The electronic structure and the band gap of nano-sized Si particles: competition between quantum confinement and surface reconstruction

Abstract

The electronic structure and especially the band gap of Si n clusters (n = 3-45 atoms) is studied by photoelectron spectroscopy. Contrary to expectations of quantum conf nement, almost all clusters studied here have a band gap smaller than that of crystalline Si or even display a continuous (metallic) density of states. We attribute this to covalent bond formation analogous to the reconstructions observed on single-crystal surfaces. Additionally, for Si 30 and Si 33 a gap size of 0.6 eV (0.4 eV) is observed, supporting the prediction of stable, spherically symmetric structures of these particular clusters.

Ultraviolet photoelectron spectroscopy of Si 4- to Si 1000-

“…The results for the smallest clusters (n = 4 to 20) are in very good agreement with UV-spectra published earlier [4,7,8], although some details are better resolved here, which is probably due to the low cluster temperature. Furthermore the spectra of these small clusters are in excellent agreement with simulations [2,4].…”

Fluorescent silicon clusters and nanoparticles

“…The energy difference between the two corresponds to the band gap in the bulk picture. Small clusters between n=3 and 13 were found to have band gap energies smaller than those typically seen for bulk crystals (Maus et al, 2000). This is incommensurate with the idea of quantum confinement, which would predict larger band gap energies for anything smaller than the bulk.…”

“…Helium produces smaller clusters because it is much lighter than silicon and energy transfer is less efficient. The pulsed arc cluster ion source is another variation of a gas aggregation-type cluster source that has been used for silicon cluster production (Maus et al, 2000).…”

“…These spectra exhibit rich features for small clusters, which become smoother as the clusters become larger. Maus, Ganteför and Eberhardt (Maus et al, 2000) assign the low binding energy features to the extra electron occupying the conduction bands. The higher energy features observed are attributed to the valence electrons.…”

Isomer-Resolved Ion Spectroscopy

“…Disentangling the features from different geometries and those from different transitions in the same geometry is, generally, a challenge. In practice, finite experimental resolution commonly makes the spectra of isomeric mixtures featureless; PES of large Si n anions is an example [26].…”