Transcript Distinct electron-phonon couplings in chemically doped and

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.
K
Г
• Energy band model:
– Zero gap semiconductor
M
Phonon dispersion of 2D graphene
Real space
R. Saito et al., “Physical Properties of Carbon Nanotubes” Imperial College Press (1998)
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
q~0
• 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
Electrons and Phonons
R. Saito et al, Physical Properties of Carbon Nanotubes, Imperial College Press (1998)
Electrons
Tight-Binding
Electrons
G
g0=3.033eV
q ~ 2k


inter-valley
k
k
Phonons
Force Constant


G
Phonons
q
q
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 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)
ClO4- (cyan)
Li+ (magenta)
A. Das, Nature 3, 210 (2008)
Raman shift as a function of gate voltage
Dirac point
A. Das, Nature 3, 210 (2008)
Back gate field effect tuning in Graphene
Vg > 0
n-type doping
Vg < 0
p-type doping
Ec is the onset energy for vertical electron-hole pair transitions.
J. Yan, PRL 98, 166802 (2007)
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
Softening?
(was not mentioned in this paper)
• 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
hardening
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 transfer
Chemical doping in SWNT
Up shift
• Electron acceptor
(Iodine, Bromine)
p
– P-type doing
– Vapor reactant at RT
• Electron donor
(Potassium, Rubidium)
– N-type doping
– T(Alkali-metal) =120°C
T (SWNT) =160°C
n
A.M.Rao, Nature 388, 257 (1997)
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
P-type doping
Up shift
Up shift
Urszula Dettlaff-Weglikowska, JACS 127, 5125 (2005)
N-type doping of SWNT via amine group adsorption
• Amine-rich (NH2) polymers:
– Polyethylenimine
N-type doping
Down shift
Moonsub Shim, JACS 128, 7522 (2006)
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.
p
n
p
Barun Das, ChemCommun, 5155 (2008)
n
Changes in the electronic structure of graphene by
molecular charge-transfer
Electron-withdrawing
Nitrobenzene, NO2 (p)
Aniline, NH2 (n)
Barun Das, ChemCommun, 5155 (2008)
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
SOCl2
for 30min. (-COCl)
• Rinse in Aceton for few second.
(a)
A
B
C
– Amino functionalized: 80°C Monoethanolamine for 24hrs. (-CONH-R)
• Rinse in Aceton for few second.
H2NCH2CH2OH
120°
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.
408
Graphene -NH-R
Intensity (a.u)
-C-O
400
(b)
(a)
N-C=O
404
396
392
N 1s
284.9
N1s/C1s = 0.0917
283.8
Graphene -Cl
284.2
(c)
Cl 2p
200.2
Graphene -COOH
284.4
C-Cl bonding
Cl2p/C1s = 0.2787
Pristine
290
288
286
284
282
204
202
Binding energy (eV)
200
198
196
Observation of I(2D)/I(G) changes by tuning the
Fermi-level
– 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 Fermisurface are still below the resonant
electron energy in DR scattering.
=633nm
(a) pristine
I(2D)/I(G) = 3
2D
FWHM(2D) = 24.5 cm-1
G
(b) graphene -NH-R =633nm
Intensity (a.u.)
• 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)/I(G) = 0.13
FWHM(2D) = 43.2 cm-1
(c) graphene -NH-R =488nm
I(2D)/I(G) = 1.19
FWHM(2D) = 35.1 cm-1
1200
1600
2000
2400
Raman shift (cm-1)
2800
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.
Intensity (a.u.)
(a)
n
2D
G
(b)
nG
2D
Vg
70 V
graphene-NH-R
50 V
pristine
30 V
graphene-COOH
p
0V
-20 V
p
graphene-Cl
1570 1580 1590 1600
2620 2640 2660 2680
-30 V
1570 1580 1590 1600
Raman shift (cm-1)
2620 2640 2660 2680
Theoretical explanation – Field-effect doping
• ГG is G phonon band width.
Residual band width
Broadening of the G phonon.
• D is the e-p coupling strength.
• G band energy:
– When graphene is chargeneutral, 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)
Pauli principle
Non-adiabatic perturbation
Electronic band
DFT
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 frequency
Relaxation time of liquid surface = electron relaxation time
The higher the Fermi level => the larger the difference between ΔE => Δω .
S. Pisana, Nature Mater. 6, 198 (2007)
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