We have studied the optical transition energies of single-wall carbon nanotubes over broad diameter (0.7-2.3 nm) and energy (1.26-2.71 eV) ranges, using their radial breathing mode Raman spectra. We establish the diameter and chiral angle dependence of the poorly studied third and fourth optical transitions in semiconducting tubes. Comparative analysis between the higher lying transitions and the first and second transitions show two different diameter scalings. Quantum mechanical calculations explain the result showing strongly bound excitons in the first and second transitions and a delocalized electron wave function in the third transition.

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We investigate, through first-principles calculations, the stability and electronic structure of self-interstitials and vacancies in both hexagonal ͑graphite-like͒ and cubic boron nitride. We find that the self-interstitials N i and B i in hexagonal boron nitride (h-BN) have low formation energies, comparable to those of the vacancies V N and V B . For instance, we find that N i is the most stable defect in h-BN under N-rich and p-type conditions followed by the nitrogen vacancy. This is consistent with experimental findings of large concentrations of nitrogen interstitials and vacancies, and of the trapping of nitrogen in the hexagonal phase of BN thin films grown by ion-bombardment assisted deposition techniques. In contrast, in cubic boron nitride (c-BN) the self-interstitials have high formation energies as compared to those of the vacancies. As a consequence, the formation of vacancy-interstitial pairs in kickout processes would typically require much more energy in c-BN than in h-BN. This suggests that a possible role of the ion bombardment in favoring the growth of c-BN films is to generate a much larger amount of defects in the hexagonal phase than in the cubic phase.

We investigate, through first-principles calculations, lattice instabilities induced in diamond by the application of high shear stresses. For shear stresses as low as 95 GPa a lattice instability will occur, leading to graphitelike layered structures. This effect is highly anisotropic. The reversal of the direction of the applied shear forces may cause a change of 80 GPa in the shear stress value at which the instability develops. The same reversal also causes different bonds to be broken, resulting in a drastic change in the orientation of the resulting graphitelike structures. We also find that an additional compressive stress of 50 GPa along the (111) direction does not eliminate the shear-induced instability.

We report ab initio calculations for positively charged fragments of dry poly(dC)-poly(dG) DNA, with up to 4 C-G pairs. We find a strong hole-lattice coupling and clear evidence for the formation of small polarons. The largest geometry distortions occur in only one or two base pairs. They involve the stretching of weak bonds within each base pair, increasing the distance of positive hydrogens, and decreasing that of negative oxygens, to the region in which the hole localizes. We obtain an energy of approximately 0.30 eV for the polaron formation, nearly independent of the chain size. From it, we can estimate an activation energy for polaron hopping of approximately 0.15 eV, consistent with the available experimental value.