Large band gaps, ferromagnetism, and anomalous magnetoresistance oscillations derived from edge states; Nanoribbons and Antidot-lattice graphenes

Transcription

1 Large band gaps, ferromagnetism, and anomalous magnetoresistance oscillations derived from edge states; Nanoribbons and Antidot-lattice graphenes Junji Haruyama Aoyama Gakuin University, Tokyo, Japan

2 Contents Non-lithographic 1. Introduction 2.GNRs fabricated by unzipping of carbon nanotubes and 3-stepped annealing Low defects and 7-times larger energy band gaps Arm Chair 3.Antidot-lattice graphenes fabricated using nanoporous alumina templates as etching masks Anomalous magnetoresistance oscillations Room-temperature Ferromagnetism zigzag Nature Nanotech & Latest Highlights (10 layers) (Monolyer) PRL Submitted to Nature 4.Future plans: (Quantum ) Spin-Hall effect

3 Edge atomic structures of Graphene (Graphite) ( 超伝導 強磁性 ) 電極 Arm chair ジグザグ端 ( 超伝導 強磁性電極 zigzag Graphene nanoribbon

4 Edge states of Graphene (Graphite) K.Nakata et al., Phys. Rev. B 54, (1996) Tight-binding Band gap Flat band Arm chair Zigzag Strong Electron localization High EDOS Spin polarization

5 Spin polarization and ferromagnetism at zigzag edges with hydrogen termination local-spin-density approximation Kusakabe and Maruyama, Phys. Rev. B 67, (2003) Hydrogen Up spin Down spin

6 Why graphene edges are important and interesting?? 1. Band gap engineering 2. Electron correlation with localized edge electrons 3. All carbon magnetism (magnets) 4. Spin Current & (Quantum) Spin Hall Effect So many theoretical reports, but no experimental reports Large damages by lithographic methods Non-lithographic methods

7 Contents Non-lithographic 1. Introduction 2.GNRs fabricated by unzipping of carbon nanotubes and 3-stepped annealing Nature Nanotech & Latest Highlights Low defects and 7-times larger energy band gaps

8 Current issue December Vol 5 No 12 Quantum tunnelling through single bases FREE RNA nanotechnology: Best of both worlds Current issue table of contents Impact Factor Latest highlights Advance online publication Nanoelectronics Article by Shimizu et al. Graphene nanoribbons manufactured by annealing unzipped carbon nanotubes have been measured to have a large energy bandgap. Nature Nanotechnology News and Views Nanoelectronics: Graphene gets a better gap Stephan Roche Journal name: Nature Nanotechnology Volume: 6, Pages: 8 9 Year published: (2011) DOI:doi: /nnano Published online 23 December 2010

9 Introduction of energy band gaps Semi metal, Zero- gap semiconductor Absence of energy band gaps Destruction of symmetry in bilayer graphenes Voltages, Carrier doping, Substrate Carrier confinement into 1D space GNRs

10 Disordered Graphene Nano-ribbon(Lithographic) Large transport gaps Large difference between & E a Stochastic Coulomb diamond Hopping conductance Han, Kim et al., Phys. Rev. Lett., 104, (2010) E c =e 2 /2C

11 Low-defect GNR formed by unzipping of MWCNTs Rice University, Smalley Institute for Nanoscale Science and Technology J.Tour et al., Nature 458, 872 (2009) Our originality 1Formation of GNRs on substrate by air blow 23stepped annealing(high vacuum, H 2 )

12 Our originality 1Formation of GNRs on substrate by air blow As-depo 1μm 200nm

13 Formation of GNRs on Si-substrate by air blow 1by brushing 2by air blowing to droplet AFM Number of GNRs within 5mm 2 -substrate Brush Air blow Isolated Rectangle Monolayer 5 15

14 Our originality for deoxidization and carrier doping 23-stepped annealing during FET formation process Right after formation of GNRs on substrate High vacuum 800 C for long time For deoxidization & Recovery of defects Right before formation of EB mark H 2 atmosphere 800 C For carrier doping Right before formation of FET electrode High vacuum 300 C For cleaning

15 Quality of GNRs: low defects AFM As-grown nanoribbon HRTEM Before annealing AIST Suenaga After annealing FET Raman Nature Nanotech(Dec.19, 2010)

16 Electronic transport for 4 different-type GNRs N Layer number W Width (nm) Nature Nanotech(Dec.19, 2010)

17 Correlation of ambipolar feature with annealing time at high vacuum t = 0 t = 24h Deoxidization:p-type Ambipolar

18 Electronic transport:zero-bias anomaly & Transport gap W=75nm N=1 V BG = 1V Small transport gap Low defects Nature Nanotech(Dec.19, 2010)

19 Single-electron Spectroscopy Low-defects GNR Coulomb diamonds Disordered GNRs W=75nm N=1 No stochastic diamonds Low defects Stochastic diamonds due to defects (Q-dots connected in series) Nature Nanotechnology

20 Energy band gap in thermalactivation relationships 55m ev Transport gap close to E a values 7-times larger E a No hopping conductance Nature Nanotech(Dec.19, 2010)

21 Q1: The large band gaps are relevant for large-width GNRs? Theory for energy band gaps of GNRs with arm chair edge Louie et al., UC Berkeley l e 300nm 3 ev for W=1nm W < 300nm Remaining 1D LDA GWA E a 30 mev for W 100nm

22 Contents Non-lithographic 1. Introduction 2.GNRs fabricated by unzipping of carbon nanotubes and 3-stepped annealing Nature Nanotech & Latest Highlights Low defects and 7-times larger energy band gaps 3.Antidot-lattice graphenes fabricated using nanoporous alumina templates as etching masks PRL Anomalous magnetoresistance oscillations (10 layers)

23 Antidot-lattice graphene Graphene GNR Antidots edge 100nm

24 Antidot Lattice on Semiconductor 2DEG ( 1990) Commensurability peak Aharonov-Bohm-type Oscillation ΔB ABT = (h/e)/s Cyclotron orbit R c = ( n S ) 1/2 (h/2 )/eb - Antidots Antidot Low B D. Weiss, K. von Klitzing et al., PRL 70, 4118 (1993) Under enough antidot spacing High B M. Kato,S. Katsumoto, Y. Iye, PRB 77, (2008).

25 Antidot Lattice as a scattering center for electrons on 2DEG Anomalous FQHEs 200nm/Ls 600nm = 1/3 Kang, Stormer, PRL71, 3850 (1993) Anomalous filling factor Antidots No antidots Composite Fermion In Graphenes: How edge-localized electrons are interacted with cyclotron-moton electrons?

26 Antidot Lattice Graphene Only a few publications No reports for edges S.Russo et al., PRB 77, (2008). T. Shen et al., APL 93, (2008). J. Bai et al., Nature Nanotech. 5, 190 (2010).

27 Formation of low-defect antidot-lattice graphene by porous alumina templates Graphene GNR Antidots Zigzag edge Hydrogen annealing

28 Porous alumina templates 80nm Pore spacing 20nm 50nm Pore spacing 40nm 15nm Pore spacing 20nm

29 FESEM images of ADLGs

30 AFM and STM images of Hydrogen-terminated ADLGs T = 80K 100nm AFM 500nm STM 100nm Hiroshi Fukuyama Tokyo University

31 M(T) [emu] Magnetization (emu/100 m 2 ) 6.0e e e e e e-5 All-carbon Ferromagnetism in ADLG with Hydrogen-terminated edges T = 2K M-H@2K H[Oe] M(T) [emu] e-4 5.0e e-5 M-H@4K T = 2K e H[Oe] M(T) [emu] -6.0e-4 (a) (b) (c) Magnetic field (gauss) Mono-layer graphene e-4 6.0e e-4 2.0e e e-4 M-H@2K T = 2K e H[Oe] Hydrogen Oxygen No antidots Evidence for zigzag at antidot edge Correlation of localized electrons with MR oscillations??

32 FFT (arb. units) Aharonov-Bohm-type Oscillations in H 2 -terminated ADLGs Commensurability peak = 80 nm AD Space 80 nm 2R c = ( n S ) 1/2 (h/2 )/eb = a n S cm -2 Sample B Sample A 0 < B < 2.5 Low B Fourier Spectrum B = 200 mt (b) l e = 2D/v F 800 nm > 2 (a/2) = 540 nm (c) 2.5 < B < 5 High B / B (T -1 ) ΔB ABT = (h/e)/(s) AB-type oscillation

33 Electron trajectories on honey-comb ADL and magnetoresistance oscillations Runaway orbit SDHO orbit a 1st Unit cell Graphene AB-type Anti-dots oscillation Low B B 200 mt 2R c = a (Commensurability MR peak orbit) 2nd Unit cell ΔB ABT =(h/e)/s S = 6(3-1/2 /2)(a/2) 2 Graphene nanoribbons a = 160 nm zigzag edges Quantized electron orbitals around antidots in an unit cell E n = h 2 /2mL 2 (n - Φ/φ 0 )

34 Aharonov-Bohm-type effect ΔB ABT =(h/e)/s 2R c = ( n S ) 1/2 (h/2 )/eb = a n S cm -2 l e = 2D/v F 800 nm > 2 (a/2) = 540 nm E n = h 2 /2mL 2 (n - Φ/φ 0 )

35 FFT (arb. units) Anomalous MR Oscillations in ADL- multi-layered Graphenes Commensurability peak = 80 nm 2R c = ( n S ) 1/2 (h/2 )/eb = a Sample B 0 < B < 2.5 Fourier Spectrum n S cm -2 Sample A (b) l e = 2D/v F 800 nm > 2 (a/2) = 540 nm (c) 2.5 < B < 5 B 260 mt / B (T -1 ) High B

36 Electron trajectories on honey-comb ADL and magnetoresistance oscillations Runaway orbit SDHO orbit 2R c = a (Commensurability MR peak orbit) 2nd Unit cell a 1st Unit cell Graphene High B B 260 mt Anti-dots ΔB=(h/e)/( r 2 ) with r = 40 nm S: r for pore radius Antidot radius Graphene nanoribbons Absent zigzag AB effect edges Edge- Localized electrons - Bohr Sommerfeld quantization condition =B r 2 =m(h/e) m: integer Quantization of magnetic flux Like flux quanta in superconductor

37 E = h 2 /2mL 2 (n - Φ/φ ) Aharonov-Bohm-type effect Ensemble average Disappearance Aharonov-Bohm effect Vector potential A ΔB ABT =(h/e)/s 2R c = ( n S ) 1/2 (h/2 )/eb = a ΔB AB = (h/e)/( r 2 ) n S cm -2 l e = 2D/v F 800 nm > 2 (a/2) = 540 nm

38 FFT (arb. units) Contribution of larger unit cell (2nd unit cell) Sample B 0.6 < B < 1 Fourier Spectrum (e) B 2 = 70 mt 1/ B (T -1 ) ΔB ABT =(h/e)/s = 50 mt

39 Contents 1. Introduction Non-lithographic 2.GNRs fabricated by unzipping of carbon nanotubes and 3-stepped annealing Nature Nanotech & Latest Highlights Low defects and 7-times larger energy band gaps 3.Antidot-lattice graphenes fabricated using nanoporous alumina templates as etching masks Anomalous magnetoresistance oscillations (10 layers) PRL Room-temperature Ferromagnetism (Monolyer) Submitted to Nature

40 Magnetization ( emu/100 m 2 ) M(T) [emu] M(T) [emu] M(T) [emu] 6.0e e-5 2.0e e e e-5 4.0e e e e-5 All-carbon Ferromagnetism in ADLG with Hydrogen-terminated edges T = 2K Hydrogen M-H@2K H[Oe] M(T) [emu] T = 300K Hydrogen M-H@40K e-4 5.0e e-5 M-H@4K T = 2K Oxygen e e-5 60 H[Oe] M(T) [emu] -6.0e-4 (a) (b) (c) (d) M(T) [emu] H[Oe] e e e e-5-6.0e-5 M-H@300K (e) T = 300K Oxygen H[Oe] e-4 6.0e e-4 2.0e e e-4 M-H@2K T = 2K No antidots -800 H[Oe] -8.0e M-H@300K -3.0e-5 30 H[Oe] M(T) [emu] e-5-1.0e e e M-H@300K H[Oe] -3.0e-5 Magnetic Field (gauss) (f) T = 300K No antidots Hydrogen Oxygen No antidots

41 Estimation of magnetic moment at edge-carbon atoms Only dangling-bonds at zigzag edges have magnetization Saturation M/one carbon 100 B theory 100-times larger than All carbon atoms within 7nm region from the edges 1.2 B

42 Magnetization ( emu/100 m 2 ) Weak Ferromagnetism in ADL-Graphite with Hydrogen-terminated edges M (T)[emu] 4.0e-5 2.0e Hydrogen T = 2K M-H@2K M(T) [emu] 1.0e-5 5.0e M-H@300K Hydrogen T = 300K e e e H [Oe] H[Oe] -4.0e-5 Magnetic Field (gauss)

43 2D defects array in Graphite and Room-temperature Ferromagnetism Arm chair ZIGZAG Cervenka et al., Nature Physics 5, 840 (2009) Ambiguous system and poor reproducibility

44 Zigzag-edge related Ferromagnetism in Activated carbon Fibers T. Enoki et al., Sol. Stat. Comm. 149, 1144 (2009)

45 Spin polarization and ferromagnetism at zigzag edges with hydrogen termination Kusakabe and Maruyama, Phys. Rev. B 67, (2003) Hydrogen Up spin Down spin

46 Group-theoretical consideration

47 Spin polarization and magnetism of No termination zigzag-edge nanoribbon On one edge first-principles densityfunctional calculations On both edges H. Lee et al.,. Phys. Rev. B 72, (2005)

48 Correlation of Flat band and Spin polarization with Hydrogen termination 1 Hydrogen 2Hydrogen 2 & 1 Hydrogen Ferromagnetic Majority Spin Minority Spin Antiferromagnetic Up Spin Down Spin H. Lee et al.,. Phys. Rev. B 72, (2005)

49 Elimination of Magnetic moment at zigzag edges with Oxygen termination Edge No oxygen Oxygen R.G.A. Veiga, et al., J. Chem. Phys. 128, (2008)

50 Elimination of Magnetic moment by Interlayer coupling in Zigzag-edge graphite with Hydrogen termination Lee, H. et al. Chem.Phys.Lett (2004) No termination AB Stack with no termination

51 Advantage of porous alumina template for formation of low-defect ADLGs Non-lithographic Low damages Hexagonal-shaped ADs placed like honeycomb array Alignment of the same edge structures to each boundary Six ADs and GNRs/one AD Large ensemble of GNRs GNRs If zigzag structure is the most stiff, the advantages give a large volume of zigzag-gnrs and Ferromagnetism. zigzag

52 Contents 1. Introduction Non-lithographic 2.GNRs fabricated by unzipping of carbon nanotubes and 3-stepped annealing Low defects and 7-times larger energy band gaps 3.Antidot-lattice graphenes fabricated using nanoporous alumina templates as etching masks Anomalous magnetoresistance oscillations Room-temperature Ferromagnetism (10 layers) (Monolyer) 4.Future plans: (Quantum ) Spin-Hall effect Controlling edge-spins by electric fields

53 Spin Current & Filter Y-W. Son, S.Louie et al., Nature 444, (2006) E ext E ext = VA -1 J s y = (h/2e)(j y J y )

54 (Quantum) Spin Hall Effect in Graphene QSHE regime Insulating regime Over estimation of SOI?? Kane, C. L. and Mele, E. J.,. Phys.Rev. Lett. 95, (2005)

55 Spin Hall Effect in graphen/graphen junction with hydrogen termination H-C H-C No SO Interaction sp 3 SOI M.Schmidt & D.Loss, Phys. Rev. B 81, (2010)

56 Spin Hall Effect in graphen/graphen junction with hydrogen termination SO Interaction Edge Bulk M.Schmidt & D.Loss, Phys. Rev. B 81, (2010)

57 Conclusions Non-lithographic 1.GNRs fabricated by unzipping of carbon nanotubes and 3-stepped annealing Low defects and 7-times larger energy band gaps 2.Antidot-lattice graphenes fabricated using nanoporous alumina templates as etching masks Anomalous magnetoresistance oscillations Room-temperature Ferromagnetism 3.Future plans: (Quantum) Spin-Hall effect Controlling edge-spins by electric fields

58 So many thanks to MIT: Millie Dresselhaus Colombia University: Philip Kim Rice University: James Tour Tokyo Institute of Technology: T.Ando, T.Enoki Tokyo University: S.Tarucha, M.Yamamoto, H.Fukuyama, T.Matsui, H. Aoki Tokyo University, ISSP: Y.Iye, S.Katsumoto, T.Otsuka AIST: K. Suenaga My students and staff Japan Science and Technology Agency:CREST Hidetoshi Fukuyama Jun Akimitsu