RESUMO
We report on Kelvin probe force microscopy (KPFM) and density functional theory (DFT) investigations of charge transfers in vertical heterojunctions between tungsten diselenide (WSe2) layers and graphene on silicon carbide substrates. The experimental data reveal the existence of an interface dipole, which is shown by DFT to originate from the neutralization of the graphene n-doping by an electron transfer towards the transition metal dichalcogenide (TMD) layer. The relative vacuum level shift probed by KPFM between the TMD and the substrate stays constant when passing from monolayer to bilayer graphene, which confirms that the Schottky-Mott model can be rigorously applied to these interfaces by taking into account the charge transfer from the substrate to the TMD. DFT calculations show that the first TMD layer absorbs almost all the excess charges contained in the graphene, and that the second TMD layer shall not play a significant role in the electrostatics of the system. Negatively charged defect at the TMD edges contribute however to the electrostatic landscape probed by KPFM on both TMD layers.
RESUMO
We explore the behaviour of nitrogen doping in carbon nanomaterials, notably graphene, nanotubes, and carbon thin films. This is initially via a brief review of the literature, followed by a series of atomistic density functional calculations. We show that at low concentrations, substitutional nitrogen doping in the sp(2)-C graphenic basal plane is favoured, however once the nitrogen concentration reaches a critical threshold there is a transition towards the formation of the more thermodynamically-favoured nitrogen terminated 'zigzag' type edges. These can occur either via formation of finite patches (polycyclic aromatic azacarbons), strips of sp(2) carbon with zigzag nitrogen edges, or internal nitrogen-terminated hole edges within graphenic planes. This transition to edge formation is especially favoured when the nitrogen can be partially functionalised with, e.g. hydrogen. By comparison with available literature results, notably from electron energy loss spectroscopy and X-ray spectroscopy, the current results suggest that much of the nitrogen believed to be incorporated into carbon nanoobjects is instead likely to be present terminating the edges of carbonaceous impurities attached to nanoobject's surface. By comparison to nitrogen-doped tetrahedrally amorphous carbon, we suggest that this transition at around 10-20% nitrogen concentration and above towards sp(2) coordination via internal nitrogen-terminated edge formation may be a general property of nitrogen-doped carbon materials.
RESUMO
A prospective survey involving a group of 95 general practitioners (GPs) in France was conducted to describe antibiotic therapy prescribed for out-patients with community-acquired pneumonia (CAP). A total of 173 cases of CAP, defined by the association of fever and pulmonary focal crackles and/or radiological changes consistent with a pulmonary infection, were reported between February 1993 and March 1994: 84 males and 89 females (mean age: 48 yrs) of whom 45% had no underlying disease. Nineteen (11%) were immediately hospitalized and the remaining 154 out-patients were treated without microbiological investigation. First-line antibiotic therapy was amoxicillin or amoxicillin-clavulanic acid combination (57%), a first or second generation cephalosporin (12%), ceftriaxone (8%), oral broad-spectrum cephalosporin (3%), a macrolide (16%), a tetracycline (1%) and a fluoroquinolone (2%). A total of 120 (78%) patients recovered with no change in treatment and 34 (22%) patients failed to improve: 18 were hospitalized and 16 had a second-line therapy, mainly a macrolide or a quinolone. Five patients died at hospital. The overall mortality was 3%, and 14% in hospitalized patients. Empirical therapy using a betalactam to target a presumed pneumococcal infection, in agreement with European guidelines, is appropriate for out-patients with mild lobar community-acquired pneumonia.
