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 sp(3)-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 Crown Copyright (c) 2010 Published by Elsevier B V All rights reserved