Combined spectroscopic and ab initio investigation of monolayer-range Cr oxides on Fe(001) : the effect of ordered vacancy superstructure

We investigated the electronic structure of an ultrathin Cr oxide film prepared by growing about 0.8 monolayers of Cr on the oxygen-terminated Fe(001) 12p(1 71)O surface and characterized by the formation of an ordered array of Cr vacancies producing a ( 1a5 7 1a5)R27 18 superstructure. We combined experimental techniques such as angle- and spin-resolved photoemission spectroscopy, low-energy electron diffraction, and scanning tunneling spectroscopy with ab initio calculations, focusing on (i) the peculiar energy dispersion of O2p states and (ii) the orbital and spin character of Cr3d states. We show that the experimental O2p dispersion can be related to the presence of an ordered vacancy lattice. The comparison with the existing literature on the oxidation of bulk Cr(001), where a network of Cr vacancies with a short-range crystallographic order is present, reveals a similar effect on O states. The valence electronic structure of the Cr oxide layer is mostly composed by spin-minority Cr states, consistent with an antiferromagnetic coupling with the Fe substrate

Stratigraphic analysis of intercalated graphite electrodes in aqueous inorganic acid solutions

A detailed stratigraphic investigation of the intercalation mechanism when graphite electrodes are immersed inside diluted perchloric HClO4 and sulfuric H2SO4 electrolytes is obtained by comparing results when graphite crystals are simply immersed in the same acid solutions. By combining time of flight secondary ion mass spectrometry ToF SIMS and in situ atomic force microscopy AFM , we provide a picture of the chemical species involved in the intercalation reaction. The depth intensity profile of the ion signals along the electrode crystal clearly shows a more complex mechanism for the intercalation process, where the local morphology of the basal plane plays a crucial role. Solvated anions are mostly located within the first tens of nanometers of graphite, but electrolytes also diffuse inside the buried layers for hundreds of nanometers, the latter process is also aided by the presence of mesoscopic crystal defects. Residual material from the electrolyte solution was found localized in well defined circular spots, which represent preferential interaction areas. Interestingly, blister like micro structures similar to those observed on the highly oriented pyrolytic graphite HOPG surface were found in the buried layers, confirming the equivalence of the chemical condition on the graphite surface and in the underneath layer

Synergistic and competitive aspects of the adsorption of Poly(ethylene glycol) and Poly(vinyl alcohol) onto Na-Bentonite

Graph Presented) The competitive adsorption of poly(ethylene glycol) (PEG) and poly(vinyl alcohol) (PVOH) onto Na-bentonite has been assessed quantitatively. Particular emphasis was focused on the amount of organic located within the bentonite interlayer and any subsequent eff ects on the extent of layer expansion. The individual isotherms showed strong adsorption for both PVOH and PEG at amounts lower than the quantities required to produce a fully loaded bilayer (0.33 g of PVOH/g of clay) and single layered structures (0.10 g of PEG/g of clay), respectively. Above these concentrations, the incremental amounts adsorbed were smaller, and the concentration of adsorbates in solution gradually increased. Na-bentonite adsorbed more PVOH than PEG at any given concentration. In the competitive study, the amount of PVOH adsorbed was enhanced in the presence of PEG (0.10 and 0.30 g/g of clay), but less PEG was adsorbed. At low loadings of PVOH (0.02-0.10 g/g of clay), the amount of adsorbed PEG was increased but at higher PVOH levels PEG adsorption was reduced. The XRD data showed stepped changes in the d-spacing as the adsorbed amounts of both PEG and PVOH increased. The PEG-bentonite samples did not expand beyond a bilayer structure (18 A˚), but the XRD data for PVOH-treated samples indicated the formation of multilayer structures (d ≥ 44 A˚)

Tuning spin-charge interconversion with quantum confinement in ultrathin bismuth films

Spin-charge interconversion (SCI) phenomena have attracted a growing interest in the field of spintronics as means to detect spin currents or manipulate the magnetization of ferromagnets. The key ingredients to exploit these assets are a large conversion efficiency, the scalability down to the nanometer scale and the integrability with opto-electronic and spintronic devices. Here we show that, when an ultrathin Bi film is epitaxially grown on top of a Ge(111) substrate, quantum size effects arising in nanometric Bi islands drastically boost the SCI efficiency, even at room temperature. Using x-ray diffraction (XRD), scanning tunneling microscopy (STM) and spin- and angle-resolved photoemission (S-ARPES) we obtain a clear picture of the film morphology, crystallography and electronic structure. We then exploit the Rashba-Edelstein effect (REE) and inverse Rashba-Edelstein effect (IREE) to directly quantify the SCI efficiency using optical and electrical spin injection.Comment: 18 pages, 5 figure

Modification of large area Cu2O/CuO photocathode with CuS non-noble catalyst for improved photocurrent and stability

In this work, a three-layered heterostructure Cu2O/CuO/CuS was obtained through a low-cost and large-area fabrication route comprising electrodeposition, thermal oxidation, and reactive annealing in a sulfur atmosphere. Morphological, microstructural, and compositional analysis (AFM, SEM, XRD, EDS, XPS) were carried out to highlight the surface modification of cuprous oxide film after oxidation and subsequent sulfurization. Impedance, voltammetric, and amperometric photoelectrochemical tests were performed on Cu2O, Cu2O/CuO, and Cu2O/CuO/CuS photocathodes in a sodium sulfate solution (pH 5), under 100 mW cm−2 AM 1.5 G illumination. A progressive improvement in terms of photocurrent and stability was observed after oxidation and sulfurization treatments, reaching a maximum of − 1.38 mA cm−2 at 0 V versus RHE for the CuS-modified Cu2O/CuO electrode, corresponding to a ~ 30% improvement. The feasibility of the proposed method was demonstrated through the fabrication of a large area photoelectrode of 10 cm2, showing no significant differences in characteristics if compared to a small area photoelectrode of 1 cm2

Disclosing the Graphite Surface Chemistry in Acid Solutions for Anion Intercalation

Graphitic electrodes in ion batteries and graphite electrochemical exfoliation for graphene production are technological processes based on the anion intercalation mechanism in the stratified crystal structure upon immersion in acid solutions. To the detriment of the graphite mother-crystal, decomposition of the electrolyte solution or production of unexpected graphite intercalation compounds (GICs) is a side effect related to the anion intercalation mechanism. In this work, we studied the surface of highly oriented pyrolytic graphite (HOPG) samples treated with electrolyte solutions of 2 M HClO4 and 1 M H2SO4 in order to identify elemental and molecular species involved in the intercalation processes and, at the same time, find local surface defects through which solvated ions easily reach the HOPG subsurface. Results from cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) are described and discussed. Large blister formation and more general HOPG surface damage induced by treatment with acid solutions were revealed and mapped. In both HOPG samples, oxidized in 2 M HClO4 and 1 M H2SO4, the surface distribution of perchlorate and hydrogen sulfate ions gives direct evidence that the anion intercalation happens via surface defects among HOPG crystallites. These results pave the way for a complex intercalation process where more than one molecular species could be responsible for blister formation and carbon detriment