High-resolution cathodoluminescence hyperspectral imaging of nitride nanostructures

Hyperspectral cathodoluminescence imaging provides spectrally and spatially resolved information on luminescent materials within a single dataset. Pushing the technique toward its ultimate nanoscale spatial limit, while at the same time spectrally dispersing the collected light before detection, increases the challenge of generating low-noise images. This article describes aspects of the instrumentation, and in particular data treatment methods, which address this problem. The methods are demonstrated by applying them to the analysis of nanoscale defect features and fabricated nanostructures in III-nitride-based materials

Study of Surface Morphology and Microstructure of Electrodeposited Polycrystalline Cu Films

The applications of polycrystalline films range from interconnects in the electronics and semiconductor industry to solar cells and as corrosion protection. Despite their significance, factors that determine their microstructure and morphology remain largely unsolved. The surface and microstructure of electrodeposited polycrystalline Cu films were investigated. This involves looking at the later growth stages of Cu films using different surface and bulk characterization techniques. The surface evolution of an electrodeposited Cu film was imaged in real-time using a Highspeed Atomic Force Microscope (HS-AFM). This provides details about how the film structure coarsens with time. The high-resolution video showed accelerated local grain growth and grain overgrowth at different locations of the film. A combination of both of these mechanisms could drive structural coarsening. The microstructure could play a role in inducing faster growth in certain grains. How the local and large-scale roughness varies with film thickness is studied by scaling analysis. As a complement to scaling analysis, variation in the local slope with thickness is calculated using slope analysis. Rapid growth was observed in the regions where the HS-AFM tip was scanning. The removal of oxygen adlayer from the surface by the tip could promote faster growth in these regions. Pulsed electrodeposition produced Cu films with hexagonal structures. They are known to be twinned which is a desirable feature in applications that require superior mechanical and electrical properties. The effect of electrode potential on grain size was studied. Using a watershed segmentation algorithm, the grain area was calculated from the AFM images. The grain area showed an increasing trend with increasing overpotential. Slope analysis on the ’hexagons’ and the complete films electrodeposited at higher potential revealed higher slopes and distinct slope distribution. Cross-sectional Focused Ion Beam (FIB) milling confirmed that horizontal twins are present in the pulse-deposited Cu films. The hexagonal pyramids with twins could be produced by one of the two mechanisms, stress relaxation during the ’OFF’ period of pulsing or driven by screw dislocation. We attribute the origin of the hexagons to spiralling screw dislocations. A template matching algorithm was developed to try and correlate the surface and microstructural data of a Cu film grown on a microelectrode. It involved matching the AFM and Electron Back Scatter Diffraction (EBSD) data on the later FIB milled sample, thus relating surface topography to crystallographic orientation. The crystallographic orientation of the edge of the microelectrode and its centre showed different orientations, switching from (111) to (110). Twinning was investigated at the edge and the centre of the microelectrode revealing the presence of stacking fault twins in both of these regions

Quantitative optical mapping of two-dimensional materials

The pace of two-dimensional materials (2DM) research has been greatly accelerated by the ability to identify exfoliated thicknesses down to a monolayer from their optical contrast. Since this process requires time-consuming and error-prone manual assignment to avoid false-positives from image features with similar contrast, efforts towards fast and reliable automated assignments schemes is essential. We show that by modelling the expected 2DM contrast in digitally captured images, we can automatically identify candidate regions of 2DM. More importantly, we show a computationally-light machine vision strategy for eliminating false-positives from this set of 2DM candidates through the combined use of binary thresholding, opening and closing filters, and shape-analysis from edge detection. Calculation of data pyramids for arbitrarily high-resolution optical coverage maps of two-dimensional materials produced in this way allows the real-time presentation and processing of this image data in a zoomable interface, enabling large datasets to be explored and analysed with ease. The result is that a standard optical microscope with CCD camera can be used as an analysis tool able to accurately determine the coverage, residue/contamination concentration, and layer number for a wide range of presented 2DMs

Examining the relationship of variables related to litigation regarding students with significant cognitive disabilities

Non-null interferometry offers a viable alternative to traditional interferometric testing of aspheric micro-lenses since computer generated holograms or null optics whose fabrication and testing are very expensive, are not required. However, due to the violation of the Nyquist sampling theorem these non-null tests provide limited dynamic range. The dynamic range of these non-null tests can be extended by implementing an index liquid which allows the measurement of micro-lenses with several microns of departure from a sphere. The first objective of this dissertation was to test important micro-lens properties such as the sag, radius of curvature and form errors for a micro-lens by using an index liquid. The results compared favorably to measurements taken on a Twyman-Green interferometer, a contact profilometer and an optical non-contact profilometer. Also, retrace errors, which are aberrations caused by altered ray paths of the test beam through a micro-lens were investigated. Reverse ray-trace and reverse optimization techniques are typically used to calibrate retrace errors, but in depth knowledge of the interferometer optics is assumed, and hence cannot be used for systems containing commercial optics. In this dissertation, re-trace errors are quantified and a novel calibration procedure derived to experimentally compensate for these errors. This retrace error calibration led to agreement of within 1% for the sag values between the index liquid technique and a profilometer. The second objective of this dissertation was to enable measurements of arbitrary geometries and to reduce testing time compared to profilometry. The index liquid technique was applied to faceted microstructured optical products which are becoming more widespread due to advances in manufacturing. Many of these structures contain faceted surfaces with steep slopes. Adequate metrology for such surfaces is lacking. The use of the index liquid technique achieved high quality, high speed measurements of such faceted microstructures. Refraction is accounted for at the interfaces, rather than consider only optical path length changes due to the index liquid, and this significantly improves the facet angle measurement. The technique is demonstrated with the measurement of an array of micro-pyramids and show that our results are in good agreement with measurements taken on a contact profilometer. The index liquid measurements took approximately five seconds to complete compared to a measurement time of six hours for the contact profilometer. The technique was also extended to measure opaque micro-corner cubes by implementing an intermediate replication step. This allowed a measurement of the angle between facets of a nickel micro-corner cube hexagonal array, a combination not previously demonstrated in the literature. A first order uncertainty analysis was carried out on the index liquid technique to determine any limiting factors that need to be taken into account when assessing such parameters as the sag and facet angle. The uncertainties in the sag and facet angle were found to be well below 1%. Lastly secondary factors such interferometer bias, refraction, masking effects and pixel calibration were investigated to understand the possible implications on the sag and facet angle calculation

The role of sequence in the structure of self-assembling 3D DNA crystals

DNA is a widely used biopolymer for the construction of nanoscale objects due to its programmability and structural predictability. DNA oligonucleotides can, however, exhibit a great deal of local structural diversity. DNA conformation is strongly linked to both environmental conditions and the nucleobase identities inherent in the oligonucleotide sequence, but the exact relationship between sequence and local structure is not completely understood. We previously determined the X-ray crystal structure of a DNA 13-mer that forms a continuously hydrogen bonded three-dimensional lattice through Watson-Crick and non-canonical base pairs. In the current work I examined how the sequence of the Watson-Crick duplex region influenced crystallization of this 13-mer. I screened all possible self-complementary sequences in the hexameric duplex region and found 21 oligonucleotides that crystallized. Sequence analysis showed that one specific Watson-Crick base pair influenced the crystallization propensity and the speed of crystal self-assembly. I determined X-ray crystal structures for 13 of these oligonucleotides and found sequence-specific structural changes suggesting that this base pair may serve as a structural anchor during crystal assembly. I explored the crystal self-assembly and nucleation process and demonstrated that crystals grown from mixtures of two different oligonucleotide sequences contained both the oligonucleotides. These results suggested that crystal self-assembly is nucleated by the formation of Watson-Crick duplexes. Finally, I also examined how a single nucleotide addition to the DNA 13-mer leads to a significantly different overall structure under identical crystallization conditions. The 14-mer crystal structures described here showed that all of the predicted Watson-Crick base pairs were present, but the major difference as compared to the parent 13-mer structure was a significant rearrangement of non-canonical base pairs. This included the formation of a sheared A-G base pair, a junction of strands formed from base triple interactions, and tertiary interactions that generated structural features similar to tandem sheared G-A base pairs. The adoption of this alternate non-canonical structure was dependent in part on the sequence of the Watson-Crick duplex region. These results provided important new insights into the sequence/structure relationship of short DNA oligonucleotides and demonstrated a unique interplay between Watson-Crick and non-canonical base pairs that are responsible for crystallization fate

Characterization, modeling, and simulation of multiscale directed-assembly systems

Nanoscience is a rapidly developing field at the nexus of all physical sciences which holds the potential for mankind to gain a new level of control of matter over matter and energy altogether. Directed-assembly is an emerging field within nanoscience in which non-equilibrium system dynamics are controlled to produce scalable, arbitrarily complex and interconnected multi-layered structures with custom chemical, biologically or environmentally-responsive, electronic, or optical properties. We construct mathematical models and interpret data from direct-assembly experiments via application and augmentation of classical and contemporary physics, biology, and chemistry methods. Crystal growth, protein pathway mapping, LASER tweezers optical trapping, and colloid processing are areas of directed-assembly with established experimental techniques. We apply a custom set of characterization, modeling, and simulation techniques to experiments to each of these four areas. Many of these techniques can be applied across several experimental areas within directed-assembly and to systems featuring multiscale system dynamics in general. We pay special attention to mathematical methods for bridging models of system dynamics across scale regimes, as they are particularly applicable and relevant to directed-assembly. We employ massively parallel simulations, enabled by custom software, to establish underlying system dynamics and develop new device production methods