Amorphous nature of small CdS nanoparticles: Molecular dynamics simulations

The semiconductor CdS is generally found in the wurtzite structure. Prior experimental and theoretical results confirm that the semiconductor CdS nanoparticles maintain a wurtzite structure for diameters greater than 6 nm. There is disagreement in the literature for sizes smaller than 6 nm. We use the density-functional theory FIREBALL code and perform finite-temperature molecular dynamics simulations on nanoparticles that are approximately 2 nm in diameter, considering different sized structures and different simulation temperatures. To determine the structure of the nanoparticles we analyze the radial distribution of the atoms about the center of the nanoparticle, the nearest-neighbor and next-nearest-neighbor bond lengths, and the radial distribution function about individual atoms. Comparing the molecular dynamics simulations of relaxed nanocrystals against bulklike wurtzite and zinc-blende nanocrystals, we find that small uncapped CdS nanoparticles are not nanocrystals but are amorphous.