Novel nanostructures in transition metal chalcogenide systems

This thesis discusses the synthesis of transition metal chalcogenide nanostructures (where the chalcogen is either sulfur, selenium or tellurium) through the use of standard chemical vapour transport (CVT) and chemical vapour deposition (CVD) techniques. The resultant structures are characterised with a variety of methods and comparisons of their properties are made with their bulk counterparts. A discussion into how some of these structures form during the reaction is also given. Highly symmetrical, isotropic, nickel disulfide (NiS2) nanocubes have been synthesised via a Physical Vapour Transport (PVT) method in which sulfur vapour generated in situ is reacted with nickel-coated silica substrates. Systematic studies demonstrate the effect of the reactant ratio, substrate, metal layer thickness and reaction temperature on the synthesis and growth process. The evolution of structure and composition has been followed by diffraction and scanning electron microscopy (SEM). The size of the NiS2 cubes can be varied from below 200 nm to 1 -2 1m across. Magnetic properties of the disulfide nanomaterials have been determined using superconducting quantum interference device (SQUID) magnetometry. Initial experiments also demonstrate that related CVT techniques can be exploited to produce alternative compositions in the Ni-S system with varying morphologies that can be controlled via chemical and physical reaction parameters. Surface Assisted Chemical Vapour Transport (SACVT) methods have been employed to grow flower-like nanostructures of titanium disulfide (TiS2) and titanium trisulfide (TiS3) on titanium coated silica substrates. Systematic studies demonstrate the role of the reactant ratio and reaction temperature on the synthesis and growth process. The evolution of structure and composition has been followed by powder X-ray diffraction (PXD) and electron microscopy techniques such as and transmission electron microscopy (TEM). Magnetic properties of the disulfide nanomaterials have been determined using SQUID and Raman spectroscopy has been used to confirm the identity of the sulfides. Investigations into nanostructured materials of the group IV transition metals zirconium and hafnium resulted in the successful synthesis of nanostructures of zirconium trisulfide/selenide (ZrS3/Se3) and hafnium trisulfide/selenide (HfS3/Se3). The unusual effects on structure that can occur when reactant time and synthesis temperature are varied and when a balance between these two factors is successfully found, nanostructures other than tubes and wires can be formed. Each of these systems were characterised with a variety of techniques including, TEM, PXD and SQUID

Pressure-induced phase transition for single crystalline LaO0.5F0.5BiSe2

We have demonstrated a pressure-induced phase transition from a low-Tc phase to a high-Tc phase in a single crystal of the superconductor LaO0.5F0.5BiSe2. The high-Tc phase appears at 2.16 GPa and the maximum superconducting transition temperature (Tc) is observed at 6.7 K under 2.44 GPa. Although the anisotropy ({\gamma}) for the low-Tc phase is estimated to be 20, it is reduced by around half (9.3) in the high-Tc phase. This tendency is the same for the BiS2 system. The Tc of LaO0.5F0.5BiSe2 has continued to increase up to the maximum pressure of this study (2.44 GPa). Therefore applied further pressure has the potential to induce a much higher Tc in this system.Comment: 12 pages, 4 figure

Evolution of superconductivity in isovalent Te-substituted KxFe2-ySe2 crystals

We report the evolution of superconductivity and the phase diagram of the KxFe2-ySe2-zTez (z=0-0.6) crystals grown by a simple one-step synthesis. No structural transition is observed in any crystals, while lattice parameters exhibit a systematic expansion with Te content. The Tc exhibits a gradual decrease with increasing Te content from Tconset = 32.9 K at z = 0 to Tconset = 27.9 K at z = 0.5, followed by a sudden suppression of superconductivity at z = 0.6. Upon approaching a Te concentration of 0.6, the shielding volume fraction decreases and eventually drops to zero. Simultaneously, hump positions in r-T curve shift to lower temperatures. These results suggest that isovalent substitution of Te for Se in KxFe2-ySe2 crystals suppresses the superconductivity in this system.Comment: 10 pages, 1 table, 8 figure