Transcript Field Enhancement

Near-field Optical
Imaging of
Carbon Nanotubes
Achim Hartschuh, Huihong Qian
Tobias Gokus,
Department Chemie und Biochemie
and CeNS,
Universität München
Neil Anderson, Lukas Novotny
The Institute of Optics,
University of Rochester,
Rochester, NY
Nice, 25.09.2006
Achim Hartschuh, Nano-Optics München
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Why High Spatial Resolution Near-field Optics?
Why Optics?
• electronic and vibronic energies
• DOS
• excited state dynamics....
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Why Near-field Optics?
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Why Near-field Optics?
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Why Near-field Optics?
spatial resolution is
limited by diffraction l/2
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 Diffraction Limit
Abbé, Arch. Mikrosk.
Anat. 9, 413 (1873)
Uncertainty relation:
Δx  Δk x  2
2π
Δk x  k  sin α  
sin α 
λ
λ
2π
Δx 

Δk x sin α 
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Tip-Enhanced Spectroscopy
Wessel, JOSA B
2, 1538 (1985)
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Tip-Enhanced Spectroscopy
laser illuminated metal tip
Novotny et al. PRL 79, 645 (1997)
Theory: Enhanced electric field confined to tip apex
Mechanism: Lightning rod and antenna effect, plasmon resonances
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Tip-Enhanced Spectroscopy
SEM micrograph
diameter = 22 nm
enhanced electric field confined within 20 nm ?
……………….Optical imaging with 20 nm resolution?!
……………….Signal enhancement !?
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Experimental Setup
Confocal microscope
+ Tip-sample distance control
a sharp metal tip is held at constant
height (~2nm) above the sample
using a tuning-fork feedback
mechanism
K. Karrai et al., APL 66, 1842 (1995)
2 nm
Optical Images and Spectra
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Topography of the sample
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Tip-enhanced Spectroscopy
(Examples…………)
Raman scattering
S.M. Stoeckle et al., Chem. Phys. Lett. 318, 131 (2000)
N. Hayazawa et al., Chem. Phys. Lett. 335, 369 (2001)
A. Hartschuh et al. Phys. Rev. Lett. 90, 95503 (2003)
B. Pettinger et al., Phys. Rev. Lett. 92, 96101 (2004)
N. Anderson et al., J. Am. Chem. Soc. 127, 2533 (2005)
C.C. Neacsu et al., Phys. Rev. B 73, 193406 (2006)
D. Methani, N. Lee, R. D. Hartschuh et al. J. Raman Spectrosc. 36, 1068 (2005)
.................................
Two-photon excited fluorescence
E.J. Sánchez et al., Phys. Rev. Lett. 82, 4014 (1999)
Fluorescence
H.G. Frey et al., Phys. Rev.Lett. 81, 5030 (2004)
J. Farahani et al. Phys. Rev. Lett. 95, 017402 (2005)
A. Hartschuh et al. Nano Lett. 5, 2310 (2005)
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Single-Walled Carbon Nanotubes (SWCNT)
single-walled carbon nanotube
Graphene sheet + roll up
model
diameter ~ 1-2 nm
length up to mm
structure (n,m)
determines
properties:
(n-m) mod 3
=0 metallic
else semiconducting
from
Shigeo
Maruyama
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Near-field Raman Imaging of SWCNTs
Raman image
(G’ band)
Topography
500 nm
only SWCNT detected in optical image
optical contrast with 25 nm resolution
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500 nm
Hartschuh et al.
PRL 90, 95503 (2003)
chemically specific
enhanced field confined to tip
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Near-field Raman Spectroscopy
Raman image
(G band)
Topography
image
height:
0 - 1.9 nm
G
RBM
RBM at 199 cm-1
diam = 1.2 nm
structure
(n,m)(14,2)
metallic SWCNT
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Resolution enhancement
Farfield
no tip
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Near-field
same area
with tip
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Signal Enhancement
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Signal Enhancement
tip-enhanced signal > signal * 2500
Hartschuh et al. Phil. Trans. R. Soc. Lond A, 362 (2004)
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Distance Dependence
Enhanced Raman scattering signal
~ d-12
d
tip-enhancement is near-field effect => tip has to be close to sample
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Near-field Raman Spectroscopy
N. Anderson, unpub.
RBM
30 nm steps
G
wRBM=251 cm-1  (10,3)
(semiconducting)
wRBM=191 cm-1  (12,6)
(metallic)
 intramolecular junction
(IMJ) pn-junction
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Simultaneous Raman and PL Spectroscopy
(7,5)
(8,3)
(9,1)
(6,4)
Photoluminescence
Raman
Emission
Raman
Excitation at 633 nm  Emission and Raman signals are spectrally isolated
A. Hartschuh et al. Science 301, 1394 (2003)
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Near-field optical imaging of SWCNTs
Farfield Raman image
DNA-wrapped SWCNTs on mica
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Topography
Raman (G-band) Photoluminescence
 correlate Raman and photoluminescence
properties
 length of emissive segment ~ 70 nm
• changes in chirality (n,m)?
• coupling to substrate?
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Localized PL-Emisssion on Glass
Photoluminescence Raman scattering
SWCNTs on glass
Emission spatially
confined to within
10 – 20 nm
Photoluminescence
 Localized excited states
Bound excitons?
•role of defects?
•substrate?
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Near-field PL-Spectroscopy
Topography
Photoluminescence
4
5
6
1
2
3
PL spectra:
30 nm steps
between spectra
 Emission energy can
vary on the nanoscale
(caused by changes in local dielectric
environment e)
A. Hartschuh et al. Nano Lett. 5, 2310 (2005)
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Tip-enhanced Microscopy
 Spatial resolution < 15 nm
 Signal amplification
 Tip as nanoscale „light source“
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Signal Enhancement
Raman scattering
Photoluminescene
Enhancement of incident field
and scattered field
SRaman ~ (Elocal / E0)2 (Elocal / E0) 2=f4
local field
at tip
Ein
field without
tip
Eout
kex
krad
knonrad
 kex: enhanced excitation field
SPL~ (Elocal / E0)2=f2
 krad: Purcell-effect
krad
Q=
krad + knonrad
Q is increased (Q0~10-4)
cycling rate is increased
 SPL~ f2 Q/Q0
PL Enhancement depends on Q0!
 knonrad: dissipative energy transfer
to metal quenching of PL
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Signal Enhancement
Raman enhancement
PL enhancement
500 nm
500 nm
Far-field Raman ~ 2000 counts/s
No far-field PL < 200 counts/s
Near-field Raman ~ 4000 counts/s
Near-field PL ~17000 counts/s
Raman-enhancement ~ 6000 / 2000
PL-enhancement >17000 / 200
~ 85
~3
SPL~ f2 Q/Q0
SRaman ~ f4
 PL quantum yield is increased by tip (SEF)
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Signal Enhancement
(First data)
d
knon-rad
kex
krad
 PL quenching for very small distances
 optimum distance for PL enhancement
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Signal Enhancement
(First data)
P. Anger et al.
d
knon-rad
kex
krad
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Signal Enhancement
Raman scattering
Topography
290nm
Photoluminescence
290nm
290nm
DNA-wrapped SWCNTs
Raman scattering
Photoluminescence
tip-nanotube-distance
(schematic)
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Near-field Interactions
Uncertainty relation:
Diffraction limit for propagating waves:
Δx  Δk x  2
k  2π  0.01nm1
λ
Nanotubes:
k || 
2dr
 1nm1
3d t
High resolution provided by evanescent
fields that have higher k-vectors:
Δk x  2π
Δx
 2π
5  10nm
 1nm1
 k-vectors of tip enhanced fields extend
through BZ !
 selection rules for optical transitions?
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Summary
High-resolution optical microscopy of carbon
nanotubes using a sharp laser-illuminated metal tip
• PL and Raman spectroscoy and imaging
• Spatial resolution < 15 nm
• Signal enhancement
Results
• Resolved RBM variations (IMJ) on the nanoscale
• Non-uniform emission energies that result from local
variations of dielectric environment
• Strongly confined emission signals  bound excitons?
• PL-Quantum yield can be enhanced by metal tip
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Thanks to
PC Tuebingen:
Alfred J. Meixner
Mathias Steiner
Antonio Virgilio Failla
PC Siegen:
Gregor Schulte
Funding: DFG, Cm Siegen, NSF, FCI
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Acknowledgement
PC Tuebingen:
Alfred J. Meixner
Mathias Steiner
Antonio Virgilio Failla
PC Siegen:
Gregor Schulte
Funding: DFG, Cm Siegen, NSF, FCI
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