AuxAg1-x alloy seeds: A way to control growth, morphology and defect formation in Ge nanowires

Germanium (Ge) nanowires are of current research interest for high speed nanoelectronic devices due to the lower band gap and high carrier mobility compatible with high K-dielectrics and larger excitonic Bohr radius ensuing a more pronounced quantum confinement effect [1-6]. A general way for the growth of Ge nanowires is to use liquid or a solid growth promoters in a bottom-up approach which allow control of the aspect ratio, diameter, and structure of 1D crystals via external parameters, such as precursor feedstock, temperature, operating pressure, precursor flow rate etc [3, 7-11]. The Solid-phase seeding is preferred for more control processing of the nanomaterials and potential suppression of the unintentional incorporation of high dopant concentrations in semiconductor nanowires and unrequired compositional tailing of the seed-nanowire interface [2, 5, 9, 12]. There are therefore distinct features of the solid phase seeding mechanism that potentially offer opportunities for the controlled processing of nanomaterials with new physical properties. A superior control over the growth kinetics of nanowires could be achieved by controlling the inherent growth constraints instead of external parameters which always account for instrumental inaccuracy. The high dopant concentrations in semiconductor nanowires can result from unintentional incorporation of atoms from the metal seed material, as described for the Al catalyzed VLS growth of Si nanowires [13] which can in turn be depressed by solid-phase seeding. In addition, the creation of very sharp interfaces between group IV semiconductor segments has been achieved by solid seeds [14], whereas the traditionally used liquid Au particles often leads to compositional tailing of the interface [15] . Korgel et al. also described the superior size retention of metal seeds in a SFSS nanowire growth process, when compared to a SFLS process using Au colloids [12]. Here in this work we have used silver and alloy seed particle with different compositions to manipulate the growth of nanowires in sub-eutectic regime. The solid seeding approach also gives an opportunity to influence the crystallinity of the nanowires independent of the substrate. Taking advantage of the readily formation of stacking faults in metal nanoparticles, lamellar twins in nanowires could be formed.

Small angle scattering characterisation of micellar systems and templated architectures

The work described in this thesis reports the structural changes induced on micelles under a variety of conditions. The micelles of a liquid crystal film and dilute solutions of micelles were subjected to high pressure CO2 and selected hydrocarbon environments. Using small angle neutron scattering (SANS) techniques the spacing between liquid crystal micelles was measured in-situ. The liquid crystals studied were templated from different surfactants with varying structural characteristics. Micelles of a dilute surfactant solution were also subjected to elevated pressures of varying gas atmospheres. Detailed modelling of the in-situ SANS experiments revealed information of the size and shape of the micelles at a number of different pressures. Also reported in this thesis is the characterisation of mesoporous materials in the confined channels of larger porous materials. Periodic mesoporous organosilicas (PMOs) were synthesised within the channels of anodic alumina membranes (AAM) under different conditions, including drying rates and precursor concentrations. In-situ small angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) was used to determine the pore morphology of the PMO within the AAM channels. PMO materials were also used as templates in the deposition of gold nanoparticles and subsequently used in the synthesis of germanium nanostructures. Polymer thin films were also employed as templates for the directed deposition of gold nanoparticles which were again used as seeds for the production of germanium nanostructures. A supercritical CO2 (sc-CO2) technique was successfully used during the production of the germanium nanostructures.

The impacts of past land-use on the ecology of an ancient woodland in south-west Ireland

A multi-disciplinary study was conducted to compare stands of ancient and secondary origin within a single wood, the Gearagh woodland, County Cork. These sites were compared with adjacent areas of grassland, which provided a reference for the former land-use (pasture) of the secondary woodland. A historical study confirmed that while the core of the Gearagh has been subject to minimal human interference, other sections have been cleared in the past for agricultural purposes. Investigations into soil structure and composition showed that soil properties in these secondary woodland areas were significantly altered by this past woodland clearance and conversion to agriculture, while the soil of the ancient woodland showed little signs of disturbance. The vegetation community also differed between the two woodland areas, partly due to altered environmental conditions. Many of the ancient woodland plant species were unable to form a persistent seed bank, while there was increased representation of species associated with more open-habitat conditions in the seed bank of the secondary woodland. While germination of woodland species was low in all sites, overall, seeds tended to germinate more successfully in the ancient woodland. The ancient woodland also provided a suitable habitat for many soil and ground detritivores, most notably enchytraeids, although earthworms were not abundant. Past agricultural use, however, changed the decomposer community considerably, with increased representation of earthworm species and a decline in the abundance of enchytraeids in the secondary stands. In conclusion, the legacies of historical agricultural activities can continue to significantly affect the structure and composition of present-day woodlands so that they may differ considerably from undisturbed ancient woodland stands, even within the same woodland. A greater understanding of the origin, development and ecological functioning of ancient woodlands should aid in determining future conservation and management requirements.

Synthesis & characterisation of group IV semiconducting nanowires

Semiconductor nanowires, particularly group 14 semiconductor nanowires, have been the subject of intensive research in the recent past. They have been demonstrated to provide an effective, versatile route towards the continued miniaturisation and improvement of microelectronics. This thesis aims to highlight some novel ways of fabricating and controlling various aspects of the growth of Si and Ge nanowires. Chapter 1 highlights the primary technique used for the growth of nanowires in this study, namely, supercritical fluid (SCF) growth reactions. The advantages (and disadvantages) of this technique for the growth of Si and Ge nanowires are highlighted, citing numerous examples from the past ten years. The many variables involved in this technique are discussed along with the resultant characteristics of nanowires produced (diameter, doping, orientation etc.). Chapter 2 outlines the experimental methodologies used in this thesis. The analytical techniques used for the structural characterisation of nanowires produced are also described as well as the techniques used for the chemical analysis of various surface terminations. Chapter 3 describes the controlled self-seeded growth of highly crystalline Ge nanowires, in the absence of conventional metal seed catalysts, using a variety of oligosilylgermane precursors and mixtures of germane and silane compounds. A model is presented which describes the main stages of self-seeded Ge nanowire growth (nucleation, coalescence and Ostwald ripening) from the oligosilylgermane precursors and in conjunction with TEM analysis, a mechanism of growth is proposed. Chapter 4 introduces the metal assisted etching (MAE) of Si substrates to produce Si nanowires. A single step metal-assisted etch (MAE) process, utilising metal ion-containing HF solutions in the absence of an external oxidant, was developed to generate heterostructured Si nanowires with controllable porous (isotropically etched) and non-porous (anisotropically etched) segments. In Chapter 5 the bottom-up growth of Ge nanowires, similar to that described in Chapter 3, and the top down etching of Si, described in Chapter 4, are combined. The introduction of a MAE processing step in order to “sink” the Ag seeds into the growth substrate, prior to nanowire growth, is shown to dramatically decrease the mean nanowire diameters and to narrow the diameter distributions. Finally, in Chapter 6, the biotin – streptavidin interaction was explored for the purposes of developing a novel Si junctionless nanowire transistor (JNT) sensor.

Fabrication and characterisation of semiconductor nanowires and metal interconnects

One-dimensional semiconductor nanowires are considered to be promising materials for future nanoelectronic applications. However, before these nanowires can be integrated into such applications, a thorough understanding of their growth behaviour is necessary. In particular, methods that allow the control over nanowire growth are deemed especially important as it is these methods that will enable the control of nanowire dimensions such as length and diameter (high aspect ratios). The production of nanowires with high-aspect ratios is vital in order to take advantage of the unique properties experienced at the nanoscale, thus allowing us to maximise their use in devices. Additionally, the development of low-resistivity interconnects is desirable in order to connect such nanowires in multi-nanowire components. Consequently, this thesis aims to discuss the synthesis and characterisation of germanium (Ge) nanowires and platinum (Pt) interconnects. Particular emphasis is placed on manipulating the nanowire growth kinetics to produce high aspect ratio structures. The discussion of Pt interconnects focuses on the development of low-resistivity devices and the electrical and structural analysis of those devices. Chapter 1 reviews the most critical aspects of Ge nanowire growth which must be understood before they can be integrated into future nanodevices. These features include the synthetic methods employed to grow Ge nanowires, the kinetic and thermodynamic aspects of their growth and nanowire morphology control. Chapter 2 outlines the experimental methods used to synthesise and characterise Ge nanowires as well as the methods used to fabricate and analyse Pt interconnects. Chapter 3 discusses the control of Ge nanowire growth kinetics via the manipulation of the supersaturation of Ge in the Au/Ge binary alloy system. This is accomplished through the use of bi-layer films, which pre-form Au/Ge alloy catalysts before the introduction of the Ge precursor. The growth from these catalysts is then compared with Ge nanowire growth from standard elemental Au seeds. Nanowires grown from pre-formed Au/Ge alloy seeds demonstrate longer lengths and higher growth rates than those grown from standard Au seeds. In-situ TEM heating on the Au/Ge bi-layer films is used to support the growth characteristics observed. Chapter 4 extends the work of chapter 3 by utilising Au/Ag/Ge tri-layer films to enhance the growth rates and lengths of Ge nanowires. These nanowires are grown from Au/Ag/Ge ternary alloy catalysts. Once again, the supersaturation is influenced, only this time it is through the simultaneous manipulation of both the solute concentration and equilibrium concentration of Ge in the Au/Ag/Ge ternary alloy system. The introduction of Ag to the Au/Ge binary alloy lowers the equilibrium concentration, thus increasing the nanowire growth rate and length. Nanowires with uniform diameters were obtained via synthesis from AuxAg1-x alloy nanoparticles. Manifestation of the Gibbs-Thomson effect, resulting from the dependence of the mean nanowire length as a function of diameter, was observed for all of the nanowires grown from the AuxAg1-x nanoparticles. Finally, in-situ TEM heating was used to support the nanowire growth characteristics. Chapter 5 details the fabrication and characterisation of Pt interconnects deposited by electron beam induced deposition of two different precursors. The fabrication is conducted inside a dual beam FIB. The electrical and structural characteristics of interconnects deposited from a standard organometallic precursor and a novel carbon-free precursor are compared. The electrical performance of the carbon-free interconnects is shown to be superior to that of the organometallic devices and this is correlated to the structural composition of both interconnects via in-situ TEM heating and HAADF-STEM analysis. Annealing of the interconnects is carried out under two different atmospheres in order to reduce the electrical resistivity even further. Finally, chapter 6 presents some important conclusions and summarises each of the previous chapters.

Studies on quinoa (Chenopodium quinoa) for novel food and beverage applications

Quinoa (Chenopodium quinoa) is a seed crop native to the Andes, that can be used in a variety of food product in a similar manner to cereals. Unlike most plants, quinoa contains protein with a balanced amino acid profile. This makes it an interesting raw material for e.g. dairy product substitutes, a growing market in Europe and U.S. Quinoa can however have unpleasant off-flavours when processed into formulated products. One means of improving the palatability is seed germination. Also, the increased activities of hydrolytic enzymes can have a beneficial influence in food processing. In this thesis, the germination pattern of quinoa was studied, and the influence of quinoa malt was evaluated in a model product. Additionally, to explore its potential for dairy-type products, quinoa protein was isolated from an embryo-enriched milling fraction of non-germinated quinoa and tested for functional and gelation properties. Quinoa seeds imbibed water very rapidly, and most seeds showed radicle protrusion after 8-9 h. The α-amylase activity was very low, and started to increase only after 24 hours of germination in the starchy perisperm. Proteolytic activity was very high in dry ungerminated seeds, and increased slightly over 24 h. A significant fraction of this activity was located in the micropylar endosperm. The incorporation of germinated quinoa in gluten-free bread had no significant effect on the baking properties due to low α-amylase activity. Upon acidification with glucono-δ-lactone, quinoa milk formed a structured gel. The gelation behaviour was further studied using a quinoa protein isolate (QPI) extracted from an embryoenriched milling fraction. QPI required a heat-denaturation step to form gel structures. The heating pH influenced the properties drastically: heating at pH 10.5 led to a dramatic increase in solubility, emulsifying properties, and a formation of a fine-structured gel with a high storage modulus (G') when acidified. Heating at pH 8.5 varied very little from the unheated protein in terms of functional properties, and only formed a randomly aggregated coagulum with a low G'. Further study of changes over the course of heating showed that the mechanism of heat-denaturation and aggregation indeed varied largely depending on pH. The large difference in gelation behaviour may be related to the nature of aggregates formed during heating. To conclude, germination for increased enzyme activities may not be feasible, but the structure-forming properties of quinoa protein could possibly be exploited in dairy-type products.

Evaluation of rhizobacterial colonisation and the ability to induce Globodera pallida hatch

Three bacterial isolates, SB13 (Acinetobacter sp.), SB14 (Arthrobacter sp.) and SB15 (Bacillus sp.), were previously isolated from the rhizosphere of sugar beet (Beta vulgaris ssp. vulgaris) plants and shown to increase hatch of potato cyst nematodes in vitro. In this study, the three isolates were assayed for rhizosphere competence. Each isolate was applied to seeds at each of four concentrations (105-108 CFU ml−1) and the inoculated seeds were planted in plastic microcosms containing coarse sand. All three isolates were shown to colonise the rhizosphere, although to differing degrees, with the higher inoculation densities providing significantly better colonisation. The isolates increased sugar beet root and shoot dry weight. Isolates SB14 and SB15 were analysed for their ability to induce in vivo hatch of Globodera pallida in non-sterile soil planted with sugar beet. After 4 and 6 weeks, both isolates had induced significantly greater percentage hatch compared to controls.

Nanosize effect in germanium nanowire growth with binary metal alloys

This article describes feasible and improved ways towards enhanced nanowire growth kinetics by reducing the equilibrium solute concentration in the liquid collector phase in a vapor-liquid-solid (VLS) like growth model. Use of bi-metallic alloy seeds (AuxAg1-x) influences the germanium supersaturation for a faster nucleation and growth kinetics. Nanowire growth with ternary eutectic alloys shows Gibbs-Thompson effect with diameter dependent growth rate. In-situ transmission electron microscopy (TEM) annealing experiments directly confirms the role of equilibrium concentration in nanowire growth kinetics and was used to correlate the equilibrium content of metastable alloys with the growth kinetics of Ge nanowires. The shape and geometry of the heterogeneous interfaces between the liquid eutectic and solid Ge nanowires were found to vary as a function of nanowire diameter and eutectic alloy composition.