Transport properties of single vacancies in nanotubes

Abstract

We present density of states and electronic transport calculations of single vacancies in carbon nanotubes. We confirm that the defect reconstructs into a pentagon and a nonagon, following the removal of a single carbon atom. This leads to the formation of a dangling bond. Finally, we demonstrate that care must be taken when calculating the density of states of impurities in one-dimensional systems in general. Traditional treatments of these systems using periodic boundary conditions leads to the formation of minigaps even in the limit of large unit cells.Since their discovery by Iijima in 1991, 1 carbon nanotubes ͑CNTs͒ have attracted quite some interest from the scientific community. The prospect of using them for electronic applications ranging from nanotransistors 2 to sensors for hazardous gases 3,4 and viruses 5 offers tantalizing possibilities. In that aspect, the presence of defects 6 in CNTs can be used to alter their mechanical 7,8 and electrical properties, 9,10 enhancing the potential for applications.In the case of sensors, Andzelm et al. 11 showed that impurities interact little with the walls of pristine CNTs. Hence, it is generally believed that the interaction of various molecules is, in fact, mediated by defects generated either during the growth process or by later irradiation of the CNT. 6 For that reason, the design of nanotube-based devices entails the determination of the electronic and transport properties of CNTs in the presence of defects. In the case of irradiated CNTs, a combination of molecular dynamics ͑MD͒ and empirical potentials have been used to determine that the most common type of defect is a single vacancy. 12,13 Tight-binding electronic transport calculations of single vacancies have been performed for ͑4,4͒ ͑Ref. 14͒ and ͑5,5͒ ͑Ref. 15͒ CNTs. In both aforementioned cases, the authors removed one C atom from the CNT and the transport coefficients showed a dip symmetric about the Fermi level E F . This feature is also observed as a contribution to the density of states ͑DOS͒ of ͑5,5͒ CNTs. 16 Later calculations for a ͑10,10͒ nanotube using a Hamiltonian that provenes from density functional theory 17,18 ͑DFT͒ showed that the dip is lowered in energy, whereas there are two sharp antiresonances around the Fermi level. 19 However, the authors did not perform structural relaxations of the region surrounding the vacancy site. The removal of a single atom from the CNT leaves three C atoms twofold coordinated in an energetically unfavorable situation. Therefore, reconstruction of the defect is likely to occur. In fact, both tight-binding MD calculations 20,21 and DFT-based atomic relaxations show that the single vacancy reconstructs. 22,23 In this Brief Report, we performed DFT structural relaxations of ͑5,5͒ nanotubes using both plane waves and localized basis sets. We model our device as a CNT with a single C atom removed, as shown in Fig. 1͑a͒. Because we are interested in the effects to the electronic and transport properties of isolated vacancies, we neglect the scattering due...

Modifying the electronic structure of semiconducting single-walled carbon nanotubes byAr+ion irradiation

“…In a recent study 85 of the transport properties of metallic SWNTs with SVs, it was pointed out that the size of the simulation supercell may affect the position of the midgap peak and thus one has to be careful when stating which changes are due to the defect and which are just simulation artifacts. We did not observe any of these effects caused by the changes in supercell size of the semiconducting systems we studied.…”

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

“…A primeira classe consiste nos defeitos que não possuem vacâncias, mas sua redeé distorcida através de uma rotação da ligação química, e o defeito que estudaremos desta classeé conhecido como Stone-Wales [17][18][19][20][21]. A segunda classe, por sua vez,é caracterizada pela falta de umúnicoátomo, e este defeitoé conhecido na literatura como monovacância [22][23][24][25][26]. E por fim, a terceira classe de defeitosé caracterizada pela falta de doisátomos ou divacâncias [27][28][29][30] e, para estaúltima classe, foram investigados dois tipos destes defeitos: 585 e 555777.…”

“…Podemos citar alguns exemplos dessa classe de defeitos, tais como monovacâncias [22][23][24][25][26] e divacâncias [24,[27][28][29][30]. A terceira classe de defeitos que estudaremos nesta tese consiste nos defeitos de Wigner [31][32][33], defeito este queé formado por um par de Frenkel -vacância-interstício (V-I), onde umátomo do cristalé deslocado para a região intersticial.…”

Propriedades eletrônicas de nanofios semicondutores

“…Estes problemas são de fato importantes, e há discussões na literatura de como corrigi-los [44,45]. Estas correções propostas se aplicam sobretudo a sistemas cristalinos ou moleculares, sendo a situação mais complicada em superfícies e fios, conforme discutido nas referências [46,47] e no Apêndice A desta tese. Nestes casos, o problema pode ser minimizado de forma a se extraírem dados, principalmente de natureza comparativa, que nos permitem fazer certas constatações com relação ao sistema que se propõe estudar.…”

“…Isso porque estados de natureza semelhante, ou mesma simetria, tendem a se repelir e afastar. Em um trabalho recente [47], demonstrou-se que em sistemas dopados, em particular unidimensionais como nanotubos de carbono, a existência de um potencial periódico gera mini-gaps na estrutura de bandas, que contudo não tem significado físico, pois desaparecem quando a aproximação de supercélulaé suprimida através de cálculos com funções de Green. Apesar de nossas estruturas de bandas mostradas em 4.16 terem sido obtidas com a presença de um potencial periódico associadoà impureza, comparamos duas situações análogas observando os vãos entre estados em um caso e não no outro.…”