GAS-PHASE SYNTHESIS OF NANOPARTICLES OF GROUP-12 CHALCOGENIDES

The effect of pyridine addition upon the gas-phase reactions of hydrog en sulfide (H2S) or hydrogen selenide (H2Se) with either dimethylcadmi um (Me2Cd) or dimethylzinc (Me2Zn) has been investigated. The deposits of CdS, CdSe, ZnSe or ZnS which form have been analysed by powder X-r ay diffraction (PXRD), elemental analysis and transmission electron mi croscopy (TEM). At ambient temperatures the deposits consist of partic les in the nanocrystalline size range of the hexagonal phase. The aver age particle size within the deposits is dependent upon the concentrat ion of pyridine in the gas phase, the temperature at which the reactan ts are mixed and, in the case of ZnSe, whether an inert (He) or reduci ng (H-2) carrier gas is used. At ambient temperatures in an inert carr ier gas, the control of particle size exerted by pyridine decreases in the order ZnS > CdS > CdSe > ZnSe, although in hydrogen, the prereact ion between Me2Zn and H2Se could be almost completely eliminated by ra ising the temperature. Further investigation of CdS deposits have been carried out by photoacoustic spectroscopy (PAS) to access the band-ga p, and solid-state C-13 and Cd-113 NMR to probe the surface state of t he particles. Elemental analysis and the NMR studies suggest that pyri dine binds through the lone pair of the nitrogen to surface metal atom s on growing particles, inhibiting further particle growth. The partic le 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 propos ed that amide species may form and be responsible for growth inhibitio n.