By simulating the dissociation of O2 molecules on the Be(0001) surface using the first-principles molecular dynamics approach, we propose a new atomistic model for the surface oxidation of sp metals. In our model, only the dissociation of the first oxygen molecule needs to overcome an energy barrier, while the subsequent oxygen molecules dissociate barrierlessly around the adsorption area. Consequently, oxide islands form on the metal surface, and grow up in a lateral way. We also discover that the firstly dissociated oxygen atoms are not so mobile on the Be(0001) surface, as on the Al(111) surface. Our atomistic model enlarges the knowledge on metal surface oxidations by perfectly explaining the initial stage during the surface oxidation of Be, and might be applicable to some other sp metal surfaces.
Employing electrons for direct control of nanoscale reaction is highly desirable since it provides fabrication of nanostructures with different properties at atomic resolution and with flexibility of dimension and location. Here, applying in situ transmission electron microscopy, we show the reversible oxidation and reduction kinetics in Ag, well controlled by changing the dose rate of electron beam. Aberration-corrected high-resolution transmission electron microscopy observation reveals that O atoms are preferably inserted and extracted along the {111} close-packed planes of Ag, leading to t…
Present knowledge of the function of materials is largely based on studies (experimental and theoretical) that are performed at low temperatures and ultra-low pressures. However, the majority of everyday applications, like e.g. catalysis, operate at atmospheric pressures and temperatures at or higher than 300 K. Here we employ ab initio, atomistic thermodynamics to construct a phase diagram of surface structures in the (T,p)-space from ultra-high vacuum to technically-relevant pressures and temperatures. We emphasize the value of such phase diagrams as well as the importance of the reaction ki…
A thorough understanding of native oxides is essential for designing semiconductor devices. Here we report a study of the rate and mechanisms of spontaneous oxidation of bulk single crystals of ZrS$_x$Se$_{2-x}$ alloys and MoS$_2$. ZrS$_x$Se$_{2-x}$ alloys oxidize rapidly, and the oxidation rate increases with Se content. Oxidation of basal surfaces is initiated by favorable O$_2$ adsorption and proceeds by a mechanism of Zr-O bond switching, that collapses the van der Waals gaps, and is facilitated by progressive redox transitions of the chalcogen. The rate-limiting process is the formation a…
We present a detailed atomistic picture of the oxidation mechanism of Co nanoparticles and its impact on magnetism by experimentally following the evolution of the structure, chemical composition, and magnetism of individual, gas-phase grown Co nanoparticles during controlled oxidation. The early stage oxidation occurs in a twostep process characterized by (i) the initial formation of small CoO crystallites randomly distributed across the nanoparticle surface, until their coalescence leads to structural completion of the oxide shell and passivation of the metallic core; (ii) progressive conver…
The oxidation and corrosion of metals are fundamental problems in materials science and technology that have been studied using a large variety of experimental and computational techniques. Here we review some of the recent studies that have led to significant advances in our atomic-level understanding of copper oxide, one of the most studied and best understood metal oxides. We show that a good atomistic understanding of the physical characteristics of cuprous (Cu$_2$O) and cupric (CuO) oxide and of some key processes of their formation has been obtained. Indeed, the growth of the oxide has b…
To help understand the high activity of silver as an oxidation catalyst, e.g., for the oxidation of ethylene to epoxide and the dehydrogenation of methanol to formaldehyde, the interaction and stability of oxygen species at the Ag(111) surface has been studied for a wide range of coverages. Through calculation of the free energy, as obtained from density-functional theory and taking into account the temperature and pressure via the oxygen chemical potential, we obtain the phase diagram of O/Ag(111). Our results reveal that a thin surface-oxide structure is most stable for the temperature and p…
Understanding surface reactivity is crucial in many fields, going from heterogeneous catalysis to materials oxidation and corrosion. In order to decipher the surface reactions of ZrB2 exposed to the harsh environment of aerospace components, the chemical activity of both Zr- and B-surfaces is predicted and compared by using density functional theory and nudged elastic band methods. In particular the adsorption, dissociation and diffusion of O2, CO and H2O are extensively examined through the calculation of surface adsorption energies and reaction pathways. We find the dissociative adsorption o…
The oxygen dissociation and the oxidized structure on the pristine C3N monolayer in exposure to air are the inevitably critical issues for the C3N engineering and surface functionalization yet have not been revealed in detail. Using the first-principles calculations, we have systematically investigated the possible O2 adsorption sites, various O2 dissociation pathways and the oxidized structures. It is demonstrated that the pristine C3N monolayer shows more O2 physisorption sites and exhibits stronger O2 adsorption than the pristine graphene. Among various dissociation pathways, the most prefe…
Titanium is commonly added to nickel superalloys but has a well-documented detrimental effect on oxidation resistance. The present work constitutes the first atomistic-scale quantitative measurements of grain boundary and bulk compositions in the oxide scale of a current generation polycrystalline nickel superalloy performed through atom probe tomography. Titanium was found to be particularly detrimental to oxide scale growth through grain boundary diffusion.
Understanding oxidation mechanisms of layered semiconducting transition-metal dichalcogenide (TMDC) is important not only for controlling native oxide formation but also for synthesis of oxide and oxysulfide products. Here, reactive molecular dynamics simulations show that oxygen partial pressure controls not only the ZrS2 oxidation rate but also the oxide morphology and quality. We find a transition from layer-by-layer oxidation to amorphous-oxide-mediated continuous oxidation as the oxidation progresses, where different pressures selectively expose different oxidation stages within a given t…
The oxidation of α-Ti(0001) surface was studied using density functional theory. In order to enhance the oxidation resistance, we substituted Ti atoms with Si atoms in Ti(0001) surface. We observe that Si prefers to segregate at the surface layer of Ti(0001) compared to other sub-surface layers. The Si solubility on the Ti(0001) surface was estimated to be around 75 at.%. While Si segregation was found to reduce the binding between the oxygen atom and the Ti(0001) surface, the barrier for oxygen to diffuse into the slab increases. The dissociation of the oxygen molecule on the clean and Si seg…
We report on the oxidation of self-assembled silicene nanoribbons grown on the Ag(110) surface using Scanning Tunneling Microscopy and High-Resolution Photoemission Spectroscopy. The results show that silicene nanoribbons present a strong resistance towards oxidation using molecular oxygen. This can be overcome by increasing the electric field in the STM tunnel junction above a threshold of +2.6 V to induce oxygen dissociation and reaction. The higher reactivity of the silicene nanoribbons towards atomic oxygen is observed as expected. The HR-PES confirm these observations: Even at high exposu…
Hydration properties of graphene oxide (GO) are essential for most of its potential applications. In this work, we employ atomistic molecular dynamics simulations to investigate seven GO compositions with different levels of oxygenation. Two atomic charge models for GO are compared: (1) sp2 carbons are purely Lennard-Jones sites; (2) sp2 carbon charges are consistent with the CHELPG scheme. Structural properties were found to depend insignificantly on the charge model, whereas thermodynamics appeared very sensitive. In particular, the simplified model provides systematically stronger GO/water …
Layered indium selenide (InSe), a new two-dimensional (2D) material with a hexagonal structure and semiconducting characteristic, is gaining increasing attention owing to its intriguing electronic properties. Here, by using first-principles calculations, we reveal that perfect InSe possesses a high chemical stability against oxidation, superior to MoS2. However, the presence of intrinsic Se vacancy (VSe) and light illumination can markedly affect the surface activity. In particular, the excess electrons associated with the exposed In atoms at the VSe site under illumination are able to remarka…
Supported vanadia catalysts are modeled within the cluster DFT approach to get an insight into the mechanism in which the support affects the activity of vanadia in the oxidation processes. The energy of the V=O group dissociation chosen as a descriptor of the oxidation activity is estimated using two aligned divanadate V2O3(OH)4 particles at various distances. Separation between particles allows to imitate (i) the various supporting materials (e.g. TiO2, SiO2, etc.), and (ii) the coverage of vanadia on a particular support. A substantial compensation of the energy loss upon the vanadyl oxygen…
The oxidation of the Pd(100) surface at oxygen pressures in the 10^-6 to 10^3 mbar range and temperatures up to 1000 K has been studied in-situ by surface x-ray diffraction (SXRD). The results provide direct structural information on the phases present in the surface region and on the kinetics of the oxide formation. Depending on the (T,p) environmental conditions we either observe a thin sqrt(5) x sqrt(5) R27 surface oxide or the growth of a rough, poorly ordered bulk oxide film of PdO predominantly with (001) orientation. By either comparison to the surface phase diagram from first-principle…
The paper reports on a theoretical description of work function of TiN, which is one of the most used materials for the realization of electrodes and gates in CMOS devices. Indeed, although the work function is a fundamental quantity in quantum mechanics and also in device physics, as it allows the understanding of band alignment at heterostructures and gap states formation at the metal/semiconductor interface, the role of defects and contaminants is rarely taken into account. Here, by using first principles simulations, we present an extensive study of the work function dependence on nitrogen…
Using density-functional theory we assess the stability of bulk and surface oxides of the late 4d transition metals in a ``constrained equilibrium'' with a gas phase formed of O2 and CO. While the stability range of the most stable bulk oxide extends for ruthenium well into gas phase conditions representative of technological CO oxidation catalysis, this is progressively less so for the 4d metals to its right in the periodic system. Surface oxides could nevertheless still be stable under such conditions. These thermodynamic considerations are discussed in the light of recent experiments, empha…
We have performed electronic state calculations to clarify the initial stage of the oxidation of the Si- and C-faces in 4H-SiC based on the density-functional theory. We investigate how each Si and C atomic site is oxidized on C- and Si-face, and explore most probable reaction pathways, corresponding energy barriers, and possible defects generated during the oxidation. We have found that carbon annihilation process is different between on Si- and on C-face, and this difference causes different defects in interface; In C-face case, (1), carbon atoms are dissociated directly from the substrate a…
Water absorption and dissociation processes on pristine low-index TiO$_2$ interfaces are important but poorly understood outside the well-studied anatase (101) and rutile (110). To understand these, we construct three sets of machine learning potentials that are simultaneously applicable to various TiO$_2$ surfaces, based on three density-functional-theory approximations. We compute the water dissociation free energies on seven pristine TiO$_2$ surfaces, and predict that anatase (100), anatase (110), rutile (001), and rutile (011) surfaces favor water dissociation, anatase (101) and rutile (10…
Silicon together with its native oxide SiO$_2$ was recognized as an outstanding material system for the semiconductor industry in the 1950s. In state-of-the-art device technology, SiO$_2$ is widely used as an insulator in combination with high-$k$ dielectrics such as HfO$_2$, demanding fabrication of ultra-thin interfacial layers. The classical standard model derived by Deal and Grove accurately describes the oxidation of Si in a progressed stage, however, strongly underestimates growth rates for thin oxide layers. Recent studies report a variety of oxidation mechanisms during the growth of ox…
Role of Oxygen during Methane Oxidation on Pd$_1$/PdO$_1$@CeO$_2$ Surface: A Combined Density Functional Theory, Microkinetic, and Machine Learning Approach
The results of DFT supercell calculations of oxygen behavior upon the UN (001) and (110) surfaces as well as at the tilt grain boundary are presented. Oxygen adsorption, migration, incorporation into the surface N vacancies on (001) and (110) surfaces have been modeled using 2D slabs of different thicknesses and supercell sizes. The temperature dependences of the N vacancy formation energies and oxygen incorporation energies are calculated. We demonstrate that O atoms easily penetrate into UN surfaces and grain boundaries containing N vacancies, due to negative incorporation energies and a sma…
Empirical Tight Binding (TB) methods are widely used in atomistic device simulations. Existing TB methods to passivate dangling bonds fall into two categories: 1) Method that explicitly includes passivation atoms is limited to passivation with atoms and small molecules only. 2) Method that implicitly incorporates passivation does not distinguish passivation atom types. This work introduces an implicit passivation method that is applicable to any passivation scenario with appropriate parameters. This method is applied to a Si quantum well and a Si ultra-thin body transistor oxidized with SiO2 i…
Conceptual 2D group III nitrides and oxides (e.g., 2D InN and 2D InO) in heterostructures with graphene have been realized by metalorganic chemical vapor deposition (MOCVD). MOCVD is credited with being central to fabrication of established semiconductor materials and by purpose for an advance in emergent semiconductor materials at the 2D limit. A defining characteristic of MOCVD is the employment of metalorganic precursors such as trimethyl-indium, -gallium, and -aluminum, which contain (strong) metal-carbon bonds. Mechanisms that regulate MOCVD processes at the atomic scale are largely unkno…
We report static and dynamic first-principles calculations that provide atomistic pictures of the initial stage of the oxidation processes occurring at the ($11\bar20$) surface of 4H-SiC. Our results unveil reaction pathways and their associated free-energy barriers for the adsorption of oxygen and the desorption of carbon atoms. We find that oxygen adsorption shows structural multi-stability and that the surface-bridge sites are the most stable and crucial sites for subsequent oxidation. We find that an approaching O$_2$ molecule is adsorbed, then dissociated and finally migrates toward these…
The uranium mononitride (UN) is a material considered as a promising candidate for Generation-IV nuclear reactor fuels. Unfortunately, oxygen in air affects UN fuel performance and stability. Therefore, it is necessary to understand the mechanism of oxygen adsorption and further UN oxidation in the bulk and at surface. Recently, we performed a detailed study on oxygen interaction with UN surface using density functional theory (DFT) calculations. We were able to identify an atomistic mechanism of UN surface oxidation consisting of several important steps, starting with the oxygen molecule diss…
The turn-over frequency of the catalytic oxidation of CO at RuO2(110) was calculated as function of temperature and partial pressures using ab initio statistical mechanics. The underlying energetics of the gas-phase molecules, dissociation, adsorption, surface diffusion, surface chemical reactions, and desorption were obtained by all-electron density-functional theory. The resulting CO2 formation rate [in the full (T, p_CO, p_O2)-space], the movies displaying the atomic motion and reactions over times scales from picoseconds to seconds, and the statistical analyses provide insights into the co…
We propose a scheme for global optimization with first-principles energy expressions (GOFEE) of atomistic structure. While unfolding its search, the method actively learns a surrogate model of the potential energy landscape on which it performs a number of local relaxations (exploitation) and further structural searches (exploration). Assuming Gaussian Processes, an acquisition function is used to decide on which of the resulting structures is the more promising. Subsequently, a single point first-principles energy calculation is conducted for that structure. The method is demonstrated to outp…
Surface oxidation processes are crucial for the functionality of Cu-based catalytic systems used for methanol synthesis, partial oxidation of methanol or the water-gas shift reaction. We assess the stability and population of the "8"-structure, a $|\begin{smallmatrix} 3&2\\ -1&2 \end{smallmatrix}|$ oxide phase, on the Cu(111) surface. This structure has been observed in x-ray photoelectron spectroscopy and low-energy electron diffraction experiments as a Cu(111) surface reconstruction that can be induced by a hyperthermal oxygen molecular beam. Using density-functional theory calculations in c…
The phase diagram of surface structures for the model catalyst RuO2(110) in contact with a gas environment of O2 and CO is calculated by density-functional theory and atomistic thermodynamics. Adsorption of the reactants is found to depend crucially on temperature and partial pressures in the gas phase. Assuming that a catalyst surface under steady-state operation conditions is close to a constrained thermodynamic equilibrium, we are able to rationalize a number of experimental findings on the CO oxidation over RuO2(110). We also calculated reaction pathways and energy barriers. Based on the v…
During aqueous corrosion, atoms in the solid react chemically with oxygen, leading either to the formation of an oxide film or to the dissolution of the host material. Commonly, the first step in corrosion involves an oxygen atom from the dissociated water that reacts with the surface atoms and breaks near surface bonds. In contrast, hydrogen on the surface often functions as a passivating species. Here, we discovered that the roles of O and H are reversed in the early corrosion stages on a Si terminated SiC surface. O forms stable species on the surface, and chemical attack occurs by H that b…
Using scanning tunneling microscopy (STM), we investigate oxide-induced growth pits in Si thin films deposited by molecular beam epitaxy. In the transition temperature range from 2D adatom islanding to step-flow growth, systematic controlled air leaks into the growth chamber induce pits in the growth surface. We show that pits are also correlated with oxygen-contaminated flux from Si sublimation sources. From a thermodynamic standpoint, multilayer growth pits are unexpected in relaxed homoepitaxial growth, whereas oxidation is a known cause for step pinning, roughening, and faceting on element…
Engineering NV Centers via Hydrogen-Driven Defect Chemistry in CVD Diamonds for Quantum Applications: NVHx Dissociations into NV, Origin of 468nm Center, and Cause of Brown Coloration
