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Metallic-Enriched Single-Walled Carbon Nanotubes for Electronics Applications. Erik H. Hároz NASA-Rice Nanotechnology Forum May 18, 2010. a 2. Single-Walled Carbon Nanotubes. a 1. ( n , m ) = (4,2). Chiral vector:. C h = n a 1 + m a 2. n – m = 3 M + q. 1) M = q = 0.

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metallic enriched single walled carbon nanotubes for electronics applications

Metallic-Enriched Single-Walled Carbon Nanotubes for Electronics Applications

Erik H. Hároz

NASA-Rice Nanotechnology Forum

May 18, 2010

single walled carbon nanotubes

a2

Single-Walled Carbon Nanotubes

a1

(n,m) = (4,2)

Chiral vector:

Ch = na1 + ma2

n – m = 3M + q

1) M = q = 0

Metal (Armchair)

2) M≠0,q = 0

Narrow Gap Semicond.

3) M≠0,q = ±1

Metallic

Semiconducting

Wide Gap Semicond.

metallic carbon nanotubes
“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.

density gradient ultracentrifugation
Density Gradient Ultracentrifugation

Metallic-enriched

SWNTs

As-produced SWNTs

DGU

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 @ 200,000g

Run in a 40-20% iodixanol gradient in 1.5% sodium dodecyl sulfate, 1.5% sodium cholate

Centrifugated for 18 hrs @ 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
absorption spectroscopy
Absorption Spectroscopy
  • 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.
  • 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.

x 0.1

x 2

photoluminescence excitation spectroscopy
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.
resonant raman spectroscopy
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 Ti:Sapphire laser (695-850 nm)
    • Kiton red laser dye (610-685 nm),
    • Rhodamine 6G+B laser dye (562-615 nm),
    • Ar+ laser (514.5, 501.7, 496.5, 488, 476.5, & 457.8 nm)
    • doubled CW Ti:Sapphire (500-440 nm)
    • 5 weeks and 230 spectra later….
raman 562 670 nm
Raman (562-670 nm)

Metallic-enriched HiPco

As-produced HiPco

raman 440 500 nm
Raman (440-500 nm)

Metallic-enriched HiPco

As-produced HiPco

enrichment of armchair nanotubes
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 Nano4, 1955 (2010).

absorption of films uv vis nir
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.
rbm raman of dgu films
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)

absorption of films terahertz
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.
how can these materials be used in research
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
armchair quantum wire program
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?
acknowledgements
Acknowledgements
  • Prof. JunichiroKono (Rice, advisor)
  • Prof. R. Bruce Weisman (Rice)
  • Dr. Stephen K. Doorn (LANL)
  • Dr. Robert H. Hauge (Rice)
  • Mr. William D. Rice (Rice)
  • Mr. SaunabGhosh (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.
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