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Chemical vapor deposition (CVD) is one of the best ways to scalably grow low cost, high quality graphene on metal substrates; unfortunately, it not ideal for producing graphene on dielectric substrates. Here, we demontrate production of a high quality graphene layer on Sapphire using CVD with a copper catalyst. The catalyst consists of a thin copper film grown epitaxially on α- Al 2 O 3 (0001). After CVD growth of Graphene, the copper can be removed by simple evaporation in the presence of a carbon source ( C 2 H 4 ). We characterized the resulting graphene layer using Raman spectroscopy, atomic force microscopy (AFM), optical transmission and helium atom scattering (HAS). The sample exhibited a reduced Raman D peak and an excellent 2D to G ratio. AFM and HAS show large graphene domains over a macroscopic region. We measured > 86 % transparency over the visible spectrum.

The modulation of electrical and Raman properties of ultrafine (∼4 nm) Ag nanoparticle/graphene/SiO hybrid material at low coverage is evaluated with gradual nanoparticle incorporation by the gas aggregation deposition technique. The different contributing factors, such as doping, impurity scattering or strain, are assessed. Incorporation of Ag nanoparticles produce a very efficient n-type doping of graphene (∼7.5 e per particle) maintaining a constant mobility for particle coverage below ∼0.3 monolayers. Doping efficiency is determined by the probability for nanoparticles to be deposited in contact with graphene. The Fermi level upshift is modeled within the charged impurity scattering mechanism in the whole coverage range. A crossover to the regime where impurity scattering dominates is evidenced at large particle concentration. Surface-enhanced Raman scattering is detected in graphene phonons in the limit of vanishing plasmon resonance and very low coverage (∼0.08) that correspond to ∼1500 nanoparticles at the laser spot. Small distortions of the graphene lattice (±0.012%) induced by the nanoparticles overcome the predicted changes in Raman phonons related to carrier doping and originate I/I damping. This evolution of physical properties with gradual incorporation of Ag nanoparticles is anticipated to provide valuable hints to tune the optical and electronic performance of these graphene-based hybrid systems., This work is supported by MINECO (MAT2012-37276-C03-01) and from Comunidad de Madrid, project S2013/MIT-2740 (PHAMA_2.0-CM).

A cheap and green technology based on electro-erosion, performed at room conditions and scalable to large area (cm2) is demonstrated for structuring graphene and graphene oxide films on any substrate. This one-step technique based on electrical discharges produced by a direct current voltage source competes favorably with laser patterning. The threshold voltage for complete graphene elimination as determined by Raman mapping is around 20 V. At low relative humidity conditions (30%) the transformation to graphene oxide is also detected for operating voltages above 40 V, the oxidation being probably mediated by the residual adsorbed water at the surface. We also show the close correlation of atomic force microscopy (AFM) phase images with the modified graphene characteristics while AFM topographic images are dominated by extrinsic aspects. The use of a spring probe relaxes the requirement of precision for the tip-sample distance and sample flatness to around 10 μm which is very convenient for large scale applications. Furthermore, this technique allows the formation of high quality nanographene, with size around 18 nm, and graphene micro-ribbon lattices in a very fast way and very well defined edges, with dimensions down to 1 μm width and mm length, very promising for terahertz graphene plasmonic applications., X.D. acknowledges a FPI scholarship from Spanish MINECO and C.M. thanks financial support from the “Juan de la Cierva” postdoctoral program JCI-2011-08815. The research leading to these results has received funding from Spanish MINECO under projects MAT2012-37276-C03-01, MAT2012-37276-C03-03 and MAT2011-27470-C02-02, from Comunidad de Madrid under Project S2013/MIT-2740 (PHAMA_2.0-CM) as well as from the European Union Seventh Framework Program under grant agreement No. 604391 “Graphene Flagship.”

The detection, identification, and quantification of different types of molecules and the optical imaging of, for example, cellular processes are important challenges. Here, we present how interference-enhanced Raman scattering (IERS) in adequately designed heterostructures can provide amplification factors relevant for both detection and imaging. Calculations demonstrate that the key factor is maximizing the absolute value of the refractive indices' difference between dielectric and metal layers. Accordingly, Si/Al/AlO/graphene heterostructures have been fabricated by optimizing the thickness and roughness and reaching enhancement values up to 700 for 488 nm excitation. The deviation from the calculated enhancement, 1200, is mainly due to reflectivity losses and roughness of the Al layer. The IERS platforms are also demonstrated to improve significantly the quality of white light images of graphene and are foreseen to be adequate to reveal the morphology of 2D and biological materials. A graphene top layer is adequate for most organic molecule deposition and often quenches possible fluorescence, permitting Raman signal detection, which, for a rhodamine 6G (R6G) monolayer, presents a gain of 400. Without graphene, the nonquenched R6G fluorescence is similarly amplified. The wavelength dependence of the involved refractive indices predicts much higher amplification (around 10) for NIR excitation. These interference platforms can therefore be used to gain contrast and intensity in white light, Raman, and fluorescence imaging. We also demonstrate that surface-enhanced Raman scattering and IERS amplifications can be efficiently combined, leading to a gain of >10 (at 488 nm) by depositing a Ag nanostructured transparent film on the IERS platform. When the plasmonic structures deposited on the IERS platforms are optimized, singlemolecule detection can be actively envisaged., L.A.-F. acknowledges a FPI grant (BES-2013-062759) from Spanish Ministerio de Economía y Competitividad (MINECO). The research leading to these results has received funding from MINECO (MAT2012-37276-C03-01 and MAT2015-65356-C3-1-R) and Comunidad de Madrid (S2013/MIT-2740, PHAMA2.0-CM).

The oxidation and corrosion of copper are fundamental issues studied for many decades due to their ubiquitous and transversal impact. However, the oxidation of copper used as catalyst for graphene synthesis has opened a singular problem not yet solved. Contradictory results are reported about the protecting or enhancing role of graphene in copper oxidation. We study short- and long-term oxidation of copper with different characteristics, such as oxygen content and morphology, with and without graphene, and in polycrystalline copper foils and almost totally textured (100) and (111) copper films on MgO and sapphire substrates, respectively. We propose a mechanism to explain the enhanced oxidation of polycrystalline copper originated by oxygen encapsulated by the graphene impermeable layer during graphene growth. The initial oxygen content and the existence of grain boundaries are the main factors that determine the relevance of this process. Graphene is shown to prevent oxidation from the atmosphere for any of the copper substrates but also promotes slow oxidation derived by the release of out-of-equilibrium encapsulated oxygen. The formation of bubbles after several months evidence this slow release. The occluded oxygen in graphene covered copper is demonstrated by comparing the oxygen to copper ratio at different depths using hard X-ray photoelectron spectroscopy for samples with and without graphene., Funding by the Spanish Ministerio de Economía y Competitividad (MINECO) under Projects MAT2015-65356-C3-1-R and MAT2012-37276-C03-01 and Comunidad de Madrid Excellence Network under Project S2013/MIT-2740 is acknowledged. We also acknowledge the MINECO for financial support and provision of synchrotron radiation facilities at BM25 SpLine The Spanish CRG Beamline at the ESRF. L.A-F. acknowledges a FPI grant (BES-2013-062759) from MINECO.

In addition to the known effect of the substrate on the interfacial properties of perovskite films, here we show that the bulk properties of hybrid lead halide perovskite films depend on the type of substrate used for film growth. Despite the relative large film thickness, ∼600 nm, the roughness and nature of the substrate layer (glass, FTO, TiO and PEDOT:PSS) affect not just the degree of preferential orientation and crystal grain size but also the lattice parameters of CHNHPbI films synthesized from the PbCl precursor. The obtained changes in lattice parameters indicate that the Pb-Pb distance varies by around 0.7%. We suggest that the substrate roughness and chemical nature determine the concentration of defects mainly by varying the chlorine content and probably by the incorporation of oxygen and iodine vacancies during film nucleation and growth. These differences also have consequences in the observed light induced transformations. Upon laser illumination, the formation of additional defects, most probably related to oxygen, is revealed by 110 and 165 cm Raman peaks. With increasing laser power the chemical transformation into PbO is clearly identified by the 140 and 275 cm Raman peaks. The irreversible photoluminescence enhancement observed at low power with illumination time, also dependent on the substrate nature, is proposed to be due to the localization of the electron-hole excitons created in the vicinity of the light generated defects. These results shed light on the performance of the perovskite layer and help to understand how bulk processes, where ion migration is a conspicuous example, are severely affected by interfacial properties such as those imposed by the substrate., Funding by the Spanish Ministerio de Economía y Competitividad (MINECO) under Projects MAT2015-65356-C3-1-R and 2-R, MAT2014-54852-R and MAT2015-70611-ERC and Comunidad de Madrid Excellence Network under Project S2013/MIT-2740 (and Associated Lab LABCADIO belonging to CM net labs ref. 351) is acknowledged. We also acknowledge the MINECO for financial support and provision of synchrotron radiation facilities at ESRF, and thank María Vila for her assistance in using beamline BM25-SpLine. B. C. H. is grateful to the support of the National Council of Technological and Scientific Development (CNPq), Brazil, through the Science without Borders program.

Stable Graphene/gas/Cu bubbles are formed by the spontaneous oxidation at room conditions of the Cu substrates used as the catalyst agent in chemical vapor deposition growth of graphene. The non homogeneous copper oxidation produces discontinuous CuO films where atmospheric molecules get trapped by the impermeable graphene layer forming bubbles. Raman Interference effects are calculated both for graphene and CuO phonons in graphene/CuO/Cu system using the transfer matrix method finding that CuO thickness is limited to 20 nm and maximum Raman enhancement of graphene is 10. Nevertheless, enhancement values up to 60 are detected for micrometric graphene bubbles on copper which, according to the simulations, are 70 nm height. AFM measurements confirm the bubble dimensions and show that they are mechanically softer than graphene on copper. Consistently, the Raman characteristics of the graphene bubbles coincide with free standing undoped graphene with small strain, in the range 0-0.3%, while graphene on CuO presents hole doping and larger strain fields. The simulations indicate that interference Raman enhancement can reach values >10 in graphene/dielectric/Aluminum trilayers (dielectric = gas, oxide or polymer). These systems are therefore interesting platforms for optical sensing where graphene is the adequate bio-compatible layer for metallic nanoparticles and molecules deposition., A-F. acknowledges a FPI grant from Spanish Ministerio de Economía y Competitividad (MINECO) (BES-2013-062759) and E.C-P from the European Union Seventh Framework Program under grant agreement n° 604391 “Graphene Flagship”. The research leading to these results has received funding from Spanish MINECO (MAT2012-37276-C03-01 and MAT2015-65356-C3-1-R) and Comunidad Autónoma de Madrid (S2013/MIT-2740, PHAMA2.0-CM).

CoFe2O4 particles of 16 nm and 17 nm embedded in a silica matrix have been prepared through the hydrothermal method and the sol-gel method, respectively. From neutron powder diffraction a cation distribution of (Fe(0.72)Co(0.28))[Fe(1.28)Co(0.72)]O4 has been determined for Co-ferrite particles of 17 nm, which is in agreement with its particle size taking into account the reported x values for other nanometric Co-ferrite particles. Magnetic measurements were performed up to 700 K as the prepared ferrite samples present blocking temperatures above room temperature. The temperature dependence of the superparamagnetic moment has been analyzed and presents for both samples an abrupt drop in the magnitude once the blocking temperature is overcome. The temperature dependence of the calculated magnetic field needed to reach the magnetic saturation of the samples allows us to determine the temperature range for which the nanoparticles show superparamagnetic behaviour. The ordering temperature is in both cases lower than the tabulated one for bulk Co-ferrite (793 K) which has been ascribed mainly to two factors: a different cation distribution and the nanometric particle size, both contributing to lowering of the strength of the superexchange interactions.

The scheelite-type polymorph of GdCrO 4 has been obtained from the corresponding zircon-type compound under high pressure and temperature conditions, namely 4 GPa and 803 K. The crystal structure has been determined by X-ray powder diffraction. This GdCrO 4 scheelite crystallizes in a tetragonal symmetry with space group I 4 1 / a (No. 88, Z =4), a =5.0501(1) A, c =11.4533(2) A and V =292.099(7) A 3 . The thermal decomposition leads to the formation of the zircon–polymorph as intermediate phase at 773 K to end in the corresponding GdCrO 3 distorted perovskite-structure at higher temperatures. Magnetic susceptibility and magnetization measurements suggest the existence of long-range antiferromagnetic interactions which have been also confirmed from specific heat measurements. Neutron powder diffraction data reveal the simultaneous antiferromagnetic Gd 3+ and Cr 5+ ordering in the scheelite-type GdCrO 4 with a T N ∼20 K. The magnetic propagation vector was found to be k =(0 0 0). Combined with group theory analysis, the best neutron powder diffraction fit was obtained with a collinear antiferromagnetic coupling in which the m Cr 5 + and m Gd 3 + magnetic moments are confined in the tetragonal basal plane according to the mixed representation Γ 6 ⊕ Γ 8 .

Spinel-type (S.G.= Fd 3 m ) ZnFe 2 O 4 fine particles with sizes from 4 to 19 nm prepared by solvothermal and microwave-assisted solvothermal methods have been studied by neutron powder diffraction at room temperature. The cation distribution corresponding to mixed spinel structure (Zn 2+ 1− x Fe 3+ x )[Fe 3+ 2− x Zn 2+ x ]O 4 along with the unit cell parameter has been estimated after Rietveld refinement of the obtained neutron diffraction data for all the samples. It has been found that the inversion degree parameter ( x ) takes values between 0.11 and 0.20 depending not only on the particle size but also on the synthesis conditions as well. All the samples behave as superparamagnetic with an effective magnetic moment per particle ( μ SP ) from 7.0×10 2 to 7.7×10 3 μ B . The sample obtained by microwave assistance displays a different magnetic behavior as the ZFC and FC magnetic susceptibility and the magnetization versus applied field hysteresis loop measured at 5 K suggest. This is related with the dipole interactions that are a consequence of the higher inversion degree and μ SP .

We report a new method to produce high-quality, transparent graphene/sapphire samples, using Cu as a catalyst. The starting point is a high-quality graphene layer prepared by CVD on Cu(111)/AlO. Graphene on sapphire is obtained in situ by evaporation of the Cu film in UHV. He-diffraction, atomic force microscopy (AFM), Raman spectroscopy and optical transmission have been used to assess the quality of graphene in a metal free area. We used helium atom scattering as a sensitive probe of the crystallinity of the graphene on sapphire. The observation of high reflectivity and clear diffraction peaks demonstrates the presence of flat and homogeneous graphene domains over lateral scales of microns, consistent with the AFM results. Surprisingly, putting graphene on sapphire improves the quality of the He-diffraction spectra. Graphene forms a moiré pattern with a (11 × 11) periodicity, aligned with the (1 × 1) sapphire unit cell. The lattice constant of graphene on sapphire is a = (2.44 ± 0.02) Å. The phonon dispersion of the graphene flexural mode has been measured. This allowed the determination of the bending rigidity k = 0.61 ± 0.15 eV, and the graphene-sapphire coupling strength g = (5.8 ± 0.4) × 10 N m. The uniformity of the graphene has also been investigated by Raman mapping. Judging by the ratio of the 2D to G peaks, the quality of the graphene is not degraded by Cu removal. The high transparency (80%) measured in the visible range makes this system suitable for many applications that require hybrid properties commonly associated with metals (conductivity) and insulators (transparency). Our study shows that He-diffraction and Raman provide crucial information on quite different, complementary aspects of the same samples., This project was sponsored by the European Union, Seventh Framework Programme: Theme NMP.2012.1.4-3 Grant no. 309672. AMW and HKY acknowledge support from the Alexander von Humboldt Foundation, the Max Planck EPFL Center for Molecular Nanoscience and Technology as well as the Deutsche Forschungsgemeinschaft under CRC 1073. Additionally, a part of the research has been supported by the Spanish MINECO, under project MAT2012-37276-C03-01, and by Comunidad de Madrid, under project S2013/MIT-2740 (PHAMA_2.0-CM). S2013/MIT-2740/PHAMA_2.0-CM

This work was supported by the Spanish MINECO (FIS2014-54498-R, MAT2011-27470-C02-02, and CSD-2009-00013), by the European Union (Graphene Flagship Contract No. CNECT-ICT-604391 and COST MP1201 action), and by the Comunidad de Madrid through programs Nanofrontmag-CM (S2013/MIT-2850) and MAD2D-CM (S2013/MIT-3007).We acknowledge MINECO and CSIC for financial support and for provision of synchrotron radiation facilities and would like to thank the SpLine BM25 staff for assistance in using the beamline

We propose a strategy to study the elastic properties of extremely thin graphene oxide (GO) films using Brillouin spectroscopy. The dependence of the surface acoustic wave of a gold capping layer on the structural, chemical and morphological changes occurring to the underneath GO film with temperature is reported and analyzed. At room temperature the shear constant c44 is ∼17 GPa and hardens up to 28 GPa at 100 °C due to the partial elimination of embedded water layers and to interlayer distance shrinking. At 200 °C the almost complete elimination of water induces layer stacking disorder, further GO–GO distance reduction and a significant increase of all elastic constants. The in-plane constants harden due to the partial restoration of the sp2 C network (c11: from 268 to 620 GPa) and the out of plane constants harden due to the H bonds that now directly connect the neighbouring GO layers (c44 ≈ 80 GPa). The obtained Young's moduli are significantly higher than those reported for GO paper because the ultra-thin GO films are highly ordered and there is no macroscopic applied strain during the measurement. The results obtained here are associated with the intrinsic properties of GO as in-plane and inter-layer bonding., X.D. acknowledges a FPI scholarship from Spanish MINECO. The research leading to these results has received funding from Spanish MINECO under project MAT2012-37276-C03-01, from Comunidad de Madrid, project S2013/MIT-2740 (PHAMA_2.0-CM) and from the European Union Seventh Framework Program under grant agreement no. 604391 “Graphene Flagship.” info:eu-repo/grantAgreement/EC/FP7/604391

We report the characterization of Bi${}_{2}$Te${}_{2}$Se crystals obtained by the modified Bridgman and Bridgman-Stockbarger crystal-growth techniques. X-ray-diffraction study confirms an ordered Se-Te distribution in the inner and outer chalcogen layers, respectively, with a small amount of mixing. The crystals displaying high resistivity ($g1\phantom{\rule{0.28em}{0ex}}\ensuremath{\Omega}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$) and low carrier concentration ($\ensuremath{\sim}\phantom{\rule{-0.16em}{0ex}}5\ifmmode\times\else\texttimes\fi{}{10}^{16}$/cm${}^{3}$) at 4 K were found in the central region of the long Bridgman-Stockbarger crystal, which we attribute to very small differences in defect density along the length of the crystal rod. Analysis of the temperature-dependent resistivities and Hall coefficients reveals the possible underlying origins of the donors and acceptors in this phase.

WO${}_{3\ensuremath{-}x}$F${}_{x}$ ($x$ $\ensuremath{\le}$ 0.45) perovskite-like oxyfluorides were prepared by a chemically reducing fluorination route using the polymer polytetrafluoroethylene (teflon). The symmetry of the crystal structures of WO${}_{3\ensuremath{-}x}$F${}_{x}$ changes from monoclinic to tetragonal to cubic as the fluorine content increases. Fluorine doping changes insulating WO${}_{3}$ to a metallic conductor, and superconductivity (${T}_{c}$ $=$ 0.4 K) was discovered in the samples with fluorine contents of 0.41 $\ensuremath{\le}$ $x$ $\ensuremath{\le}$ 0.45. This easy fluorination method may be applicable to other systems and presents an opportunity for finding new oxyfluoride superconductors.

7 páginas, 6 figuras, 3 tablas.-- PACS number(s): 75.25.-j, 62.50.-p, 61.66.Fn, 75.30.Cr.-- et al., The scheelite form of the TbCrO4 oxide has been obtained by treating TbCrO4-zircon at 4 GPa and 823 K. X-ray and neutron diffraction data reveal that the high-pressure polymorph of TbCrO4 crystallizes with tetragonal symmetry space group I41/a and lattice parameters a=5.036 74(10) Å and c=11.3734(4) Å. Although bisdisphenoids [TbO8] and tetrahedra [CrO4] are present in both scheelite and zircon polymorphs, the remarkable changes observed in both polyhedra appear to support the reconstructive model for the zircon-scheelite first-order phase transition. Specific heat and neutron diffraction measurements confirm the antiferromagnetic ordering previously proposed from magnetic susceptibility measurements, in which both Tb3+ and Cr5+ sublattices are involved. The magnetic structure has been determined and can be described on the basis of the coincidence between the chemical and magnetic cells, k=(0,0,0), where the magnetic moments of the Tb3+ and Cr5+ cations are antiferromagnetically aligned along the c axis of the structure, with TN=29 K. Their values at 2 K are 8.97(3)μB and 1.089(9)μB, respectively., The authors are grateful to Spanish MICINN and FEDER for financial support under Research Projects No. MAT2007- 63497 and No. MAT2007-61621, CONSOLIDER-Ingenio 2010 (Grant No. CSD 2007-00010).

Neutron diffraction data of DyCrO4 oxide, prepared at 4 GPa and 833 K from the ambient pressure zircon-type, reveal that crystallize with the scheelite-type structure, space group I41/a. Accompanying this structural phase transition induced by pressure the magnetic properties change dramatically from ferromagnetism in the case of zircon to antiferromagnetism for the scheelite polymorph with a T N= 19 K. The analysis of the neutron diffraction data obtained at 1.2 K has been used to determine the magnetic structure of this DyCrO4-scheelite oxide which can be described with a k = [0, 0, 0] as propagation vector, where the Dy and Cr moments are lying in the ab-plane of the scheelite structure. The ordered magnetic moments are 10 µB and 1 µB for Dy+3 and Cr+5 respectively