Defect energetics, thermal stability and localized electronic states in carbon nanotubes
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Defect energetics, thermal stability and localized electronic states in carbon nanotubes Roberto Conversano(1), Fabrizio Cleri(2,3), Gregorio D’Agostino(2), Vittorio Rosato(1,3), Manuela Volpe(2,4) (1) ENEA, Casaccia Research Center, HPCN Project, 00100 Roma (Italy) (2) ENEA, Casaccia Research Center, Divisione Nuovi Materiali, 00100 Roma (Italy) (3) Istituto Nazionale di Fisica della Materia (INFM), Unità di Ricerca di Roma I (4) Dip.to di Scienze e Tecnologie Chimiche, Università di Roma “Tor Vergata”, 00133 Roma (Italy)
ABSTRACT Tight Binding molecular dynamics simulations have been performed on single wall carbon nanotubes, in order to evaluate thermal stability and the effect of the most relevant defects (the single vacancy and a Stone-Wales -SW- defect). The nanotubes are stable up to the graphite instability temperature. Both the considered defects have a large formation energy (EF(vac)=6.10 eV, EF(SW)= 5.55 eV).
INTRODUCTION Single Wall Carbon Nanotubes (SWNT) constitute prototype systems of one dimensional carbon structures with sp2 bonding. The theoretical relevance of such objects has been discussed in several papers [1-3]. We limit ourselves here to recall their use as elemental building blocks for functional nanostructures, as host for intercalation structures, and for their possible role in microelectronics. Although several methods to produce carbon SWNT have been proposed, much remains to be understood on the mechanism of their formation and on the role played by the defects in enhancing or degrading certain functional properties. In this work we have studied the thermal behavior of these objects; furthermore we evaluated the formation energy of some defects (the single vacancy and the 5/7 rings pair, Stone-Wales SW defects hereafter) and assessed the extent of their perturbation on the SWNT structure and properties.
MODEL AND COMPUTATIONS The atomic structures selected for the computation are SWNT (6,6) and (10,0). They are considered as prototypes of zig-zag and armchair structures, respectively. The two structures have a similar diameter d: d (10, 0)= 7.83 Å, d (6,6)= 8.14 Å. Finite temperature simulations have been carried out on model systems containing N=408 and N=400 for the (6,6) and the (10,0) SWNT, respectively. Periodic boundary conditions have been used in the direction parallel to the tube axis. An orthogonal Tight Binding (TB) model [4] has been used to describe particles interaction both in the O(N3) formulation [5] and in the O(N) linearly scaling formulation [6]. TB molecular dynamics in the O(N) formulation has been used for the high temperature simulations, where extensive calculations (for times of the order of tens of picoseconds) were necessary to assess the system thermal stability. A14.8.1
The behavior of the defect-free (6,6) SWNT has been studied, at different temperatures and at constant vanishing pressure, in order to locate possible instability regions at temperature below the graphite instability temperature. In fact, it has been recently pointed out [7], , that thr
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