Preparation And Characterization Of Chalcogenide Nanomaterials

Transition metal chalcogenide (sulfides or selenides) have a variety of special properties hence it is very important in both fundamental research and commercial applications. In this thesis, several metal chalcogenide nanomaterials have been synthesized via chemical approach and their morphologies, structures and properties were systematically investigated.Monodispersed CdSe nanocrystals (quantum dots, QDs) with high photoluminescence intensity were synthesized using CdO and Se powder as the precursors together with oleic acid used as the ligand. The absorption spectroscopy and photoluminescence spectroscopy were employed to characterize the as prepared nanocrystals. It was found that excitonic features were clearly evident in the spectra.Monodispersed cross-linked polymer microspheres (PS) with the diameters ranged between 250 and 700nm were firstly prepared by means of emulsifier-free emulsion copolymerization. And the effects of the contents of initiator and monomer, the ionic strength of the reaction solution on the particle size were also investigated. Then the CdSe or CdSe/ZnS - polymer composites were successfully obtained by using PS microspheres and surfactant-coated semiconductor quantum dots through swelling technique. The photoluminescences (PL) of as- prepared QDs-PS composites were investigated. Their good PL properties may find potential applications in displaying and biological labeling.The monodispersed CdSe nanocrystals were further coated with silica through a modified stO|¨ber method. The SEM and TEM results indicate that these spheres with silica shell are in the size of 150-200 nm containing cores of pre-synthesized CdSe nanoparticle. Near-field scanning optical microscopy (NSOM) was used to measure the topography and the luminescence phase of the samples. The silica-coated quantum dots (QDs) have vastly improved their photostability and chemical stability.PbS microcrystals with different shapes including dendrites, multipods, flowers and cubes were prepared by the reaction of Pb(NO3)2 or Pb(CH3COO)2 and thiourea (NH2CSNH2) with acrylamide (AA) as the surfactant through a simple hydrothermal process. It is found that thiourea, acrylamide and reaction temperature play important roles on the shape evolution of PbS microcrystals. A possible formation mechanism of PbS microcrystals was discussed based on their shape evolutions.Large-scale synthesis of nanowires of lead sulfide with a length/diameter ratio of about 200 was carried out in a two-step strategy. In the first step, elemental Pb was produced as the intermediate of the whole reaction via thermal decomposition of lead acetate with ethylene glycol (EG) as the solvent and in the presence of poly (vinylpyrrolidone) (PVP). In the second step, the elemental Pb was directly combined with S power under heating, leading to the formation of the final product. The process is known as "polyol process" and temperature is a dominant factor in affecting the reactivity. The PbS nanocubes and PbS nanostructures with a hole have been prepared by changing the reaction conditions.Cobalt-doped disulfide nanotubes were fabricated by a technique named as exfoliation- intercalation-hydrothermal adulteration. WS2 and M0S2 powders were used as primary materials and were exfoliated into single layer by n-butyllithium. Then Co2+ cations entered into M0S2 and WS2 matrix through a hydrothermal process in C0CI2 aqueous solution. XRD results indicate that multi-metal disulfide can be prepared by this approach. TEM images show that the morphology of the products is bamboo-like nanotubes with open-end, tip-end and joints. HRTEM images show that the tip-end, joint and wall of bamboo-like nanotubes are of inorganic fullerene-like (IF-like) nanostructures. EDX results show that the components of bamboo-like nanotubes are Co, Mo, W and S. A possible formation mechanism of bamboo-like nanotubes was suggested as well.Selenium (Se) and tellurium (Te) nanorods or nanotubes with trigonal crystal structure were successfully prepared by one-step polyol process using commercial selenium or telenium powder as precursors. Other solution-phase approach (solvothermal method) was also used to obtain Te nanotubes. It was found that themorphologies of Se and Te nanomaterials obtained under similar reaction conditions were different. Moreover, SexTei-x nanorods were successfully prepared with the mixture of Se and Te powder in ethylene glycol (EG) solvent. The products wereanalyzed by XRD, SEM, EDS and Raman spectra, respectively.