Surface Phase Diagram Research Articles

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.

Wetting of prototypical one- and two-dimensional systems: thermodynamics and density functional theory.

Consider a two-dimensional capped capillary pore formed by capping two parallel planar walls with a third wall orthogonal to the two planar walls. This system reduces to a slit pore sufficiently far from the capping wall and to a single planar wall when the side walls are far apart. Not surprisingly, wetting of capped capillaries is related to wetting of slit pores and planar walls. For example, the wetting temperature of the capped capillary provides the boundary between first-order and continuous transitions to condensation. We present a numerical investigation of adsorption in capped capillaries of mesoscopic widths based on density functional theory. The fluid-fluid and fluid-substrate interactions are given by the pairwise Lennard-Jones potential. We also perform a parametric study of wetting in capped capillaries by a liquid phase by varying the applied chemical potential, temperature, and pore width. This allows us to construct surface phase diagrams and investigate the complicated interplay of wetting mechanisms specific to each system, in particular, the dependence of capillary wetting temperature on the pore width.

Systematic Theoretical Investigations on Surface Reconstruction and Adatom Kinetics on AlN Semipolar Surfaces

The surface reconstructions and adatom kinetics on AlN(1-102) and AlN(1-101) surfaces in the presence of H2 molecules are investigated on the basis of density-functional theory calculations. Our surface phase diagram calculations demonstrate that the H-incorporated surfaces are invariably stable over the wide range of H2 pressure. We find that under the growth condition the adsorption energy of Al adatom on AlN(1-102) surface is much larger than that on AlN(1-101) surface. Furthermore, we find that the diffusion behavior of Al adatom on AlN(1-101) surface is significantly influenced by the presence of hydrogen. These results suggest that impact of hydrogen ambient during the metal-organic vapor-phase epitaxy growth on AlN(1-102) surface is quite different from that on AlN(1-101) surface. [DOI: 10.1380/ejssnt.2015.239]

Structural and electronic properties of submonolayer-thick Sn films on Ru(0 0 0 1)

The existing experimental data on the arrangement of Sn adatoms deposited on the Ru(0001) substrate at room temperature are inconsistent: the earlier LEED, AES and LEISS measurements of Paffett et al. [J. Phys. Chem. 97 (1993) 690] recognized long-range ordered c(2×8) and (3×3)rect adsorbate patterns corresponding to local coverage θ=1/4ML and θ=2/3ML, respectively, whereas recent LEED and STM observations of Yuhara et al. [Surf. Sci. 616 (2013) 131] indicated the formation of a c(2×8) structure with θ=3/8ML, followed by a (3×7) structure with θ=2/5ML and an incommensurate Sn phase of somewhat similar geometry at larger overlayer densities. Motivated by this controversy, we employed first-principles DFT calculations to simulate the growth of Sn/Ru(0001) films at submonolayer coverages. The optimal adatom configurations were found for increasing Sn coverage and contrasted with the structural models proposed in the literature. The computed surface phase diagram indicates that the characteristic ‘zigzag–zigzag–zagzig–zagzig’ arrangement of the c(2×8) phase corresponding to the local coverage θ=1/2ML is energetically preferred at low Sn depositions, followed by the (3×7) structure with θ=3/5ML and the (3×3)rect structure with θ=2/3ML, the latter constituting the most closely packed arrangement of the first wetting Sn layer. The DFT-predicted thermodynamically stable Sn/Ru(0001) adsorbate configurations match well the available high-resolution STM images. The effect of Sn adsorption on the electronic characteristics of the Ru(0001) surface is also explored, with special emphasis on adsorbate-induced modifications of the electron density-of-states distributions within the surface region.

Model Realization and Numerical Studies of a Three-Dimensional Bosonic Topological Insulator and Symmetry-Enriched Topological Phases

We study a topological phase of interacting bosons in (3+1) dimensions which is protected by charge conservation and time-reversal symmetry. We present an explicit lattice model which realizes this phase and which can be studied in sign-free Monte Carlo simulations. The idea behind our model is to bind bosons to topological defects called hedgehogs. We determine the phase diagram of the model and identify a phase where such bound states are proliferated. In this phase we observe a Witten effect in the bulk whereby an external monopole binds half of the elementary boson charge, which confirms that it is a bosonic topological insulator. We also study the boundary between the topological insulator and a trivial insulator. We find a surface phase diagram which includes exotic superfluids, a topologically ordered phase, and a phase with a Hall effect quantized to one-half of the value possible in a purely two-dimensional system. We also present models that realize symmetry-enriched topologically-ordered phases by binding multiple hedgehogs to each boson; these phases show charge fractionalization and intrinsic topological order as well as a fractional Witten effect.

Ab initio study of molecular and atomic oxygen on GeTe(111) surfaces

Oxidation of the phase-change material germanium telluride (GeTe) is an atomic-scale process of fundamental importance, as it is detrimental to the stability of GeTe-based data-storage devices. Here, we present comprehensive density-functional theory simulations of molecular and atomic oxygen in contact with GeTe(111) surfaces. Molecular O2 is predicted to readily adsorb on the Ge-terminated (111) surface; the pristine Te-terminated counterpart, by contrast, appears quite inert. The coverage-dependent adsorption of O atoms is then investigated, and based on these data, a surface phase diagram for GeTe(111)/O is constructed. These results afford a detailed, atom-resolved picture of the initial surface oxidation of GeTe, and they harmonize well with a previous X-ray photoelectron spectroscopy study on this very topic.

Controlling CH2 dissociation on Ru(0001) through surface site blocking by adsorbed hydrogen

Understanding the relative stability of CHx species on surfaces is necessary for mechanistic description of much important catalytic chemistry. Here, we experimentally quantify the barrier of the reaction CH2→CH+H on Ru(0001) in UHV and find an activation energy, 65±6kJ/mol, that is >4× higher than previous computational results with 0, 1, or 2 coadsorbed H atoms per CH2, i.e. 16kJ/mol. Employing density functional theory calculations, we show that this disagreement can be reconciled if 3 coadsorbed H atoms per CH2 are present in our experiment. We further demonstrate, by calculating the surface phase diagram for one carbon species on Ru(0001) as a function of H2 chemical potential, that the additional hydrogen surface coverage requires non-equilibrium conditions. Such conditions may be important at the high temperatures and pressures of real catalytic systems.

Critical and Tricritical Wetting in the Two-Dimensional Blume–Capel model

The two-dimensional Blume–Capel model with free surfaces where a surface field $$H_1$$ acts and the “crystal field” (controlling the density of the vacancies) takes a value $$D _s$$ different from the value $$D$$ in the bulk, is studied by Monte Carlo methods. Using a recently developed finite size scaling method that studies thin films in a $$L \times M$$ geometry with antisymmetric surface fields $$(H_L=-H_1)$$ and keeps a generalized aspect ratio $$c = L^2/M$$ constant, surface phase diagrams are computed for several typical choices of the parameters. It is shown that both second order and first order wetting transitions occur, separated by tricritical wetting behavior. The special role of vacancies near the surface is investigated in detail.

Theoretical Study of Carbon Adsorption on Re Surfaces: Morphological Instability

We report results from new experiments on C/Re( $$11\bar{2}1$$ ) to identify threshold conditions for morphological instability of Re( $$11\bar{2}1$$ ). We have found that adsorption of carbon from 0.35 to 0.85 ML (0.3–6.0 L exposure of C2H2) at T ≥ 800 K leads to faceting of Re( $$11\bar{2}1$$ ) with formation of three-sided pyramids. Using density functional theory we have investigated binding sites and binding energies of C on planar and faceted Re surfaces as well as generated a surface phase diagram of C/Re to obtain an atomistic understanding of C-induced pyramidal faceting of Re( $$11\bar{2}1$$ ). The calculations reveal that at low to intermediate coverage, C atoms prefer binding at four-fold sites on the Re surfaces and formation of three-sided pyramids is thermodynamically favored. Using density functional theory and thermodynamic considerations as well as AES and LEED measurements we studied the structure of Re(11–21) surfaces in contact with C2H2. The experiments show that adsorption of carbon from 0.35 to 0.85 ML (0.3–6.0 L exposure of C2H2) at T ≥ 800 K leads to faceting of Re(11–21) with formation of three-sided pyramids. The calculations reveal that at low to intermediate coverage, C atoms prefer binding at four-fold sites on the Re surfaces and formation of three-sided pyramids is thermodynamically favored.

Prediction on the Surface Phase Diagram and Growth Morphology of Nanocrystal Ruthenium Dioxide

Surface energy has an important role in controlling the exposed facets and growth morphology of nanocrystals. In this study, we employed first‐principle thermodynamic modeling and calculations to evaluate the substantial effects of environmental factors (temperature and oxygen partial pressure), on the surface structure, stability, and nanocrystal morphology of rutile‐type ruthenium dioxide (RuO2). Both stoichiometric and nonstoichiometric surfaces with ideal bulk terminations were assessed. The relative ordering of stoichiometric surface stabilities was predicted as (110) > (101) > (100) > (001). The sensitive environment dependence of nonstoichiometric surface stabilities was evaluated by calculating the surface phase diagram, and partially validated by comparing with available experimental observations. The predicted surface stabilities were further coupled with the Gibbs–Wulff construction of equilibrium crystal shape, to predict the morphological evolutions of RuO2 nanocrystals under practical growth conditions. A morphology‐controlled growth technique was finally suggested for designing and developing hierarchical nanostructures by intelligently adjusting the thermodynamic growth conditions.

Surfactant-induced phases in water-supported alkane monolayers: I. Thermodynamics.

Alkanes longer than n = 6 carbons do not spread on the water surface, but condense in a macroscopic lens. However, adding trimethylammonium-based surfactants, C(m)TAB, in submillimolar concentrations causes the alkanes to spread and form a single Langmuir-Gibbs (LG) monolayer of mixed alkanes and surfactant tails, which coexists with the alkane lenses. Upon cooling, this LG film surface-freezes at a temperature T(s) above the bulk freezing temperature T(b). The thermodynamics of surface freezing (SF) of these LG films is studied by surface tension measurements for a range of alkanes (n = 12-21) and surfactant alkyl lengths (m = 14, 16, 18), at several concentrations c. The surface freezing range T(s)-T(b) observed is up to 25 °C, an order of magnitude larger than the temperature range of SF monolayers on the surface of pure alkane melts. The measured (n,T) surface phase diagram is accounted for well by a model based on mixtures' theory, which includes an interchange energy term ω. ω is found to be negative, implying attraction between unlike species, rather than the repulsion found for SF of binary alkane mixtures. Thus, the surfactant/alkane mixing is a necessary condition for the occurrence of SF in these LG films. The X-ray derived structure of the films is presented in an accompanying paper.

Investigations on the morphological evolution of zinc oxide nanostructures and their optical properties

The morphological evolution of ZnO nanostructures grown under various argon sputtering pressures and substrate temperatures has been systematically investigated. A surface phase diagram depicting compact two-dimensional thin films to various types of one-dimensional structures as a function of pressure and temperature is constructed. Structural studies of ZnO nanostructures reveal the preferential orientation along the c-axis of a hexagonal wurtzite crystal structure with a highly crystalline nature. However, the as-grown ZnO nanostructures slightly experience compressive strain, which increases with the argon sputtering pressure. Exciton recombination and point defects have been extensively analyzed by temperature-dependent photoluminescence spectroscopy. The observation of donor-bound exciton emission and its replicas reveals the high optical quality of the nanostructures. Nevertheless, the observed visible emission indicates the presence of point defects which significantly depend on the deposition pressure. The characteristic visible emission bands at 2.28 and 2.02 eV provide strong evidence for the existence of oxygen vacancies and interstitial sites in ZnO nanostructures irrespective of growth temperature and pressure.

Structural properties of ultrathin Pb layers on Ru(0001) revealed by LEED, AES and DFT

Abstract The structure and energetics of ultrathin Pb films deposited on Ru(0 0 0 1) are investigated by LEED and AES experiments combined with first-principles DFT simulations. AES measurements indicate the Stranski–Krastanov growth mode with three-dimensional Pb islands nucleating upon completion of the first wetting layer. DFT computations support this scheme, yielding adatom binding energy of |Eads| ≈ 3.1 eV and |Eads| ≈ 1.5 eV, i.e., notably larger and smaller than the cohesive energy of lead, for Pb adsorbing on the bare substrate and on top of the wetting layer, respectively. LEED patterns reveal the formation of long-range ordered c(4 × 2) structure for a wide range of Pb doses up to Θ = 1/2 ML, followed by the ( 7 × 7 ) structure at around Θ = 4/7 ML, which constitutes the densest identified arrangement within the first adlayer. Further Pb deposition leads to appearance of reproducible, although yet unidentified overlayer structure. Increasing the deposition temperature reduces the Pb sticking coefficient down to zero at around 600 K for Pb adsorbing on a completed wetting layer, which prevents nucleation of multilayer Pb islands at higher substrate temperatures, but formation of well-ordered first Pb adlayer is observed up to 890 K. The optimal adatom arrangement of the Pb/Ru(0 0 0 1)-c(4 × 2) phase postulated by DFT corresponds to local coverage of 1/2 ML and consists of Pb adatoms occupying the hcp and fcc sites in the 1:1 proportion, yielding a slightly off-centered rectangular adsorbate pattern. For the (4/7 ML)-Pb/Ru(0 0 0 1)- ( 7 × 7 ) phase, two hexagonal-like adlayer configurations containing either one Pb adatom in the hcp site and three Pb adatoms in distorted fcc-like positions, or one fcc and three hcp-like adatoms within the surface unit cell, are predicted to coexist. The calculated surface phase diagram indicates that other low-coverage ordered overlayer structures, identified for other adsorbates on Ru(0 0 0 1), are thermodynamically unfavorable for the Pb/Ru(0 0 0 1) system.

Phase stability analysis of the InAs/GaAs (001) wetting layer from first principles

The large atomic-size mismatch between In and Ga and the large lattice-mismatch strain between InAs and GaAs make the InAs/GaAs (001) growth interface a complex alloy system, the understanding of which can enhance control of device synthesis and nanostructure self-assembly. We present a detailed first-principles analysis of the full progression of surface reconstructions observed on the InAs/GaAs(001) wetting layer during early stages of In deposition. We use systematic techniques to identify the most likely surface reconstruction prototypes of the InAs wetting layer on GaAs(001) using density functional theory (DFT) and then develop several cluster expansion Hamiltonians in order to thoroughly explore surface alloy disorder due to species substitution of In, Ga, and As at the surface. We use these results to construct a first principles 0-K surface phase diagram of the InAs wetting layer on GaAs(001) and test the sensitivity of our predicted phase diagram to DFT approximations and convergence errors. We find two alloy configurations of the $(4\ifmmode\times\else\texttimes\fi{}3)$ structural prototype that are likely ground-state surface reconstructions, and simulated scanning tunneling micrographs (STM) of these reconstructions indicate that they can explain prominent features of experimentally obtained STM of the InAs/GaAs $(4\ifmmode\times\else\texttimes\fi{}3)$ surface.

Oxygen chemisorption-induced surface phase transitions on Cu(110)

From an interplay between variable temperature scanning tunneling microscopy and density–functional theory calculations, the evolution of oxygen chemisorption-induced surface reconstructions of the Cu(110) surface is determined. The surface reconstructions proceed via a sequential pathway with increasing oxygen surface coverage. The (2×1) reconstruction occurs first and then transits to the c(6×2) phase with a higher oxygen coverage through a mechanism that consumes the existing (2×1) phase with the supply of Cu adatoms from step edges and terraces. The temperature dependence of the (2×1)→c(6×2) transition demonstrates that the surface phase transition is an activated process for breaking up added Cu–O–Cu rows in the (2×1) structure. Comparison between the experimental observations and the theoretical surface phase diagram obtained from first-principles thermodynamic calculations reveals that the (2×1)→c(6×2) transition takes place at the oxygen chemical potentials that are far above the chemical potential for Cu2O bulk oxide formation, reflecting the existence of kinetic limitations to the surface phase transition and the bulk oxide formation.

Surface phase diagram of the three-dimensional kinetic Ising model in an oscillating magnetic field.

We study the surface phase diagram of the three-dimensional kinetic Ising model below the equilibrium critical point subjected to a periodically oscillating magnetic field. Changing the surface interaction strength as well as the period of the external field, we obtain a nonequilibrium surface phase diagram that in parts strongly resembles the corresponding equilibrium phase diagram, with an ordinary transition, an extraordinary transition, and a surface transition. These three lines meet at a special transition point. For weak surface couplings, however, the surface does not order. These results are found to remain qualitatively unchanged when using different single-spin flip dynamics.

Understanding and controlling indium incorporation and surface segregation on InxGa1−xN surfaces: Anab initioapproach

The incorporation of In into the technologically relevant $(0001)$ (Ga-polar) and $(000\overline{1})$ (N-polar) surfaces of ${\mathrm{In}}_{0.25}{\mathrm{Ga}}_{0.75}\mathrm{N}$ is investigated using density functional theory. The cases of coherent pseudomorphic growth on GaN and on lattice-matched heterointerfaces are considered. For pseudomorphic growth on GaN, In incorporation into the ${0001}$ surface layers is limited to a tiny growth window corresponding to extreme In-rich growth conditions and at the In-rich/Ga-poor region of the metal chemical potentials. Lattice-matched growth, however, allows for a wider growth window. Surface phase diagrams are constructed as a function of growth conditions and reveal similarities between the two polar growth planes. However, a strong driving force is found for segregation of In atoms to the first III-N layer for Ga-polar growth, but not for N-polar growth. The former was found to be mainly due to chemical effects (stronger Ga-N as compared to In-N bonds), absent in the case of N-polar growth. Furthermore, finite-temperature calculations show that In incorporated into the first III-N layer is stable to $\ensuremath{\approx}150$ K higher temperatures in the N-polar surface than in the Ga-polar surface, indicating that for a given level of In incorporation, higher temperatures can be used for N-polar growth as compared to Ga-polar growth.

The (0001)A surface phase diagram

AbstractThe (0001)A surface phase diagram of InxGa1‐xN is investigated by using different approximations of the cluster variation method with the one‐, two‐ and three‐point basic clusters. The surface layer is considered in the total composition range as a binary regular solution on the triangular lattice. The miscibility gap temperature and concentration of the nearest neighbor surface atoms at this temperature are estimated as functions of the composition. The miscibility gap temperatures of In0.5Ga0.5N obtained by one‐, two‐ and three‐point cluster approximations are 710 °C, 535 °C and 370 °C, respectively. The strong InIn and GaGa pair clustering in comparison with InGa pair formation at temperatures higher than the miscibility gap one is demonstrated in the three‐point approximation. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nitrogen-induced reconstruction and faceting of Re($11\bar 21$112¯1)

The surface morphology of Re(11̄21), tailored on the nanometer scale by kinetic control of nitrogen, has been investigated using low energy electron diffraction, scanning tunneling microscopy, Auger electron spectroscopy, and density functional theory (DFT) in combination with the ab initio atomistic thermodynamics approach. Experiments show that when exposing to NH3 (>0.5 L) at 300 K followed by annealing in ultra-high vacuum at 700 K or 900 K, the initially planar Re(11̄21) surface becomes (2 × 1) reconstructed or partially faceted, respectively. Upon annealing in 100 L NH3 at 900 K, Re(11̄21) becomes fully faceted and covered by N. The fully faceted surface consists of two-sided ridges formed by (13̄42) and (31̄42) facets. The (2 × 1) reconstruction may serve as a precursor state for faceting of Re(11̄21). The DFT calculations provide an atomistic understanding of facet formation in terms of binding sites and energies of N on Re surfaces of the substrate and facets as well as the corresponding surface phase diagram. The N-covered faceted Re(11̄21) surfaces are promising nanoscale model catalysts and nanotemplates. Our findings should be of importance for the design and development of Re-based heterogeneous catalysts operating under nitrogen-rich conditions.