Monolayer Of Oxygen Research Articles

Interface Properties of MoS2 van der Waals Heterojunctions with GaN.

The combination of the unique physical properties of molybdenum disulfide (MoS2) with those of gallium nitride (GaN) and related group-III nitride semiconductors have recently attracted increasing scientific interest for the realization of innovative electronic and optoelectronic devices. A deep understanding of MoS2/GaN interface properties represents the key to properly tailor the electronic and optical behavior of devices based on this heterostructure. In this study, monolayer (1L) MoS2 was grown on GaN-on-sapphire substrates by chemical vapor deposition (CVD) at 700 °C. The structural, chemical, vibrational, and light emission properties of the MoS2/GaN heterostructure were investigated in detail by the combination of microscopic/spectroscopic techniques and ab initio calculations. XPS analyses on as-grown samples showed the formation of stoichiometric MoS2. According to micro-Raman spectroscopy, monolayer MoS2 domains on GaN exhibit an average n-type doping of (0.11 ± 0.12) × 1013 cm-2 and a small tensile strain (ε ≈ 0.25%), whereas an intense light emission at 1.87 eV was revealed by PL analyses. Furthermore, a gap at the interface was shown by cross-sectional TEM analysis, confirming the van der Waals (vdW) bond between MoS2 and GaN. Finally, density functional theory (DFT) calculations of the heterostructure were carried out, considering three different configurations of the interface, i.e., (i) an ideal Ga-terminated GaN surface, (ii) the passivation of Ga surface by a monolayer of oxygen (O), and (iii) the presence of an ultrathin Ga2O3 layer. This latter model predicts the formation of a vdW interface and a strong n-type doping of MoS2, in closer agreement with the experimental observations.

Unraveling the atomic and electronic structure of nanocrystals on superconducting Nb(110): Impact of the oxygen monolayer

The niobium surface is almost always covered by a native oxide layer which greatly influences the performance of superconducting devices. Here we investigate the highly stable niobium oxide overlayer of Nb(110), which is characterized by its distinctive nanocrystal structure as observed by scanning tunneling microscopy (STM). Our ab initio density functional theory (DFT) calculations show that a subtle reconstruction in the surface niobium atoms gives rise to rows of fourfold coordinated oxygen separated by regions of threefold coordinated oxygen. The fourfold oxygen rows are determined to be the source of the nanocrystal pattern observed in STM and the two chemical states of oxygen observed in core-level x-ray photoelectron spectroscopy (XPS) are ascribed to the threefold and fourfold oxygens. Furthermore, we find excellent agreement between the DFT calculated electronic structure with scanning tunneling spectroscopy and valence XPS measurements.

Improving MgO/Fe insulator-metal interface structure through oxygen-precoating of Fe(0 0 1)

We grew atomically flat and electronically uniform MgO island terraces on a Fe(001) whisker substrate precoated by a p(1 × 1) oxygen monolayer film. This interface engineering process increased about 10 times the size of MgO islands compared to the size of MgO islands directly grown on Fe(001), showing that the interface oxygen monolayer plays an essential role in MgO growth. Importantly, each squared-shape atomically flat MgO island includes only a few atomic defects, which provides high-quality insulating layers. Further, the MgO islands have an electronic decoupling functionality from the metal substrate. Surface morphology and local electronic structures were studied using a home-built ultrahigh vacuum scanning tunneling microscopy (STM) at a temperature of 5 K.

Creation of Gases with Interplanetary Oxygen Concentration at Atmospheric Pressure by Nanoparticle Aerosol Scavengers: Implications for Metal Processing from nm to mm Range

A gas purification setup (GPS) is presented that is capable of producing ultrapure inert gases, such as nitrogen, at ambient pressure with residual oxygen concentrations of 10–13 ppm and below. The GPS relies on the interaction of metallic aerosol nanoparticles that capture residual oxygen, resulting in metal oxide particle formation. The finally reached residual oxygen level can be illustrated by the 6.8 billion years needed for the creation of one monolayer of oxygen on a substrate. Consequently, the processing of oxygen-affine materials of any size may be shifted from vacuum processes toward machining steps at atmospheric pressure, maintaining an oxygen-free environment in the future. As an outlook, the presented work includes measurements of the electron work function of nanoparticles under purified (10–13 ppm-level) and unpurified process gas (100 ppm-level), pointing at the strong influence of residual oxygen on nanostructured materials with reference to copper nanoparticles. Future syntheses and applications of nanoscaled structures will be inspired by the opportunity to work with true inert gases at ambient pressure, which has been inaccessible so far.

Lattice distortion of oxygen monolayer on Ag(111) observed by scanning probe microscopy

Oxygen molecules (${\mathrm{O}}_{2}$) physically adsorbed on solid surfaces are of interest as low-dimensional spin systems. Monolayers of ${\mathrm{O}}_{2}$ adsorbed on an Ag(111) surface at a low density of less than 1 ML are observed at 4.5 K using scanning tunneling microscopy combined with atomic force microscopy with ${\mathrm{O}}_{2}$ terminated tips. Various domains with their specific density are mixed, each with different rotation angles to the substrate. The shape of the ${\mathrm{O}}_{2}$ lattice correlates with the rotation angle between the monolayer and the substrate. The domains with a rotation angle of greater than $1.{4}^{\ensuremath{\circ}}$ can be explained by the model of on-line coincidence, which had previously been confirmed only for organic molecules. The coincidence with the substrate naturally explains the observed lattice distortion quantitatively. On the other hand, in domains at the rotation angle near ${0}^{\ensuremath{\circ}}$, the lattice distortion is smaller than that of previous study that attributed it to spin. Nevertheless, the length of the shortest distance between two ${\mathrm{O}}_{2}$ implies antiferromagnetic order.

The effect of oxygen-containing species on corrosion behavior of Ta (1 1 0) surface: A DFT study with an experimental verification

Adsorption of oxygen-containing species on the surface of tantalum (Ta) electrode significantly affects its electrochemical corrosion behavior. Density-functional theory (DFT) is employed to investigate the adsorption energies, structural properties and electronic structures of atomic oxygen (O) and molecular water (H2O) on Ta (110) surface. The adsorption behavior of H2O at room temperature is also studied based on ab initio molecular dynamics (AIMD). We find the passivation of Ta metal is mainly attributed to the strong adsorption of oxygen atoms. Thermodynamic results show that bulk Ta2O5 is easily formed at room temperature, which is the fundamental reason for the spontaneous passivation of Ta (110) surface. The formation of an oxygen monolayer (1.00 ML) on Ta (110) surface dramatically increases the work function, making the equilibrium potential of Ta electrode move in the positive direction, thus slowing down the corrosion rate of Ta metal. However, the adsorption of H2O causes a negative work function change, which promotes its anodic dissolution. The electrochemical impedance spectra (EIS) of tantalum foil in three different NH4F-methanol electrolytes (pure, 0.01 M water and oxygen saturated) shows that the charge transfer resistance increases in the sequence RH2O < Rpure < Roxygen, which can be well explained by the results of DFT calculations.

A magnetic map of O/Fe/Mn/Fe(001) multilayer with DFT+U scheme

Zenia et al. (Surface Science 584 (2005) 146152) have shown that a Mn monolayer on Fe(001) is in-plane antiferromagnetic. They have also obtained a ferromagnetic as well as an antiferromagnetic coupling between Mn and Fe in the subsurface layer but those couplings are found higher in energy. When an oxygen monolayer is added this in-plane antiferromagnetic solution becomes unstable. Besides Meza-Aguilar and Demangeat (Eur. Phys. J. B 93 (2020) 107 have shown that the interlayer exchange coupling in Fe/Cr/Fe(001) can be modified by the inclusion of one monolayer of oxygen i.e. O/Fe/Cr/Fe(001). In the present communication, we study the magnetic stability of a Mn monolayer on Fe(001) and the interlayer exchange coupling in Fe/O/Mn/Fe(001) and O/Fe/Mn/Fe(001) multilayers with a DFT+U scheme. The Fe/O/Mn/Fe(001) multilayer is energetically clearly unstable and convergency leads to the displacement of the oxygen monolayer on top of the Fe surface atoms so that we restrict ourselves to the determination of the magnetic map of O/Fe/Mn/Fe(001). The interlayer exchange coupling in Fe/Mn/Fe(001) is modified by adding a monolayer of oxygen atoms on it. This is very similar to the result obtained recently on Fe/Cr/Fe(001) multilayers.

Orbital Rashba effect in a surface-oxidized Cu film

Recent experimental observation of unexpectedly large current-induced spin-orbit torque in surface oxidized Cu on top of a ferromagnet suggested a possible role of the orbital Rashba effect (ORE). With this motivation, we investigate the ORE from first principles by considering an oxygen monolayer on top of a Cu(111) film. We show that surface oxidization of Cu film leads to gigantic enhancement of the ORE for states near the Fermi surface. The resulting chiral orbital texture in the momentum space is exceptionally strong, reaching $\sim 0.5\hbar$ in magnitude. We find that resonant hybridization between O $p$-states and Cu $d$-states is responsible for the emergence of the ORE. We demonstrate that application of an external electric field generates huge orbital Hall current, which is an order of magnitude larger than the spin Hall current found in heavy metals. This implies that "orbital torque" mechanism may be significant in surface oxidized Cu/ferromagnet structures. It also encourages experimental verification of the orbital texture in surface oxidized Cu through optical measurements such as angle-resolved photoemission spectroscopy.

Effect of catalyst preparation and storage on chemical looping epoxidation of ethylene

Chemical looping epoxidation (CLE) of ethylene to ethylene oxide (EO) presents an exciting alternative to the incumbent technology of direct epoxidation of ethylene with O2(g). In CLE, the reaction is still catalysed by Ag but oxygen is provided as Olattice from a solid metal oxide, eliminating the need and limitations of using O2(g). Here, the influence of catalyst preparation in CLE is investigated. The Ag catalyst was impregnated on a solid oxide, SrFeO3, which was used as the donor of Olattice in CLE. Temperature programmed reduction in H2 indicated that O-species participating in CLE can be attributed to the removal of the first monolayer of oxygen in SrFeO3. By changing the temperature of calcination in Ag-SrFeO3 preparation, the size of Ag particles was varied, resulting in a simultaneous increase of selectivity for EO (up to 60%) and conversion of C2H4 (up to 10%). Then, an assessment of the effects of impurities (carbonates, hydroxides), which deposit over time on the Ag-SrFeO3 surface, was performed, showing that impurities deteriorate the catalyst performance in CLE. Finally, the doping of the SrFeO3 with Ce to the A-site of the perovskite was carried out, and a substantial improvement of the conversion of C2H4 was achieved, reaching 15%, while maintaining the 60% selectivity for EO.

Effect of an interfacial oxygen monolayer on the spin-polarization at the Fe/MgO interface: First-principles calculations

Density functional theory is applied to calculate the structural, electronic, and magnetic properties of the Fe/MgO/Fe (001) magnetic tunnel junctions (MTJs) as well as the influence of an interfacial oxygen monolayer by the addition of oxygen atoms at the Fe/MgO interface. The formation of Fe–O bonded atoms, as well as the changes in the spin polarization around the Fermi level of the Fe/MgO/Fe (001) with an interfacial FeO layer, is observed. The calculated spin-polarized density of states of the Fe/MgO/Fe with a Fe–O interface layer showed that the peaks of the O 2s, O 2p, and Fe 3d states are shifted due to the coupling between the O atoms and Fe layer. The Fe/MgO/Fe structure exhibits metallic behavior due to the position of the density of states (DOS) peak at the Fermi level with both majority and minority spins. The calculated spin polarization (P) is only 7% for the metallic Fe/MgO/Fe structure, whereas the metallic Fe/MgO/Fe structure with FeO interfacial layer has P of 24%. Thus, the polarization is found to be extremely sensitive to the nature of the interface layer. Likewise, the nature of spin-charge transfer in both investigated MTJ structures was found to be different at the Fe/MgO interface due to the hybridization interaction between Fe and O. We have also calculated the structural, magnetic, and electronic properties of Fe/MgO/Fe with FeO layer with different sizes of the junction by varying the number of MgO layers, with Fe thickness fixed at 5 ML.

High-temperature oxygen monolayer structures on W(110) revisited

Systematic studies of the two high-temperature monolayer oxygen structures that exist on the (110) tungsten surface were performed using low-energy electron microscopy and diffraction measurements. Our work questions the commonly accepted interpretation from the literature that striped oxygen superstructures arise from alternating site-exchanged (S-E) domains. We postulate that the superstructures originate from a misfit between tungsten and oxygen lattices while the striped appearance corresponds to a moiré pattern. Moreover, we show that the two structures, indicated as 113- and 337-phases due to the characteristic directions of the respective moiré patterns, differ considerably in their symmetry properties. This suggests that oxygen atoms in the two overlayers occupy different adsorption sites on average. In particular, the 113-phase features rotational domains that retain mirror symmetries with respect to the [001] and [11¯0] directions, whereas the 337-phase is characterized by the appearance of additional domains due to the breaking of these symmetries. We propose structural models for both phases that consistently explain their unusual properties and suggest a universal mechanism for the thermal evolution of oxygen monolayer adsorbed on W(110).

Kinetics of the Thermal Oxidation of Ir(100) toward IrO2 Studied by Ambient-Pressure X-ray Photoelectron Spectroscopy.

Using time-lapsed ambient-pressure X-ray photoelectron spectroscopy, we investigate the thermal oxidation of single-crystalline Ir(100) films toward rutile IrO2(110) in situ. We initially observe the formation of a carbon-free surface covered with a complete monolayer of oxygen, based on the binding energies of the Ir 4f and O 1s core level peaks. During a rather long induction period with nearly constant oxygen coverage, the work function of the surface changes continuously as sensed by the gas phase O 1s signal. The sudden and rapid formation of the IrO2 rutile phase with a thickness above 3 nm, manifested by distinct binding energy changes and substantiated by quantitative XPS analysis, provides direct evidence that the oxide film is formed via an autocatalytic growth mechanism that was previously proposed for PbO and RuO2.

Oxygen species stabilized on the cobalt spinel nano-octahedra at various reaction conditions and their role in catalytic CO and CH4 oxidation, N2O decomposition and oxygen isotopic exchange

Periodic spin unrestricted DFT+U calculations joined with atomistic thermodynamics and model catalytic experiments (TPD-O2, 18O2/16O4 exchange, N2O decomposition, CO and CH4 oxidation) were used to study the structure, stability and reactivity of various surface oxygen species and oxygen vacancies, produced under different thermodynamic conditions on the (1 1 1) surface exposed by the cobalt spinel nanooctahedra. We thoroughly analyzed the stability of differently oxygen covered terminations starting from ΘO = 0.86 nm−2 (isolated monoatomic species) up to ΘO = 5.21 nm−2 (full oxygen monolayer). The constructed 3-dimensional thermodynamic (γ, T, pO2) surface redox state diagram revealed that in typical catalytic pressures of O2 (pO2/p° ∼ 0.01– 1) three principal states of the spinel surface, including a region associated with the presence of μ-superoxo CoO3c–O2–CoT3c and metal-oxo CoO3c-O species (T < 450– 500 °C), a bare surface region (450– 500 °C to 550–600 °C), and a region were surface oxygen vacancies appear (T > 600 °C) may be distinguished. This diagram was used for quantitative assignment of the experimental TPD-O2 desorption peaks, and also, for interpretation of the role that various adoxygen and lattice oxygen species play in the investigated model catalytic reactions. It was shown that CO is primarily oxidized by the suprafacial CoO3c-O2-CoT3c diatomic oxygen and/or cobalt-oxo CoO3c-O species, whereas in low temperature activation of CH4 only the CoO3c-O adducts are involved. With the increasing temperature, the suprafacial methane oxidation route gradually changes into the interfacial Mars van Krevelen scheme, and the later pathway is accelerated above 600 °C, due to the onset of oxygen vacancy formation (in line with 18O2/Co316O4 isotopic exchange experiments). In the case of N2O decomposition the reaction occurs preferably on a bare surface, and the favorable thermodynamics provides a steady driving force for efficient removal of the recombined adoxygen intermediates in the form of gaseous O2. The phase diagram of the alterable surface oxygen states on the (1 1 1) termination was compared to the corresponding diagram for the (1 0 0) facet, to explain the nature and differences in catalytic redox behavior of the cobalt spinel nano-octahedra and nano-cubes. This allows also for prediction of oxygen speciation anisotropy on cubo-octahedral nanocrystals at various temperatures.

Tuning Nanoscale Friction by Applying Weak Magnetic Fields to Reorient Adsorbed Oxygen Molecules

Sliding friction levels of thin (1–2 monolayers) and thick (~10 monolayers) oxygen films adsorbed on nickel and gold at 47.5 K have been measured by means of a quartz crystal microbalance (QCM) technique. Friction levels for the thin (thick) films on nickel in the presence of a weak magnetic field were observed to be approximately 30% (50%) lower than those recorded in the absence of the external field. Friction levels for thin films on gold were meanwhile observed to be substantially increased in the presence of the field. Magnetically-induced structural reorientation (magnetostriction) and/or realignment of adlayer spins, which respectively reduce structural and magnetic interfacial corrugation and commensurability, appear likely mechanisms underlying the observed field-induced reductions in friction for the nickel samples. Eddy current formation in the gold substrates may account for the increased friction levels in this system. The work demonstrates the role of magnetic effects in model systems that are highly amenable to theoretical studies and modeling.

Measuring Surface Energies of GaAs (100) and Si (100) by Three Liquid Contact Angle Analysis (3LCAA) for Heterogeneous Nano-BondingTM

ABSTRACTAnalysis of the total surface energy γTand its three components as established by the van Oss-Chaudhury-Good Theory (vOCG) is conducted via Three Liquid Contact Angle Analysis (3LCAA). γTis correlated with the composition of the top monolayers (ML) obtained from High-Resolution Ion Beam Analysis (HR-IBA). Control of γTenables surface engineering for wafer bonding (Nano-BondingTM) and/or epitaxial growth. Native oxides on boron-doped p-Si(100) are found to average γTof 53 ± 1.4 mJ/m2) and are always hydrophilic. An HF in methanol or aqueous HF etch for 60 s always renders Si(100) hydrophobic. Its γTdecreases by 20% to 44 ± 3 mJ/m2in HF in methanol etch and by 10% to 48 ± 3 mJ/m2in aqueous HF. On the contrary, GaAs(100) native oxides are found to always be hydrophobic. Tellurium n+-doped GaAs(100) yields an average of γTof 37 ± 2 mJ/m2, 96% of which is due to the Lifshitz-Van der Waals molecular interactions (γLW= 36 ± 1 mJ/m2). However, hydrophobic GaAs(100) can be made highly hydrophilic. After etching, γTincreases by almost 50% to 66 ± 1.4 mJ/m2. 3LCAA shows that the γTincrease is due to electron acceptor and donor interactions, while the Lifshitz-van der Waals energy γLWremains constant. IBA combining the 3.039 ± 0.01 MeV oxygen nuclear resonance with &lt;111&gt; channeling, shows that oxygen on Si(100) decreases by 10% after aqueous HF etching, from 13.3 ± 0.3 monolayers (ML) to 11.8 ± 0.4 ML 1 hour after etch.Te-doped GaAs(100) exhibits consistent oxygen coverage of 7.2 ± 1.4 ML, decreasing by 50% after etching to a highly hydrophilic surface with 3.6 ± 0.2 oxygen ML. IBA shows that etching does not modify the GaAs surface stoichiometry to within 1% . Combining 3LCAA with HR-IBA provides a quantitative metrology to measure how GaAs and Si surfaces can be altered to a different hydroaffinity and surface termination.

Stereospecific Autocatalytic Surface Explosion Chemistry of Polycyclic Aromatic Hydrocarbons.

Autocatalytic processes are important in many fields of science, including surface chemistry. A better understanding of its mechanisms may improve the current knowledge on heterogeneous catalysis. The thermally induced decomposition of eight different polycyclic aromatic hydrocarbons (PAHs) on a saturated monolayer of atomic oxygen on a Cu(100) surface is studied using temperature-programmed reaction spectroscopy (TPRS), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). 9-Bromo-heptahelicene decomposes autocatalytically in a narrow temperature range into CO2 and H2O, while non-halogenated heptahelicene decomposes into the same products but does not show autocatalytic behavior. Fixation of the hydrocarbon to the surface via the organometallic bond after elimination of the bromine is identified as a prerequisite for the autocatalytic reaction mechanism. Of all the hydrocarbons studied, only those being sterically overcrowded decompose autocatalytically. Such an observation can be explained by facile dehydrogenation of the overcrowded PAHs. The reaction of such hydrogen with oxygen creates vacancies in the oxygen layer which act as active sites and catalyze further decomposition.

(Invited) Enhanced Oxygen Reduction Reaction Activity on Pt-Monolayer-Shell PdIr/Ni-Core Catalysts

Platinum monolayer oxygen reduction reaction catalysts present promising way of reducing the Pt content without scarifying its fuel cell performance. Here, we present a facile way of preparing Pt monolayer shell PdIr-based core catalysts, which showed much higher activity for oxygen reduction reaction than that of TKK 46.6% Pt/C catalyst. Among tested samples, PtMLPd2Ir/Ni/C performs the best with Pt and Platinum Group Metal mass activity around 9 and 0.25 times higher of that of TKK 46.6% Pt/C. In addition, accelerated aging test indicates its excellent durability.

Suppression of material transfer at contacting surfaces: the effect of adsorbates on Al/TiN and Cu/diamond interfaces from first-principles calculations

The effect of monolayers of oxygen (O) and hydrogen (H) on the possibility of material transfer at aluminium/titanium nitride (Al/TiN) and copper/diamond (Cu/Cdia) interfaces, respectively, were investigated within the framework of density functional theory (DFT). To this end the approach, contact, and subsequent separation of two atomically flat surfaces consisting of the aforementioned pairs of materials were simulated. These calculations were performed for the clean as well as oxygenated and hydrogenated Al and Cdia surfaces, respectively. Various contact configurations were considered by studying several lateral arrangements of the involved surfaces at the interface. Material transfer is typically possible at interfaces between the investigated clean surfaces; however, the addition of O to the Al and H to the Cdia surfaces was found to hinder material transfer. This passivation occurs because of a significant reduction of the adhesion energy at the examined interfaces, which can be explained by the distinct bonding situations.

Enhanced Oxygen Reduction Reaction Activity on Pt-Monolayer-Shell PdIr/Ni-Core Catalysts

Platinum monolayer oxygen reduction reaction catalysts offer the most promising possibility of reducing the Pt content without sacrificing its fuel cell performance. Here, we present a facile way of preparing Pt monolayer shell on PdIr-based core catalysts with and without Ni sub-core. They showed much higher activity for oxygen reduction reaction than commercial Pt/C catalyst. Among tested samples, PtMLPd2Ir/Ni/C performs the best having Pt mass activity 14 times and Platinum Group Metal mass activity 0.8 times higher than that of Pt/C. In addition, accelerated durability test indicates its excellent durability, which was demonstrated by a 7.5% decrease in Platinum Group Metal mass activity compared to 64% for the commercial Pt/C catalyst. The role of Ni sub-core in oxygen reduction reaction activity and durability of PtML on PdIr-based catalysts was also elucidated.

Chemical and kinetic insights into the Thermal Decomposition of an Oxide Layer on Si(111) from Millisecond Photoelectron Spectroscopy

Despite thermal silicon oxide desorption is a basic operation in semiconductor nanotechnology, its detailed chemical analysis has not been yet realized via time-resolved photoemission. Using an advanced acquisition system and synchrotron radiation, heating schedules with velocities as high as 100 K.s−1 were implemented and highly resolved Si 2p spectra in the tens of millisecond range were obtained. Starting from a Si(111)-7 × 7 surface oxidized in O2 at room temperature (1.4 monolayer of oxygen), changes in the Si 2p spectral shape enabled a detailed chemical analysis of the oxygen redistribution at the surface and of the nucleation, growth and reconstruction of the clean silicon areas. As desorption is an inhomogeneous surface process, the Avrami formalism was adapted to oxide desorption via an original mathematical analysis. The extracted kinetic parameters (the Avrami exponent equal to ~2, the activation energy of ~4.1 eV and a characteristic frequency) were found remarkably stable within a wide (~110 K) desorption temperature window, showing that the Avrami analysis is robust. Both the chemical and kinetic information collected from this experiment can find useful applications when desorption of the oxide layer is a fundamental step in nanofabrication processes on silicon surfaces.