Reactive Phase Separation during Methanol Oxidation on a V‑Oxide-Promoted Rh(110) Surface

The distribution of ultrathin layers of vanadium oxide on Rh(110) (θ_V ≤ 1 MLE, one monolayer equivalent corresponds to the number of Rh atoms in the topmost Rh(110) surface layer) after exposure to catalytic methanol oxidation in the 10^–4 mbar range has been investigated with x-ray photoelectron spectroscopy and spectroscopic low-energy electron microscopy (SPELEEM). The reaction is shown to cause a macroscopic phase separation of the VO_<i>x</i> film into VO_<i>x</i>-rich and into V-poor phases. For θ_V = 0.8 MLE compact VO_<i>x</i> islands develop whose substructure exhibits several ordered phases. At θ_V = 0.4 MLE the VO_<i>x</i>-rich phase consists of many small VO_<i>x</i> islands (0.1–1 μm). Laterally resolved x-ray photoelectron spectroscopy of V 2p_3/2 shows an oxidic component at 515.5 eV binding energy (BE) and a component at 513.0 eV BE attributed to metallic or strongly reduced V. On the V-poor phase only the reduced/metallic component is present. The results are compared with the distribution of ultrathin layers of vanadium oxide on Rh(111) after catalytic methanol oxidation. The presence of the metallic V on Rh(110) is at variance with the behavior of Rh(111), where V is found to be present only in high oxidation states during methanol oxidation

Temperature-Driven Reversible Rippling and Bonding of a Graphene Superlattice

In order to unravel the complex interplay between substrate interactions and film configuration, we investigate and characterize graphene on a support with non-three-fold symmetry, the square Ir(100). Below 500 °C, distinct physisorbed and chemisorbed graphene phases coexist on the surface, respectively characterized by flat and buckled morphology. They organize into alternating domains that extend on mesoscopic lengths, relieving the strain due to the different thermal expansion of film and substrate. The chemisorbed phase exhibits exceptionally large one-dimensional ripples with regular nanometer periodicity and can be reversibly transformed into physisorbed graphene in a temperature-controlled process that involves surprisingly few C–Ir bonds. The formation and rupture of these bonds, rather than ripples or strain, are found to profoundly alter the local electronic structure, changing graphene behavior from semimetal to metallic type. The exploitation of such subtle interfacial changes opens new possibilities for tuning the properties of this unique material

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 °C 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 Neél 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

Spectromicroscopy of a Model Water–Gas Shift Catalyst: Gold Nanoparticles Supported on Ceria

Nanometer-sized gold particles supported on ceria are an important catalyst for the low-temperature water–gas shift reaction. In this work, we prepared a model system of epitaxial, ultrathin (1–2 nm thick) CeO_2–<i>x</i>(111) crystallites on a Rh(111) substrate. Low-energy electron microscopy (LEEM) and X-ray photoemission electron microscopy (XPEEM) were employed to characterize the in situ growth and morphology of these films, employing Ce 4f resonant photoemission to probe the oxidation state of the ceria. The deposition of submonolayer amounts of gold at room temperature was studied with scanning tunneling microscopy (STM) and XPEEM. Spatially resolved, energy-selected XPEEM at the Au 4f core level after gold adsorption indicated small shifts to higher binding energy for the nanoparticles, with the magnitude of the shift inversely related to the particle size. Slight reduction of the ceria support was also observed upon increasing Au coverage. The initial oxidation state of the ceria film was shown to influence the Au 4f binding energy; more heavily reduced ceria promoted a larger shift to higher binding energy. Understanding the redox behavior of the gold/ceria system is an important step in elucidating the mechanisms behind its catalytic activity

Unraveling the Structural and Electronic Properties at the WSe₂–Graphene Interface for a Rational Design of van der Waals Heterostructures

WSe₂ thin films grown by chemical vapor deposition on graphene on SiC(0001) are investigated using photoelectron spectromicroscopy and electron diffraction. By tuning of the growth conditions, micrometer-sized single or multilayer WSe₂ crystalline islands preferentially aligned with the main crystallographic directions of the substrate are obtained. Our experiments suggest that the WSe₂ islands nucleate from defective WSe_<i>x</i> seeds embedded in the support. We explore the electronic properties of prototypical van der Waals heterostructures by performing μ-angle resolved photoemission spectroscopy on WSe₂ islands of varying thickness (mono- and bilayer) supported on single layer, bilayer, and trilayer graphene. The experiments are substantiated by DFT calculations indicating that the interaction between WSe₂ and graphene is weak and the electronic properties of the resulting heterostructures are unaffected by the thickness of the supporting graphene layer or by the crystallographic orientation. Yet the WSe₂–graphene distance and the WSe₂/WSe₂ interlayer separation strongly influence the electronic band alignment at the high symmetry points of the Brillouin zone. The values of technology relevant quantities such as splitting of spin polarized bands and effective mass of electrons at band valleys are extracted from experimental angle resolved spectra. These findings establish further strategies for tuning the morphology and electronic properties of artificially fabricated van der Waals heterostructures that may be used in the fields of nanoelectronics and valleytronics

Unraveling the Structural and Electronic Properties at the WSe₂–Graphene Interface for a Rational Design of van der Waals Heterostructures

WSe₂ thin films grown by chemical vapor deposition on graphene on SiC(0001) are investigated using photoelectron spectromicroscopy and electron diffraction. By tuning of the growth conditions, micrometer-sized single or multilayer WSe₂ crystalline islands preferentially aligned with the main crystallographic directions of the substrate are obtained. Our experiments suggest that the WSe₂ islands nucleate from defective WSe_<i>x</i> seeds embedded in the support. We explore the electronic properties of prototypical van der Waals heterostructures by performing μ-angle resolved photoemission spectroscopy on WSe₂ islands of varying thickness (mono- and bilayer) supported on single layer, bilayer, and trilayer graphene. The experiments are substantiated by DFT calculations indicating that the interaction between WSe₂ and graphene is weak and the electronic properties of the resulting heterostructures are unaffected by the thickness of the supporting graphene layer or by the crystallographic orientation. Yet the WSe₂–graphene distance and the WSe₂/WSe₂ interlayer separation strongly influence the electronic band alignment at the high symmetry points of the Brillouin zone. The values of technology relevant quantities such as splitting of spin polarized bands and effective mass of electrons at band valleys are extracted from experimental angle resolved spectra. These findings establish further strategies for tuning the morphology and electronic properties of artificially fabricated van der Waals heterostructures that may be used in the fields of nanoelectronics and valleytronics

The Selective Species in Ethylene Epoxidation on Silver

Silver’s unique ability to selectively oxidize ethylene to ethylene oxide under an oxygen atmosphere has long been known. Today it is the foundation of ethylene oxide manufacturing. Yet, the mechanism of selective epoxide production is unknown. Here we use a combination of ultrahigh vacuum and in situ experimental methods along with theory to show that the only species that has been shown to produce ethylene oxide, the so-called <i>electrophilic oxygen</i> appearing at 530.2 eV in the O 1s spectrum, is the oxygen in adsorbed SO₄. This adsorbate is part of a 2D Ag/SO₄ phase, where the nonstoichiometric surface variant, with a formally S­(V+) species, facilitates selective transfer of an oxygen atom to ethylene. Our results demonstrate the significant and surprising impact of a trace impurity on a well-studied heterogeneously catalyzed reaction

Imaging Phase Segregation in Nanoscale Li_<i>x</i>CoO₂ Single Particles

LixCoO2 (LCO) is a common battery cathode material that has recently emerged as a promising material for other applications including electrocatalysis and as electrochemical random access memory (ECRAM). During charge–discharge cycling LCO exhibits phase transformations that are significantly complicated by electron correlation. While the bulk phase diagram for an ensemble of battery particles has been studied extensively, it remains unclear how these phases scale to nanometer dimensions and the effects of strain and diffusional anisotropy at the single-particle scale. Understanding these effects is critical to modeling battery performance and for predicting the scalability and performance of electrocatalysts and ECRAM. Here we investigate isolated, epitaxial LiCoO2 islands grown by pulsed laser deposition. After electrochemical cycling of the islands, conductive atomic force microscopy (c-AFM) is used to image the spatial distribution of conductive and insulating phases. Above 20 nm island thicknesses, we observe a kinetically arrested state in which the phase boundary is perpendicular to the Li-planes; we propose a model and present image analysis results that show smaller LCO islands have a higher conductive fraction than larger area islands, and the overall conductive fraction is consistent with the lithiation state. Thinner islands (14 nm), with a larger surface to volume ratio, are found to exhibit a striping pattern, which suggests surface energy can dominate below a critical dimension. When increasing force is applied through the AFM tip to strain the LCO islands, significant shifts in current flow are observed, and underlying mechanisms for this behavior are discussed. The c-AFM images are compared with photoemission electron microscopy images, which are used to acquire statistics across hundreds of particles. The results indicate that strain and morphology become more critical to electrochemical performance as particles approach nanometer dimensions