Edge state and unconventional magnetism of nanographene/nanographite

Transcription

1 Edge state and unconventional magnetism of nanographene/nanographite TOSHIAKI ENOKI Dept. of Chem. Tokyo Institute of Technology NT06, Nagano June 18-23, 2006

2 nanographite aromatic molecules bulk graphite nanographite π-electron electron system with open edges fullerenes carbon nanotubes closed π-electron system

3 nanographite (nanographene) # shape effect edge states localized π-spins enhanced magnetism non-kékule str. nonbonding π-state (s=1/2) Yamabe et al. Fujita et al. zigzag edge armchair edge

4 (a) (b) (c) (d) E κ = π 8π / 9 7π / 9 2π / k π E N=30 π* π D(E) (Fujita et al.) zig-zag edge edge state ferromagnetic edge state nonbonding π-state large density of states (DOS) spin paramagnetism contrast to bulk graphite

5 Klein edge extra π-bond connected to the carbon atom at a zigzag edge edge state flat band edge states with localized spins appear at -2/3 < k < 2/3 K. Kusakabe, et al. 2001

6 combination of Fujita s and Klein s edges K F completely localized edge state appears at E F nano-magnetism K. Kusakabe, et al. 2001

7 Carbon-only only ferromagnet Kusakabe et al. HC< Fujita edge -CH 2 Klein edge Fujita edge FC< -CF 2 spatially maldistributed ferromagnetism Klein edge

8 Present talk # Nanographene # single nanographene sheet nanodiamond electrophoretic technique + heat-treatment # nanographene ribbon resonance Raman experiments # electronic structure of edge state at edges hydrogen-terminated # Controllable nanoscopic magnetism gas-adsorption adsorption induced magnetic switch

9 single nanographene sheet

10 SEM and AFM images of diamond nanoparticles. 0.3 μm nanodiamond particles (ca. 5 nm) deposited by electrophoretic technique spherical shape with particle sizes larger than those observed by TEM absorbed solvent molecules on the surface of particles

11 nanographene and STM analysis STM images after heat-treatment at 1600 C nanographene flat single layer sheet mean in-plane size of 10nm

12 nanographene on HOPG substrate and STM Analysis current image ~ 10 nm 0.36 nm HOPG cross-sectional profile monolayer of nanographene

13 nanographene ribbon observed by resonance Raman experiments

14 AFM image of single nanographene ribbon single sheet of nanographene ribbon at a step edge ribbon size 8 nm x >1 μm single sheet

15 Resonance Raman experiments with polarized light G band (intralayer C-C stretching) G2: substrate HOPG G1: nanographene ribbon θ polarization alignments of individual ribbons are determined G2 G1 small nanographene ribbon can be easily heated by laser beam

16 edge states observed by STM/STS and analyzed by tight-binding calculations

17 electronic state of graphene edges experimental evidence of edge state zigzag edge edge state armchair edge π π armchair zigzag edge: edge: short long and defective, continuous, energetically unstable

18 3 zigzag carbon sites 5 zigzag carbon sites finite length zigzag edge edge state band structure localized edge state tight binding calculation

19 electron confinement effect in zigzag edges (6 6 nm 2 ) edge-state-absent site at zigzag edge (small local density of states (LDOS)) small LDOS node of the wave function

20 magnetic switch of nanographite

21 Activated carbon fibers (ACFs ) nanopore localized edge-state spin nanographite 3 nm 3-4 graphene sheets stacked 3D random network of nanographite domains nanopores adsorption of guest molecules edge-state spins

22 Gas physisorption (O 2,N 2,H 2 O etc.) in ACF nanopore space guest molecules effective pressure inter-nanographene layer distance reduced What effect on magnetism? nanographite + H 2 O

23 Comparison between H 2 O adsorption isotherm and magnetic susceptibility ity H 2 O/ACF (mg/g) χ(p)/χ(0) high spins low 0.5 adsorption desorption spins # physisorption isotherm adsorption threshold at P/P 0 ~ 0.5 (hydrophobic pore) # magnetic susceptibility drops at P/P 0 >0.5 P/P 0 ON/OFF magnetic switch

24 effective pressure induced by H 2 O guests d T.Suzuki et al., Carbon(1988) d ~0.38 nm d ~ 0.34 nm change in magnetism (high spin low spin transition) inter-graphene interaction J ~ t 2 /U enhanced effective magnetic moment reduced controllable nanoscopic magnetism in nanographite

25 Analysis by the Hubbard type model intralayer transfer int.: t on-site Coulomb int.: U # closed shell electron case: increase of interactions increase of total magnetic moment # open shell electron case: increase of interactions decrease of total magnetic moment in agreement with experiments interlayer hopping t 1 K. Harigaya, J. Phys.: Condens. Matter 13, 1295 (2001); K. Harigaya, Chem. Phys. Lett. 340, 123 (2001); K. Harigaya and T. Enoki, Chem. Phys. Lett. 351, 128 (2002).

26 Ar adsorption in micropores M [emu/g] 0.04 B =1T in vacuo Ar magnetization M Ar - M Vac [emu/g] boiling point 87 K B =1T M Ar - M Vac T [K] T [K] magnetization affected well above the boiling point Ar guest atoms (diamagnetic) adsorbed well above the boiling point

27 zero-field muon spin relaxation (ZF-μSR) in Ar T >130 K (above T b =87 K) exp ( λ t ) = exp ( γ σ ) ν t exchange narrowing T <84 K(below T b ) 1 3 σ B : dipolar field ν ex : exchange frequency Kubo-Toyabe formula 2 3 { ( ) } γ σ t exp ( γ σ ) μ B μ B t inhomogeneous broadening due to the dipolar field μ ex B 2 2 Asymmetry [%] K 95 K 84 K H = 0 G t [μs] exchange narrowing suppressed upon Ar adsorption

28 ΔH pp [mt] T b Ar vacuo ESR line width ΔH d : dipolar field H ex : exchange filed exchange narrowing suppressed below T b 2 d / ΔH ~ ΔH H ex 4 consistent with μsr T [K] magnetic switching effect J inter Ar J inter reduced J intra J intra enhanced

29 Summary Nanographene/Nanographite importance of geometry of edges non-bonding π-electron state (edge state) STM/STS observations for well defined edges nanoscopic magnetism spin glass magnetic switch gas sensor

30 Nanographene-based molecular devices zigzag edge armchair edge electron beam lthography magnetic line chemical modifications CH itinerant magnetism CH 2 localized magnetism CF nonmagnetic C=O conducting line future promising molecular devices (compared with nanotubes)

31 Y. Kobayashi, K. Takahara,, N. Kawatsu,, M. Affoune, B. L. V. Prasad, K. Takai,, H. Sato, K. Fukui, Tokyo Inst. of Tech. H. Harigaya, Nanotechnology Res. Inst., AIST Y. Kaburagi,, Y. Hishiyama, Musashi Inst. of Technology K. Kusakabe, Osaka University L. G. Cancado,, B. R. A. Neves,, A. Jorio,, M. A. Pimenta, Univ. Fed. Minas Gerais M. S. Dresselhaus, Massachusetts Inst. of Tech. R. Saito, Tohoku Univ. H. Suzuki, I. Watanabe, Riken Contributors F. Pratt, Rutherford Appleton Laboratory R. Kobori,, S. Maruyama, K. Kaneko, Chiba University

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33 Nanographene ribbon zigzag ribbon armchair ribbon

34 magnetic nanocarbons with Klein and Fujita edges C=C sp 3 F K nanocarbon-based based ferromagnetism F(or H)-terminated edge graphite-diamond interface K. Kusakabe, et al. 2001

35 triangular lattice image on a particle a) b) Particle Substrate same A-B A B stacking mode with HOPG substrate small irregularity

36 Electronic properties graphene sheet-substrate substrate interaction interlayer distance nm γ 1 interlayer resonance integral γ 1 = eV 0.29eV (bulk γ 1 =0.39 ev) quantum size effect N ~ 3000 largest polycyclic aromatic molecule band width W = 6γ 0 in-plane resonance integral γ 0 =3.16 ev energy discreteness N/W~75 K edge state effect scanning tunneling spectroscopy experiment

37 one zigzag edge atom in armchair edge zigzag part increase by one row ( nm 2 ) calculated LDOS near the defect point dispersed LDOS near the defect point at which an armchair line is added

38 A two zigzag edge atoms added to armchair edge B ( nm 2 ) increase by two armchair lines ( nm 2 ) arrays of bright spots near defects run in different directions

39 A B calculated LDOS near the defect point

40 A B the difference comes from one extra-carbon atom

41 B effect of inter-graphene layer interaction on the edge state 2D LDOS mapping 6 6 nm 2 two C atoms missing edge state at a defect point α site Z β site A X A Y

42 effect of various physisorbed molecules on nanomagntism 0.2 adsorption desorption MeOH hydrophobic graphene surface χ (P) / χ (0) EtOH Acetone Benzene -OH hydrocarbon with no OH CHCl 3 CCl 4 threshold pressure of magnetic switch effect is lowered upon the decrease in the weight of OH P / P 0

43 magnetic switch effect created at the solid-liquid liquid transition of Br 2 guests χt (emu g -1 K) magnetic moment freezing temp Br/C=0 Br/C=0.067 Br/C=0.038 Br/C=0.083 Br/C=0.1 Br/C=0.14 Br/C=0.16 Br/C=0.27 Br 2 content increases magnetic moment reduced ρ (g cm -3 ) 4 Br 2 high density solid low density liquid effective pressure T (K) magnetic switch effect

44 magnetoresitance with paramgnetic O 2 molecules { R(B) - R(0) } / R(0) 4 O 2 molecules S= T =3.00K in vacuo He Ar N 2 O 2 1 atm at R.T B [T] wave function overlap with graphitic π-electron strong exchange int. chemical bond with S=0 singlet state physisorbed weak dipole-dipole int. exchange int. between O 2 spins and edge-state spins internal field from O 2 molecules ~20 K

45 Zero field muon spin relaxation (μsr( SR) ) in Ar atmosphere Asymmetry [%] t [μs] Ar pressure: ~ 1 atm at 87 K (B.P.) : 84 K : 95 K : 294 K Freezing P. 84 K Boiling p. 87 K increase of the static component is prominent below the F.P. of Ar freezing of Ar condensed in the nanopores modification of the magnetism of the edge-state spins

46 nano-graphite network Coulomb blockade effect spin-glass Activated carbon fibers 3D nano-graphite network heat treatment insulator-metal transition

47 Heat-treatment effect on conductivity 4 2 metallic insulator percolation threshold P c ca. HTT 1200 C log (σ/sm -1 ) MI threshold T (K) no heat treatment 400 e - e - semiconductive metallic

48 semiconductive region conductivity of ACFs and iodine-doped ACFs I 2 -doped Coulomb gap variable-range hopping conduction non-doped charging effect e 2 εr e - α -1 ε [ ( T ) 1/ 2 ] σ = σ T T 0 = 0 exp 0 / 2 6e πk B 1 1 4πε α ε: dielectric constant α -1 : localization length 1

49 Heat-treatment effect on magnetic susceptibility χ obs /10-6 emu g H = 10kOe HTT / C a 800 b 1000 insulator c 1100 P C d 1200 e 1300 f 1400 metallic g 1500 h 1500 ( HTT 1h) T/ K a b c d e f g h HTT < P C Curie-Weiss behavior with localized spins and negative Weiss temperature HTT > P C less temp. dependent enhanced diamagnetism

50 Magnetism around the percolation threshold region 0.5 χ/10-6 emu g (1h) T / K HTT susceptibility peak ~ 7K antiferromagnetic ordering? negative Weiss temperature

51 Field cooling effect large field cooling effect around the MI threshold χ/ 10-6 emu g H=1T HTT800 H =1T HTT1100 FC ZFC FC ZFC χ/ 10-6 emu g -1 J disordered magnetism spin-glass glass-like like behavior T /K exchange interaction random distribution ΔJ 2 J ~ 0.8

52 ACFs nanopore localized edge state spin nanographite # 3D random network of nanographite domains # nanopores # edge-state spins

53 Huge helium condensation in micropores

54 Activated carbon fibers (ACF) 3D random network of nanographites micropore localized spin edge spin probe 3-4 layers nano-graphite L a ~ 3 nm nanographite; metallic domain 3-4 graphene sheets with in-plane size 2-3nm localized spins of edge origin several spins / nanographite micropores ~ 1-2nm, networked

55 Helium in micropores ACF; specific surface areas ~3000 m 2 /g gas adsorption He, Ne, Ar, H 2, N 2, O 2 ESR spin-lattice relaxation saturation technique edge-localized spins probe for guest-host int. hω spin system T 1 lattice

56 Peff(atm) He effective pressure in micropores at room temp. large condensation of guest gaseous species P (Torr) extremely large condensation ~ 10 4 times He atom, exceptional nature Peff(atm) N 2 Ar O 2 P(Torr)

57 ESR saturation curves with guest gases I (arb.units) independent of internal degree of freedom rotation / vibration q (mw 1/ 2 ) spin-lattice relaxation rate T -1 1 is accelerated by gas uptake remarkable enhancement in He atmosphere

58 spin-lattice relaxation rate T 1-1 He electric dipole-dipole int (van der Waals) edge-state spin phonon He collisional process process heat reservoir 1/T 1 =nσ υ n: density of helium atoms σ: cross section related to the spin-flop process υ: velocity of an He atom υ 2 = 3RT M 1/T 1 ~ n T

59 1 T = ( π ) 1/ 2 h 4 nm 3 / 2 p ΔE 2 2 ( e IJ ) R w 1/ 2 ( k T ) M: mass of He atom, Ro: min.dist.(he atom and edge spin) λ: spin-orbit int. of carbon p state, D: difference ( gr. and ex. states of C) B ΔE: difference ( 1 P and 1 S states of He atom) p = λ / Δ I = Ψ Ψ 1s 2pz dr J = Ψ Ψ 2s 2pz dr Ψ: He wave function helium atoms in activated carbon fibers extremely large condensation at room temperature remarkable acceleration of T -1 1 He He atom in ultra-micropore enhanced interaction He-nanographite

60 nano-graphite aromatic molecules bulk graphite nano-graphite π-electron system with open edges fullerenes carbon nanotubes closed π-electron system

61 Nanographene ribbon zigzag ribbon armchair ribbon

62 combination of Fujita s and Klein s edges K F completely localized edge state appears at E F nano-magnetism K. Kusakabe, et al. 2001

63 Klein edge π-bond connected to the carbon atom at a zigzag edge edge states with localized spins appear at -2/3 k< 2/3 K. Kusakabe, et al. 2001

64 magnetic nanocarbons with Klein and Fujita edges C=C sp 3 F K nanocarbon-based based ferromagnetism F(or H)-terminated edge graphite-diamond interface K. Kusakabe, et al. 2001

65 Electron wave diffractions by STM wave function diffraction patterns observed by STM theoretical characterization

66 Variations in diffraction period nm Diffraction patterns of electron waves 0 V=200(mV) I=0.7(nA) nm

67 graphene sheet tilted with respect to the substrate potential gradient on the graphene plane free electron model linear potential

68 Electron density by the k p model Hamiltonian around K-point Solution with the infinite well 13 linear potential electron density at the A-sublattice y envelope function as a long range component Local density of states x 0.5

69 Contributors Y. Kobayashi, A. M. Affoune, B. L. V. Prasad, J. Ravier, N. Kawatsu,, Y. Shibayama, O. E. Andersson,, A. Nakayama, K. Takai, H. Sato Tokyo Institute of Technology K. Sugihara Nihon University M. Endo Shinshu University R. Kobori,, S. Maruyama, K. Kaneko Chiba University H. Harigaya Nanotechnology Research Institute, AIST Y. Kaburagi,, Y. Hishiyama Musashi Institute of Technology

70 He pressure dependence of spin-lattice relaxation rate at room temperature enhancement of T -1 1 collisional process of He atom

71 Present talk # Nano-graphite heat treatment of nano-diamond at 1600ºC electrophoretic technique single nano-graphene sheet electronic structure # Nano-graphite network activated carbon fibers (ACF) Coulomb blockade effect spin-glass # Controllable nanoscopic magnetism water-adsorption induced magnetic switch