Developing epitaxial graphene for the purpose of nanoelectronics

Work presented here has been centered around the growth of epitaxial graphene via the thermal decomposition of 4H silicon carbide wafers. Improvements to ultra high vacuum growth procedures used within the research group have been made via the optimization of annealing times and temperatures. The optimization involved the use of surface science techniques such as low energy electron diffraction, atomic force microscopy, low energy electron microscopy and Raman spectroscopy amongst others to monitor changes in surface reconstructions, lateral grain sizes of graphene domains and graphene coverage on the surface as the growth parameters were varied. Improvements observed via the surface science techniques such as increasing the lateral domain grain sizes from 10s nm to 100s nm and increasing the graphene film coverage were linked to the betterment of the electronic properties of the graphene films (electronic measurements carried out by Graham Creeth), this linking lead to published work. The mechanical properties of these films were also measured via the use of Raman spectroscopy to probe the formation of strains within the graphene and compare growth carried out on the silicon carbide (000�1) face to literature work carried out on the (0001) face to show evidence of graphene-substrate decoupling within the films grown here, this work also lead to a publication. Alternate growth procedures have also been investigated. This involved carrying out annealing processes in inert argon gas atmospheres. Atomically terraced substrates were produced via annealing in argon gas atmospheres at temperatures of ~1500°C. These terraced substrates where then subsequently graphitised by increasing the annealing temperature to ~1600°C allowing for a single stage substrate preparation and graphitisation process. A result not published elsewhere. A Nanoprobe system has been used to manipulate the graphene films grown under argon atmosphere and make 4-probe electrical transport measurements allowing sheet resistance measurements to be made

Vibrational fingerprints of residual polymer on transferred CVD-graphene

The use of vibrational sum-frequency spectroscopy (VSFS) to study transferred graphene, produced by chemical vapour deposition, is presented. The VSF spectrum shows a clear CH stretching mode at ∼2924 cm−1, which is attributed to residue of the polymer used for the transfer. This makes VSFS a powerful tool to identify adsorbates and contaminants affecting the properties of graphene

The Effects of Exfoliation, Organic Solvents and Anodic Activation on Catalytic Hydrogen Evolution Reaction of Tungsten Disulfide

International audienceThe rational design of transition metal dichalcogenide electrocatalysts for efficiently catalyzing hydrogen evolution reaction (HER) is believed to lead to the generation of a renewable energy carrier. To this end our work has made three main contributions. At first, we have demonstrated that exfoliation via ionic liquid assisted grinding combined with gradient centrifugation is an efficient method to exfoliate bulk WS2 to nanosheets with a thickness of a few atomic layers and lateral size dimensions in the range of 100 nm to 2 nm. These WS2 nanosheets decorated with scattered nanodots exhibited highly enhanced catalytic performance for HER with an onset potential of-130 mV vs. RHE, an overpotential of 337 mV at 10 mA cm-2 and a Tafel slope of 80 mV dec-1 in 0.5 M H2SO4. Secondly, we found a strong aging effect on the electrocatalytic performance of WS2 stored in high boiling point organic solvents such as dimethylformamide (DMF). Importantly, the HER ability could be recovered by removing the organic (DMF) residues, which obstructed the electron transport, with acetone. Thirdly, we established that the HER performance of WS2 nanosheets/nanodots could be significantly enhanced, by activating the electrode surface at a positive voltage for a very short time (60 s), decreasing the kinetic overpotential by more than 80 mV at 10 mA cm-2. The performance enhancement was found to arise primarily from the ability of a formed proton-intercalated amorphous tungsten trioxide (a-WO3) to provide additional active sites and favourably modify the immediate chemical environment of the WS2 catalyst, rendering it more favorable for local proton delivery and/or transport to the active edge site of WS2. Our results provide new insights into the effects of organic solvents and electrochemical activation on the catalytic performance of two-dimensional WS2 for HER

COVID-19 symptoms at hospital admission vary with age and sex: results from the ISARIC prospective multinational observational study

Background: The ISARIC prospective multinational observational study is the largest cohort of hospitalized patients with COVID-19. We present relationships of age, sex, and nationality to presenting symptoms. Methods: International, prospective observational study of 60 109 hospitalized symptomatic patients with laboratory-confirmed COVID-19 recruited from 43 countries between 30 January and 3 August 2020. Logistic regression was performed to evaluate relationships of age and sex to published COVID-19 case definitions and the most commonly reported symptoms. Results: ‘Typical’ symptoms of fever (69%), cough (68%) and shortness of breath (66%) were the most commonly reported. 92% of patients experienced at least one of these. Prevalence of typical symptoms was greatest in 30- to 60-year-olds (respectively 80, 79, 69%; at least one 95%). They were reported less frequently in children (≤ 18 years: 69, 48, 23; 85%), older adults (≥ 70 years: 61, 62, 65; 90%), and women (66, 66, 64; 90%; vs. men 71, 70, 67; 93%, each P < 0.001). The most common atypical presentations under 60 years of age were nausea and vomiting and abdominal pain, and over 60 years was confusion. Regression models showed significant differences in symptoms with sex, age and country. Interpretation: This international collaboration has allowed us to report reliable symptom data from the largest cohort of patients admitted to hospital with COVID-19. Adults over 60 and children admitted to hospital with COVID-19 are less likely to present with typical symptoms. Nausea and vomiting are common atypical presentations under 30 years. Confusion is a frequent atypical presentation of COVID-19 in adults over 60 years. Women are less likely to experience typical symptoms than men

Raman analysis of both Graphene and bare Ag/AgCl electrodes

A number of Raman spectroscopy images were obtained for graphene coated Ag/AgCl and bare Ag/AgCl are shown in these images.  A summary figure which explains  the difference of  the graphene coating also presented in the data set

Evidence of altered prefrontal-thalamic circuitry in schizophrenia: An optimised diffusion MRI study

MRI diffusion tensor imaging (DTI), optimized for measuring the trace of the diffusion tensor, was used to investigate microstructural changes in the brains of 12 individuals with schizophrenia compared with 12 matched control subjects. To control for the effects of anatomic variation between subject groups, all participants' diffusion images were non-linearly registered to standard anatomical space. Significant statistical differences in mean diffusivity (MD) measures between the two groups were determined on a pixel-by-pixel basis, using Gaussian random field theory. We found significantly elevated MD measures within temporal, parietal and prefrontal cortical regions in the schizophrenia group (P > 0.001), especially within the medial frontal gyrus and anterior cingulate. The dorsal medial and anterior nucleus of the thalamus, including the caudate, also exhibited significantly increased MD in the schizophrenia group (P > 0.001). This study has shown for the first time that MD measures offer an alternative strategy for investigating altered prefrontal-thalamic circuitry in schizophrenia. (c) 2006 Elsevier Inc. All rights reserved

Determining the Level and Location of Functional Groups on Few-Layer Graphene and Their Effect on the Mechanical Properties of Nanocomposites

Graphene is a highly desirable material for a variety of applications; in the case of nanocomposites, it can be functionalized and added as a nanofiller to alter the ultimate product properties, such as tensile strength. However, often the material properties of the functionalized graphene and the location of any chemical species, attached via different functionalization processes, are not known. Thus, it is not necessarily understood why improvements in product performance are achieved, which hinders the rate of product development. Here, a commercially available powder containing few-layer graphene (FLG) flakes is characterized before and after plasma or chemical functionalization with either nitrogen or oxygen species. A range of measurement techniques, including tip-enhanced Raman spectroscopy (TERS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and NanoSIMS, were used to examine the physical and chemical changes in the FLG material at both the micro- and nanoscale. This is the first reported TERS imaging of commercially available FLG flakes of submicron lateral size, revealing the location of the defects (edge versus basal plane) and variations in the level of functionalization. Graphene-polymer composites were then produced, and the dispersion of the graphitic material in the matrix was visualized using ToF-SIMS. Finally, mechanical testing of the composites demonstrated that the final product performance could be enhanced but differed depending on the properties of the original graphitic material

Correlation of quantitative EEG in acute ischemic stroke with 30-day NIHSS score: Comparison with diffusion and perfusion MRI

Background and Purpose-Magnetic resonance imaging (MRI) methods such as diffusion-(DWI) and perfusion-weighted (PWI) imaging have been widely studied as surrogate markers to monitor stroke evolution and predict clinical outcome. The utility of quantitative electroencephalography (qEEG) as such a marker in acute stroke has not been intensively studied. The aim of the present study was to correlate ischemic cortical stroke patients' clinical outcomes with acute qEEG, DWI, and PWI data

Chemical Vapor Deposition of High Quality Graphene Films from Carbon Dioxide Atmospheres

The realization of graphene-based, next-generation electronic applications essentially depends on a reproducible, large-scale production of graphene films <i>via</i> chemical vapor deposition (CVD). We demonstrate how key challenges such as uniformity and homogeneity of the copper metal substrate as well as the growth chemistry can be improved by the use of carbon dioxide and carbon dioxide enriched gas atmospheres. Our approach enables graphene film production protocols free of elemental hydrogen and provides graphene layers of superior quality compared to samples produced by conventional hydrogen/methane based CVD processes. The substrates and resulting graphene films were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and Raman microscopy, sheet resistance and transport measurements. The superior quality of the as-grown graphene films on copper is indicated by Raman maps revealing average G band widths as low as 18 ± 8 cm^–1 at 514.5 nm excitation. In addition, high charge carrier mobilities of up to 1975 cm²/(V s) were observed for electrons in transferred films obtained from a carbon dioxide based growth protocol. The enhanced graphene film quality can be explained by the mild oxidation properties of carbon dioxide, which at high temperatures enables an uniform conditioning of the substrates by an efficient removal of pre-existing and emerging carbon impurities and a continuous suppression and <i>in situ</i> etching of carbon of lesser quality being co-deposited during the CVD growth