Atom-Scale Insights into III-V Semiconductor Nanowires

As the feature size of MOSFET is scaling down to nano-size, series of problems need to be overcome to continue Moore’ Law, which seems an impossible task with traditional bulk Si technology due to the physical limitation and various negative effects being subject to small feature size. The critical issues for both further improving the devices’ performance and lowering their cost lie in the exploration of substitutions for Si and the control of morphological and compositional properties of materials. group III-V nanowires due to its unique properties are considered as the building block for next-generation electronic devices. To fulfill these commercial applications with group III-V nanowires, a fundamental and quantitative understanding of growth-structure-property relationships is central to applications where nanowires exhibit clear advantages. Therefore, this doctoral research systematically investigates three different semiconductor nanowires: Au-seeded, self-seed and planar nanowires, in terms of elemental, morphological and structural aspects by taking advantage of cutting-edge technique atom probe tomography, and endeavor to unveil the correlation between nanowires’ intrinsic properties and performance. Based on the atom probe findings, the growth mechanism of Au-seeded and self-seeded nanowire have been systematically discussed, and new model has been proposed to explain the phenomena on the basis of density functional calculation. Moreover, the doping distribution in planar nanowires has also been carefully investigated, and the results demonstrate that the dopants can diffuse into the substrate which subsequently degrade the device performance due to parasitic channel effect, and accordingly, suggestions have been given to optimize the planar nanowire growth for improved dopant distribution. The outcomes of this project are expected to theoretically support high-quality nanowire synthesis for specific applications

Dynamics and control of gold-encapped gallium arsenide nanowires imaged by 4D electron microscopy

Eutectic related reaction is a special chemical/physical reaction involving multiple phases, solid and liquid. Visualization of phase reaction of composite nanomaterials with high spatial and temporal resolution provides a key understanding of alloy growth with important industrial applications. However, it has been a rather challenging task. Here we report the direct imaging and control of the phase reaction dynamics of a single, as-grown free-standing gallium arsenide nanowire encapped with a gold nanoparticle, free from environmental confinement or disturbance, using four-dimensional electron microscopy. The non-destructive preparation of as-grown free-standing nanowires without supporting films allows us to study their anisotropic properties in their native environment with better statistical character. A laser heating pulse initiates the eutectic related reaction at a temperature much lower than the melting points of the composite materials, followed by a precisely time-delayed electron pulse to visualize the irreversible transient states of nucleation, growth and solidification of the complex. Combined with theoretical modeling, useful thermodynamic parameters of the newly formed alloy phases and their crystal structures could be determined. This technique of dynamical control and 4D imaging of phase reaction processes on the nanometer-ultrafast time scale open new venues for engineering various reactions in a wide variety of other systems

An analysis of surface area estimates of binary volumes under three tilings

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1997.Includes bibliographical references (leaves 77-79).by Erik G. Miller.M.S

Anisotropic nanomaterials: structure, growth, assembly, and functions

Comprehensive knowledge over the shape of nanomaterials is a critical factor in designing devices with desired functions. Due to this reason, systematic efforts have been made to synthesize materials of diverse shape in the nanoscale regime. Anisotropic nanomaterials are a class of materials in which their properties are direction-dependent and more than one structural parameter is needed to describe them. Their unique and fine-tuned physical and chemical properties make them ideal candidates for devising new applications. In addition, the assembly of ordered one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) arrays of anisotropic nanoparticles brings novel properties into the resulting system, which would be entirely different from the properties of individual nanoparticles. This review presents an overview of current research in the area of anisotropic nanomaterials in general and noble metal nanoparticles in particular. We begin with an introduction to the advancements in this area followed by general aspects of the growth of anisotropic nanoparticles. Then we describe several important synthetic protocols for making anisotropic nanomaterials, followed by a summary of their assemblies, and conclude with major applications

Comparison of segmentation software packages for in-hospital 3D print workflow

Purpose: In-hospital three-dimensional (3D) printing of patient-specific pathologies is increasingly being used in daily care. However, the efficiency of the current conversion from image to print is often obstructed due to limitations associated with segmentation software. Therefore, there is a need for comparison of several clinically available tools. A comparative study has been conducted to compare segmentation performance of Philips IntelliSpace Portal(®) (PISP), Mimics Innovation Suite (MIS), and DICOM to PRINT(®) (D2P). Approach: These tools were compared with respect to segmentation time and 3D mesh quality. The dataset consisted of three computed tomography (CT)-scans of acetabular fractures (ACs), three CT-scans of tibia plateau fractures (TPs), and three CTA-scans of abdominal aortic aneurysms (AAAs). Independent-samples [Formula: see text]-tests were performed to compare the measured segmentation times. Furthermore, 3D mesh quality was assessed and compared according to representativeness and usability for the surgeon. Results: Statistically significant differences in segmentation time were found between PISP and MIS with respect to the segmentation of ACs ([Formula: see text]) and AAAs ([Formula: see text]). Furthermore, statistically significant differences in segmentation time were found between PISP and D2P for segmentations of AAAs ([Formula: see text]). There were no statistically significant differences in segmentation time for TPs. The accumulated mesh quality scores were highest for segmentations performed in MIS, followed by D2P. Conclusion: Based on segmentation time and mesh quality, MIS and D2P are capable of enhancing the in-hospital 3D print workflow. However, they should be integrated with the picture archiving and communication system to truly improve the workflow. In addition, these software packages are not open source and additional costs must be incurred

Bubble size, coalescence and particle motion in flowing foams

In minerals processing, froth flotation is used to separate valuable metal minerals from ore. The efficiency of a froth to recover these valuable minerals is closely related to the bubble size distribution through the depth of the froth. Measurement of the bubble size entering the froth and at the froth surface has been achieved previously; however measurement of the bubble size within the froth is extremely difficult as the mineral laden bubble surfaces are opaque and fragile. This work developed a flowing foam column to enable new measurement techniques, in particular visual measurement of the bubble size distribution and velocity profile throughout the depth of the foam. Two phase foam systems share their structure with three phase froth flotation systems, but are transparent in a thin layer. A foam column was constructed to contain a monolayer of overflowing and coalescing foam. This enabled direct measurement of the dynamic bubble size and coalescence through image analysis. The results showed a strong link between column geometry and the foam behaviour. In addition, the measured bubble streamlines closely matched simulated results from a foam velocity model. Positron Emission Particle Tracking (PEPT) is the only existing technique to measure particle behaviour inside froths. In this work, tracer particles with different size and hydrophobicity were tracked in a foam flowing column with PEPT. The particle trajectories were verified with image analysis, thereby increasing confidence in PEPT measurements of opaque flotation systems. The results showed that as hydrophilic tracer particles passed through the foam, their trajectory was determined by the local structure and changes of the foam, such as coalescence events. A hydrophobic tracer particle was involved in drop–off and reattachment events, however in the majority of cases still overflowed with the foam. The tracer particle did not always follow the bubble streamlines of the flowing foam, taking instead the shortest path to overflow which cut across streamlines. This work has developed an experimental methodology to validate flowing foam and coalescence models and has developed the necessary techniques to interpret PEPT trajectories in froth flotation

Fluid dynamics and mass transfer in porous media: Modelling fluid flow and filtration inside open-cell foams

L'abstract è presente nell'allegato / the abstract is in the attachmen

Multi-Microscopy Characterisation of III-Nitride Devices and Materials

III-nitride optoelectronic devices have become ubiquitous due to their ability to emit light efficiently in the blue and green spectral ranges. Specifically, III-nitride light emitting diodes (LEDs) have become widespread due to their high brightness and efficiency. However, III-nitride devices such as single photon sources are also the subject of research and are promising for various applications. In order to improve design efficient devices and improve current ones, the relationship between the structure of the constituent materials and their optical properties must be studied. The optical properties of materials are often examined by photoluminescence or cathodoluminescence, whilst traditional microscopy techniques such a transmission electron microscopy and scanning electron microscopy are used to elucidate their structure and composition. This thesis describes the use of a dual-beam focussed ion beam/scanning electron microscope (FIB/SEM) in bridging the gap between these two types of techniques and providing a platform on which to perform correlative studies between the optical and structural properties of III-nitride materials. The heteroepitaxial growth of III-nitrides has been known to produce high defect densities, which can harm device performance. We used this correlative approach to identify hexagonal defects as the source of inhomogeneous electroluminescence (EL) in LEDs. Hyperspectral EL mapping was used to show the local changes in the emission induced by the defects. Following this the FIB/SEM was used to prepare TEM samples from the apex of the defects, revealing the presence of p-doped material in the active region caused by the defect. APSYS simulations confirmed that the presence of p-doped material can enhance local EL. The deleterious effects of defects on the photoelectrochemical etching of cavities were also studied. We performed TEM analysis of an edge-defect contained in unetched material on the underside of a microdisk using FIB/SEM sample preparation methods. The roughness and morphology of microdisk and nanobeam cavities was studied using FIB-tomography (FIBT), demonstrating how the dual-beam instrument may be used to access the 3D morphology of cavities down to the resolution of the SEM and the slicing thickness of the FIB. This tomography approach was further extended with electron tomography studies of the nanobeam cavities, a technique which provided fewer issues in terms of image series alignment but also the presence of reconstruction artefacts which must be taken into account when quantitatively analysing the data. The use of correlative techniques was also used to establish the link between high Si content in an interlayer running along the length of microrods with changes in the optical emission of these rods. The combination of CL, FIB/SEM and TEM-based techniques has made it possible to gain a thorough understanding of the link between the structural and optical properties in a wide variety of III-nitride materials and devices.EPSR