Liplip Interaction Models for Multiwalled Nanotube Growth

Additional carbon atoms (spot-weld), bridging the dangling bonds between shells of a multilayered structure, have also been proposed for the stabilization of the open growing edge of multishell tubes [21]. For multiwall species, it is quite likely that the presence of the outer walls should stabilize the innermost wall, keeping it open for continued growth (Fig. 6). Static tight-binding calculations performed on multilayered structures where the growing edge is stabilized by bridging carbon adatoms, show that such a mechanism could prolong the lifetime of the open structure [21].

Quantum molecular dynamics simulations were also performed to understand the growth process of multi-walled carbon nanotubes [22,23]. Within such calculations, the topmost atoms (dangling bonds) of the inner and outer edges of a bilayer tube rapidly move towards each other, forming several bonds to bridge the gap between the adjacent edges, thus verifying the assumption that atomic bridges could keep the growing edge of a nanotube open without the need of "spot-weld" adatoms (Fig. 7). At ~3000K (a typical experimental growth temperature), the "lip-lip" interactions stabilize the open-ended bilayer structure and inhibit the spontaneous dome closure of the inner tube as observed in the simulations of single-shell tubes. These calculations also show that this end geometry is highly active chemically, and easily

Fig. 6. Schematic ball-and-stick representation of the top of a multiwall nanotube with an open zigzag edge. Only two of many layers are shown for clarity. Three-coordinated carbon atoms are represented by white spheres, while low coordinated carbon atoms (dangling bonds and bridging atoms) are represented as light grey spheres on the top of the structure. Several "spot-weld" adatoms are shown occupying sites between doubly coordinated edge atoms of adjacent layers. The nanotube growth is enabled by these adatom spot-welds which stabilize the open configuration [21]

Fig. 6. Schematic ball-and-stick representation of the top of a multiwall nanotube with an open zigzag edge. Only two of many layers are shown for clarity. Three-coordinated carbon atoms are represented by white spheres, while low coordinated carbon atoms (dangling bonds and bridging atoms) are represented as light grey spheres on the top of the structure. Several "spot-weld" adatoms are shown occupying sites between doubly coordinated edge atoms of adjacent layers. The nanotube growth is enabled by these adatom spot-welds which stabilize the open configuration [21]

accommodates incoming carbon clusters, supporting a model of growth by chemisorption from the vapor phase.

In the "lip-lip" interaction model, the strong covalent bonds which connect the exposed edges of adjacent walls are also found to be highly favorable energetically within ab initio static calculations [24]. In the latter work, the open-ended growth is stabilized by the "lip-lip" interactions, involving rearrangement of the carbon bonds, leading to significant changes in the growing edge morphology. However, when classical three-body potentials are used, the role of the "lip-lip" interactions is relegated to facilitate tube closure by mediating the transfer of atoms between the inner and outer shells [25]. This example shows that very-accurate ab initio techniques are required to simulate the quantum effects in a "fluctuating dangling bond network", which is likely present at the growing open edge of a multiwall carbon nanotube.

Successful synthesis of multi-wall nanotubes raises the question why the growth of such tubular structures often prevail over their more stable spherical fullerene counterpart [24]. It is furthermore intriguing that these nanotubes are very long, largely defect-free, and (unless grown in the presence of a metal catalyst) always have multiple walls. The "lip-lip" model explains that the sustained growth of defect-free multi-wall carbon nanotubes

Fig. 7. Creation (a) and stabilization (b) of a double-walled (10,0)@(18,0) nano-tube open edge by "lip-lip" interactions at ~3000K. The notation (10,0)@(18,0) means that a (10,0) nanotube is contained within an (18,0) nanotube. The direct incorporation (c) of extra single carbon atoms and a dimer with thermal velocity into the fluctuating network of the growing edge of the nanotube is also illustrated [22,23]. The present system contains 336 carbon atoms (large white spheres) and 28 hydrogen atoms (small dark grey spheres) used to passivate the dangling bonds on one side of the cluster (bottom). The other low coordinated carbon atoms (dangling bonds) are represented as light grey spheres on the top of the structure

Fig. 7. Creation (a) and stabilization (b) of a double-walled (10,0)@(18,0) nano-tube open edge by "lip-lip" interactions at ~3000K. The notation (10,0)@(18,0) means that a (10,0) nanotube is contained within an (18,0) nanotube. The direct incorporation (c) of extra single carbon atoms and a dimer with thermal velocity into the fluctuating network of the growing edge of the nanotube is also illustrated [22,23]. The present system contains 336 carbon atoms (large white spheres) and 28 hydrogen atoms (small dark grey spheres) used to passivate the dangling bonds on one side of the cluster (bottom). The other low coordinated carbon atoms (dangling bonds) are represented as light grey spheres on the top of the structure is closely linked to efficiently preventing the formation of pentagon defects which would cause a premature dome closure. The fluctuating dangling bond network present at the nanotube growing edge will also help topological defects to heal out, yielding tubes with low defect concentrations. With nonzero probability, two pentagon defects will eventually occur simultaneously at the growing edge of two adjacent walls, initiating a double-dome closure. As this probability is rather low, carbon nanotubes tend to grow long, reaching length to diameter ratios on the order of 103-104.

Semi-toroidal end shapes for multi-wall nanotubes are sometimes observed experimentally [11,17]. The tube, shown in Fig. 8a does not have a simple double sheet structure, but rather consists of six semi-toroidal shells. The lattice images turn around at the end of the tube, so that an even number of lattice fringes is always observed. Another example is shown in Fig. 8b, in which some of the inner tubes are capped with a common carbon tip structure, but outer shells form semi-toroidal terminations. Such a semi-toroidal termination is extremely informative and supports the model of growth by "lip-lip" interactions for multi-wall nanotubes.

Fig. 8. Transmission electron micrographs of the semi-toroidal termination of multi-wall tubes, which consists of six graphitic shells (a). A similar semi-toroidal termination, where three inside tube shells are capped (b) [17]
0 0

Post a comment