near-field optical imaging of carbon nanotubes

Nice, 25.09.2006 Achim Hartschuh, Nano-Optics München 1

Near-field Optical Imaging of

Carbon Nanotubes

Achim Hartschuh, Huihong QianTobias Gokus,

Department Chemie und Biochemie and CeNS,

Universität München

Neil Anderson, Lukas NovotnyThe Institute of Optics, University of Rochester,

Rochester, NY

Nice, 25.09.2006 Achim Hartschuh, Nano-Optics @ LMU 2

Why High Spatial Resolution Near-field Optics?

Why Optics?• electronic and vibronic energies• DOS• excited state dynamics....


Why Near-field Optics?



spatial resolution is limited by diffraction


Diffraction Limit

Abbé, Arch. Mikrosk. Anat. 9, 413 (1873)

Uncertainty relation:

2ΔkΔx x

αsinλ

2παsinkΔkx

αsin

λΔk

2πΔxx


Tip-Enhanced Spectroscopy

Wessel, JOSA B 2, 1538 (1985)



Theory: Enhanced electric field confined to tip apex

Mechanism: Lightning rod and antenna effect, plasmon resonances

laser illuminated metal tip

Novotny et al. PRL 79, 645 (1997)



SEM micrograph

diameter = 22 nm

enhanced electric field confined within 20 nm ?

……………….Optical imaging with 20 nm resolution?!

……………….Signal enhancement !?


Experimental Setup

Confocal microscope

Optical Images and Spectra

a sharp metal tip is held at constant height (~2nm) above the sampleusing a tuning-fork feedback mechanism

Topography of the sample

2 nm

K. Karrai et al., APL 66, 1842 (1995)

Tip-sample distance control+


Tip-enhanced Spectroscopy

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).................................

Raman scattering

Two-photon excited fluorescenceE.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, 017402 (2005)A. Hartschuh et al. Nano Lett. 5, 2310 (2005)

(Examples…………)

Fluorescence


Single-Walled Carbon Nanotubes (SWCNT)

Graphene sheet + roll up single-walled carbon nanotube

fromShigeo Maruyama

length up to mm

diameter ~ 1-2 nm model

structure (n,m) determines properties:

(n-m) mod 3 =0 metallicelse semiconducting


Near-field Raman Imaging of SWCNTs

only SWCNT detected in optical image chemically specificoptical contrast with 25 nm resolution enhanced field confined to tip

Hartschuh et al. PRL 90, 95503 (2003)500 nm 500 nm

Raman image (G’ band)

Topography


Near-field Raman Spectroscopy

height: 0 - 1.9 nm

Raman image(G band)

Topography image

RBM at 199 cm-1

diam = 1.2 nmstructure (n,m)(14,2) metallic SWCNT

G

RBM


Resolution enhancement

Farfield Near-field

no tip same area with tip


Signal Enhancement


Signal Enhancement

tip-enhanced signal > signal * 2500Hartschuh et al. Phil. Trans. R. Soc. Lond A, 362 (2004)


Distance Dependence

tip-enhancement is near-field effect => tip has to be close to sample

~ d-12 d

Enhanced Raman scattering signal


Near-field Raman Spectroscopy

RBM=251 cm-1 (10,3)

RBM=191 cm-1 (12,6)

(semiconducting)

(metallic)

N. Anderson, unpub.

RBMRBM GG30 nm steps

intramolecular junction (IMJ) pn-junction


Simultaneous Raman and PL Spectroscopy

Photoluminescence

Raman

(7,5) (8,3) (9,1)

Raman Emission

Excitation at 633 nm Emission and Raman signals are spectrally isolated

(6,4)

A. Hartschuh et al. Science 301, 1394 (2003)


Near-field optical imaging of SWCNTs

Farfield Raman image

DNA-wrapped SWCNTs on mica

Topography Raman (G-band) Photoluminescence

correlate Raman and photoluminescence properties length of emissive segment ~ 70 nm

• changes in chirality (n,m)?• coupling to substrate?


Localized PL-Emisssion on Glass

Emission spatiallyconfined to within 10 – 20 nm

Localized excited statesLocalized excited statesBound excitons?Bound excitons?

•role of defects?role of defects?•substrate?substrate?

Photoluminescence Raman scattering

Photoluminescence

SWCNTs on glass


Near-field PL-Spectroscopy

Topography Photoluminescence

PL spectra:30 nm steps between spectra

12

34

56

A. Hartschuh et al. Nano Lett. 5, 2310 (2005)

Emission energy can vary on the nanoscale

(caused by changes in local dielectric environment )


Spatial resolution < 15 nm Signal amplification

Tip as nanoscale „light source“

Tip-enhanced Microscopy


Signal Enhancement

Enhancement of incident fieldand scattered field

SRaman ~ (Elocal / E0)2 (Elocal / E0) 2=f4

Raman scattering Photoluminescene

dissipative energy transfer to metal quenching of PL

knonrad:

kex: enhanced excitation fieldSPL~ (Elocal / E0)2=f2

Q is increased (Q0~10-4)cycling rate is increased

krad: Purcell-effect

krad + knonrad

kradQ =

kex krad knonrad

local fieldat tip

field without tip

PL Enhancement depends on Q0!

SPL~ f2 Q/Q0

Ein Eout


Signal Enhancement

SPL~ f2 Q/Q0

Raman enhancement

500 nm

Far-field Raman ~ 2000 counts/s

Near-field Raman ~ 4000 counts/s

Raman-enhancement ~ 6000 / 2000

~ 3

SRaman ~ f4

PL enhancement

500 nm

No far-field PL < 200 counts/s

Near-field PL ~17000 counts/s

PL-enhancement >17000 / 200

~ 85

PL quantum yield is increased by tip (SEF)


Signal Enhancement

(First data)

PL quenching for very small distances optimum distance for PL enhancement

d

kex

krad

knon-rad


Signal Enhancement

(First data)

d

kex

krad

knon-rad

P. Anger et al.


Signal Enhancement

Topography Raman scattering Photoluminescence

290nm 290nm 290nm

tip-nanotube-distance

Raman scatteringPhotoluminescence

DNA-wrapped SWCNTs

(schematic)


Near-field Interactions

Uncertainty relation:Diffraction limit for propagating waves:

2ΔkΔx x 10.01nmλ

2πk

k-vectors of tip enhanced fields extend through BZ !

selection rules for optical transitions?

1x 1nm10nm5

2πΔx

2πΔk

High resolution provided by evanescent fields that have higher k-vectors:

Nanotubes:

1

t

r|| 1nm

d3

2dk


Summary

High-resolution optical microscopy of carbon High-resolution optical microscopy of carbon nanotubes using a sharp laser-illuminated metal tipnanotubes using a sharp laser-illuminated metal tip

• PL and Raman spectroscoy and imaging• Spatial resolution < 15 nm• Signal enhancement

ResultsResults• 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

Nice, 25.09.2006 Achim Hartschuh, Nano-Optics München 32

Thanks to

PC Tuebingen:Alfred J. MeixnerMathias SteinerAntonio Virgilio Failla

Funding: DFG, C Siegen, NSF, FCI

PC Siegen: Gregor Schulte


Acknowledgement

PC Tuebingen:Alfred J. MeixnerMathias SteinerAntonio Virgilio Failla

Funding: DFG, C Siegen, NSF, FCI

PC Siegen: Gregor Schulte

near-field optical imaging of carbon nanotubes

Documents

field optics

field effect

nm resolution enhanced

nmenhanced electric

raman signals

nm emission

nano lett

tip apexmechanism