Synthesis, Structural Characterisation and Self Assembly of Nanoparticles

<p>This thesis is concerned with the synthesis, structural characterisation and self assembly of various nanocrystalline materials. These materials include gold, lead sulfide and lead selenide with substantial focus given to the noble metal palladium. The aim of this research was to obtain size and shape control over nanoparticles formed from solution phase synthesis for various applications. This was realised with chemical techniques using organic surfactants as growth controlling agents. The morphology, composition, internal crystal structure and applicable properties of the as synthesised nanoparticles were fully investigated to give a complete characterisation. Characterisation was carried out using a number of techniques including Super and High Resolution Transmission Electron Microscopy (SHREM, HREM), Synchrotron Powder X-Ray Diffraction (XRD), Selected Area Electron Diffraction (SAED) and Energy Dispersive X-Ray Spectroscopy (EDS). The first chapter in this thesis focuses on the synthesis and self assembly of monodisperse gold nanoparticles into nanoparticle superlattices (NPSLs), an exciting new type of material. The nanoparticles were prepared using a well known chemical method at room temperature. They were then arranged into NPSLs by a simple evaporation technique. Intermediate structures to the SLs were isolated which gave an insight into their formation. This showed that the NPs first self assembled into an energetically unfavourable bilayer before forming the most thermodynamically preferred three dimensional structure. This behaviour was due to the presence of organic capping ligands. The second chapter is concerned with the synthesis and characterisation of lead chalcogenide nanoparticles (lead sulfide and lead selenide). These are semiconductor materials which can provide a photocurrent when illuminated with infra-red radiation which makes them ideal candidates for solar cell technology. The nanoparticles were synthesised using a bench top solvothermal method. By varying the nature of the surfactant system, the precursor and the reaction time and temperature a wide range of nanoparticles with different sizes and shapes were prepared. A type of lead sulfide nanoparticles was then chosen for capping ligand exchange experiments. The new method developed here provides a facile route to water soluble lead chalcogenide nanoparticles and a means to more easily extract a photocurrent when used in solar cell applications. The remainder of this thesis is focussed on the synthesis and structural characterisation of palladium nanoparticles. Palladium is a very important catalytic metal therefore control over its size and shape on the nanoscale is of primary concern. In the third chapter of this thesis various types of palladium nanoparticles were produced using solution phase techniques in a pressure reaction vessel. By varying the nature of the surfactant system, the precursor and the reaction pressure, temperature and time the size and shape of the resulting nanoparticles could be controlled. These included spherical and worm-like nanoparticles as well as novel pod-like and highly branched palladium nanostructures. These complex shapes were the first evidence of this kind of morphology for palladium and provide a new and exciting material for catalytic applications. The final chapter in this thesis features a full structural characterisation and growth mechanism for the novel, complex palladium nanostructures along with an investigation into their catalytic and hydrogen absorption properties. The structural characterisation of a palladium tripod provides the first direct evidence of complex growth from a symmetrical nanoparticle core possessing the face centred cubic crystal structure. The growth of the highly branched palladium nanostructures is then tracked in real time. It is shown that the growth involves the formation of nuclei followed by tripod intermediates and finally highly branched nanostructures. By varying the nature of the surfactant system the kinetics of the reaction and hence the morphology of the resulting nanostructures can be controlled. A full growth mechanism is therefore proposed. The catalytic activity of the highly branched palladium nanostructures towards a simple organic transformation reaction is investigated. Finally, the hydrogen absorption and desorption properties of the highly branched nanostructures is explored. The results presented here regarding palladium nanoparticles are applicable to other industrially important noble metals such as gold, silver and platinum. A final conclusion chapter is then presented along with ideas for future research.</p>

Branched High Aspect Ratio Nanostructures Fabricated by Focused Helium Ion Beam Induced Deposition of an Insulator

Helium ion beam induced deposition using the gaseous precursor pentamethylcyclopentasiloxane is employed to fabricate high aspect ratio insulator nanostructures (nanopillars and nanocylinders) that exhibit charge induced branching. The branched nanostructures are analyzed by transmission electron microscopy. It is found that the side branches form above a certain threshold height and that by increasing the flow rate of the precursor, the vertical growth rate and branching phenomenon can be significantly enhanced, with fractalesque branching patterns observed. The direct-write ion beam nanofabrication technique described herein offers a fast single-step method for the growth of high aspect ratio branched nanostructures with site-selective placement on the nanometer scale.

Unveiling General Rules Governing Dimensional Evolution of Branched TiO2 and Impacts on Photo-electrochemical Behaviours

Branched nanostructures represent a unique group of nanoarchitectures exhibiting advantageous high surface area and excellent charge transport for energy conversion application compared to their bulk counterparts. Especially, branched titanium dioxide...

Multiplicative Degree Based Topological Indices of Nanostar Dendrimers

A topological index is a numerical quantity connected with a graph describing the molecular topology of the graph. It can predict different physicochemical properties such as boiling point, entropy, acentric factor etc. of chemical compounds. Dendrimers are highly branched nanostructures that are regarded as a building block in nanotechnology having wide applications. In this paper, multiplicative degree-based topological indices are computed for some nanostar dendrimers. The derived results have the potential for implementation in the chemical, biological, and pharmaceutical sciences.

Peptide-Based Soft Hydrogels Modified with Gadolinium Complexes as MRI Contrast Agents

Poly-aromatic peptide sequences are able to self-assemble into a variety of supramolecular aggregates such as fibers, hydrogels, and tree-like multi-branched nanostructures. Due to their biocompatible nature, these peptide nanostructures have been proposed for several applications in biology and nanomedicine (tissue engineering, drug delivery, bioimaging, and fabrication of biosensors). Here we report the synthesis, the structural characterization and the relaxometric behavior of two novel supramolecular diagnostic agents for magnetic resonance imaging (MRI) technique. These diagnostic agents are obtained for self-assembly of DTPA(Gd)-PEG8-(FY)3 or DOTA(Gd)-PEG8-(FY)3 peptide conjugates, in which the Gd-complexes are linked at the N-terminus of the PEG8-(FY)3 polymer peptide. This latter was previously found able to form self-supporting and stable soft hydrogels at a concentration of 1.0% wt. Analogously, also DTPA(Gd)-PEG8-(FY)3 and DOTA(Gd)-PEG8-(FY)3 exhibit the trend to gelificate at the same range of concentration. Moreover, the structural characterization points out that peptide (FY)3 moiety keeps its capability to arrange into β-sheet structures with an antiparallel orientation of the β-strands. The high relaxivity value of these nanostructures (~12 mM−1·s−1 at 20 MHz) and the very low in vitro cytotoxicity suggest their potential application as supramolecular diagnostic agents for MRI.

Synthesis of InP branched nanostructures by controlling the intermediate nanoclusters

Herein, we present the syntheses of branched, hyper-branched and dendrimer-like InP nanostructures from InP magic-sized clusters and additives.

Assembly of silica rods into tunable branched living nanostructures mediated by coalescence of catalyst droplets

Branched nanostructures with tunable arm numbers are prepared through the assembly of silica rods mediated by coalescence of catalyst droplets on the end of the rods.

Understanding the Role of AgNO3 Concentration and Seed Morphology to Achieve Tunable Shape Control in Gold Nanostars

<div>Gold nanostars are one of the most fascinating anisotropic nanoparticles. Nanostar morphology can be controlled by changing various synthetic parameters; however, the detailed</div><div>growth mechanisms are not fully understood. Herein, we investigate this process in six-branched nanostars, focusing first on the properties of the single crystalline seed, which evolves to include penta-twinned defects as the gateway to anisotropic growth into 6-branched nanostars. In particular, we report on a high-yield seed-mediated protocol for the synthesis of these particles with high monodispersity in the presence of Triton-X, ascorbic acid, and AgNO3. Detailed</div><div>spectroscopic and microscopic analyses have allowed the identification of several key intermediates in the growth process, revealing that it proceeds via penta-twinned intermediate seeds. Importantly, we report the first experimental evidence tracking the location of silver with</div><div>sub-nanometer resolution and prove its role as stabilizing agent in these highly branched nanostructures. Our results indicate that metallic silver on the spikes stabilizes the nanostar morphology, and that the remaining silver, present when AgNO3 is added at high concentration, deposits on the core and between the base of neighboring spikes. Importantly, we also demonstrate the possibility to achieve monodispersity, reproducibility, and tunability in colloidal gold nanostars that are substantially higher than previously reported, which could be leveraged to carry out holistic computational-experimental studies to understand, predict, and tailor their plasmonic response.</div><div><br></div>