Mechanism for bias-assisted indium mass transport on carbon nanotube surfaces

In this paper, results of ab initio pseudopotential density-functional calculations of indium adsorption on graphitelike surfaces are presented. The adsorption energy was calculated as a function of In coverage, and it is shown that, for low surface densities, In becomes positively charged by donating about one electron to the underlying nanotube surface. This is consistent with experimental observations of bias-assisted In flux in the direction opposite to that of electron flow. The effects of nanotube surface curvature on In adsorption are shown to be small. Based on the calculated energy barrier between two neighboring energy minima and the calculated vibrational frequencies, the hopping rate for In adsorbed on graphene is estimated. It is also shown that In adsorption is stronger on and around a Stone-Wales defect on a graphene sheet, which suggests that defects can work as nucleation centers for crystal growth. Adsorption of In on BN sheets and of Au on graphene is also discussed. No significant charge transfer is present in these two alternative systems and the adsorption energies are weaker.

Figure 1

Four transmission electron microscope images, spaced by $1\min$, showing left-to-right indium transport on a single-carbon-multiwall nanotube (courtesy of B. C. Regan). The anode is out of view to the left and the cathode to the right. The In moves from left to right, opposite to the direction of electron flow. The numbers are a rough estimate of the number of In atoms of each nanocrystal assuming that they are spherical (M for million, K for thousand).

Figure 2

(Color online) (a) Projection of the grid points at which the energy of an adsorbed atom on a hexagonal lattice was calculated (see text). Minimum-energy landscape for (b) In adsorption on graphene at $1∕32$ $\mathrm{In}∕\mathrm{C}$ atoms density, (c) one In atom adsorbed on a BN sheet with 16 B and 16 N atoms/cell, (d) In adsorption on a (5,5) carbon nanotube at $1∕60$ $\mathrm{In}∕\mathrm{C}$ atoms surface density (arrow shows the direction of the nanotube axis), and (e) Au adsorption on graphene at $1∕32$ $\mathrm{Au}∕\mathrm{C}$ atom density. Black is the energy minimum while white is the energy maximum. For energy values refer to Table .

Figure 3

Energy versus optimal height at different sites of several adsorption systems computed within the LDA. The points are plotted with energies relative to the energy of an isolated adsorbate (either In or Au). For the In adsorption on a Stone-Wales defect, the energies were minimized at the centers of the pentagon, heptagon, and several hexagon sites, while for the other systems, the lowest-energy point is at the minimum energy, the highest-energy point is at the maximum energy, and the middle energies are at the saddle points of the energy landscapes shown in Fig. 2.

Figure 4

(a) LDA energy and (b) height of In adsorbates on graphene as a function of $\mathrm{In}–\mathrm{In}$ distance. In (a) and (b) the dots correspond to the calculated energy minima for several surface densities of In on top of graphene, and the solid line is a cubic spline fit. In (a) the zero of energy corresponds to the energy of an isolated In atom, and the dashed line represents the energy of a single isolated sheet of In in a triangular lattice as a function of In separation.

Figure 5

Comparison between densities of states of several adsorption systems (solid lines) and the DOS of an isolated adsorption surface (dashed lines) and the isolated adsorbate (dotted lines), all within the LDA. (a) In adsorbed on graphene at $1∕32$ $\mathrm{In}∕\mathrm{C}$ atoms density, (b) In adsorbed on graphene at the center of one pentagon of a Stone-Wales defect (the inset shows the unit cell of the Stone-Wales defect used), (c) In adsorbed on a BN sheet at $1∕32$ In atoms/surface atom, and (d) Au adsorbed on graphene at $1∕32$ $\mathrm{Au}∕\mathrm{C}$ atom density. The DOSs are convoluted with Gaussians of $0.4\mathrm{eV}$ standard deviation. The vertical lines are positioned at the Fermi level of the corresponding systems, where the Fermi level for the adsorption system is set at $0\mathrm{eV}$, and the Fermi levels of the isolated systems are adjusted to match the features of the DOS of the adsorption system.

Figure 6

(Color online) Isosurface plots of charge densities for the electronic charge associated with an In atom on top of a graphene layer and on top of a pentagon of a Stone-Wales defect. (a) Electronic charge density of the In@graphene system minus the charge density of an isolated graphene layer (total of three electrons), (b) planar cut of the electronic charge density associated with the electron donated by the indium to graphene, (c) electronic charge density of the In@Stone-Wales defect minus the charge density of an isolated Stone-Wales defect (total of three electrons), and (d) electronic charge density associated with the electron donated by the indium to graphene Stone-Wales defect.