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

1. Distinct electron-phonon couplings in chemically doped and field-effect doped graphenes 林永昌 20 Feb, 2009

2. 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.

3. 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).

4. Electronic structure of graphene • π band of graphene. • Energy band model: • Zero gap semiconductor K Г M

5. Phonon dispersion of 2D graphene k space Real space R. Saito et al., “Physical Properties of Carbon Nanotubes” Imperial College Press (1998)

6. 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)

7. Resonance Raman intensity • 1st order Raman (G): q~0 • 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

8. 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 K¢ inter-valley K k k Phonons Force Constant K K¢ G Phonons q q NT05-Raman-Tutorial, M . Dresselhaus

9. 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.

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

11. Background review I Field-effect tuning of electron-phonon coupling

12. Electrochemical gating in Carbon nanotube • Electrochemical doping in aqueous medium. • H2SO4 S. B. Cronin, APL 84, 2052 (2004)

13. 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)

14. Electrochemical doping in Graphene • Polymer electrolyte:(PEO + LiClO4) ClO4- (cyan) Li+ (magenta) A. Das, Nature 3, 210 (2008)

15. Raman shift as a function of gate voltage Dirac point A. Das, Nature 3, 210 (2008)

16. 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)

17. 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)

18. 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). softening hardening L. M. Malard, PRL 101, 257401 (2008)

19. 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)

20. Background review II Chemical functionalization and charge transfer

21. Chemical doping in SWNT Up shift p • 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 n Down shift A.M.Rao, Nature 388, 257 (1997)

22. 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)

23. Conductivity increasing by SOCl2 adsorption • Chemical modification: • SOCl2 P-type doping Up shift Up shift Urszula Dettlaff-Weglikowska, JACS 127, 5125 (2005)

24. 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)

25. 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 n Barun Das, ChemCommun, 5155 (2008)

26. Changes in the electronic structure of graphene by molecular charge-transfer Electron-withdrawing Nitrobenzene, NO2 (p) Aniline, NH2 (n) Barun Das, ChemCommun, 5155 (2008)

27. Experiment Result

28. 120° 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. (a) A B SOCl2 C H2NCH2CH2OH

29. 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. N-C=O N1s/C1s = 0.0917 Intensity (a.u) 200.2 C-Cl bonding -C-O Cl2p/C1s = 0.2787 Binding energy (eV)

30. 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. I(2D)/I(G) = 3 FWHM(2D) = 24.5 cm-1 I(2D)/I(G) = 0.13 FWHM(2D) = 43.2 cm-1 Intensity (a.u.) I(2D)/I(G) = 1.19 FWHM(2D) = 35.1 cm-1 Raman shift (cm-1)

31. 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. n n Intensity (a.u.) p p Raman shift (cm-1)

32. 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. Residual band width Broadening of the G phonon. Pauli principle J. Yan, PRL 98, 166802 (2007)

33. 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)

34. 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.

35. 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.

36. Thank you