The amorphous-crystalline transition in SiH₂ nanoclusters

Silicon nanocrystals (NCs) have great potential for applications in optoelectronics, photovoltaics and biomedicine. The photo-physical characteristics of these particles strongly depend on whether they are crystalline or amorphous. This structural order is sensitive to the synthesis details. To understand the morphology of hydrogen-passivated silicon clusters and find how it depends on the passivation degree, we calculated the optimal structures of SiH₂ clusters with n ≤ 21 and 2m ≤ 30. We found that as the hydrogen amount increases, clusters run through three structural types: (i) amorphous clusters with dangling bonds (DBs), (ii) amorphous clusters without DBs at intermediate passivation, and (iii) crystalline clusters. We describe a mechanism which removes dangling bonds in the amorphous clusters of the second type and shows its key importance for cluster structure formation. The crystalline lattice (diamond or lonsdaleite) is found to emerge when all broken bonds at the NC surface are passivated. We constructed the phase P-T diagram of Si-H clusters, compared it with the available experimental data and discussed the transfer of our results to large Si nanoparticles.