Analysis of the chiral composition of a carbon nanotube surface by means of second harmonic generation

The second-order susceptibility tensor χ(2)(2ω;ω, ω) of a carbon nanotube (CNT) surface was determined as a function both of the average tube orientation and CNT topology distribution. The transformation properties of the hyperpolarizability β tensor, as rooted in the CNT isogonal symmetry point group, make non-vanishing only the chiral component of χ(2)(2ω;ω, ω), thus denying second harmonic generation (SHG) in racemic samples. Nevertheless, the rupture of the CNT spatial symmetry due to local deformations and to finite size can act as a source for SHG even in racemic samples. The study of SHG polarization and of the anisotropic dependence of the signal intensity on the angle Ω between the surface normal and the exciting laser beam direction are proposed as methods to discriminate between the two contributions. An experimental test of such a statement was performed on a sample grown by the laser annealing technique. A massive presence of chiral CNTs with strong local deformations was revealed by the low- and medium-wavenumber Raman spectra measured on the sample. The SHG polarization and the anisotropic dependence of the signal intensity as a function of Ω were studied experimentally, allowing the racemic nature of the investigated sample to be elucidated. Copyright © 2005 John Wiley & Sons, Ltd.