Bulk and surface properties of liquid X-Zr (X=Ag, Cu) compound forming alloys

The phase diagrams of the Ag-Zr and Cu-Zr systems exhibit the existence of different intermetallic compounds in the solid state, and since the structure of a liquid alloy is in some respects similar to that of a crystal, the compound formation phenomenon in these liquid alloy systems has been analysed through the study of surface properties (surface tension and surface composition), dynamic properties (chemical diffusion and viscosity) and microscopic functions (concentration fluctuations in the long-wavelength limit and chemical short-range order parameter) in the frame of the compound formation model (CFM). Moreover, the associative tendency between unlike constituent elements qualitatively expressed by the microscopic functions indicates the glass-forming ability of both systems at higher Zr-concentrations. These results are in agreement with reported experimental data and confirm the applicability of a statistical mechanical theory in conjunction with the CFM to describe the mixing behaviour of compound forming alloys.     

Disordered flat phase separation of the Lennard-Jones fcc(111) surface

Recent lattice model calculations have suggested that a full-layered crystal surface may undergo, under canonical (particle-conserving) conditions, a preroughening-driven two-dimensional phase separation into two disordered flat regions, of opposite order parameter. We have carried out extensive classical molecular dynamics (MD) simulations of the Lennard-Jones fcc(111) surface, to check whether these predictions are relevant or not for a realistic continuous system. Very long simulation times, a grid of temperatures from to T_m, and unusually large system sizes are employed to ensure full equilibrium and good statistics. By examining layer-by-layer occupancies, height fluctuations, sublattice order parameter and X-ray structure factors, we find a clear anomaly at 0.83T_m. The anomaly is distinct from roughening (whose incipiency is also detected at 0.94T_m), and is seen to be consistent with the preroughening plus phase separation scenario.     

Statistical thermodynamics of soft surfaces

We review the continuum, statistical thermodynamics of surfaces and interfaces in soft matter where both the energy and entropy of the surface are comparable. These systems include complex fluids that are dominated by either surface tension or the interfacial curvature, such as: fluid and solid interfaces, colloidal dispersions, macromolecular solutions, membranes, and other self-assembling aggregates such as micelles, vesicles, and microemulsions. The primary focus is on the theoretical concepts, their universality, and the role of fluctuations and inhomogeneities with connections to relevant experimental systems.     

Catalysis and corrosion: the theoretical surface-science context

Numerous experiments in ultra-high vacuum as well as (T=0 K, p=0) theoretical studies on surfaces have been performed over the last decades in order to gain a better understanding of the mechanisms, which, for example, underlie the phenomena of catalysis and corrosion. Often the results achieved this way cannot be extrapolated directly to the technologically relevant situation of finite temperature and high pressure. Accordingly, modern surface science has realized that bridging the so-called pressure gap (getting out of the vacuum) is the inevitable way to go. Of similar importance are studies in which the temperature is changed systematically (warming up and cooling down). Both aspects are being taken into account in recent experiments and ab initio calculations.

In this paper we stress that there is still much to learn and important questions to be answered concerning the complex atomic and molecular processes which occur at surfaces and actuate catalysis and corrosion, although significant advances in this exciting field have been made over the years. We demonstrate how synergetic effects between theory and experiment are leading to the next step, which is the development of simple concepts and understanding of the different modes of the interaction of chemisorbed species with surfaces. To a large extent this is being made possible by recent developments in theoretical methodology, which allow to extend the ab initio (i.e., starting from the self-consistent electronic structure) approach to poly-atomic complexes with 10,000 and more atoms, time scales of seconds, and involved statistics (e.g., ab initio molecular dynamics with 10,000 and more trajectories). In this paper we will

1. sketch recent density–functional theory based hybrid methods, which bridge the length and time scales from those of electron orbitals to meso- and macroscopic proportions, and

2. present some key results on properties of surfaces, demonstrating their role in corrosion and heterogeneous catalysis. In particular we discuss

◦ the influence of the ambient gas phase on the surface structure and stoichiometry,

◦ adsorbate phase transitions and thermal desorption, and

◦ the role of atoms' dynamics and statistics for the surface chemical reactivity.

Study of the adsorption of interacting dimers on square lattices by using a cluster-exact approximation

A theoretical approach, based on exact calculation of the partition function on finite rectangular clusters, is introduced to study the adsorption of interacting homonuclear dimers on square lattices. An efficient algorithm allows us to calculate the detailed structure of the configuration space for m=(k×l) clusters with m varying from 8 to 48. The adsorption process has been monitored by following thermodynamic properties such as coverage versus chemical potential, internal energy and specific heat of the adlayer, etc. The analytical results are compared with those in [Surf. Sci. 411 (1998) 294], which were obtained by using Monte Carlo simulation and finite-size scaling techniques. The theoretical adsorption isotherms and phase diagrams (critical temperature versus coverage) for both attractive and repulsive lateral interactions are in good qualitative agreement with the computational data. This agreement between simulated and theoretical results supports the validity of the cluster-exact approximation proposed in this paper.     

Surface and transport properties of Au-In liquid alloys

Thermodynamic quantities on Au-In liquid alloys have been used as the input data for the interaction parameter calculations in the framework of the complex formation model (CFM). Once the interaction energies are computed the surface (surface tension and surface composition) and transport properties (chemical diffusion and viscosity) as well as the microscopic functions (concentration fluctuations in the long-wavelength limit and chemical short-range order parameter) have been calculated. The concentration and temperature dependent surface tension values have been compared with our new set of experimental data, obtained by the large drop method in the temperature range of T = 1273-1493 K. The anomalous change of surface tension for some alloy compositions may be attributed to a retention of order in the Au-In melts which is similar to the atomic arrangement in solid Au-In.     

Shape cycle of Ga clusters on GaAs during coalescence growth

GaAs(1 0 0) was heated above its decomposition temperature of 585 °C bringing it into a phase separation regime where the thermodynamic favoured state is liquid Ga clusters on the surface. Varying the annealing times and temperatures provided an overview of the clustering at all stages from transitioning ripening at lower temperatures to coalescence at higher temperatures. We observed a shape cycle between round and rectangular shaped clusters during the growth. This cycle is driven by subcluster etching where pits are formed under clusters during the growth due to preferential loss of As through the liquid Ga cluster. The newly observed shape cycle is compared to a shape cycle observed previously in In on InP illustrating that shape cycles are a common feature of the decomposition of Group III-V semiconductors.     

The surface phase transition and low-temperature phase of α-Ga(0 1 0) studied by SPA-LEED

The order–disorder phase transition on the α-Ga(0 1 0) structure was studied by spot-profile analysis low energy electron diffraction (SPA-LEED). A low temperature diffraction pattern reveals a small splitting of the overlayer spots which corresponds to a real-space distance of 81 Å, equivalent to 18 unit cells. The splitting is interpreted as caused by a regular ordering of anti-phase domains of the low-temperature phase. Due to the low symmetry of the surface, the domain boundaries are aligned only in one direction, giving rise to a regular, one-dimensional grid. The temperature dependence of the intensity and width of the reconstruction-induced diffraction spots is also investigated. It suggests that the phase transition takes place at a critical temperature T_c=232 K and that anti-phase boundary proliferation plays a role.     

Non-universal critical behaviour of heteronuclear dimers on a square lattice––A Monte Carlo study

Monte Carlo simulation within the grand canonical ensemble, the histogram reweighting technique, and finite size scaling analysis are used to explore the phase behaviour of heteronuclear dimers, composed of A and B type atoms, on a square lattice. We have found that for the models with attractive BB and AB nearest-neighbour energy, u_BB=u_AB=−1, and for non-repulsive energy between AA nearest-neighbour sites, u_AA<0, the system belongs to the universality class of the two-dimensional Ising model. However, when u_AA>0, the system exhibits a non-universal critical behaviour. We have evaluated the dependences of the critical point characteristics on the value of u_AA.     

A Monte Carlo study of the thermal properties of Cu3Au low index surfaces

The thermal dependencies of composition and order of the (111), (100) and (110) Cu₃Au surfaces are studied at the atomic scale by means of Monte Carlo simulations in the “transmutational” ensemble at constant volume. The question addressed is the extent to which such simulations carried on with a model N-body potential designed on the basis of bulk energetic and mechanical properties allow predictions consistent with experimental observations of the surface. Although the currently available experimental data still leave unanswered questions, many of them allow for comparison with modelling. Qualitative agreement is found for temperature dependencies of both surface composition and order, and the simulation results are discussed in detail. Some clear discrepancies are found as well, in particular (but not only) in the case of the (110) surface and its first neighbouring layer. Although the origin of such differences is not yet clear, it is suggested that they may serve to assess and to improve the model in the light of quantitative surface studies.     

Phase transition of thiophene molecules on Au(1 1 1) in solution

The orientation and packing arrangement of thiophene molecules on a well-defined Au(1 1 1) surface in 0.1 M HClO₄ solution have been investigated as a function of applied potentials by in situ scanning tunneling microscopy (STM) and electrochemical method. Thiophene molecules are found to form highly ordered adlayers in the double layer region. High-resolution STM images reveal different adlayer structures. Thiophene molecules take flat-lying and vertical orientation at 0.3 and 0.6 V, respectively. Compared with the results of electrochemical measurement, we concluded that the phase transition of thiophene on Au(1 1 1) occurs as the potential is changed between 0.1 and 0.65 V.     

Coalescence kinetics of two-dimensional TiN islands on atomically smooth TiN(0 0 1) and TiN(1 1 1) terraces

In situ high-temperature scanning tunneling microscopy (STM) was employed to follow the coalescence kinetics of two-dimensional TiN islands on atomically smooth TiN(0 0 1) and TiN(1 1 1) terraces. The results are analyzed using a numerical approach based upon Mullin’s theory for surface relaxation and experimentally determined absolute orientation-dependent step stiffnesses. From the calculated time-dependent island shapes during coalescence, in combination with the STM data, we obtain quantitative values for edge mobilities.     

The instability of vicinal electrode surfaces against step bunching II: Theory

It is shown that vicinal surfaces with a regular step array in contact with an electrolyte are intrinsically unstable against a phase separation into areas of flat terraces and areas of step bunches. The effect is caused by the step dipole moment which lowers the potential of zero charge (pzc) of stepped surfaces. Specific calculations performed for vicinal surfaces of silver are qualitatively in keeping with experimental observations. Step bunching is likewise expected for vicinal surfaces of gold and platinum.     

The instability of vicinal electrode surfaces against step bunching I: Experiment

Using electrochemical STM we have studied the stability of arrays of parallel, single atom height steps on vicinal Ag(1 1 1) electrodes in electrolyte. We find that the steps for Ag(1 1 1) electrodes are unstable and form first double-steps and later multiple steps, separated by wide, flat terraces. In this paper denoted as “I: Experiment” we deal with the experimental aspects whereas theoretical aspects are discussed in the following paper “II: Theory”.     

Molecular scale structure formation by folding

A conception of a structure formation suitable for nano-technology is proposed, which is programmable and suitable for mass production-like lithography. This conception utilizes the controlled folding of chains like the scan-lines of television. Its possibility and property were studied theoretically using the modeled chains consist of beads. By adopting the interaction among the beads which can distinguish the kind of the partner by its polarity and is chiral to break the chiral symmetry of the folded state, the special chains which have the unique ground states could be designed. In these ground states, the chains are folded like the scan-lines of television. The thermodynamic properties of the suggested chains were studied by the Monte Carlo simulations and the suggested chains showed the phase-transition-like behavior which is distinct compared to both the random chains and the chain that has only the non-specific attraction. The size dependence and the effects of adding the non-specific attraction and modifying the border of the folded conformation were also studied.     

Surface oxides of the oxygen-copper system: Precursors to the bulk oxide phase?

To gain an initial understanding of the copper-based catalysts in commercially important chemical reactions such as the oxygen-assisted water-gas shift reaction, we performed density-functional theory calculations, investigating the interaction of oxygen and copper, focusing on the relative stability of surface oxides and oxide surfaces of the O/Cu system. By employing the technique of “ab initio atomistic thermodynamics”, we show that surface oxides are only metastable at relevant pressures and temperatures of technical catalysis, with no stable chemisorption phase observed even at very low coverage. Although exhibiting only metastability, these surface oxides resemble the bulk oxide material both geometrically and electronically, and may serve as a precursor phase before onset of the bulk oxide phase. Having identified the bulk oxide as the most stable phase under realistic catalytic conditions, we show that a Cu₂O(1 1 1) surface with Cu vacancies has a lower free energy than the stoichiometric surface for the considered range of oxygen chemical potential and could be catalytically relevant.     

Step structure on the fivefold Al-Pd-Mn quasicrystal surface, and on related surfaces

We compare step morphologies on surfaces of Al-rich metallic alloys, both quasicrystalline and crystalline. We present evidence that the large-scale step structure observed on Al-rich quasicrystals after quenching to room temperature reflects equilibrium structure at an elevated temperature. These steps are relatively rough, i.e., have high diffusivity, compared to those on crystalline surfaces. For the fivefold quasicrystal surface, step diffusivity increases as step height decreases, but this trend is not obeyed in a broader comparison between quasicrystals and crystals. On a shorter scale, the steps on Al-rich alloys tend to exhibit local facets (short linear segments), with different facet lengths, a feature which could develop during quenching to room temperature. Facets are shortest and most difficult to identify for the fivefold quasicrystal surface.