 1350 C
Interfacial layer
(anneal at 1150 C)
SiC
Auger, graphene growth on SiC, deHeer et al.
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Inverse photoemission and LEED (Forbeaux, et al, PRB, 58 (1998) 16396)
Growth of graphite on SiC(0001)
π*
feature
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Angle resolved UPS (Emtsev, et al, PRB 77(2008) 155303)
shows transition to graphene band structure
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Adjacent layers on graphene /SiC are decoupled from each other,
Due to azimuthal rotation
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Graphene on SiC(0001) Not uniform on an atomic level, different regions due
to different #s of layers, orientations
M
B
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Graphene/SiC photoemission: varying hv can vary the
sampling depth (Emtsev, et al, PRB 77 (2008) 155303
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The covalently bound stretched graphene (CSG model)
Emtsev, et al., PRB 77 (2008) 155303
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Pertinent Questions: How do Adjacent Graphene Sheets couple electronically?
Single layer Graphene (good)
Many layerGraphite (meh!?)
When/how this transition occurs
is very pertinent to devices
Answer: On SiC, Adjacent Sheets apparently not coupled due to azimuthal
rotation
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Core (left) and valence band (right) PES
graphene growth on SiC (Emtsev, et al)
Explain the implications of this for graphene coupling between
layers
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Motivation: Direct Growth on Dielectric Substrates: Toward Industrially
Practical, Scalable Graphene—Based Devices
Graphene Growth: Conventional Approaches
CVD graphene monolayer
transfer
SiO2
Metal or HOPG
SiC(0001)
Our Focus: Direct
CVD, PVD or MBE
On Dielectrics
Si
Si evaporation
> 1500 K
SiC(0001)
Charge-based devices
Result: graphene
monolayer, interfacial inhomogeneities
Result: graphene
monolayer or multilayer
on SiC(0001)
FET: Band gap
n
Spintronics
graphene
MgO(111)
Si(100)
graphene
Coherent-Spin FET:
Top Gate
Co3O4(111)
Multi-functional, nonvolatile devices
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Direct Growth of Graphene on Dielectric Substrates: Summary
Substrate
Growth
Temperature
Method
MgO(111)
~ 1000 K
L. Kong, et al. J. Phys. Chem.
C. 114 (2010) 21618
Co3O4(111)
1000 K
Mica
~1000 K
Al2O3(0001)
1800 K
CVD, PVD Interfacial
reaction, band
gap
MBE
Incommensurate
interface,
Ferromagnetism1
MBE
Oxidation at
C(111) edge
sites?
CVD
High temp.
required for fewdefect films
BN(0001)
1000 K
CVD
C. Bjelkevig, et al., J. Phys.:
Cond. Matt. 22 (2010) 302002
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Remarks
Monolayer BN
by ALD, strong
BN graph
charge transfer
References
M. Zhou, et al., J. Phys.:
Cond. Matt. 24 (2012) 072201
G. Lippert, et al. Phys. Stat.
Sol. B. 248 (2011) 2619
M. Fanton,et al., Conf.
Abstract (Graphene 2011,
Bilbao, Spain)
Unpublished result
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Gate
valves
BCl3
NH3
Butterfly valve
Turbo
MBE
Intro/
transfer
deposition
Sample heating to
1000 K @ 1 Torr
Auger
Graphene/Co3O4
STM
Graphene/MgO(111)
UHV chamber, 10-11 Torr
LEED I(V)
ALD or PVD
Free
radical
source
Hemispheri
cal analyzer
(XPS)
LEED
Graphene growth &
characterization without
ambient exposure
Sample
Intro
chamber
P = 103 Sample processing P = 10-9
Torr –
-10-3 Torr
UHV Analysis
10-6 Torr
Chamber
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P ~ 5 x 10-10 Torr
Graphene/BN/Ru(0001): Bjelkevig, et al
LEED shows BN and Graphene NOT azimuthally rotated!
Orbital
hybridization
with Ru 3d!
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Gr/BN/Ru(0001): Inverse photoemission. π* not observed!
BN layer does NOT screen graphene from orbital hybridization and
charge transfer from Ru!
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Graphene on Co3O4(111): Molecular Beam Epitaxy
Substrate Preparation
Evaporator
P~ 10-8 Torr
750 K
Co(111)+ dissolved O
Sapphire(0001)
Sapphire(0001)
1000 K/UHV
~3 ML Co3O4(111)
Co(111)
O segregation
Sapphire(0001)
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Graphene growth on Co3O4(111)/Co(0001)
MBE (graphite source)@1000 K:
Layer-by-layer growth
1st ML
3 ML
2nd ML
0.4 ML
M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201
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Graphene Domain
Sized (from FWHM) (c)
65eV
~1800 Å (comp. to
HOPG)
Oxide spots
attenuated
with increasing
Carbon
coverage
(b)
G1
40000
G2
graphene
35000
0.4 ML
Intensity
30000
Co3O4(111)
O1
25000
O2
20000
15000
10000
5000
400
300
200
100
0
Pixel Position
(d)
65 eV beam energy
40000
3 ML
G1
35000
Intensity
LEED:
(a) 65eV
Oxide/Carbon
Interface is
incommensurate:
Different than
graphene on SiC or
BN!
G2
30000
2.5 Å
25000
O1
20000
2.8 Å
O2
15000
10000
5000
400
300
200
100
0
Pixel Position
M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201
2.8 Å O-O surface repeat distance on
Co3O4(111)
W. Meyer, et al. JPCM 20 (2008) 265011
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XPS: C(1s) Shows π system:
Binding Energy indicates graphene oxide
charge transfer
XPS Intensity (CPS)
8000
XPS (separate chamber):
Al Kα
source
C(1s)
284.9(±0.1) eV
binding energy:
Interfacial
polarization/charge
transfer to oxide
π→π*
No C-O bond
formation
x75
300
297
294
291
288
0
300
297
294
291
288
285
282
Binding Energy (eV)
279
276
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M. Zhou, et al., J. Phys.: Cond.
Matt. 24 (2012) 072201
Directly grown graphene/metals and
dielectrics:
Inverse photoemission and charge
transfer
n-type
Ef
charge
transfer
p-type
Position of * (relative to EF) indicates
direction of interfacial charge transfer
(Kong, et al., J.Phys. Chem. C. 114 (2010)
21618
Forbeaux,
et al.
Multilayers
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Generalization, Directly Grown Graphene and Charge Transfer: Oxides
(p-type) vs. Metals (n-type)
e-
graphene
Transition metals
(Ru, Ni, Cu, Ir…)
e-
graphene
Oxides, SiC
EF
n-type; metal to
graphene charge transfer
p-type; graphene to
substrate charge
transfer
EF
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Suspended
graphene
Graphene (few layer) on Co3O4:
Much more conductive than
suspeneded graphene
Why??
•Significant doping?????
•High mobility (How high)?????
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Conclusion:
Graphene:
Large area growth on practical
substrates critical for device
development.
Interactions with substrates and
(maybe) other graphene layers are
critical to device properties
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