DMR 0244290:Electronic Correlationsin Carbon Nanotube

Apparao M. RaoJosé Menéndez

Damping of vibrations in carbon nanotubes•Tunneling and optical experiments give wildly different lifetimes for nanotube vibrations.•We measure the lifetimes using Raman spectroscopy and explain the puzzle.

Recent electron tunneling experiments indicate that the radial breathing mode (RBM) in carbon nanotubes has lifetimes as long as 10 ns. This is contradicted by Raman scattering experiments (both in the frequency and time domains) which yield lifetimes of less than 2 ps. How can this be?

Suspended SWNTs were prepared via a chemical vapor deposition method on silicon. Trenches measuring 5 m wide, 10 m in length and 3 m in depth were etched in silicon substrates using focused ion beam microscopy. Individual suspended SWNTs were then grown across these trenches using the “rapid-heating” method.

Raman phonons decay into other phonons. In one-dimensional systems there are very few secondary phonons, and they can create a decay bottleneck. The simulated spectrum (a) has a sharp spike (long lifetime of secondary phonons) and a broad tail (short lifetime of primary phonon). Instrumental broadening, simulated in (b), eliminates the spike. But the time-domain transformation of the spike determines the tunneling lifetimes.

High-resolution Raman spectra of individual SWNTs were obtained under extreme resonance conditions. Line-widths as small as 0.7 cm-1 were found. This implies lifetimes 3 times longer than previously assumed but still much shorter than the tunneling results.

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41.3 mW

2.1 mW

1. 7 mW

2.6 mW

2.6 mW

0.9 mW

0.4 mW

(a) (b)

DMR 0244290:Electronic Correlationsin Carbon Nanotube

Apparao M. RaoJosé Menéndez

The group maintains close collaborations with the group of Prof. E. Molinari at the University of Modena in the field of modeling the Raman cross section of carbon nanotubes including excitonic effects.

Broader impacts•Research could impact understanding of transport in one-dimensional systems.•Strong ASU-Clemson collaboration has been established.

The study of anharmonic effects in one-dimensional systems is not only fascinating from a fundamental science perspective but could also have a significant impact on our understanding of transport properties.

Carbon nanotubes have been proposed as interconnects. The presence of large non-equilibrium phonon populations as a result of decay bottlenecks could increase the scattering of electrons by phonons and reduce the electrical conductivity of these materials.

A close collaboration has been established between Clemson University and Arizona State University. Graduate student Rahul Rao from Clemson has spent time in Tempe, and the superb semiconductor fabrication capabilities at ASU are now being used to engineer new silicon substrates for the growth of improved suspended nanotube samples by the Clemson group. A collaboration with the University of Michigan (Roberto Merlin) is also planned in the near future.

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