graphene single atomic layer of graphite castro-neto, et al. rev. mod. phys. 81 (2009) 109 1
TRANSCRIPT
Graphene
Single atomic layer of graphite
Castro-Neto, et al. Rev. Mod. Phys. 81 (2009) 109
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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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Castro Neto
EF
Graphene’s band structure yields unusual properties
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 grapheneBoth holes and electrons can be carriers
The velocity of an electron at the Fermi level (vF)Is inversely related to meff
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Castro-Neto, et al. Rev. Mod. Phys. 81 (2009) 109Effective mass for graphene does get very
small as n~ 1012
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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. !@#$%%
The Big Problem with graphene: an imagined conversation:
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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!)
SiC
Interfacial layer (anneal at 1150 C)
Anneal at 1350 C
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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MB
Graphene on SiC(0001) Not uniform on an atomic level, different regions due to different #s of layers, orientations
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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 layerGraphite (meh!?)
Answer: On SiC, Adjacent Sheets apparently not coupled due to azimuthal rotation
When/how this transition occurs is very pertinent to devices
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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
DevicesGraphene Growth: Conventional Approaches
Metal or HOPG
CVD graphene monolayer
SiO2
Si
transfer
Result: graphene monolayer, interfacial inhomogeneities
SiC (0001)> 1500 K
SiC (0001)
Result: graphene monolayer or multilayer
on SiC(0001)
Si evaporation
Our Focus: Direct CVD, PVD or MBE
On Dielectrics graphene
Si(100)
MgO(111)
nTop Gate
FET: Band gap
Coherent-Spin FET:
Spintronics
Charge-based devices
Multi-functional, non-volatile devices
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graphene Co3O4(111)
Co(111) or
Si(100)-gate
Direct Growth of Graphene on Dielectric Substrates: Summary
Substrate Growth Temperature
Method Remarks References
MgO(111) ~ 1000 K CVD, PVD Interfacial reaction, band gap
L. Kong, et al. J. Phys. Chem. C. 114 (2010) 21618
Co3O4(111) 1000 K MBE Incommensurate interface, Ferromagnetism1
M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201
Mica ~1000 K MBE Oxidation at C(111) edge sites?
G. Lippert, et al. Phys. Stat. Sol. B. 248 (2011) 2619
Al2O3(0001) 1800 K CVD High temp. required for few-defect films
M. Fanton,et al., Conf. Abstract (Graphene 2011, Bilbao, Spain)
BN(0001) 1000 K CVD Monolayer BN by ALD, strong BN graph charge transfer
C. Bjelkevig, et al., J. Phys.: Cond. Matt. 22 (2010) 302002
1Unpublished result
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Auger
LEED I(V)
STM
Intro/ transfer deposition
Gate valves BCl3
NH3
Turbo
Butterfly valve
Sample heating to 1000 K @ 1 Torr
UHV chamber, 10-11 Torr
MBE
LEED
Hemispherical analyzer (XPS)
Sample processing P = 10-9
-10-3 Torr UHV Analysis Chamber
P ~ 5 x 10-10 Torr
Free radical source
ALD or PVD
Sample Intro chamber P = 103 Torr – 10-6 Torr
Graphene/Co3O4
Graphene/MgO(111)
Graphene growth & characterization without ambient exposure 20
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 EpitaxySubstrate Preparation
Evaporator
P~ 10-8 Torr
Sapphire(0001)
750 K
Sapphire(0001)
Co(111)+ dissolved O
Sapphire(0001)
Co(111)
1000 K/UHV
~3 ML Co3O4(111)
O segregation
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Graphene growth on Co3O4(111)/Co(0001)
MBE (graphite source)@1000 K: Layer-by-layer growth
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1st ML
2nd ML0.4 ML
3 ML
M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201
LEED: Oxide/Carbon Interface is incommensurate:Different than graphene on SiC or BN!
Graphene Domain Sized (from FWHM) ~1800 Å (comp. to HOPG)
400 300 200 100 05000
10000
15000
20000
25000
30000
35000
40000
O1
O2
G2
Inte
nsi
ty
Pixel Position
G1
400 300 200 100 05000
10000
15000
20000
25000
30000
35000
40000
Inte
nsi
ty
Pixel Position
O1O2
G2
G1
Oxide spots attenuated with increasing Carbon coverage
2.8 Å O-O surface repeat distance on Co3O4(111)W. Meyer, et al. JPCM 20 (2008) 265011
2.8 Å
2.5 Å
0.4 ML
3 ML
graphene
Co3O4(111)
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65 eV beam energy
M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201
XPS (separate chamber):
284.9(±0.1) eV binding energy:Interfacial polarization/charge transfer to oxide
No C-O bond formation
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300 297 294 291 288 285 282 279 276
0
8000
XP
S I
nte
ns
ity
(C
PS
)
Binding Energy (eV)
C(1s)
x75
300 297 294 291 288
π→π*
XPS: C(1s) Shows π system: Binding Energy indicates graphene oxide charge transfer
Al Kαsource
M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201
Ef
charge transfer
Forbeaux, et al.
n-type p-typeDirectly grown graphene/metals and dielectrics:
Inverse photoemission and charge transfer
Position of * (relative to EF) indicates direction of interfacial charge transfer(Kong, et al., J.Phys. Chem. C. 114 (2010) 21618
Multilayers
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Generalization, Directly Grown Graphene and Charge Transfer: Oxides (p-type) vs. Metals (n-type)
Transition metals(Ru, Ni, Cu, Ir…)
n-type; metal to graphene charge transfer
Oxides, SiC
p-type; graphene to substrate charge transfer
EF
EF
graphene
graphene
e-
e-
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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