Curvature and chirality dependence of the properties of point defects in nanotubes

Abstract

This article presents a systematic study of how point defects, such as SW-defects and vacancies, influence the properties of nanotubes. The DFT calculations show that large atomic relaxations at vacancies leads to a contraction of the nanotube. The formation energy ÓÖÑ , has a curvature, chirality and a family dependence, where ÓÖÑ is slightly lower in metallic compared to semiconducting nanotubes. Vacancies become electrically active due to defect states close to¯ and its population depend on the chirality and the position of¯ . A model based on the heat of formation of defective nanotubes furthermore provide an upper estimate for the defect concentration, which is in better agreement with values from AFM experiments than the standard equilibrium approximation.

Extraordinarily Long-Ranged Structural Relaxation in Defective Achiral Carbon Nanotubes

“…Since previous ab initio studies employing periodic boundary conditions could not take this relaxation mode into account they substantially overestimate the defect formation energy (see Table I). The a-tilt formation energy determined in the approximate tight-binding calculations of Lu and Pan [14] appear to be in closer agreement with our results than ab initio studies [13,[15][16][17][18]. However, this is the result of an underestimated formation energy canceling uncompensated strain energy arising from the application of periodic boundary conditions and a small supercell: Lu and Pan use the same combination of supercell size and periodic boundary conditions as in the ab initio calculations of Wang and Wang [16] for both a-tilt and z-axial defects and in both find the formation energy %0:7 eV lower.…”

“…Moreover, even with open boundary conditions exceptionally large supercells are required for full relaxation and accurate determination of the defect formation energy, demonstrating the extraordinarily long-ranged structural distortion associated with the formation of even the simplest vacancy structure in a SWCNT. Our results imply that calculated values for the energies (and hence the derived properties) of reconstructed monovacancies in the literature [13][14][15][16][17][18] are in error and that the reported formation energies of a wide spectrum of other defects in nanoscale carbon systems need further evaluation.…”

“…The formation energy of isolated 5-1db defects in achiral (armchair and zigzag) SWCNTs has been theoretically determined by a number of researchers [13][14][15][16][17][18] in calculations employing periodic boundary conditions with super-cells up to 10 unit cells long. Such approaches simulate an infinitely long tube with periodically repeating defects that introduce unphysical defect-defect interactions.…”

Reconstruction and alignment of vacancies in carbon nanotubes

“…An ideal single vacancy is energetically unstable with respect to a reconstructed structure with a new bond formed by the elimination of two dangling bonds. [16][17][18]20,21 The orientation and bond length of the reconstructed bond ͑henceforth referred to as the rebond͒ strongly depend on the tube chirality and diameter. In nanotubes, the reconstructed bond can have three different orientations: parallel ͑ ʈ ͒, diagonal ͑//͒, and perpendicular ͑Ќ͒ with respect to the tube axis.…”

Estudo da influência de defeitos estruturais nas propriedades de nanotubos de carbono

“…Esse estudo também foi realizado para nanotubos armchair e para zigzag, e mostra que, para nanotubos zigzag, a energia de formação oscila conforme o seu tipo (metal ou semicondutor), uma vez que os nanotubos zigzag podem assumir as duas formas. Este efeito já foi reportado anteriormente [53]. Num outro trabalho sobre divacância que merece destaque, entretanto para grafeno [27], os autores mostram que a divacância pode assumir duas formas (585 e 555777).…”