Structure transformation of Ir clusters on Ir surfaces

A small Ir cluster can assume either a one-dimensional linear-chain structure or a two-dimensional island-like structure. We present a study of the energetics of the 1 D to 2 D structure transformation of three-atom Ir clusters on the Ir(111) and (001) surfaces. On the (111) plane, the temperature dependence of the ratio of the probabilities of observing a three-atom cluster in the 1 D and 2 D structures exhibits a simple linear Arrhenius behavior. The 2 D island structure is found to be more stable with the cluster binding energy lower by 0.098±0.004 eV. On the (001) plane, the 1 D chain structure is more stable with the cluster binding energy lower by 0.335±0.015 eV. From these energies, the relative pair interaction at three different bond lengths can be derived. The relative pair potential is found to be non-monotonic in distance dependence. We explain the (1×5) reconstruction of the Ir(001) surface as being caused by the large difference in the pair binding energy of the first and second nearest-neighbor bonds. In addition, we find a significant deviation from the simple linear Arrhenius behavior at low temperatures for the three-atom Ir cluster on the Ir (001) plane, indicating that the entropy factor is temperature dependent.     

Free energies of steps on (111) fcc surfaces

For a hexagonal two-dimensional lattice we derive, using a surprisingly simple route, exact expressions for the step free energies along the high symmetry directions, 〈1-10〉 and 〈11-2〉. If we consider only nearest-neighbor interactions, ε, and ignore step overhangs the step free energy vanishes at a temperature . In a more sophisticated model that incorporates step overhangs we find a reduction of T_R to about 0.87ε/k_b. The obtained step free energy expressions are also valid for the free energy of walls between two regions of opposite spins of the triangular 2D Ising system.     

Calculation of the surface energy of hcp metals by using the modified embedded atom method

With MEAM, the surface energies of three kinds of representative surfaces, (h 0 l), (h h l) and (h k 0) belong to [0 1 0], [] and [0 0 1] crystal band, respectively, have been calculated for 13 closed-packed hexagonal (hcp) metals Co, Dy, Er, Gd, Ho, Mg, Nd, Pr, Re, Sc, Tb, Tl and Zr. For all 13 hcp metals, the basal plane (0 0 1) has the minimum surface energy. So from surface energy minimization, the (0 0 1) texture should be favored in the hcp films, this is consistent with the experimental results. The fact that the short termination corresponds to much lower surface energy than long one implies the former is more stable for those surfaces having two possible terminations. Such as the prism plane (1 0 0), only the short termination was observed in experiment.     

Calculation of the surface energy of bcc transition metals by using the second nearest-neighbor modified embedded atom method

The surface energies for 24 surfaces of all bcc transition metals Fe, Cr, Mo, W, V, Nb and Ta have been calculated by using the second nearest-neighbor modified embedded atom method. The results show that, for all bcc transition metals, the order among three low-index surface energies E_(1 1 0) < E_(1 0 0) < E_(1 1 1) is in agreement with experimental results and E_(1 1 0) is also the lowest surface energy for various surfaces. So that from surface energy minimization, the (1 1 0) texture should be favorable in the bcc films. This is also consistent with experimental results. The surface energy for the other surfaces increases linearly with increasing angle between the surfaces (h k l) and (1 1 0). Therefore, a deviation of a surface orientation from (1 1 0) can be used to estimate the relative values of the surface energy.     

Surface magnetic phase transitions in films

Using a simple Landau model, we discuss the different possibilities of generating magnetic effects at a second-order transition for films. Varying the sample size d$d$ and/or surface coupling γ$\gamma$ one can decrease or increase substantially the surface critical temperature _Ts$T_{\mathrm{s}}$ and the saturation magnetization _Ms$M_{\mathrm{s}}$. In the case of γ>0$\gamma >0$, _Ms$M_{\mathrm{s}}$ and _Ts$T_{\mathrm{s}}$ decrease from the bulk values as the film thickness is reduced. These theoretical results are in nice agreement with the experimental data on superconducting _MgB2${\mathrm{MgB}}_2$ thin films. By contrast, for γ<0$\gamma <0$, an enhancement of both quantities is expected. This extraordinary transition has rarely been observed experimentally and, usually, the situation is far from being clear. We analyze a new experiment on _NiFe2O4${\mathrm{NiFe}}_2{\mathrm{O}}_4$ ultra-thin films, where a very strong enhancement of the saturation magnetization is observed.     

Towards an understanding of surfactant action in the epitaxial growth of metals: The case of Sb on Ag (111)

We address the role of surfactant adsorbates in determining changes in the homoepitaxial growth mode of metals, discussing the case of Sb on Ag (111). From ab initio calculations, we extract evidence that the mechanism operative in this system is that Sb induces an irregular shape and an increase in density of the growing Ag islands, and an ensuing increase of the number of attempts for an adatom to descend to a lower terrace. This results from a combination of peculiar properties of this system: Sb is adsorbed insubstitutional surface sites, leading to the formation of a Sb–Ag surface alloy; deposited Ag has reduced mobility on Sb-covered Ag (111), from which follows a higher nucleation probability. The island shape is irregular since the surface alloy is disordered. Surface seggregation of Sb once the growing layer is completed furthers the phenomenon for many deposited Ag layers. Our explanation of the surfactant action of Sb on Ag (111) does not require a reduction of the downstep diffusion barrier, which may, however, be a concurrent factor helpful to interlayer mass transport and layerby-layer growth.     

Critical phenomena in nonequilibrium phase transitions

We discuss a number of models associated with phase transitions in purely kinetic models where detailed balance does not hold as in thermal equilibrium systems. These models include some of the features of heterogeneous catalysis on a surface, and are used to examine the effect of local correlations on the reaction process. We argue that many models which do not include desorption will show the same kind of critical behavior if they have a continuous transition to a poisoned state. We also present results for a model with a continuous transition to a non-poisoned state. The effect of adding desorption, diffusion, and anisotropy to these idealized models is also discussed.     

In vitro biological performance of nano-particles on the surface of hydroxyapatite coatings

The biocompatibility of a kind of heat-treated bilayer hydroxyapatite (HA) coatings with nano-particles was investigated, mainly in terms of the immersion in simulated body fluid (SBF) and osteoblast adhesion. Scanning electron microscopy (SEM) was used to observe the morphology of coatings and cellular adhesion. The phases present in the coatings were determined by X-ray diffraction (XRD). Calcium ion (Ca²⁺) concentration in SBF was measured by Atomic absorption spectrophotometer. The results show nano-HA heat-treated at 650 °C for 0.5 h (BBCs) is comparatively stable during immersion in SBF and favor of the adhesion of osteoblasts. Cellular filopodia adhere firmly to the nano-particles and stretch in various direction.     

New phenomena in the adsorption of alkali metals on Al surfaces

Recent studies of the adsorption and co-adsorption of alkali metals on Al surfaces have revealed a wealth of hitherto unexpected phenomena, including: order-preserving phase transformations between metastable and stable structures; reconstruction of the substrate by adsorption at room temperature; and formation of binary and ternary surface alloys. The results of experimental structural determinations are reviewed, with the major emphasis on Low-Energy Electron Diffraction (LEED) studies of the ordered structures formed by adsorption of the alkalis on Al(111). The experimental results are compared where possible with corresponding results of ab initio calculations of total energy by Scheffler and co-workers. The experiment-theory comparisons provide valuable insight into the adsorption mechanisms, including the possible role of surface steps as both sources and sinks.     

Short-range order in Valenta model of thin films

The present paper deals with the Valenta model for magnetic thin films extended to the entropy construction in its pair representation. The problem of the thin films theory is old, but the Valenta model introduced fifty years ago seems to be still an effective approach requiring however, some modifications. The most important one is connected with the improvement of the entropy construction in the self-consistent way when the correlations are taken into account. The idea is introduced in the present paper and discussed in detail.     

Nonlinear optics for surface phase transitions

A general approach of second-harmonic generation (SHG) studies for surface phase transitions (PTs) is presented, with a thermodynamic classification of surface PTs and their relation to SHG parameters. The symmetry aspects of SHG near a surface PT are discussed, including issues connected with separation of surface and bulk contributions and the role of atomic and mesoscopic inhomogeneities. This approach is illustrated by applying it to two systems revealing a (near-) surface PT: single-crystalline SrTiO₃ near a bulk structural PT and single-crystalline Au in an electrochemical cell revealing an order–disorder and a reconstructive PT. Received: 15 January 2002 / Published online: 11 June 2002     

An atomic view of crystal growth

The ability to routinely observe individual metal atoms adsorbed on metal surfaces using field ion microscopy has made it possible to directly examine atomic events important in the growth of crystals from the vapor. On the atomically smooth (111) plane of iridium, there are now available observations of the binding sites for atoms and of condensation from the vapor, together with measurements of the diffusion of atoms over the surface, as well as studies of their subsequent incorporation at both ascending and descending steps. These results are briefly reviewed, revealing a picture generally more varied and interesting than envisioned in classical theories of crystal growth.     

Growth of Cu Films on Si(111)-7×7 Surfaces at Low Temperature: A Scanning Tunnelling Microscopy Study

Morphologies of Cu(111) films on Si(111)-7×7 surfaces prepared at lowtemperature are investigated by scanning tunnelling microscopy (STM) andreflection high-energy electron diffraction (RHEED). At the initial growth stage, Cu films are flat due to the formation of silicide at the interface that decreases the mismatch between Cu films and the Si substrate. Different from the usual multilayer growth of Cu/Cu(111), on the silicide layer a layer-by-layer growth is observed. The two dimensional (2D) growth is explained by the enhanced high island density at low deposition temperature. Increasing deposition rateproduces films with different morphologies, which is the result of Ostwald ripening.     

Surface phase diagram and temperature induced phase transitions of Sn/Cu(1 0 0)

Room temperature deposition of Sn on Cu(1 0 0) gives rise to a rich variety of surface reconstructions in the submonolayer coverage range. In this work, we report a detailed investigation on the phases appearing and their temperature stability range by using low-energy electron diffraction and surface X-ray diffraction. Previously reported reconstructions in the submonolayer range are p(2 × 2) (for 0.2 ML), p(2 × 6) (for 0.33 ML), ()R45° (for 0.5 ML), and c(4 × 4) (for 0.65 ML). We find a new phase with a structure for a coverage of 0.45 ML. Furthermore, we analyze the temperature stability of all phases. We find that two phases exhibit a temperature induced reversible phase transition: the ()R45° phase becomes ()R45° phase above 360 K, and the new phase becomes p(2 × 2) also above 360 K. The origin of these two-phase transitions is discussed.     

Molecular organization of phospholipid monolayers on the water surface by Maxwell displacement current measurement

The monolayer of organic molecules at the air-water interface has been studied using the Maxwell displacement current (MDC) technique. The materials used in this study were the biological materials of phosphatidyl ethanolamine (PE) and phosphatidic acids (PA). The configuration of the experimental set-up consists of the metal/air-gap/monolayer/metal coupled with the Langmuir method. This measurement enables the detection of current without destroying the monolayer. The phase transition and molecular orientation of the phospholipid monolayers were investigated using MDC measurement without mechanical contact between electrodes and the materials. Direct evidence of phase transition from gaseous to the polar ordering phase can be obtained across phospholipid monolayers even though at very low surface pressure. Relaxation process of the phospholipid monolayers was investigated by using the step compression on the MDC signals.     

Interdiffusion,reaction, and stability in a thin film iron-aluminium bilayered system

In this work we describe the results of Rutherford backscattering spectrometry, sheet resistivity measurements, X-ray diffractometry and conversion electron Mössbauer spectroscopy performed on thin film Fe-Al bilayered samples submitted to high vacuum furnace annealing. Isothermal anneals were performed at 570 K for time intervals ranging from 60 to 600 min. It is demonstrated that the diffusion of Al into Fe is smaller than the diffusion of Fe into Al for temperatures below 600 K. Sequential isochronous thermal anneals of 60 min were performed at temperatures ranging from 570 to 870 K, in order to study the stability of the formed phases. The stable Fe₂Al₅ intermetallic compound formed at 570 K decomposes at about 650 K, and the FeAl₆ intermetallic compound appears at temperatures around 750 K.Work supported in part by CNPq and FINEP     

Depth dependence of the anomalous thermal behavior of Cu(110)

We report the results of a mirror electron microscope low energy electron diffraction (MEM-LEED) study of anomalous thermal behavior caused by enhanced thermal vibrational motion on a clean Cu(110) surface. The enhanced vibrational amplitudes cause diffracted intensities to deviate from simple Debye-Waller behavior. These deviations become apparent between 550 K and 600 K; i.e. at about 0.45 of the bulk melting temperature. Because of the finite penetration of low energy electrons, LEED intensities contain information about both surface and subsurface order. Our LEED results are analyzed to extract this information using a kinematical model in which the electron attenuation and the depth and temperature dependent vibrational amplitudes are parametrized. For a large range of model parameters, we conclude that by 850 K large anharmonic vibrations are present in several layers and at least two subsurface layers may have melted.     

Surface structure of β-FeSi2(101) epitaxially grown on Si(111)

The surface morphology and structure of -FeSi₂(101) films epitaxially grown on Si(111) has been studied by means of Scanning Tunneling Microscopy (STM). The films are formed by large crystallites which are single domain. Each crystallite has only one of the three possible azimuthal orientations with respect to the substrate. A large density of planar defects, however, is detected on top of each crystallite. They are assigned to intrinsic stacking faults and their existence seems hard to avoid. This high density of intrinsic defects casts serious doubts on the use of -FeSi₂ as an optoelectronic material.     

Phase explosion induced by high-repetition rate pulsed laser

The characteristics and mechanisms of the damage to absorbing glass with high-repetition laser pulses (several kHz) are discussed. The results show that: (1) in the range of comparatively low-repetition rate, the damage is characterized by material melting and a small crater on the surface of substrate; (2) with the increase in repetition rate, a bigger and deeper crater is surrounded by re-deposition and crystalline granules originating from the cooling of vapor; and (3) the crater, surrounded by evaporation and an large number of solid particulates which is obviously the characters of phase explosion, becomes even bigger and deeper when the repetition rate is further increased. We modeled the temperature distribution in different repetition rate regime and found that heat accumulation plays a significant role in damage process. Because of the temperature dependence of damage mechanism, the temperature of the area irradiated by laser beam will ramp up with increasing the repetition rate, which triggers the melting and evaporation of dielectrics and phase explosion successively. Our results may benefit the understanding of laser-induced damage in optical materials.