Ground states of diatomic molecules adsorbed on single-walled carbon nanotubes

We have studied the adsorption of diatomic molecules on graphite sheets and carbon nanotubes using Monte Carlo simulations. We have obtained the ground states for two-site dimer adsorption. On the high-coverage side, the ground states for dimer adsorption are highly degenerate. The complementary vacancy structures and the variation tendency of the ordered structures with increase of nanotube size are similar to those for single-site adsorption. On the low-coverage side, most dimer ordered structures are not degenerate. The confinement of nanotubes leads to helical ground-state structures for dimer adsorption.

Figure 1

Ground-state phase diagrams for dimer adsorption on (a) graphite, (b) (6,0) nanotube, (c) (7,0) nanotube, (d) (8,0) nanotube, (e) (9,0) nanotube, and (f) (10,0) nanotube.

Figure 2

(a) Vacancy-ordered structures of the $O_1^G$, $O_2^G$, $O_3^G$, and $O_4^G$ phases with coverage $\theta >1∕2$. The white circles represent vacancies. The other hexagon centers are occupied by dimers. (b) Examples of dimer coverings of the $O_1^G$, $O_2^G$, $O_3^G$, and $O_4^G$ phases. Black circles joined by black lines represent adsorbed dimers, and hollow circles represent vacancies. (c) Ground-state structures of dimer adsorbates on the graphite with coverage $\theta ⩽1∕2$.

Figure 3

Ground-state structures on the (6,0) nanotube. The structure in the dashed rectangle is the repeating helical unit of the $O_{6c}$ phase. The axis direction of the nanotube and the helical direction of the repeating helical units are also shown.

Figure 4

Ground-state structures on the (7,0) nanotube. The structures in the dashed rectangles are the repeating helical units of the corresponding phases. The axis direction of the nanotube and the helical direction of the repeating helical units are also shown.

Figure 5

Ground-state structures on the (8,0) nanotube. The structures in the dashed rectangles are the repeating helical units of the corresponding phases. The axis direction of the nanotube and the helical direction of the repeating helical units are also shown.

Figure 6

Ground-state structures on the (9,0) nanotube. The structures in the dashed rectangles are the repeating helical units of the corresponding phases. The axis direction of the nanotube and the helical direction of the repeating helical units are also shown.

Figure 7

Ground-state structures on the (10,0) nanotube. The structure in the dashed rectangle is the repeating helical unit of the corresponding phase. The axis direction of the nanotube and the helical direction of the repeating helical units are also shown.

Figure 8

(a) Vacancy basic substructures of the $O_{2i}$, $O_3$, and $O_{4i}$ phases. The white circles represent vacancies. (b) Vacancy-derived substructures of the $O_{2i}$ phases in the case of $n=2k+1$. (c) Vacancy-derived substructures of the $O_{4i}$ phases in the case of $n=3k+1$. (d) Vacancy-derived substructures of the $O_{4i}$ phases in the case of $n=3k+2$. (e) Representation of the vacancy-ordered structure of the $O_4$ phase by two $B_4$ vacancy basic substructures in the case of $n=6$. (f) Representation of the vacancy-ordered structure of the $O_{4b}$ phase by two $B_4$ vacancy basic substructures and one vacancy-derived substructure $D_{4a}^I$ in the case of $n=7$. (g) Representation of the vacancy-ordered structure of the $O_{4b}$ phase by two $B_4$ vacancy basic substructures and one vacancy-derived substructure $D_{4b}^{II}$ in the case of $n=8$.

Figure 9

Variation of the coverage as a function of $n$.