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

Deborah PrezziDeborah Prezzi

Research Center S3 on nanoStructures and bioSystems at SurfacesCNR – Nanoscience InstituteModena, Italy

Graphene:Co(0001) – Motivation

Epitaxial growth of graphene lattice mismatch < 2% no superstructures

Graphene:Ir(111) (a 11%)

N’Diaye et al, PRL 97, 215501 (2006)

25x25supercell

Graphene:Ru(0001) (a 10%)

Martoccia et al, PRL 101, 126102 (2008)

Spintronics application spin injection from FM contact

Tombros et al, Nature 448, 571 (2007)

Graphene islands on Co(0001)

6 nm

0.03V

0.03V

2 nm

From contorted hexabenzocoronene (HBC) to graphene...

2 nm

Thermal annealing

Deposition of carbon-based molecular precursors on clean Co(0001)

In situ thermal annealing at 600 K

Graphene nanoislands( 1-10 nm)

Different shapesWell-ordered edges

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 Letters 9, 2844 (2009)

1 nm

2 nm

•Mainly triangular (60) and hexagonal (120) Growth along preferential direction

• Zigzag edges in all cases

•STS tunneling spectra: edge-localized state at about -150 mV

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4

Sample bias (V)

dI/

dV

- 160 mV

- 151 mV

120

60

STM measurements at the edges

1 nm

2 nm

•Mainly triangular (60) and hexagonal (120) Growth along preferential direction

• Zigzag edges in all cases

•STS tunneling spectra: edge-localized state at about -150 mV

- 160 mV

- 151 mV

120

60

STM measurements at the edges

Prototype systems: graphene nanoribbons

Armchair Zigzag

C Co 1st layer Co 2nd layer H

• Periodic boundary conditions • Plane-wave basis set• LSDA approximation• 4-layer Co slab• Passivated and non-pass ribbons

DFT calculations

P. Giannozzi et al. J. Phys. Condens. Matter 21, 395502 (2009).

Edge stability

Isolated graphene nanoribbons:

Armchair edges are more stableSee: Wassman et al., PRL 101, 096402 (2008)

Zigzag Armchair

Edge stabilization on Co(0001):

Zigzag edges are more stable

C Co 1st layer Co 2nd layer

H:Co

Edge formation energy

Magnetic properties: zigzag edge

Spin polarization ρ(↑) - ρ(↓)

Isolated zigzag graphene nanoribbons

Magnetic ordering with AF ground stateSee: Son et al, PRL (2006); Pisani et al., PRB (2007)

top view

side view

with H w/o H

Zigzag graphene nanoribbons on Co(0001)

Strong suppression of edge-related features

Edge-localized states

Edge Top Edge Hollow

2 nm

Edge stability and magnetic properties of graphene islands on Co(0001)

Other on-going activities

Daejin Eom, Mark S. Hybertsen,Tony F. Heinz, George W. Flynn

Spin injection and transportat the graphene/Co interface

Andrea Ferretti Mark S. Hybertsen

Other on-going activities

L C R

Gr:Co Gr Gr:Co

Designing band-offset by chemical functionalization

Caterina Cocchi, Alice Ruini, Marilia Caldas, Elisa Molinari

Optical properties: edge modulation and functionalizationDaniele Varsano, Caterina Cocchi, Alice Ruini, Elisa Molinari

Back-up slides

STM Measurements: Registry

AB AC BC

130 meV/atom

deq = 2.07 Å

30 meV/atom

deq = 3.48 Å

DFT–LSDA calculations• Periodic boundary conditions • Plane-wave basis set• Slab geometry

C

Co 1st layer

Co 2nd layer

On top Hollow

P. Giannozzi et al. J. Phys. Condens. Matter 21, 395502 (2009).

2nm

-0.5

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6 7 8

Lateral position (nm)

He

igh

t (A

)

V=-3 mV

V=-400 mV

STM Measurements: Tunneling Conductance

2nm

1 nm

Clean Co(0001)

Graphene:Co(0001)

Differential conductance spectra

Electronic properties from DFT calculations

Band structure (AC):

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)

Strong coupling with the substrate Disruption of the graphene -bands

Effective n-doping Rigid downshift of -bands of about 1.1 eV

UP

gray lines: majority-spin bands red dots: projection on C shaded area: bulk Co(0001)black lines: ideal graphene (-1.1 eV)

Electronic properties from DFT calculations

Band structure (AC): UP

DW

Hybridization scheme

K point:

C A

Electronic properties from DFT calculations

Tunneling conductance:

(*) Y. Zhang et al., Nature Phys. 4, 627 (2008); T. O. Wehling et al., Phys. Rev. Lett. 101, 216803 (2008).

P1 P2

P3

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Sample bias (V)

No

rmal

ized

co

nd

uct

ance

P1

P2

P3

- 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 (*)

Edge functionalization (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)

Exploring the effects of edge functionalization with different organic groups:

Edge functionalization (II)

Exploring the effects of edge functionalization with different organic groups:

Edge functionalization (III)

Decrease of the energy gap EG corresponding increase of the effective width

Up- (down-) shift of the EA and IP in presence of electron-donating

(-withdrawing) functional group

IP increases almost linearly with the number of functional groups

Family behaviour of the energy gap also for functionalized flakes

Concentration and width dependence

EG shows 1/w behaviour

IP and EA show faster decay compatible with a local dipole mechanism

Designing type-II graphene nanojunctions

Results on functionalized GNFs suggest the possibility to realize type-I or type-II graphene nanojunctions with tunable EA and IP

-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)

Outline

Optical properties: edge modulation and functionalization

2 nm

Edge stability and magnetic properties of graphene edges on Co(0001)

Optical properties: edge modulation and functionalization

Optical properties: edge modulation 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

Optical excitations in width modulated GNRs

Single particle localized states

LUMOHOMO

Prototype system

Egap = 3.8 eV Egap = 1.0 eV 2.8 eV

D. Prezzi, D. Varsano, A. Ruini, E. Molinari, submitted (2010)

Optical response

Wannier-like exciton localized in the width

modulation (dot)

Large binding energy enhanced by the

confinement potential

Egap = 3.8 eV Egap = 1.0 eV 2.8 eV

A7;8

h

Optically active graphene QDs

Optically active graphene QDs

Optical response

Wannier-like exciton localized in the width

modulation (dot)

Large binding energy enhanced by the

confinement potential

Egap = 3.8 eV Egap = 1.0 eV 2.8 eV

Dark excitations

Optical response

Dark states with different localization

properties

Egap = 3.8 eV Egap = 1.0 eV 2.8 eV

a)

b)

c)

h

Optical excitations in graphene nanojunctions

-H -COCH3

Single-particle states

Optical excitations in graphene nanojunctions

C. Cocchi, D. Prezzi, A. Ruini, M. J. Caldas, E. Molinari, in preparation (2010)

-H -COCH3

Optical response

Both from localized and resonant states

Need to find a way to visualize the excited state

-H -COCH3

Optical excitations in graphene nanojunctions

Gives information about the spatial localization of the excitation

|e|2

| h|2

Weighted transitions

|e|2

|h|2

|e|2

|h|2

Optical excitations in graphene nanojunctions

-H -COCH3

-NH2 -F