Silicon nanocrystals (n-Si) in amorphous silicon oxide (n-Si)O2 is being intensively investigated as promising material for optoelectronics after the discovery of its photoluminescence at room temperature [1].

Two hypothesis have been raised to explain the photoluminescence of n-Si: i) the confinement of wavefunctions as due to the size of nanocrystals and, ii) the presence of defects that generate even more confined orbitals.

Recent photoluminescence experiments under intense magnetic fields [2] show that the emission from silicon nanocrystals grown in amorphous SiO2 (n-Si/a-SiO2) is due to the presence of very localized states rather than to quantum confinement. Experiments also shown than the presence of the defects depend on the size of the nanoparticles.

Even though the presence and nature of defects in n-Si in vacuum have been already investigated [3, 4], a study of n-Si in a-SiO2 under experimental conditions, using realistic potentials, is still lacking. In this work, using state-of-the-art empirical force models [5] in combination with molecular dynamics, we have identified various defects in nanoparticle of different size and at various temperatures. These defects can be divided into two classes: i) coordination defects, ii) strain defects. Coordination defects consist in under or over-coordinated silicon atoms. Strain defects consist of stretched Si-Si bonds. In agreement with experiments, our simulations demonstrate that the number and the 'intensity' of these defects decrease with the increase of the of n-Si size. We also observed the diffusion of defects into the SiO2 matrix via a bond switching mechanism.

[1] B. Averboukh et al.,  J. Appl. Phys. 92 (2002), 3564
[2] S. Goodefroo et al., Nature nanotechnology, 3 (2008), 174
[3] A. Puzder et al., Phys. Rev. Lett. 88 (2002), 097401

[4] Hadjisavvas et al., Phys. Rev. Lett. 93 (2004), 226104
[5] S. R. Billeter et al., Phys. Rev. B 73 (2006), 155329

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1. Molecular dynamics results showing continuum theory failure in describing the elastic behavior of nanoparticles embedded in Si-based systems
2. Growth phenomena and optoelectronic properties of mixed-phase amorphous-crystalline nanosystems