A quantitative method for extracting minority carrier diffusion and drift lengths is developed and demonstrated in a heavily-doped semiconductor heterostructure. This method advances the high resolution transport imaging technique, yielding key material parameters with a single, non-destructive measurement. This is the first demonstration of an SEM-based, contact-free, non-destructive technique for high-resolution minority carrier lifetime measurement. The measured values are in excellent agreement with theoretical calculations. The imaging transport technique is also employed to image the nature of the generation region as a function of beam energy, probe current and sample atomic number. These types of images should be useful to allow for experimental verification of resolution limits in CL and EBIC associated with interaction volume effects in bulk materials and can be obtained without additional sample preparation. Finally, several suggestions for further research are offered, including mapping of radiation damage in solar cells, near-contact E field mapping and studies of low-dimensional structures such as superlattices and quantum wires. These nanoscale structures are poised to usher-in the next revolution in solid-state electronic devices.

<p> Carbon nanotubes (CNTs) are materials with significant potential applications due to their desirable mechanical and electronic properties, which can both vary based on their structure. Electronic applications for CNTs are still few and not widely available, mainly due to the difficulty in the control of fabrication. Carbon nanotubes are grown in batches, but despite many years of research from their first discovery in 1991, there are still many unanswered questions regarding how to control the structure of CNTs. This work attempts to bridge some of the gap between question and answer by focusing on the catalyst particle used in common CNT growth procedures. Ostwald ripening studies on iron nanoparticles are performed in an attempt to link catalyst morphology during growth and CNT chirality (the structure aspect of a nanotube that determines its electrical properties). These results suggest that inert gas dynamics play a critical role on the catalyst morphology during CNT growth. A novel method for CNT catalyst activation by substrate manipulation is presented. Results of this study build upon prior knowledge of the role of the chemistry of the substrate supporting CNT catalysts. By bombarding sapphire, a substrate known to not support CNT growth, with an argon ion beam, the substrate is transformed into an active CNT growth support by modifying both the structure and chemistry of the sapphire surface. Finally, catalyst formation is studied with transmission electron microscopy by depositing an iron gradient film in order to identify a potential critical catalyst size and morphology for CNT growth. A relationship between catalyst size and morphology has been identified that adds evidence to the hypothesis that a catalysts activity is determined by its size and ability to properly reduce.</p>

<p>Printed electronics, including transparent conductors, currently rely on expensive materials to generate high conductivity devices. Conductive inks for thick film applications utilizing inkjet, aerosol, and screen printing technologies are often comprised of expensive and rare silver particles. Thin film applications such as organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs) predominantly employ indium tin oxide (ITO) as the transparent conductive layer which requires expensive and wasteful vapor deposition techniques. Thus an alternative to silver and ITO with similar performance in printed electronics warrants considerable attention. Copper nanomaterials, being orders of magnitude cheaper and more abundant than silver or indium, solution-coatable, and exhibiting a bulk conductivity only 6 % less than silver, have emerged as a promising candidate for incorporation in printed electronics.</p><p> First, we examine the effect of nanomaterial shape on the conductivity of thick films. The inks used in such films often require annealing at elevated temperature in order to sinter the silver nanoparticles together and obtain low resistivities. We explore the change in morphology and resistivity that occurs upon heating thick films of silver nanowires (of two different lengths, Ag NWs), nanoparticles (Ag NPs), and microflakes (Ag MFs) deposited from water at temperatures between 70 and 400 °C. At the lowest temperatures, longer Ag NWs exhibited the lowest resistivity (1.8 × 10-5 Ω cm), suggesting that the resistivity of thick films of silver nanostructures is dominated by the contact resistance between particles.</p><p> This result supported previous research showing that junction resistance between Ag NWs in thin film conductors also dominates optoelectronic performance. Since the goal is to replace silver with copper, we perform a similar analysis by using a pseudo-2D rod network modeling approach that has been modified to include lognormal distributions in length that more closely reflect experimental data collected from the nanowire transparent conductors. In our analysis, we find that Cu NW-based transparent conductors are capable of achieving comparable electrical performance to Ag NW transparent conductors with similar dimensions. We also synthesize high aspect ratio Cu NWs (as high as 5700 in an aqueous based synthesis taking less than 30 minutes) and show that this increase in aspect ratio can result results in transparent conducting films with a transmittance >95% at a sheet resistance <100 Ω sq−1, optoelectronic properties similar to that for ITO.</p><p> Two of the major barriers preventing the further use of Cu NWs in printed electronics are the necessity to anneal the nanowires under H2¬ at higher temperatures and copper’s susceptibility to oxidation. The former issue is solved by removing the insulating oxide along the Cu NWs with acetic acid and pressing the nanowires together to make H2 annealing obsolete. Finally, several methods of preventing copper oxidation in the context of transparent conductors were successfully developed such as electroplating zinc, tin, and indium and electrolessly plating benzotriazole (BTAH), nickel, silver, gold, and platinum. While all of the shells lessened or prevented oxidation both in dry and humid conditions, it was found that a thin layer of silver confers identical optoelectronic properties to the Cu NWs as pure Ag NWs. These results are expected provide motivation to replace pure silver and ITO in printed electronics.</p> / Dissertation

The ultimate intent of this research program is to produce nanosized magnetic tunneling junctions, and to study the physical properties of such devices. The physical phenomena of nanosized tunneling junctions are significantly different than that of currently popular micro-sized junctions. There is a considerable amount of work that must be done prior to producing these new junctions to ensure that good measurements can be carried out once the structures have been built. This thesis describes the efforts taken to design a measurement platform that will accurately measure tunneling magnetoresistance (TMR) in nanosized Magneto-tunneling Junctions (MTJ). The testing done with this system at various stages throughout the design and testing process confirm the expectations for the performance of the system. Voltage-current measurements can be performed on objects ranging from a few nanometers in size to micrometer sized. Traditional micro-sized MTJs have not been excluded in this design.

MSc., Faculty of Science, University of the Witwatersrand, 2011 / Gold has for many years been regarded as being inert and catalytically inactive compared to the PGMs (platinum group metals). However, in the past decade it has attracted a lot of interest as both a heterogeneous and a homogenous catalyst and has been shown to catalyse a wide range of reactions e.g. oxidation, hydrogenation and reduction among others. Highly dispersed gold nanoparticles on metal oxides, like titanium oxide (Degussa, P25) have predominantly been studied because they yield some of the most active and stable catalysts. Modification of the catalysts and/or supports has been shown to affect their catalytic properties. Likewise, perovskites, which can be manipulated by partial substitution, are reported to be active supports for CO oxidation, but only at high temperatures with no activity shown for temperatures below 200°C. In this study, these perovskites were investigated at low temperatures (below 100°C) with improved activity found upon gold deposition. The presence of gold nanoparticles therefore significantly enhanced the catalytic activity, while the support itself was suspected to be involved in the reaction mechanism. A series of perovskites of the type ABO3 (LaMnO3, LaFeO3, LaCoO3 and LaCuO3) were prepared using the citrate method, while the gold was deposited on them using the deposition-precipitation method. The supports were calcined at different temperatures for optimisation. The catalysts were tested for carbon monoxide oxidation and the active catalysts characterised by XRF, XPS, XRD, Raman spectroscopy and BET surface area measurements. With the support calcined at 800ºC, the best catalyst was then modified and compared with the unmodified catalyst. The 1-wt%Au supported on LaFeO3 was found to give the best catalytic performance. This support was then modified with various weight loadings of calcium to determine the effect of calcium on the catalytic activity. Calcium-doped materials showed decreased surface area, poorer crystallinity and a drop in catalytic activity relative to the Au-LaFeO3 which indicated the best results for CO oxidation. In addition, Au-LaFeO3 showed online stability over 21 hours. Calcining the support improved the incorporation of gold nanoparticles into the perovskite lattice, resulting in superior catalytic activity. Nevertheless, at higher calcination temperatures, the catalytic activity of Au-CaTiO3 was depressed while that of Au-LaFeO3 was enhanced. The activity of perovskites increased upon gold deposition. XPS, revealed that in the active catalysts, both cationic and metallic gold co-existed, whilst in the inactive catalysts the gold existed predominantly either as cationic or metallic gold.

Carbon nanotubes (CNTs) are known to exhibit outstanding mechanical, electrical, thermal, and coupled electromechanical properties. CNTs can be employed towards the design of an innovative strain sensor with enhanced multifunctionality due to their load carrying capability, sensing properties, high thermal stability, and outstanding electrical conductivity. All these features indicate the prospect to use CNTs in a very wide range of applications, for instance, highly sensitive resistance-type strain/force sensors, wearable electronics, flexible microelectronic devices, robotic skins, and in-situ structural health monitoring. CNT-based strain sensors can be divided into two different types, the individual CNT- based strain sensors and the ensemble CNT-based strain sensors e.g. CNT/polymer nanocomposites and CNT thin films. In contrast, to individual CNT-based strain sensors with very high gauge factor (GF) e.g. ~3000, the ensemble CNT-based strain sensors exhibit very low GFs e.g. for a SWCNT thin film strain sensor, GF is ~1. This research discusses the mechanisms and the optimizing principles of a SWCNT thin film piezoresistive sensor, and provide an experimental validation of the numerical/analytical investigations. The dependence of the piezoresistivity on key parameters like alignment, network density, bundle diameter (effective tunneling area), and SWCNT length is studied. The tunneling effect is significant in SWCNT thin films showing higher degrees of alignment, due to greater inter-tube distances between the SWCNTs as compared to random oriented SWCNT thin films. It can be concluded that SWCNT thin films featuring higher alignment would have a higher GF. On the other hand, the use of sparse network density which comprises of aligned SWCNTs can as well intensify the tunneling effect which can result to a further increase in the GF. In addition, it is well-known that percolation is greatly influenced by the geometry of the nanotubes e.g. bundle diameter and length. A study on the influence of bundle diameter of SWCNTs on the piezoresistivity behavior of mechanically drawn SWCNT thin films showed the best performance with an improved GF of ~10 when compared to the randomly oriented SWCNT thin films with GF of ~1. The non-linear piezoresistivity of the mechanically drawn SWCNT thin films is considered to be the main mechanism behind the high strain sensitivity. Furthermore, information about the average length and length distribution is very essential when examining the influence of individual nanotube length on the strain sensitivity. With that in mind, we use our previously developed preparative ultracentrifuge method (PUM), and our newly developed gel electrophoresis and simultaneous Raman and photolumiscence spectroscopy (GEP-SRSPL) to characterize the average length and length distribution of SWCNTs respectively. / A Dissertation submitted to the Department of Industrial and Manufacturing Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester 2015. / September 28, 2015. / Alignment, Gauge Fcator, Network density, Piezoresistivity, Single Walled Carbon nanotubes, Strain sensor / Includes bibliographical references. / Tao Liu, Professor Directing Dissertation; Sachin Shanbhag, University Representative; Mei Zhang, Committee Member; Okenwa Okoli, Committee Member; William Oates, Committee Member.

Carbon nanotube (CNT) fibers are very promising materials for many applications. Strong interactions among individual CNTs could produce a dense yarn results in exceptional properties. These properties are used in the application of high-performance reinforcement for composites. As the reinforcement, the primary function is to provide outstanding load bearing capability. Currently literatures use a variety of measurement techniques and gauge lengths that have not been uniform for CNT yarn tests. The need for a standardized testing method for characterization is necessary in generating reproducible and comparable data for CNT yarn or fiber materials. In this work, the strength of CNT fibers was characterized using three different types of tensile test method: the film and fiber test fixtures from dynamics mechanic analysis (DMA), and TS 600 tensile fixture. Samples that underwent the film and TS 600 tensile fixture were attached with a thick paper tabbing methodology based on ASTM standard D3379. As for the fiber fixture was performed with the test material attached directly to the fixture based on the fiber test instruction from TA Instrument. The results of the three different methods provided distinct variance in stress, strain, and modulus. A design of experiment (DoE) was established and performed on the DMA film fixture as determined from the preliminary experiment. The DoE was successful in quantifying the critical parameters' ranges that attributed to standard deviation of average stress. These parameters were then tested on 30 more samples with an improved additive manufactured tab. The results significantly decreased all mechanical testing parameters' standard deviations. Most importantly, the results prove the probability of a valid gauge break increased to more than 400%. / A Thesis submitted to the Department of Industrial and Manufacturing Engineering in partial fulfillment of the Master of Science. / Fall Semester 2015. / November 13, 2015. / Additive Manufacturing, Manufacturing, Material Science, Quality Engineering, Statistics / Includes bibliographical references. / Zhiyong Liang, Professor Directing Thesis; Mei Zhang, Committee Member; Arda Vanli, Committee Member.

University of Macau / Faculty of Science and Technology. / Department of Electromechanical Engineering

Chen, Qianjin. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Nanotechnology is the manipulation of matter at the supramolecular, molecular and atomic scale. As a result, nanotechnology is included in various fields of science including surface science, organic chemistry, molecular biology, semiconductor physics, microfabrication and molecular engineering. One of the ambitions for nanotechnology is to develop electrical devices where the active component is a single molecule or nanomoiety. In order to fabricate such devices, it is of paramount importance to develop strategies beyond the current top-down lithographic approaches typically employed in the semiconductor industry. In this regard, the ability to control the assembly of single-molecules and individual nanomoieties directly in solution can allow for the development of solution-processable approaches in nanotechnology, towards the fabrication of single-molecule devices. In this thesis, it will be discussed how molecular junctions with functional single molecules are fabricated in aqueous solutions employing single-walled carbon nanotubes as potential nanoelectrodes. Furthermore, it will be demonstrated how the assembly of molecular junctions can facilitate other functions and the construction of both nanostructures and microstructures. To begin, relevant work will be discussed that has been done in this field to date and outline clear ambitions of the study presented here. Subsequently, the key characterisation techniques that underpin all the results in this study will be described. In this work, it will be reported how metallic carbon nanotubes can act as nanoelectrodes in molecular junction assemblies and how conductive measurements of individual molecules are performed. Therefore, for the first time, the molecular junction conductance of a series of oligophenyls were successfully measured, which were formed via a solution-based assembly method. Measured molecular conductance values of the series of oligophenyls resulted in a β value of 0.5 Å−1. Furthermore, it will be described how the approach outlined previously can be extended to the synthesis of tri-amine molecular linkers as well as the formation of three-terminal junctions as the foundation of carbon nanotube-based single-molecule electronic devices. This research resulted in an increase in the formation of Y-shape molecular junctions by ~25%. Next, this report will outline the formation of molecular junctions in two-dimensional structures, which can allow for the development of electrical devices into networks. Utilising modified DNA sequences, "click" chemistry can lead to nanotube network with dimensions ranging into the micrometre scale. Building on this work, it will be further report on the change in physical properties when these two-dimensional superstructures are embedded into polymeric thin films. Finally, conclusions of the research will be drawn and it will be discussed how the findings obtained in this work can contribute to the development of novel single-molecule electronic devices.