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