Structural properties of hydrogenated silicon nanocrystals and nanoclusters

Abstract

The structures of fully and partially hydrogenated Si nanocrystals and nanoclusters are studied by geometric optimizations and molecular dynamics simulations based on an empirical tight-binding approach. It is shown that the structural properties of the hydrogen saturated Si nanocrystals have little size effect, contrary to their electronic properties. The surface relaxation is quite small in the fully hydrogen saturated Si nanocrystals, with a lattice contraction of 0.01 to 0.02 Å residing in the outermost two or three layers. Inside the hydrogenated Si nanocrystals, there is only very small strain (lattice expansion) of the order 10−4 to 10−3, in agreement with the x-ray diffraction measurement. The fully hydrogenated Si nanocrystals are the most stable structures compared to those partially hydrogenated. For the smaller SimHx (m⩽151) nanocrystals, removing up to 50% of the surface terminating H atoms only causes distortions to the crystal structure, with the basic tetrahedral structural features still retained. However, removal of more than 70%–80% of the surface terminating H atoms will lead to a change to more compact structures.