Synthesis and Characterisation of Metal Chalcogenide Nanocrystals

Abstract

Nanomaterials are defined as materials which possess features with dimensions of less than 100 nm. Nanocrystals are a subclass of nanomaterials, where the absolute dimensions of individual particles are within this range. Various effects become evident at such small scales, including notably: alterations in electronic structure and magnetic behaviour; and the predominance of surface chemistry. Consequently, the synthesis of nanocrystals with tailored properties via chemical methodology has become an area of some interest. Metal chalcogenides form an important class of inorganic materials, which includes many technologically important semiconductors. Metal chalcogenides feature prominently among semiconductor nanocrystals synthesised to date, but the development of magnetic nanocrystals has focused primarily on metal, and metal oxide phases. Thus the aim of this project was the investigation and development of synthetic methodology for producing nanocrystals, focusing on the metal chalcogenides, with specific emphasis on magnetic metal chalcogenides (iron sulfides). Properties of nanocrystals and metal chalcogenides are discussed in Chapter 1. As described in Chapter 2, metal chalcogenide nanocrystals were synthesised by high temperatures solution-phase reactions, and all samples were characterised by Transmission Electron Microscopy (TEM), Energy Dispersive X-ray Spectroscopy (EDS) and Electron Diffraction (ED). Powder X-ray Diffraction (XRD), Scanning Quantum Interference Device magnetometry (SQUID), Thermogravimetric analysis (TGA), Ultraviolet-visible (UV-vis) absorption and fluorescent emission spectroscopy were also used extensively. CdSe nanocrystals with diameters <10 nm are noted for their size-dependent absorption and emission in the visible region. As described in Chapter 3, an established synthesis was used to produce CdSe nanocrystals in order to explore the size-dependence of the optical properties of the nanocrystals, and to explore the possibility of transferring the nanocrystals to aqueous media. As described in Chapter 4, high temperature reaction of iron salts and elemental sulfur in non-aqueous coordinating solvents was used to produce Fe1-xS and Fe3S4 nanocrystals. The factors affecting phase-selectivity, particle size and morphology were ascertained; and the magnetic properties of pure Fe1-xS, pure Fe3S4 and mixtures of Fe1-xS and Fe3S4 were investigated. As described in Chapter 5, thermal decomposition of iron salts in a coordinating solvent was used to synthesis iron metal or iron oxide intermediates, which could either be oxidised to iron oxide spinel; or sulfidised in situ to iron thiospinel (Fe3S4) nanocrystals. This approach proved to be a good source of small, monodisperse iron oxide spinel and iron thiospinel nanocrystals with the same average dimensions. The magnetic properties of the highly-researched iron oxide spinel nanocrystals were determined, and contrasted to those of the their far less investigated thioanalogues. As described in Chapter 6, metal polysulfido complexes of the type [M(N-MeIm)x]Sy/MSy(N-MeIm)x (M = Fe, Zn, Mg; N-MeIm = N-methylimidazole) were synthesised from metal powders, elemental sulfur and N-MeIm; then thermolysed in coordinating solvents to afford metal sulfide nanocrystals. Thus establishing a new general route for synthesis of metal sulfide nanocrystals from low-cost starting materials.