Synthesis And Characterization Of Metal Sulfide Monodisperse Nanocrystals

Due to their size and shape-dependent optical, electrical, magnetic, and catalytic properties, colloidal nanocrystals have been the promising building blocks for functional materials and devices, such as light emitting diodes, solar cells, telecommunication amplifiers, and biomedical fluorescent labels. Although the strategies of colloidal nanocrystals have attracted a special attention because of their fundamental role in their properties and applications, it was inevitable to use expensive, toxic, and environment-polluted organometallic precursor in the previous report. Moreover, the exploration of novel approach would be in favor of fabricating intriguing nanostructures with new properties or application. Hence, a novel strategy has been developed to synthesize metal sulfide colloidal nanocrystals using inorganic salt as precursor. It is indicated that the above-mentioned approach is low-cost, simple, effective, environment friendly and general for the synthesis of metal sulfide colloid nanocrystals. Furthermore, the properties of the as-synthesized metal sulfide colloid nanocrystals have also been investigated. The main results achieved in this disquisition are listed as below:(1) Monodisperse hexagonal CuS (covellite) and monoclinic Cu_1.75S (roxbyite) disk-like nanocrystals were synthesized via high-temperature chemical reaction, respectively. Moreover, they could be self-assembled into the ribbon-like nanostructures of copper sulfide by the face-to-face stacking along [001] direction under the certain condition. On the different substrate such as carbon film or silicon wafer, the stacking angle of the disk-like copper sulfide nanocrystals is different. It was further indicated that van der Waals force caused by rich sulfur condition (special crystallographic orientation of CuS nanocrystals), the complexed ability of capping ligand (oleylamine), and dipole-dipole interaction played critical roles in the self-assembly of CuS nanocrystal. While, ferroelectric properties and dipole-dipole interaction were responsible for the self-assembly of Cu_1.75S nanocrystal. Meanwhile, the effects of binary surfactants on the morphology and size of monodisperse CuS nanocrystals were also investigated. FT-IR spectrum revealed that there were the oleylamine molecules on the surface of thecopper sulfide nanocrystals, resulting in the formation of oil-soluble copper sulfide nanocrystals. UV-vis analysis indicated that the band gap of the as-prepared copper sulfide nanocrystals varied from 2.40 eV to 2.50 eV. Comparing with the band gap (2.0 eV) of bulk CuS, the obvious blue-shift was observed due to the quantum confinement effect.(2) Tetragonal CuInS₂ (chalcopyrite) nanocrystals with the average diameter of about 5 nm were fabricated by high-temperature chemical reaction. Moreover, they exhibited the narrow size distribution and good monodispersity. The UV-vis and PL analysis indicated that the obvious blue-shift of the absorption and emission peaks was observed due to the quantum confinement effect.(3) High-quality monodisperse Mn-doped ZnS nanocrystals with zinc-blende crystal structure were prepared by high-temperature chemical reaction. UV-vis spectrum revealed that the absorption peak of Mn-doped ZnS nanocrystals was within the range of 312～326 nm. Moreover, there were two emission peaks in their PL spectrum. The strong and narrow emission peak at about 583 nm attributed to the emission of Mn ion, which was unchanged with the increase of Mn content. While, the weak and broad emission peak belonged to the intrinsic defects in ZnS. It was further indicated that the electronic radiative transition at shallow level generated from the sulfur vacancy related defects contributed to the blue emission (410 nm) and the orange emission (583 nm) of Mn-doped ZnS nanocrystals is due to the Mn^{2+ 4}T₁-⁶A₁ transition.