Formation of silicon-fullerene-linked nanowires inside carbon nanotubes: A molecular-dynamics and first-principles study

We study the formation of Si nanowires inside carbon nanotubes by using a combination of empirical molecular-dynamics and first-principles approaches. Molecular-dynamics simulations demonstrate that liquid Si encapsulated into a (13,0) nanotube crystallizes into a nanowire composed of linked ${\mathrm{Si}}_{16}$ fullerene cages. On the other hand, a nanowire composed of linked ${\mathrm{Si}}_{20}$ fullerene cages forms inside a (14,0) nanotube. The stabilities of these nanowires are further confirmed by first-principles calculations. We also find that the freestanding ${\mathrm{Si}}_{16}$-linked nanowire is a metal, while the ${\mathrm{Si}}_{20}$-linked nanowire is a semiconductor. The present findings suggest that the choice of the nanotube size allows us to control the structure of Si nanowires, and therefore to tailor the material properties.

Figure 1

(Color online) (a) Time evolution of the interaction energy of Si atoms for ${\mathrm{Si}}_{52}@\left[(13,0)\mathrm{CNT}\right]$ (solid line) and ${\mathrm{Si}}_{65}@\left[(14,0)\mathrm{CNT}\right]$ (dashed line). Snapshots of the atomic configurations corresponding to the filled and open circles are given in (b) and (c).

Figure 2

(Color online) (a) Structure of ${\mathrm{Si}}_{52}@\left[(13,0)\mathrm{CNT}\right]$ optimized by using the DFT calculations. (b) Isosurface $(0.05\text{electrons}∕{\text{bohr}}^3)$ of the electron density for ${\mathrm{Si}}_{52}@\left[(13,0)\mathrm{CNT}\right]$ (Ref. 42). (c) Isosurface $(0.035\text{electrons}∕{\text{bohr}}^3)$ near the Si atoms. Results for ${\mathrm{Si}}_{65}@\left[(14,0)\mathrm{CNT}\right]$ are given in (d)–(f).

Figure 3

(Color online) Electronic band structures of (a) $C_{4v}\text{−}{\mathrm{Si}}_{16}$- and (b) $C_{5v}\text{−}{\mathrm{Si}}_{20}$-linked NWs. Energy is measured from the Fermi energy. Bands drawn in bold (red) lines are the $\pi$ bands of $s{p}^2$ bonded Si atoms. The unit cell of the nanowires is depicted in the upper parts. The vertical lines represent the optimized lengths of the unit cell, which are $1.048\mathrm{nm}$ ($C_{4v}\text{−}{\mathrm{Si}}_{16}$-linked NW) and $1.068\mathrm{nm}$ ($C_{5v}\text{−}{\mathrm{Si}}_{20}$-linked NW). The light gray (yellow) atoms are joined by $s{p}^2$-like bonds. The other atoms, which are the gray (orange) and dark gray (red) atoms, are joined by $s{p}^3$ bonds.