Gas-phase synthesis of nanoparticles of group 12 chalcogenides

The effect of pyridine addition upon the gas-phase reactions of hydrogen sulfide (H2S) or hydrogen selenide (H2Se) with either dimethylcadmium (Me2Cd) or dimethylzinc (Me2Zn) has been investigated. The deposits of CdS, CdSe, ZnSe or ZnS which form have been analysed by powder X-ray diffraction (PXRD), elemental analysis and transmission electron microscopy (TEM). At ambient temperatures the deposits consist of particles in the nanocrystalline size range of the hexagonal phase. The average particle size within the deposits is dependent upon the concentration of pyridine in the gas phase, the temperature at which the reactants are mixed and, in the case of ZnSe, whether an inert (He) or reducing (H-2) carrier gas is used. At ambient temperatures in an inert carrier gas, the control of particle size exerted by pyridine decreases in the order ZnS > CdS > CdSe > ZnSe, although in hydrogen, the prereaction between Me2Zn and H2Se could be almost completely eliminated by raising the temperature. Further investigation of CdS deposits have been carried out by photoacoustic spectroscopy (PAS) to access the band-gap, and solid-state C-13 and Cd-113 NMR to probe the surface state of the particles. Elemental analysis and the NMR studies suggest that pyridine binds through the lone pair of the nitrogen to surface metal atoms on growing particles, inhibiting further particle growth. The particle size is greatly dependent upon the strength of the pyridine-surface metal atom interaction, the acidity of the EH bond (E = S or Se) and the polarity of the MMe bond (M = Zn or Ca). In hydrogen, it is proposed that amide species may form and be responsible for growth inhibition.