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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