Electroabsorption Spectroscopy of Carbon Nanotubes

Elucidation of the Electronic Structure of Semiconducting Single Walled Carbon Nanotubes by Electroabsoprtion Spectroscopy

Hongbo Zhao and Sumit Mazumdar

PRL 98, 166805 (2007)

URL: http://link.aps.org/abstract/PRL/v98/e166805

That title is quite a mouthful!

The paper discusses how electroabsorption might be used to determine exciton binding energies and the free-particle excitation gap in carbon nanotubes.

The basic idea of electroabsorption spectroscopy is very simple. You make an absorption measurement on a nanotube sample, then you turn on an electric field and repeat the measurement. By subtracting off the free nanotube data, you can observe the effect of the electric field.

da(E,w) = a(E,w) - a(0,w)

Mazumdar and Zhao identify three noteworthy features in the nanotube spectrum, two of which can be used to determine the exciton binding energy. Surprisingly, it seems this quantity is not known. The splitting of different exciton peaks are usually studied in photoluminescence experiments, but it seems the free particle excitation gap is harder to probe. Without knowing the free particle gap, one cannot infer the exciton binding energy.

The most prominent feature in the graphs are the oscillations in the free-particle continuum. I believe these are the Franz-Keldysh oscillations studied by Perebeinos, et alii. I feel that Perebeinos description of the features and the physical mechanisms responsible are much clearer. That paper was published in Nanoletters, but I've only read the arXiv version so far.

After reading this letter, I finally understand what a Fano resonance is. It is a coupling between bound an continuum states. I don't know what its effects are, however. It seems like these would occur often in semiconductors for excitons in any band higher than the first. I don't see how it could occur for an atomic system, except in Rydberg systems where the ionization energy is very small.