Ab-initio Calculations Of The Electronic Properties of Hydrogenated and Oxidized Silicon Nanocrystrals: Ground and Excited States

The electronic and optical properties of hydrogenated silicon nanocrystals (H-Sinc) have been investigated through ab-initio techniques (Pseudopotential approach in ground and excited state electronic configurations, TDDFT, GW) as a function of size and symmetry. The presence of an electron-hole pair in the nanocrystals causes a strong deformation of the structures with respect to the ground-state configuration, and this is more evident for smaller systems and at the surface of the H-Si-nc. Also the nature of the distorsion changes: for small clusters it is strongly localized, while as the size increases the distortion is spread out over the entire structure. The structural modifications are immediately reflected into the electronic structure. Actually, we have found the expected decrease of the energy gap on increasing the nanocrystal dimension for the ground state and, for the excited state configuration, a reduction of the energy gap the more significant the smaller is the nanocrystal. For the excited nanoparticles the HOMO and LUMO become strongly localised in correspondence of the distortion, giving rise to defect-like states which reduce the gap; these effects are stronger in smaller clusters. Thus, we can deduce that the absorption of resonant radiation by the nanocrystal in its ground state configuration induces a transition between the HOMO and LUMO levels, which for all these nanocrystals is optically allowed. Such a transition is followed by a cluster relaxation in the excited state configuration giving rise to distorted geometries and to new LUMO and HOMO, whose energy difference is smaller than that in the ground-state geometry. It is between these two last states that emission occurs; the Stokes Shift between absorption and emission changes as a function of the dimension. Thus we have substituted some H with O both double bonded or in a bridge configuration with respect to Si. The substitution of H with O as passivating agent results in a different cluster geometry and in a reduction of the energy band gap depending on the type of O-Si bond. Moreover also the optical properties strongly depend on the different O-Si bond type. The results provide a consistent interpretation of the photoluminescence redshift observed in oxidized samples and of recent outcomes on Si single quantum dot photoluminescence bandwidth. Three fundamental aspects come out: first, there is a strong interplay between structural and electronic properties, mostly when excited configurations are concerned. Second, a consistent explanation of emission processes can be carried out only if such excited configurations are accounted for. Third, optical gaps cannot be calculated simply as the HOMO-LUMO energy separation without introducing errors as larger as smaller is the nanocrystal.