Graphene growth on h-BN by Van der Waals MBE

García Martínez, Jorge Manuel et al.Comunicación presentada en el PDI Topical Workshop on MBE-Grown Graphene 2013, celebrado en Berlín el 19 y 20 de septiembre de 2013.Graphene growth on dielectric substrates has potential to enable new kinds of devices and applications. We explore graphene growth via direct depositing carbon in a MBE environment on different dielectric substrates, such as h-BN and sapphire. The growth of single layer graphene nanometer size domains by solid carbon source molecular beam epitaxy on hexagonal boron nitride (h-BN) flakes is demonstrated. These results are consistent with a Van der Waals growth mode of graphene on dielectric substrates. Atomic-force microscope images and Raman maps reveal that the graphene grown depends on the surface morphology of the h-BN substrates. On h-BN substrates, high quality single layer growth occurs as nano-domains. The growth is governed by a high mobility of the carbon atoms on the h-BN surface, in a manner that is consistent with van der Waals epitaxy. The successful growth of graphene layers depends on the substrate temperature, but is fairly independent of the incident flux of carbon atoms. Formation of single-layer graphene is clearly apparent in Raman spectra that display sharp optical phonon bands. We will show also results of graphene growth by MBE on sapphire, where large area growth occurs with monolayer thickness fluctuations. The successful growth of graphene on hexagonal boron-nitride flakes distributed over a large area allows the exploration of arrange of growth parameters and suggests the capability for large area growth.Work supported by ONR (N000140610138 and Graphene MURI), AFOSR (FA9550-11-1-0010), EFRC Cente fo Re-De_ning Photovoltaic E_ciency through Molecule Scale Control (award DE-SC0001085), NSF (CHE-0641523), NYSTAR, CSIC-PIF (200950I154), Spanish CAM (Q&C Light (S2009ESP-1503), Numancia 2 (S2009/ENE-1477)) and Spanish MEC (ENE2009-14481-C02-02, TEC2011-29120-C05-04, MAT2011-26534).Peer Reviewe

Graphene growth on h‐BN by Molecular Beam Epitaxy

Jorge M. Garcia...et al.The growth of single layer graphene nanometer size domains by solid carbon source molecular beam epitaxy on hexagonal boron nitride (h‐BN) flakes is demonstrated. Formation of single‐layer graphene is clearly apparent in Raman spectra which display sharp optical phonon bands. Atomic‐force microscope images and Raman maps reveal that the graphene grown depends on the surface morphology of the h‐BN substrates. The growth is governed by the high mobility of the carbon atoms on the h‐BN surface, in a manner that is consistent with van der Waals epitaxy. The successful growth of graphene layers depends on the substrate temperature, but is independent of the incident flux of carbon atoms.This work is supported by ONR (N000140610138 and Graphene MURI), AFOSR (FA9550‐11‐1‐ 0010), EFRC Center for Re‐Defining Photovoltaic Efficiency through Molecule Scale Control (award DE‐SC0001085), NSF (CHE‐0641523), NYSTAR, CSIC‐PIF (200950I154), Spanish CAM (Q&C Light (S2009ESP‐1503), Numancia 2 (S2009/ENE‐1477)) and Spanish MEC (ENE2009‐14481‐C02‐02, TEC2011‐29120‐C05‐04, MAT2011‐26534).Peer reviewe

Graphene growth on h-BN by molecular beam epitaxy

García Martínez, Jorge Manuel et al.Comunicación presentada en el 17th European Molecular Beam Epitaxy Workshop (EUROMBE), celebrado en Levi (Finlandia) del 10 al 13 de marzo de 2013.Graphene growth on dielectric substrates has potential to enable new kinds of devices and applications. We explore graphene growth via direct depositing carbon in a MBE environment on different dielectric substrates, such as h-BN and sapphire. The growth of single layer graphene nanometer size domains by solid carbon source molecular beam epitaxy on hexagonal boron nitride (h-BN) flakes is demonstrated [1]. Formation of single-layer graphene is clearly apparent in Raman spectra which display sharp optical phonon bands (see Fig. 1 (a)). Atomic-force microscope images and Raman maps (Fig. 2) reveal that the graphene grown depends on the surface morphology of the h-BN substrates. On h-BN substrates, high quality single layer growth occurs as nano-domains. The growth is governed by the high mobility of the carbon atoms on the h-BN surface, in a manner that is consistent with van der Waals epitaxy. The successful growth of graphene layers depends on the substrate temperature, but is independent of the incident flux of carbon atoms. We will show also results of graphene growth by MBE on sapphire, where large area growth occurs with monolayer thickness fluctuations.This work is supported by ONR (N000140610138 and Graphene MURI), AFOSR (FA9550-11-1-0010), EFRC Center for Re-Defining Photovoltaic Efficiency through Molecule Scale Control (award DE-SC0001085), NSF (CHE-0641523), NYSTAR, CSIC-PIF (200950I154), Spanish CAM (Q&C Light (S2009ESP-1503), Numancia 2 (S2009/ENE-1477)) and Spanish MEC (ENE2009-14481-C02-02, TEC2011-29120-C05-04, MAT2011-26534).Peer Reviewe

In(Ga)As self-assembled quantum ring formation by molecular beam epitaxy

The effect of growth conditions on the morphological properties of InAs/GaAs(001) quantum dots covered by a thin (<3 nm) GaAs cap has been studied by atomic force microscopy. Each dot turns into an elongated nanostructure at 540 °C upon deposition of the cap in As4 atmosphere, while structures with two humps are obtained when capping at 500 °C. The use of As2 atmosphere instead of As4 at 500 °C leads to the formation of quantum rings. Photoluminescence spectroscopy and polarization photoluminescence (PL) at 15 K show dramatic changes due to the different kinds of confinement. This allows the possibility of tailoring PL emission by controlling the size and shape.The authors would like to thank J. L. Costa Kramer for help during writing and Axel Lorke and Richard Warburton for fruitful discussions. This work was partially supported by Project No. 07T/0062/2000 of the Comunidad Autónoma de Madrid, Project No. TIC99-1035-C02, Nanoself Project No. TIC2002-04096-C03 and by Nanomat of the EC Growth Program, Contract no. G5RD-CT-2001-00545.Peer reviewe

Optical transitions and excitonic recombination in InAs/InP self-assembled quantum wires

InAs self-assembled quantum wire structures have been grown on InP substrates and studied by means of photoluminescence and polarized-light absorption measurements. According to our calculations, the observed optical transitions in each sample are consistent with wires of different heights, namely from 6 to 13 monolayers. The nonradiative mechanism limiting the emission intensity at room temperature is related to thermal escape of carriers out of the wires.This work has been supported under project No. TIC99-1035-C02 of the Spanish CICYT.Peer reviewe

Absorptive capacity from foreign direct investment in Spanish manufacturing firms

This paper deals with the determinants of absorptive capacity from foreign direct investment (FDI) spillovers. We study how firm behavior, capabilities, and structure drive absorptive capacity such as research and development (R&D) activities and expenditures, R&D results, internal organization of innovation, external relationships of innovation, human-capital quality, family management, business complexity, and market concentration. Our results enhance and complement previous evidence of the determinants of absorptive capacity, particularly with different approaches to innovative activities as mediators of the capability

Fast and robust phase-shift estimation in two-dimensional structured illumination microscopy

A method of determining unknown phase-shifts between elementary images in two-dimensional Structured Illumination Microscopy (2D-SIM) is presented. The proposed method is based on the comparison of the peak intensity of spectral components. These components correspond to the inherent structured illumination spectral content and the residual compo- nent that appears from wrongly estimated phase-shifts. The estimation of the phase-shifts is carried out by finding the absolute maximum of a function defined as the normalized peak intensity difference in the Fourier domain. This task is performed by an optimization method providing a fast estimation of the phase-shift. The algorithm stability and robustness are tested for various levels of noise and contrasts of the structured illumination pattern. Furthermore, the proposed approach reduces the number of computations compared to other existing techniques. The method is supported by the theoretical calculations and validated by means of simulated and experimental results

The selective reduction of NOx with propene on Pt-beta catalyst: a transient study

The mechanism of the NO/C3H6/O2 reaction has been studied on a Pt-beta catalyst using transient analysis techniques. This work has been designed to provide answers to the volcano-type activity behaviour of the catalytic system, for that reason, steady state transient switch (C3H6/NO/O2 → C3H6/Ar/O2, C3H6/Ar/O2 → C3H6/NO/O2, C3H6/NO/O2 → Ar/NO/O2, Ar/NO/O2 → C3H6/NO/O2, C3H6/NO/O2 → C3H6/NO/Ar and C3H6/NO/Ar → C3H6/NO/O2) and thermal programmed desorption (TPD) experiments were conducted below and above the temperature of the maximum activity (Tmax). Below Tmax, at 200 °C, a high proportion of adsorbed hydrocarbon exists on the catalyst surface. There exists a direct competition between NO and O2 for Pt free sites which is very much in favour of NO, and therefore, NO reduction selectively takes place over hydrocarbon combustion. NO and C3H6 are involved in the generation of partially oxidised hydrocarbon species. O2 is essential for the oxidation of these intermediates closing the catalytic cycle. NO2 is not observed in the gas phase. Above Tmax, at 230 °C, C3H6 ads coverage is negligible and the surface is mainly covered by Oads produced by the dissociative adsorption of O2. NO2 is observed in gas phase and carbon deposits are formed at the catalyst surface. From these results, the state of Pt-beta catalyst at Tmax is inferred. The reaction proceeds through the formation of partially oxidised active intermediates (CxHyOzNw) from C3H6 ads and NOads. The combustion of the intermediates with O2(g) frees the Pt active sites so the reaction can continue. Temperature has a positive effect on the surface reaction producing active intermediates. On the contrary, formation of NOads and C3H6 ads are not favoured by an increase in temperature. Temperature has also a positive effect on the dissociation of O2 to form Oads, consequently, the formation of NO2 is favoured by temperature through the oxygen dissociation. NO2 is very reactive and produces the propene combustion without NO reduction. These facts will determine the maximum concentration of active intermediates and consequently the maximum of activity.The authors thank the Spanish Ministry of Education and Science (project CTQ2005-01358) for the financial support

Size quantization effects in InAs self-assembled quantum dots

3 páginas, 3 figuras. PACS: 73.21.-b; 78.55.Cr; 78.66.Fd; 72.40.+w; 68.35.Ct; 73.40.KpWe study size quantization effects in InAs self-assembled quantum dots (QDs) that are embedded in GaAs. Using capacitance, photoluminescence and photovoltage spectroscopy, we correlate the measured quantized level energies with the quantum dot sizes and densities obtained from transmission electron microscopy. With increasing dot size, we observe a strong redshift of the QD features in all our data. In the capacitance spectra, a band gap renormalization of the two-dimensional wetting layer system appears when the first excited QD state crosses the wetting layer ground state. The relative size dependence and absolute energetic position of the QD transitions determined with photoluminescence provide some information about the influence of lateral confinement and height of the QD.This work was supported by QUEST, a NSF science and technology center (Grant No. DMR91-20007), the Alexander von Humboldt Foundation (KHS), and the Brazilian agency CNPq (GMR).Peer reviewe

Free surface energy evaluation in the laser texturing of a carbon steel s275

Currently, the manufacture of hybrid structures of dissimilar materials is generating great interest. These allow to combine mechanical properties of different materials to achieve a higher functional performance. To obtain these structures, different joining technologies are used, especially the use of adhesives. Nevertheless, for the correct application of an adhesive, the surface of the metal to be bonded must be prepared. Laser texturing has become a widely studied technology for this purpose because it allows very defined textures to be obtained. This technology allows to activate the surface and improve the application of the adhesive afterwards. In that sense, hydrophilic surfaces are generated and evaluated by wettability tests to know the surface free energy obtained in the texturing. In order to improve the bonding of dissimilar materials in the form of hybrid structure, a study has been carried out on the laser texturing of a carbon steel S275 and the relationship between the texturing parameters with its surface activation through wettability and surface free energy tests. Thus, the energy density obtained by the combination of power and scanning speed allows the generation of very defined textures that increase the surface activation of the steel. This has made it possible to establish a mathematical relationship between the texturing parameters and the results obtained. © 2022 The Authors