Very sharp zinc blende-wurtzite phase transition of CdS nanoparticles

Cadmium sulfide nanoparticles, in the size range of 6-20 nm, were prepared on glass substrates in the growth temperature (Tb) interval of 60-97 °C by microwaves assisted chemical bath synthesis. The nanoparticle size was controlled by the bath temperature. The crystalline phase, shown in the X ray diffraction patterns, depends on the Tb range. In the intervals 60≤ Tb ≤ 93 °C and 95≤ Tb ≤ 97 °C the lattice is cubic zinc-blende (ZB) and hexagonal wurtzite (W), respectively. In Tb = 94 °C, the critical temperature of transition (Tbc), the films have crystals in both cubic and hexagonal phases. The films grow with preferred orientation in (111) for ZB and (002) for W, perpendiculars to the substrate. Lattice interplanar spacing indicates that the lattice experiences uniaxial contraction along the (111) direction in films grown at Tb in a region near to Tbc. Raman measurements reveal softening of LO in other directions. This compression on the lattice is probably caused by the formation of Cd-vacancies and Cd-interstitials originated by the zincblende-wurtzite phase transformation around Tbc.