Silicon nanocrystals structure

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In a Si zero-dimensional system the strong quantum confinement can increase the optical infrared gap of bulk Si and consequently shift the optical transition energies towards the visible range [65,66]. This is the reason for which silicon nanocrystals (Si-NCs) with a passivated surface are used as the natural trial model for theoretical simulations on Si based light emitting materials, such as porous Si or Si nanocrystals dispersed in a matrix. In this section we present a comprehensive analysis of the structural, electronic and optical properties of Si-NCs as a function of size, symmetry and surface passivation. We will also point out the main changes induced... 

Structural Properties of Hydrogenated Silicon Nanocrystals and Nanoclusters... 

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LIGHT-EMITTING PROPERTIES OF ERBIUM-DOPED STRUCTURES OF SILICON NANOCRYSTALS... 

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Most of the semiempirical tight-binding methods for nanostructures are based on the parametrization of bulk systems. It consists of an iterative fitting procedure, performed on the tight-binding parameters, to match the bulk silicon band structure calculated using the most advanced techniques [21]. The as-calculated parameters are then applied to the study of the electronic properties of silicon nanostructures. When the nanostructures are well passivated, the surface is expected to play a minor role, and the main electronic and optical properties are determined by the nanocrystal core. 

We report a representative picture to show the tight-binding results for silicon nanocrystals. Figure 5.1 shows the absorption cross section for a set of silicon nanocrystals upon increasing their size. It can be seen that the absorption spectra move from a multipeak structure that is typical of molecules to a broad, continuous curve that is typical of bulk systems. This is due to the increase in the number of transitions, which makes a nanocrystal as a molecular system that is in the middle between a small molecule and a bulk system. An interesting feature emerges from the analysis of the first transition, defined as the transition between the HOMO and LUMO energy levels. 

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