Effects of the passivating coating on the properties of silicon nanocrystals

The effects of the hydrogen-coating of silicon nanocrystals (Si:H NCs) on the chemical and physical properties are theoretically investigated. The empirical tight-binding (TB) method, within the minimal sp3-basis set and second nearest-neighbor interaction scheme, is employed to calculate the electronic structures, oscillator strength (OS) and recombination rates (RR). The coating is found to induce numerous effects: (i) the full chemical passivation of the dangling bonds existing on the surface of the silicon NCs; (ii) the charge-carrier quantum-confinement (QC) enhancement, which yields direct bandgap character, distinguished with strong and fast photoluminescence (PL) emissions. In this perspective, based on the modeling of the PL data, the QC rules are derived and found to be power-low like, similar to the case of a single particle confined in a 3D box; and (iii) the enhancement of the optical properties (i.e., OS and RR). Furthermore, to deepen our understanding of the coating effects, we have considered the Si29NC under three different situations: (a) un-coated; (b) the surface-dangling bonds being partially hydrogenated and the rest being dimerized (i.e., Si29H24 NC); (c) all the surface-dangling bonds being fully hydrogenated (i.e., Si29H36 NC). Using the density-functional-theory (DFT), the total energy calculation has confirmed that the occurrence of to hydrogenization is more probable than the dimerization (i.e., Si29H36 has lower energy and is, thus, more stable than Si29H24). On one hand, these results corroborate the experimental findings presenting the enhancement of the optical efficiency with the increasing hydrogen content. On the other hand, the atomic relaxation is also shown to further enhance the optical properties and this should in turn corroborate the results of the experimental heat treatment of Si:H NC films, recently reported in literature.