Laser excitation of carbon nanostructures

Defects in carbon nanotubes

Carbon nanotubes (CNTs), single- and multi-walled exhibit fascinating electronic and mechanical properties but the presence of defects such as vacancies or pentagon-heptagon (5-7) pairs have a serious impact on these properties. We investigate the possibility of removing pentagon-heptagon defects with the help of ultrashort laser pulses. Using non-adiabatic molecular dynamics simulations we show that pentagon-heptagon defects become instable in the presence of a strong electron-hole plasma of app. 7% of the valence electrons. This leads to an ultrafast healing of the defects. Observations that resonant Raman spectra show a ten-fold intensity increase after intense laser irradiation of a carbon nanotube ensemble support our prediction of laser-induced healing of defects.

Figure: Snapshots of the response of a (7,7) carbon nanotube to an ultrashort laser pulse leading to the elimination of the pentagon-heptagon defect.

A. H. Romero, M. E. Garcia, F. Valencia, H. Terrones, M.Terrones, and H. O. Jeschke
Femtosecond Laser Nanosurgery of Defects in Carbon Nanotubes
Nano Lett. 5, 1361 (2005). (doi,pdf).

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Stability of carbon nanotubes in intense laser fields

We investigate the stability carbon nanotubes within a a large range of diameters and chiralities with respect to intense laser irradiation.

Figure: Damage thresholds of (n,n) carbon nanotubes as a function of diameter and laser pulse duration.

H. O. Jeschke, A. H. Romero, M. E. Garcia, and A. Rubio
Microscopic investigation of laser-induced structural changes in single-wall carbon nanotubes
Phys. Rev. B 75, 125412 (2007). (doi,pdf).

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Coherent phonons in capped carbon nanotubes

The growth process of carbon nanotubes is often spontaneously terminated by the formation of a spherical cap, ending the growth of the nanotube. A method to open capped nanotubes could have important applications in the control of the growth process. We investigate the response of capped carbon nanotubes to intense laser fields. We find that short laser pulses are suitable for an efficient removal of spherical caps from carbon nanotubes. The mechanism of this process can be identified as the excitatation of two coherent phonons in the tube body and caps, respectively. The different frequencies of these phonons lead to large strain and fracture in the region where tube and cap are joined.

Figure: Response of a (10,0) carbon nanotube to an intense laser pulse. The caps are cleanly separated from the tube body.

T. Dumitrica, M. E. Garcia, H. O. Jeschke and B. I. Yakobson
Selective cap opening in carbon nanotubes driven by laser-induced coherent phonons
Phys. Rev. Lett. 92, 117401 (2004). (doi,pdf)