Instrumentation Unit

Unit Leader: Gustavo Ceballos

Publications

2018

  • Bottom-up synthesis of multifunctional nanoporous graphene

    Moreno C., Vilas-Varela M., Kretz B., Garcia-Lekue A., Costache M.V., Paradinas M., Panighel M., Ceballos G., Valenzuela S.O., Peña D., Mugarza A. Science; 360 (6385): 199 - 203. 2018. 10.1126/science.aar2009. IF: 41.058

    Nanosize pores can turn semimetallic graphene into a semiconductor and, from being impermeable, into the most efficient molecular-sieve membrane. However, scaling the pores down to the nanometer, while fulfilling the tight structural constraints imposed by applications, represents an enormous challenge for present top-down strategies. Here we report a bottom-up method to synthesize nanoporous graphene comprising an ordered array of pores separated by ribbons, which can be tuned down to the 1-nanometer range. The size, density, morphology, and chemical composition of the pores are defined with atomic precision by the design of the molecular precursors. Our electronic characterization further reveals a highly anisotropic electronic structure, where orthogonal one-dimensional electronic bands with an energy gap of ∼1 electron volt coexist with confined pore states, making the nanoporous graphene a highly versatile semiconductor for simultaneous sieving and electrical sensing of molecular species. 2017 © The Authors


  • Characterization of Carbon-Contaminated B4C-Coated Optics after Chemically Selective Cleaning with Low-Pressure RF Plasma

    Moreno Fernández H., Rogler D., Sauthier G., Thomasset M., Dietsch R., Carlino V., Pellegrin E. Scientific Reports; 8 (1, 1293) 2018. 10.1038/s41598-018-19273-6. IF: 4.122

    Boron carbide (B4C) is one of the few materials that is expected to be most resilient with respect to the extremely high brilliance of the photon beam generated by free electron lasers (FELs) and is thus of considerable interest for optical applications in this field. However, as in the case of many other optics operated at light source facilities, B4C-coated optics are subject to ubiquitous carbon contaminations. Carbon contaminations represent a serious issue for the operation of FEL beamlines due to severe reduction of photon flux, beam coherence, creation of destructive interference, and scattering losses. A variety of B4C cleaning technologies were developed at different laboratories with varying success. We present a study regarding the low-pressure RF plasma cleaning of carbon contaminated B4C test samples via inductively coupled O2/Ar, H2/Ar, and pure O2 RF plasma produced following previous studies using the same ibss GV10x downstream plasma source. Results regarding the chemistry, morphology as well as other aspects of the B4C optical coating before and after the plasma cleaning are reported. We conclude that among the above plasma processes only plasma based on pure O2 feedstock gas exhibits the required chemical selectivity for maintaining the integrity of the B4C optical coatings. © 2018 The Author(s).


  • Control of the Polarization of Ferroelectric Capacitors by the Concurrent Action of Light and Adsorbates

    Liu F., Fina I., Sauthier G., Sánchez F., Rappe A.M., Fontcuberta J. ACS Applied Materials and Interfaces; 10 (28): 23968 - 23975. 2018. 10.1021/acsami.8b05751. IF: 8.097

    Ferroelectric perovskites hold promise of enhanced photovoltaic efficiency and photocatalytic activity. Consequently, the photoresponse of oxide ferroelectric thin films is an active field of research. In electrode/ferroelectric/electrode devices, internal charge in the ferroelectric, free charge in the electrodes, and buried adsorbates at interfaces combine to screen the ferroelectric polarization and to stabilize the polar state. Under illumination, photoinduced carriers and photodissociated adsorbates may disrupt the screening equilibrium, modifying the switchable polarization and altering its expected benefits. Here, we explore the photoresponse of BaTiO3 thin films in a capacitor geometry, focusing on the effects of visible illumination on the remanent polarization. By combining ferroelectric and X-ray photoelectron spectroscopy, we discover that photoreaction of charge-screening H2O-derived adsorbates at the buried metal-ferroelectric Pt/BaTiO3 interface plays an unexpected pivotal role, enabling a substantial modulation (up to 75%) of the switchable remanent polarization by light. These findings illustrate that the synergy between photochemistry and photovoltaic activity at the surface of a ferroelectric material can be exploited to tune photoferroelectric activity. © 2018 American Chemical Society.


  • Design, fabrication, and characterisation of wire grid polarizers for the deep UV spectral range

    Rodríguez-De Marcos L., Ong Bin L., Citra Asmara T., Heussler S.P., Guerrero A., Mas R., Borrise X., Breese M.B.H., Rusydi A. Proceedings of SPIE - The International Society for Optical Engineering; 10691 ( 1069124) 2018. 10.1117/12.2314459. IF: 0.000

    In this communication, we show preliminary results on transmissive TiO2 wire-grid polarizers (WGP) operating in the deep ultraviolet (DUV) range. WGP are devices based on strips of materials with large values of the modulus of the dielectric constant along with high absorption in the operational range. The merit function I is introduced as a new tool to find the optimum material for WGPs in a given spectral range. The experimental dielectric constant of TiO2 thin films deposited by pulsed laser deposition are obtained through spectroscopic ellipsometry, and the function indicates that TiO2 is the best candidate for WGP in the DUV range when it is compared with other oxides. Once the material selection for WGP is done, we present and compare two different design approaches for WGP: one using an effective medium theory for the periodic structure, and the second using finite-difference time-domain (FDTD) analysis. A prototype of WGP is fabricated by electron beam (e-beam) lithography followed by lift-off process; the topography of the sample is analyzed by AFM, and we found noticeable deviations in the grating from the designed values. In preliminary characterization work the effective dielectric constant in two perpendicular orientations is obtained by ellipsometry and the contrast is compared with the design. © 2018 COPYRIGHT SPIE.


  • Geometric frustration in a hexagonal lattice of plasmonic nanoelements

    Conde-Rubio A., Rodríguez A.F., Borrisé X., Perez-Murano F., Batlle X., Labarta A. Optics Express; 26 (16): 20211 - 20224. 2018. 10.1364/OE.26.020211. IF: 3.356

    We introduce the concept of geometric frustration in plasmonic arrays of nanoelements. In particular, we present the case of a hexagonal lattice of Au nanoasterisks arranged so that the gaps between neighboring elements are small and lead to a strong near-field dipolar coupling. Besides, far-field interactions yield higher-order collective modes around the visible region that follow the translational symmetry of the lattice. However, dipolar excitations of the gaps in the hexagonal array are geometrically frustrated for interactions beyond nearest neighbors, yielding the destabilization of the low energy modes in the near infrared. This in turn results in a slow dynamics of the optical response and a complex interplay between localized and collective modes, a behavior that shares features with geometrically frustrated magnetic systems. © 2018 Optical Society of America.


  • On-surface synthesis of superlattice arrays of ultra-long graphene nanoribbons

    Moreno C., Paradinas M., Vilas-Varela M., Panighel M., Ceballos G., Peña D., Mugarza A. Chemical Communications; 54 (68): 9402 - 9405. 2018. 10.1039/c8cc04830d. IF: 6.290

    We report the on-surface synthesis of graphene nanoribbon superlattice arrays directed by the herringbone reconstruction of the Au(111) surface. The uniaxial anisotropy of the zigzag pattern of the reconstruction defines a one dimensional grid for directing the Ullmann polymerization and inducing periodic arrays of parallel ultra-long nanoribbons (>100 nm), where the periodicity is varied with coverage at discrete values following a hierarchical templating behavior. © 2018 The Royal Society of Chemistry.


2017

  • Electric-Field-Adjustable Time-Dependent Magnetoelectric Response in Martensitic FeRh Alloy

    Fina I., Quintana A., Padilla-Pantoja J., Martí X., Macià F., Sánchez F., Foerster M., Aballe L., Fontcuberta J., Sort J. ACS Applied Materials and Interfaces; 9 (18): 15577 - 15582. 2017. 10.1021/acsami.7b00476. IF: 7.504

    Steady or dynamic magnetoelectric response, selectable and adjustable by only varying the amplitude of the applied electric field, is found in a multiferroic FeRh/PMN-PT device. In-operando time-dependent structural, ferroelectric, and magnetoelectric characterizations provide evidence that, as in magnetic shape memory martensitic alloys, the observed distinctive magnetoelectric responses are related to the time-dependent relative abundance of antiferromagnetic-ferromagnetic phases in FeRh, unbalanced by voltage-controlled strain. This flexible magnetoelectric response can be exploited not only for energy-efficient memory operations but also in other applications, where multilevel and/or transient responses are required. © 2017 American Chemical Society.


  • Epitaxial Growth of SrTiO3 Films on Cube-Textured Cu-Clad Substrates by PLD at Low Temperature Under Reducing Atmosphere

    Padilla J.A., Xuriguera E., Rodríguez L., Vannozzi A., Segarra M., Celentano G., Varela M. Nanoscale Research Letters; 12 (1, 226) 2017. 10.1186/s11671-017-1997-9. IF: 2.833

    The growth of epitaxial {001}<100> SrTiO3 (STO) on low-cost cube-textured Cu-based clad substrate at low temperature was carried out by means of pulsed laser deposition (PLD). STO film was deposited in one step under a reducing atmosphere (5% H2 and 95% Ar mixture) to prevent the oxidation of the metal surface. The optimization of PLD parameters leads to a sharpest biaxial texture at a temperature as low as 500 °C and a thickness of 500 nm with a (100) STO layer. The upper limit of highly textured STO thickness was also investigated. The maximum thickness which retains the best quality {001}<100> texture is 800 nm, since the texture is preserved not only through the layer but also on the surface. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements showed that STO films are continuous, dense, and smooth with very low roughness (between 5 and 7 nm). This paper describes the development of STO layer by means of PLD in absence of oxygen throughout the process, suggesting an alternative and effective method for growing highly {001}<100> textured STO layer on low-cost metal substrates. © 2017, The Author(s).


  • Low-pressure RF remote plasma cleaning of carbon-contaminated B4C-coated optics

    Moreno Fernández H., Thomasset M., Sauthier G., Rogler D., Dietsch R., Barrett R., Carlino V., Pellegrin E. Proceedings of SPIE - The International Society for Optical Engineering; 10236 ( 102360E) 2017. 10.1117/12.2269374. IF: 0.000

    Boron carbide (B4C)-due to its exceptional mechanical properties-is one of the few existing materials that can withstand the extremely high brilliance of the photon beam from free electron lasers (FELs) and is thus of considerable interest for optical applications in this field. However, as in the case of many other optics operated at modern accelerator-, plasma-, or laser-based light source facilities, B4C-coated optics are subject to ubiquitous carbon contaminations. These contaminations-that are presumably produced via cracking of CHx and CO2 molecules by photoelectrons emitted from the optical components-represent a serious issue for the operation of the pertinent high performance beamlines due to a severe reduction of photon flux and beam coherence, not necessarily restricted to the photon energy range of the carbon K-edge. Thus, a variety of B4C cleaning technologies have been developed at different laboratories with varying success [1]. Here, we present a study regarding the low-pressure RF plasma cleaning of a series of carbon-contaminated B4C test samples via an inductively coupled O2/Ar and Ar/H2 remote RF plasma produced using the IBSS GV10x plasma source following previous studies using the same RF plasma source [2, 3]. Results regarding the chemistry, morphology as well as other aspects of the B4C optical coatings and surfaces before and after the plasma cleaning process are reported. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.


  • Symmetry forbidden morphologies and domain boundaries in nanoscale graphene islands

    Parreiras S.O., Gastaldo M., Moreno C., Martins M.D., Garcia-Lekue A., Ceballos G., Paniago R., Mugarza A. 2D Materials; 4 (2, 025104) 2017. 10.1088/2053-1583/aa70fa. IF: 6.937

    The synthesis of graphene nanoislands with tailored quantum properties requires an atomic control of the morphology and crystal structure. As one reduces their size down to the nanometer scale, domain boundary and edge energetics, as well as nucleation and growth mechanisms impose different stability and kinetic landscape from that at the microscale. This offers the possibility to synthesize structures that are exclusive to the nanoscale, but also calls for fundamental growth studies in order to control them. By employing high-resolution scanning tunneling microscopy we elucidate the atomic stacking configurations, domain boundaries, and edge structure of graphene nanoislands grown on Ni(1 1 1) by CVD and post-annealed at different temperatures. We find a non-conventional multistep mechanism that separates the thermal regimes for growth, edge reconstruction, and final stacking configuration, leading to nanoisland morphologies that are incompatible with their stacking symmetry. Whole islands shift their stacking configuration during cooling down, and others present continuous transitions at the edges. A statistical analysis of the domain structures obtained at different annealing temperatures reveals how polycrystalline, ill-defined structures heal into shape-selected islands of a single predominant stacking. The high crystallinity and the control on morphology and edge structure makes these graphene nanoislands ideal for their application in optoelectronics and spintronics. © 2017 IOP Publishing Ltd.


2015

  • Substrate-induced stabilization and reconstruction of zigzag edges in graphene nanoislands on Ni(111)

    Garcia-Lekue A., Olle M., Sanchez-Portal D., Palacios J.J., Mugarza A., Ceballos G., Gambardella P. Journal of Physical Chemistry C; 119 (8): 4072 - 4078. 2015. 10.1021/jp511069y. IF: 4.772

    We combine experimental observations by scanning tunneling microscopy (STM) and density functional theory (DFT) to reveal the most stable edge structures of graphene on Ni(111) as well as the role of stacking-driven activation and suppression of edge reconstruction. Depending on the position of the outermost carbon atoms relative to hollow and on-top Ni sites, zigzag edges have very different energies. Triangular graphene nanoislands are exclusively bound by the more stable zigzag hollow edges. In hexagonal nanoislands, which are constrained by geometry to alternate zigzag hollow and zigzag top edges along their perimeter, only the hollow edge is stable, whereas the top edges spontaneously reconstruct into the (57) pentagon-heptagon structure. Atomically resolved STM images are consistent with either top-fcc or top-hcp epitaxial stacking of graphene and Ni sites, with the former being favored by DFT. Finally, we find that there is a one-to-one relationship between the edge type, graphene stacking, and orientation of the graphene islands. © 2015 American Chemical Society.


2014

  • Atomic monolayer deposition on the surface of nanotube mechanical resonators

    Tavernarakis, A.; Chaste, J.; Eichler, A.; Ceballos, G.; Gordillo, M.C.; Boronat, J.; Bachtold, A. Physical Review Letters; 2014. 10.1103/PhysRevLett.112.196103. IF: 7.728


  • Spin tuning of electron-doped metal-phthalocyanine layers

    Stepanow, S.; Lodi Rizzini, A.; Krull, C.; Kavich, J.; Cezar, J.C.; Yakhou-Harris, F.; Sheverdyaeva, P.M.; Moras, P.; Carbone, C.; Ceballos, G.; Mugarza, A.; Gambardella, P. Journal of the American Chemical Society; 136 (14): 5451 - 5459. 2014. 10.1021/ja501204q. IF: 11.444


  • Spin-dependent electron scattering at graphene edges on Ni(111)

    Garcia-Lekue, A.; Balashov, T.; Olle, M.; Ceballos, G.; Arnau, A.; Gambardella, P.; Sanchez-Portal, D.; Mugarza, A. Physical Review Letters; 2014. 10.1103/PhysRevLett.112.066802. IF: 7.728


2012

  • A nanomechanical mass sensor with yoctogram resolution

    Chastem J. ; Eichler, A.; Moser, J.; Ceballos, G.; Rurali, R.; Bachtold, A. Nature Nanotechnology; 7: 301 - 304. 2012. .


  • Yield and Shape Selection of Graphene Nanoislands Grown on Ni(111)

    Olle, M. ; Ceballos, G.; Serrate, D.; Gambardella, P. Nano Letters; 12(9): 4431 - 4436. 2012. DOI: 10.1021/nl300897m.


2011

  • Localization, splitting, and mixing of field emission resonances induced by alkali metal clusters on Cu(100)

    Stepanow, S.; Mugarza, A.; Ceballos, G.; Gambardella, P.; Aldazabal, I.; Borisov, A.G.; Arnau, A. Physical Review B - Condensed Matter and Materials Physics; 83 2011. 10.1103/PhysRevB.83.115101.


  • Mixed-valence behavior and strong correlation effects of metal phthalocyanines adsorbed on metals

    Stepanow, S.; Miedema, P. S.; Mugarza, A.; Ceballos, G. ; Moras, P. ; Cezar, J. C.; Carbone, C. ; de Groot, F. M. F.; Gambardella, P. Physical Review B; 2011. .


  • Spin coupling and relaxation inside molecule-metal contacts.

    Mugarza, A.; Krull, C.; Robles, R.; Stepanow, S.; Ceballos, G.; Gambardella, P. Nature Communications; 2 2011. 10.1038/ncomms1497.


2010

  • Giant spin and orbital moment anisotropies of a Cu-phthalocyanine monolayer

    Stepanow, S.; Mugarza, A.; Ceballos, G.; Moras, P.; Cezar, J.C.; Carbone, C.; Gambardella, P. Physical Review B - Condensed Matter and Materials Physics; 82 2010. 10.1103/PhysRevB.82.014405.


  • Orbital specific chirality and homochiral self-assembly of achiral molecules induced by charge transfer and spontaneous symmetry breaking

    Mugarza A., Lorente N., Ordejón P., Krull C., Stepanow S., Bocquet M.-L., Fraxedas J., Ceballos G., Gambardella P. Physical Review Letters; 105 (11, 115702) 2010. 10.1103/PhysRevLett.105.115702.

    We study the electronic mechanisms underlying the induction and propagation of chirality in achiral molecules deposited on surfaces. Combined scanning tunneling microscopy and ab initio electronic structure calculations of Cu-phthalocyanines adsorbed on Ag(100) reveal the formation of chiral molecular orbitals in structurally undistorted molecules. This effect shows that chirality can be manifest exclusively at the electronic level due to asymmetric charge transfer between molecules and substrate. Single molecule chirality correlates with attractive van der Waals interactions, leading to the propagation of chirality at the supramolecular level. Ostwald ripening provides an efficient pathway for complete symmetry breaking and self-assembly of homochiral supramolecular layers. © 2010 The American Physical Society.


2007

  • Insulating ground state of Sn/Si(111)-(¿30 x¿30)R30°

    S. Modesti; L. Petaccia; G. Ceballos; I. Vobornik; G. Panaccione; G. Rossi; L. Ottaviano; R. Larciprete; S. Lizzit; A. Goldoni Physical Review Letters; 2007. .


  • Role of the electric field in surface electron dynamics above the vacuum level

    J. I. Pascual; C. Corriol; G. Ceballos; I. Aldazabal; H.-P. Rust; K. Horn; J. M. Pitarke; P. M. Echenique; A. Arnau Physical Review B; 2007. .