Understanding the effect of hydrogen surface passivation and etching on the shape of silicon nanocrystals

One of the significant challenges in the use of nanocrystals, is the control of crystal shape when grown from the gas-phase. Recently, the Kortshagen group has succeeded in generating cubic Si nanocrystals in a nonequilibrium plasma. In this paper we consider the energetics of various shaped Si nanocrystals, and the role that hydrogen surface termination plays. We consider cube, truncated octahedron, icosahedron, and spherical shapes for both bare and hydrogen coated silicon nanocrystals for sizes between 2 and 10 nm. From our molecular dynamics (MD) simulations, show that for bare Si crystals, icosahedron crystals are the most energetically stable, and cubic the least. On the other hand, when hydrogenated, the cubic structure comes about because 1) the cubic structure is energetically favored when hydrogen terminated and 2) the plasma that operates with hydrogen also provides a steady source of hydrogen atoms for etching.