Influence of Synthesis Conditions on the Structural and Optical Properties of Passivated Silicon Nanoclusters

First-principles molecular dynamics and quantum Monte Carlo techniques are employed to gain insight into the effect of preparation conditions on the structural and optical properties of silicon nanoparticles. Our results demonstrate that (i) kinetically limited nanostructures form different core structures than bulk-derived crystalline clusters, (ii) the type of core structure that forms depends on how the cluster is passivated during synthesis, and (iii) good agreement with measured optical gaps can be obtained for nanoparticles with core structures different from those derived from the bulk.

Figure 1

Results of FPMD synthesis for ${\mathrm{S}\mathrm{i}}_{29}$ and ${\mathrm{S}\mathrm{i}}_{30}$, compared with low-energy ideal structures. Interior core atoms are dark blue. (a) Ideal ${\mathrm{S}\mathrm{i}}_{29}{\mathrm{H}}_{24}$ structure proposed in Ref. 12. (b) Representative final ${\mathrm{S}\mathrm{i}}_{29}{\mathrm{H}}_{21}$ structure from FPMD synthesis at $T=1000\mathrm{K}$, relaxed at $T=0\mathrm{K}$. (c) Proposed ideal ${\mathrm{S}\mathrm{i}}_{30}{\mathrm{H}}_{22}$ structure. (d) Representative final ${\mathrm{S}\mathrm{i}}_{30}{\mathrm{H}}_{22}$ structure from FPMD synthesis at $T=1000\mathrm{K}$, relaxed at $T=0\mathrm{K}$.

Figure 2

Maximum solid angle unoccupied by other silicon atoms ${\Omega }_i$ (see text) vs time in an ${\mathrm{S}\mathrm{i}}_{29}$ FPMD synthesis run at $T=1000\mathrm{K}$, after 2 ps at $T=1500\mathrm{K}$, with a hydrogen atom added to the surface of the cluster every 0.3 ps. The blue and red curves correspond to the two final interior core atoms, and the grey curves are surface silicon atoms. The inset shows how $\Omega$ is defined, in this case for an interior atom (dark blue).

Figure 3

Interior parameter ${\Omega }_i$ vs time for the silicon atoms in (a) ${\mathrm{S}\mathrm{i}}_{29}{\mathrm{H}}_6$, (b) ${\mathrm{S}\mathrm{i}}_{29}{\mathrm{H}}_{12}$, and (c) ${\mathrm{S}\mathrm{i}}_{29}{\mathrm{H}}_{18}$ FPMD simulations at $T=1000\mathrm{K}$. The colored curves correspond to atoms which are in the interior of the cluster at some point in the simulation, and the grey curves are surface atoms.

Figure 4

Two different reconstructions of a ${\mathrm{S}\mathrm{i}}_{29}{\mathrm{H}}_{24}$ nanocluster: (a) ${\mathrm{S}\mathrm{i}}_{29}{\mathrm{H}}_{24}^{\mathrm{n}\mathrm{a}\mathrm{n}\mathrm{o}1}$; (b) ${\mathrm{S}\mathrm{i}}_{29}{\mathrm{H}}_{24}^{\mathrm{n}\mathrm{a}\mathrm{n}\mathrm{o}2}$.