C. Ballesteros

Production of ordered silicon nanocrystals by low-energy ion sputtering

We report on the production of ordered assemblies of silicon nanostructures by means of irradiation of a Si (100) substrate with 1.2 keV Ar+ ions at normal incidence. Atomic force and high-resolution transmission electron microscopies show that the silicon structures are crystalline, display homogeneous height, and spontaneously arrange into short-range hexagonal ordering. Under prolonged irradiation (up to 16 h) all dot characteristics remain largely unchanged and a small corrugation develops at long wavelengths. We interpret the formation of the dots as a result of an instability due to the sputtering yield dependence on the local surface curvature.

Solid-phase crystallization of amorphous SiGe films deposited by LPCVD on SiO2 and glass

The crystallization kinetics and film microstructure of poly-SiGe layers obtained by solid-phase crystallization (SPC) of amorphous SiGe with Ge fractions (x) in the 0 to 0.42 range have been studied in detail. Amorphous SiGe layers 100 nm thick were deposited by LPCVD at 450°C on thermally oxidized Si afers and 7059 Corning glasses, using Si 2 H 6 and GeH 4 as gas sources. The samples were crystallized at 550°C and low pressure (below 9 Pa The evolution of the crystallization and the resulting film microstructure were characterized by X-ray diffractometry and transmission electron microscopy. The experimental results on growth kinetics fit the Avramis model. The characteristic crystallization time decreases with x, slowly for x < 0.3 and more abruptly for higher values of x. The transient time depends exponentially on x in all the intervals. The crystallized films have a (111) preferred orientation for low values of x and evolve to a randomly oriented polycrystal as x increases. The grain size in the fully crystallized layers decreases with increasing x. The results are similar for the films deposited on silicon dioxide and glass.

Magnetism and superconductivity in La0.7Ca0.3MnO3/YBa2Cu3O7−δ superlattices

We report on the magnetic and superconducting properties of La0.7Ca0.3MnO3/YBa2Cu3O7 (LCMO/YBCO) superlattices. For a constant LCMO layer thickness of 6 unit cells (u.c.), resistance and susceptibility measurements show superconductivity for YBCO layer thickness in excess of 4 unit cells. The critical temperature increases with YBCO thickness, and a Tc of 58 K is found for a YBCO thickness of 10 unit cells. Magnetization measurements show a ferromagnetic transition at 100 K in a (LCMO6 u.c./YBCO5 u.c.)15 bilayer superlattice, and a depressed value of the saturation magnetization of 20 emu cm−3. These results are discussed in terms of interface disorder (analyzed by x-ray diffraction and transmission electron microscopy) and of the possible interaction between magnetism and superconductivity.

Methicillin-resistant Staphylococcus aureus carrying the mecC gene: emergence in Spain and report of a fatal case of bacteraemia

OBJECTIVES Methicillin-resistant Staphylococcus aureus (MRSA) strains carrying the mecC gene have been reported from humans and animals from several European countries, but never from Spain. We describe the first isolates of mecC-positive MRSA of human origin collected in Spain and report a fatal case of bacteraemia. METHODS Isolates were tested for phenotypic resistance using cefoxitin, tested for the mecA/mecC genes and toxin genes by PCR, and typed by staphylococcal cassette chromosome mec (SCCmec), PFGE, spa, multilocus sequence typing and agr. RESULTS During 2008-13 five MRSA isolates showing resistance to cefoxitin and carrying the mecC gene were recovered at one hospital in Spain. In a review of 5505 S. aureus strains received at the Spanish National Reference Centre for Staphylococci from the same period, we found two additional mecC-positive isolates. The isolates were recovered from blood (two), wounds (two), joint fluid (one), urine (one) and a nasal swab (one). All MRSA were mecA negative, presented SCCmecXI, belonged to agr group III and to clonal complex 130, and were negative for the production of the toxin genes tst1, eta, etb, etd and Panton-Valentine leucocidin. Six isolates belonged to spa type t843 (ST130 and ST1945, where ST stands for sequence type) and one to spa type t6220 (ST1945). One patient with mecC-positive MRSA sepsis died in the emergency department. CONCLUSIONS We confirm the presence of MRSA carrying the mecC gene in Spain, the ability of this livestock-associated MRSA to cause severe infections in humans and the need to perform culture-based susceptibility testing methods in order to detect these emerging strains.

Nanopatterning of silicon surfaces by low-energy ion-beam sputtering: dependence on the angle of ion incidence

We report on the production of nanoscale patterning on Si substrates by low-energy ion-beam sputtering. The surface morphology and structure of the irradiated surface were studied by atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM). Under ion irradiation at off-normal incidence angle (∼50 ◦ ), AFM images show the formation of both nanoripple and sawtooth-like structures for sputtering times longer than 20 min. The latter feature coarsens appreciably after 60 min of sputtering, inducing a large increase in the surface roughness. This behaviour is attributed to the preferential direction determined on the substrate by the ion beam for this incidence angle, leading to shadowing effects among surface features in the sputtering process. Under irradiation at normal incidence, the formation of an hexagonal array of nanodots is induced for irradiation times longer than 2 min. The shape and crystallinity of the nanodots were determined by HRTEM. At this incidence angle, the surface roughness is very low and remains largely unchanged even after 16 h of sputtering. For the two angle conditions studied, the formation of the corresponding surface structures can be understood as the interplay between an instability due to the sputtering yield dependence on the local surface curvature and surface smoothing processes such as surface diffusion.

Wet etching of GaN grown by molecular beam epitaxy on Si(111)

A wet etching method for GaN and AlxGa1-xN, based on aqueous solutions of KOH, is presented. A detailed analysis of the etching rate dependence with temperature and concentration is described. This etching has been used for the fabrication of high optical quality pyramidal nanostructures in wurtzite N-face GaN grown on AlN-buffered Si(111) substrates by molecular beam epitaxy. These nanostructures have been studied by high-resolution transmission and scanning electron microscopy and their optical quality has been analysed by low-temperature photoluminescence (PL) measurements. The pyramids are parallel to the basal plane and limited by {111} planes and its presence improves the overall PL response of the sample. The relationship between the polarity of GaN and the characteristics of the AlN buffer has also been analysed.

Effect of electron beam on the cathodoluminescence from indented MgO in the scanning electron microscope

The effect of an electron beam on the cathodoluminescence from indented MgO single crystals has been studied in the scanning electron microscope. It has been found that the luminescence intensity and spectrum vary with irradiation time. The behavior is attributed to changes in impurities and vacancies defects.

Luminescent nanostructures based on Ge nanoparticles embedded in an oxide matrix

Ge nanoparticles embedded in an oxide matrix have been obtained by (a) steam thermal oxidation at 650 °C of polycrystalline SiGe layers and (b) deposition of discontinuous Ge films/SiO2 multilayers by low pressure chemical vapour deposition at 390 °C and thermal annealing at 700 °C. These two approaches are compared in terms of the composition and size of the nanoparticles and the luminescence properties of the structures. In both cases violet luminescence peaking at 3.1 eV is detected. The origin of this emission in both types of structures is the same and it will be related to defects at the interface between the nanocrystalline Ge and the dielectric matrix. Thinking about future applications, the second approach is found to be much more attractive from the technological point of view, even considering that the first one gives a more intense luminescence emission, since the structure can be fabricated in a single-run process; these structures are currently being investigated to improve their luminescence emission.

Comparative study of the luminescence of structures with Ge nanocrystals formed by dry and wet oxidation of SiGe films

The luminescence emission of structures containing Ge nanocrystals embedded in a dielectric matrix obtained by dry and wet oxidation of polycrystalline SiGe layers has been studied as a function of the oxidation time and initial SiGe layer thickness. A clear relationship between the intensity of the luminescence, the structure of the sample, the formation of Ge nanocrystals and the oxidation process parameters that allows us to select the appropriate process conditions to get the most efficient emission has been established. The evolution of the composition and thickness of the growing oxides and the remaining SiGe layer during the oxidation processes has been characterized using Raman spectroscopy, x-ray diffraction, Fourier-transform infrared spectroscopy, Rutherford backscattering spectrometry and transmission electron microscopy. For dry oxidation, the luminescence appears suddenly, regardless of the initial SiGe layer thickness, when all the Si of the SiGe has been oxidized and the remaining layer of the segregated Ge starts to be oxidized forming Ge nanocrystals. Luminescence is observed as long as Ge nanocrystals are present. For wet oxidation, the luminescence appears from the first stages of the oxidation, and is related to the formation of Ge-rich nanoclusters trapped in the mixed (Si and Ge) growing oxide. A sharp increase of the luminescence intensity for long oxidation times is also observed, due to the formation of Ge nanocrystals by the oxidation of the layer of segregated Ge. For both processes the luminescence is quenched when the oxidation time is long enough to cause the full oxidation of the Ge nanocrystals. The intensity of the luminescence in the dry oxidized samples is about ten times higher than in the wet oxidized ones for equal initial thickness of the SiGe layer.

Molecular epidemiology of community-associated methicillin-resistant Staphylococcus aureus in Spain: 2004–12

OBJECTIVES In Spain, despite the high rates of healthcare-associated methicillin-resistant Staphylococcus aureus (MRSA), the incidence of community-associated (CA) MRSA seems to be low on the basis of a small number of studies. We analysed the evolution of CA-MRSA in Spain from 2004 to 2012, and identified the clonal lineages and population structure. METHODS The study included 8326 MRSA strains. Susceptibility to 18 antimicrobials was determined. Isolates were tested for the presence of mecA, Panton-Valentine leucocidin (PVL) and the arginine catabolic mobile element (ACME) by PCR, and typed by staphylococcal cassette chromosome mec, PFGE, spa, multilocus sequence typing and agr. RESULTS Among the 8326 isolates, 246 (2.9%) were CA-MRSA. We identified genotypically 226 PVL-positive CA-MRSA isolates (88% agr type I, 10.2% agr type III and 1.8% agr type II) and 20 PVL-negative CA-MRSA isolates (all agr type I) from children and adults (82.1% from wounds) from 13 different geographical areas. A significant increase in the rates of CA-MRSA was observed when comparing 2004-07 (0.43%) with 2008-12 (5.44%). Resistance rates were as follows: only β-lactams, 84.5%; erythromycin, 12.8%; tetracycline, 8.8%; clindamycin, 4.9%; ciprofloxacin, 3.1%; fusidic acid, 2.0%; others, 0.4%; and multiresistant, 6.2% (six isolates USA300). The strains belonged to the PVL-positive clones ST8-IVc (69.9%), ST8-IVa-ACME-positive (USA300, 8.9%), ST8-IVa-ACME-negative (0.8%), ST30-IVc (4.5%), ST80-IVc (2.0%), ST5-IVc (1.2%) and others (ST59, ST72, ST88, ST642, ST1472 and ST1829; 4.5%) and to the PVL-negative ST398-V (8.1%). CONCLUSIONS We confirm an increase in CA-MRSA in Spain, the predominance of the ST8-IVc clone, the emergence of the USA300 clone, a high genetic diversity among PVL-positive CA-MRSA isolates and the recent emergence of the pig-associated ST398-V clone.