We report large electroabsorption susceptibilities in the ultraviolet region for single-walled carbon nanotubes (SWNTs) supported on fused quartz substrates that are approximately 103 larger than the highest values reported to date for any system. The SWNT films were prepared on fused quartz substratres by a vacuum filtration method using a diluted suspension of HiPco SWNTs in H2O with 1 wt% sodium dodecyl sulfate. From electroabsorption spectra, the characteristic oscillatory behavior in the SWNT films was observed in the ultraviolet region, which decayed exponentially to the low energy side. It is easily described using a convolution of two sets of Airy functions in photon energy, attributing the effect to Franz-Keldysh oscillations [1]. The one-dimensional SWNT wave functions tunnel into the classically forbidden bandgap of the quartz substrate, creating a confined interfacial state demonstrating unusually strong Franz-Keldysh modulation. To address the origin of the unusually large susceptibility, the SWNT films with different ratios of metal to semiconductor content were prepared using a newly developed separation method [2]. By systematically varying the metallic and semiconducting SWNT composition of a series of films, the confinement energy is shown to correlate with the mean bandgap of semiconductors in the film. It is confirmed that the unusually large susceptibilities arise from a sub-percolated network of metallic SWNT that enhance the electric field by increasing the field gradient and decreasing the distance between conductive junctions.

References

[1] H. Shen and M. Dutta, J. Appl. Phys. 78, 2151 (1995).

[2] W.J. Kim, N. Nair, C.Y. Lee, and M.S. Strano, J. Phys. Chem. C in press.