Electronically Homogeneous, Photonic Semiconducting Single-Walled Carbon Nanotubes Bundles In Solid Phase

The ability to synthesize and manipulate uniform bundles of electronically pure single walled carbon nanotubes (SWNTs) remains central to the advancement of these systems in nano-electronic applications. The assembly of separated SWNT into bundles (parallel aggregates) that are electronically homogeneous and of controlled size is currently beyond engineering capabilities. Using the advance of both density gradient^1,2 and dielectrophoresis methods³, we have performed the first synthesis and measurements of electronically homogeneous bundles of SWNTs in solid state.⁴ Several interesting properties are noted. We find that bundles of semi-conducting SWNT that are free of all metallic impurities emit photoluminescence (PL) in the near infrared, as anticipated by theory.⁵ The emission is an important quality control metric, and allows for photophysical characterization of such systems. TEM image reveals a dielectrophoretically synthesized 36 nm bundle of (6,5)-enriched SWNTs bundle attached to the very end of a supporting chemically etched tungsten tip. A vertical conductance measurement characterizes the resistance with length and reveals non-ohmic, step changes in current at constant potential characteristic of ballistic conductance. A scattering length, L, of approximate 4 to 18 nm is calculated from the trace. We also find that such solid bundles show unprecedented significantly bright PL emission as imaged using an InGaAs camera at both 658 and 785 nm laser excitations, confirming a high degree of semiconductor purity. Controls with un-enriched samples show no emission. The G/G₀, where G and G₀ are conductance and quantum conductance 2e²/h ≈ 1/13 (k W)^-1 respectively, ranges from 0.3 to 2.2 for a typical un-enriched SWNT bundle. High purity semi-conducting bundles exhibit significantly low ratios at about 0.002 to 0.004. Some interesting applications involving environmentally-related PL shift from these semiconducting solid bundles will be discussed.

References

1. Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam M. C. Nature Nanotech. 2006, 1, 60-65.

2. Kim, W. -J.; Usrey M. L.; Strano, M. S. Chem. Mater. 2007, 19, 1571-1576.

3. Lee, C. Y.; Baik, S.; Zhang, J.; Masel, R. I.; Strano, M. S. J. Phys. Chem. B 2006, 110, 11055-11061.

4. Han, J. H.; Maruyama, R.;, Kim, W. -J.; Lee, C. Y.; Choi, J. H.; Heller, D. A.; Strano, M. S. 2008 (submitted).

5. Graff, R. A.; Swanson, J. P.; Barone, P. W.; Baik, S.; Heller, D. A.; Strano, M. S. Adv. Mater. 2005, 17, 980-984.