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=3x10 10 (cm/s) =λ (cm) ·ν (1/s) ν (1/cm) =1/ λ (cm) E (eV) =1240/ λ (nm) =1240·ν (1/cm) · E (G=1600) =1240 x 1600 x = (meV) S. Piscanec, PRL 93, (2004)

Resonance Raman intensity 1 st order Raman (G): 2 nd 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  q  

R. Saito et al, Physical Properties of Carbon Nanotubes, Imperial College Press (1998) ElectronsTight-Binding Phonons Force Constant Electrons Phonons kk qq    eV      inter-valley q  kq  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 sp 2 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 E 2g phonons. The E 2g 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. – H 2 SO 4 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 + LiClO 4 ) A. Das, Nature 3, 210 (2008) ClO 4 - (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, (2007) E c is the onset energy for vertical electron-hole pair transitions. Vg > 0 n-type doping Vg < 0 p-type doping

Low-temperature Raman spectra Increases in charge density of either sign result in stiffening of G mode. Г G band width sharply decreases as |V g -V Dirac | increase. J. Yan, PRL 98, (2007)

Distinct E-P coupling in gated bilayer Graphene The bilayer graphene is formed in AB Bernal staking. The phonon branch (E 2g mode ) gives rise to two branches for bilayer graphene, one S (in-phase, E g ) and other AS (out-of-phase, E u ). L. M. Malard, PRL 101, (2008) Softening? (was not mentioned in this paper) hardening softening

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, (2008)

Background review II Chemical 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 N 2 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 P-type doping

N-type doping of SWNT via amine group adsorption Amine-rich (NH 2 ) 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 n p n

Changes in the electronic structure of graphene by molecular charge-transfer Nitrobenzene, NO 2 (p) Aniline, NH 2 (n) Electron-withdrawing

Experiment Result

Sample Preparation Graphenes were transferred from HOPG onto Si substrate with 300 nm SiO 2 on the top by mechanical exfoliation. Chemical functionalization: – Oxidization: 80°C HNO 3 for 30min. (-COOH) Rinse in H 2 O. – 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 SOCl 2

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 C-Cl bonding N-C=O -C-O Binding energy (eV) Intensity (a.u) N1s/C1s = Cl2p/C1s =

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. Raman shift (cm -1 ) Intensity (a.u.) I(2D)/I(G) = 3 FWHM(2D) = 24.5 cm -1 I(2D)/I(G) = 0.13 FWHM(2D) = 43.2 cm -1 I(2D)/I(G) = 1.19 FWHM(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 ) Intensity (a.u.) p n p n

Theoretical explanation – Field-effect doping Г G 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, (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 τ m ~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.

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.

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