Optical Spectroscopy of Single-Walled Carbon Nanotubes

Louis BrusChemistry Department, Columbia University

Groups: Heinz, O’Brien, Hone, Turro, Friesner, Brus

1. SWNT Luminescence dynamics – psec pump-probe spectroscopy (F. Wang, G. Dukovic)• Radiative lifetime ~ 100ns• Efficient Auger recombination

3. Nanotube surface oxides and hole-doping (G. Dukovic, B. White, Z. Zhou, F. Wang, S. Jockusch, M. Steigerwald)• Neutral and protonated endoperoxides

5. Rayleigh scattering from individual metallic and semiconducting SWNTs (M. Sfeir, F. Wang, L. Huang, C. Chuang)

Constructing a (10,10) SWNT

Electronic Structure of SWNTs

Nanotube: Ideal 1-D system; Many unique properties: Electrical, mechanical and optical.

Metallic:

Semiconducting:

10 frontier orbitals of (5,5) Nanotube fragments

2 4 6 8 10 12 14 16 18 20 22-7

-6

-5

-4

-3

-2

-1

# of sections(10 C each sect.)

Ener

gy (e

V)

3 nm

Light Emission from SWNTs Band Gap Fluorescence [1]

Electroluminescence[2]

[1] O’Connel et al. [2] Misewich, Martel, Avouris [3] Lefebvre, Homma, Finnie

[Science 297 593 (2002)] [Science 300 783 (2003)] [PRB 69 075403 (2004)]

Hartschuh, Pedrosa, Novotny, Krauss

[Science 301 1354 (2003)]

• Photonic application.• Useful tool for probing electronic structure and carrier dynamics.

Single Tube Fluorescence [3]

AFM Image

[1] O’Connell et al., Science 297 553 (2002)

A single nanotube embedded in SDS micelle

Ref. [1]

Psec dynamics in SWNT ensembles

8nm6420

1 µm

Isolated SWNTs in aqueous micelles

Micellar Fluorescence Spectra

400 600 800 1000 1200 14000.2

0.4

0.6

0.0

0.2

0.4

0.6

0.8

1.0

Fluo

resc

ence

(a.u

.)

Abs

orba

nce

Wavelength (nm)

absorption spectra

fluorescence(7,6)

(12,1)

(11,3)

(10,5)

(9,7)

[(n,m) assignment according to S.M. Bachilo et al. Science 298, 2361 (2002)]

Pump wavelength 800 nm

Spectrally Integrated Time-Resolved Fluorescence

• Fluorescence decay: Principal decay time ~ 7 ps

• Also fluorescence in long-time tails.

longer time scale

γtotal=7ps-1

Radiative Recombination Lifetime

Absolute fluorescence Q. E. (fast comp.): η = 0.7*10-4

γrad = η . γtotal

τrad = 1/γ rad ~ 110 ns

Implication of the Radiative Lifetime

τrad ~ 100 ns

• Comparable to CdSe semiconductor nanoparticles with

high fluorescence quantum yield:

Low fluorescence efficiency is due to the fast trapping

In 10 ps carrier at thermal velocity travels ~ 3 μm

New Channel with Multiple e-h Pairs

• Additional fast decay initially for multiple excitation per nanotube

• Similar decay at later times, which saturates at high excitation density.Similar results recently reported by Fleming et al., UC Berkeley (J. Chem. Phys. (2004) 120, 3368)

e

e

e

h

Auger Recombination

At low excitation density:

Fluorescence ∝ # of excitated nanotubes ∝ pump fluence

At high excitation density: Fast auger process until one excitation per nanotube

Initial decay differs, tail part of decay is equivalent.

Multi-Carrier Interaction: Auger Process

Modeling vs. Experiment

Two electron-hole pairs in 400nm long tube:Auger rate ~ 0.8 ps-1

Theoretical Estimation of Auger Rate

Simplified model:

e-h e-h

( ) ( )e h e hV r r U r rδ− = − −

Simplified Coulomb interaction:2

2 ~ 2002bindUE meVµ= −h

Exciton binding energy:

Auger Recombination ProcessesFeynman Diagrams:

( )K P+ ↔

Calculation Results

2, , ,

,

2 ( )e h

e h

K P K P c k v kk k

M E Eπ δ ε εΓ = ⋅ + − −∑h

11 0.8( )

Auger Augerpsl

τ−

− = Γ =

Doesn’t depend strongly on temperature.

In comparison, Auger rate for free electron picture without including excitonic effect has an activated temperature behavior, and is orders of magnitude smaller.

M determined by U and Feynman diagrams

Implication of the Rapid Auger Recombination Rate

• Limit on e-h pairs density:Constraints for population inversion and light amplification.

• Auger recombination due to the presence of free electrons or holes arising from defects or surface molecular species: Environmental sensitivity of photoluminescence.

• Rapid Auger recombination:Limit electroluminescence efficiency at high electron-hole injection rate.

Intraband

< 200fs

Summary: Carrier Dynamics in SWNT

e

e

e

h

Auger ~ 1psNonradiative: Defect ~ 7ps

Radiative: ~ 100ns

ee

hh

e

h

Experimental Observations:

Some Implications:• Fast defect trapping limits the fluorescence yield

• Efficient Auger process excludes multiple sustained electron-hole pairs in nanotube, constraint for light emission and amplification in nanotubes

Absorption bleaching and luminescence quenching at low pH

ABSORPTIONLUMINESCENCE

1.2

1.0

0.8

0.6

0.4

0.2

Abso

rban

ce

12001000800600400λ / nm

pH 3 pH 5 pH 7 pH 9 pH 12

7

6

5

4

3

2

1

0

Fluo

resc

ence

(a.u

)

14001300120011001000900λ / nm

(8,3

)(6

,5)

(7,5

)(1

0,2)

(9,4

)

(12,

1)(1

1,3)

(10,

5)

(9,7

)

Assignment

• Overall increase in intensity with increasing pH – hole doping at acid pH due to a protonated surface oxide(??) (also observed by Strano et al, J. Phys. Chem, 2003, 107, 6979)

• Luminescence more sensitive to H+ than optical absorption

Direct observation of oxygen desorption(surface oxide in equilibrium with atmospheric oxygen)

• Heating to 97 °C under argon results in luminescence recovery – surface oxide decomposition

• Data fits unimolecular decomposition kinetics after induction period

• Estimate of Ea for concerted decomposition – 1.2eV

8

6

4

2

0

Fluo

resc

ence

(a.u

.)

12080400Time (min)

(8,3)

(6,5)

(9,7)(7,5)

(10,2)

(9,4)

(10,5)(12,1)

(11,3)

What is the structure of SWNT surface oxide? B3LYP DFT calculation

ENDOPEROXIDE PROTONATED OXIDE

Positive charge delocalized on SWNT

+

carbocation

SWNT re-oxidation with 1∆ O2 at pH 3

80

60

40

20

0

Fluo

resc

ence

(a.u

.)

14001300120011001000900λ / nm

[DMN] = 0.3 mM (control) [DMN-O2] = 0.1 µM [DMN-O2] = 1.0 µM [DMN-O2] = 3.3 µM

0.40

0.35

0.30

0.25

0.20

0.15

0.10

Abso

rban

ce

1300120011001000900λ / nm

pH 3; air-equilibrated pH 3; oxygen removed;

[DMN-O2] = 3.7 µM; t = 0 pH 3; [DMN-O2] = 3.7 µM; t = 18 hours

Absorption NOT bleachedLuminescence quenched

O O + 1∆ O2

h e a tendoperoxide

Effect of oxide on SWNT optical properties

• Fluorescence quenching – ~ 10 holes per 400 nm tube

• Absorption bleaching – ~ 250 holes per 400 nm tube

Difference in sensitivities to holes in absorption and luminescence explained by the Auger effect

Non-radiative recombination: e--h+ + h+ h+ + kinetic energy

First quantitative study of SWNT oxidation and its effect on SWNT optical properties

Optical Spectroscopy of Single Nanotubes

Existing techniques: • Resonance Raman spectroscopy. • Fluorescence Excitation Spectroscopy.

We perform single tube Rayleigh scattering spectroscopy.

Advantages: • Direct probe of electronic transitions, intrinsically stronger than Raman Scattering.• Present for both semiconductor and metallic nanotubes.

Challenge: Characterize and identify an individual carbon nanotube by their Rayleigh scattering.

Rayleigh Scattering from Bundles (~100 tubes)

Previous Work: (Z. Yu 2001)

Ensemble Rayleigh scattering from laser-ablation material.

Shown to closely resemble ensemble absorption spectra.

Intensity of features shown to be highly polarization dependent.

€

σ∝r4ε−1

2

λ3€

σ∝r6

λ4

ε−1ε+2

Sphere Infinite Cylinder

Scaling of Nanotube Rayleigh Cross Section

σ (dt = 2 nm) ~ 10-14 cm2

λ

High brightness – like laser Large spectrum bandwidth – like a light bulb

450 - 1450 nm

Supercontinuum Radiation

Microstructured fiber: core ~ 2 m

Spectral range:

Experimental SetupSupercontinuum Generation

Mode-locked Ti:Saph coupled to microstructured fiber optic.

spectrograph and CCD

spatial filter (pinhole)

reference beam

scattered light

piezo (oscillating in z)

450-1550 nmSpectral range:

excitation and collection objectives

polarizers

polarizers

sample

nonlinear fiber

Laser brightness

supercontinuum light

laser system

Scattered light is corrected by the supercontinuum spectral profile giving the Rayleigh spectrum

transmitted light

CVD Growth Process

Imaging

Directional growth determined by flow direction of feed gas, lengths > 100 microns:

• CO, methane, and ethanol gas

• Fe, FeMo, and CoMo catalysts

Si/SiO2 substrates with slits patterned by optical lithography and wet etching.

Look at total integrated intensity on CCD to find tubes. Correlates to SEM images.

Single tubes scatter light much less than bundles. Distinguishable from the number of peaks in the spectra and width of features.

Growth and Imaging

Isolated SWNT

nanotube scattering

slit edges

10 µm

Types of Rayleigh Spectra from Individual Nanotubes

Sharp, well-separated two peaks

Two closely-positioned peaks.

Interpreting the Rayleigh Spectra

•Resonance structure corresponds to van Hove singularities of nanotube, reflecting the electronic signature of specific nanotubes.

2

3

1εσ

λ−

∝Rayleigh scattering from an infinitely long cylinder:

Theory: (23,0) tube

E33 E44

E11s

E22s

E11m

E33s

E44s

E22m

E33 and E44 of semiconductor nanotubes E22 of metallic nanotubes in our dt range

eV

eV

DOSScattering

Metallic Carbon Nanotube

Semiconducting Carbon Nanotube

Single E22 transition observed in the visible – sometimes split into two very close peaks by trigonal warping effect

Two well separated E33 and E44 transitions for larger diameter tubes, E33 and E22 for smaller diameters.

E22

E33 E44

Rayleigh Spectra

Polarization Dependence of The Rayleigh Scattering

Light scattering is strongly polarized along the nanotube.

5

E//0

E//total E 0

+++

- - -

E ind

P = ( -1) 21+

E 0

P//=( -1) E//0

Scattering Spectra along the Nanotube: Single Tube

Does the nanotube keep the same chirality along the entire length?

40 μm

substrate

Yes. At least up to 40 μm, a chain of several millions of carbon atoms.

Bundling Effect, Tube Interactions

Four or more peaks, broadened compared to single nanotube spectra.

Small Carbon Nanotube Bundle

When in bundles, spectra broaden and red-shift by approximately 15 meV.

Scattering Spectra along the Nanotube: Single Tube to Small Bundle

40 μm

substrate

16401600156015204002000-200-400

2.62.42.22.01.81.61.4

Rayleigh

Resonance Raman

RBM

cm-1 cm-1

G mode

1.92eV 2.10eV

radial breathing mode d=1.89nm(21,4) nanotube: d=1.85nm. E33=1.87 , E44=2.10 (tight binding model)

Correlated Raman and Rayleigh Scattering from the Same Nanotube

Summary• Nonradiative Auger recombination is extremely fast in SWNTs

• Nanotubes exposed to air form surface endoperoxides which can protonate to quench luminescence by hole-doping– Just a few holes in a 400 nm tube sufficient for quenching– Excited state sensitive along the entire length

• In open-slit geometry, it is possible to detect very strong Rayleigh scattering in short times– Rayleigh scattering clearly shows strong resonant optical

transitions and distinguishes between metallic and semiconducting tubes and provides a structure identification tool