Chemistry-dependent magnetic properties at the FeNi oxide–metal interface

Fe and Ni compounds and their oxides offer stoichiometry dependent magnetic properties, exploitable for the design of magnetic heterojunctions

Black or red phosphorus yields the same blue phosphorus film

After the discovery of graphene, many other 2D materials have been predicted theoretically and successfully prepared. In this context, single-sheet black phosphorus - phosphorene - is emerging as a viable contender in the field of (2D) semiconductors. Phosphorene offers high carrier mobility and an anisotropic structure that gives rise to a modulation of physical and chemical properties. This opens the way to many novel and fascinating applications related to field-effect transistors and optoelectronic devices. In previous studies, a single layer of blue phosphorene intermixed with Au atoms was grown using purified black phosphorus as a precursor. Starting from the observation that phosphorus vapor mainly consists of P clusters, in this work we aimed at obtaining blue phosphorus using much less expensive purified red phosphorus as an evaporant. By means of microscopy, spectroscopy and diffraction experiments, we show that black or red phosphorus deposition on Au(111) substrates yields the same blue phosphorus film

Coupling of morphological instability and kinetic instability: Chemical waves in hydrogen oxidation on a bimetallic Ni/Rh(111) surface

The oxidation and reduction of a bimetallic Ni/Rh model catalyst during the water forming O2+H2 reaction is studied with low-energy electron microscopy, microspot-low-energy electron diffraction, and x-ray photoemission electron microscopy. Oxidation of a submonolayer Ni film results in the formation of three-dimensional (3D) NiO nanoparticles. Reduction of 3D-NiO in H2 produces a dispersed two-dimensional film of metallic Ni. Chemical waves during the O2+H2 reaction involve a cyclic transformation between 3D-NiO and 2D-NiO

Stimulated CO Dissociation and Surface Graphitization by Microfocused X-ray and Electron Beams

The irradiation with photons or electrons can dramatically influence the chemical stability of a molecule, either free or adsorbed on a surface, inducing its fragmentation or desorption. We revisit here the exostimulated dissociation of CO, a prototypical case, choosing hcp thin cobalt films as model support. Intense, microfocused soft X-rays or electron beams are used to locally stimulate CO dissociation. Fast-XPS gives direct access to the adsorbates' chemical state and coverage during irradiation, enabling the kinetics of the process to be monitored in real time. The energy-dependent cross sections for photon and electron stimulated molecular dissociation and desorption are estimated for a fixed initial CO coverage of 1/3 ML. In the soft X-ray regime, the desorption channel always prevails over dissociation and is significantly enhanced above the O K edge. The relative dissociation probability increases steadily with increasing photon energy, reaching 30% at 780 eV. Furthermore, we show that low energy electrons in the range 50 to 200 eV dissociate CO more efficiently than X-rays. The prolonged irradiation of the Co surface in CO ambient is found to produce a continuous increase of the carbon coverage, initially promoting the formation of carbides and subsequently accumulating sp2 carbon on the surface. Far from being a detrimental effect, the CO stimulated dissociation can be exploited to lithographically graft carbon-rich microscopic patterns on Co, with resolution well into the nanometer scale. A brief thermal treatment following irradiation results in the formation of a graphitic carbon overlayer, which effectively protects Co from oxidation upon exposure to ambient conditions, preserving its out-of-plane magnetic anisotropy and domain configuration

Toward the perfect membrane material for environmental x ray photoelectron spectroscopy

We outline our achievements in developing electron transparent, leak tight membranes required for environmental photoelectron spectroscopy PES . We discuss the mechanical constraints limiting the achievable membrane size and review the development of growth protocols for the chemical vapor deposition CVD of single crystalline graphene on highly 111 textured Cu foils serving as membrane material. During CVD growth, Cu tends to develop a mesoscopic staircase morphology consisting of alternating inclined surface planes, irrespective of whether the covering graphene film or the substrate are single crystalline. This morphology remains imprinted even when converting the film into freestanding graphene, which affects its mechanical properties. Determining the number of carbon layers in freestanding graphene, we show that membranes reported to suspend over distances larger than 20 m most likely consist of few layer graphene. The Raman band signature often used to confirm monolayer graphene rather relates to graphene with turbostratic stacking. The vertical corrugation of freestanding graphene was shown to be almost absent for tri and four layer thick graphene but substantial for bilayer and especially for monolayer graphene. The corrugation is reduced when mechanically straining the freestanding graphene through thermal expansion of the supporting frame, especially flattening membrane areas with imprinted staircase morphology. The electron signal attenuation through supported and freestanding graphene was determined as a function of the electron kinetic energy, verifying that large area graphene based electron windows have sufficient electron transparency required for environmental PES. Meanwhile, we managed to cover 100 m sized single holes by few layer graphene up to a coverage fraction of over 99.9998 , as deduced when applying 10 mbar air on one side of the sealing membrane without detecting any measurable pressure increase on its ultrahigh vacuum side. The reported achievements will pave the way toward the development of laboratory based environmental PE

Magnetic Patterning by Electron Beam-Assisted Carbon Lithography

We report on the proof of principle of a scalable method for writing the magnetic state by electron-stimulated molecular dissociative adsorption on ultrathin Co on Re(0001). Intense microfocused low-energy electron beams are used to promote the formation of surface carbides and graphitic carbon through the fragmentation of carbon monoxide. Upon annealing at the CO desorption temperature, carbon persists in the irradiated areas, whereas the clean surface is recovered elsewhere, giving origin to chemical patterns with nanometer-sharp edges. The accumulation of carbon is found to induce an in-plane to out-of-plane spin reorientation transition in Co, manifested by the appearance of striped magnetic domains. Irradiation at doses in excess of 1000 L of CO followed by ultrahigh vacuum annealing at 380 \ub0C determines the formation of a graphitic overlayer in the irradiated areas, under which Co exhibits out-of-plane magnetic anisotropy. Domains with opposite magnetization are separated here by chiral Ne\ue9l walls. Our fabrication protocol adds lateral control to spin reorientation transitions, permitting to tune the magnetic anisotropy within arbitrary regions of mesoscopic size. We envisage applications in the nano-engineering of graphene-spaced stacks exhibiting the desired magnetic state and properties

Revisiting the Chemical Stability of Germanium Selenide (GeSe) and the Origin of its Photocatalytic Efficiency

Recently, germanium selenide (GeSe) has emerged as a promising van der Waals semiconductor for photovoltaics, solar light harvesting, and water photoelectrolysis cells. Contrary to previous reports claiming perfect ambient stability based on experiments with techniques without surface sensitivity, here, by means of surface-science investigations and density functional theory, it is demonstrated that actually both: i) the surface of bulk crystals; and ii) atomically thin flakes of GeSe are prone to oxidation, with the formation of self-assembled germanium-oxide skin with sub-nanometric thickness. Surface oxidation leads to the decrease of the bandgap of stoichiometric GeSe and GeSe1−x, while bandgap energy increases upon surface oxidation of Ge1−xSe. Remarkably, the formation of a surface oxide skin on GeSe crystals plays a key role in the physicochemical mechanisms ruling photoelectrocatalysis: the underlying van der Waals semiconductor provides electron–hole pairs, while the germanium-oxide skin formed upon oxidation affords the active sites for catalytic reactions. The self-assembled germanium-oxide/germanium-selenide heterostructure with different bandgaps enables the activation of photocatalytic processes by absorption of light of different wavelengths, with inherently superior activity. Finally, it is discovered that, depending on the specific solvent-GeSe interaction, the liquid phase exfoliation of bulk crystals can induce the formation of Se nanowires. © 2021 The Authors. Advanced Functional Materials published by Wiley-VCH GmbHA.P. thanks CERIC-ERIC for the access to the NAP-XPS facility and Elettra Sincrotrone Trieste for providing access to its synchrotron radiation facilities. S.N. and F.B. acknowledge funding from EUROFEL project (RoadMap Esfri), J.D.S., V.P., and A.P. thank Maria Giammatteo for technical support in SEM experiments at Microscopy Centre of University of L'Aquila. Open access funding provided by Universita degli Studi dell'Aquila within the CRUI-CARE Agreement

Charge Redistribution Mechanisms in SnSe2Surfaces Exposed to Oxidative and Humid Environments and Their Related Influence on Chemical Sensing

Tin diselenide (SnSe2) is a van der Waals semiconductor, which spontaneously forms a subnanometric SnO2 skin once exposed to air. Here, by means of surface-science spectroscopies and density functional theory, we have investigated the charge redistribution at the SnO2-SnSe2 heterojunction in both oxidative and humid environments. Explicitly, we find that the work function of the pristine SnSe2 surface increases by 0.23 and 0.40 eV upon exposure to O2 and air, respectively, with a charge transfer reaching 0.56 e-/SnO2 between the underlying SnSe2 and the SnO2 skin. Remarkably, both pristine SnSe2 and defective SnSe2 display chemical inertness toward water, in contrast to other metal chalcogenides. Conversely, the SnO2-SnSe2 interface formed upon surface oxidation is highly reactive toward water, with subsequent implications for SnSe2-based devices working in ambient humidity, including chemical sensors. Our findings also imply that recent reports on humidity sensing with SnSe2 should be reinterpreted, considering the pivotal role of the oxide skin in the interaction with water molecules. ©PID2019-109525RB-I00; Horizon 2020 Framework Programme, H2020: 730872; Ministerio de Economía y Competitividad, MINECO: CEX2018-000805-M, E12H1800010001; Ministero dell’Istruzione, dell’Università e della Ricerca, MIUR; Ministry of Education and Science of the Russian Federation, Minobrnauka: FEUZ-2020-0060This work has been partially supported by the Spanish Ministerio de Ciencia e Innovación under Project PID2019-109525RB-I00. D.F. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M). D.F. and A.A.T. acknowledge the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. A.P. and G.D. acknowledge the CERIC–ERIC Consortium for the access to the Nanospectroscopy facility and financial support. G.D. acknowledges funding of a Ph.D. fellowship from PON Ricerca e Innovazione 2014–2020 (Project E12H1800010001) by the Italian Ministry of University and Research (MIUR). D.W.B. acknowledges the support by the Ministry of Science and Higher Education of the Russian Federation (through the basic part of the government mandate, Project No. FEUZ-2020-0060)