Abner de Siervo

Lithium insertion and electrochromism in polycrystalline molybdenum oxide films

In this work, molybdenum oxide thin films were deposited by r.f. reactive sputtering of metallic molybdenum target in an Ar + O 2 atmosphere. Thin films with different compositions and crystal structures were obtained by varying the oxygen flow (o). All samples, with the exception of the MoO 2 film, showed an electrochromic behavior, due to the reversible Li + /e insertion or extraction process. The transmittance change (λ = 632.8 nm), for the best sample, was ∼ 70%. The film having the best optical behavior is mostly formed by the β-MoO 3 phase. The composition, optical and mechanical properties were determined and analyzed in as-grown and in Li intercalated films. The local order structure was analyzed by X-ray absorption spectroscopy measurements at the Mo L 2,3 -edges. The valence band was studied by Photoelectron Spectroscopy.

Robust perpendicular exchange coupling in an ultrathin CoO/PtFe double layer: Strain and spin orientation

We report on the exchange coupling and magnetic properties of a strained ultrathin CoO/PtFe double-layer with perpendicular magnetic anisotropy. The cobalt oxide growth by reactive molecular beam epitaxy on a Pt-terminated PtFe/Pt(001) surface gives rise to an hexagonal surface and a monoclinic distorted CoO 3nm film at room temperature. This distorted ultrathin CoO layer couples with the PtFe(001) layer establishing a robust perpendicular exchange bias shift. Soft x-ray absorption spectroscopy provides a full description of the spin orientations in the CoO/PtFe doublelayer. The exchange bias shift is preserved up to the Neel antiferromagnetic ordering temperature of TN=293 K. This unique example of selfsame value for blocking and ordering temperatures, yet identical to the bulk ordering temperature, is likely related to the original strain induced distortion and strengthened interaction between the two well-ordered spin layers.

Observation of ferromagnetism in PdCo alloy nanoparticles encapsulated in carbon nanotubes

Carbon nanotubes terminated by PdCo catalyst nanoparticles were grown by plasma-enhanced chemical vapor deposition. Transmission electron microscopy reveals that these nanoparticles have a droplike shape and are completely encapsulated inside multiwalled nanotubes. Magnetization measurements showed the existence of a permanent magnetization with a small shape anisotropy effect. The magnetization of both cobalt and palladium was confirmed by x-ray magnetic circular dichroism. These results show that nanotubes ended by magnetic PdCo nanoparticles can be grown and could be used in magnetic storage media and electrical spin injection.

Electronic And Structural Study Of Pt-modified Au Vicinal Surfaces: A Model System For Pt-au Catalysts.

Two single crystalline surfaces of Au vicinal to the (111) plane were modified with Pt and studied using scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) in ultra-high vacuum environment. The vicinal surfaces studied are Au(332) and Au(887) and different Pt coverage (θPt) were deposited on each surface. From STM images we determine that Pt deposits on both surfaces as nanoislands with heights ranging from 1 ML to 3 ML depending on θPt. On both surfaces the early growth of Pt ad-islands occurs at the lower part of the step edge, with Pt ad-atoms being incorporated into the steps in some cases. XPS results indicate that partial alloying of Pt occurs at the interface at room temperature and at all coverage, as suggested by the negative chemical shift of Pt 4f core line, indicating an upward shift of the d-band center of the alloyed Pt. Also, the existence of a segregated Pt phase especially at higher coverage is detected by XPS. Sample annealing indicates that the temperature rise promotes a further incorporation of Pt atoms into the Au substrate as supported by STM and XPS results. Additionally, the catalytic activity of different PtAu systems reported in the literature for some electrochemical reactions is discussed considering our findings.

From quenched to unquenched orbital magnetic moment on metallic@oxide nanoparticles: dc magnetic properties and electronic correlation

In this study, the correlation between magnetic, structure, and electronic properties of Ag@Fe3O4 hetero nanostructures are presented. These nanostructures were prepared using a two-step new chemical approach. Three different nanoparticle systems with different Ag concentrations have been prepared and characterized using high resolution transmission electron microscopy, dc magnetization (magnetization and coercive field as a function of temperature), X-ray absorption near edge spectroscopy, and magnetic circular dichroism studies (XMCD). From the correlation between XMCD and dc magnetic measurements (Verwey transition) the presence of non-stoichiometric magnetite in Ag@Fe3O4 nanoparticle systems was confirmed. From the spin and orbital contribution to the total magnetic moment, we conclude that the sample with less Ag seeds particle concentration presents a non-quenched orbital contribution. These phenomena were analyzed based on the actual models and correlated with dc magnetic properties. From these, we conclude that the enhancement on the orbital contribution increases the spin orbital interaction, also increasing the magnetocrystalline anisotropy reflected on the dc magnetic properties.

L123M45M45 Auger satellites of the 4d metals: shake-up or Coster–Kronig?

Abstract Interpretation of the satellites of the Cu L 2 , 3 M 4 , 5 M 4 , 5 Auger spectrum has involved considerable controversy in recent years. Reaching unambiguous conclusions is impeded by the contribution of Coster–Kronig (CK) transitions, requiring considerable experimental investigation before clarifying the relative roles of shake-up and CK processes. The L 2 , 3 M 4 , 5 M 4 , 5 spectra of Ag, Pd and Rh excited by bremsstrahlung also have satellite features and manifest experimental systematics consistent with 4 d spectator vacancies produced by shake-up. The contribution of CK processes in the formation of these satellites, which, in this case, would have to involve the L 1 level, is consistent with theoretical predictions, however. A direct determination of the role of CK processes is necessary, therefore, to resolve this issue. One possibility would be to measure the L 1 M 4 , 5 M 4 , 5 spectum directly: were CK processes dominant, the spectrum should manifest no satellite. Direct measurement is difficult with bremsstrahlung excitation, however, because of the transitions low intensity. We report measurements of this spectrum in Ag, Pd and Rh and in Pd–Ag alloys using a Ti anode and demonstrate the presence of a satellite which is consistent in energy position and relative intensity with those of the L 2 , 3 MM satellites. This result supports the interpretation of these satellites as shake-up in nature.

Experimental and computational investigation of reduced graphene oxide nanoplatelets stabilized in poly(styrene sulfonate) sodium salt

The production of large-area interfaces and the use of scalable methods to build up designed nanostructures generating advanced functional properties are of high interest for many materials science applications. Nevertheless, large-area coverage remains a major problem even for pristine graphene, and here we present a hybrid, composite graphene-like material soluble in water that can be exploited in many areas such as energy storage, electrodes fabrication, selective membranes and biosensing. Graphene oxide (GO) was produced by the traditional Hummers’ method being further reduced in the presence of poly(styrene sulfonate) sodium salt (PSS), thus creating stable reduced graphene oxide (rGO) nanoplatelets wrapped by PSS (GPSS). Molecular dynamics simulations were carried out to further clarify the interactions between PSS molecules and rGO nanoplatelets, with calculations supported by Fourier transform infrared spectroscopy analysis. The intermolecular forces between rGO nanoplatelets and PSS lead to the formation of a hybrid material (GPSS) stabilized by van der Waals forces, allowing the fabrication of high-quality layer-by-layer (LbL) films with poly(allylamine hydrochloride) (PAH). Raman and electrical characterizations corroborated the successful modifications in the electronic structures from GO to GPSS after the chemical treatment, resulting in (PAH/GPSS) LbL films four orders of magnitude more conductive than (PAH/GO).

Surface Structure Determination of Black Phosphorus Using Photoelectron Diffraction

Atomic structure of single-crystalline black phosphorus was studied by high resolution synchrotron-based photoelectron diffraction (XPD). The results show that the topmost phosphorene layer in the black phosphorus is slightly displaced compared to the bulk structure and presents a small contraction in the direction perpendicular to the surface. Furthermore, the XPD results show the presence of a small buckling among the surface atoms, in agreement with previously reported scanning tunneling microscopy results. The contraction of the surface layer added to the presence of the buckling indicates an uniformity in the size of the sp3 bonds between P atoms at the surface.

Controlling the Self-Metalation Rate of Tetraphenylporphyrins on Cu(111) via Cyano Functionalization

The reaction rate of the self-metalation of free-base tetraphenylporphyrins (TPPs) on Cu(111) increases with the number of cyano groups (n=0, 1, 2, 4) attached at the para positions of the phenyl rings. The findings are based on isothermal scanning tunneling microscopy (STM) measurements. At room temperature, all investigated free-base TPP derivatives adsorb as individual molecules and are aligned with respect to densely packed Cu substrate rows. Annealing at 400 K leads to the formation of linear dimers and/or multimers via CN-Cu-CN bonds, accompanied by self-metalation of the free-base porphyrins following a first-order rate equation. When comparing the non-cyano-functionalized and the tetracyano-functionalized molecules, we find a decrease of the reaction rate by a factor of more than 20, corresponding to an increase of the activation energy from 1.48 to 1.59 eV. Density functional theory (DFT) calculations give insights into the influence of the peripheral electron-withdrawing cyano groups and explain the experimentally observed effects.

Estudo dos mecanismos de formação de nanofios moleculares de Tetra-Fenil-Ciano Porfirinas

Resumo Neste projeto de IC estudamos a coordenação molecular – formação de nanofios moleculares de 2H-TCNPP na superfície de Cu(111) mediados por átomos de Fe ou Pd, além de analisar a sua variação com a temperatura. O estudo envolveu a preparação deste particular substrato, evaporação das moléculas, aquecimento da amostra e sua investigação pelas técnicas de microscopia de tunelamento eletrônico (STM) e espectroscopia de elétrons excitados por raios X (XPS). No trabalho procuramos desvendar o papel do Fe ou Pd na formação dos nanoestruturas moleculares.