Ab-initio density functional study of O on the Ag(001) surface

The adsorption of oxygen on the Ag(001) is investigated by means of density functional techniques. Starting from a characterization of the clean silver surfaces oxygen adsorption in several modifications (molecularly, on-surface, sub-surface, Ag$_2$O) for varying coverage was studied. Besides structural parameters and adsorption energies also work-function changes, vibrational frequencies and core level energies were calculated for a better characterization of the adsorption structures and an easier comparison to the rich experimental data.Comment: 26 pages, 8 figures, Surf. Sci. accepte

Second harmonic generation on self-assembled GaAs/Au nanowires with thickness gradient

Here we investigated the SH generation at the wavelength of 400 nm (pump laser at 800 nm, 120 fs pulses) of a "metasurface" composed by an alternation of GaAs nano-grooves and Au nanowires capping portions of flat GaAs. The nano-grooves depth and the Au nanowires thickness gradually vary across the sample. The samples are obtained by ion bombardment at glancing angle on a 150 nm Au mask evaporated on a GaAs plane wafer. The irradiation process erodes anisotropically the surface, creating Au nanowires and, at high ion dose, grooves in the underlying GaAs substrate (pattern transfer). The SHG measurements are performed for different pump linear polarization angle at different positions on the "metasurface" in order to explore the regions with optimal conditions for SHG efficiency. The pump polarization angle is scanned by rotating a half-wave retarder plate. While the output SH signal in reflection is analyzed by setting the polarizer in s or p configuration in front of the detector. The best polarization condition for SHG is obtained in the configuration where the pump and second harmonic fields are both p polarized, and the experiments show a SH polarization dependence of the same symmetry of bulk GaAs. Thus, the presence of gold contributes only as field localization effect, but do not contributes directly as SH generator

Conditions and mechanism for the formation of iron-rich Montmorillonite in deep sea sediments (Costa Rica margin): Coupling high resolution mineralogical characterization and geochemical modeling

International audienceIron-rich smectite is commonly described in the diagenetic fraction of deep-sea sediment, as millimeter to centimeter aggregates dispersed in the sediment, or as a coating on sedimentary particles or nodules. This study examines several factors to elucidate formation mechanisms of a particular iron-rich smectite and its potential transformation to glauconite. The study combines a detailed mineralogical investigation on natural samples and a chemical modeling approach to assess mineralogical reactions and pathways. Transmission electron microscopy (TEM) observations and analytical electron microscopy (TEM-AEM) analyses were conducted on microtomed samples of millimeter- to centimeter-long green grains. These grains are widespread in pelagic calcareous sediment from the Costa Rica margin. They are composed of pyrites that are partially dissolved and are surrounded by amorphous or very poorly crystallized iron-rich particles. Iron-rich montmorillonite grows from an amorphous precursor and its formation requires the input of Si, O, Mg, K, Na and Ca; our results suggest that these inputs are supported by the dissolution of sedimentary phases such as volcanic glasses, siliceous fossils and silicates. Thermodynamic modeling of fluid-sediment interactions was conducted with the geochemical computer code PhreeqC, using mineralogical and pore fluid compositions from sediment samples and calculated estimates for thermodynamic constants of smectites that are not maintained by the computer code. Simulations confirm the possibility that the green grains are the product of pyrite alteration by seawater under oxidizing conditions. The extent of smectite production is controlled by the kinetics of pyrite dissolution and fluid migration. The absence of aluminum in the Costa Rica margin system explains the formation of an iron-rich montmorillonite instead of glauconite, whereas the presence of calcite that buffers the system explains the formation of an iron-rich montmorillonite instead of iron oxides

Zinc-rich clays in supergene non-sulfide zinc deposits

International audienceThe nature and the origin of zinc clays are poorly understood. With the example of the Bou Arhous Zn-Pb ore deposit in the Moroccan High Atlas, this study presents new data for the mineralogical and chemical characterization of barren and zinc clays associated with non-sulfide zinc ores. In the field, white to ocher granular clays are associated with willemite (Zn2SiO4), while red clays fill karst-related cavities cutting across the non-sulfide ore bodies. Red clays (kaolinite, chlorite, illite, and smectite) present evidence of stratification that reflects internal sedimentation processes during the karst evolution. White clays contain 7-Å clay mineral/smectite irregular interstratified minerals with less than 20 % of smectite layers. Willemite is partially dissolved and is surrounded by authigenic zinc clay minerals. Together with XRD results, WDS analyses on newly formed clay aggregates suggest that this interstratified mineral is composed of fraipontite and sauconite. CEC measurements support that zinc is only located within the octahedral sheets. These new results support the following process: (i) dissolution of willemite, leading to release of Si and Zn, (ii) interaction between Zn-Si-rich solutions and residual-detrital clays, and (iii) dissolution of kaolinite and formation of interstratified zinc clay minerals that grew over detrital micas

Characterization of metalliferous sediment from a low-temperature hydrothermal environment on the Eastern Flank of the East Pacific Rise

Metalliferous deposits are described from the eastern flank of the East Pacific Rise (EPR) offshore Costa Rica, close to a basaltic seamount called “Dorado high”. Based on heat-flow data and porewater profiles, the site is an area of active low-temperature hydrothermal discharge. We focus on the mineralogical and chemical analysis from a 124 cm long gravity core (GC50), located on the northwestern slope of the 100 m high Dorado. In this core, the sediments consist of detrital clay minerals as well as authigenic minerals such as zeolites, apatites, and Fe/Mn-rich oxyhydroxides. In contrast, the reference sediments from adjacent areas without hydrothermal activity are olive gray hemipelagic muds composed of volcanic glass particles, clay minerals, siliceous microfossils, and some detrital quartz and feldspar. Bulk sediment chemistry and chemical enrichment factors calculated with respect to the reference sediment indicate that the most important chemical changes occurred at the base of the core from 100 to 124 cm bsf, with strong enrichments in MnO, CaO, P2O5, and Fe2O3. These enrichments are correlated with the occurrence of authigenic Fe-oxyhydroxide (goethite) and Mn oxide (todorokite and vernadite, at 100 cm bsf), and hydrothermal apatite (110–124 cm bsf). In the upper section of the core from 12 to 70 cm, the sediment is composed of abundant smectite and authigenic phillipsite, and only minor chemical changes can be observed with respect to the reference sediments. The ubiquitous presence of phillipsite suggests that the entire sedimentary column of core GC50 was first affected by diagenesis. However, below 70 cm bsf, these phillipsites are partially dissolved and Fe oxides occur from 110 to 124 cm, followed upward by Mn oxides at 100 cm. This transition from Fe to Mn-rich sediments can be interpreted in terms of an upward increasing redox potential. PAAS-normalized REY patterns of GC50 sediments present clearly negative Ce and positive Y anomalies inherited from seawater at the base of core GC50. These anomalies decrease upward, which we interpret together with the transition from Fe to Mn-rich sediments by an upward migrating low-temperature hydrothermal fluid. Thus, after a first stage of diagenesis, the discharge of a low-temperature hydrothermal fluid occurred through the sedimentary column, leading to the precipitation of hydrothermal compounds that are lacking towards the surface

Deterministic Thermal Sculpting of Large-Scale 2D Semiconductor Nanocircuits

Two-dimensional (2D) Transition Metal Dichalcogenide semiconductor (TMDs) nanocircuits are deterministically engineered over large-scale substrates. The original approach combines large-area physical growth of 2D TMDs layer with high resolution thermal - Scanning Probe Lithography (t-SPL), to reshape the ultra-thin semiconducting layers at the nanoscale level. We demonstrate the additive nanofabrication of few-layer MoS2 nanostructures, grown in the 2H-semiconducting TMD phase, as shown by their Raman vibrational fingerprints and by their optoelectronic response. The electronic signatures of the MoS2 nanostructures are locally identified by Kelvin probe force microscopy providing chemical and compositional contrast at the nanometer scale. Finally, the potential role of the 2D TMD nanocircuits as building blocks of deterministic 2D semiconducting interconnections is demonstrated by high-resolution local conductivity maps showing the competitive transport properties of these large-area nanolayers. This work thus provides a powerful approach to scalable nanofabrication of 2D nano-interconnects and van der Waals heterostructures, and to their integration in real-world ultra-compact electronic and photonic nanodevices.Comment: 17 pages, 4 figure

Self-Organized Nanogratings for Large-Area Surface Plasmon Polariton Excitation and Surface-Enhanced Raman Spectroscopy Sensing

Surface plasmon polaritons (SPP) are exploited due to their intriguing properties for the fabrication and miniaturization of photonic circuits, for surface-enhanced spectroscopy and imaging beyond the diffraction limit. However, excitation of these plasmonic modes by direct illumination is forbidden by energy/momentum conservation rules. One strategy to overcome this limitation relies on diffraction gratings to match the wavevector of the incoming photons with that of propagating SPP excitations. The main limit of the approaches so far reported in the literature is that they rely on highly ordered diffraction gratings fabricated by means of demanding nanolithographic processes. In this work, we demonstrate that an innovative, fully self-organized method based on wrinkling-assisted ion-beam sputtering can be exploited to fabricate large-area (cm2 scale) nanorippled soda lime templates, which conformally support ultrathin Au films deposited by physical deposition. The self-organized patterns act as quasi-one-dimensional (1D) gratings characterized by a remarkably high spatial order, which properly matches the transverse photon coherence length. The gratings can thus enable the excitation of hybrid SPP modes confined at the Au/dielectric interfaces, with a resonant wavelength that can be tuned by modifying the grating period, photon incidence angle, or, potentially, the choice of the thin-film conductive material. Surface-enhanced Raman scattering experiments show promising gains in the range of 103, which are competitive, even before a systematic optimization of the sample fabrication parameters, with state-of-the art lithographic systems, demonstrating the potential of such templates for a broad range of optoelectronic applications aiming at plasmon-enhanced photon harvesting for molecular or biosensing

Controlling resonant surface modes by arbitrary light induced optical anisotropies

In this work the sensitivity of Bloch Surface Waves to laser-induced anisotropy of azo-polymeric thin layers is expe rimentally shown . The nanoscale reshaping of the films via thermal-Scanning Probe Lithography allows to couple light to circular photonic nanocavities, tailoring on-demand resonant BSW confined within the nanocavity

Free-standing plasmonic nanoarrays for leaky optical waveguiding and sensing

Flat optics nanogratings supported on thin free-standing membranes offer the opportunity to combine narrowband waveguided modes and Rayleigh anomalies for sensitive and tunable biosensing. At the surface of high-refractive index Si3N4 membranes we engineered lithographic nanogratings based on plasmonic nanostripes, demonstrating the excitation of sharp waveguided modes and lattice resonances. We achieved fine tuning of these optical modes over a broadband Visible and Near-Infrared spectrum, in full agreement with numerical calculations. This possibility allowed us to select sharp waveguided modes supporting strong near-field amplification, extending for hundreds of nanometres out of the grating and enabling versatile biosensing applications. We demonstrate the potential of this flat-optics platform by devising a proof-of-concept nanofluidic refractive index sensor exploiting the long-range waveguided mode operating at the sub-picoliter scale. This free-standing device configuration, that could be further engineered at the nanoscale, highlights the strong potential of flat-optics nanoarrays in optofluidics and nanofluidic biosensing. (C) 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreemen