Nano-sized Crystals of Silicon Embedded in Silica Glass: Large Scale Models and Aspects of the Electronic Structure
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Nano-Sized Crystals of Silicon Embedded in Silica Glass: Large Scale Models and Aspects of the Electronic Structure Peter Kroll and Hendrik J. Schulte Inorganic Chemistry, RWTH Aachen, Landoltweg 1, Aachen, 52056, Germany
ABSTRACT We construct quasi-spherical Si nanocrystallites consisting of 17, 29, 47, 71, and 99 atoms with diameters from 0.8 to 1.6 nm embedded in an SiO2 network. All atoms have saturated bonds: Si is four-fold connected and O is two-fold connected. The models comprise 400-600 atoms and have lattice parameters of about 2 nm. The networks are subjected to a bond switching algorithm yielding models of nanocrystalline Si embedded in amorphous silica. Subsequently, we employ density functional methods. As a result of the DFT-optimization we find that the Si nanocrystals are free of defects at the interface to the host matrix. Si-Si distances within the Si nanocrystallites are strained, the strain itself tailors off to the suboxide interface. The excess energy of the optimized models with respect to crystalline silicon and vitreous silica scales linearly with the surface of the Si nanoparticles. The interfacial energy of the nc-Si/SiO2 interface is calculated to 1.5 J/m2. We observe an increase of the band gap with decreasing cluster size due to the quantum-confinement effect. The highest occupied states of the valence band are located at Si-Si bonds close to the interface; the corresponding charge density forms a shell-like structure around the central core of the nanocrystal. The lowest unoccupied states are centered within the nanocrystal. INTRODUCTION The strong confinement of a nanoparticle in three dimensions has dramatic influence on its properties with reference to the infinite crystal. Hence, the physics of nanoparticles present particularly interesting aspects. Nanocrystalline silicon (nc-Si) embedded in silica glass, for example, exhibits enhanced optoelectronic properties, namely the emission of visible light on laser excitation [1]. With the advent of density-functional theory tremendous advances have been made in computation of materials. State-of-the-art techniques allow the treatment of hundreds and even thousands of atoms within a quantum mechanical methods. Though first row elements, and especially oxides, are more difficult to compute, much insight has been gained by employing computational tools. In this study we give a first account of nc-Si embedded in vitreous silica. A recent study of Luppi and Ossicini [2] treated a 10-atom Si-cluster embedded in a highly symmetrized structure of crystalline cristobalite-SiO2. Our models, however, constructed using a network approach to provide a realistic model for silica glass and subsequently fully optimized with DFT methods, go beyond their study in both size and accuracy.
COMPUTATIONAL The combined computational procedure can be outlined as follows. We start from a 3x3x3 supercell of diamond silicon. We then define all Si atoms within a radius around a central Si atom as belonging to the inner core region, all remaining Si
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