Molecular dynamics simulation of icosahedral Si quantum dot formation from liquid droplets

The present paper reports on molecular dynamics simulations of the formation process of Si quantum dots (Si QDs). Icosahedral Si QDs are formed spontaneously by freezing 274-, 280-, and 323-atom Si droplets. We find that the initialization of pentagonal channels leads into the overall icosahedral structure. We also study the melting behavior of the 280-atom icosahedral Si QD. We find that the melting point is reduced more than 15% compared with that of bulk Si. A possible approach to synthesize icosahedral Si QDs is discussed. The formation of the icosahedral structure could be expected in other systems characterized by tetrahedral bonding network.

Figure 1

(Color online) Time evolution of the potential energy for the 280-atom Si nanoparticle during the freezing procedure. The nanoparticle is liquidlike at $0\mathrm{ns}$. The freezing transition takes place at 1500 and $1800\mathrm{K}$. The snapshots of the atomic structures for the open circles and the filled circle are given in Figs. 2 and Fig. 3, respectively.

Figure 2

(Color) Typical formation process of the $i\text{−}\mathrm{Si}$ QD. (a) pentagonal network fragment; (b) crystalline fragment; (c)–(f) snapshots of a 280-atom Si nanoparticle corresponding to the open circles in Fig. 1. Two snapshots from different angles are given in (c)–(f). Si atoms less than $2.85\mathrm{\AA }$ apart are connected by bonds. Each atom and bond is colored according to the procedures described below. (1) All atoms and bonds are first shown in gray. (2) The atoms and bonds making up the pentagonal network fragment [shown in (a)] are in green. (3) The atoms making up the crystalline fragment [shown in (b)] are in blue. The (111) facets of the $i\text{−}\mathrm{Si}$ QD are indicated by light orange triangles for clarity. The same identifications are used in the other figures.

Figure 3

(Color) The snapshot of a 280-atom Si nanoparticle corresponding to the filled circle in Fig. 1. Si atoms related to surface defects are shown in red for clarity. These defects move around on the surface. When they meet each other, the formation of the icosahedral structure is completed.

Figure 4

(Color) Snapshots of (a) 274- and (b) 323-atom $i\text{−}\mathrm{Si}$ QDs. Although 274 and 323 are the numbers of atoms for perfect $c\text{−}\mathrm{Si}$ QDs, 274- and 323-atom droplets freeze into icosahedral structures with defects.

Figure 5

(Color) Time evolution of the potential energy for the 280-atom Si nanoparticle during the melting procedure. The nanoparticle is a solidlike $i\text{−}\mathrm{Si}$ QDs at $0\mathrm{ns}$. The melting transition takes place at $2000\mathrm{K}$. Snapshots corresponding to the open circles at 9 and $17\mathrm{ns}$ in the $2000\mathrm{K}$ simulation are included.