Diapositiva 1

Report
Electronic Coupling and Edge Effects in
Graphene Nanoislands grown on Co(0001)
Deborah Prezzi
Research Center S3 on nanoStructures and
bioSystems at Surfaces
CNR – Nanoscience Institute
Modena, Italy
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)
Graphene islands on Co(0001)
From contorted hexabenzocoronene (HBC) to graphene...
0.03V
0.03V
6 nm
Thermal
annealing
2 nm
2 nm
Deposition of carbon-based molecular
precursors on clean Co(0001)
In situ thermal annealing at  600 K
Graphene nanoislands
( 1-10 nm)
Different shapes
Well-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)
STM measurements at the edges
•Mainly triangular (60) and hexagonal (120)
 Growth along preferential direction
120
• Zigzag edges in all cases
60
•STS tunneling spectra: edge-localized state
at about -150 mV
- 151 mV
dI/dV
1 nm
2 nm
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
Sample bias (V)
0.2
0.3
0.4
- 160 mV
STM measurements at the edges
•Mainly triangular (60) and hexagonal (120)
 Growth along preferential direction
120
• Zigzag edges in all cases
•STS tunneling spectra: edge-localized state
at about -150 mV
60
- 151 mV
1 nm
2 nm
- 160 mV
DFT calculations
Prototype systems: graphene nanoribbons
Armchair
Zigzag
H
• Periodic boundary conditions
• Plane-wave basis set
• LSDA approximation
• 4-layer Co slab
• Passivated and non-pass ribbons
C
Co 1st layer
Co 2nd layer
P. Giannozzi et al. J. Phys. Condens.
Matter 21, 395502 (2009).
Edge stability
Edge formation energy
Isolated graphene nanoribbons:
 Armchair edges are more stable
See: Wassman et al., PRL 101, 096402 (2008)
Edge stabilization on Co(0001):
 Zigzag edges are more stable
H:Co
C
Co 1st layer
Co 2nd layer
Zigzag
Armchair
Magnetic properties: zigzag edge
Spin polarization ρ(↑) - ρ(↓)
top view
Isolated zigzag graphene nanoribbons
 Magnetic ordering with AF ground
state
side view
See: Son et al, PRL (2006); Pisani et al., PRB (2007)

Zigzag graphene nanoribbons on Co(0001)
 Strong suppression of edge-related features
with H
w/o H
Edge-localized states
Edge Top
Edge Hollow
Other on-going activities
Edge stability
Daejinand
Eom,
magnetic
Mark S. properties
Hybertsen,
ofTony
graphene
F. Heinz,
islands
George
on Co(0001)
W. Flynn
2 nm
Other on-going activities
Spin injectionAndrea
and transport
Ferretti
at the graphene/Co
Mark S. Hybertsen
interface
Gr:Co
Gr
Gr:Co
L
C
R
Daniele properties:
Optical
Varsano, Caterina
edge modulation
Cocchi,
and functionalization
Alice
Ruini, Elisa Molinari
Designing
band-offset
Caterina Cocchi,
Alice
by
chemical
Ruini,
Marilia Caldas,
functionalization
Elisa Molinari
Back-up slides
STM Measurements: Registry
On top
AB
AC
Hollow
BC
C
Co 1st layer
Co 2nd layer
30 meV/atom
deq = 2.07 Å
deq = 3.48 Å
DFT–LSDA calculations
• Periodic boundary conditions
• Plane-wave basis set
• Slab geometry
P. Giannozzi et al. J. Phys. Condens.
Matter 21, 395502 (2009).
2nm
2.5
V=-400 mV
2
Height (A)
130 meV/atom
1.5
1
0.5
V=-3 mV
0
-0.5
0
1
2
3
4
5
6
Lateral position (nm)
7
8
STM Measurements: Tunneling Conductance
Clean Co(0001)
1 nm
Differential conductance spectra
2nm
Graphene:Co(0001)
Electronic properties from DFT calculations
Band structure (AC):
UP
Strong coupling with
the substrate
 Disruption of the graphene
p-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)
Electronic properties from DFT calculations
Band structure (AC):
UP
K point:
C
A
DW
Hybridization scheme
Electronic properties from DFT calculations
P3
P1
P2
Normalized conductance
Tunneling conductance:
P3
P2
P1
-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)
- 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 zonecenter states (*)
(*) Y. Zhang et al., Nature Phys. 4, 627 (2008); T. O. Wehling et al., Phys. Rev. Lett. 101, 216803 (2008).
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)
Edge functionalization (II)
Exploring the effects of edge functionalization with different organic
groups:
Edge functionalization (III)
Exploring the effects of edge functionalization with different organic
groups:
 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
Concentration and width dependence
 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
Designing type-II graphene nanojunctions
 Results on functionalized GNFs suggest the possibility to realize type-I
or type-II graphene nanojunctions with tunable DEA and DIP
 -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
Edge stability and magnetic properties
of graphene edges on Co(0001)
2 nm
Designing band-offset by
chemical functionalization
Optical properties: edge modulation
and functionalization
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 BetheSalpeter 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
Prototype system
Egap = 3.8 eV
Single particle localized states
HOMO
LUMO
D. Prezzi, D. Varsano, A. Ruini, E. Molinari, submitted (2010)
2.8 eV
Egap = 1.0 eV
Optically active graphene QDs
Optical response
Egap = 3.8 eV
2.8 eV
Egap = 1.0 eV
Eb
Large binding energy
enhanced by the
confinement potential
A7;8
h
Wannier-like exciton
localized in the width
modulation (dot)
Optically active graphene QDs
Optical response
Egap = 3.8 eV
2.8 eV
Egap = 1.0 eV
Eb
Large binding energy
enhanced by the
confinement potential
Wannier-like exciton
localized in the width
modulation (dot)
Dark excitations
Optical response
Egap = 3.8 eV
2.8 eV
Egap = 1.0 eV
Eb
a)
b)
c)
h
Dark states with
different localization
properties
Optical excitations in graphene nanojunctions
-H
-COCH3
Single-particle states
Optical excitations in graphene nanojunctions
-H
-COCH3
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)
Optical excitations in graphene nanojunctions
-H
-COCH3
|e|2
| h|2
Weighted transitions
Gives information about the spatial localization of the excitation
Optical excitations in graphene nanojunctions
-H
-COCH3
|e|2
|h|2
-NH2
-F
|e|2
|h|2

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