59 research outputs found
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
Growth and characterisation of titanium sulphide nanostructures by surface-assisted vapour transport methods; from trisulphide ribbons to disulphide nanosheets
Surface Assisted Chemical Vapour Transport (SACVT) methods have been employed to grow nanostructures of titanium disulphide (TiS2) and titanium trisulphide (TiS3). SACVT reactions occur between titanium and sulphur powders to form TiSx species transported in the vapour phase to grow nanometric flower-like structures on titanium-coated silica substrates. The evolution of structure and composition has been followed by powder X-ray diffraction, electron microscopy and Raman spectroscopy. At 1 : 2 Ti : S ratios, the size and shape of the hexagonal 1T-TiS2 titanium disulphide structures formed can be varied from flower-like growths with 'petals' formed from nanosheets 10 nm thick to platelets microns across. Increasing the proportion of sulphur (Ti : S 1 : 4) enables TiS3 flower-like structures composed of radiating nanoribbons to grow at elevated temperatures without decomposition to TiS2. TEM/SAED suggests that individual trisulphide ribbons grow along the [010] direction. Magnetic properties of the disulphide nanomaterials have been determined using SQUID magnetometry and Raman spectra for disulphides suggest that their crystal and electronic structures may be more complex than expected for bulk, stoichiometric, CdI2-structured TiS2
Evolution of superconductivity in LaO1-xFxBiS2 prepared by high pressure technique
Novel BiS2-based superconductors LaO1-xFxBiS2 prepared by the high pressure
synthesis technique were systematically studied. It was found that the high
pressure annealing strongly the lattice as compared to the LaO1-xFxBiS2 samples
prepared by conventional solid state reaction at ambient pressure. Bulk
superconductivity was observed within a wide F-concentration range of x = 0.2 ~
0.7. On the basis of those results, we have established a phase diagram of
LaO1-xFxBiS2.Comment: 11 pages, 6 figure
Fermiological Interpretation of Superconductivity/Non-superconductivity of FeTe_{1-x}Se_{x} Thin Crystal Determined by Quantum Oscillation Measurement
We have successfully observed quantum oscillation (QO) for FeTe_{1-x}Se_{x}.
QO measurements were performed using non-superconducting and superconducting
thin crystals of FeTe_{0.65}Se_{0.35} fabricated by the scotch-tape method. We
show that the Fermi surfaces (FS) of the non-superconducting crystal are in
good agreement with the rigid band shift model based on electron doping by
excess Fe while that of the superconducting crystal is in good agreement with
the calculated FS with no shift. From the FS comparison of both crystals, we
demonstrate the change of the cross-sectional area of the FS, suggesting that
the suppression of the FS nesting with the vector Q_{s} = (\pi, \pi) due to
excess Fe results in the disappearance of the superconductivity.Comment: 8 pages, 4 figure
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
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