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Electronic Coupling and Edge Effects in Graphene Nanoislands grown on Co(0001)

Electronic Coupling and Edge Effects in Graphene Nanoislands grown on Co(0001). Deborah Prezzi. Research Center S3 on nano S tructures and bio S ystems at S urfaces CNR – Nanoscience Institute Modena, Italy. Graphene :Co(0001) – Motivation.

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Electronic Coupling and Edge Effects in Graphene Nanoislands grown on Co(0001)

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  1. Electronic Coupling and Edge Effects in GrapheneNanoislands grown on Co(0001) Deborah Prezzi Research Center S3 on nanoStructures and bioSystems at Surfaces CNR – Nanoscience Institute Modena, Italy

  2. Graphene:Co(0001) – Motivation Epitaxial growth of graphene lattice mismatch < 2%  no superstructures Graphene:Ru(0001) (Da 10%) Graphene:Ir(111) (Da 11%) 25x25 supercell Martoccia et al, PRL 101, 126102 (2008) N’Diaye et al, PRL 97, 215501 (2006) Spintronics application  spin injection from FM contact Tombros et al, Nature 448, 571 (2007)

  3. Grapheneislands on Co(0001) 0.03V 0.03V 2 nm 2 nm 6 nm From contorted hexabenzocoronene (HBC) to graphene... Thermal annealing Graphene nanoislands ( 1-10 nm) Different shapes Well-ordered edges Deposition of carbon-based molecular precursors on clean Co(0001) In situ thermal annealing at  600 K D. Eom, D. Prezzi, K. T. Rim, H. Zhou, M. Lefenfeld, S. Xiao, C. Nuckolls, M. S. Hybertsen, T. F. Heinz, and G. W. Flynn, Nano Letters9, 2844 (2009)

  4. STM measurements at the edges - 151 mV 1 nm 2 nm - 160 mV • Mainly triangular(60) and hexagonal(120) Growth along preferential direction • Zigzagedges in all cases • STS tunneling spectra: edge-localizedstateat about-150 mV 120 60

  5. STM measurements at the edges - 151 mV 1 nm 2 nm - 160 mV • Mainly triangular(60) and hexagonal(120) Growth along preferential direction • Zigzagedges in all cases • STS tunneling spectra: edge-localizedstateat about-150 mV 120 60

  6. DFT calculations Prototype systems: graphene nanoribbons Armchair Zigzag • Periodic boundary conditions • Plane-wave basis set • LSDA approximation • 4-layer Co slab • Passivated and non-pass ribbons P. Giannozzi et al. J. Phys. Condens. Matter 21, 395502 (2009). H C Co 2nd layer Co 1st layer

  7. Edge stability Edge stabilization on Co(0001): Zigzag edges are more stable Zigzag Armchair Edge formation energy Isolated graphene nanoribbons: Armchair edges are more stable See: Wassman et al., PRL 101, 096402 (2008) H:Co C Co 1st layer Co 2nd layer

  8. Magnetic properties: zigzag edge top view side view  Zigzag graphene nanoribbons on Co(0001)  Strong suppression of edge-related features with H w/o H Spin polarization ρ(↑) - ρ(↓) Isolated zigzag graphene nanoribbons Magnetic ordering with AF ground state See: Son et al, PRL (2006); Pisani et al., PRB (2007)

  9. Edge-localized states Edge Top Edge Hollow

  10. Other on-going activities 2 nm Edge stability and magnetic properties of graphene islands on Co(0001) Daejin Eom, Mark S. Hybertsen, Tony F. Heinz, George W. Flynn

  11. Other on-going activities Spin injection and transport at the graphene/Co interface Andrea Ferretti Mark S. Hybertsen Gr:Co Gr Gr:Co L C R Daniele Varsano, Caterina Cocchi, Alice Ruini, Elisa Molinari Designing band-offset by chemical functionalization Optical properties: edge modulation and functionalization Caterina Cocchi, Alice Ruini, Marilia Caldas, Elisa Molinari

  12. Back-up slides

  13. STM Measurements: Registry On top Hollow AB AC BC C Co 1st layer Co 2nd layer 130 meV/atom deq = 2.07 Å 30 meV/atom deq = 3.48 Å V=-400 mV • DFT–LSDA calculations • Periodicboundaryconditions • Plane-wavebasis set • Slabgeometry V=-3 mV 2nm P. Giannozziet al. J. Phys. Condens. Matter 21, 395502 (2009).

  14. STM Measurements: TunnelingConductance Clean Co(0001) 1 nm Differential conductance spectra 2nm Graphene:Co(0001)

  15. Electronic properties from DFT calculations Band structure (AC): UP Strong coupling with the substrate Disruption of the graphenep-bands Effective n-doping  Rigid downshift of s-bands of about 1.1 eV gray lines: majority-spin bands red dots: projection on C shaded area: bulk Co(0001) black lines: ideal graphene (-1.1 eV) Karpan et al., PRL 99, 176602 (2007); Giovannetti et al., PRL 101, 026803 (2008); Varykhalov et al., PRL 101, 157601 (2008); Rader et al., PRL 102, 057602 (2009); Varyakhalov and Rader, PRB 80, 035437 (2009)

  16. Electronic properties from DFT calculations Band structure (AC): K point: C A UP DW Hybridization scheme

  17. Electronic properties from DFT calculations Tunneling conductance: P3 P3 P2 P1 P1 P2 • Projected density of states (pDOS) onto • the carbon pz orbitals  LDOS near the • surface  major contribution from the edge • region of the BZ • LDOS far from the surface (4 Å) •  featureless Mechanism which mixes zone-edge and zone-center states (*) (*) Y. Zhang et al., Nature Phys. 4, 627 (2008); T. O. Wehling et al., Phys. Rev. Lett. 101, 216803 (2008).

  18. Edgefunctionalization (I) Exploring the effects of edge functionalization with different organic groups:  Sub-nm wide graphene nano-flakes (GNFs) as prototypical systems  Hartree-Fock based semiempirical method (*)to evaluate: - ground state properties - electron affinity: EA = E0 – E-1 - ionization potential: IP = E+1 - E0 (*) Further information on AM1 parametrization: M. J. S. Dewar et al., J. Am. Chem. Soc. 107, 3902 (1985)

  19. Edgefunctionalization (II) Exploring the effects of edge functionalization with different organic groups:

  20. Edgefunctionalization (III) Exploring the effects of edge functionalization with different organic groups: • Decrease of the energy gap EGcorresponding increase of the effective width • Up- (down-) shift of the EA and IP in presence of electron-donating (-withdrawing) functional group

  21. Concentration and widthdependence  IP increases almost linearly with the number of functional groups  Family behaviour of the energy gap also for functionalized flakes • EG shows 1/w behaviour •  DIP and DEA show faster decay compatible with a local dipole mechanism

  22. Designingtype-IIgraphenenanojunctions • Results on functionalized GNFs suggest the possibility to realize type-I or type-II graphene nanojunctions with tunable DEA andDIP •  -H / -COCH3: frontier orbitals localized on the two sides of the junction indicating a type-II level alignment C. Cocchi, A. Ruini, D. Prezzi, M.J. Caldas, and E. Molinari, (hopefully) J. Phys. Chem. C (2010)

  23. Outline 2 nm Edge stability and magnetic properties of graphene edges on Co(0001) Optical properties: edge modulation and functionalization Designing band-offset bychemicalfunctionalization

  24. Opticalproperties: edgemodulation and functionalization

  25. Opticalproperties: edgemodulation and functionalization • Ab initio Many-Body Perturbation Theory scheme: • Self-energy correction to the band structure in the GW approximation •  Solution for the Bethe-Salpeter equation for the inclusion of excitonic effects • Semiempirical Configuration Interaction approach: • ZINDO/1: ground state properties • ZINDO/S: optical excitations

  26. Opticalexcitations in widthmodulatedGNRs Prototype system Egap = 3.8 eV 2.8 eV Egap = 1.0 eV Single particle localized states HOMO LUMO D. Prezzi, D. Varsano, A. Ruini, E. Molinari, submitted (2010)

  27. OpticallyactivegrapheneQDs Optical response Egap = 3.8 eV 2.8 eV Egap = 1.0 eV Large binding energy enhanced by the confinement potential A7;8 Wannier-like exciton localized in the width modulation (dot) h Eb

  28. OpticallyactivegrapheneQDs Optical response Egap = 3.8 eV 2.8 eV Egap = 1.0 eV Large binding energy enhanced by the confinement potential Wannier-like exciton localized in the width modulation (dot) Eb

  29. Dark excitations Optical response Egap = 3.8 eV 2.8 eV Egap = 1.0 eV h a) Dark states with different localization properties b) c) Eb

  30. Opticalexcitations in graphenenanojunctions -COCH3 -H Single-particle states

  31. Opticalexcitations in graphenenanojunctions -COCH3 -H Optical response Both from localized and resonant states  Need to find a way to visualize the excited state C. Cocchi, D. Prezzi, A. Ruini, M. J. Caldas, E. Molinari, in preparation (2010)

  32. Opticalexcitations in graphenenanojunctions -COCH3 -H |e|2 | h|2 Weighted transitions Gives information about the spatiallocalizationof the excitation

  33. Opticalexcitations in graphenenanojunctions -H -COCH3 |e|2 |h|2 -NH2 -F |e|2 |h|2

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