Columnar-to-Disk Structural Transition in Nanoscale $({\mathrm{SiO}}_2{)}_N$ Clusters

Extensive large-scale global optimizations refined by ab initio calculations are used to propose $({\mathrm{SiO}}_2{)}_N$ $N=14–27$ ground states. For $N<23$ clusters are columnar and show $N\mathrm{\text{−}}\mathrm{odd}–N\mathrm{\text{−}}\mathrm{even}$ stability, energetically and electronically. At $N=23$ a columnar-to-disk structural transition occurs reminiscent of that observed for ${\mathrm{Si}}_N$. These transitions differ in nature but have the same basis, linking the nanostructural behavior of an element (Si) and its oxide (${\mathrm{SiO}}_2$). Considering the impact of devices based on the nanoscale manipulation of $\mathrm{Si}/{\mathrm{SiO}}_2$ the result is of potential technological importance.

Figure 1

Lowest energy $({\mathrm{SiO}}_2{)}_N$ clusters, $N=14–27$ (dashed line separates the columnar and disklike clusters).

Figure 2

For $({\mathrm{SiO}}_2{)}_N$ $N=14–27$: (a) Cluster energies ($\mathrm{eV}/{\mathrm{SiO}}_2$) with respect to the bulk extrapolated limit (left), nucleation energies (eV) (right). (b) HOMO-LUMO gap (eV). (c) Average $(\mathrm{SiO}{)}_R$ ring size $⟨R⟩$. (d) Aspect ratio ($I_b/I_c$).