Ru Substrate Research Articles

Interaction of Mg with the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide—An experimental and computational model study of the electrode–electrolyte interface in post-lithium batteries

We report results of a combined experimental and computational model study on the interaction of the battery-relevant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI) with a Mg thin film model electrode grown on a Ru(0001) substrate, which aims at a fundamental understanding of the solid electrolyte interphase formation at the electrode–electrolyte interface in postlithium batteries. Scanning tunneling microscopy, x-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy were employed for the characterization of the Mg thin film model electrode, revealing oxygen-free and atomically flat Mg films. Room temperature XPS measurements after vapor deposition of a (sub)monolayer of BMP-TFSI on the Mg film revealed the formation of a “contact layer” on Mg(0001), created by the reactive decomposition of the IL. In agreement with computationally determined core level binding energies of stable reaction products (dispersion corrected density functional theory calculations), we identified mainly inorganic MgF2-, MgO-, and MgS-like surface compounds, but also other more complex (Mg2+-free) F-, O-, and/or S-containing “TFSI-like” and carbon-containing adsorbed species. The deposition of higher IL amounts (up to 6 monolayers) results in the overgrowth of the direct “contact layer” by molecularly adsorbed BMP-TFSI. Heating of the adsorbate covered surface to around 470 K leads to desorption of multilayer BMP-TFSI and the partial desorption and transformation of adsorbed (Mg2+-free) “TFSI-like” decomposition products on the Mg substrate into MgF2-, MgO-, and MgS species or the respective adsorbed Fad, Oad, and Sad species.

Adsorption behavior of 9,9′-Dixanthylidene and moiré superstructure on Ru(0001)

This paper reports the adsorption behavior of the 9,9′-Dixanthylidene on Ru(0001), the bottom-up fabrication of graphene is also investigated through a scanning tunneling microscope (STM). We analyze the spatial structure of 9,9′-Dixanthylidene molecules which are randomly dispersed on a Ru(0001) substrate in the sub-monolayer coverage. Then we bottom-up fabricate the graphene on the Ru(0001) substrate with the 9,9′-Dixanthylidene molecules as the precursor. Three kinds of moiré superstructures with different rotation angles (6.3°, 13.9°, and 16.1°) are found after implementing high temperature annealing in ultrahigh vacuum. This result provides data support for the study of moiré superstructures on Ru(0001) substrate and lays a solid foundation for further scientific research based on graphene/Ru(0001).

Adsorption Behavior of 9,9'-Dixanthylidene and Moiré Superstructure on Ru(0001)

This paper reports the adsorptions behavior of the 9,9'-Dixanthylidene on Ru(0001), the bottom-up fabrication of graphene was also investigated through a Scanning Tunneling Microscope (STM). We analyzed the spatial structure of 9,9'-Dixanthylidene molecules which were randomly dispersed on the Ru(0001) substrate in the sub-monolayer coverage. Then we bottom-up fabricated the graphene on Ru(0001) substrate with the 9,9'-Dixanthylidene molecules as the precursor. Three kinds of moiré superstructures with different rotation angles (6.3°, 13.9°, and 16.1°) were found after high temperature annealing in an ultrahigh vacuum. This experiment provides data support for the study of moiré superstructures on Ru(0001) substrate and lays a solid foundation for further scientific research based on graphene/Ru(0001).

Depolarization of Cu electrodeposition in the presence of Ag: A cyclic-voltammetry study

Cyclic voltammetry profiles of Cu-Ag acidic sulfate deposition baths at Cu, Cu-Ag, Au or Ru substrates have been systematically studied, revealing the existence of an early Cu reduction peak before the bulk Cu deposition peak. The Cu deposit thickness produced by this early Cu reduction peak is estimated to decrease with scan rate and increase with the [Ag+] concentration in the electrolyte. The early Cu reduction peak generates Cu deposits thicker than 3 nm for the Cu-Ag electrolyte with 2 mM Ag(I). Electrochemical quartz crystal microbalance (EQCM) and CV reveal that the early Cu reduction peak is associated with a Cu(II)/Cu(0) reaction at 65% Coulombic efficiency, slightly smaller than that of the main Cu(II)/Cu(0) reduction peak (∼78%). Directly observing the linear regions in the polarization curves produced by CV, the early Cu reduction peak is estimated to show a Tafel slope ∼30 mV/dec. We hypothesize that the early Cu reduction peak is caused by the catalytic effect of Ag in the substrate or in the initial deposit on the intermediate reduction step of Cu(II)/Cu(I), leading to a shift of the rate determining step of Cu(II)/Cu(0) reduction.

Electrodeposition of Cu-Ag Alloy Films at n-Si(001) and Polycrystalline Ru Substrates

Electrodeposition of Cu-Ag films from acidic sulfate bath was conducted at n-Si(001) and polycrystalline Ru substrates. Significant nucleation overpotential of 0.4 V is observed with the Cu-Ag bath at n-Si(001) substrate, whereas the electrodeposition of Cu-Ag at Ru substrate is influenced by Ru oxides at the surface. Incomplete coverage of Si substrate by Cu-Ag deposit was observed from the deposition systems without Ag(I), or with 0.1 mM Ag(I), comparing with the compact Cu-Ag film obtained with the deposition bath containing 0.01 mM Ag(I). Layered and faceted Cu-Ag deposit was observed at small Cu deposition overpotential with the Ru substrate. Phase composition of the Cu-Ag deposits at n-Si(001) substrate from electrolyte with various Ag(I) concentrations is examined by XRD. Limited solubility of Ag (0.4 at.%) was observed in fcc-Cu until phase separation occurs. The classical model for nucleation kinetics in electrodeposition was used to examine the potentiostatic transients of the Cu-Ag electrodeposition at n-Si(001) substrate.

Optimizing Hydrogen Binding on Ru Sites with RuCo Alloy Nanosheets for Efficient Alkaline Hydrogen Evolution.

Ruthenium (Ru)‐based catalysts, with considerable performance and desirable cost, are becoming highly interesting candidates to replace platinum (Pt) in the alkaline hydrogen evolution reaction (HER). The hydrogen binding at Ru sites (Ru−H) is an important factor limiting the HER activity. Herein, density functional theory (DFT) simulations show that the essence of Ru−H binding energy is the strong interaction between the 4dz2 orbital of Ru and the 1s orbital of H. The charge transfer between Ru sites and substrates (Co and Ni) causes the appropriate downward shift of the 4dz2 ‐band center of Ru, which results in a Gibbs free energy of 0.022 eV for H* in the RuCo system, much lower than the 0.133 eV in the pure Ru system. This theoretical prediction has been experimentally confirmed using RuCo alloy‐nanosheets (RuCo ANSs). They were prepared via a fast co‐precipitation method followed with a mild electrochemical reduction. Structure characterizations reveal that the Ru atoms are embedded into the Co substrate as isolated active sites with a planar symmetric and Z‐direction asymmetric coordination structure, obtaining an optimal 4dz2 modulated electronic structure. Hydrogen sensor and temperature program desorption (TPD) tests demonstrate the enhanced Ru−H interactions in RuCo ANSs compared to those in pure Ru nanoparticles. As a result, the RuCo ANSs reach an ultra‐low overpotential of 10 mV at 10 mA cm−2 and a Tafel slope of 20.6 mV dec−1 in 1 M KOH, outperforming that of the commercial Pt/C. This holistic work provides a new insight to promote alkaline HER by optimizing the metal‐H binding energy of active sites.

Optimizing Hydrogen Binding on Ru Sites with RuCo Alloy Nanosheets for Efficient Alkaline Hydrogen Evolution

AbstractRuthenium (Ru)‐based catalysts, with considerable performance and desirable cost, are becoming highly interesting candidates to replace platinum (Pt) in the alkaline hydrogen evolution reaction (HER). The hydrogen binding at Ru sites (Ru−H) is an important factor limiting the HER activity. Herein, density functional theory (DFT) simulations show that the essence of Ru−H binding energy is the strong interaction between the orbital of Ru and the 1s orbital of H. The charge transfer between Ru sites and substrates (Co and Ni) causes the appropriate downward shift of the ‐band center of Ru, which results in a Gibbs free energy of 0.022 eV for H* in the RuCo system, much lower than the 0.133 eV in the pure Ru system. This theoretical prediction has been experimentally confirmed using RuCo alloy‐nanosheets (RuCo ANSs). They were prepared via a fast co‐precipitation method followed with a mild electrochemical reduction. Structure characterizations reveal that the Ru atoms are embedded into the Co substrate as isolated active sites with a planar symmetric and Z‐direction asymmetric coordination structure, obtaining an optimal modulated electronic structure. Hydrogen sensor and temperature program desorption (TPD) tests demonstrate the enhanced Ru−H interactions in RuCo ANSs compared to those in pure Ru nanoparticles. As a result, the RuCo ANSs reach an ultra‐low overpotential of 10 mV at 10 mA cm−2 and a Tafel slope of 20.6 mV dec−1 in 1 M KOH, outperforming that of the commercial Pt/C. This holistic work provides a new insight to promote alkaline HER by optimizing the metal‐H binding energy of active sites.

Estimating electrodeposition properties and processes: Cu-Ag alloy at n-Si(001) and Ru substrates from acidic sulfate bath

In alloy electrodeposition, estimations of the initial transient and the potential vs. composition trend of the film system is important for material design, deposition procedure and the deposit characterization. However, solving the diffusion-migration-convection problem based on common simulation methods is often too burdensome and too resource-consuming for ordinary tasks in the context of alloy electrodeposition works. In this work, based on a stagnant diffusion layer model, we have proposed simple estimates on (1) the potential-composition curve, (2) the thickness of the initial deposit, and (3) the time interval to reach the steady state deposition. These estimates were validated by using a Cu-Ag acidic sulfate electrolyte, which upon electrodeposition results in an immiscible Cu-Ag alloy, both at n-Si(001) or at polycrystallline Ru substrates. The limiting current of Cu-Ag electrodeposition is compared with the numerical results given by Selman and Newman in 1970s [17], whereas the film composition deposited at the limiting current from various electrolyte constitutions is compared with the model suggest by Brenner based on stagnant diffusion layer model [21]. The proposed estimation methods show good agreement with the observed values, despite being highly simplified, since they are compared with the actual mass-transfer problem of ions under convection and migration.

Large-Scale Layer-by-Layer Synthesis of Borophene on Ru(0001)

Borophene, a two-dimensional (2D) layer of elemental boron, has attracted attention as a metallic 2D material with rich polymorphism, as well as potentially interesting electronic, optical, chemical, and catalytic properties. While controllable adjustments of the thickness beyond a single layer promise a further tunability of the properties, the growth of borophene on metal substrates has largely remained limited to monolayers. Recently, bilayer borophene has been realized on Ag(111), albeit so far limited to domains a few tens of nanometers in size. The broader exploration of the properties of borophene calls for the development of synthesis methods that produce borophene with controlled thickness beyond a single layer. Here, we report layer-by-layer borophene growth on Ru(0001) substrates up to a coverage of two atomic layers using surface segregation of interstitial boron during cooling from high temperatures. Real-time surface microscopy follows borophene nucleation and growth, establishes the growth mode of the first and second layers, and demonstrates coverage across large surface areas. Diffraction shows highly crystalline borophene layers that grow in a high-order superstructure on the Ru(0001) surface lattice. Our results add to the capabilities of synthesizing borophene and point out avenues for growing high-quality few-layer borophene on metal substrates.

The Prospect of the Electroless Atomic Layer Deposition (ALD) for Applications in Semiconductor Technology

The metallic thin films are widely used for in semiconductor technology. Surface morphology, thickness and purity of the film are the key attributes determining its functionality 1 , 2. Recent developments indicate that Cu/liner/TaN structures where liner could be either Co, Ru or Rh and TaN serves as barrier are of particular interest. The electrochemical doping of Zn with liner material to develop barrier-less Cu structures has been demonstrated3. In this talk we demonstrate a protocol to deposit conformal Cu thin film on Co by replacement of Zn overlayer without etching Co. EDTA and BTA are used for corrosion inhibition of Co film once all Zn capping layer is consumed in galvanic displacement with Cu. TEM, XPS and EDS indicate conformal Cu layer deposition Co layer underneath being intact. The second part of the talk focuses on electroless ALD application for deposition of Pt and Rh layers on Cu, Au or Ru substrates. 4 , 5. Surface reflectivity changes during electroless ALD serves as an indicator of surface roughness evolution after each cycle. Surface morphology after 20 electroless ALD cycles is studied by STM and AFM. Cyclic voltammetry illustrates catalytically active Pt and Rh surface after deposition. All electroless ALDs are performed at room temperature in glove box with inert atmosphere. Surface selective, self-terminating deposition and precise control over the film thickness are achieved demonstrating potential of this deposition method for scale up and industrial applications. Ahmadi, K., Wu, D., Dole, N., R. Monteiro, O. & R. Brankovic, S. Tuning Surface Chemoresistivity of Au Ultrathin Films Using Metal Deposition via Surface-Limited Redox Replacement of the Underpotentially Deposited Pb Monolayer. ACS Sensors 4, 2442–2449 (2019).Wu, D. et al. Pb Monolayer Mediated Thin Film Growth of Cu and Co: Exploring Different Concepts. Journal of The Electrochemical Society 166, D3013–D3021 (2019).Kailash Venkatraman, Yezdi Dordi, A. J. Electrochemical doping of thin metal layers employing underpotential deposition and thermal treatment. 1 (2019).Wu, D. et al. Electroless Deposition of Pb Monolayer: A New Process and Application to Surface Selective Atomic Layer Deposition. Langmuir 34, 11384–11394 (2018).Wu, D. et al. Underpotential and Electroless Pb Monolayer Deposition on Ru(0001). Journal of The Electrochemical Society 166, D359–D365 (2019). Figure 1

Sub-Poissonian distribution of Cs and K ions in the valleys of hBN/Ru(0001)

The moir\'e pattern of hBN/Ru(0001) provides a template for the adsorption of the alkali metals Cs and K: Scanning tunneling microscopy reveals that the adatoms prefer to adsorb in the valley areas of the moir\'e unit cell. Distinctive from transition metal adatoms that tend to agglomerate, the alkali metal adatoms avoid each other within the valleys, leading to a sharper distribution than the Poisson statistics expected for noninteracting particles. This sub-Poissonian distribution is further explored via Monte Carlo simulations. The average charge $q$ of the alkali metal adatoms is extracted for different alkali metal concentrations $\ensuremath{\lambda}$, which reveals a monotonically decreasing $q(\ensuremath{\lambda})$ dependence. We explain this dependence with the energy trade-off between the charge transfer from the alkali metals to the more electron-affine Ru(0001) substrate and the static electric field created by this charge transfer.

Dissociation of water on atomic oxygen-covered Rh nanoclusters supported on graphene/Ru(0001).

We studied the dissociation of water (H2O*, with * denoting adspecies) on atomic oxygen (O*)-covered Rh nanoclusters (RhO* ) supported on a graphene film grown on a Ru(0001) surface [G/Ru(0001)] under ultrahigh-vacuum conditions and with varied surface-probe techniques and calculations based on density-functional theory. The graphene had a single rotational domain; its lattice expanded by about 5.7% to match the Ru substrate structurally better. The Rh clusters were grown by depositing Rh vapors onto G/Ru(0001); they had an fcc phase and grew in (111) orientation. Water adsorbed on the Rh clusters was dissociated exclusively in the presence of O*, like that on a Rh(111) single-crystal surface. Contrary to the case on Rh(111)O* , excess O* (even at a saturation level) on small RhO* clusters (diameter of 30-34 Å) continued to promote, instead of inhibiting, the dissociation of water; the produced hydroxyl (OH*) increased generally with the concentration of O* on the clusters. The difference results from more reactive O* on the RhO* clusters. O* on RhO* clusters activated the dissociation via both the formation of hydrogen bonds with H2O* and abstraction of H directly from H2O*, whereas O* on Rh(111)O* assisted the dissociation largely via the formation of hydrogen bonds, which was readily obstructed with an increased O* coverage. As the disproportionation (2 OH* → H2O* + O*) is endothermic on the RhO* clusters but exothermic on Rh(111)O* , OH* produced on RhO* clusters showed a thermal stability superior to that on the Rh(111)O* surface-thermally stable up to 400K.

The growth of CeOx(111) thin films on Ru(0001)

Ceria has attracted great attention in recent years in the catalysis community. Metal catalysts supported on ceria exhibit a promising catalytic activity due to the ceria’s unique redox properties and oxygen storage capacity. The purpose of this paper is to look at the fundamental aspect of CeOx(111) thin films on Ru(0001). The investigation presented in this paper utilizes the surface science method to understand the properties of CeOx(111). In one component, through a series of XPS studies, it is shown that the deposition of Ce in the presence of oxygen on Ru(0001) can produce ceria thin films. Fully oxidized CeO2 can be obtained with an oxygen pressure of 2 x 10−7 Torr. With the decrease in the oxygen pressure, partially reduced CeOx films can be prepared. Moreover, STM images are collected from the CeO2 growth in the presence of 2×10−7 Torr of oxygen and partially reduced ceria with oxygen pressures of 8×10−8 Torr. These films can completely cover the Ru(0001) substrate. The measured step height is about 0.3 nm, which is consistent with the 0.313 nm spacing of O-Ce-O tri-layers in the CeO2(111) fluorite bulk structure.

Same-Energy UV Photons and Low-Energy Electrons Activating Methane and Ammonia Frozen in Amorphous Solid Water.

UV photons and low-energy electrons play an important role in the evolution of various molecules in the interstellar medium (ISM). Here, we examined the product molecule formation as a result of irradiation of 193 nm photons and 6.4 eV electrons (same energy under identical laboratory conditions) on D2O|CH4 + ND3|D2O sandwiched films deposited on Ru(0001) substrate at 25 K in ultrahigh vacuum as a model for processes in the ISM. Temperature-programmed desorption spectra performed following the irradiation revealed the signature of hydrazine and formamide product molecules. These molecules were, however, formed exclusively following the photons' irradiation. These results were compared with the products obtained from a D2O|CH4|D2O sample without ammonia, where deuterated formaldehyde was the dominant product, formed also by photons only. Our results indicate that the photon-induced activation of the cofrozen molecules within D2O occurs via direct (partial) dissociation of the host and embedded molecules, followed by sample annealing. The electron-induced activation occurs through a direct dissociative electron attachment mechanism. The results presented here suggest possible pathways to generate various C-N, C-O, C-C, N-O, and N-H bonds containing molecules in the ISM.

Vacuum ultraviolet enhanced atomic layer etching of ruthenium films

Vacuum ultraviolet (VUV) enhanced atomic layer etching (ALE) of thin (∼8 nm) Ru films is demonstrated. Oxidation half-cycles of 2–5 min VUV/O2 co-exposure are used to oxidize near-surface Ru to RuO2 at 1 Torr O2 and 100–150 °C. In situ x-ray photoelectron spectroscopy measurements indicate that RuO2 formation saturates after ∼5 min of VUV/O2 exposure at 100 and 150 °C. The depth of Ru oxidation is limited by the rate of oxidation and can be controlled with substrate temperature and exposure time. Etching half-cycles are performed by exposing the oxidized Ru film to HCOOH vapor at 0.50 Torr for 30 s isothermally, which results in the removal of the oxidized Ru layer. The amount of Ru removed per ALE cycle is determined by comparing ex situ x-ray reflectivity (XRR) measurements of the film before and after etching. When using 2 min VUV/O2 co-exposure, approximately 0.8 and 0.9 Å of Ru is etched per cycle at 100 and 150 °C, respectively. XRR and atomic force microscopy measurements indicate that the as-deposited and sputtered Ru film surface becomes smoother as ALE is performed. The etch rate decreases with ALE cycles and corresponds to a slowing oxidation rate, which is likely associated with the decrease in surface roughness. Density functional theory is used to study the adsorption of oxidants in a model Ru system, and nudged elastic band (NEB) calculations describe O diffusion into the Ru substrate by following an O “probe” atom as it moves between Ru(002) atomic planes with 0.50 monolayer (ML) O on the surface. NEB results reveal an approximate energetic barrier to diffusion, Ea, of 5.10 eV for O to move through the second and third atomic Ru layers when O, which can form an RuOx species, is subsurface. This Ea is in excess of the energetic gain of 4.23 eV in adsorbing an O atom to Ru(002) with 0.50 ML O. The difference in Ea and the adsorption energy likely contributes to the self-limiting nature of the oxidation and explains the observation that VUV/O2 co-exposure time must be increased to allow additional time for O diffusing into the subsurface as it overcomes the barrier to subsurface O diffusion. The self-limiting oxidation of Ru arising from VUV/O2 at low temperatures, in turn, enables an ALE process for Ru.

The role of thermal history on spontaneous polarization and phase transitions of amorphous solid water films studied by contact potential difference measurements.

Monitoring thermal processes occurring in molecular films on surfaces can provide insights into physical events such as morphology changes and phase transitions. Here, we demonstrate that temperature-programmed contact potential difference (TP-∆CPD) measurements employed by a Kelvin probe under ultrahigh vacuum conditions and their temperature derivative can track films' restructure and crystallization occurring in amorphous solid water (ASW) at temperatures well below the onset of film desorption. The effects of growth temperature and films' thickness on the spontaneous polarization that develops within ASW films grown at 33 K-120 K on top of a Ru(0001) substrate are reported. Electric fields of ∼106 V/m are developed within the ASW films despite low average levels of molecular dipole alignment (<0.01%) normal to the substrate plane. Upon annealing, an irreversible morphology-dependent depolarization has been recorded, indicating that the ASW films keep a "memory" of their thermal history. We demonstrate that TP-∆CPD measurements can track the collapse of the porous structure at temperatures above the growth and the ASW-ice Ic and ASW-ice Ih transitions at 131 K and 157 K, respectively. These observations have interesting implications for physical and chemical processes that take place at the interstellar medium such as planetary formation and photon- and electron-induced synthesis of new molecules.

Spontaneous polarization of thick solid ammonia films.

Ammonia molecules have an important role in the radiation-induced chemistry that occurs on grains in the cold interstellar medium and leads to the formation of nitrogen containing molecules. Such grains and surfaces are primarily covered by water ices; however, these conditions allow the growth of solid ammonia films as well. Yet, solid ammonia know-how lags the vast volume of research that has been invested in the case of films of its "sibling" molecule water, which, in the porous amorphous phase, spontaneously form polar films and can cage coadsorbed molecules within their hydrogen-bonded matrix. Here, we report on the effect of growth temperature on the spontaneous polarization of solid ammonia films (leading to internal electric fields of ∼105 V/m) within the range of 30 K-85 K on top of a Ru(0001) substrate under ultra-high vacuum conditions. The effect of growth temperature on the films' depolarization upon annealing was recorded as well. By demonstrating the ability of ammonia to cage coadsorbed molecules, as water does, we show that temperature-programmed contact potential difference measurements performed by a Kelvin probe and especially their temperature derivative can track film reorganization/reconstruction and crystallization at temperatures significantly lower than the film desorption.

Facet Dependence of Photocatalytic Activity in Anatase TiO2: Combined Experimental and DFT Study

Abstract For enhancing the photocatalytic performance of anatase TiO2, proper control of the reactive facets and its molecular-level understanding are crucial. This experimental and theoretical study clarifies the facet dependence of the photocatalytic reaction at the anatase TiO2 surface. The 〈112〉-oriented anatase TiO2 layer is deposited on a Ru(0001) substrate at 360 °C by low-pressure chemical vapor deposition. The deposition rate is three times higher than that of the multi-orientation layer on the Pyrex glass. The photocatalytic activity induced by ultraviolet (UV) irradiation around 365 nm in methylene blue aqueous solutions is significantly high, and the rate constant is 6.1 × 10−1 min−1, which is two orders of magnitude greater than that on the multi-orientation TiO2. The density functional theory (DFT) calculations using the constrained DFT method and the hybrid functionals show that the (112) surface stabilizes the adsorbed water molecule most strongly. The photogenerated hole is stably trapped not at the bare surface but at the hydroxylated surface, especially at the hydroxyl group of the hydroxylated (112) and (001) surfaces. The experimental and theoretical findings consistently indicate the high photocatalytic activity at the anatase TiO2(112) surface.

Reversible and irreversible structural changes in FeO/Ru(0 0 0 1) model catalyst subjected to atomic oxygen

Islands and thin films of metal oxides grown on metal single-crystal supports constitute well-defined model systems for studying elementary steps of catalytic reactions that take place at the surface of metal-oxide catalysts. Fundamental studies carried out under idealized ultra-high vacuum (UHV) conditions have to be, however, linked to the industrial ones by overcoming the so called “pressure gap”. In the case of oxidation reactions, experiments using high (millibar) oxygen pressures or moderate pressures of highly oxidative gaseous species (such as atomic oxygen) are commonly performed to bridge (at least partially) the two catalytic worlds. We studied the structural changes that occur in ultrathin iron monoxide (FeO) islands and films grown on single-crystal Ru(0 0 0 1) substrate upon exposure to atomic oxygen. The results reveal that room-temperature exposure leads to the formation of oxygen-rich and structurally ill-defined iron oxide and ruthenium oxide species. The transformation can be, however, reverted by vacuum annealing. Exposing FeO/Ru(0 0 0 1) to atomic oxygen at 700 K leads, on the other hand, to an irreversible transformation of the iron oxide to a Fe2O3-like phase. The results contribute to the general understanding of the oxidation reactions over metal-oxide catalysts.

Sizable Band Gap in Epitaxial Bilayer Graphene Induced by Silicene Intercalation.

Opening a band gap in bilayer graphene (BLG) is of significance for potential applications in graphene-based electronic and photonic devices. Here, we report the generation of a sizable band gap in BLG by intercalating silicene between BLG and Ru substrate. We first grow high-quality Bernal-stacked BLG on Ru(0001) and then intercalate silicene to the interface between the BLG and Ru, which is confirmed by low-energy electron diffraction and scanning tunneling microscopy. Raman spectroscopy shows that the G and 2D peaks of the intercalated BLG are restored to the freestanding-BLG features. Angle-resolved photoelectron spectroscopy measurements show that a band gap of about 0.2 eV opens in the BLG. Density functional theory calculations indicate that the large-gap opening results from a cooperative contribution of the doping and rippling/strain in the BLG. This work provides insightful understanding on the mechanism of band gap opening in BLG and enhances the potential of graphene-based device development.