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Graphene. Castro-Neto, et al. Rev. Mod. Phys. 81 (2009) 109. Single atomic layer of graphite. Graphene Electronic Properties (isolated graphene sheets) Graphene Formation—Growth on SiC Graphene Growth on BN, Co 3 O 4 , etc. Castro-Neto, et al. Rev. Mod. Phys. 81 (2009) 109.

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Graphene

Castro-Neto, et al. Rev. Mod. Phys. 81 (2009) 109

Single atomic layer of graphite


Graphene Electronic Properties (isolated graphene sheets)

Graphene Formation—Growth on SiC

Graphene Growth on BN, Co3O4, etc.



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


Castro-Neto, et al. Rev. Mod. Phys. 81 (2009) 109

Effective mass for graphene does get very small as n~ 1012


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. [email protected]#$%%


How do you “grow” graphene?

You can evaporate Si from SiC(0001) (either face)

Popularized by the de Heer group at Georgia Tech.


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.


Inverse photoemission and LEED (Forbeaux, et al, PRB, 58 (1998) 16396)

Growth of graphite on SiC(0001)

π* feature


Angle resolved UPS (Emtsev, et al, PRB 77(2008) 155303) shows transition to graphene band structure



Graphene on SiC(0001) Not uniform on an atomic level, different regions due to different #s of layers, orientations

M

B


Graphene/SiC photoemission: varying hv can vary the sampling depth (Emtsev, et al, PRB 77 (2008) 155303


The covalently bound stretched graphene (CSG model) sampling depth (Emtsev, et al, PRB 77 (2008) 155303

Emtsev, et al., PRB 77 (2008) 155303


Pertinent Questions: How do Adjacent Graphene Sheets couple electronically?

Single layer Graphene (good)

Many layerGraphite (meh!?)

Answer: On SiC, Adjacent Sheets apparently not coupled due to azimuthal rotation

When/how this transition occurs is very pertinent to devices


Core (left) and valence band (right) PES graphene growth on SiC (Emtsev, et al)

Explain the implications of this for graphene coupling between layers


Motivation SiC (Emtsev, et al): Direct Growth on Dielectric Substrates: Toward Industrially Practical, Scalable Graphene—Based Devices

Graphene Growth: Conventional Approaches

transfer

CVD graphene monolayer

Result: graphene

SiO

monolayer, interfacial inhomogeneities

2

Co3O4(111)

Metal or HOPG

Si

graphene

Si evaporation

Result: graphene

> 1500 K

monolayer or multilayer

SiC

(0001)

SiC

(0001)

on

SiC

(0001)

Co(111) or Si(100)-gate

FET: Band gap

Our Focus: Direct CVD, PVD or MBE

On Dielectrics

Charge-based devices

Top Gate

n

graphene

Spintronics

MgO(111)

Si(100)

Coherent-Spin FET:

Multi-functional, non-volatile devices



Hemispherical analyzer (XPS) SiC (Emtsev, et al)

ALD or PVD

Free radical source

LEED

Sample Intro chamber P = 103 Torr – 10-6 Torr

Sample processing P = 10-9 -10-3 Torr

UHV Analysis Chamber

P ~ 5 x 10-10 Torr

Butterfly valve

Gate

NH3

BCl3

Graphene/Co3O4

valves

Turbo

Graphene/MgO(111)

Intro/

deposition

transfer

MBE

Sample heating to 1000 K @ 1 Torr

STM

Auger

UHV chamber, 10-11 Torr

Graphene growth & characterization without ambient exposure

LEED I(V)


Graphene/BN/Ru(0001): Bjelkevig, et al SiC (Emtsev, et al)

LEED shows BN and Graphene NOT azimuthally rotated!

Orbital hybridization with Ru 3d!


Gr/BN/Ru(0001): Inverse photoemission. SiC (Emtsev, et al)π* not observed!

BN layer does NOT screen graphene from orbital hybridization and charge transfer from Ru!


Graphene on Co SiC (Emtsev, et al)3O4(111): Molecular Beam Epitaxy

Substrate Preparation

Evaporator

P~ 10-8 Torr

750 K

Co(111)+ dissolved O

Sapphire(0001)

Sapphire(0001)

Sapphire(0001)

1000 K/UHV

~3 ML Co3O4(111)

Co(111)

O segregation


Graphene growth on Co SiC (Emtsev, et al)3O4(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


LEED: Oxide/Carbon Interface is incommensurate: SiC (Emtsev, et al)

Different than graphene on SiC or BN!

Graphene Domain Sized (from FWHM) ~1800 Å (comp. to HOPG)

65eV

(a)

(b)

graphene

0.4 ML

Co3O4(111)

65 eV beam energy

(d)

(c)

3 ML

65eV

Oxide spots attenuated with increasing Carbon coverage

2.5 Å

2.8 Å

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


XPS: C(1s) Shows π system: SiC (Emtsev, et al)

Binding Energy indicates graphene oxide charge transfer

XPS (separate chamber):

Al Kα

source

284.9(±0.1) eV binding energy:

Interfacial polarization/charge transfer to oxide

No C-O bond formation

π→π*

M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201


Directly grown graphene/metals and dielectrics: SiC (Emtsev, et al)

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

n-type

charge transfer

p-type

Ef

Forbeaux, et al.

Multilayers


Generalization, Directly Grown Graphene and Charge Transfer: Oxides (p-type) vs. Metals (n-type)

graphene

graphene

EF

e-

n-type; metal to graphene charge transfer

Transition metals

(Ru, Ni, Cu, Ir…)

p-type; graphene to substrate charge transfer

e-

Oxides, SiC

EF


Suspended graphene Oxides (p-type) vs. Metals (n-type)

  • Graphene (few layer) on Co3O4:

  • Much more conductive than suspeneded graphene

  • Why??

  • Significant doping?????

  • High mobility (How high)?????


Conclusion: Oxides (p-type) vs. Metals (n-type)

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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