Nucleation and growth mechanisms of Fe on Au(111) in the sub-monolayer regime

The growth of Fe on Au(111) at 300 K in the sub-monolayer regime has been investigated using scanning tunneling microscopy, focusing on the mechanisms of nucleation, coalescence and interlayer diffusion. Below a coverage of 0.1 ML, Fe growth proceeds in a well-ordered fashion producing regular arrays of islands, while approaching the island coalescence threshold (above 0.35–0.4 ML), we observed a consistent increasing of random island nucleation. These observations have been interpreted through rate equation models for the island densities in the presence of preferred nucleation sites. The evolution of the second layer fraction has also been interpreted in a rate equation scheme. Our results show that the ordered to random growth transition can be explained by including in the model bond breaking mechanisms due to finite Fe–Fe bond energy. A moderate interlayer diffusion has been inferred from data analysis between the second and the first layer, which has been used to estimate the energy barrier of the adatoms descending process.     

高阻GaN的MOCVD外延生长

利用金属有机化合物气相沉积(MOCVD)在蓝宝石衬底上生长了高阻GaN薄膜。对GaN成核层生长的反应室压力、生长时间和载气类型对GaN缓冲层电学特性的影响进行了分析。实验结果表明,延长GaN成核层的生长时间,降低成核层生长时的反应室压力,载气由H₂换为N₂都会得到高阻的GaN缓冲层。样品的方块电阻R_s最高为2.49×10¹¹ Ω/□。以高阻GaN样品为衬底制备了AlGaN/AlN/GaN结构HEMT器件,迁移率最高达1 230 cm²/(V·s)。     

Multilayer Cooperative Sequential Adsorption

Cooperative sequential adsorption is here extended to multilayer coverages. We discuss two different growth rules with cooperativity either restricted to only the first layer of coverage or applied in all layers. The unrestricted variant is considered in the case where lateral growth dominates over the nucleation of terraces. The limit of completely suppressed nucleation corresponds to a morphological transition to a flat interface from one governed by the Kardar–Parisi–Zhang equation. With the restricted growth rule we find interesting behavior resulting from a competition between lateral growth at the first layer and growth on the top of nucleated islands. There is an intermediate regime between random deposition at the submonolayer coverage and asymptotic random deposition behavior. In this regime the kinetic roughening can be studied by applying sequential adsorption rate equations for cluster lengths in the first layer, with an additional geometric argument.     

Feature of solid–liquid metals reaction revealed by conversion electron Mössbauer spectrometry

Selective electrolytic dissolution of the η-zinc layer of continuous galvanized steel sheet has been utilized to uncover the Fe–Al intermetallic layer which inhibits the formation of other binary Fe–Zn intermetallic compounds. The characterization of the Fe–Al–Zn compounds by scanning electron microscopy, Mössbauer spectrometry and semi-grazing X-ray diffraction brings new enlightment about the conditions which control nucleation and growth as well as collapse of the inhibition layer and the resulting morphology of the coating.     

Valence variations in the monolayer regime of Sm on the Nb(110) surface

Sm overlayers in the monolayer regime, deposited on a Nb(110) single-crystal surface at room temperature, have been studied by means of high-resolution photoelectron spectroscopy of the Nb 3d_5/2 core level and the Sm valence band. In the submonolayer regime, the Sm valence varies from mainly divalent for very low coverages to essentially trivalent close to a complete monolayer. Above 1 ML, a new divalent component appears in the Sm 4f spectra, corresonding to divalent Sm in the second layer. The mixed valence in this overlayer system is concluded to be heterogeneous (all Sm atoms have integer but site-dependent valence). Sm forms ordered overlayers on Nb(110) and the Sm growth is consistent with a layer-by-layer growth mode (Frank–van der Merwe growth).     

Stochastic theories and scaling relations for early-stage surface growth

Recent theories for the surface growth are reviewed, which describe the nucleation kinetics of binary alloys in the first layer and the nucleation on top of islands in the second layer. In the first part, describing the submonolayer regime, scaling relations for the number density of stable islands in dependence of the growth parameters are derived from an extension of the classical rate equation approach to binary systems. In the second part a general stochastic theory is presented that allows one to predict whether thin films become smooth or rough under non-equilibrium growth conditions.     

Classification of regimes determining ultrasonic cavitation erosion in solid particle suspensions

Although the factors that influence ultrasonic cavitation erosion in solid particle suspensions have been extensively studied, the role that solid particles play in the cavitation process remains poorly understood. The ultrasonic cavitation erosion of AISI 1045 carbon steel was studied in the presence of monodisperse silica particles (10–100 μm, 0.5–20 vol%) suspended in transformer oil. Based on our results, we propose an overview of the possible influencing mechanisms of particle addition for specific particle sizes and concentrations. Four major regimes, namely a viscosity-enhancing regime (V), a particle-impinging regime (I), a particle-shielding regime (S), and a nuclei-adding regime (A) are identified, and their dependence on suspended particle characteristics is analyzed. The VISA regimes, in essence, reflect the viscous and inertial effects of suspended particles, and the way in which particle–particle interactions and heterogeneous nucleation affect erosion. This regime-based framework provides a better understanding of the dominant factors controlling the erosive wear caused by cavitation in the presence of solid particles, and provides a guide for erosion prediction and prevention.     

On the theory of phase transformations with a nonuniform nucleation rate

An expression for the phase volume fraction in a system with a nonuniform nucleation rate is derived by using the geometrical-probabilistic approach. Examples of such systems considered here are (1) a plane layer (with nucleation in the midplane) and random planes in space, (2) an infinitely long cylinder (with nucleation on the axis) and random lines in space, and (3) a sphere (with nucleation at the center) and nucleation at random points. In each case, an expression for the phase volume fraction is derived for the time-dependent rates of nucleation and growth. The equivalence of homogeneous nucleation and nucleation at points is established.     

Nucleation of damage centres during ion implantation of silicon

Nucleation of damage centres in single crystal silicon held at room temperature and bombarded in a “random” direction with light ions is not at all times heterogeneous, as has sometimes been assumed. Homogeneous nucleation of interstitial clusters occurs at low ion fluences and this is characterized, for an extremely short period, by a linear dependence of the state of disorder on ion fluence, followed by a longer period during which disorder a (fluence)^1/2. During these periods of ‘nucleation’ and ‘primary growth’ small interstitial clusters behave as unsaturable traps. For higher fluences there is a smooth transfer to a form of damage increase which is not of displacement cascade origin. In this period of ‘secondary growth’ the state of disorder varies linearly with ion fluence, and interstitial clusters behave as nucleation traps.

It is shown how marked effects on the state of disorder due to changes in the mass of the bombarding ion, in the flux of the impinging beam, or in the temperature and impurity content of the bombarded crystal, can be simply traced to an early assistance for or a resistance to the onset of hetero- over homogeneous nucleation.     

The initial oxidation of molybdenum: V. A model for the nucleation of epitaxial oxide and its application to molybdenum

A new model for the phenomenon of epitaxial oxidation nucleation is presented growth characteristics of the oxide suggest that the prenucleation growth is governed by the oxygen adsorption rate of the oxide/gas boundary. As the oxide grows the calculated rate of dissolution of cations at the oxide/metal interface falls until this becomes rate determining. At this point low work function facet sided pits are etched in the substrate with corresponding bulges in the outer oxide film. These bulges are identified as epitaxial nuclei. Based on this model several parameters relating to oxide growth may be calculated. The agreement between theory and experiment is very good. In particular a lower and upper temperature limit for nucleation are found and the pressure dependence is shown to be quite complicated. The basis of the model can be extended to the chemisorption regime. It is shown that above a surface potential of about 1 V cations migrate rapidly through the oxygen layer. The onset of this rapid migration is identified with molecular oxide production and the start of faceting.     

X-ray photoelectron spectroscopy study of the growth kinetics of biomimetically grown hydroxyapatite thin-film coatings

Hydroxyapatite (HA) thin-film coatings grown biomimetically using simulated body fluid (SBF) are desirable for a range of applications such as improved fixation of fine- and complex-shaped orthopedic and dental implants, tissue engineering scaffolds and localized and sustained drug delivery. There is a dearth of knowledge on two key aspects of SBF-grown HA coatings: (i) the growth kinetics over short deposition periods, hours rather than weeks; and (ii) possible difference between the coatings deposited with and without periodic SBF replenishment. A study centred on these aspects is reported. X-ray photoelectron spectroscopy (XPS) has been used to study the growth kinetics of SBF-grown HA coatings for deposition periods ranging from 0.5 h to 21 days. The coatings were deposited with and without periodic replenishment of SBF. The XPS studies revealed that: (i) a continuous, stable HA coating fully covered the titanium substrate after a growth period of 13 h without SBF replenishment; (ii) thicker HA coatings about 1 μm in thickness resulted after a growth period of 21 days, both with and without SBF replenishment; and (iii) the Ca/P ratio at the surface of the HA coating was significantly lower than that in its bulk. No significant difference between HA grown with and without periodic replenishment of SBF was found. The coatings were determined to be carbonated, a characteristic desirable for improved implant fixation. The atomic force and scanning electron microscopies results suggested that heterogeneous nucleation and growth are the primary deposition mode for these coatings. Primary osteoblast cell studies demonstrated the biocompatibility of these coatings, i.e., osteoblast colony coverage of approximately 80%, similar to the control substrate (tissue culture polystyrene).     

Direct visualisation of homogeneous and heterogeneous crystallisation in an ensemble of confined domains of poly(ethylene oxide)

We present a study of homogeneous and heterogeneous nucleation in polymer crystallisation. In bulk samples the crystallization is typically dominated by nucleation from defects (heterogeneous nucleation), and consequently studies must rely on sample preparation to minimize this effect. We present a study of nucleation within discrete droplets of poly(ethylene oxide) that are formed by the dewetting of a thin film on an unfavourable substrate. The samples provide an ensemble of impurity-free droplets, with length scales that can easily be measured. We show that the data for heterogeneous and homogeneous nucleation is qualitatively different, and that the data mirrors the fundamental differences in the underlying mechanisms for the two nucleation processes. The experiments presented here provide a simple method that can be used to study heterogeneous and homogeneous nucleation in great detail.Received: 1 January 2003, Published online: 30 October 2003PACS: 61.41. + e Polymers, elastomers, and plastics - 68.55.-a Thin film structure and morphology - 81.10.-h Methods of crystal growth; physics of crystal growth     

Surface morphology for ion-beam sputtered Al layer with varying sputtering conditions

We study the growth morphology of thin macrostructure films which is known to be largely affected by the deposition conditions as thin film nucleation and formation is dependent on the kinetic energy and chemical free energy of the atoms. The ion-beam sputtering technique used for depositing thin layers is due to the advantage over other techniques, e.g. the independent control of many process parameters, such as the pressure and/or the energy of the ion-beam and the substrate temperature. Therefore, the dependence of various sputtering parameters such as: (i) sputtering pressure and/or the rate of deposition and (ii) the effect of substrate temperature on the growth has been studied by depositing a single layer of Al. The variations show some interesting dependencies on the structural parameters for the Al layer deposited which has been understood in terms of thin film growth and nucleation theory.     

Nonisomorphic nucleation pathways arising from morphological transitions of liquid channels

Motivated by unexpected morphologies of the emerging liquid phase (channels, bulges, droplets) at the edge of thin, melting alkane terraces, we propose a new heterogeneous nucleation pathway. The competition between bulk and interfacial energies and the boundary conditions determine the growth and shape of the liquid phase at the edge of the solid alkane terraces. Calculations and experiments reveal a "precritical" shape transition (channel-to-bulges) of the liquid before reaching its critical volume along a putative shape-conserving path. Bulk liquid emerges from the new shape, and, depending on the degree of supersaturation, the new pathway may have two, one, or zero energy barriers. The findings are broadly relevant for many heterogeneous nucleation processes because the novel pathway is induced by common, widespread surface topologies (scratches, steps, etc.).     

Grain growth and collective migration of grain boundaries during plastic deformation of nanocrystalline materials

A theoretical model is proposed for the collective migration of two neighboring grain boundaries (GBs) in a nanocrystalline material under applied elastic stress. By analyzing the change in the energy of the system, it is shown that GBs can remain immobile or migrate toward each other depending on the values of the applied shear stress and misorientation angles. The process of GB migration can proceed either in a stable regime, wherein the GBs occupy equilibrium positions corresponding to a minimum of the energy of the system under relatively small applied stress, or in an unstable regime, wherein the motion of GBs under relatively high stress is accompanied by a continuous decrease in the system energy and becomes uncontrollable. The stable migration of GBs leads to a decrease of the grain bounded by them at the cost of growth of the neighbor grains and can result in complete or partial annihilation of the GBs and the collapse of this grain. Unstable migration leads either to annihilation of GBs or to passage of them through each other, which can be considered as the disappearance of the grain and nucleation and growth of a new grain.     

Homogeneous and heterogeneous clustering in the accretion regime

Condensation of nano-droplets in a supersaturating vapor decomposes in two steps: the formation of a nucleation center, also called critical nuclei or nucleation seed, and the growth sequence, by accretion of further atoms on the nucleation center. These two steps have been investigated separately through the clustering of homogeneous particles Na_n and heterogeneous particles Na_nX in a helium buffer gas (X = (Na₂O)₂ or (NaOH)₂). The growth sequence is analyzed with preformed molecules X injected in a supersaturating sodium vapor and driving production of Na_nX clusters. Cluster distribution mean sizes are controlled by sodium concentration and by the condensation cell effective length. The signal intensities observed for homogeneous and heterogeneous clusters are proportional to the homogeneous and heterogeneous nucleation center numbers respectively. We can measure the efficiency for the homogeneous nucleation center production versus sodium concentration. This process is the onset of the condensation phase transition.     

Delayed plasticity in nanoindentation of annealed crystals

Continuum constitutive relations used in the design of macro-sized components assume that the elastic limit of a crystalline solid is time independent. Recent experiments using the nanoindentation technique, however, reveal that the elastic limit of submicron-sized metallic volumes decreases as time under load increases. A submicron metallic volume can sustain a static load in the elastic regime initially, but transition to plastic deformation may occur after some waiting time. In this paper, the characteristics of this type of delayed plasticity are reviewed. The available experimental data suggest that homogeneous nucleation of the plasticity events, which was frequently discussed in the recent literature, occurs only at sufficiently high loads within a narrow range. In a lower and broader load range, the nucleation of the plasticity events occurs at a history dependent rate, thus via a damage-accumulation mechanism not compatible with the homogeneous nucleation theory. A model based on the diffusion-controlled, subcritical growth of a Frank loop just underneath the indenter is proposed in this work to explain the history dependent nucleation of instability observed at lower loads. By fitting to the available nanoindentation data in Ni₃Al, it is apparent that self-diffusion along the indenter-sample interface, rather than through the bulk, is likely to be the controlling factor for the growth of the Frank loop to a critical size to yield a dislocation avalanche.