electron energy loss spectra of graphite, graphene and...
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Electron Energy Loss Spectra of Graphite,Graphene and Carbon Nanotubes: PlasmonDispersion and Crystal Local Field Effects.
Ralf Hambach1, C. Giorgetti1, X. Lopez1, F. Sottile1, F. Bechstedt2,C. Kramberger3 , T. Pichler3, N. Hiraoka4, Y.Q. Cai4, L. Reining1.
1 LSI, Ecole Polytechnique, CNRS-CEA/DSM, Palaiseau, France2 IFTO, Friedrich-Schiller-Universitat Jena, Germany
3 University of Vienna, Faculty of Physics, Vienna, Austria4 National Synchrotron Radiation Research Center, Hsinchu, Taiwan
20. 11. 2008 — MORE08 Vienna/Austria
R. Hambach Collective Excitations in Carbon Systems
What do we want to describe?
collective excitationsI Coulomb interactionI probed by EELS, IXS
k0, E0 k0−
q, E0−
hω
q
contributions to the spectra
I elastic scattering → crystal structureI inelastic scattering → collective excitations
Key quantity: S(q, ω) ∝ −q2={ε−1(q, ω)}
R. Hambach Collective Excitations in Carbon Systems
How do we calculate ε−1?
ab initio calculations
1. ground state calculation gives φKSi
2. independent-particle polarisability χ0
3. RPA full polarisability χ = χ0 + χ0vχ
4. dielectric function ε−1 = 1 + vχ
(ε−1: no retardation, no multiple scattering,no Bragg reflection of the incident electron)
χ0
Codes:ABINIT: X. Gonze et al., Comp. Mat. Sci. 25, 478 (2002)DP-code: www.dp-code.org; V. Olevano, et al., unpublished.
R. Hambach Collective Excitations in Carbon Systems
Self-Consistent Hartree Potentials
long range v0
I difference between EELS and absorptionI vanishes for large qI vanishes for localised systems
short range v
I crystal local field effects in solidsI depolarisation in finite systems
R. Hambach Collective Excitations in Carbon Systems
Outlook
dimensionality
1. induced Hartree potentials in low dimensional systems⇒ linear plasmon dispersion in SWCNT + Graphene
2. assembling nanoobjects⇒ role of interaction
inhomogenitiy
3. crystal local field effects in solids⇒ enhanced anisotropy in Graphite
R. Hambach Collective Excitations in Carbon Systems
Vertical Aligned SWNT
EEL spectraspecimen
I oriented SWCNTI diameter: 2 nmI nearly isolated
spectroscopy
I angular-res. EELSI resolution:
∆E = 0.2 eV∆q = 0.05 A−1
[ C. Kramberger, M.Rummeli, M. Knupfer, J. Fink, B.Buchner,T. Pichler, IFW Dresden, Germany ]
R. Hambach Collective Excitations in Carbon Systems
Vertical Aligned SWNT
EEL spectra
π plasmon at 9eV in Graphite
[ C. Kramberger, M.Rummeli, M. Knupfer, J. Fink, B.Buchner,T. Pichler, IFW Dresden, Germany ]
R. Hambach Collective Excitations in Carbon Systems
Vertical Aligned SWNT
EEL spectra
0 0.2 0.4 0.6 0.84
5
6
7
8
9
(a) Experiment
0momentum transfer q (1/Å)
4en
ergy
loss
(eV
)
[ C. Kramberger, M.Rummeli, M. Knupfer, J. Fink, B.Buchner,T. Pichler, IFW Dresden, Germany ]
R. Hambach Collective Excitations in Carbon Systems
Vertical Aligned SWNT
I linear π plasmondispersion
I SWNT ⇔ graphene
0 0.2 0.4 0.6 0.84
5
6
7
8
9
(a) Experiment
0momentum transfer q (1/Å)
4en
ergy
loss
(eV
)
[ C. Kramberger, M.Rummeli, M. Knupfer, J. Fink, B.Buchner,T. Pichler, IFW Dresden, Germany ]
R. Hambach Collective Excitations in Carbon Systems
ε−1 in isolated nanosystems
IPA: ={ε−1} ∝ ={χ0}→ sum of interband transitions
RPA: ={ε−1} ∝ ={χ}full susceptibility χ = χ0(1− vχ0)−1
contains self-consistent response→ mixing of interband transitions
χ0
R. Hambach Collective Excitations in Carbon Systems
IPA: independent particles
energy loss in graphene(in-plane, q = 0.41 A−1)
0 2 4 6 8 10energy loss (eV)
- Im
ε-1
(a
rb. u
.)
IPA=⇒ given by ={χ0}:interpretation in terms ofband-transitions
χ0
R. Hambach Collective Excitations in Carbon Systems
IPA: independent particles
energy loss in graphene(in-plane, q = 0.41 A−1)
0 2 4 6 8 10energy loss (eV)
- Im
ε-1
(a
rb. u
.)
IPAπ−π∗ at K
bandstructure
M K Γ M-20
-15
-10
-5
0
5
Ene
rgie
(eV
)
π∗
π
σ
σ∗
R. Hambach Collective Excitations in Carbon Systems
RPA: random phase approximation
energy loss in graphene(in-plane, q = 0.41 A−1)
0 2 4 6 8 10energy loss (eV)
- Im
ε-1
(a
rb. u
.)
IPAπ−π∗ at KRPA
I given by ={χ}:no interpretation byband-transitions
I contributions from KI mixing of dispersion
R. Hambach Collective Excitations in Carbon Systems
RPA: random phase approximation
energy loss in graphene(in-plane, q = 0.41 A−1)
0 2 4 6 8 10energy loss (eV)
- Im
ε-1
(a
rb. u
.)
IPAπ−π∗ at KRPAwithout "K"
I given by ={χ}:no interpretation byband-transitions
I contributions from KI mixing of dispersion
R. Hambach Collective Excitations in Carbon Systems
Plasmon dispersion
01-Im(1/ε) [arb. u.]
0
2
4
6
8
10
Ene
rgy
loss
[eV
]
0 0.2 0.4 0.6 0.8 1momentum transfer q (1/Å)
0
2
4
6
8
10
π-pl
asm
on p
ositi
on -
Ene
rgy
(eV
)
RPAIPA
q=0.408 1/Å
1P. Longe, and S. M. Bose, Phys. Rev. B 48, 18239 (1993)2F. L. Shyu and M. F. Lin, Phys. Rev. B 62,8508 (2000)
R. Hambach Collective Excitations in Carbon Systems
SWCNT vs. Graphene
0 0.2 0.4 0.6 0.84
5
6
7
8
9
VASWCNT
(a) Experiment
0 0.2 0.4 0.6 0.8 1
(b) Calculation
0momentum transfer q (1/Å)
4
ener
gy lo
ss (
eV)
R. Hambach Collective Excitations in Carbon Systems
SWCNT vs. Graphene
0 0.2 0.4 0.6 0.84
5
6
7
8
9
VASWCNT
(a) Experiment
0 0.2 0.4 0.6 0.8 1
graphene-1L
(b) Calculation
0momentum transfer q (1/Å)
4
ener
gy lo
ss (
eV)
R. Hambach Collective Excitations in Carbon Systems
Role of Interactions
0 0.2 0.4 0.6 0.84
5
6
7
8
9
VASWCNTbulk-SWCNT
(a) Experiment
0 0.2 0.4 0.6 0.8 1
graphene-1L
(b) Calculation
0momentum transfer q (1/Å)
4
ener
gy lo
ss (
eV)
R. Hambach Collective Excitations in Carbon Systems
Role of Interactions
0 0.2 0.4 0.6 0.84
5
6
7
8
9
VASWCNTbulk-SWCNTgraphite
(a) Experiment
0 0.2 0.4 0.6 0.8 1
graphene-1Lgraphite
(b) Calculation
0momentum transfer q (1/Å)
4
ener
gy lo
ss (
eV)
R. Hambach Collective Excitations in Carbon Systems
Role of Interactions
0 0.2 0.4 0.6 0.84
5
6
7
8
9
VASWCNTbulk-SWCNTgraphite
(a) Experiment
0 0.2 0.4 0.6 0.8 1
graphene-1Lgraphene-2Lgraphite
(b) Calculation
0momentum transfer q (1/Å)
4
ener
gy lo
ss (
eV)
R. Hambach Collective Excitations in Carbon Systems
Conclusions
SWCNT ⇐⇒ graphene
I isolated SWCNT ⇐⇒ graphene-1LI bundled SWCNT ⇐⇒ graphene-2L
graphene
I induced Hartree potentials importantI picture of independent transitionsI mixing of transitions/dispersions→ leads to linear dispersion
0 0.2 0.4 0.6 0.84
5
6
7
8
9
VASWCNTbulk-SWCNTgraphite
(a) Experiment
0 0.2 0.4 0.6 0.8 1
graphene-1Lgraphene-2Lgraphite
(b) Calculation
0momentum transfer q (1/Å)
4
ener
gy lo
ss (
eV)
0 2 4 6 8 10energy loss (eV)
- Im
ε-1
(a
rb. u
.)
IPAπ−π∗ at KRPAwithout "K"
R. Hambach Collective Excitations in Carbon Systems
(3,3) Single Wall Carbon nanotube
Experiment: oriented SWCNT(Diameter 20 A, nearly isolated)
Calculation: (3,3) SWCNT(Diameter 4 A, low interaction)
R. Hambach Collective Excitations in Carbon Systems
Graphitecalculation: revealing an angular anomalyexplication: in terms of crystal local field effectsexperiment: verification by inelastic x-ray scattering
R. Hambach Collective Excitations in Carbon Systems
On-axis: continuous
inelastic elastic
2 4 6 8 10energy loss (eV)
0
1
2
3
4
5
S(q
,ω)
(1/ keV
)
3
8/3
7/3
2
5/3
4/3
1
2/3
1/3
0
q1=0 q
3
c-a
xis
I energy loss S(q, ω)in graphite (AB)
I q along c-axisI weak dispersion1
I and off-axis?
1Y. Q. Cai et. al., Phys. Rev. Lett. 97, 176402 (2006)R. Hambach Collective Excitations in Carbon Systems
On-axis: continuous
inelastic elastic
2 4 6 8 10energy loss (eV)
0
1
2
3
4
5
S(q
,ω)
(1/ keV
)
3
8/3
7/3
2
5/3
4/3
1
2/3
1/3
0
q1=0 q
3
c-a
xis
I energy loss S(q, ω)in graphite (AB)
I q along c-axisI weak dispersion1
I and off-axis?
1Y. Q. Cai et. al., Phys. Rev. Lett. 97, 176402 (2006)R. Hambach Collective Excitations in Carbon Systems
Off-axis: discontinuous!
inelastic elastic
2 4 6 8 10energy loss (eV)
0
1
2
3
4
5
S(q
,ω)
(1/ k
eV)
3
8/3
7/3
2
5/3
4/3
1
2/3
1/3
0
q1=0 q3
c-a
xis
2 4 6 8 10energy loss (eV)
0
1
2
3
4
5
S(q
,ω)
(1/ k
eV)
3
8/3
7/3
2
5/3
4/3
1
2/3
1/3
0
q1=1/8 (~0.37 1/Å) q3
R. Hambach Collective Excitations in Carbon Systems
What is the origin?
RPA with full v RPA with v = 0
2 4 6 8 10energy loss (eV)
0
1
2
3
4
5
S(q
,ω)
(1/ k
eV)
3
8/3
7/3
2
5/3
4/3
1
2/3
1/3
0
q1=1/8 (with LFE)
2 4 6 8 10energy loss (eV)
3
8/3
7/3
2
5/3
4/3
1
2/3
1/3
0
q1=1/8 (without LFE)q3q3
R. Hambach Collective Excitations in Carbon Systems
Crystal Local Field Effects
dielectric function in crystals
I recall: ε−1 = 1 + vχ, χ = χ0 + χ0vχ
I RPA: ε = 1− vχ0
I ε is a matrix: ε(q, q′;ω) =(εGG′(qr , ω)
)I energy loss function (EELS, IXS)
S(q, ω) ∝ −={ε−1GG(qr , ω)}, q = qr+G
⇒ mixing of all transitions in χ0
⇒ crystal local field effects (LFE)
χ0
R. Hambach Collective Excitations in Carbon Systems
Physical picture of LFE: Dipoles
Two ways of understanding crystal local field effects:
dipole pictureperturbation induced dipoles
ϕt sc ++ϕikk
I induced local fieldsI crystal structure important
R. Hambach Collective Excitations in Carbon Systems
Physical picture of LFE: Coupled modes
plane wave pictureperturbing mode induced mode
eiq · r --ei(q+G) · rscll
I Bragg-reflection inside the crystalI couples modes with same qr
I εGG′(qr ) describes coupling δϕi (G)δϕt (G′)
⇒ key for understanding the discontinuity
R. Hambach Collective Excitations in Carbon Systems
Simple 2×2 model for LFE
I dominant coupling between the twomodes 0 and G = (0, 0, 2)
0BBBB@ε00 ... ε0G ......
...εG0 ... εGG ......
...
1CCCCA −→(
ε00 ε0GεG0 εGG
)
we introduce an effective 2×2-matrix ε
I Remember: the loss function was
S(q, ω) ∝ −={ε−1GG(qr , ω)}
R. Hambach Collective Excitations in Carbon Systems
Simple 2×2 model for LFE
I inverting the effective 2×2-matrix ε
ε−1GG(qr , ω) = 1
εGG+ εG0ε0G
(εGG)2 ε−100 (qr , ω)
without LF correction ...
(known as two plasmon-band model2)
2L. E. Oliveira, K. Sturm, Phys. Rev. B 22, 6283 (1980).R. Hambach Collective Excitations in Carbon Systems
1. Recurring excitations
2 4 6 8 10energy loss (eV)
0
1
2
3
4
5
S(q
,ω)
(1/ k
eV)
3
8/3
7/3
2
5/3
4/3
1
2/3
1/3
0
q1=1/8 (~0.37 1/Å) q3
ε−1GG(qr , ω) = 1
εGG+ εG0ε0G
(εGG)2 ε−100 (qr , ω)
S(qr +G) = SNLF(qr +G) + f ·S(qr )
coupling of excitations of momentumqr 1. Brillouin zone
qr + G higher Brillouin zone
⇒ reappearance3 of the anisotropicspectra from q → 0
3K. Sturm, W. Schulke, J. R. Schmitz, Phys. Rev. Lett. 68, 228 (1992).R. Hambach Collective Excitations in Carbon Systems
1. Recurring excitations
2 4 6 8 10energy loss (eV)
0
1
2
3
4
5
S(q
,ω)
(1/ k
eV)
3
8/3
7/3
2
5/3
4/3
1
2/3
1/3
0
q1=1/8 (~0.37 1/Å) q3
ε−1GG(qr , ω) = 1
εGG+ εG0ε0G
(εGG)2 ε−100 (qr , ω)
S(qr +G) = SNLF(qr +G) + f ·S(qr )
coupling of excitations of momentumqr 1. Brillouin zone
qr + G higher Brillouin zone
⇒ reappearance3 of the anisotropicspectra from q → 0
3K. Sturm, W. Schulke, J. R. Schmitz, Phys. Rev. Lett. 68, 228 (1992).R. Hambach Collective Excitations in Carbon Systems
1. Recurring excitations
2 4 6 8 10energy loss (eV)
0
1
2
3
4
5
S(q
,ω)
(1/ k
eV)
3
8/3
7/3
2
5/3
4/3
1
2/3
1/3
0
q1=1/8 (~0.37 1/Å) q3
ε−1GG(qr , ω) = 1
εGG+ εG0ε0G
(εGG)2 ε−100 (qr , ω)
S(qr +G) = SNLF(qr +G) + f ·S(qr )
coupling of excitations of momentumqr 1. Brillouin zone
qr + G higher Brillouin zone
⇒ reappearance3 of the anisotropicspectra from q → 0
3K. Sturm, W. Schulke, J. R. Schmitz, Phys. Rev. Lett. 68, 228 (1992).R. Hambach Collective Excitations in Carbon Systems
2. Strength of coupling
2 4 6 8 10energy loss (eV)
0
1
2
3
4
5
S(q
,ω)
(1/ k
eV)
3
8/3
7/3
2
5/3
4/3
1
2/3
1/3
0
q1=1/8 (~0.37 1/Å) q3
ε−1GG(qr , ω) = 1
εGG+ εG0ε0G
(εGG)2 ε−100 (qr , ω)
strength of coupling εG0 depends on:I angle ∠(qr , qr +G) andI structure factor ∝ density nG
⇒ enhances the angular anomaly
θ′η
(0,0,2
)q
R. Hambach Collective Excitations in Carbon Systems
Experimental verificationby inelastic x-ray scattering
R. Hambach Collective Excitations in Carbon Systems
IXS experiments
c-a
xis
2 4 6 8 10energy loss (eV)
2
4
6
8
10
12
14
S(q
,ω)
(1/ k
eV)
3
17/6
16/6
15/6
14/6
13/6
2
11/6
10/6
9/6
8/6
7/6
1
q1=1/8 (~0.37 1/Å) q3
I IXS experimentsN. Hiraoka
(Spring8, Taiwan)I elastic tail removedI uniform scaling
R. Hambach Collective Excitations in Carbon Systems
IXS experiments
c-a
xis
2 4 6 8 10energy loss (eV)
2
4
6
8
10
12
14
S(q
,ω)
(1/ k
eV)
3
17/6
16/6
15/6
14/6
13/6
2
11/6
10/6
9/6
8/6
7/6
1
q1=1/8 (~0.37 1/Å) q3
2 4 6 8 10energy loss (eV)
2
4
6
8
10
12
14
S(q
,ω)
(1/ k
eV)
3
17/6
16/6
15/6
14/6
13/6
2
11/6
10/6
9/6
8/6
7/6
1
q1=1/8 (~0.37 1/Å) q3
I IXS experimentsN. Hiraoka
(Spring8, Taiwan)I elastic tail removedI uniform scaling
R. Hambach Collective Excitations in Carbon Systems
Conclusions
graphite
I angular anomaly close to Bragg reflectionsI originates from local field effects (coupling to 1. BZ):
1. spectrum from q → 0 reappears (direction of qr )2. coupling εG0(qr ) enforces anisotropy
other systems
I all systems with strong local field effectse. g. layered systems, 1D structures
⇒ caution with loss experiments close to Bragg reflections
R. Hambach Collective Excitations in Carbon Systems
Summary
dimensionality
1. strong mixing of transitions (largeenergy range) in low dimensionalsystems (→ linear plasmon dispersion)
2. interactions important for small q
inhomogenity
3. discontinouity in S(q, ω) close to Braggreflections (result of plasmon bands)
0 0.2 0.4 0.6 0.84
5
6
7
8
9
VASWCNTbulk-SWCNTgraphite
(a) Experiment
0 0.2 0.4 0.6 0.8 1
graphene-1Lgraphene-2Lgraphite
(b) Calculation
0momentum transfer q (1/Å)
4
ener
gy lo
ss (
eV)
2 4 6 8 10energy loss (eV)
0
1
2
3
4
5
S(q
,ω)
(1/ k
eV)
3
8/3
7/3
2
5/3
4/3
1
2/3
1/3
0
q1=1/8 (~0.37 1/Å) q3
R. Hambach Collective Excitations in Carbon Systems
Thank you for your attention!
publications:Linear Plasmon Dispersion in Single-Wall Carbon Nanotubes and theCollective Excitation Spectrum of GrapheneC. Kramberger, R. H., C. Giorgetti, M. Rummeli, M. Knupfer, J. Fink,B. Buchner, Lucia Reining, E. Einarsson, S. Maruyama, F. Sottile,K. Hannewald, V. Olevano, A. G. Marinopoulos, and T. Pichler[Phys. Rev. Lett. 100, 196803 (2008)]
Anomalous Angular Dependence of the Dynamic Structure Factornear Bragg Reflections: GraphiteR. H., C. Giorgetti, N. Hiraoka, Y. Q. Cai, F. Sottile, A. G. Marinopoulos,F. Bechstedt, and Lucia Reining[accepted by Phys. Rev. Lett. (2008)]
codes:ABINIT: X. Gonze et al., Comp. Mat. Sci. 25, 478 (2002)DP-code: www.dp-code.org; V. Olevano, et al., unpublished.
R. Hambach Collective Excitations in Carbon Systems
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