Graphene and Carbon Nanotubes. R. Saito Tohoku University

Transcription

1 東北大学 TOHOKU UNIVERSITY The 1 st GCOE international symposium "Weaving Science Web beyond Particle-Matter Hierarchy" , Sendai, Japan Graphene and Carbon Nanotubes K. Nomura R. Saito Tohoku University k x k y

2 Why do we need nano-materials? Carbon Nanotubes and Graphene are Important For Motivating the Nanoworld The Shrinking of Electronics and Moore s Law Rapid advances in molecular Biology at the nanoscale Evolution of Materials Chemistry from a molecule to a quantum dot 100 nm DRAM 1/2 pitch, 3-yr cycle DRAM 1/2 pitch, 2-yr cycle MPU gate length 10 nm 1 nm Moore s Law Biology at the nanoscale Molecule quantum dot

3 Moore s Law For Semiconductor Electronics Soon, All Microchips Will Be nanoscale Devices 100 nm DRAM 1/2 pitch, 3-yr cycle DRAM 1/2 pitch, 2-yr cycle MPU gate length 10 nm 1 nm Semiconductor devices were approaching the nano scale when nanotubes had their official birthday. Semiconductor Research Corporation

4 Biological Building Blocks in the Nanoworld Artist s conception of a DNA strand being packed inside a virus capsid by a molecular motor. The rotary motor of the virus exerts a force of 60 pn to compress the DNA to 6000 times its normal volume. The internal pressure thus generated is later used by the virus to launch this DNA into a cell. (Credit: C. Bustamante, UCBerkeley)

5 A Potential Quantum Dot Building Block For Electronics Applications Left: Lithographically-patterned coupled quantum dot molecule formed in a GaAs/AlGaAs heterostructure. Using voltages applied to the gates, a double-well potential is created where individual electrons can be trapped in the left or right hand well. The charge state of the molecule is read out using the quantum point contact of the confining gates as a sensor. Right: Measured phase diagram of the molecule, where (nl,nr) indicates the number of electrons in the left and right wells. The slanted lines indicate places where the charge state changes. (Credit: C. Marcus, Harvard U.)

6 Carbon Nanotubes - a Prototype 1D (one-dimensional) material of the Nanoworld Left: Electron spiraling down a nanotube. The nanotube energy levels are quantized, as in an atom, so that every (n,m) nanotube species has a different fluorescence peak energy determined by its diameter and chirality, as shown in the right panel. (Credits: left, P. McEuen, Cornell U; right, R.B. Weisman, Rice U)

7 Carbon Allotropes with various dimensions Diamond graphite graphene nanotube nanowire fullerene D. O. S. D. O. S. D. O. S. D. O. S. E E E E 3D Bulk Semiconductor 2D Quantum Well 1D Quantum Wire 0D Quantum Dot Hill, J. W., and Petrucci, R. H., General Chemistry, 3rd edition, Prentice Hall, NJ, (2002)

8 Diamond Family of carbon hybridization and geometry R. Saito, in Carbon Alloy Elsevier (2003) sp 3 : Diamond (3D) sp 2 : Graphite (2D) Fullerene (0D) Fullerene Polymer(1-3D) Nanotube (1D) Peapod (0D+1D) sp: Carbyne (1D) Not crystal: HOPG, VGCF, glassy, whisker, fiber, nanohorn, turbostratic, ribbon Nanotube Polymer Graphite Fullerene ribbon Peapod

9 Nanotube Structure Rolling graphene into a cylinder Chiral vector: C h = na 1 + ma 1 (n,m) Physical properties depend on (n,m): 2 2 dt = 3aC C ( n + m + Diameter 3p n m = 3 p ± 1 nm) 1/ 2 /π metal semiconductor Chirality 3m θ = tan 1 2n + m (4,2) (10,10) achiral, metallic (10,5) chiral, semiconducting Shigeo MARUYAMA's Fullerene and Nanotube Site

10 18 years of Carbon Nanotubes S. Iijima, (1991), M. S. Dresselhaus et al. (1991) Arc Method:(by Y. Saito) J. Mintmire et al., R. Saito et al., N. Hamada et al, K. Tanaka et al, Laser Ablation:(by H. Kataura) (5,5) Armchair Nanotube (9,0) Zigzag Nanotube (6,5) Chiral Nanotube

11 Carbon Nanotubes & Graphene both are rapidly growing fields Nanotube publications go back to 1952 Graphene publications go back to 1947 Number of nanotube publications is still growing rapidly SWNT number of publications containing Carbon Nanotube vs. time

12 Why now graphene? High performance nano-device high mobility 100,000cm 2 /Vs at 300K Improvement of nanotechnology Fabricated easier than nanotubes Long spin coherence µm Exotic physics (QHE) at 300K Stable, chemically inactive Challenge for graphene devices Integration, process technique Synthesis (SiC, CVD, Scotch tape) Opening energy gap, Edges, defects Graphene = single layer of graphite = 2D graphite

13 Quantum Hall Effect of Graphene

14 Hall Effect in 1878 In a magnetic field R H = V H I B

15 Quantum Hall Effect in 1980

16 Quantum Hall Effect (Classical) Hall effect Quantum Hall effect von Klitzing ν= σ xy = 1 = n R xy e h 2 ν Landau level Filling factor ν N 2π 2 = n = 2π Bn N eb φ

17 QHE of Graphene in 2005 σ ( n yx = + 1/ 2) [4e 2 / h] Nature 438 (2005) 04233, Half-integer quantum Hall effect Zheng-Ando, Gusynin-Sharapov, Peres et al.

18 Additional Quantum Hall Plateaus Valley and spin splitting in the strong field limit -4-6

19 Graphene Quantum Hall Ferromagnet K B=0 K K K K K SU ( 2) SU (2) = SU (4) spin valley disordered [weak B] clean [strong B] E C << Γ E C >> Γ

20 QHE in zero-field limit Non-relativistic Relativistic σ xy = 0 weak B field (strong disorder)? σ xy 2 e = ± 2h a a strong B field (weak disorder) -3/2 1/2-1/2 3/2

21 QHE: Localization in B-fields ( p ea( r) ) H = + V ( r) 2m 2 V(r) 0

22 Localization and QHE Topological metal k y QHE in a vanishing B-field k x B = 0 massless Dirac fermions B 0 Relativistic (Dirac eq.) σ xy 2 e = ± 2h Non-relativistic (Schrodinger)

23 Singularity at Dirac point K-K decoupled case (long-range potential) QHE of Dirac fermions in Bg0 σ xy = ± 2 e 2h (per valley, per spin) Phase diagram ν = h eb n Phys. Rev. Lett. 100, (2008)

24 Why are nanotube and graphene special? 1.Quantum effect at 300K 2.Constant Fermi velocity v F = c/ Large variety of geometries more than 300. A B Hexagons make graphene special. Similarity to elementary particle physics

25 A and B carbon atoms in graphene Two component wave function a 1 a 2 X C 2 Pseudo spin A Lattice Structure of Graphene 2 fold symmetry between A and B B

26 Pseudo Spin vs Real Spin α β A B only A site only B site up spin down spin

27 Energy Band of Graphene Tight Binding Hamiltonian Schrodinger Eq. Conduction band K and K : Dirac points Valence band Γ K M K K point K point

28 Taylor expansion of f(k) around the K point Eigenvalue Eigenvector = Θ Θ Θ Θ i i i i e e e e = Θ Θ Θ Θ i i i i e e e e ± = Ψ Θ i e 1 (k) K point Hamiltonian around the K point CC 0 F a v γ = k y k x

29 The phase of the wavefunction and Berry s phase T. Ando et al., J. Phys. Soc. Jpn. 67, 2857 (1998) γ (Θ) Ψ Ψ 1 ( 1 k) = Θ( e i k + ) Ψ Θ: 0 2π around the K point, ( k + 2π ) = Ψ1 ( 1 k We must add the Berry Phase γ: Θ ) = e iθ( k)/ 2 ( k ) 2 + iθ( k) / 2 e iθ( k)/ 2 ( = e Ψ1( k) Ψ2 ( k + 2π ) = Ψ2 ( k) Θ+γ ) γ (Θ) = π 0 Ψ1 ( k + 2π ) = Ψ1 ( k) Ψ2 ( k + 2π ) = Ψ2 ( k)

30 4 π rotational symmetry of Pseudo Spin vs Real Spin A B α β 4 π rotation in the k space rotation around the K (K ) point 4 π rotation in the real space rotation around z-axes

31 Absence of Back scattering T. Ando et al., J. Phys. Soc. Jpn, 67, 2857 (1998) A time reversal pair of back scatterimg Cancel scattering amplitude to each other by the phase difference of π.

32 Question: what is the field that polarizes Pseudo Spin or Real Spin? α β A B only A site only B site up spin down spin A: Magnetic field B B

33 Effective Hamiltonian around the K point K point K point

34 Dirac s equation Mass Hamiltonian of a relativistic electron with electronic real spin Paul Adrien Maurice Dirac When m=0 A B Similar Hamiltonian for graphene with pseudo-spin σ: operator for pseudo spin

35 Vector field Real spin and the magnetic field Zeeman term Magnetic field does not polarize the pseudo spin. Similar field A q for pseudo spin

36 Edge electronic states at the zigag edge of graphene M. Fujita et al, J. Phys. Soc. Jpn. 65 (1996) 1920 Delocalized states: Edge states: Pseudo-spin polarized states W.F. has a value only either A or B site. Spacialy localized at zigzag edge Amplitude of WF

37 Vector potential for pseudo spin K. Sasaki, R. Saito, Prog. Theor. Phys. Suppl. 176, 253 (2008) A q is expressed by δγ 0 modification of nearest neighbor interaction Why? modification of off-diagonal element with time reversal symmetry A B

38 Analytic solution of zigzag edge states by A q K. Sasaki et al., J. Phys. Soc. Japan, 75, (2006) Edge states zigzag edge formation γ 0(1 c) tight binding results A q field results up: energy dispersion, down: localization length

39 Zigzag edge vs Armchair edge K. Sasaki et al., J. Phys. Soc. Japan, 75, (2006) Zigzag edge Armchair edge B z =0 No pseudo-spin polarization no edge states Useful for considering edge states for any edge

40 Direction of Pseudo-Spin = σ z ( ) ) ( ) ( = = Θ + Θ k i k i e e σ z Extended states Edge states σ z A B Pauli matrices

41 Parity anomaly in graphene origin of the mass K. Sasaki, R. Saito, J. Phys. Soc. Japan, 77, (2008) Weyl equation (periodic system) Weyl equation (finite system) Zigzag edge Broken A and B symmetry Partial polarization of pseudo-spin Occurrence of the mass Most simple realization of Parity anomaly 1. Deformation-induce magnetic field 2. Coulomb-induce mass term

42 Electron-phonon interaction is expressed by A q K. Sasaki, R. Saito, G. Dresselhaus, M. S. Dresselhaus, H. Farhat, J. Kong, Phys. Rev. B, 77, (2008) Electron-photon interaction Electron-phonon interaction K point K point Broken time-reversal symmetry Time-reversal symmetry over the k space

43 Electro-chemical doping and Raman spectrosocpy of single wall carbon nanotubes Farhat et al., Phys. Rev. Lett., (2007) TO LO isolated metallic SWNT E F dependent upshift of TO? Broadening

44 Chiral angle dependence of KA K. Sasaki, R. Saito, et al. Phys. Rev. B, 77, (2008) Zigzag Chiral Armchair Why TO becomes hard for zigzag NT? Θ dependent el-ph interaction LO: TO: K. Ishikawa and T. Ando, J. Phys. Soc. Jpn. 75, (2006). T. Ando, J. Phys. Soc. Jpn. 77, (2008).

45 Gate voltage dependence of G band Raman spectra J. S. Park, et al. unpublished Armchair SWNT Chiral SWNT Zigzag SWNT K. Sasaki, et al. Phys. Rev. B 77, (2008) TO E F >0 LO LO TO TO E F <0 For LO mode of zigzag tube, No Electron-phonon int.

46 Edge states superconductivity of graphene K. Sasaki, et al., J. Phys. Soc. Japan, 76, , (2007) Eliashberg equation Tang et al., Science 292 (01) for SWNT (0.4nm diameter) Takesue et al., PRL 96 (06) for MWNT (10nm diameter) Two problems 1. no D(E F ) 2. suppression of el-ph interaction U : repulsive Coulomb interaction Edge states could overcome the problems. Energy bandwidth of edge states K. Sasaki et al., Appl. Phys. Lett., 88, (2006)

47 Pseudo Spin and A q Summary Edge states: polarized pseudo spin A B Strong electron-phonon interaction superconductivity similarity to real spin σ z : Edge states parameter Application of A q to el-ph interaction

48 Thank you.

49 局在解 Sasaki et al., JPSJ75 (2006) ほとんどゼロエネルギー状態 局在長がエッジに沿った方向の波数に依存

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51 Single peak or double peaks? Spin Polarization STS experiments for a step edge in graphite Klusek et al., Appl. Surf. Sci. 161 (2000) Niimi et al., Appl. Surf. Sci. 241 (2005) Kobayashi et al., PRB 71 (2005)

52 Sasaki et al., arxiv: to appear in J. Phys. Soc. Jpn. (2008)

53 Mass from Coulomb interaction

54 Mean field approximation neglect Equal densities at A and B atoms Pseudo-spin polarization disappears, and Real-spin is polarized

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56 後方散乱の抑制 不純物による散乱 波数に依存した相対位相 多重散乱でも後方散乱が抑制 Nakanishi et al., JPSJ67, 2857(98) 2 成分間の干渉効果 Suzuura and Ando, PRB65, (02) エッジ状態 二成分間の相対位相や大きさの比 ( 擬スピン ) が物性に関係

57 A 原子 エッジ状態の性質をすべて再現する Sasaki et al., JPSJ75 (2006) B 原子

58 グラフェンの擬スピンとは何か まとめ 二成分間の相対位相 実スピン 後方散乱の抑制エッジ状態 ディラック方程式と電磁場 擬スピンをそろえる場

59 Ishikawa et al., JPSJ75 (2006) Sasaki et al., PRB77 (2008)

60 エッジ状態は擬スピンが傾いている B 原子 A 原子

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62 Calculation of G band Raman spectra G band Raman spectra = Intensity + Spectral width a 1. Raman Intensity I(ω, E L ) optical matrix el-ph matrix optical matrix k b K E L phonon energy j Phonon energy Laser energy resonance window Original frequency Correction frequency Including el-ph coupling 2. Spectral width Electron-hole pair creation matrix by el-ph interaction electron energy hole energy Spectral width Fermi distribution function K. Sasaki, et al. Phys. Rev. B 77, (2008) Spectral width is given by the decay length Γ.

63 Gate voltage dependence of G band Raman spectra Armchair SWNT Chiral SWNT Zigzag SWNT K. Sasaki, et al. Phys. Rev. B 77, (2008) TO E F >0 LO LO TO TO E F <0 J. S. Park, et al. unpublished For LO mode of zigzag tube, Electron-phonon coupling Almost zero

64 BN nano ribbon

65 Zigzag BN nanoribbon F. Zheng et al. (unpublished) Zone boundary k = π/a, f = 0

66 Energy band of BN nanoribbon F. Zheng et al. (unpublished) B Edge state around B edge N Edge state around N edge Pseudo spin polarized everywhere.

67 Coulomb interaction in BN ribbon F. Zheng et al. (unpublished) BN B Energy gap decreases (BN) increases (G) One spin direction appears at E F G N U B = U N = 7.5eV

68 Half metalicity in BN ribbon F. Zheng et al. (unpublished) Only down spin can flow at N zigzag edge Half metalicity D: dangling bond E: edge states

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