Synthèse de nanoparticules de ZnS et études de leurs propriétés structurales et optiques

Abstract

This work focuses on the synthesis by the sol-gel method with nano precipitation ZnS powder, process carried out without any constraints related to the environment or the temperature or atmospheric pressure that is called process soft chemistry. In the preparation of the powder of ZnS was chosen the precursor sodium sulfide (Na2S) (99% pure Aldrich) as crystals as the sulfur source and the bi hydrated zinc acetate (Zn-C2H3O2-C2H3O2) 2H2O) (99% pure Aldrich), a powdered zinc source .These two components are mixed in suitable proportions. This method allowed us to recover a large amount of ZnS powder. The prepared powder was subsequently undergoes a heat treatment at different temperature to 450 ° C and 650 ° C, followed by the development of thin layers of the nanocomposite material ZnS / PMMA consisting of crystallites of pure nano ZnS dispersed in the poly methyl methacrylate. These materials were analyzed by X-ray diffraction for the structural characterization of the pure powder and annealed at a temperature 450 ° C and 650 ° C for one hour under the pressure of the atmosphere, and the optical characterization by absorption spectroscopy and emission (photoluminescence) explicitly for pure ZnS nano powders and thin layers of composite nano ZnS / PMMA. The thin layers of ZnS / PMMA deposited on glass dimensions 1.5cm x 1.5cm substrates has been developed by the so-called spin-coating. Was used for the analysis of all our samples a diffractometer 'Bruker D8 Advance' 'equipped with a Cu Kα source. = 1.54148A °. XRD showed that: -a- The structure of the pure ZnS powder is of the type zinc blende (cubic) with a lattice parameter a = 5.400A ° (sheet No. 010 792) and the average crystallite size estimated by Scherer's formula is: D = 2R = 4.08nm (quantum dots). -b- The ZnS powder annealed at the temperature 450 ° C for one hour under the pressure of the atmosphere is an overlap of two phases, one phase of ZnS and ZnO phase. The latter is due to the effect of oxidation during annealing (oxygen is snuff in the air rate of 20%). The structure of nano crystallites changes to become hexagonal ZnS (JCPDS card No. 720163) with increase in the average size of the crystallites and becomes: D = 2R = 12.08nm. The structure of nano ZnO is hexagonal crystallites (JCPDS card No. 891397), their average size: D = 2R = 12.48nm. -c- The oxidation is almost complete of the nano ZnS crystallites during annealing at 650 ° C for one hour under the pressure of the atmosphere, the structure of nano ZnO remains hexagonal crystallites (JCPDS card No. 891 397) with parameters a = 3.253A ° mesh; c = 5.213A °, therefore the crystallite size becomes greater: D = 2R = 31.26nm. We used a UV-visible spectrophotometer Shimadzu UV3101PC brand dual beam whose spectral range extending from 190 nm (UV) to 3200 nm (near IR) with a pitch of 0.5Nm The study by optical absorption spectroscopy and photoluminescence in the UV-visible allowed to deduct the gap from the optical absorption spectrum of the nano-crystallites of pure ZnS, equal to Eg = 3.85 eV, the latter is subjected to a shift 0.3EV to major energy compared to the gap in the bulk state (3.55 eV) for the cubic structure of ZnS. This increase is due to the reduction in the size inducing quantum confinement. The size of the nano crystallites estimated pure ZnS by using the model of the effective mass, it is equal to D = 2R = 2x2.33nm = 4.66nm. This result is in good agreement with that found from X-ray diffraction spectra calculated by the formula Scherer. The radius of pure ZnS crystallites is of the same order of magnitude as the Bohr radius of the exciton (rB = 2.5Nm), which corresponds to an intermediate confinement. Emission spectra (PL) made using a Nd-YAG laser source with an excitation of 355 nm at the temperature of liquid nitrogen (77 ° K) can be observed three bands in the visible range attributed to the recombination of electrons of the sulfur levels in vacant locations with the holes in the valence band and also the surface defects and impurities. In perspective, we can consider the use of nano ZnS crystallites in the field of nanotechnology as fluorescent nano-objects, sensors in the near infrared and also as a heart shell of another semiconductor because they present a wide gap and it is less sensitive to the effect of oxidation. In addition, the annealing mechanism nano ZnS powder under the pressure of the atmosphere may be used for the synthesis of nano ZnO crystallites.