Determining GaN Nanowire Polarity and its Influence on Light Emission in the Scanning Electron Microscope.

The crystal polarity of noncentrosymmetric wurtzite GaN nanowires is determined nondestructively in the scanning electron microscope using electron backscatter diffraction (EBSD). The impact of the nanowire polarity on light emission is then investigated using cathodoluminescence (CL) spectroscopy. EBSD can determine polarity of noncentrosymmetric crystals by interrogating differences in the intensity distribution of bands of the EBSD pattern associated with semipolar planes. Experimental EBSD patterns from an array of GaN nanowires are compared with theoretical patterns produced using dynamical electron simulations to reveal whether they are Ga- or N-polar or, as in several cases, of mixed polarity. CL spectroscopy demonstrates the effect of the polarity on light emission, with spectra obtained from nanowires of known polarity revealing a small but measurable shift (≈28 meV) in the GaN near band edge emission energy between those with Ga and N polarity. We attributed this energy shift to a difference in impurity incorporation in nanowires of different crystal polarity. This approach can be employed to nondestructively identify polarity in a wide range of noncentrosymmetric nanoscale material systems and provide direct comparison with their luminescence.

Rapid nondestructive analysis of threading dislocations in wurtzite materials using the scanning electron microscope.

We describe the use of electron channeling contrast imaging in the scanning electron microscope to rapidly and reliably image and identify threading dislocations (TDs) in materials with the wurtzite crystal structure. In electron channeling contrast imaging, vertical TDs are revealed as spots with black-white contrast. We have developed a simple geometric procedure which exploits the differences observed in the direction of this black-white contrast for screw, edge, and mixed dislocations for two electron channeling contrast images acquired from two symmetrically equivalent crystal planes whose g vectors are at 120° to each other. Our approach allows unambiguous identification of all TDs without the need to compare results with dynamical simulations of channeling contrast.

Metrology of crystal defects through intensity variations in secondary electrons from the diffraction of primary electrons in a scanning electron microscope.

Understanding defects and their roles in plastic deformation and device reliability is important for the development of a wide range of novel materials for the next generation of electronic and optoelectronic devices. We introduce the use of gaseous secondary electron detectors in a variable pressure scanning electron microscope for non-destructive imaging of extended defects using electron channelling contrast imaging. We demonstrate that all scattered electrons, including the secondary electrons, can provide diffraction contrast as long as the sample is positioned appropriately with respect to the incident electron beam. Extracting diffraction information through monitoring the modulation of the intensity of secondary electrons as a result of diffraction of the incident electron beam, opens up the possibility of performing low energy electron channelling contrast imaging to characterise low atomic weight and ultra-thin film materials. Our methodology can be adopted for large area, nanoscale structural characterisation of a wide range of crystalline materials including metals and semiconductors, and we illustrate this using the examples of aluminium nitride and gallium nitride. The capability of performing electron channelling contrast imaging, using the variable pressure mode, extends the application of this technique to insulators, which usually require conducting coatings on the sample surface for traditional scanning electron microscope based microstructural characterisation.

Hysteretic photochromic switching of Eu-Mg defects in GaN links the shallow transient and deep ground states of the Mg acceptor.

Although p-type activation of GaN by Mg underpins a mature commercial technology, the nature of the Mg acceptor in GaN is still controversial. Here, we use implanted Eu as a 'spectator ion' to probe the lattice location of Mg in doubly doped GaN(Mg):Eu. Photoluminescence spectroscopy of this material exemplifies hysteretic photochromic switching (HPS) between two configurations, Eu0 and Eu1(Mg), of the same Eu-Mg defect, with a hyperbolic time dependence on 'switchdown' from Eu0 to Eu1(Mg). The sample temperature and the incident light intensity at 355 nm tune the characteristic switching time over several orders of magnitude, from less than a second at 12.5 K, ~100 mW/cm2 to (an estimated) several hours at 50 K, 1 mW/cm2. Linking the distinct Eu-Mg defect configurations with the shallow transient and deep ground states of the Mg acceptor in the Lany-Zunger model, we determine the energy barrier between the states to be 27.7(4) meV, in good agreement with the predictions of theory. The experimental results further suggest that at low temperatures holes in deep ground states are localized on N atoms axially bonded to Mg acceptors.

Correction: Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods.

Correction for 'Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods' by M. Conroy et al., Nanoscale, 2016, 8, 11019-11026.

Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods.

We report a method of growing site controlled InGaN multiple quantum discs (QDs) at uniform wafer scale on coalescence free ultra-high density (>80%) nanorod templates by metal organic chemical vapour deposition (MOCVD). The dislocation and coalescence free nature of the GaN space filling nanorod arrays eliminates the well-known emission problems seen in InGaN based visible light sources that these types of crystallographic defects cause. Correlative scanning transmission electron microscopy (STEM), energy-dispersive X-ray (EDX) mapping and cathodoluminescence (CL) hyperspectral imaging illustrates the controlled site selection of the red, yellow and green (RYG) emission at these nano tips. This article reveals that the nanorod tips' broad emission in the RYG visible range is in fact achieved by manipulating the InGaN QD's confinement dimensions, rather than significantly increasing the In%. This article details the easily controlled method of manipulating the QDs dimensions producing high crystal quality InGaN without complicated growth conditions needed for strain relaxation and alloy compositional changes seen for bulk planar GaN templates.

Analysis of molecular recognition using optical biosensors.

Optical biosensors have made the analysis of molecular interactions rapid and convenient. The field is still young, with commercial instrumentation having been available for less than ten years. For this reason instrument development is still an important factor and the past year has seen continuing advances in instrumentation and particularly in novel sensor surfaces.

Diminished thyroxine-binding globulin in pubertal diabetic children.

OBJECTIVE: To determine the effect of diabetes on thyroid hormone and thyroxine-binding globulin (TBG) concentrations during puberty. RESEARCH DESIGN AND METHODS: Total thyroxine (TT4), free thyroxine (FT4), and TBG levels of 171 thyroid microsomal antibody-negative subjects with normal thyroid-stimulating hormone (TSH) levels were measured and compared with those of nondiabetic adolescents. A random subset of 68 diabetic patients (40 boys and 28 girls) and 51 control subjects (24 boys and 27 girls) were analyzed for puberty-related changes. RESULTS: Most TT4 levels of diabetic subjects (80% of girls and 63% of boys) were below the 50th percentile for the normal range. TT4 increased with age in girls (r = 0.25, P < 0.04) but not in boys. FT4 was within normal limits in both sexes. TBG measurements were below the 50th percentile and 20% were below the 95% CI for both sexes; TT4 correlated with TBG in boys (r = 0.54, P < 0.001) and in girls (r = 0.58, P < 0.001). Duration of diabetes had no effect, whereas TT4 and FT4 levels were higher in girls with the lowest HbA1 levels (r = -0.29, P < 0.01 and r = -0.45, P < 0.01). Levels of TBG were reduced for all male pubertal stages (P < 0.01) and for early and late female pubertal stages (P < 0.01). There was no direct relationship between glucose control or the duration of diabetes and levels of TBG. CONCLUSIONS: Because TT4 levels are low and correlate with the low levels of TBG, it is important to measure free thyroid hormone and TSH levels in diabetic adolescents to establish euthyroidism.

Photoluminescence studies of a perceived white light emission from a monolithic InGaN/GaN quantum well structure.

In this work we demonstrate by photoluminescence studies white light emission from a monolithic InGaN/GaN single quantum well structure grown by metal organic chemical vapour deposition. As-grown and thermally annealed samples at high temperature (1000 °C, 1100 °C and 1200 °C) and high pressure (1.1 GPa) were analysed by spectroscopic techniques, and the annealing effect on the photoluminescence is deeply explored. Under laser excitation of 3.8 eV at room temperature, the as-grown structure exhibits two main emission bands: a yellow band peaked at 2.14 eV and a blue band peaked at 2.8 eV resulting in white light perception. Interestingly, the stability of the white light is preserved after annealing at the lowest temperature (1000 °C), but suppressed for higher temperatures due to a deterioration of the blue quantum well emission. Moreover, the control of the yellow/blue bands intensity ratio, responsible for the white colour coordinate temperatures, could be achieved after annealing at 1000 °C. The room temperature white emission is studied as a function of incident power density, and the correlated colour temperature values are found to be in the warm white range: 3260-4000 K.

Maternal hypothyroidism in the rat influences placental and liver glycogen stores: fetal growth retardation near term is unrelated to maternal and placental glucose metabolic compromise.

Maternal hypothyroidism impairs fetal growth in the rat, but the mechanisms by which this occurs are unknown. Since the fetus derives its glucose supply from the mother, and maternal thyroidectomy may disturb maternal and placental glucose metabolism, we postulated that maternal and/or placental glucose metabolic compromise may contribute to fetal growth retardation in hypothyroid dams. Feto-placental growth, tissue glycogen stores and glucose levels in sera and amniotic fluid were determined in rat dams partially thyroidectomized (TX) before pregnancy and in euthyroid controls. Fetal body weight at 16, 19 and 21 days gestation (d.g.) was related to pre-mating maternal serum total thyroxine (TT(4)) levels; permanent fetal growth retardation occurred in severely (TX(s); pre-mating maternal serum TT(4)

Maternal hypothyroxinemia disrupts neurotransmitter metabolic enzymes in developing brain.

Maternal thyroid status influences early brain development and, consequently, cognitive and motor function in humans and rats. The biochemical targets of maternal thyroid hormone (TH) action in fetal brain remain poorly defined. A partially thyroidectomized rat dam model was therefore used to investigate the influence of maternal hypothyroxinemia on the specific activities of cholinergic and monoaminergic neurotransmitter metabolic enzymes in the developing brain. Maternal hypothyroxinemia was associated with reduced monoamine oxidase (MAO) activity in fetal whole brain at 16 and 19 days gestation (dg). A similar trend was observed for choline acetyltransferase (ChAT) activity. In contrast, DOPA decarboxylase (DDC) activity was markedly elevated at 21 dg. Further study of these enzymes at 14 dg showed no differences between normal and experimental progeny - suggesting they become TH sensitive after this age. Tyrosine hydroxylase (TyrH) and acetylcholinesterase (AChE) activities were unaffected prenatally. During postnatal development, the activities of TyrH, MAO, DDC and, to a lesser extent, AChE were increased in a brain region- and age-specific manner in experimental progeny. The prenatal disturbances noted in this study may have wide-ranging consequences since they occur when neurotransmitters have putative neurotropic roles in brain development. Furthermore, the chronic disturbances in enzyme activity observed during postnatal life may affect neurotransmission, thereby contributing to the behavioural dysfunction seen in adult progeny of hypothyroxinemic dams.

Maternal hypothyroxinemia influences glucose transporter expression in fetal brain and placenta.

The influence of maternal hypothyroxinemia on the expression of the glucose transporters, GLUT1 and GLUT3, in rat fetal brain and placenta was investigated. Fetal growth was retarded in hypothyroxinemic pregnancies, but only before the onset of fetal thyroid hormone synthesis. Placental weights were normal, but placental total protein concentration was reduced at 19 days gestation (dg). Immunoblotting revealed a decreased abundance of GLUT1 in placental microsomes at 16 dg, whereas GLUT3 was increased. Fetal serum glucose levels were reduced at 16 dg. In fetal brain, the concentration of microsomal protein was deficient at 16 dg and the abundance of parenchymal forms of GLUT1 was further depressed, whereas GLUT3 was unaffected. Northern hybridization analysis demonstrated normal GLUT1 mRNA levels in placenta and fetal brain. In conclusion, maternal hypothyroxinemia results in fetal growth retardation and impaired brain development before the onset of fetal thyroid function. Glucose uptake in fetal brain parenchyma may be compromised directly, due to deficient GLUT1 expression in this tissue, and indirectly, as a result of reduced placental GLUT1 expression. Though corrected by the onset of fetal thyroid hormone synthesis, these deficits are present during the critical period of neuroblast proliferation and may contribute to long term changes in brain development and function seen in this model and in the progeny of hypothyroxinemic women.

SHORT COMMUNICATION maternal thyroid dysfunction and c- fos and c- jun expression in rat placenta.

Maternal thyroid dysfunction is associated with perturbed fetal brain development and neurological deficits in adulthood in rat and human. To investigate whether these effects occur secondary to placental dysfunction, c- fos and c- jun expression in placenta from normal (euthyroid) and moderately hypothyroid rat dams were investigated by Northern hybridization analysis. In normal placenta, c- fos expression increased by 74 per cent between 16 and 21 days of gestation (dg) whereas c- jun expression declined by 46 per cent. Moderate maternal hypothyroidism depressed placental c- fos expression by 32 per cent at 19 dg, but elevated c- fos and c- jun expression by 139 and 86 per cent, respectively, at 21 dg. Maternal hypothyroidism may therefore induce c- fos/c- jun -related placental dysfunction, but only relatively late in gestation when fetal thyroid function is already established.

Management of gallstone disease in the elderly.

AIM: To determine the outcome of management of symptomatic gallstone disease (GSD) in patients aged 80 years or more. PATIENTS AND METHODS: A retrospective review of the outcome of 79 patients admitted to 2 district general hospitals with symptomatic GSD over a 1-year period was undertaken. Patients were grouped according to method of management: non-operative, ERCP, and cholecystectomy. POSSUM scores for the ERCP and cholecystectomy groups were calculated and observed, and predicted outcome compared. RESULTS: Obstructive jaundice and biliary colic were the most common presenting symptoms. Each patient had been admitted at least once before the study period (median, 2; range, 1-3). Outcomes are detailed in Table 1. Non-operative management failed in 18 of 23 patients, with 17.4% mortality. ERCP was successful in 40 of 47 patients with 3 complications (0.24 of predicted) and no mortality. In all, 11 laparoscopic and 12 open cholecystectomies were performed with 6 complications and 1 mortality (0.95 and 0.83 of predicted, respectively): 4 complications and the only death occurring after emergency cholecystectomy. Table 1 Outcomes Management Number Mortality Morbidity Outcome Non-operative 23 4 9 5 OK, 13 re-admitted, 4 still symptomatic ERCP 47 0 3 1 PTC, 6 operated Operative 23 1 6 CONCLUSIONS: This study suggests that recurrent GSD in elderly patients managed non-operatively may have fatal outcome. Elective cholecystectomy has acceptable morbidity and mortality in this age group and there is often ample opportunity to avoid emergency surgery, but a prospective randomised study is required to improve clinical algorithms.

Methods and prognosis of non-invasive ventilation in neuromuscular disease.

Ventilatory failure is the commonest terminal event in many neuromuscular disorders. Increasingly, non-invasive methods of ventilation, employing both negative and positive pressure techniques, have been used, with the aim of prolonging life and reducing morbidity. The use of non-invasive ventilation is reviewed in three main groups of neurological diseases, and the degree of success assessed. It is concluded that non-invasive methods can be effective in reducing morbidity in selected patients, especially those with more slowly progressive disorders. A reduction in mortality is suggested by comparison with historical controls. The institution of ventilatory support is a major undertaking for the medical team, patient and family, and should only be initiated after a full and frank discussion of the overall prognosis, in terms of both mortality and quality of life. The patient and principal caregivers can then make an informed decision as to the desirability of home ventilation.

Are periods of low atmospheric pressure associated with an increased risk of abdominal aortic aneurysm rupture?

INTRODUCTION: Seasonal variation in rates of abdominal aortic aneurysm (AAA) rupture has previously been described. However, only two studies, to date, have suggested that periods of low atmospheric pressure may account for this observation and both returned apparently contradictory findings. The objective of this study was to demonstrate whether periods of low atmospheric pressure are indeed associated with an increased likelihood of AAA rupture presentation. PATIENTS AND METHODS: A total of 182 cases of ruptured AAA were identified retrospectively from two centres over a 6-year period from January 2000 to December 2005. Local meteorological data for the corresponding period was obtained from the UK Meteorological Office which was recorded daily at a local weather station. Statistical analysis using Student's t-test, ANOVA (Kruskal-Wallis), linear regression and multiple logistic regression was conducted to identify significant relationships from the data. RESULTS: Days of rupture presentation were associated with a significantly lower daily mean atmospheric pressure when compared with days when no rupture occurred (P = 0.025). Multiple logistic regression demonstrated a significant association between low daily atmospheric pressure and rupture incidence (P = 0.033) which appeared to be independent of temperature. A significant seasonal trough in atmospheric pressure was observed in October and November (Kruskal-Wallis, P < 0.001); however, there was no corresponding autumnal peak in rupture incidence (P = 0.232). CONCLUSIONS: The findings of this study appear to confirm the assertion that low atmospheric pressure is associated with an increased risk of AAA rupture on a day-to-day basis; however, no evidence was identified to support a change in policy regarding prioritising elective surgery based on seasonal risk.