1. 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
let me start my thanking the organizers for inviting me to this excellent conference and for giving me the opportunity to report on our efforts on near-field optics
The work I am going to talk about was done in roc, tueb, and siegen
just recently we moved to munich
Nice,
Achim Hartschuh, Nano-Optics München

2. Why High Spatial Resolution Near-field Optics?
Why Optics?
electronic and vibronic energies
DOS
excited state dynamics....
why should we be interested in near-field optics on nanotubes that provides high spatial resolution?
why optics? energy of light quanta is the region of the electronic and vibronic transitions
If we have a straight, uniform and defect free nanotube, the optical properties will not vary along the nanotube and we do not need to have high spatial resolution
Nice,
Achim Hartschuh, LMU

3. Achim Hartschuh, Nano-Optics @ LMU
Why Near-field Optics?
however, very often we have something like this, a kink
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Achim Hartschuh, LMU

4. Achim Hartschuh, Nano-Optics @ LMU
Why Near-field Optics?
or even worse, a bent nanotube with particles, carbon sticking on the surface
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Achim Hartschuh, LMU

5. Why Near-field Optics? spatial resolution is
limited by diffraction l/2
or even worse, a bent nanotube with particles, carbon sticking on the surface
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Achim Hartschuh, LMU

6. Achim Hartschuh, Nano-Optics @ LMU
 Diffraction Limit
Abbé, Arch. Mikrosk.
Anat. 9, 413 (1873)
Uncertainty relation:
1873 Abbe wrote down this formula telling us that two objects that are closer than dx can not be distinguished
the formula can be easily derived from an uncertainty relation linking the uncertainty in space to the uncertainty in momentum
in a microscope k varies with the focusing angle
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Achim Hartschuh, LMU

7. Tip-Enhanced Spectroscopy
Wessel, JOSA B
2, 1538 (1985)
we take a sharp metal tip to interact with the objects independly
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Achim Hartschuh, LMU

8. 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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Achim Hartschuh, LMU

9. 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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Achim Hartschuh, LMU

10. 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
Topography of the sample
2 nm
K. Karrai et al., APL 66, 1842 (1995)
Tip-sample distance control +
Optical Images and Spectra
Nice,
Achim Hartschuh, LMU

11. 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, (2003)
B. Pettinger et al., Phys. Rev. Lett. 92, (2004)
N. Anderson et al., J. Am. Chem. Soc. 127, 2533 (2005)
C.C. Neacsu et al., Phys. Rev. B 73, (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)
H.G. Frey et al., Phys. Rev.Lett. 81, 5030 (2004)
J. Farahani et al. Phys. Rev. Lett. 95, (2005)
A. Hartschuh et al. Nano Lett. 5, 2310 (2005)
Fluorescence
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Achim Hartschuh, LMU

12. Single-Walled Carbon Nanotubes (SWCNT)
Graphene sheet + roll up
single-walled carbon nanotube
diameter ~ 1-2 nm
model
length up to mm
structure (n,m)
determines
properties:
(n-m) mod 3
=0 metallic
else semiconducting
we can think of a cnt as being formed by a graphene sheet that has been rolled up to form a seemless zylinder
from
Shigeo
Maruyama
Nice,
Achim Hartschuh, LMU

13. Near-field Raman Imaging of SWCNTs
Raman image
(G’ band)
Topography
Hartschuh et al.
PRL 90, (2003)
500 nm
500 nm
this is one of the first near-field raman images we detected, about 3 years ago
only SWCNT detected in optical image chemically specific
optical contrast with 25 nm resolution enhanced field confined to tip
Nice,
Achim Hartschuh, LMU

14. Near-field Raman Spectroscopy
Raman image
(G band)
Topography
image
RBM at 199 cm-1
diam = 1.2 nm
structure
(n,m)(14,2)
metallic SWCNT G
RBM
height: nm
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Achim Hartschuh, LMU

15. Resolution enhancement
Farfield
Near-field
no tip same area with tip
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Achim Hartschuh, LMU

16. Achim Hartschuh, Nano-Optics @ LMU
Signal Enhancement
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Achim Hartschuh, LMU

17. Achim Hartschuh, Nano-Optics @ LMU
Signal Enhancement
tip-enhanced signal > signal * 2500
Hartschuh et al. Phil. Trans. R. Soc. Lond A, 362 (2004)
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Achim Hartschuh, LMU

18. Achim Hartschuh, Nano-Optics @ LMU
Distance Dependence
Enhanced Raman scattering signal
~ d-12 d
development of new techniques and their applicaton
tip-enhancement is near-field effect => tip has to be close to sample
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Achim Hartschuh, LMU

19. Near-field Raman Spectroscopy
N. Anderson, unpub.
RBM G
30 nm steps
wRBM=251 cm-1  (10,3)
(semiconducting)
wRBM=191 cm-1  (12,6)
meanwhile the group at rochester is performing detailed near-field Raman studies
the topo of a nanotube
spectra taken in steps of about 30 nm initially show a rbm at indicating sc tube
disappears and an rbm at 191 corresponding to a metallic nt appears
we detect corresponding changes in g band
in essence we believe to observe an imj
(metallic)
intramolecular junction
(IMJ) pn-junction
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Achim Hartschuh, LMU

20. Simultaneous Raman and PL Spectroscopy
(7,5)
(8,3)
(9,1)
Photoluminescence
(6,4)
Raman
Emission
Raman
cnt are very particular material, at least for spectroscopists since we can observe both flu and Ra on the single object level with identical experimental conditions
the fluorescence from cnt results from exc rec. and ist energy is directly determined by the structure, an 7,5 tube....
Excitation at 633 nm  Emission and Raman signals are spectrally isolated
A. Hartschuh et al. Science 301, 1394 (2003)
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Achim Hartschuh, LMU

21. Near-field optical imaging of SWCNTs
Farfield Raman image
Topography
Raman (G-band)
Photoluminescence
 correlate Raman and photoluminescence
properties
 length of emissive segment ~ 70 nm
changes in chirality (n,m)?
coupling to substrate?
DNA-wrapped SWCNTs on mica
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Achim Hartschuh, LMU

22. Localized PL-Emisssion on Glass
Photoluminescence Raman scattering
SWCNTs on glass
Emission spatially
confined to within
10 – 20 nm
 Localized excited states
Bound excitons?
role of defects?
substrate?
Photoluminescence
if we deposite the raw nanotube material directly on glass, we detect extremely bright but highly confined PL signals
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Achim Hartschuh, LMU

23. Near-field PL-Spectroscopy
Topography
Photoluminescence
PL spectra:
30 nm steps
between spectra 1 2 3 4 5 6
no SDS in the optical images.....
 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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Achim Hartschuh, LMU

24. Achim Hartschuh, Nano-Optics @ LMU
Tip-enhanced Microscopy
Spatial resolution < 15 nm  Signal amplification  Tip as nanoscale „light source“
whole story?
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Achim Hartschuh, LMU

25. Achim Hartschuh, Nano-Optics @ LMU
Signal Enhancement
Raman scattering
Photoluminescene
Enhancement of incident field
and scattered field
SRaman ~ (Elocal / E0)2 (Elocal / E0) 2=f4
kex: enhanced excitation field
SPL~ (Elocal / E0)2=f2
local field
at tip
field without
tip
Q is increased (Q0~10-4)
cycling rate is increased
krad: Purcell-effect
krad + knonrad
krad
Q =
the enhancement of the Raman signal results from the enhancement of the incident field and the scattered field and can be approximated by the fourth power of the field enhancement factor
for fluorescence signal I think it is useful to consider the transitions rates involved
the excitation rate is enhanced by the enhanced excitation field, this it the first part
the tip will also modify the radiative rate because of a modified mode density. most likely it will be increased.
an increase in the radiative rate corresponds to an increase in the Q and cycling rate and both will increase the detected signal
however, the non-radiative rate will increase also because of energy transfer to the metal and quench the fluorescence
kex
krad
knonrad
Ein
Eout
PL Enhancement depends on Q0!
 SPL~ f2 Q/Q0
dissipative energy transfer
to metal quenching of PL
knonrad:
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Achim Hartschuh, LMU

26. Achim Hartschuh, Nano-Optics @ LMU
Signal Enhancement
Raman enhancement
PL enhancement
500 nm
500 nm
Far-field Raman ~ 2000 counts/s
Near-field Raman ~ 4000 counts/s
Raman-enhancement ~ 6000 / 2000
~ 3
No far-field PL < 200 counts/s
Near-field PL ~17000 counts/s
PL-enhancement >17000 / 200
~ 85
the enhancement of the Raman signal results from the enhancement of the incident field and the scattered field and can be approximated by the fourth power of the field enhancement factor
for fluorescence signal I think it is useful to consider the transitions rates involved
the excitation rate is enhanced by the enhanced excitation field, this it the first part
the tip will also modify the radiative rate because of a modified mode density. most likely it will be increased.
an increase in the radiative rate corresponds to an increase in the Q and cycling rate and both will increase the detected signal
however, the non-radiative rate will increase also because of energy transfer to the metal and quench the fluorescence
SRaman ~ f4
SPL~ f2 Q/Q0
 PL quantum yield is increased by tip (SEF)
Nice,
Achim Hartschuh, LMU

27. Achim Hartschuh, Nano-Optics @ LMU
Signal Enhancement
(First data) d
knon-rad
kex
krad
 PL quenching for very small distances
 optimum distance for PL enhancement
Nice,
Achim Hartschuh, LMU

28. Achim Hartschuh, Nano-Optics @ LMU
Signal Enhancement
(First data)
P. Anger et al. d
knon-rad
kex
krad
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Achim Hartschuh, LMU

29. Achim Hartschuh, Nano-Optics @ LMU
Signal Enhancement
Topography
Raman scattering
Photoluminescence
290nm
290nm
290nm
DNA-wrapped SWCNTs
tip-nanotube-distance
Raman scattering
Photoluminescence
(schematic)
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Achim Hartschuh, LMU

30. Near-field Interactions
Nanotubes:
Uncertainty relation:
Diffraction limit for propagating waves:
High resolution provided by evanescent
fields that have higher k-vectors:
uncertainty relation
for propagating light we have fixed |k|=2pi/lambda, diffraction limit tells us that we can not localize fields / intensities better than dx
what does it mean for nts. the max wv has typical values on the order of 1 inverse nm, the states we can acces are limited to the photon cone
here high resolution is provided by evanescent fields with higher k
here: higher localization achieved with optical near-fields, fields are evanescent and can basically have arbitrary k
the numbers we have are k_nf=0.2 nm-1 vs. k_BZ=1 nm-1
up to now no indication for other transitions or selection rules
 k-vectors of tip enhanced fields extend through BZ !
 selection rules for optical transitions?
Nice,
Achim Hartschuh, LMU

31. Achim Hartschuh, Nano-Optics @ LMU
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
strong variation consequence of sample material, there is certainly better defined material for which no such variations occur, on the other hand, interaction with the local environment also essential for perfect nanotube
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Achim Hartschuh, LMU

32. Achim Hartschuh, Nano-Optics München
Thanks to
PC Tuebingen:
Alfred J. Meixner
Mathias Steiner
Antonio Virgilio Failla
PC Siegen:
Gregor Schulte
Funding: DFG, Cm Siegen, NSF, FCI
Nice,
Achim Hartschuh, Nano-Optics München

33. Achim Hartschuh, Nano-Optics @ LMU
Acknowledgement
PC Tuebingen:
Alfred J. Meixner
Mathias Steiner
Antonio Virgilio Failla
PC Siegen:
Gregor Schulte
Funding: DFG, Cm Siegen, NSF, FCI
Nice,
Achim Hartschuh, LMU