Distinct electron-phonon couplings in chemically doped and field-effect doped

graphenes

林永昌20 Feb, 2009

Outline• Basic physical properties of graphene.• Raman spectroscopy of graphene.• Back ground review.

– Field-effect tuning of electron-phonon coupling.– Chemical functionalization and charge transfer.

• Experiment result.• Theory explanation.• Summary.• Reference.

Carbon family

3-dimensional diamond and graphite

2-dimensional graphene

1-dimensional carbon nanotubes

0-dimensional buckyballs

I.K. Mikhail, Mater. today 10, 20 (2007).

Electronic structure of graphene• π band of graphene.

• Energy band model: – Zero gap semiconductor

Г

K

M

Phonon dispersion of 2D graphene

R. Saito et al., “Physical Properties of Carbon Nanotubes” Imperial College Press (1998)

Real space k space

Phonon frequency and energy• C=3x1010(cm/s)=λ(cm)·ν(1/s)

• ν(1/cm) =1/ λ(cm) • E(eV)=1240/ λ(nm)=1240·ν(1/cm) ·10-10

• E(G=1600)=1240 x 1600 x 10-10 = 198.4 (meV)

S. Piscanec, PRL 93, 185503 (2004)

Resonance Raman intensity

• 1st order Raman (G):

• 2nd order Raman (D, 2D):

NT06-Tutorial “Chirality and energy dependence of first and second order resonance Raman intensity” R. Saito, Tohoku Univ., June 18-23, 2006 Nagano, JAPAN

q

R. Saito et al, Physical Properties of Carbon Nanotubes, Imperial College Press (1998)R. Saito et al, Physical Properties of Carbon Nanotubes, Imperial College Press (1998)

ElectronsElectronsTight-BindingTight-Binding

PhononsPhononsForce ConstantForce Constant

Electrons

Phonons

kk

qq

eV

inter-valleyinter-valley

q k

Electrons and Phonons

NT05-Raman-Tutorial, M . Dresselhaus

Relation between Raman shift and Excitation energy

• In Graphene:– Raman D peak is

proportional to the excitation laser energy.

– But Raman G peak does not sensitive to the excitation energy.

Raman G peak

• The G peak is due to the bond stretching of all pairs of sp2 atoms in both rings and chains which consist of in-plane displacement of the carbon atoms.

• The phonon frequencies near Г point which are called long wavelength optical phonons or E2g phonons.

• The E2g phonon energy will be influenced by not only the C-C force constant and also electron-phonon coupling strength.

Background review I

Field-effect tuning of electron-phonon couplingField-effect tuning of electron-phonon coupling

Electrochemical gating in Carbon nanotube

• Electrochemical doping in aqueous medium.– H2SO4

S. B. Cronin, APL 84, 2052 (2004)

Raman G peak shift• The Raman G peak of tangential mode (TM)

vibrational frequency up shift for both positive and negative applied electrochemical gate voltages.

S. B. Cronin, APL 84, 2052 (2004)

Electrochemical doping in Graphene• Polymer electrolyte: (PEO + LiClO4)

A. Das, Nature 3, 210 (2008)

ClO4- (cyan)

Li+ (magenta)

Raman shift as a function of gate voltage

A. Das, Nature 3, 210 (2008)

Dirac point

Back gate field effect tuning in Graphene

J. Yan, PRL 98, 166802 (2007)

Ec is the onset energy for vertical electron-hole pair transitions.

Vg > 0n-type doping

Vg < 0p-type doping

Low-temperature Raman spectra• Increases in charge density of either sign result in stiffening of

G mode.• ГG band width sharply decreases as |Vg-VDirac| increase.

J. Yan, PRL 98, 166802 (2007)

Distinct E-P coupling in gated bilayer Graphene

• The bilayer graphene is formed in AB Bernal staking.

• The phonon branch (E2g mode ) gives rise to two branches for bilayer graphene, one S (in-phase,

Eg) and other AS (out-of-phase, Eu).

L. M. Malard, PRL 101, 257401 (2008)

Softening? (was not mentioned in this paper)

hardeningsoftening

Raman shift of the S and AS component of G band

• S: symmetric displacements of the atoms.• AS: antisymmetric displacements.

L. M. Malard, PRL 101, 257401 (2008)

Background review II

Chemical functionalization and charge transferChemical functionalization and charge transfer

Chemical doping in SWNT

• Electron acceptor (Iodine, Bromine)– P-type doing– Vapor reactant at RT

• Electron donor (Potassium, Rubidium)– N-type doping– T(Alkali-metal) =120°C

T (SWNT) =160°C

A.M.Rao, Nature 388, 257 (1997)

p

n

Up shift

Down shift

Covalent bonding and charge transfer

• Diazonium reagents extract electrons, thereby evolving N2 gas leaving a stable C-C covalent bond with the nanotube surface.

• The amounts of electron transfer are dependent on the density of bonding reactants.

M. S. Strano, Science 301, 1519 (2003)

Conductivity increasing by SOCl2 adsorption

• Chemical modification: – SOCl2

Urszula Dettlaff-Weglikowska, JACS 127, 5125 (2005)

Up shift

Up shift

P-type doping

N-type doping of SWNT via amine group adsorption

• Amine-rich (NH2) polymers: – Polyethylenimine

Moonsub Shim, JACS 128, 7522 (2006)

Down shift

N-type doping

Changes in the electronic structure of graphene by molecular charge-transfer

• The stiffening or softening of the G band depends on the electron-donating (n) or –withdrawing (p) power of the substituent on benzene.

Barun Das, ChemCommun, 5155 (2008)

p

np

n

Barun Das, ChemCommun, 5155 (2008)

Changes in the electronic structure of graphene by molecular charge-transfer

Nitrobenzene, NO2 (p)

Aniline, NH2 (n)

Electron-withdrawing

Experiment Result

Sample Preparation• Graphenes were transferred from HOPG onto Si substrate with 300 nm

SiO2 on the top by mechanical exfoliation.

• Chemical functionalization:– Oxidization: 80°C HNO3 for 30min. (-COOH)

• Rinse in H2O .

– Converted into acylchloride groups : 80°C thionylchloride for 30min. (-COCl)

• Rinse in Aceton for few second.

– Amino functionalized: 80°C Monoethanolamine for 24hrs. (-CONH-R)• Rinse in Aceton for few second. 120°

A

C

B

(a)

H2NCH2CH2OH

SOCl2

Binding energy of different functional group bonding• Cl group extract out electron from carbon atom and shifted by 0.4 eV to lower

binding energy.• Amine groups stand in opposite function and shifted by 0.5 eV to higher BE.

290 288 286 284 282

408 404 400 396 392

204 202 200 198 196

(c)

(b)

Graphene -NH-R(a)

284.9

283.8

284.4

284.2

Graphene -Cl

Pristine

Graphene -COOH

N 1s

Cl 2p

200.2 C-Cl bonding

N-C=O

-C-O

Binding energy (eV)

Inte

nsity

(a.

u)

N1s/C1s = 0.0917

Cl2p/C1s = 0.2787

Observation of I(2D)/I(G) changes by tuning the Fermi-level

• For pristine monolayer graphene, the linear behavior energy band at K point leads the sharp Raman 2D peak due to DR scattering.

• After the Amino functionalization, the graphene was chemically n-doped. The DR is forbidden by the Pauli exclusion principle.

– I(2D) decreased apparently.

• Change the excitation Laser energy from 1.95 eV (633nm) to 2.54 eV (488nm), the DR thus revive. Because the Fermi-surface are still below the resonant electron energy in DR scattering.

1200 1600 2000 2400 2800

=488nm

=633nm

(c)

(a)

=633nm(b)

graphene -NH-R

graphene -NH-R

2D

pristine

G

Raman shift (cm-1)

Inte

nsity

(a.

u.)

I(2D)/I(G) = 3FWHM(2D) = 24.5 cm-1

I(2D)/I(G) = 0.13FWHM(2D) = 43.2 cm-1

I(2D)/I(G) = 1.19FWHM(2D) = 35.1 cm-1

Different Raman G peak shift in chemically and field-effect doping

• For p-doping, the Raman G peak will both up-shift.• For chemically n-doping, the Raman G peak will down-shift,

but it will up-shift by field-effect n-doping.

Raman shift (cm-1)

Inte

nsity

(a.

u.)

1570 1580 1590 1600 2620 2640 2660 2680

(b)

-30 V

-20 V

0 V

30 V

50 V

70 VG 2DVg

1570 1580 1590 1600 2620 2640 2660 2680

2D

(a) G

graphene-NH-R

pristine

graphene-COOH

graphene-Clp

n

p

n

Theoretical explanation – Field-effect doping

• ГG is G phonon band width.

• D is the e-p coupling strength.

• G band energy:

– When graphene is charge-neutral, the onset energy is zero.

– If graphene is doped with electrons or holes, the onset energy is twice the Fermi energy.J. Yan, PRL 98, 166802 (2007)

Broadening of the G phonon.

Pauli principle

Residual band width

Non-adiabatic perturbation

S. Pisana, Nature Mater. 6, 198 (2007)

DFT

Electronic band

Non-adiabatic Born-Oppenheimer

The G peak pulsation is ~ 3fs, which is much smaller than e-momentum relaxation time τm ~100fs.The electrons do not have time to relax their momenta to reach the instantaneous adiabatic ground state.

Shaking frequency = phonon frequencyRelaxation time of liquid surface = electron relaxation timeThe higher the Fermi level => the larger the difference between ΔE => Δω .

Theoretical explanation – Chemical doping

• An real covalent bonding exist on the carbon atom which will change the C-C bond length.

• Acylchloride group will withdraw electron form carbon atom (p-dope) to form a covalent bond C-Cl, therefore the C-C bond at the edge will become shorter which will directly cause Raman stiffening.

• Amine group will donate electron into carbon atom

(n-dope) and extend the C-C bond so the Raman softened.

Summary

• We demonstrate the chemical functionalization on graphene ribbons, furthermore the charge transfer phenomenon was observed by Raman spectroscopy.

• An apparent distinct electron-phonon coupling occurred on the electrical field-effect doping and chemical doping.

Thank you