Hexagonal boron nitride (h-BN) is a key material for 2D heterostructures and electronic devices. It has been widely employed as encapsulant, dielectric barrier and, mainly, as a substrate for graphene since it significantly enhances its electronic properties and in particular the mobility of the charge carriers. However, these advances are to a great extent restricted to laboratory-scale experiments due to the limited scalability of current h-BN growth methods. In this work, we present an in-depth study of h-BN thin films grown by ion beam assisted deposition (IBAD), a versatile and scalable technique that enables the synthesis of high-quality ultra-flat layers with tunable thickness on metallic, semiconducting, or insulating substrates. Comprehensive structural, optical, and morphological characterization of the h-BN films was performed using complementary spectroscopic and advanced microscopy techniques. Our results reveal the growth of highly in-plane textured nanocrystalline h-BN layers exhibiting very low defect density and ultraflat surfaces with a wide bandgap around 5.35 eV. These findings make IBAD a promising method for the scalable growth of highly performing h-BN thin films for integration as substrates in graphene-based devices.

We present a benchmarking protocol that combines the characterization of boron nitride (BN) crystals and films with the evaluation of the electronic properties of graphene on these substrates. Our study includes hBN crystals grown under different conditions and scalable BN films deposited by either chemical or physical vapor deposition (CVD or PVD). We explore the complete process from boron nitride growth, over its optical characterization by time-resolved cathodoluminescence (TRCL), to the optical and electronic characterization of graphene by Raman spectroscopy after encapsulation and Hall bar processing. Within our benchmarking protocol we achieve a homogeneous electronic performance within each Hall bar device through a fast and reproducible processing routine. We find that a free exciton lifetime of 1 ns measured on as-grown hBN crystals by TRCL is sufficient to achieve high graphene room temperature charge carrier mobilities of 80,000 cm^2/(Vs) at a carrier density of |n| = 10^12 cm^-2, while respective exciton lifetimes around 100 ps yield mobilities up to 30,000 cm^2/(Vs). For scalable PVD-grown BN films, we measure carrier mobilities exceeding 10,000 cm^2/(Vs) which correlates with a graphene Raman 2D peak linewidth of 22 cm^-1. Our work highlights the importance of the Raman 2D linewidth of graphene as a critical metric that effectively assesses the interface quality (i.e. surface roughness) to the BN substrate, which directly affects the charge carrier mobility of graphene. Graphene 2D linewidth analysis is suitable for all BN substrates and is particularly advantageous when TRCL or BN Raman spectroscopy cannot be applied to specific BN materials such as amorphous or thin films. This underlines the superior role of spatially-resolved spectroscopy in the evaluation of BN crystals and films for the use of high-mobility graphene devices.

We report the impact of flash-lamp-annealing (FLA) on the structural evolution of amorphous titania (TiO2) films produced by DC reactive magnetron sputtering. TiO2 films were grown at room-temperature at different oxygen partial pressure (PO2) and subsequently annealed as a function of the FLA energy density. X-ray diffraction confirms that FLA induces phase formation from the initial amorphous state with a general transition from anatase to rutile by increasing the FLA energy density (temperature). Interestingly, the transformation onset of anatase to rutile is achieved at lower energy densities for higher PO2. On the contrary, films with a highly resilient anatase phase can be produced at relatively low PO2. A detailed analysis of the pristine amorphous structure carried out by X-ray absorption near-edge structure indicates the role of oxygen sites in the observed phase transformation. In particular, oxygen vacancies seem to stabilize the anatase phase at high temperatures. The results show the relevance of subtle changes in the initial amorphous structure for phase selectivity in TiO2 films upon FLA.

It is shown how to efficiently convert solar into electrical energy, taking advantage of laser amplification and intra-cavity use of a low-efficiency converter. The latter may consist of a low-efficiency transparent photovoltaic cell or a thermoelectric cell integrated into a metallic laser-cavity mirror, constituting a minor intra-cavity loss for the laser operation. The overall power conversion efficiency is derived and discussed for a variety of current solid-state laser materials. It is shown that power conversion efficiencies comparable with commercial silicon photovoltaic cells are obtained with current standard laser materials.

Graphene-hexagonal boron nitride (hBN) scalable heterostructures are pivotal for the development of graphene-based high-tech applications. In this work, we demonstrate the realization of high-quality graphene-hBN heterostructures entirely obtained with scalable approaches. hBN continuous films were grown via ion beam-assisted physical vapor deposition directly on commercially available SiO2/Si and used as receiving substrates for graphene single-crystal matrixes grown by chemical vapor deposition on copper. The structural, chemical, and electronic properties of the heterostructure were investigated by atomic force microscopy, Raman spectroscopy, and electrical transport measurements. We demonstrate graphene carrier mobilities exceeding 10,000 cm2/Vs in ambient conditions, 30% higher than those directly measured on SiO2/Si. We prove the scalability of our approach by measuring more than 100 transfer length method devices over a centimeter scale, which present an average carrier mobility of 7500 ± 850 cm2/Vs. The reported high-quality all-scalable heterostructures are of relevance for the development of graphene-based high-performing electronic and optoelectronic applications.

Un-doped (uZO) and silver-doped zinc oxide (SZO) films were prepared by oblique incidence sputtering deposition under different process parameters. The crystalline structure, chemical composition, and surface morphology were correlated with the optical properties, as well as with the wettability of the films. In the case of uZO films, the orientation, inclination, and morphology of the columnar structure determined the wettability of the layer, moving from a hydrophilic- to hydrophobic-like character. In the case of SZO films, although almost all of them displayed hydrophobic behavior, the hydrophobic character increased with the Ag content. The most hydrophobic surface was obtained when the Ag content in the layers was greater than 7 at.% and, in these cases, the structural results indicate that the layers were formed by a disordered mixture of Zn and Ag oxides.

The bonding structure of tin oxide (SnOx) films grown by reactive DC magnetron sputtering has been studied by the combination of X-ray diffraction (XRD) and soft X-ray absorption near-edge structure (XANES). The oxygen incorporation in the films has been controlled by the O2 partial pressure (PO2) in the O2/Ar discharge mixture. In addition, the impact of substrate heating and post-deposition flash lamp annealing (FLA) on crystal growth has been studied. In general, it has been stablished a transition from SnO to SnO2 arrangements by increasing PO2, where XRD and XANES provide complementary results about the formation of single- and mixed-phase films. In samples produced at room temperature, XANES gives unique information about such structural evolution, as well as related to defects like the incorporation of O2 molecules at high PO2. FLA on samples grown at room temperature promotes crystal growth and the phase evolution follows the initial structural selectivity. Finally, the optical properties and surface morphology of the films have been correlated with the structural identification. (c) 2022 The Author(s). Published by Elsevier B.V. CC_BY_NC_ND_4.0

The optical and electrical properties of fluorinated tin oxide (FTO) films deposited at room temperature by sputtering have been investigated varying the fluorine content and the hydrogen atmosphere. The complex behavior of the obtained films is disclosed using a wide set of characterization techniques that reveals the combined effects of these two parameters on the generated defects. These defects control the electrical transport (carrier density, mobility and conductivity), the optical properties (band gap and defects-related absorption and photoluminescence) and finally promote the amorphization of the samples. H-2 in the sputtering gas does not modify the H content in the films but induces the partial reduction of tin (from Sn4+ to Sn2+) and the consequent generation of oxygen vacancies with shallow energy levels close to the valence band. A variation of up to four orders of magnitude in electrical conductivity is reported in samples with the appropriate fluorine doping and hydrogen fraction in the sputtering gas, maintaining excellent optical transparency. Optimized room temperature grown electrodes reach sheet resistance similar to 20 Omega/square and transparency > 90%. This room temperature deposition process enables film preparation on flexible organic substrates, such as polyethylene terephthalate (PET), with identical performance of doubtless interest in flexible and large scale electronics.

Disruptive technologies are usually characterised by universal, versatile applications, which change many aspects of our life simultaneously, penetrating every corner of our existence. In order to become disruptive, a new technology needs to offer not incremental, but dramatic, orders of magnitude improvements. Moreover, the more universal the technology, the better chances it has for broad base success. Significant progress has been made in taking graphene and related materials from a state of raw potential to a point where they can revolutionize multiple industries. When it comes to electrochemical applications, Raman spectroscopy is an ideal non-destructive technique to study degradation in graphite anodes, as it is sensitive to doping, strain, defects, and interlayer coupling. I will discuss how in-situ Raman spectroscopy can unravel the signatures of Li-ion induced doping, intercalation staging and degradation upon cycling.

In this paper, we report on the phase selectivity in Cr and N co-doped TiO2 (TiO2:Cr,N) sputtered films by means of interface engineering. In particular, monolithic TiO2:Cr,N films produced by continuous growth conditions result in the formation of a mixed-phase oxide with dominant rutile character. On the contrary, modulated growth by starting with a single-phase anatase TiO2:N buffer layer, can be used to imprint the anatase structure to a subsequent TiO2:Cr,N layer. The robustness of the process with respect to the growth conditions has also been investigated, especially regarding the maximum Cr content (<5 at.%) for single-phase anatase formation. Furthermore, post-deposition flash-lamp-annealing (FLA) in modulated coatings was used to improve the as-grown anatase TiO2:Cr,N phase, as well as to induce dopant activation (N substitutional sites) and diffusion. In this way, Cr can be distributed through the whole film thickness from an initial modulated architecture while preserving the structural phase. Hence, the combination of interface engineering and millisecond-range-FLA opens new opportunities for tailoring the structure of TiO2-based functional materials.

We address the impact of chromium (Cr) incorporation (< 15 at.%) in the structure of titanium dioxide (TiO2: Cr) films for as-grown and after flash-lamp-annealing (FLA) states. Samples were produced by DC magnetron sputtering on either unheated or heated (400 degrees C) substrates. Complementary medium-and local-order information was extracted by X-ray diffraction and absorption near-edge structure, respectively. TiO2: Cr grown on unheated substrates are amorphous with the major contribution from Cr3+ and progressive formation of Cr6+ with Cr. On heated substrates, anatase phase is dominant for low Cr levels (<= 7 at.%) and the structure evolves with Cr towards a disordered mixed-oxide with rutile structure. By tuning the FLA energy density, customized (single or mixed) phase formation is achieved from (initially amorphous) Cr-free TiO2. For amorphous TiO2: Cr with low Cr (<= 7 at.%), FLA induces a short-range rutile structure but structural ordering is not observed at higher Cr levels. Nonetheless, FLA annihilates Cr6+ sites and promotes Cr4+, which is associated to the mixed-oxide rutile. FLA also improves the pristine structure of anatase TiO2: Cr grown on heated substrates. These results provide relevant information about the atomic structure of mixed oxides and the use of FLA for the synthesis of band-gap engineered TiO2-based materials. (C) 2017 Elsevier B.V. All rights reserved.

Poster presented at the Nanoscale Pattern Formation at Surfaces Formation of 3D Nanostructures by Ion Beams, held in Helsinki (Finland) on June 26-30th, 2017.

We have studied the tribological properties of multilayers composed of nanoscale hexagonal carbon (h-C) and hexagonal boron nitride (h-BN) layers. These two materials are soft and lubricant in bulk, with poor wear resistnace under high loads. However, when a stacking of nanoscale layers is formed, the wear resistance improves drastically, showing wear rates below 1x10-7 mm3/Nm. the C/BN multilayers were formed by sequential evaporation of (i) carbon and (ii) boron with a concurrent nitrogen ion beam. In this way, a series of multilayers with period between 3 and 80 nm is obtained [1]. Details on film preparation and characterization by SEM and TEM microscopies are discussed. The friction and wear resistance are evaluated by pin-on-disk tests under different conditions. To perform a Photoemission Electron Microscopy (PEEM) analysis of the tribochemistry taking place on these multilayered films, erosion by pin-on-disk was performed to create a measurable weartrack where only a few of the topmost sublayers were exposed to the environment. In this way, layers with nanometric thickness appear as micrometric and nanometric bands along the sliding direction in the wear track. The spectroscopic analysis of these compositional features in the weartrack constitutes a perfect playground to evaluate the microscopy and spectroscopy capabilities of PEEM analysis. Several interesting observations are made on the tribochemistry between the stainless steel balls and the C/BN film, among them the preferential reactivity of Fe towards boron and Cr towards C. Reference [1] R. Torres et al. Reversed texture in nanometric carbon/boron nitride multilayers, Carbon, 74, 374 (2014)

Rights: © 2003 American Physical Society (APS). This is the accepted version of the following article: Fraxedas, J. & Lee, Y. J. & Jiménez, I. & Gago, R. & Nieminen, Risto M. & Ordejón, P. & Canadell, E. 2003. Characterization of the unoccupied and partially occupied states of TTF-TCNQ by XANES and first-principles calculations. Physical Review B. Volume 68, Issue 19. 195115-1-11. ISSN 1550-235X (electronic). DOI: 10.1103/physrevb.68.195115, which has been published in final form at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.68.195115.

Silica microcapsules encapsulating an epoxy compound (CAP) and silica nanoparticles functionalized by an amine group (NS) are synthesized to be used as self-healing system for smart cementitious composites. The innovative character of this system comes from the use of silica shell microcapsules to improve the durability and compatibility with the cement and from the use of functionalized nanosilica to obtain an amine functionalized cementitious matrix. Characterization of the particles indicates that they are amorphous and possess a proper morphology and size to be considered as additions to cement. The stability of the epoxy compound inside the microcapsules and the presence of amine groups bonded to silica nanoparticles are also confirmed. Moreover, NS shows a pozzolanic activity superior to that of the silica fume used as reference, while CAP is to a high degree stable upon reaction with lime. The results confirm that the synthesized particles are a suitable starting point to address the development of a smart self-healing concrete.

Two innovative additions are considered for the development of self-healing concrete: epoxy-containing silica microcapsules and amine-functionalized nanosilica. The effect of two concentrations of the additions on the microstructure of a cement paste with silica fume is studied. The results indicate a proper dispersion of the additions within the matrix, a pozzolanic reaction induced by nanosilica and the stability of the microcapsules that reliably isolate the epoxy from the paste. As the concentration of additions increases, a preferential orientation of the portlandite phase is observed, together with a decrease of the compressive strength due to the presence of a minor content of macropores and to the low strength of the capsules. The self-healing efficiency is confirmed in concrete specimens for 150μm wide cracks and a particular concentration of the additions. These results will be essential for the subsequent development of a reliable self-healing concrete based in the epoxy-amine adhesive.

The use of carbon-based coatings (hydrogenated and non-hydrogenated DLC, doped and alloyed-DLC) is of wide interest due to its applications in mechanical components submitted to friction and wear including sliding parts in automotive engines. A tribological comparative analysis using a reciprocating (SRV) tester in lubricated and unlubricated conditions with a 4-stroke motor oil has been carried out on six currently relevant state-of-the-art coatings (namely WC/a-C, TiBC/a-C and TiC/a-C:H nanocomposites, Ti-doped DLC, BCN film and a crystalline monolithic TiC film as reference). The quantification of the fraction of the sp2-bonded matrix has been done by fitting of C 1s XPS peak and the mechanical properties evaluated by nanoindentation. The comparative analysis has allowed us to identify the capabilities of each system depending on the testing conditions and the possible synergies as a function of the chemical composition and film nature. Under lubricated harsh conditions (max. contact pressure 1.7 GPa) only coatings displaying hardness superior to 20 GPa could stand the sliding motion without failure. At lower contact pressures, a significant fraction of sp2 carbon (≥ 75%) is advantageous for reducing wear in boundary lubrication. WC/a-C, BCN and Ti-DLC films showed the best tribological response in dry sliding conditions. This fundamental information would be of relevance for assisting engineers in selecting best partnership for lubrication systems.

Transnational access (TNA) to national radiation sources is presently provided via programmes of the European Commission by BIOSTRUCT-X and CALIPSO with a major benefit for scientists from European countries. Entirely based on scientific merit, TNA allows all European scientists to realise synchrotron radiation experiments for addressing the Societal Challenges promoted in HORIZON2020. In addition, by TNA all European users directly take part in the development of the research infrastructure of facilities. The mutual interconnection of users and facilities is a strong prerequisite for future development of the research infrastructure of photon science. Taking into account the present programme structure of HORIZON2020, the European Synchrotron User Organization (ESUO) sees considerable dangers for the continuation of this successful collaboration in the future.

Pulsed and Continuous-wave (CW) Electron Paramagnetic Resonance (EPR) in combination with X-ray absorption near edge structure (XANES) analysis is used to study the bulk and surface modification of isotactic polypropylene (i-PP) conducted by X-ray irradiation and O-2, Ar and N-2 plasma treatments. In all cases, a midchain peroxy radical is the major species detected by CW-EPR, with little delocalization onto the i-PP main chain, as revealed by X-band Mims electron nuclear double resonance and hyperfine sublevel correlation. Besides, surface functionalization of plasma treated i-PP is discussed on the light of XANES results with emphasis in the more complex case of N2 plasma modification. The article gives a comprehensive picture of the chemical changes that occur in irradiated i-PP, with implications in sterilization, surface activation, and adhesion enhancement procedures. (C) 2014 Elsevier Ltd. All rights reserved.

A structure-controlled series of carbon/boron nitride multilayers, with bilayer thicknesses from 1.25 to 160nm has been grown by sequential evaporation of carbon and boron assisted with nitrogen ions. The minimum bilayer thickness for a stable stack is 2.9nm. A turbostratic texture of the carbon and BN phases is evidenced even for small periods of the bilayers. Interestingly, BN and C basal planes of adjacent sub-layers exhibit perpendicular alignment between them: along the growth direction for h-BN rich layers, and parallel to the surface for the C rich ones.