Material Physics Center

Donostia / San Sebastián, Spain

Material Physics Center

Donostia / San Sebastián, Spain
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Aguirregabiria G.,Material Physics Center | Marinica D.C.,University Paris - Sud | Esteban R.,Donostia International Physics Center | Esteban R.,Ikerbasque | And 4 more authors.
ACS Photonics | Year: 2017

The nonlinear response of metallic nanoparticles is obtained from quantum time dependent density functional theory calculations. Without any aprioristic assumption our calculations allow us to identify high-order harmonic generation in canonical plasmonic structures such as spherical single particles and dimers. Furthermore, we demonstrate that under currently available experimental conditions, the application of an external polarizing field to the nanoparticles allows to actively control even-order harmonic generation in otherwise symmetry forbidden situations. Our quantum calculations provide quantitative access to the high-order response of metallic nanoantennas, which is of utmost importance in the design, control, and exploitation of optoelectronic devices as well as in the generation of extreme ultraviolet radiation. © 2017 American Chemical Society.

Borca B.,Max Planck Institute for Solid State Research | Borca B.,National Institute of Materials Physics Bucharest | Michnowicz T.,Max Planck Institute for Solid State Research | Petuya R.,Donostia International Physics Center | And 16 more authors.
ACS Nano | Year: 2017

The ability to elucidate the elementary steps of a chemical reaction at the atomic scale is important for the detailed understanding of the processes involved, which is key to uncover avenues for improved reaction paths. Here, we track the chemical pathway of an irreversible direct desulfurization reaction of tetracenothiophene adsorbed on the Cu(111) closed-packed surface at the submolecular level. Using the precise control of the tip position in a scanning tunneling microscope and the electric field applied across the tunnel junction, the two carbon-sulfur bonds of a thiophene unit are successively cleaved. Comparison of spatially mapped molecular states close to the Fermi level of the metallic substrate acquired at each reaction step with density functional theory calculations reveals the two elementary steps of this reaction mechanism. The first reaction step is activated by an electric field larger than 2 V nm-1, practically in absence of tunneling electrons, opening the thiophene ring and leading to a transient intermediate. Subsequently, at the same threshold electric field and with simultaneous injection of electrons into the molecule, the exergonic detachment of the sulfur atom is triggered. Thus, a stable molecule with a bifurcated end is obtained, which is covalently bound to the metallic surface. The sulfur atom is expelled from the vicinity of the molecule. © 2017 American Chemical Society.

Marinica D.-C.,University Paris - Sud | Aizpurua J.,Material Physics Center | Aizpurua J.,Donostia International Physics Center | Borisov A.G.,University Paris - Sud | Borisov A.G.,Donostia International Physics Center
Optics Express | Year: 2016

We report a quantum mechanical study of the plasmonic response of bimetallic spherical core/shell nanoparticles. The systems comprise up to 104 electrons and their optical response is addressed with Time Dependent Density Functional Theory calculations. These quantum results are compared with classical electromagnetic calculations for core/shell systems formed by Al/Na, Al/Au and Ag/Na, as representative examples of bimetallic systems. We show that for shell widths in the nanometer range, the system cannot be described as a simple stack of two metals. The finite size effect and the transition layer formed between the core and the shell strongly modify the optical properties of the compound nanoparticle. In particular this configuration leads to a frequency shift of the plasmon resonance with shell character and an increased plasmon decay into electron-hole pairs which eventually quenches this resonance for very thin shells. This effect is difficult to capture with a classical theory even upon adjustment of the parameters of a combination of metallic dielectric functions. © 2016 Optical Society of America.

Borghetti P.,Catholic University of the Sacred Heart | Borghetti P.,Material Physics Center | Santo G.D.,Elettra - Sincrotrone Trieste | Castellarin-Cudia C.,Elettra - Sincrotrone Trieste | And 7 more authors.
Journal of Chemical Physics | Year: 2013

Due to the growing interest in the ferromagnetic properties of Fe-octaethylporphyrins (Fe-OEP) for applications in spintronics, methods to produce stable Fe-porphyrins with no Cl atoms are highly demanded. Here, we demonstrate the formation of Fe-OEP layers on Ag(111) single crystal by the ultra high vacuum in situ metalation of the free-base 2H-2,3,7,8,12,13,17,18- octaethylporphyrin (2H-OEP) molecules. The metalation proceeds exactly as in the case of 2H-5,10,15,20-tetraphenylporphyrin (2H-TPP) on the same substrate. An extensive surface characterization by means of X-ray photoemission spectroscopy, valence band photoemission, and NEXAFS with synchrotron radiation light provides information on molecular conformation and electronic structure in the monolayer and multilayer cases. We demonstrate that the presence of the ethyl groups affects the tilt of the adsorbed molecules, the conformation of the macrocycle, and the polarization screening in multilayers, but has only a minor effect in the metalation process with respect to 2H-TPP. © 2013 American Institute of Physics.

Zegkinoglou I.,University of Wisconsin - Madison | Zegkinoglou I.,Lawrence Berkeley National Laboratory | Cook P.L.,University of Wisconsin - Madison | Cook P.L.,University of Wisconsin-Superior | And 16 more authors.
Journal of Physical Chemistry C | Year: 2012

Highly doped diamond films are new candidates for electrodes in reactive environments, such as electrocatalytic interfaces. Here the electronic structure of such films is investigated by X-ray absorption spectroscopy at the C 1s and B 1s edges, combined with X-ray and ultraviolet photoelectron spectroscopy, as well as optical measurements. A diamond surface functionalized covalently with Ru(tpy) 2, a model complex similar to ruthenium-based molecules used in photocatalysis and photovoltaics, is compared to a hydrogen-terminated diamond surface as a reference. Bulk-sensitive absorption spectra with photon detection reveal diamond gap states, while surface-sensitive spectra with electron detection reveal the adsorbate states and π-bonding at the diamond surface. The positions of the frontier orbitals of the dye relative to the band edges of diamond are inferred from the spectroscopic data. The implications of using diamond films as inert electron donors in photocatalysis and dye-sensitized solar cells are discussed. © 2012 American Chemical Society.

Zazpe R.,CIC Nanogune | Ungureanu M.,CIC Nanogune | Golmar F.,CIC Nanogune | Golmar F.,CONICET | And 12 more authors.
Journal of Materials Chemistry C | Year: 2014

Resistance random access memory (ReRAM) is considered a promising candidate for the next generation of non-volatile memory. In this work, we fabricate Co/HfO2/Ti devices incorporating atomic-layer-deposited HfO 2 thin films as the active material grown under different atomic layer deposition (ALD) conditions. We focus on analyzing the effect of ALD conditions on the resistive switching behaviour of the devices. Electrical characterization reveals a particular non-crossing current-voltage curve and bipolar resistive switching behaviour. Device memory properties were confirmed by stability and retention measurements as well as voltage pulses, by which logical computational processes were conducted. X-ray photoelectron spectroscopy combined with electrical measurements demonstrates that the presence of Hf sub-oxides at the interface with the underlying Ti layer is required in order to achieve a stable switching device. The ability of Ti to scavenge oxygen from the HfO2 is shown to be affected by the ALD conditions. This journal is © the Partner Organisations 2014.

Pickup D.F.,Material Physics Center | Pickup D.F.,University of the Basque Country | Zegkinoglou I.,University of Wisconsin - Madison | Zegkinoglou I.,Lawrence Berkeley National Laboratory | And 17 more authors.
Journal of Physical Chemistry C | Year: 2013

To discover how molecular changes affect the electronic structure of dye molecules for solar cells, we have investigated four titanium phthalocyanines customized by axial and peripheral ligands (monodentate oxo versus bidentate catechol and tert-butyl versus tert-butylphenoxy, respectively). X-ray absorption spectroscopy and photoelectron spectroscopy were combined with density functional theory (DFT) and crystal-field multiplet calculations to characterize the Ti 3d and N 2p valence electrons that form the frontier orbitals. When a monodentate oxo axial ligand was replaced by a bidentate catechol ligand, the multiplet structure of the Ti 2p-to-3d transitions was found to change systematically. The most noticeable change was an additional transition into the low-lying 3dxy level, which is attributed to a reduction in local symmetry from 4-fold to 2-fold at the Ti center. An increase of the Ti 2p core-level binding energy was observed in the bidentate complex and compared to a calculated core-level stabilization. DFT predicts a change of the LUMO from the inner phthalocyanine ring to the Ti dxy orbital and a reversal of the high-lying dx 2-y2 and d z 2 orbitals. The N 1s edge was calculated using time-dependent density functional theory (TDDFT) and compared to experiment. © 2013 American Chemical Society.

Borca B.,Max Planck Institute for Solid State Research | Borca B.,National Institute of Materials Physics Bucharest | Schendel V.,Max Planck Institute for Solid State Research | Petuya R.,Donostia International Physics Center | And 11 more authors.
ACS Nano | Year: 2015

Single molecular switches are basic device elements in organic electronics. The pentacene analogue anthradithiophene (ADT) shows a fully reversible binary switching between different adsorption conformations on a metallic surface accompanied by a charge transfer. These transitions are activated locally in single molecules in a low-temperature scanning tunneling microscope. The switching induces changes between bistable orbital structures and energy level alignment at the interface. The most stable geometry, the "off" state, which all molecules adopt upon evaporation, corresponds to a short adsorption distance at which the electronic interactions of the acene rings bend the central part of the molecule toward the surface accompanied by a significant charge transfer from the metallic surface to the ADT molecules. This leads to a shift of the lowest unoccupied molecular orbital down to the Fermi level (EF). In the "on" state the molecule has a flat geometry at a larger distance from the surface; consequently the interaction is weaker, resulting in a negligible charge transfer with an orbital structure resembling the highest occupied molecular orbital when imaged close to EF. The potential barrier between these two states can be overcome reversibly by injecting charge carriers locally into individual molecules. Voltage-controlled current traces show a hysteresis characteristic of a bipolar switching behavior. The interpretation is supported by first-principles calculations. © 2015 American Chemical Society.

Strozecka A.,Free University of Berlin | Li J.,Free University of Berlin | Li J.,CIC Nanogune | Schurmann R.,Free University of Berlin | And 11 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

Nanocrystals can behave as quantum boxes with confined electronic states governing their optoelectronic properties. The formation of nanometer-size crystals of copper nitride (Cu3N) grown by nitrogen sputtering of a Cu(110) surface is reported. Scanning tunneling spectroscopy shows that the nanocrystals exhibit a series of well-defined sharp electronic resonances, which correspond to confined free-electron-like states. We observe that electrons from a scanning tunneling microscope tip induce the emission of light with a larger efficiency than on the bare metal surface. The spectral analysis of the emitted photons reveals various radiative inelastic pathways enabled by the confined states, which explain the enhanced light emission. Thus, the Cu3N nanocrystals can be employed as nanometer-size light sources. © 2014 American Physical Society.

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