Synthesis and Characterization of Silicon and Germanium Nanocrystals and Titanium Disulphide Nanostructures

Abstract

Keywords: Solution synthesis, silicon nanocrystals, germanium nanocrystals, size control, hydride reducing agents, fluorescent whitening agents, titanium disulphide nanostructures

This thesis is concerned with the synthesis and characterization of nanostructured materials in the solution, in particular silicon and germanium nanocrystals, their applica-tion as fluorescent whitening agents and titanium disulphide nanostructures. The aim of this research with regards to the synthesis of silicon and germanium nanocrystals was to obtain size control and provide functionality using simple room temperature solution techniques. In the case of the nanostructures of titanium disulphide, the focus was to synthesize in the colloid using simple one-pot bench top techniques. The above were realized with chemical techniques in the solution using organic solvents and surfactants to control their size. The morphology, chemical composition and crystal structure of the synthesized nanomaterials were characterized using techniques such as High Resolution Transmission Electron Microscopy (HRTEM), Selected Area Electron Diffraction (SAED), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectrosco-py (EDX) and Optical Spectroscopies. Whilst chapter one is a brief introduction of the thesis, chapter two talks in detail about the various characterization techniques used in this research.

Chapter three of the thesis focuses on the synthesis of alkyl- and amine-functionalized silicon nanocrystals using a microemlusion technique. The effect of reducing agents, surfactants and precursors on particle size was studied. The surfactant C12E5 was found to be very effective in producing silicon nanocrystals that were freestanding and pure. Whilst the hydride reducing agents lithium aluminium hydride and lithium tri-ethyl borohydride were found to be effective in synthesizing nanocrystals of narrow size distribution, it was found that using silicon tetrachloride yielded smaller particles compared to silicon tetrabromide.

The fourth chapter in the theses is concerned with the synthesis and characterization of germanium nanocrystals by both microemulsion and high temperature techniques. Using lithium aluminium hydride; a strong reducing agent, very small nanocrystals were obtained, whilst weaker reductants such as sodium borohydride produced larger nano-crystals. Another effective method to control the particle size of germanium nanocrystals was found to be by varying the concentration of precursor. The germanium nanocrystals which were amine capped were found to luminesce in the blue and were used to image HePG2 cells. Toxicity studies on these nanocrystals proved their relative non-toxicity. The high temperature experiments, though not as flexible as the room temperature syntheses were found to facilitate a certain degree of size control.

Chapter five of the theses deal with the application of silicon and germanium nanocrys-tals as fluorescent whitening agents in wool fabrics. Both nanocrystals, when applied to the fabric were found to emit matching blue fluorescence that was demonstrated to be more suited to improving the brightness properties of fabric than the commercial fluo-rescing whitening agent Uvitex. In particular Silicon-amine and Silicon-hexene functionalized nanocrystal (low concentration) treated fabrics were found to have improved color stability against both UVA and UVB radiation. The treated fabrics were in addition found to maintain a stable color than untreated fabric. Silicon-amine treated fabrics were found to have a stable color even after 48h exposures to UVA radiation. It should be noted that this is the first evidence of the application of group IV semiconduc-tor nanocrystals as fluorescing whitening agents.

The sixth chapter of this thesis deals with the one-pot synthesis of titanium disulphide nanostructures using both coordinating and non-coordinating solvents and their subse-quent characterization. By varying the injection temperature of the titanium source into the 1-Octadecene sulphur solution, two different morphologies were synthesized. Two different pathways were suggested for the formation of the flower-like and flake-like morphologies; an instant nucleation to form titanium disulphide flakes whilst spherical nuclei to form flower-like nanostructures. The flower-like nanostructures were found to have higher BET surface area compared to the flake-like nanostructures and previously reported surface areas for analogous TiS2 nanostructures. Whilst using oleylamine as solvent, the low temperature injection yielded hollow spheres of TiS2 and the high temperature injection, fullerene-like nanoparticles of TiS2. The property of oleylamine to selectively bind to the nanostructure surface in conjunction with the effect of injection temperature was understood to be behind the growth of these nanostructures. The synthesis of flower-like and flake-like morphologies by solution phase techniques were the first evidence of this kind for titanium disulphide and provides a new and exciting material for a variety of applications.

A final chapter on conclusions and recommendations for future work is then presented.