Surface Phase Diagram Research Articles

Automated Discovery and Construction of Surface Phase Diagrams Using Machine Learning.

Surface phase diagrams are necessary for understanding surface chemistry in electrochemical catalysis, where a range of adsorbates and coverages exist at varying applied potentials. These diagrams are typically constructed using intuition, which risks missing complex coverages and configurations at potentials of interest. More accurate cluster expansion methods are often difficult to implement quickly for new surfaces. We adopt a machine learning approach to rectify both issues. Using a Gaussian process regression model, the free energy of all possible adsorbate coverages for surfaces is predicted for a finite number of adsorption sites. Our result demonstrates a rational, simple, and systematic approach for generating accurate free-energy diagrams with reduced computational resources. The Pourbaix diagram for the IrO2(110) surface (with nine coverages from fully hydrogenated to fully oxygenated surfaces) is reconstructed using just 20 electronic structure relaxations, compared to approximately 90 using typical search methods. Similar efficiency is demonstrated for the MoS2 surface.

Structure and magnetism of the(2×2)-FeO(111) surface

Based on ab initio calculations we determine the features and relative stability of different models proposed to describe the $(2 \times 2)$-FeO(111) reconstruction. Our results suggest that both wurtzite and spinel-like environments are possible, and their coexistence explains phenomena of biphase ordering. The surface phase diagram reflects a competition of charge and magnetic compensation effects, and reveals the important influence of the substrate on the final surface structure. Though antiferromagnetic couplings are dominant, frustration and the delicate balance of surface and bulk exchange interactions lead to a net surface magnetization that accounts for the large measured values.

Thermodynamic Control of Two-Dimensional Molecular Ionic Nanostructures on Metal Surfaces.

Bulk molecular ionic solids exhibit fascinating electronic properties, including electron correlations, phase transitions, and superconducting ground states. In contrast, few of these phenomena have been observed in low-dimensional molecular structures, including thin films, nanoparticles, and molecular blends, not in the least because most of such structures have been composed of nearly closed-shell molecules. It is therefore desirable to develop low-dimensional ionic molecular structures that can capture potential applications. Here, we present detailed analysis of monolayer-thick structures of the canonical TTF-TCNQ (tetrathiafulvalene 7,7,8,8-tetracyanoquinodimethane) system grown on low-index gold and silver surfaces. The most distinctive property of the epitaxial growth is the wide abundance of stable TTF/TCNQ ratios, in sharp contrast to the predominance of a 1:1 ratio in the bulk. We propose the existence of the surface phase diagram that controls the structures of TTF-TCNQ on the surfaces and demonstrate phase transitions that occur upon progressively increasing the density of TCNQ while keeping the surface coverage of TTF fixed. Based on direct observations, we propose the binding motif behind the stable phases and infer the dominant interactions that enable the existence of the rich spectrum of surface structures. Finally, we also show that the surface phase diagram will control the epitaxy beyond monolayer coverage. Multiplicity of stable surface structures, the corollary rich phase diagram, and the corresponding phase transitions present an interesting opportunity for low-dimensional molecular systems, particularly if some of the electronic properties of the bulk can be preserved or modified in the surface phases.

The structure and electronic properties of Ge/SrZrO3

High quality Ge/High-k perovskite oxide interface is essential to facilitate the high performance metal-oxide semiconductor field-effect transistors and monolithically integrated optoelectronics device performance. The atomic structure and electronic properties of perfect and defective Ge/SrZrO3 are investigated by first-principles calculations. Both SrO- and ZrO2-terminated surfaces can be formed for a comparable range of the ZrO2 chemical potential. We investigated systematically the Ge atomic initial adsorption on the SrZrO3 (001) substrate. The top sites of the oxygen atoms are favorable for 1/2 (1/3) monolayer Ge adsorption at SrO (ZrO2)-termination. The surface phase diagrams are presented by the calculated surface grand potential. The stable structure and effects of the intrinsic point defects are analyzed for the energetically favorable Ge/SrZrO3 (001) interfaces.

Theoretical investigations of initial growth processes on semipolar AlN surfaces under metal–organic vapor-phase epitaxy growth condition

The initial growth processes on semipolar AlN surfaces, such as adsorption behavior of Al adatoms, are investigated on the basis of ab initio calculations and kinetic Monte Carlo (MC) simulations. By using surface phase diagrams, which are obtained by comparing the adsorption energy from ab initio calculations with gas-phase chemical potentials, we find that the adsorption of Al adatoms under H-poor condition is much easier than that under H-rich condition. Furthermore, our kinetic MC simulations demonstrate that the surface lifetime (diffusion length) of Al adatoms under H-poor condition is found to be four (two) orders of magnitude larger than that under H-rich condition. These differences implie that the growth under H-poor condition is much faster than that under H-rich condition, qualitatively consistent with the experimental results.

Interface properties of Ge on cubic SrHfO3 (001)

High quality Ge-on-high-k oxide interface is essential to facilitate the high performance metal-oxide semiconductor field-effect transistors and monolithically integrated optoelectronics device performance. The atomic structure and electronic properties of Ge on perfect and defective (001) SrHfO3 are investigated by first-principle calculations. The amplitude of the surface rumpling for the SrO-terminated surface is much larger than that for HfO2-terminated surface, although both SrO- and HfO2-terminated surfaces are stable for a comparable range of the HfO2 chemical potential. The distance between the first and second planes compresses while that of the second and third planes expands due to the relaxation of the slab. We investigated systematically the specific adsorption sites and the atomic structure at the initial growth stage of Ge on the SrHfO3 (001) substrate. The top sites of the oxygen atoms are favorable for 1/2 (1/3) monolayer Ge adsorbate at SrO (HfO2)-terminated surface. We calculated the surface grand potential and presented the complete surface phase diagram. We also pointed out the energetically favorable interfaces among the atomic arrangements of the Ge/SrHfO3 (001) interfaces. The atomic structure and electronic properties of the intrinsic point defects were calculated and analyzed for the Ge/SrHfO3 (001) interfaces.

On the formation of two-dimensional alloys of Sn and Pb co-adsorbed on Ru(0001)

Following recent experimental work of Yuhara et al. [Surf. Sci. 616 (2013) 131], indicating that the atoms of immiscible Sn and Pb metals deposited on Ru(0001) form locally a close-packed two-dimensional alloy structure, we employ DFT simulations to establish the conditions for the occurrence of Sn–Pb ordered alloys on this substrate. To this end, we calculate the surface phase diagram of Sn and Pb co-adsorbed on Ru(0001) and determine ranges of doses of deposited Sn and Pb when the formation of a binary alloy is energetically more favorable than the growth of segregated pure phases of Sn and Pb. We find a general tendency towards alloying of Sn and Pb on Ru(0001), however, the observed dominant alloy phase depends on the assumed deposition procedure of compositionally minor and major elements. We identify the preferable Sn–Pb alloy structures in terms of their stoichiometry, lateral symmetry, as well as the optimal adatom configuration within the surface unit cell, and contrast the DFT-predicted structural models with available STM images.

Ab initio study of oxygen adsorption on the NiTi(1 1 0) surface and the surface phase diagram

The structures of the clean and oxygen atoms adsorbed austenite NiTi(110) surfaces were studied by the use of density functional theory. The results showed that the most stable sites for the adsorption of oxygen atoms were those with more Ti atoms as their nearest neighbours. Furthermore, the phase diagram of NiTi(110) surface was built. It showed that the adsorption of oxygen atoms on NiTi(110) surface can cause the segregation of Ti atoms at the surface and lead to the selective oxidation of Ti atoms on NiTi(110) surface when the oxygen chemical potential was high enough.

Ab Initio Determined Phase Diagram of Clean and Solvated Muscovite Mica Surfaces.

Focusing on muscovite mica, the most significant phyllosilicate in the mica series, we determine its surface phase diagram employing density functional theory. Surfaces in vacuum and in more realistic environmental conditions, that is, the surface in contact with water or an ionic liquid, are considered. These results naturally explain experimental observations such as the swelling of mica when it comes into contact with water.

Surfactant antimony enhanced indium incorporation on InGaN ([formula omitted]) surface: A DFT study

InGaN is an ideal alloy system for optoelectronic devices due its tunable band gap. Yet high-quality InGaN requires high In concentration, which is a challenging issue that limits its use in green-light LEDs and other devices. In this paper, we investigated the surfactant effect of Sb on the In incorporation on InGaN (0001¯) surface via first-principles approaches. Surface phase diagram was also constructed to determine surface structures under different growth conditions. By analyzing surface stress under different structures, we found that Sb adatom can induce tensile sites in the cation layer, enhancing the In incorporation. These findings may provide fundamental understandings and guidelines for the growth of InGaN with high In concentration.

Reduced overpotentials for electrocatalytic water splitting over Fe- and Ni-modified BaTiO3.

Water electrolysis is a key technology for the replacement of fossil fuels by environmentally friendly alternatives, but state-of-the-art water oxidation catalysts rely on rare elements such as Pt groups and other noble metals. In this article, we employ first-principles calculations to explore the potential of modified barium titanate (BaTiO3), an inexpensive perovskite oxide that can be synthesized from earth-abundant precursors, for the design of efficient water oxidation electrocatalysts. Our calculations identify Fe and Ni doping as a means to improve the electrical conductivity and to reduce the overpotential required for water oxidation over BaTiO3. Based on computed Pourbaix diagrams and pH/potential-dependent surface phase diagrams, we further show that BaTiO3 is stable under reaction conditions and is not sensitive with respect to poisoning by reaction intermediates and hydrogen adsorption. This proof of concept demonstrates that even minor compositional modifications of existing materials may greatly improve their catalytic activity, a fact that is often neglected when larger composition spaces are screened.

Effects of ferroelectric polarization on surface phase diagram: Evolutionary algorithm study of theBaTiO3(001) surface

We have constructed the surface phase diagram of the BaTiO$_{3}$(001) surface by employing an evolutionary algorithm for surface structure prediction, where the ferroelectric polarization is included as a degree of freedom. Among over 1000 candidate structures explored, a surface reconstruction of (2$\times$1)-TiO is discovered to be thermodynamically stable and have the \emph{p2mm} plane group symmetry as observed experimentally. We find that the influence of ferroelectric polarization on the surface free energy can be either negligibly small or sizably large (over 1 eV per ($2 \times 1$) supercell), depending strongly on the surface structure and resulting in a significant distinction of surface phase diagram with varying ferroelectric polarization. It is therefore feasible to control the surface stability by applying an external electric field. Our results may have important implications in understanding the surface reconstruction of ferroelectric materials and tuning surface properties.

Hydrogen Activation on the Promoted and Unpromoted ReS2 (001) Surfaces under the Sulfidation Conditions: A First-Principles Study

Hydrogen activation on the promoted and promoter-free ReS2(001) surfaces under the sulfidation conditions is studied by means of periodic density function theory (DFT) calculations within the generalized gradient approximation. First, surface-phase diagrams are investigated by plotting the surface free energy as a function of the chemical potential of S (μS) on the unpromoted and promoted ReS2 (001) surfaces with different loadings of nickel, cobalt, tungsten, and tantalum. The results show that on the unpromoted surface sulfur coverage of 25% and on the promoted surfaces sulfur coverage of 25% as well as 25% promoter modification are the most stable conditions, respectively, under hydrodesulfurization (HDS) reaction conditions. Second, hydrogen adsorption and dissociation are explored on these preferred surfaces. It is found that hydrogen adsorbs weakly on all the surfaces studied. The physical adsorption character makes its diffusion favorable, resulting in various adsorption sites and dissociation path...

On the electronic, structural, and thermodynamic properties of Au supported on α-Fe2O3 surfaces and their interaction with CO.

Extensive first principles calculations are carried out to investigate Au monomers and dimers supported on α-Fe2O3(0001) surfaces in terms of structure optimizations, electronic structure analyses, and ab initio thermodynamics calculations of surface phase diagrams. All computations rely on density functional theory in the generalized gradient approximation (Perdew-Burke-Ernzerhof (PBE)) and account for on-site Coulomb interactions via inclusion of a Hubbard correction (PBE+U). The relative stability of Au monomers/dimers on the stoichiometric termination of α-Fe2O3(0001) decorated with various vacancies (multiple oxygen vacancies, iron vacancy, and mixed iron-oxygen vacancies) has been computed as a function of the oxygen chemical potential. The charge rearrangement induced by Au at the oxide contact is analyzed in detail and discussed. On one hand, ab initio thermodynamics predicts that under O-rich conditions, structures obtained by replacing a surface Fe atom with a Au atom are thermodynamically stable over a wide range of temperatures. On the other hand, the complex of a CO molecule on a Au atom substituting surface Fe atoms is thermodynamically stable only in a much more narrow range of values of the O chemical potential under O-rich conditions. In the case of a Au dimer, under O-rich conditions, supported Au atoms at an O-Fe di-vacancy are more stable. However, upon CO adsorption, the complex of a CO molecule and 2 Au atoms located at a single Fe vacancy is more favorable.

CO adsorption on cobalt: Prediction of stable surface phases

Adsorption is often described by a Langmuir isotherm, sometimes accounting for a gradual decrease in the adsorption energy with coverage. Using density functional theory, we show that CO adsorption on cobalt does not follow this typical behavior. Instead, adsorption on Co(0001) is dominated by two surface phases. At low pressures, the (√3×√3)R30°–CO structure is the stable phase, and CO forms (√3×√3)R30°–CO islands for coverages below 1/3ML because of attractive CO–CO interactions. Increasing the pressure does not gradually increase the coverage beyond 1/3ML. Instead, a transition to a high coverage (2√3×2√3)R30°–7CO surface structure occurs at 0.1mbar at room temperature and at 21bar at 500K. These two phases are also the dominant structures that have been characterized experimentally on Co(0001), and the conditions where each phase was observed match the first principle surface phase diagram.

Nonuniversal surface behavior of dynamic phase transitions.

We have studied the dynamic phase transition (DPT) of the kinetic Ising model in systems with surfaces within the mean-field approximation. Varying the surface exchange coupling strength J(s), the amplitude of the externally applied oscillating field h(0), and its period P, we explore the dynamic behavior of the layer-dependent magnetization and the associated DPTs. The surface phase diagram shows several features that resemble those of the equilibrium case, with an extraordinary bulk transition and a surface transition for high J(s) values, independent from the value of h(0). For low J(s), however, h(0) is found to be a crucial parameter that leads to nonuniversal surface behavior at the ordinary bulk transition point. Specifically, we observed here a bulk-supported surface DPT for high field amplitudes h(0) and correspondingly short critical periods P(c), whereas this surface transition simultaneous to the bulk one is suppressed for slow critical dynamics occurring for low values of h(0). The suppression of the DPT for low h(0) not only occurs for the topmost surface layer, but also affects a significant number of subsurface layers. We find that the key physical quantity that explains this nonuniversal behavior is the time correlation between the dynamic surface and bulk magnetizations at the bulk critical point. This time correlation has to pass a threshold value to trigger a bulk-induced DPT in the surface layers. Otherwise, dynamic phase transitions are absent at the surface in stark contrast to the equilibrium behavior of the corresponding thermodynamic Ising model. Also, we have analyzed the penetration depth of the dynamically ordered phase for the surface DPT that occurs for large J(s) values. Here we find that the penetration depth depends strongly on J(s) and behaves identically to the corresponding equilibrium Ising model.

Ab Initio Approach for Prediction of Oxide Surface Structure, Stoichiometry, and Electrocatalytic Activity in Aqueous Solution.

The design of efficient, stable, and inexpensive catalysts for oxygen evolution and reduction is crucial for the development of electrochemical energy conversion devices such as fuel cells and metal-air batteries. Currently, such design is limited by challenges in atomic-scale experimental characterization and computational modeling of solid-liquid interfaces. Here, we begin to address these issues by developing a general-, first-principles-, and electrochemical-principles-based framework for prediction of catalyst surface structure, stoichiometry, and stability as a function of pH, electrode potential, and aqueous cation concentration. We demonstrate the approach by determining the surface phase diagram of LaMnO3, which has been studied for oxygen evolution and reduction and computing the reaction overpotentials on the relevant surface phases. Our results illustrate the critical role of solvated cation species in governing the catalyst surface structure and stoichiometry, and thereby catalytic activity, in aqueous solution.

Why graphene growth is very different on the C face than on the Si face of SiC: Insights from surface equilibria and the(3×3)−3C−SiC(1¯1¯1¯)reconstruction

We address the stability of the surface phases that occur on the C-side of 3C-SiC($\bar{1} \bar{1} \bar{1}$) at the onset of graphene formation. In this growth range, experimental reports reveal a coexistence of several surface phases. This coexistence can be explained by a Si-rich model for the unknown (3$\times$3) reconstruction, the known (2$\times$2)$_{C}$ adatom phase, and the graphene covered (2$\times$2)$_{C}$ phase. By constructing an $ab$ $initio$ surface phase diagram using a van der Waals corrected density functional, we show that the formation of a well defined interface structure like the "buffer-layer" on the Si side is blocked by Si-rich surface reconstructions.

Structural properties of Ge on SrTiO3 (001) surface and Ge/SrTiO3 interface

Germanium−perovskite oxide heterostructures have a strong potential for next-generation low-voltage and low-leakage metal-oxide semiconductor field-effect transistors. We investigated the atomic structure and electronic properties of Ge on perfect and defective (001) SrTiO3 by first-principle calculations. The specific adsorption sites at the initial growth stage and the atomic structure of Ge on the SrTiO3 (001) substrate have been systematically investigated. The surface grand potential was calculated and compared as a function of the relative chemical potential. The complete surface phase diagram was presented. The energetically favorable interfaces were pointed out among the atomic arrangements of the Ge/SrTiO3 (001) interfaces. The atomic structure and electronic properties of the intrinsic point defects were calculated and analyzed for the Ge/SrTiO3 (001) interfaces.

Ab initio atomistic thermodynamics study on the oxidation mechanism of binary and ternary alloy surfaces.

Utilizing a combination of ab initio density-functional theory and thermodynamics formalism, we have established the microscopic mechanisms for oxidation of the binary and ternary alloy surfaces and provided a clear explanation for the experimental results of the oxidation. We construct three-dimensional surface phase diagrams (SPDs) for oxygen adsorption on three different Nb-X(110) (X = Ti, Al or Si) binary alloy surfaces. On the basis of the obtained SPDs, we conclude a general microscopic mechanism for the thermodynamic oxidation, that is, under O-rich conditions, a uniform single-phase SPD (type I) and a nonuniform double-phase SPD (type II) correspond to the sustained complete selective oxidation and the non-sustained partial selective oxidation by adding the X element, respectively. Furthermore, by revealing the framework of thermodynamics for the oxidation mechanism of ternary alloys through the comparison of the surface energies of two separated binary alloys, we provide an understanding for the selective oxidation behavior of the Nb ternary alloy surfaces. Using these general microscopic mechanisms, one could predict the oxidation behavior of any binary and multi-component alloy surfaces based on thermodynamics considerations.