Luminescence study of III-nitride semiconductor nanostructures and LEDs

In this work, cathodoluminescence (CL) hyperspectral imaging, photoluminescence (PL) and electroluminescence are used to study the optical properties of III-nitride semiconductor materials. III-nitride semiconductors have successfully opened up the solid-state lighting market. Light-emitting diodes (LEDs) fabricated using III-nitrides, however, still suffer from numerous deficiencies such as high defect densities, efficiency droop and the 'green gap'. In order to investigate the type and properties of the defects, CL and electron channelling contrast imaging (ECCI) were performed on the same micron-scale area of a GaN thin film. A one-to-one correlation between isolated dark spots in CL and threading dislocations (TDs) in ECCI showed that TDs of pure edge character and TDs with a screw component act as non-radiative recombination centres. Secondary electron imaging of planar InGaN/GaN multiple quantum well (MQW) structures identified trench defects of varying width. CL imaging revealed a strong redshift (90 meV) and intensity increase for trench defects with wide trenches compared with the defect-free surrounding area. Narrower trench defects showed a small redshift (10 meV) and a slight reduction in intensity. The optical properties of nanorods fabricated from planar InGaN/GaN MQW structures were investigated using PL and CL. PL spectroscopy identified reduced strain within the MQW stack in the nanorods compared with the planar structure. CL imaging of single nanorods revealed a redshift of 18 meV of the MQW emission along the nanorod axis and provided an estimate of 55 nm for the carrier diffusion length. Colour conversion using novel organic compounds as energy down-converters was studied. The first molecules absorbed in the ultra-violet and emitted in the yellow spectral region. Further modification of the organic compound shifted the absorption into the blue and white light generation was investigated by coating blue-emitting nanorods and blue LEDs. Determination of the colour rendering index and colour temperature showed "warm white" light emission with values of 70 and 3220 K, respectively.In this work, cathodoluminescence (CL) hyperspectral imaging, photoluminescence (PL) and electroluminescence are used to study the optical properties of III-nitride semiconductor materials. III-nitride semiconductors have successfully opened up the solid-state lighting market. Light-emitting diodes (LEDs) fabricated using III-nitrides, however, still suffer from numerous deficiencies such as high defect densities, efficiency droop and the 'green gap'. In order to investigate the type and properties of the defects, CL and electron channelling contrast imaging (ECCI) were performed on the same micron-scale area of a GaN thin film. A one-to-one correlation between isolated dark spots in CL and threading dislocations (TDs) in ECCI showed that TDs of pure edge character and TDs with a screw component act as non-radiative recombination centres. Secondary electron imaging of planar InGaN/GaN multiple quantum well (MQW) structures identified trench defects of varying width. CL imaging revealed a strong redshift (90 meV) and intensity increase for trench defects with wide trenches compared with the defect-free surrounding area. Narrower trench defects showed a small redshift (10 meV) and a slight reduction in intensity. The optical properties of nanorods fabricated from planar InGaN/GaN MQW structures were investigated using PL and CL. PL spectroscopy identified reduced strain within the MQW stack in the nanorods compared with the planar structure. CL imaging of single nanorods revealed a redshift of 18 meV of the MQW emission along the nanorod axis and provided an estimate of 55 nm for the carrier diffusion length. Colour conversion using novel organic compounds as energy down-converters was studied. The first molecules absorbed in the ultra-violet and emitted in the yellow spectral region. Further modification of the organic compound shifted the absorption into the blue and white light generation was investigated by coating blue-emitting nanorods and blue LEDs. Determination of the colour rendering index and colour temperature showed "warm white" light emission with values of 70 and 3220 K, respectively

Comparing Sensory Accessibility Needs in Deaf and Low Vision Populations: an Explorative Study

Objective. The objective of the study is to compare hypothesized similar sensory accessibility needs of Lighting, Acoustics, Openness, Color/Contrast, and Services/communication for the hard of hearing and low vision population. Background. The Americans with Disability Act – Architectural Barriers Act Design Guidelines (ADA-ABA, 2004, 2015) provides guidelines to increase accessibility of public buildings for individuals covered by the ADA (1990). However, these guidelines often fall short of providing functional and practical guidelines to be accessible for all. Extant building rating tools fail to attend to the overlap of functional needs for the hard of hearing and/or low vision populations. Thus, there is a need to explore the similarities as they relate to not only accessing the environment but also staying in and engaging with the environment. We hypothesize Lighting, Openness, and Color/Contrast will be important building features to help facilitate engagement in the environment. 2) We hypothesize Acoustics and Services/Communication will have different levels of perceived importance in the facilitation of participation in the environment for hard of hearing and low vision populations. Methods. An exploratory between-group study was conducted through a survey developed specifically for this research study to compare similar sensory accessibility needs for 35 individuals who self-identify as low vision or hard of hearing. A Mann-Whitney U-test analysis (p = 0.01, 95% CI) was run on all the survey questions to analyze the statistical differences of the distributions. Results. The importance of Lighting (U = 82.0, p = .074), Acoustics (U = 192.5, p\u3c0.001), Openness (U = 80.0, p = .064), and Services/communication (U = 86.0, p = .102) showed there was not a statistically significant difference between individuals who are hard of hearing and individuals with low vision. The importance of Color/Contrast (U = 18.5, p\u3c.001) showed a statistically significant difference between individuals who are hard of hearing and individuals with low vision. Discussion. Implications for future research, and limitations are discussed

High resolution cathodoluminescence hyperspectral imaging of surface features in InGaN/GaN multiple quantum well structures

InGaN/GaN multiple quantum wells (MQWs) have been studied by using cathodoluminescence hyperspectral imaging with high spatial resolution. Variations in peak emission energies and intensities across trench-like features and V-pits on the surface of the MQWs are investigated. The MQW emission from the region inside trench-like features is red-shifted by approximately 45 meV and more intense than the surrounding planar regions of the sample, whereas emission from the V-pits is blue-shifted by about 20 meV and relatively weaker. By employing this technique to the studied nanostructures it is possible to investigate energy and intensity shifts on a 10 nm length scale.Comment: 3 pages, 3 figure

Artificial intelligence and machine learning for efficient minefield clearance

Landmines, particularly anti-personnel mines, are dreadful instruments of war. Mines can remain in the ground for decades and injure or kill long after the original conflict. Clearing mines is a dangerous, time consuming and expensive task. Fortunately, mine clearing already has well established and documented processes. To further support these efforts a new research project has started at Wrexham Glyndŵr University to explore the use of machine learning to create a prediction model able to better suggest the positions of hidden landmines based on locations of those already found. Research in psychology and computer science demonstrates the difficulty for humans and machines to create true randomness in their actions. The project will investigate whether it is possible to discover hidden patterns or sequences in mine deployment that could give hints where to look for more. The advantage of the envisioned technique is a lightweight data set only comprising numerical values and their simple acquisition in the field. The proposed system will support – not replace – conventional technology. Although machine learning and A.I. can discover structures, patterns and sequences in a huge data set, that humans cannot, it remains a form of prediction. The aim is therefore not to declare the ground safe (‘cleared’) but to give suggestions where additional explosives are likely to be found and thus, it is proposed, help to direct mine clearing resources better

Effects of dairy consumption on SIRT1 and mitochondrial biogenesis in adipocytes and muscle cells

<p>Abstract</p> <p>Background</p> <p>Recent data from this laboratory suggest that components of dairy foods may serve as activators of SIRT1 (Silent Information Regulator Transcript 1), and thereby participate in regulation of glucose and lipid metabolism. In this study, an <it>ex-vivo/in-vitro </it>approach was used to examine the integrated effects of dairy diets on SIRT1 activation in two key target tissues (adipose and muscle tissue).</p> <p>Methods</p> <p>Serum from overweight and obese subjects fed low or high dairy diets for 28 days was added to culture medium (similar to conditioned media) to treat cultured adipocytes and muscle cells for 48 hours.</p> <p>Results</p> <p>Treatment with high dairy group conditioned media resulted in 40% increased SIRT1 gene expression in both tissues (p < 0.01) and 13% increased enzyme activity in adipose tissue compared to baseline. This was associated with increased gene expression of peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC-1α), nuclear respiratory factor 1 (NRF1), cytochrome oxidase c subunit 7 (Cox 7), NADH dehydrogenase and uncoupling protein 2 (UCP2) in adipocytes as well as uncoupling protein 3 (UCP3), NRF1 and Cox 7 in muscle cells (p < 0.05). Further, direct incubation of physiological concentrations of leucine and its metabolites α-Ketoisocaproic acid (KIC) and β-hydroxy-methylbuteric acid (HMB) with recombinant human SIRT1 enzyme resulted in 30 to 50% increase of SIRT1 activity (p < 0.05).</p> <p>Conclusions</p> <p>These data indicate that dairy consumption leads to systemic effects, which may promote mitochondrial biogenesis in key target tissues such as muscle and adipose tissue both by direct activation of SIRT1 as well as by SIRT1-independent pathways.</p

Influence of substrate miscut angle on surface morphology and luminescence properties of AlGaN

The influence of substrate miscut on Al0.5Ga0.5 N layers was investigated using cathodoluminescence (CL) hyperspectral imaging and secondary electron imaging in an environmental scanning electron microscope. The samples were also characterized using atomic force microscopy and high resolution X-ray diffraction. It was found that small changes in substrate miscut have a strong influence on the morphology and luminescence properties of the AlGaN layers. Two different types are resolved. For low miscut angle, a crack-free morphology consisting of randomly sized domains is observed, between which there are notable shifts in the AlGaN near band edge emission energy. For high miscut angle, a morphology with step bunches and compositional inhomogeneities along the step bunches, evidenced by an additional CL peak along the step bunches, are observed

Crossing Cultural, National, and Racial Boundaries: Portraits of Diplomats and the pre-colonial French-Cochinchinese Exchange, 1787-1863

In this thesis, I examine portraits of diplomatic figures produced between two official embassies from Cochinchina to France in 1787 and 1863 that marked a pre-colonial period of increasing contact and exchange between the two Kingdoms. I demonstrate these portraits' departure from earlier works of diplomatic portraiture and French depictions of foreigners through a close visual analysis of their presentation of the sitters. The images foreground the French and Cochinchinese diplomats crossing cultural boundaries of costume and customs, national boundaries of loyalty, and racial boundaries of blood. By depicting these individuals as mixed or hybrid, I argue that the works both negotiated and complicated eighteenth- and nineteenth-century divides between French and foreign. The portraits' shifting form and function reveal France's vacillating attitudes towards and ambivalent foreign policies regarding pre-colonial Cochinchina, which were based on an evolving French imagining of this little-known Other within the frame of French Empire.Master of Art

Contemporary miniature painting in Pakistan

Die zeitgenössische Miniaturmalerei in Pakistan erlebte seit ihrer Wiederentdeckung im Jahr 1992 eine bemerkenswerte Evolution. Die vorliegende Dissertation verweist nicht nur auf formalistische und stilistische Veränderungen der traditionellen Kunstpraxis Südasiens in den letzten fünfzehn Jahren, sondern erörtert auch aufkommende Konflikte, die mit dem radikalen Modernisierungsprozess einhergehen. Der Suche nach möglichen Gründen für den nationalen und internationalen Erfolg dieser Bewegung folgt eine kritische Analyse der Bedeutung der zeitgenössischen Miniaturmalerei für das Kunstgeschehen im 21. Jahrhundert. Die Konzentration auf zwei wiederkehrende Metaphern, den „Schleier“ und das „Haar”, ermöglicht erstmalig eine ausführliche Aufarbeitung der Ikonografie einer Auswahl von repräsentativen Arbeiten. Hierbei wird insbesondere auf den Trend, das Bild der Frau zu „verschleiern“, eingegangen und dieser auf plausible kulturelle und soziale Zusammenhänge untersucht.In Pakistan contemporary miniature painting has experienced a phenomenonal evolution since its advent in 1992. This PhD represents not only an evaluation of formal and stylistic changes of this traditional art practice but also brings to light the problems that have arisen due to a radical contemporizing progress in the last fifteen years. An investigation of the possible reasons for the national and international success of this movement is juxtaposed with a critical analysis of the relevance of contemporary miniature painting for art practice in the twenty-first century. The focus on two frequently recurring metaphors, the “veil” and the “hair,” in the second part of this thesis is the first detailed iconographical discussion of a group of representative works. The trend of paraphrasing the female figure, in particular, is identified and examined for feasible motivations due to the social and cultural environment

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

Cathodoluminescence hyperspectral imaging of trench-like defects in InGaN/GaN quantum well structures

Optoelectronic devices based on the III-nitride system exhibit remarkably good optical efficiencies despite suffering from a large density of defects. In this work we use cathodoluminescence (CL) hyperspectral imaging to study InGaN/GaN multiple quantum well (MQW) structures. Different types of trench defects with varying trench width, namely wide or narrow trenches forming closed loops and open loops, are investigated in the same hyperspectral CL measurement. A strong redshift (90 meV) and intensity increase of the MQW emission is demonstrated for regions enclosed by wide trenches, whereas those within narrower trenches only exhibit a small redshift (10 meV) and a slight reduction of intensity compared with the defect-free surrounding area. Transmission electron microscopy (TEM) showed that some trench defects consist of a raised central area, which is caused by an increase of about 40% in the thickness of the InGaN wells. The causes of the changes in luminescences are also discussed in relation to TEM results identifying the underlying structure of the defect. Understanding these defects and their emission characteristics is important for further enhancement and development of light-emitting diodes