Structural Stability and Optical Properties of Nanomaterials with Reconstructed Surfaces

We present density functional and quantum Monte Carlo calculations of the stability and optical properties of semiconductor nanomaterials with reconstructed surfaces. We predict the relative stability of silicon nanostructures with reconstructed and unreconstructed surfaces, and we show that surface step geometries unique to highly curved surfaces dramatically reduce the optical gaps and decrease excitonic lifetimes. These predictions provide an explanation of both the variations in the photoluminescence spectra of colloidally synthesized nanoparticles and observed deep gap levels in porous silicon.

Figure 1

Unreconstructed (${\mathrm{S}\mathrm{i}}_{148}{\mathrm{H}}_{120}$) and reconstructed (${\mathrm{S}\mathrm{i}}_{148}{\mathrm{H}}_{72}$) hydrogen (white atoms) passivated 1.8 nm silicon (grey atoms) nanoclusters. Charge density isosurfaces (blue) represent 50% peak amplitude.

Figure 2

QMC calculated formation energies of 1.8 nm ${\mathrm{S}\mathrm{i}}_{148}$ clusters with different surface structures. The origin of ${\mu }_{\mathrm{H}}$ is the value at which the formation energy of ${\mathrm{S}\mathrm{i}\mathrm{H}}_4$ is zero. The value of ${\mu }_{\mathrm{S}\mathrm{i}}$ for bulk silicon was obtained from Ref. 12.

Figure 3

(a) Optical gaps of nanoclusters with different surface structures. Dashed lines are extrapolations based on LDA trends. (b) Solid circles (squares) show LDA exciton lifetimes of unreconstructed (reconstructed) clusters. Empty symbols show the lifetime of states with highest occupied molecular orbital (HOMO) [lowest unoccupied molecular orbital (LUMO)] character just below [above] the band edge.