nano-sized crystals of silicon embedded in silica glass: large models and new aspects of the...

Z. Anorg. Allg. Chem. 2006, 2089 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.zaac.wiley-vch.de 2089

DOI: 10.1002/zaac.200670028

Nano-sized crystals of siliconembedded in silica glass: large modelsand new aspects of the electronicstructure

Peter Kroll*, Hendrik J. Schulte

Institut für Anorganische Chemie, RWTH Aachen, Landoltweg1, D-52056 Aachen

Keywords: Nanocrystals; Electronic structure; DFT calculations

Confining a material to small diameters has a strong impact on itsproperties with respect to the infinite bulk compound. In this studywe present a systematic investigation of the electronic structure ofmodels of nano-crystalline Si embedded in amorphous silica [1].Our models of quasi-spherical Si nanocrystals with diameters from0.8 to 1.6 nm embedded in silica glass are obtained through acombined computational procedure of network construction anddensity-functional calculations. After relaxation the nanocrystalsexhibit a smooth interface without defects to the glass matrix. Wefind tensile strain within the Si nanocrystal, at maximum for atomsabout 300 pm below the surface, but minimal for atoms bondingto the suboxide interface.As expected, we observe the increase of the band gap with decre-asing size of the nanocrystal due to the quantum confinement ef-fect. Surprisingly, details of the electronic structure are differentfrom previous studies on isolated Si clusters, either bare, hydroge-nated, or partially oxidized. In contrast to these studies, we findthe highest occupied states of the valence band being located atSi�Si bonds close to the interface forming a shell-like structurearound the central core of the nanocrystal. The lowest unoccupiedstates are centered within the nanocrystal.

We, therefore, highlight the critical role of the interface between thenanocrystal and the embedding dielectric glass matrix that is ofuttermost importance for optical properties, but also for growthkinetics during the fabrication process.

[1] P. Kroll, H. J. Schulte, Phys. Stat. Sol. (b) 2006, 243, R47.