Metallic-Enriched Single-Walled Carbon Nanotubes for Electronics Applications

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Metallic-Enriched Single-Walled Carbon Nanotubes
for Electronics Applications

• Erik H. Hároz
• NASA-Rice Nanotechnology Forum
• May 18, 2010

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Single-Walled Carbon Nanotubes
Chiral vector
Ch na1 ma2
n m 3M q
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Metallic Carbon Nanotubes

• Metallic SWNTs can be divided into two
  subclasses, identifiable by their chiral indices
  (n,m)
• True metallic or armchair nanotubes where
  (n-m)0 such as the (7,7)
• Narrow-gap semiconducting nanotubes where (n-m)
  integer multiple of 3 such as the (12,6)
• Are ballistic conductors with electrical
  conductivity 100x greater than copper and
  electron mobility 70x that of silicon.
  Current-carrying capacity 109 A/cm2.
• Ideal materials for low-loss, high-capacity,
  power transmission cables and nanometer-sized
  electronics.

Tans et al., Nature, 386, (1997), 474

• Problems studying metallic SWNTs
• In bulk material, theoretically, 1/3 of all
  possible chiralities are metallics.
  Outnumbered by semiconductors.
• In HiPco SWNTs, E22 semiconducting transitions
  overlap with E11 metallic transitions.
• Radial breathing mode of armchair SWNTs is very
  weak in Raman spectra as compared to other
  chiralities because the electron-phonon coupling
  being weakest for armchair SWNTs.

Solution Make samples consisting of all
metallic SWNTs.
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Density Gradient Ultracentrifugation
HiPco HPR 188.1 SWNTs suspended in 1 sodium
deoxycholate (1mg/mL starting conc.) Sonicated
for about 30 min in bath sonicator, 20 hr in tip
sonicator, Decant prepared using 1hr
centrifugation _at_ 200,000g Run in a 40-20
iodixanol gradient in 1.5 sodium dodecyl
sulfate, 1.5 sodium cholate
Centrifugated for 18 hrs _at_ 200,000g

• Arnold et al., Nature Nanotechnology, 1, (2006),
  60
• -Optical Absoprtion Characterization and sheet
  conductance measurements
• Yanagi et al., Applied Physics Express, 1,
  (2008), 034003
• -Optical Absorption Single-line Excitation
  Raman characterization
• Iijima, et al., Nano Letters, 8, (2008), 3151
• - E-beam diffraction TEM chirality assignment of
  Kataura sample

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Absorption Spectroscopy

• Sharp, narrow, well-defined peaks with large
  absorbance in E11M region.
• Flat, featureless section in E22S E11S regions.
  
• Overlap between E11M E22S regions eliminated.
• Absorption features have enhanced peak-to-valley
  ratios.
• Decrease in baseline of spectrum indicates
  increase in degree of individuality.
• Based on absorption peak areas, sample is 98
  metallic.

• Peaks correspond to absorption of light at
  energies corresponding to excitonic transitions
  of specific (n,m) species of SWNTs
• Transitions are roughly proportional to inverse
  diameter but also depend on chiral angle and mod
  as well.
• Absorption is most direct optical method to
  measure (n,m) species populations.
• Sharpness of features and slope of baseline
  qualitatively indicate degree of individuality.

x 0.1
x 2
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Photoluminescence Excitation Spectroscopy
Starting SWNT material
Metallic-enriched SWNTs

• Only individualized, wide-gap semiconducting
  SWNTs fluoresce in the
• near-infrared via visible excitation.
• ?Individualized, narrow-gap semiconducting SWNTs,
  armchair SWNTs,
• and bundles of SWNTs containing metallics do
  not fluoresce.

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Resonant Raman Spectroscopy

• Resonant Raman scattering with a tunable
  excitation source is the only optical method able
  to identify all chiralities present, including
  the armchairs (n,n).
• Using excitation sources including
• CW TiSapphire laser (695-850 nm)
• Kiton red laser dye (610-685 nm),
• Rhodamine 6GB laser dye (562-615 nm),
• Ar laser (514.5, 501.7, 496.5, 488, 476.5,
  457.8 nm)
• doubled CW TiSapphire (500-440 nm)
• 5 weeks and 230 spectra later.

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Raman (562-670 nm)
Metallic-enriched HiPco
As-produced HiPco
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Raman (440-500 nm)
Metallic-enriched HiPco
As-produced HiPco
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Enrichment of Armchair Nanotubes

• Probing further using Raman scattering, we find
  not only did we enrich in metallic nanotubes but
• Of those metallic species, a large majority
  (50) are armchairs (n,n).

Results summarized in Hároz et al., ACS Nano 4,
1955 (2010).
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Heading Towards Single Armchair Species Samples
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Absorption of Films UV-vis-NIR

• 100 SWNT films produced by vacuum filtration
  from DGU-enriched solutions.
• Sharp, narrow, well-defined peaks with large
  absorbance in E11M region.
• Metallic features remain relatively unperturbed
  in film form probably due to screening.
• Broadened, redshifted peaks in E22S E11S
  regions.

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RBM Raman of DGU Films
(7,7)
514 nm excitation
Very little iodixanol

• Enrichment results in suppression of (8,5),
  (9,3) (8,2) leaves behind mostly (7,7).
• Very little density gradient medium left.

(8,5)
(8,2)
(9,3)
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Absorption of Films Terahertz

• THz examines optical conductivity.
• Response thought to be due to concentration of
  metallic SWNTs (i.e. THz absorbance proportional
  to conductivity).
• Although metallic films have lower overall
  optical absorption, they possess greater more
  metallic nanotubes.

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How can these materials be used in research?

• In the Kono group, we are using these enriched
  materials to look at
• Temp. dependent DC magneto-transport
• Optical spectroscopy
• Electron spin resonance
• Ultrafast spectroscopy
• Terahertz conductivity
• Pump-probe
• We are also looking at ways to scale separations
  using column chromatography

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Armchair Quantum Wire Program

• More broadly at Rice, we are engaged in trying to
  create macroscopic structures (films and wires)
  comprised of armchair nanotubes.
• Primary question to answer
• While individual armchair SWNTs are excellent
  ballistic conductors, how about in
  macrostructures?
• What dominates, tunneling barriers (i.e. variable
  range hopping)? Can this be overcome?

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Acknowledgements

• Prof. Junichiro Kono (Rice, advisor)
• Prof. R. Bruce Weisman (Rice)
• Dr. Stephen K. Doorn (LANL)
• Dr. Robert H. Hauge (Rice)
• Mr. William D. Rice (Rice)
• Mr. Saunab Ghosh (Rice)
• Mr. Benjamin Y. Lu (Rice)
• Mr. Budihpta Dan (Rice)
• Mr. Lei Ren (Rice)
• Funding provided by DOE, AFRL, NSF, LANL LDRD
  program, and Welch Foundation.