Formation of Nanotube-Based Quantum Dots With Strain and Addimers
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149 Mat. Res. Soc. Symp. Proc. Vol. 593 ©2000 Materials Research Society
To investigate effects of addimers on strained nanotubes, we used large-scale classical simulations based on the Tersoff-Brenner model for carbon [19]. Some of our most important results have also been confirmed with tight-binding calculations [20, 24]. ADDIMERS AND MECHANICAL TRANSFORMATIONS When an addimer is deposited on the surface of a nanotube, or alternatively, when two adatoms come together on a single hexagon, they form a (7-5-5-7) defect, as shown in Fig.1. This defect is distinct from the (5-7-7-5) defect, in that the pentagons are now back-to-back. If the tube is under strain, this (7-5-5-7) defect will undergo substantial transformations, thereby forming the new set of extended defects. The bond emanating from one of the pentagon's outward vertices rotates to form the defect structure consisting of a single, rotated hexagon which is separated from the rest of the nanotube by a "layer" of (5-7) pairs, as shown in Fig.l(d). Moreover, this process can continue, as shown in Fig.l(f), where a second bond rotation leads to a defect with two enclosed hexagons.
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Figure 1: Simulated STM images (top) and their atomic configurations of the defects (bottom) illustrating the evolution of a carbon addimer absorbed by a (17,0) tube under 10% strain: (a,b) (7-5-5-7) defect, (cd) defect with single hexagon and (e,f) defect with two hexagons. The STM images are for a Vup = -0.5 eV.
If this evolution were to continue, one ultimately obtains a defect structure consisting of layers of rotated hexagons, which would be wrapped about the circumference of the nanotube. These rotated hexagons would be separated from the rest of the tube by sets of (5-7) pairs. As these defects are most likely to be identified with STM, we present the corresponding simulated images in Fig.1. These images were obtained using a tight-binding method due to Meunier and Lambin [221 with parameters as detailed in Ref.[21]. All configurations were first fully relaxed using the Tersoff-Brenner potential. Images were then calculated in a fixed current mode with a probe tip bias voltage of -0.5 eV, as the probe tip is rastered over the surface. From these images we see that the most prominent features are bright rings, which are associated with the pentagons of the defect. The exception is the initial (7-5-5-7) defect. We note, that in these STM images the usual asymmetries associated with pristine (17,0) tube [23], are only locally disrupted by the defects. It may be shown, that these "rings" are related to enhancements of the local density of states (LDOS) of the defects [21]. 150
To further understand the transformations that these sets of defects are likely to undergo, we have studied their formation energies as a function of the number of hexagons, as shown in Fig.2. The formation energies are quoted for the most favored structures only, and are measured relative to two separate adatoms spaced far apart on the strained tubes. For instance, there are at least two d
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