Polar oxide interface stabilization by formation of metallic nanocrystals

In situ x-ray photoelectron spectroscopy and ex situ transmission electron microscopy and diffraction studies of a model Fe3O4(111)/MgO(111) polar oxide interface exclude stabilization by interface faceting, reconstruction, or by formation of a continuous interfacial layer with altered stoichiometry, and uncover stabilization by dominant formation of metallic Fe(110) nanocrystals. The iron nanocrystals nucleate both at the interface and within the magnetite film and grow in a Nishiyama-Wasserman orientation relationship with a bimodal size distribution related to twinning. Minority magnetite nanocrystals were also observed, growing in the less polar (100) orientation than the magnetite (111) film. Electron transfer and bond hybridization mechanisms are likely at the metal/oxide and oxide/oxide interfaces and remain to be explored.     

The role of the nanoscale in surface reactions: CO2 on CdSe

Cd-rich CdSe nanocrystals below a critical size, under illumination, catalyze CO2 fixation, but bulk CdSe surfaces do not. We report first-principles calculations in which we determine the roles of faceting, deviations from stoichiometry, photoexcitation, and electron confinement, and the specific physics of the nanoscale. We further establish that catalysis does not occur at the nanocrystal surface; instead, neutral molecules adsorb, desorb negatively charged, and react elsewhere. Finally, we predict that n-type doped CdSe nanocrystals would be effective catalysts without photoexcitation.     

Modeling dewetting of ultra-thin solid films

We review some models for the dynamics of dewetting of ultra-thin solid films. We discuss the similarities and the differences between faceted and non-faceted systems. The faceting of the dewetting rim leads to corrections in the velocity of dewetting fronts both in flat and axisymmetric geometries. The faceting of the edge of the dewetting rim leads to a strong anisotropy of the dewetting instability. Faceting also induces novel dewetting regimes such as layer-by-layer dewetting, and monolayer dewetting.     

Nano-faceting of fcc(1 1 0) surfaces controlled by adsorbates and atom deposition or removal

(1 1 0) surfaces of most fcc metals are only marginally stable against faceting into (1 1 1) orientations. Trace concentrations of adsorbates (surfactants) that prefer (1 1 1) over (1 1 0) facets can tip the balance and favor faceting of all or part of the (1 1 0) surface. The growth of such facets is impeded by island and vacancy-island nucleation barriers. However, during atom deposition or etching these barriers are reduced. Growth or removal conditions and facet stabilizing surfactant concentrations control the evolving faceted nano-patterns. Recent observations of hut-shaped nanocrystals formation on Al(1 1 0) are consistent with this model.     

Effect of strain on the appearance of subcritical nuclei of ge nanohuts on Si(001)

Real-time scanning tunneling microscopy observations of nucleation and heteroepitaxial growth of Ge nanocrystals (from germane) on Si(001) indicate that in the absence of Si-Ge intermixing the formation of full hut cluster islands is preceded by the nucleation of "subcritical" nuclei consisting of two adjacent truncated tetrahedral pyramids, which, upon unification, form a tiny square-based pyramidal "critical nucleus" It is suggested that such a precursor aids in surpassing the nucleation barrier and that the recently reported gradual faceting of prepyramids is characteristic of only Ge(Si) alloys.     

Adatom ascending at step edges and faceting on fcc metal (110) surfaces

Using first-principles total-energy calculations, we show that an adatom can easily climb up at monatomic-layer-high steps on several representative fcc metal (110) surfaces via a place exchange mechanism. Inclusion of such novel adatom ascending processes in kinetic Monte Carlo simulations of Al(110) homoepitaxy as a prototypical model system can lead to the existence of an intriguing faceting instability, whose dynamical evolution and kinetic nature are explored in comparison with experimental observations.     

Faceting and branching in 2D crystal growth

Using atomic scale time-dependent density functional calculations we confirm that both diffusion-controlled and diffusionless crystallization modes exist in simple 2D systems. We provide theoretical evidence that a faceted to nonfaceted transition is coupled to these crystallization modes, and faceting is governed by the local supersaturation at the fluid-crystalline interface. We also show that competing modes of crystallization have a major influence on mesopattern formation. Irregularly branched and porous structures are emerging at the crossover of the crystallization modes. The proposed branching mechanism differs essentially from dendritic fingering driven by diffusive instability.     

一种新的量子点形成机制:表面原子的向上扩散运动

在Al(110)表面的同质外延生长中，实验上观察到许多由特殊小面围成的较大的量子点．这些长方盒形的量子点要比外延膜的平均厚度至少高十倍以上，密度泛函理论的研究发现，动力学过程是导致形成具有特定小面量子点的主要原因，其关键机制是表面原子的向上扩散运动。     

Variations in shapes of outgrowths on a tungsten tip during growth in an electric field

Continuous-mode field-desorption microscopy was used to study the shape of thermal-field outgrowths on the (001) face of a tungsten tip. It is shown that the shape and faceting of the lateral surface of an outgrowth change with its height, which can be explained by reconstruction of the lateral surface of the outgrowth due to a gradual decrease in the field intensity in the bottom part of the outgrowth during its growth. The reconstruction occurs because, at high field intensity, the shape and faceting are determined by the kinetics of growth, whereas at low field intensity they are determined by the minimum free energy.     

Thermal faceting of (110) and (111) surfaces of MgO

Using LEED, we have observed the thermal faceting of MgO (110) and (111) surfaces into sets of (100) faces. Some faceting occurs under ion bombardment at room temperature, and annealing at 900–1400 K by means of electron bombardment produces complete faceting, with facets up to 1 μm across. These observations are consistent with theories of the stability of ionic crystal surfaces.     

Temperature effect on elastic modulus of thin films and nanocrystals

The stability of nanoscale devices is directly related to elasticity and the effect of temperature on the elasticity of thin films and nanocrystals. The elastic instability induced by rising temperature will cause the failure of integrated circuits and other microelectronic devices in service. The temperature effect on the elastic modulus of thin films and nanocrystals is unclear although the temperature dependence of the modulus of bulk materials has been studied for over half a century. In this paper, a theoretical model of the temperature-dependent elastic modulus of thin films and nanocrystals is developed based on the physical definition of the modulus by considering the size effect of the related cohesive energy and the thermal expansion coefficient. Moreover, the temperature effect on the modulus of Cu thin films is simulated by the molecular dynamics method. The results indicate that the elastic modulus decreases with increasing temperature and the rate of the modulus decrease increases with reducing thickness of thin films. The theoretical predictions based on the model are consistent with the results of computational simulations, semi-continuum calculations and the experimental measurements for Cu, Si thin films and Pd nanocrystals.     

Synthesis and optical properties of In-doped GaN nanocrystals

Indium-doped GaN nanocrystals with 5% and 10% In have been prepared by a low temperature solvothermal method using hexamethyldisilazane as the nitriding reagent. The nanocrystals show Raman bands at lower frequencies compared to GaN. Photoluminescence spectra of the In-doped GaN nanocrystals exhibit an increase in the FWHM with the decrease in the PL band energy, the band energy itself decreasing with increase in the In content.     

“Rotating” steps in Si(0 0 1) homoepitaxy

Steps on Si(0 0 1) surfaces which are initially not aligned along the high symmetry directions of the dimer reconstruction are observed, by scanning tunneling microscopy, to “rotate” toward [1 1 0] directions during Si growth. This step “rotation” occurs due to a faceting of the step edges. A theoretical analysis of adatom incorporation into the steps shows that this kinetic instability may be caused by a suppressed mobility of the growing species along the S_A step edge.     

Shocks and curvature dynamics: A test of global kinetic faceting in crystals

We investigate the microscopic mechanisms underlying the dynamical faceting of crystals. Partially faceted crystal shapes of CCl4 are formed from a melt contained in a Bridgman apparatus and pressure is used to control growth which is observed using optical microscopy. In contrast to predictions of models in which the local interfacial motion is greatest where the step density is the highest, the loss of rough orientations is observed to occur via a local decrease in curvature which results in the formation of discontinuities-shocks-in the surface of the growth forms, a feature predicted by a recent theory of kinetic faceting.     

Atomic structures on faceted W(111) surfaces induced by ultrathin films of Pd

The faceting of Pd/W(111) surfaces has been studied using a Scanning tunneling microscope (STM). Three-sided pyramidal facets having {211} faces with dimensions ranging from 3 to 15 nm can be induced by ultrathin Pd films (≥ 1 monolayer), upon annealing to 700 K or higher. From atomic-resolution STM-images of these surfaces, we obtain direct confirmation of the {211} structure on individual facets of the 3-sided pyramids. In addition, the atomic structure of the facet edges indicates that edge energy may play a role in faceting. When the as-deposited coverage of Pd is greater than the critical value ( 1 monolayer) for inducing faceting, the extra Pd atoms diffuse to form 3-dimensional clusters, some with discernible crystalline structures, upon annealing.     

Size-dependent melting of nanocrystals: a self-consistent statistical approach

A self-consistent statistical method is used to describe size effects on melting of free nanocrystals. The melting transition is assumed to be directly related to evolution of high-temperature instability of the phonon subsystem of the crystal, caused by strong anharmonicity of atomic vibrations. We show that depression of the melting temperature of small free particles is mainly due to presence of surface atoms which are bound to smaller numbers of atoms than those of the interior. The melting temperatures of spherical nanocrystals of Ar and Au were calculated as functions of the inverse of their radii and compared with experimental and molecular dynamics data.     

Oxygen induced reconstruction of (h11) and (100) faces of copper

A LEED and AES study on oxygen adsorption on Cu(100) and (h11) faces with 5 h 15 has been performed under various adsorption conditions (220 K T 670 K and 1 × 10⁻⁸ P 6 × 10⁻⁵ Torr of oxygen). The dependence of adsorption temp on the oxygen surface superstructures is pointed out. At least, three oxygen surface states exist on a Cu(100) face. For low temperature exposures to oxygen, under conditions of slow surface diffusion, on the (100) face, two oxygen surface phases exist: a “four spots” and a c(2 × 2) superstructure, both observed even at saturation coverage; on all the stepped faces, a c(2 × 2) appears and no faceting is observed. For high temperature exposures, on the (100) face, two oxygen superstructures are observed, a “four spots” followed by a (2√2 × √2)R45° at higher coverages; on all the stepped faces, surface diffusion is activated and oxygen induced faceting occurs. The appearance of faceting is associated with the onset of the formation of the (2√2 × √2)R45° structure on the (100) face. The oxygen induced faceting and the oxygen surface meshes are reversible with coverages. At saturation coverage, a non-reversible surface transition between the c(2 × 2) and (2√2 × √2)R45° superstructures is observed at 420 ± 20 K. The importance of impurity traces on the surface meshes is emphasized. Oxygen coverage at saturation is independent of the studied faces and adsorption temperature. Faceting occurs at a critical coverage value, whatever the stepped faces and adsorption temperature are. Models of the oxygen structure on the (h10) stepped faces are discussed.     

Stratified layer formation in particle suspensions

Initially homogeneous suspensions of colloidal particles often develop patterns during sedimentation. Commonly, the concentration profile of the particles evolves into a “staircase”: layers of nearly constant concentration, separated by sharp boundaries between successive layers, with the concentration of each successive layer increasing with depth. Siano [1] has demonstrated experimentally that uphill diffusion, diffusion against the concentration gradient, occurs during this pattern formation. Thus, these patterns appear to be the result of spinodal decomposition. We find that these staircase patterns cannot be explained by the classical spinodal decomposition theory of Cahn and Hilliard, but that they can be explained if the linear gradient-energy term of Tiller, Pound, and Hirth is added to the free energy. Such a term plays a central role in the faceting of crystals. In the present application we believe that the physical origin of this extra term may be the Rayleigh—Taylor instability.     

Surface energy anisotropy of iridium

Field-ion microscopy was used to study the faceting behavior and surface energy anisotropy of iridium in vacuum and in hydrogen. In vacuum below approximately 1300 K the order of faceting and the activation energy for growth of {111} facets agreed with previously published results of FIM studies. The unexpected faceting behavior of {210} planes was examined in terms of the geometry of field evaporated specimens. The observed anisotropy at temperatures above 1300 K was in qualitative agreement with Morse and Mie potential calculations and in nearly quantitative agreement with the pairwise bonding model using σ₂ = 0.4, σ₃ = 0.2. The observed maximum anisotropy of 8.8% for iridium at 1360 K fell within the range of extrapolated values for other metals at one-half the absolute melting temperature. Hydrogen appeared to lower the surface energy of each plane by only about 0.1%. An anisotropic effect of hydrogen on the faceting behavior, however, was observed and suggested that surface diffusion rates in {110} and {311} regions were preferentially increased in the presence of hydrogen.     

Faceting of sapphire crystals grown from a melt by the Stepanov method

The appearance of faces on cylindrical sapphire single crystals grown from a melt by the Stepanov method is studied by photogoniography and optical microscopy. The crystallographic indices of the detected faces are established, and the microstructure of the growth layers is investigated. A relationship between the faceting and the growth conditions is found. The experimental results are compared with data on the faceting of the sapphire crystals grown from a solution-melt and with the calculated specific free surface energies of the faces.