Optical Spectroscopy of Single-Walled Carbon Nanotubes

1
Optical Spectroscopy of Single-Walled Carbon
Nanotubes
Tony Heinz and Feng Wang Departments of Physics
and Electrical Engineering Columbia University,
New York, NY Supported by NSF-Nanocenter at
Columbia and DOE-BES
2
Collaborators
Overall Program and Optical Characterization Loui
s Brus Gordana Dukovic, Matt Sfeir (CU
Chemistry) Oxygen Photochemistry (for sidewall
oxidation) Nick Turro Brian White, Steffen
Jockush (CU Chemistry) Electronic Structure
Calculations (for sidewall oxidation) Rich
Friesner, Mike Steigerwald Zhiyong Zhou (CU
Chemistry) Silicon Fabrication (for single
nanotube Rayleigh scattering) Jim Hone
Chia-Chin Chuang (CU Mechanical Eng.) Nanotube
CVD Growth (for single nanotube Rayleigh
scattering) Stephen OBrien Limin Huang (CU
Materials Science) Funding Columbia NSF
Nanocenter DOE-BES
3
References to Published Papers

• F. Wang, G. Dukovic, L. E. Brus, and T. F. Heinz,
  Time-Resolved Fluorescence in Carbon Nanotubes
  and Its Implication for Radiative Lifetimes,
  Phys. Rev. Lett. 92, 177401 (2004).
• F. Wang, G. Dukovic, E. Knoesel, L.E. Brus, T.F.
  Heinz, "Observation of rapid Auger recombination
  in optically excited semiconducting carbon
  nanotubes," Phys. Rev. B 70, 241403 (2004)
• M.Y. Sfeir ,F. Wang ,L.M. Huang ,C.C. Chuang ,J.
  Hone ,S.P. O'Brien ,T.F. Heinz ,L.E. Brus,
  "Probing electronic transitions in individual
  carbon nanotubes by Rayleigh scattering," Science
  306 1540-1543 (2004)
• G. Dukovic, B. E. White, Z. Zhou, F. Wang, S.
  Jockusch, M.L. Steigerwald, T.F. Heinz, R.A.
  Friesner, N.J. Turro, L.E. Brus, " Reversible
  Surface Oxidation and Efficient Luminescence
  Quenching in Semiconductor Single-Wall Carbon
  Nanotubes," JACS 126 15269-15276 (2004)

4
Carbon NanotubesSWNT
5
Carbon NanotubesRolled-up Graphene Sheet
Graphene
Electronic structure semimetal
Pictures borrowed from Internet.
6
Electronic Structure
Metallic
Semiconducting

• Nanotubes
• Ideal 1-D system
• Many unique properties
• Electrical, mechanical and optical.

7
Electrical Transport Properties of SWNTs

• Distinctive metallic and semiconducting transport
  properties
• Ballistic transport,
• Extremely high current carrying capacity

Single nanotube transistor IBM Yorktown group
8
Optical Properties

• Signature of excited states
• Route to explore carrier dynamics
• Potential for optoelectronic applications as
  emitters, detectors, NLO elements with tunable
  response based on SWNT structure

9
Light Emission from SWNT
Electroluminescence2
Band Gap Fluorescence 1
1 Smalley et al., Rice U. 2 Misewich,
Martel, Avouris, IBM Science 297
593 (2002) Science 300 783 (2003)
10
Isolated SWNTs in Aqueous Micelles

• High-power sonication in surfactant solution
• Exfoliate bundles and suspend short sections of
  nanotubes in micelles
• Centrifugation at 110,000xg to remove bundles
  and impurities

Science 297, 593
SWNT in SDS micelle
11
Fluorescence Spectra
(n,m) assignment according to S.M. Bachilo et
al. Science 298, 2361 (2002)
12
Some Outstanding Issues

• What controls the quantum efficiency
• What are the non-radiative relaxation channesl
• How are the rates and pathways of energy
• relaxation to the band edge
• What is the nature of the excited states
• (free carriers or excitons)

13
Time-Resolved Fluorescence by Optical Kerr
Gating
14
Time-Resolved Fluorescence
longer time scale
Wang, Dukovic, Brus, and Heinz PRL (2004)

• Fluorescence decay Principal decay time 7 ps
• Also fluorescence in long-time tails.

15
Radiative Recombination Lifetime
Absolute fluorescence Q. E. (fast comp.)
? 0.710-4 Competing channels -
Radiative ?rad - Trapping ?trap ?total ?
?rad / ?total trad 1/? rad 110 ns
16
Radiative Recombination Lifetime Theoretical
Prediction
Tight Binding Model
For nanotubes with (n,m) values of (7,6), (12,1),
(11,3), (10,5) and (9,7) , their recombination
lifetimes are within 30 of each other.
Ref. 1
with l 300nm
1 S.M. Bachilo et al. Science 298 2361 (2002)
17
Implication of the Radiative Lifetime

• trad 100 ns
• Comparable to CdSe semiconductor nanoparticles
  with
• high fluorescence yield
• Low fluorescence efficiency is due to the fast
  trapping
• In 10 ps carrier at thermal velocity travels
  1 µm

• Electroluminescence injection of both electrons
  and holes
• Efficiency ltlt
  1
• Use carrier confinement

18
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
19
Multi-Carrier Interaction Auger
Process(Exciton-Exciton Annihilation)
20
Model of Carrier Evolution with Auger Process
Master Eqn

• Distribution of excitation number

radiative recombination rate
With
Auger recombination rate
trapping rate

• Nanotube emission rate

21
Modeling vs. Experiment
Two electron-hole pairs in 400nm long
tube Auger rate 0.8 ps-1
22
Summary Carrier Dynamics in SWNT
Experimental Observations
Intraband lt 200fs
e
e
e
h
h
Nonradiative Defect 7ps
Radiative 100ns
Auger 1ps
h

• Some Implications
• Fast defect trapping limits the fluorescence
  yield, rather than any intrinsic weakness of
  optical transition.
• Efficient Auger process excludes multiple
  sustained electron-hole pairs in nanotube,
  constraint for light emission and amplification
  in nanotubes

23

Nanotube electronic properties
Nanotube surface chemistry
SWNT oxide chemical structure and effect on
optical properties
Nanotube spectroscopy absorption, fluorescence
(Brus, Heinz)
Colloids nanotube solubilization 1? O2
chemistry nanotube oxidation
(Turro)
Theory Oxide structure prediction by DFT
(Friesner)
24
pH dependence of SWNT luminescence

• Luminescence quenched at low pH
• Observed only when O2 adsorbed on nanotube
  sidewall

pH 12
pH 3
Quenching due to a VERY SMALL NUMBER of holes
generated by protonated SWNT oxide
25
SWNT Oxide Experimental Observations
At pH 3

• SWNT oxide decomposes at 97 C under argon
• luminescence recovers
• Ea 1.2 eV (28 kcal/mol)
• Nanotubes re-oxidized using 1? O2
• SWNT oxide formation reversible

26
Direct observation of oxygen desorption

• 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

27
SWNT Oxide Structure

PROTONATED OXIDE
ENDOPEROXIDE
28
Hole-Induced Fluorescence Quenching

• Non-radiative (Auger) recombination important
  mechanism of fluorescence quenching in SWNTs
• Protonated SWNT oxide - 10 holes / 400 nm long
  tube (40,000 C atoms) quench the luminescence

e--h h ? h kinetic energy
29
Summary Sidewall Oxide Optical Properties

• SWNT fluorescence extremely sensitive to
  quenching by hole doping
• Hole created by hydroperoxide carbocation well
  defined chemistry

• New opportunities for
• Controlled nanotube doping
• Further chemical functionalization

30
Challenge Studying Individual Nanotubes

• Spectra from an ensemble of nanotubes
• Each peak is a different tube
• Ensemble average complicated

Challenge to identify and characterize nanotube
with a specific diameter and chiral angle ?
Study nanotubes one by one
31
Optical Spectroscopy of Single Nanotubes

• Resonance Raman spectroscopy Only with a
  resonance at the laser frequency.
• Fluorescence Only for semiconducting tubes.

We perform single tube Rayleigh scattering
spectroscopy.
Schematic
-- dark field imaging
32
I Supercontinuum Radiation

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

33
Microstructured Optical Fiber

• Ideal structure for optimized nonlinear
  interaction
• Femtosecond laser is confined into a very small
  core of 2 ?m
• Dispersion engineered minimizes pulse stretching
  and ensures a long interaction length.

Microstructured fiber core 2 ?m
Manufacturer CRYSTAL FIBRE A/S
Zero dispersion at 750nm
34
II Suspended Carbon Nanotubes

• Reduced background

Suspended nanotube across slit
SEM
35
CVD Growth of SWNTs across Slit Sample
Nanotubes across the slit
CVD growth apparatus

• CVD Growth
• Patterned bimetallic CoMo catalyst deposited
  along the slit edge
• Ethanol as the feed gas
• Temperature of the quartz tube furnace is set at
  700-900 C for 10-60 minutes.

36
Two Types of Rayleigh Spectra from Individual
Nanotubes
Sharp, well-separated two peaks
37
Rayleigh Scattering from Nanotubes
Rayleigh scattering from an infinitely long
cylinder
Theory (23,0) tube

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

38
Rayleigh Scattering from Nanotubes
For the recorded spectrum range (450-850nm) and
nanotube with d 1.8 nm
E33 and E44 of semiconductor nanotubes
E22 of metallic nanotubes
39
Understanding the Scattering Spectra
Semiconducting Tubes
E33
E44
well-separated E33 and E44 peaks.
Metallic Tubes
E22
Single peak or Split into two peakstrigonal
warping effect.
40
Rayleigh Scattering from Nanotubes
Mie scattering from infinite long cylinder

• General increase for the off resonance background
  at lower frequency
• Resonance structure corresponds to van Hove
  singularities of nanotube, reflecting the
  electronic signature of specific nanotubes.

For the recorded spectrum range (450-850nm) and
nanotube with d 1.8 µm
E33 and E44 of semiconductor nanotubes
E22m
E44s
E22 of metallic nanotubes
E33s
E11m
E22s
E11s
H. Kataura et al. Proceedings of the
International Winter School on Electronics
Properties of Novel Materials (1999)
41
Understanding the Scattering Spectra
Semiconducting Tubes
E33
E44
well-separated E33 and E44 peaks.
Metallic Tubes
E22
Single peak or Split into two peakstrigonal
warping effect.
42
Polarization Dependence of The Rayleigh Scattering
Light scattering is strongly polarized along the
nanotube.
43
Polarization Perpendicular to the SWNT
Depolarization Effect
E0
P(? -1) E0
Etotal
x5
44
Scattering Spectra along the Nanotube Single Tube
Does the nanotube keep the same chirality along
the whole growth structure?
40 ?m
substrate
Yes. At least up to 40 ?m, a chain of several
millions of carbon atoms.
45
Scattering Spectra along the Nanotube Single
Tube to Bundle of Two SWNTs
Peaks red-shifted due to tube-tube interaction.
46
Correlated Raman and Rayleigh Scatterings from
the Same Nanotube
1.9eV
Intensity (a.u.)
Rayleigh Spectra
Raman Spectra at 1.91eV radial breathing mode ?
d1.89nm
47
Summary Rayleigh Scattering

• Rayleigh spectra of individual SWNT
• General technique semiconducting and metallic
  SWNTs
• Signature of nanotube electronic structure ?
  physical structure
• Polarized along the nanotube direction
  depolarization effect
• Approach to study tube-tube interaction.

• Raman and Rayleigh scattering on specific SWNT
• Potential to combine with transport, mechanical
  measurements to study specific SWNT with known
  electronic structure.

48
Overall Summary

• Rayleigh scattering of individual SWNTs