electronic coupling and edge effects in graphene nanoislands grown on co(0001)
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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