First-principles prediction of stable SiC cage structures and their synthesis pathways

In this paper we use density functional theory calculations to investigate the structure and the stability of different SiC cagelike clusters. In addition to the fullerene family and the mixed four and six membered ring family, we introduce a family based on reconstructed nanotube slices. We propose an alternative synthesis pathway starting from SiC nanotubes.

Figure 1

(Color online) Dependence of the energetic stability from the chemical ordering. Different stoichiometric structures of SiC cages of 60 atoms. The number of alternating Si(green)-C(black) bonds and the energy per SiC pair (meV) are indicated. A partially segregated chemical ordering is energetically defavorable wrt a fully alternating one.

Figure 2

(Color online) Energetic behavior of SiC $\text{NTS}n$ families. Energy per Si-C pair of different $\text{NTS}n$ cages as a function of their diameter $m$. Each cage has $nm$ Si-C pairs. The dotted line indicates the results from the model described in the Appendix. The correct optimal diameter for each family is reproduced as a result of the balance between the energy of the body $\left[\mathcal{O}\left(1/m\right)\right]$ and of the termination of the cage $\left[\mathcal{O}\left(\sqrt{m}\right)\right]$. The structure and terminations of the NTS SiC cages are drawn for $m=6,8,10$ diameters on the bottom panel. Such terminations have stretched (red/dark gray) and compressed (blue/light gray) bonds. The termination appears more deformed as the diameter $m$ increases. An illustration of the $2\times 1$-like termination is drawn on the right panel. The latter termination is predicted to be more stable for bigger diameter $\left(m\ge 19\right)$.

Figure 3

(Color online) HOMO-LUMO gaps of some cages belonging to the $\text{NTS}n$ families, as a function of their diameter $m$.

Figure 4

(Color online) Energy per SiC pair of the 4–6 cages. We have considered octahedral structures of 4–6 SiC cages (filled diamonds) and nonoctahedral cages (empty diamonds). The main contribution to the energy of these structures comes from the spheroidal curvature as illustrated for the cages of size 96, 216, 384. This curvature decreases as the number of pair $N_c$ increases $\left[\mathcal{O}\left(1/\sqrt{N_c}\right)\right]$ (see the Appendix). These structures have also stretched (red/dark gray) and compressed (blue/light gray) bonds located around the squares whatever their size. The black dotted line indicates the result from the model fitted on cages with octahedral shape. The energies of the most stable $\text{NTS}n$ cage (filled circles) for a given number of pairs is indicated.

Figure 5

(Color online) Different structures originated from the cut of $n=6$ slices of a (5,5) armchair nanotube (60 atoms). The dangling bonds of the unreconstructed cut can recombine in a $2\times 1$-like reconstruction or into the $\text{NTS}n$ structure, which is more stable since it has a bigger number of Si-C bondings, even tough is more deformed.

Figure 6

(Color online) Fusion procedure of two NTS6 cages into one bigger NTS12 cage. As a first stage, it is energetically convenient to maximize the Si-C bonds. Eventually, the reduction of the deformation in the middle of the structure will complete the procedure.