Sphalerite-Wurtzite intermediates in nanocrystalline CdS

Abstract

Cadmium sulfide nanoparticles have been synthesized using mercaptoethanol as a capping agent. A refined analysis of the powder X-ray diffraction by Debye function analysis shows that the as synthesized material consists of both ~50 wt % hexagonal wurtzite-type nanoparticles of about 2 nm and larger cubic sphalerite-type particles of about 2.6 nm, respectively. Both species have also been identified by high-resolution electron microscopy, some of them show multiply twinning with 5-fold decahedral symmetry. The thermal stability of these nanoparticles is surprisingly high under He. The total mass-averaged size increases from 2.3 nm (as synthesized) to 3.2 nm after exposure to 315 °C in helium for 12 min. However, both polymorphs are still present. Heavy sintering occurs only above 380 °C, leading to wurtzite particles of 41 nm on average. These particles are strongly faulted by hexagonal (001) stacking faults (faulting probability = 1.1%) but show negligible twin defects. After synthesis, the particles show the optical absorption peaks at ~400 nm. The related optical band gap of 3.1 eV is in excellent agreement with the theoretical tight-binding approximation of Lippens and Lannoo for CdS particles of this size.