Oxide Monolayer Research Articles

Self-assembly of octanethiol on oxide-free cobalt electrode from aqueous solution under electrochemical control

Self-assembly of alkanethiols on Co surface from organic solution has met with limited success due to the presence of the native oxides. A way needs to be found to remove any oxide and form perfect self-assembled monolayers (SAMs). We report the development of electrochemical approach for formation of alkanethiol self-assembled monolayers on base metals in aqueous solutions. For formation of octanethiol (OT) SAMs on oxide-free cobalt surfaces, we combine the surface renewal technique with in situ electrochemical procedure of removing the native oxide, thiol adsorption, and electrochemical control of SAMs properties in one aqueous solution. The use of aqueous thiol-containing media results in removing any oxide by electrochemical reduction, accelerated monolayer formation under electrochemical control, and investigating SAMs surface coverage and monolayer integrity by voltammetry. In this work, we study the influence of different factors on self-assembly process such as the surface pretreatment, adsorption time, the presence of dissolved oxygen, solution pH, and potential cycling on polycrystalline cobalt microelectrode from aqueous 0.1 M NaClO4 solutions of 0.1 mM OT. We obtain the high-quality self-assembled monolayers which are stable in wide range of potentials and show blocking characteristics toward the following Faradaic processes: Co surface oxidation and O2 and H+ reduction.

A method to detect triplet exciton transfer from singlet fission materials into silicon solar cells: Comparing different surface treatments.

Singlet fission is one of the most promising routes to overcome the single-junction efficiency limit for solar cells. Singlet fission-enhanced silicon solar cells are the most desirable implementation, but transfer of triplet excitons, the product of singlet fission, into silicon solar cells has proved to be very challenging. Here, we report on an all optical measurement technique for the detection of triplet exciton quenching at semiconductor interfaces, a necessary requirement for triplet exciton or charge transfer. The method relies on the growth of individual, single-crystal islands of the singlet fission material on the silicon surface. The islands have different heights, and we correlate these heights to the quenching efficiency of triplet excitons. The quenching efficiency is measured by spatially resolved delayed fluorescence and compared to a diffusion-quenching model. Using silicon capped with a blocking thermal oxide and aromatic monolayers, we demonstrate that this technique can quickly screen different silicon surface treatments for triplet exciton quenching.

Chemical instability of free-standing boron monolayers and properties of oxidized borophene sheets

In this work we report results of step-by-step modeling of the oxidation of free-standing boron monolayers of different types. Results of the calculations demonstrate that the process of the oxidation is always exothermic and lead toward the formation of foam-like boron oxide films with incorporated non-oxidized small boron clusters. Some of these boron-oxide films demonstrate the presence of chemically stable magnetic centers. Evaluation of the physical properties of oxidized boprophene sheets (OBS) demonstrate it possible application in solar energy, as sensors and coating against leakage of hydrogen.

Surface Stress and Lattice Dynamics in Oxide Ultrathin Films

The lattice misfit between the substrate and an epitaxial film leads in general to static forces, which define the interface stress, and dynamic responses that modify the thin‐film lattice dynamics. Although these are both fundamental concepts that are important for film growth and thin‐film properties, they have not been investigated in a combined way so far. Therefore, herein, surface stress experiments in combination with surface phonon studies for three different, cubic oxide ultrathin film systems are reviewed. Within the class of binary oxides, NiO(001) grown on Ag(001) is chosen, which exhibits a −2.2% lattice mismatch, and BaO(001) on Pt(001), a system with a negligible lattice mismatch. For the ternary oxides, perovskite thin films of BaTiO3 grown epitaxially on Pt(001) with a lattice mismatch of −2.3% are focused upon. The surface stress experiments are conducted with an optical two‐beam curvature technique under in situ growth conditions. Surface and thin‐film phonons are determined by high‐resolution electron energy loss spectroscopy. Surface stress and lattice dynamics are discussed in the range from the oxide monolayer to thin films of about 20 unit cell in thickness.

Surface Plasmon Resonance Biosensor Performance Analysis on 2D Material Based on Graphene and Transition Metal Dichalcogenides

Since graphene was first isolated in 2004, research related to graphene-based 2D material for surface plasmon resonance (SPR) biosensor applications has increased. Recently, other types of 2D materials such as Transition Metal Dichalcogenides have also been investigated. This 2D material has exceptional optical and electronic properties and can be utilized to improve the performance of biosensors. The performance of SPR biosensors can be determined based on their sensitivity and detection accuracy (DA). To find out the sensitivity and DA, simulation approach can be done. In this paper, we perform SPR simulations on monolayer and multilayer structures of 2D material, namely graphene, graphene oxide, molibdenum disulfida, and tungsten diselenide. In addition, we also investigated the sensitivity and DA of SPR biosensors on hybrid structures. The results obtained indicate that the use of 2D material can increase the sensitivity of the SPR up to 5 times when compared to conventional structures. Of the four types of 2D material investigated, GO is the material with the best accuracy. If the SPR biosensor performance is determined based on the FOM value, the best performance is owned by the GO-based SPR structure with a sensitivity of 151.87 deg/RIU and DA 0.22 deg-1.

Structural and electronic properties of Stanene-BeO heterobilayer

Properties of Sn/BeO heterostructure formed with beryllium oxide (BeO) monolayer and 2D stanene (Sn) is studied in this work. The first-principle study is employed here to systematically investigate the structural stability and electrical properties of the Sn/BeO heterostructure. The results from simulations reveal that the introduction of BeO not only leads to a significant bandgap opening of 98 meV, but it also retains the various intrinsic electrical properties of stanene to a large extent. The effect of spin-orbit coupling (SOC) is studied both in pristine stanene as well as in Sn/BeO heterostructure. The Sn/BeO heterostructure shows the Rashba-type of spin-splitting under SOC, which is very promising for application in spintronic devices. Moreover, it is also observed that the bandgap can be tuned by applying external strain and electric field, while the characteristic Dirac cone is maintained throughout. The application of an external electric field is found to be more effective in bandgap modulation. It leads to a linear change in the bandgap, with a bandgap value of 402 meV for 4 V nm−1. The results obtained from our study indicate that Sn/BeO heterostructure can be a suitable material for the development of spintronic devices.

Flexible two-dimensional indium tin oxide fabricated using a liquid metal printing technique

Indium tin oxide (ITO) is a transparent conductor used in applications such as touch screens, smart windows and displays. A key limitation of ITO is its brittle nature, which prohibits its use in flexible electronics. The commercial deposition of high-quality ITO also currently relies on a costly vacuum manufacturing approach. Here we report the centimetre-scale synthesis of flexible two-dimensional ITO using a low-temperature liquid metal printing technique. The approach can directly deposit monolayer or bilayer ITO onto desired substrates, with the resulting bilayer samples offering a transparency above 99.3% and a sheet resistance as low as 5.4 kΩ □−1. We also show that the bilayer ITO features a stratified structure with a pronounced van der Waals spacing. To illustrate the capabilities of the technique, we develop a capacitive touch screen using centimetre-sized monolayer ITO sheets. A liquid metal printing technique can be used to create monolayer and bilayer indium tin oxide, with the bilayer samples offering a transparency above 99.3% and a sheet resistance as low as 5.4 kΩ □−1.

The reaction of oleic acid monolayers with gas-phase ozone at the air water interface: the effect of sub-phase viscosity, and inert secondary components.

Organic films that form on atmospheric particulate matter change the optical and cloud condensation nucleation properties of the particulate matter and consequently have implications for modern climate and climate models. The organic films are subject to attack from gas-phase oxidants present in ambient air. Here we revisit in greater detail the oxidation of a monolayer of oleic acid by gas-phase ozone at the air-water interface as this provides a model system for the oxidation reactions that occur at the air-water interface of aqueous atmospheric aerosol. Experiments were performed on monolayers of oleic acid at the air-liquid interface at atmospherically relevant ozone concentrations to investigate if the viscosity of the sub-phase influences the rate of the reaction and to determine the effect of the presence of a second component within the monolayer, stearic acid, which is generally considered to be non-reactive towards ozone, on the reaction kinetics as determined by neutron reflectometry measurements. Atmospheric aerosol can be extremely viscous. The kinetics of the reaction were found to be independent of the viscosity of the sub-phase below the monolayer over a range of moderate viscosities, , demonstrating no involvement of aqueous sub-phase oxidants in the rate determining step. The kinetics of oxidation of monolayers of pure oleic acid were found to depend on the surface coverage with different behaviour observed above and below a surface coverage of oleic acid of ∼1 × 1018 molecule m-2. Atmospheric aerosol are typically complex mixtures, and the presence of an additional compound in the monolayer that is inert to direct ozone oxidation, stearic acid, did not significantly change the reaction kinetics. It is demonstrated that oleic acid monolayers at the air-water interface do not leave any detectable material at the air-water interface, contradicting the previous work published in this journal which the authors now believe to be erroneous. The combined results presented here indicate that the kinetics, and thus the atmospheric chemical lifetime for unsaturated surface active materials at the air-water interface to loss by reaction with gas-phase ozone, can be considered to be independent of other materials present at either the air-water interface or in the aqueous sub-phase.

Atomic structures of V–Ti–O intermixed oxide monolayer on rutile TiO2(011) substrate predicted by extensive structural search

The formation of various structural V–Ti–O intermixed oxide monolayers on the rutile TiO2(011) substrates have been observed in experiments. The atomic structures and formation mechanism of them are still unclear. To this end, a global search of V–Ti–O ordered structures on rutile TiO2(011) surface is performed based on the evolutionary method. Nine energetically stable or metastable structures of different compositions are predicted, which can be considered as V-doping in different type of 2 × 1 reconstruction of rutile TiO2 (011) surface. The calculated electronic structures indicate that the V atoms in most structures are not fully oxidized and introduce localized impurity states in the band gap. The oxidation states of the V atoms and surface Ti atoms are determined. The stabilization regions of them are determined based on the calculated surface energies. Our results are instructive for the experimental synthesis of different structural V–Ti–O monolayers, and significant for deep understanding of the photo- or electro-catalytic properties of them.

Honeycomb silicon on alumina: Massless Dirac fermions in silicene on substrate

We predict the stability of a graphenelike silicene sheet on one monolayer of aluminum oxide. We find that the honeycomb buckled structure of silicene is not broken upon interaction with one monolayer of ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ in the kagome geometry. As a consequence, the electronic band structure shows unperturbed cones with massless Dirac fermions embedded into ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$-derived bands. The heterostructure conserves the topological character of the freestanding silicene. The optical properties of the silicene/${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ bilayers clearly maintain the characteristic infrared universal limit of $\ensuremath{\pi}\ensuremath{\alpha}$. A quantized absorbance $N\ifmmode\cdot\else\textperiodcentered\fi{}\ensuremath{\pi}\ensuremath{\alpha}$ (with $N$ silicene layers) also occurs for stacking of multilayers.

Efficient Formulation of Ab Initio Quantum Embedding in Periodic Systems: Dynamical Mean-Field Theory.

We present an efficient ab initio dynamical mean-field theory (DMFT) implementation for quantitative simulations in solids. Our DMFT scheme employs ab initio Hamiltonians defined for impurities comprising the full unit cell or a supercell of atoms and for realistic quantum chemical basis sets. We avoid double counting errors by using Hartree-Fock as the low-level theory. Intrinsic and projected atomic orbitals (IAO + PAO) are chosen as the local embedding basis, facilitating numerical bath truncation. Using an efficient integral transformation and coupled-cluster Green's function impurity solvers, we are able to handle embedded impurity problems with several hundred orbitals. We apply our ab initio DMFT approach to study a hexagonal boron nitride monolayer, crystalline silicon, and nickel oxide in the antiferromagnetic phase, with up to 104 and 78 impurity orbitals in the spin-restricted and unrestricted cluster DMFT calculations and over 100 bath orbitals. We show that our scheme produces accurate spectral functions compared to both benchmark periodic coupled-cluster computations and experimental spectra.

A Review on Common Methods for Characterizing Graphene Oxide (GO)

Graphene oxide, two-dimensional material with the thickness of 1.1±0.2 nm, has gained attention to a greater extent in the field of science for its radically distinctive properties: physical, chemical, optical as well as electrical etc. Graphene oxide (monolayer sheet) has been synthesized by oxidizing graphite (millions of layer) to graphite oxide (multilayers) which has been converted into graphene oxide via exfoliation followed by sonication and centrifugation - a method mentioned as Modified Hummer Method. I focus on the chemical structure of graphene oxide. However, I discuss the different analytical methods such as UV-Visible spectroscopy, Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR) as well as X-ray Diffraction pattern for characterizing the graphene oxide. Furthermore, this review covers the analytical evaluation of graphene oxide and discuss the past, present and future of graphene oxide in the scientific world.

High-temperature superconductivity in monolayer Bi2Sr2CaCu2O8+δ.

Although copper oxide high-temperature superconductors constitute a complex and diverse material family, they all share a layered lattice structure. This curious fact prompts the question of whether high-temperature superconductivity can exist in an isolated monolayer of copper oxide, and if so, whether the two-dimensional superconductivity and various related phenomena differ from those of their three-dimensional counterparts. The answers may provide insights into the role of dimensionality in high-temperature superconductivity. Here we develop a fabrication process that obtains intrinsic monolayer crystals of the high-temperature superconductor Bi2Sr2CaCu2O8+δ (Bi-2212; here, a monolayer refers to a half unit cell that contains two CuO2 planes). The highest superconducting transition temperature of the monolayer is as high as that of optimally doped bulk. The lack of dimensionality effect on the transition temperature defies expectations from the Mermin-Wagner theorem, in contrast to the much-reduced transition temperature in conventional two-dimensional superconductorssuch as NbSe2. The properties of monolayer Bi-2212 become extremely tunable; our survey of superconductivity, the pseudogap, charge order and the Mott state at various doping concentrations reveals that the phases are indistinguishable from those in the bulk. Monolayer Bi-2212 therefore displays all the fundamental physics of high-temperature superconductivity. Our results establish monolayer copper oxides as a platform for studying high-temperature superconductivity and other strongly correlated phenomena in two dimensions.

Copper Oxidation on Pt(111)—More than a Surface Oxide?

The growth of copper oxide thin films on a Pt(111) support has been analyzed with a combination of scanning tunneling microscopy, electron-diffraction, and X-ray photoelectron spectroscopy. At low preparation temperature, only a monolayer oxide is formed, whereas most of the precipitated copper accumulates as Cu(111) at the Pt interface. The respective films thus resemble the well-known “29” and “44” surface oxides that are composed of distorted Cu–O six-membered rings and develop in a similar manner upon low-pressure oxidation of Cu(111). With increasing temperature, unreacted Cu mixes with the Pt top layers, resulting in a well-ordered Cu3Pt surface alloy. Following these structural and chemical modifications in the support, the surface oxide undergoes an order–disorder transition, in which the Cu–O honeycomb layer forms an increasing number of 5–7 defects and later disrupts into isolated oxide islands. The most prominent island types, especially at high temperature, are Cu3O shamrock units bound to Pt surface atoms that self-assemble into highly crystalline 2√3 domains on the surface. The preparation of thicker Cu2O films, on the other hand, appears challenging on Pt(111), partly because of difficulties to oxidize copper in a vacuum experiment and partly because of an unfavorable lattice match.

Unraveling the Dynamic Network in the Reactions of an Alkyl Aryl Ether Catalyzed by Ni/γ-Al2O3 in 2-Propanol.

The reductive cleavage of aryl ether linkages is a key step in the disassembly of lignin to its monolignol components, where selectivity is determined by the kinetics of multiple parallel and consecutive liquid-phase reactions. Triphasic hydrogenolysis of 13C-labeled benzyl phenyl ether (BPE, a model compound for the major β-O-4 linkage in lignin), catalyzed by Ni/γ-Al2O3, was observed directly at elevated temperatures (150-175 °C) and pressures (79-89 bar) using operando magic-angle spinning NMR spectroscopy. Liquid-vapor partitioning in the NMR rotor was quantified using the 13C NMR resonances for the 2-propanol solvent, whose chemical shifts report on the internal reactor temperature. At 170 °C, BPE is converted to toluene and phenol with k1 = 0.17 s-1 gcat-1 and an apparent activation barrier of (80 ± 8) kJ mol-1. Subsequent phenol hydrogenation occurs much more slowly (k2 = 0.0052 s-1 gcat-1 at 170-175 °C), such that cyclohexanol formation is significant only at higher temperatures. Toluene is stable under these reaction conditions, but its methyl group undergoes facile H/D exchange (k3 = 0.046 s-1 gcat-1 at 175 °C). While the source of the reducing equivalents for both hydrogenolysis and hydrogenation is exclusively H2/D2(g) rather than the alcohol solvent at these temperatures, the initial isotopic composition of adsorbed H/D on the catalyst surface is principally determined by the solvent isotopic composition (2-PrOH/D). All reactions are preceded by a pronounced induction period associated with catalyst activation. In air, Ni nanoparticles are passivated by a surface oxide monolayer, whose removal under H2 proceeds with an apparent activation barrier of (72 ± 13) kJ mol-1. The operando NMR spectra provide molecularly specific, time-resolved information about the multiple simultaneous and sequential processes as they occur at the solid-liquid interface.

Oxidation of WS2 and WSe2 monolayers by ultraviolet-ozone treatment

We report the oxidation behavior of mechanically-exfoliated WS2 and WSe2 monolayers by ultraviolet-ozone treatment. After 1 min of ultraviolet-ozone treatment, photoluminescence and Raman measurement suggested the oxidation-induced doping of WS2 and WSe2. As the duration of ultraviolet-ozone treatment increased to 6 min, the quenching of photoluminescence and Raman spectra suggested severe oxidation of WS2 and WSe2. While x-ray photoelectron spectroscopy measurement suggested the oxidation of ultraviolet-ozone treated WS2 and WSe2 into WO2 or WO3, the formation of WO3 was confirmed by the measurement of transistor characterisitics of oxidized WS2 and WSe2. These results demonstrate that the oxidation behavior of WS2 and WSe2 monolayers significantly depends on the conditions of ultraviolet-ozone treatment, highlighting the importance of process optimization for the device integration of transition metal dichalcogenides including WS2 and WSe2.

Modeling the thickness distribution of silicon oxide thin films grown by reactive magnetron sputtering

Target poisoning in reactive sputtering is one of the biggest problems in the coatings industry. In this work, we use the co-sputtering simulation reactive mode software, Co-SS Rm, to present a different solution of the compound formation both on the target and on the substrate, based on the comparison of the simulations with the thickness measurements of the films deposited at different flows of reactive gas. In these simulations, the spatial distribution of the deposited thin films is determined by analysing the angular distribution of the atoms and/or compound ejected from the target, as well as its sputtering yield. Results indicate that the target surface poisoning suffers an evolution ejecting metallic atoms and silicon oxide until the whole racetrack area is covered by silicon oxide, but at the same time, there is a reduction in the racetrack area due to the growth of several monolayers of silicon oxide. Spectroscopic ellipsometry analysis showed the formation of SiO2 in the thin films in almost all the reactive gas flows. Simulation results were validated by thickness measurements of actual thin films and plasma optical emission spectroscopy by analysing emission transitions from the target species (Si).

Atomic and electronic structure of an epitaxial Nb2O3 honeycomb monolayer on Au(111)

The experimental discovery and theoretical analysis of an epitaxial (2 \ifmmode\times\else\texttimes\fi{} 2) honeycomb $\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{3}$ monolayer on a Au(111) surface is reported. The oxide monolayer is grown by Nb deposition and subsequent annealing in an oxidizing atmosphere. Scanning tunneling microscopy (STM) images show that the films form a well-ordered honeycomb lattice, and low energy electron diffraction patterns confirm that the films adopt a (2 \ifmmode\times\else\texttimes\fi{} 2) periodicity with respect to the Au(111) substrate. Density functional theory (DFT) modeling shows that the Nb atoms are located in Au(111) threefold hollow sites and the O atoms are located in on-top positions. DFT also demonstrates the existence of a strong interfacial interaction characterized by a large electron transfer towards the Au substrate, an increase of the Nb oxidation state, and substantial film rumpling. High-resolution STM images, supported by simulations, are able to discriminate between Nb atoms adsorbed in fcc or hcp hollow sites on the Au(111) substrate.

Chemical Immobilization of Graphene Oxide on Hydrogen Terminated Silicon via Vinyl Aniline Molecule Linking

In this work, graphene oxide (GO) monolayers were immobilized on a hydrogen terminated silicon (H-Si) substrate by applying 4-vinylaniline (VAn) molecules as the intermediate linkers. Surface prope...

Acid sites on silica-supported molybdenum oxides probed by ammonia adsorption: Experiment and theory

The origin of Brønsted acidity in a series of silica-supported molybdenum oxide catalysts with Mo loadings of 2.1–13.3 wt%, and apparent Mo surface densities of 0.2–2.5 nm−2, respectively, was analyzed by ammonia adsorption investigated by temperature-programmed desorption, infrared spectroscopy, and DFT calculations. Every surface molybdenum atom in the molybdenum oxide (sub-)monolayer is involved in the interaction with ammonia, either as Lewis or as Brønsted acid site. A model is proposed that ascribes Brønsted acidity to the interaction between silanol groups and adjacent surface molybdate species under formation of pseudo-bridging Si—O(H)---Mo(=O)2 species with a Mo---O(Si) distance of 2.1 Å and a N-H(OSi) distance of <1.1 Å in the formed adsorption complex of the ammonia molecule. The combined experimental and computational study contributes to an improved fundamental understanding of acidity in amorphous mixed metal oxides.