Effect of strain on surface diffusion and nucleation

Versatile stochastic model for predictive KMC simulation of fcc metal nanostructure evolution with realistic kinetics

Stochastic lattice-gas models provide the natural framework for analysis of the surface diffusion-mediated evolution of crystalline metal nanostructures on the appropriate time scale (often 101-104 s) and length scale. Model behavior can be precisely assessed by kinetic Monte Carlo simulation, typically incorporating a rejection-free algorithm to efficiently handle the broad range of Arrhenius rates for hopping of surface atoms. The model should realistically prescribe these rates, or the associated barriers, for a diversity of local surface environments. However, commonly used generic choices for barriers fail, even qualitatively, to simultaneously describe diffusion for different low-index facets, for terrace vs step edge diffusion, etc. We introduce an alternative Unconventional Interaction-Conventional Interaction formalism to prescribe these barriers, which, even with few parameters, can realistically capture most aspects of behavior. The model is illustrated for single-component fcc metal systems, mainly for the case of Ag. It is quite versatile and can be applied to describe both the post-deposition evolution of 2D nanostructures in homoepitaxial thin films (e.g., reshaping and coalescence of 2D islands) and the post-synthesis evolution of 3D nanocrystals (e.g., reshaping of nanocrystals synthesized with various faceted non-equilibrium shapes back to 3D equilibrium Wulff shapes).

Strain-induced enhancement of surface self-diffusion on strontium titanate (001) surfaces

We present a numerical investigation of self-diffusion on strontium titanate TiO2-terminated (001) surfaces via density functional theory. Our calculations first indicate that Ti has the highest diffusion barrier with approximately 2.20 eV, thus representing the rate-limiting step for surface self-diffusion. Furthermore, the higher energy barriers of O and Ti in comparison to O2and TiO2indicate electronic activity with the surface atoms. Under the consideration of equi-biaxial strain as it would be encountered in e.g. heteroepitaxial thin films, the diffusion barriers for surface self-diffusion decrease for both compressive and tensile strains between -6% and 2%. For larger strains, we observe plastic deformations. This possibility to lower the energy barrier paves the way for accelerated and possible new mechanisms of surface diffusion and reconstruction of strontium titanate structures in a wide range of applications.

Diffusion coefficients and MSD measurements on curved membranes and porous media

We study some geometric aspects that influence the transport properties of particles that diffuse on curved surfaces. We compare different approaches to surface diffusion based on the Laplace-Beltrami operator adapted to predict concentration along entire membranes, confined subdomains along surfaces, or within porous media. Our goal is to summarize, firstly, how diffusion in these systems results in different types of diffusion coefficients and mean square displacement measurements, and secondly, how these two factors are affected by the concavity of the surface, the shape of the possible barriers or obstacles that form the available domains, the sinuosity, tortuosity, and constrictions of the trajectories and even how the observation plane affects the measurements of the diffusion. In addition to presenting a critical and organized comparison between different notions of MSD, in this review, we test the correspondence between theoretical predictions and numerical simulations by performing finite element simulations and illustrate some situations where diffusion theory can be applied. We briefly reviewed computational schemes for understanding surface diffusion and finally, discussed how this work contributes to understanding the role of surface diffusion transport properties in porous media and their relationship to other transport processes.

Role of Graphene Topography in the Initial Stages of Pentacene Layer Growth

Using atomic force microscopy, we probed the growth of pentacene molecules on graphene that was fabricated by chemical vapor deposition and transferred onto 300 nm-thick SiO2 substrates. The topography of such graphene has two important properties. First, its surface is comprised of folds that have different orientations, and second, it has several multilayer-graphene regions distributed over the monolayer-graphene surface. On such folded graphene features, we vacuum evaporated pentacene and observed three-dimensional islands with an average height of ∼15 nm. They are oriented either parallel or perpendicular to the folds, and they are also predominantly oriented along the symmetry axes of graphene. Orientation of pentacene islands on graphene evaporated at room temperature has a wide distribution. On the contrary, most of the pentacene islands evaporated at 60 °C are oriented at 30° with respect to the fold direction. We observed that the folds act as a potential barrier for the surface transport of pentacene molecules. In addition, we interpret the 3D growth of pentacene islands on graphene in terms of reduced polar components of the surface energy on graphene investigated with contact angle measurements.

Effects of Adsorbate Diffusion and Edges in a Transition from Particle to Dendritic Morphology during Silver Electrodeposition

During silver electrodeposition on Au nanoparticle (NP)-covered highly oriented pyrolitic graphite, a transition from an initial growth of microsized particles to the growth of dendrites with pine tree shape (nanotrees) is observed, which is an advancement for material growth with hierarchical surface roughness. Using kinetic Monte Carlo simulations of an electrodeposition model, those results are explained by the interplay of diffusive cation flux in the electrolyte and relaxation of adsorbed atoms by diffusion on quenched crystal surfaces. First, simulations on NP-patterned substrates show the initial growth of faceted silver particles, followed by the growth of nanotrees with shapes similar to the experiments. Next, simulations on electrodes with large prebuilt particles explain the preferential nanotree growth at corners and edges as a tip effect. Simulations on wide flat electrodes relate the nanotree width with two model parameters describing surface diffusion of silver atoms: maximal number of random hops (G) and probability of hop per neighbor (P). Finally, simulations with small electrode seeds confirm the transition from initially compact particles to the nucleation of nanotrees and provide estimates of the transition sizes as a function of those parameters. The simulated compact and dendritic deposits show dominant (111) surface orientation, as observed in experiments. Extrapolations of simulation results to match microparticle and nanotree sizes lead to G = 4 × 10¹¹ and P = 0.03, suggesting to interpret those sizes as diffusion lengths on the growing surfaces and giving diffusion coefficients 2 to 3 × 10^-13 m²/s for deposited silver atoms. These results may motivate studies to relate diffusion coefficients with atomic-scale interactions.

Direct Measurements of Surface Strain-Mediated Lateral Interactions between Adsorbates in Colloidal Heteroepitaxy

Surface strain can alter the dynamics of adsorbates, and often, the adsorbates themselves induce and interact via their surface strain fields. In epitaxy, such strain-mediated effects get further compounded when a misfit strain exists due to lattice mismatch between the growing film and substrate. Here, we carry out particle-resolved imaging of heteroepitaxial growth of multilayer colloidal films where the particles interact via a short-range attraction. Surprisingly, although the misfit strain relaxed systematically with film thickness, the adcolloid diffusivity was nonmonotonic. We show that this nonmonotonicity stems from the competition between the spatial extent of self-induced in-layer strain and the short interaction range. Importantly, we provide direct evidence for long-ranged strain-mediated interactions between adsorbates and show that it alters the growing film's morphology.

Suspended Graphene Membranes to Control Au Nucleation and Growth

Control of nucleation sites is an important goal in materials growth: nuclei in regular arrays may show emergent photonic or electronic behavior, and once the nuclei coalesce into thin films, the nucleation density influences parameters such as surface roughness, stress, and grain boundary structure. Tailoring substrate properties to control nucleation is therefore a powerful tool for designing functional thin films and nanomaterials. Here, we examine nucleation control for metals deposited on two-dimensional materials in a situation where substrate effects are absent and heterogeneous nucleation sites are minimized. Through quantification of faceted, epitaxial Au island nucleation on graphene, we show that ultralow nucleation densities with nuclei several micrometers apart can be achieved on suspended graphene under conditions where we measure 2-3 orders of magnitude higher nucleation density on the adjacent supported substrate. We estimate diffusion distances using nucleation theory and find a strong sensitivity of nucleation and diffusion to suspended graphene thickness. Finally, we discuss the role of surface roughness as the main factor determining nucleation density on clean free-standing graphene.

Interfacial Diffusion of Hydrated Ion on Graphene Surface: A Molecular Simulation Study

Hydration plays an important role in the diffusion and sieving of ions within nanochannels. However, it is hard to quantitatively analyze the contribution of hydration to the diffusion rates due to the complex hydrogen-bond and charge interactions between atoms. Here, we quantitatively investigated the interfacial diffusion rates of a single hydrated ion with different number of water molecules on graphene surface through molecular dynamics simulation. The simulation results show the ballistic diffusion mode by analyzing the mean-square displacement, and the diffusion rates change nonmonotonically with the hydration number. The potential energy profiles with the changing position of the hydrated ion on graphene surface were further analyzed, which shows the dominant factor for interfacial diffusion changing from ion-graphene interaction to water-graphene interaction as the number of water molecules increases. Besides, it was found that the surface hydrophilicity weakened the influence of hydration number on the diffusion rates of hydrated ion. Finally, the diffusion properties of different hydrated ions on graphene surface were investigated, and the hydrated Li⁺, Na⁺, and K⁺ containing three, four, and five water molecules, respectively, show the fastest diffusion rate. This work demonstrates the interfacial diffusion behavior and mechanism of hydrated ions at the molecular level, which can provide valuable guidance in nanosensors, seawater desalination, and other hydrated ion-related fields.

Unveiling Growth Pathways of Multiply Twinned Gold Nanoparticles by In Situ Liquid Cell Transmission Electron Microscopy

A mechanistic understanding of the growth of multiply twinned nanoparticles (MTPs), such as decahedra (Dh) and icosahedra (Ih), is crucial for precisely controlled syntheses and applications. Despite previous successes, no consensus has been reached regarding the multiple competing growth pathways for MTPs proposed thus far, in part due to the lack of information about their nucleation and growth dynamics. Here, we used decahedral and icosahedral gold nanoparticles as a model system in conjunction with in situ liquid cell transmission electron microscopy (LCTEM) to investigate the nucleation and growth dynamics of MTPs in aqueous solution; two growth pathways were successfully identified: (A) nucleation-based layer-by-layer growth from a rounded multiply twinned seed and (B) the successive twinning and growth of tetrahedra. The LCTEM results enabled us to directly and conclusively identify the growth behaviors of intermediate products. The internal strain relaxation mechanisms and growth kinetics differ for the two pathways: in pathway A, a MTP grew by the opening and closing of re-entrant grooves at the twin boundaries, which was not found in pathway B. We also analyzed different MTP growth pathways from an energetic perspective and discussed how the preferred pathway (A or B) is related to factors, such as the initial seed yield and the size- and morphology-dependent formation of MTPs. Our results contextualize the current understanding of MTP formation mechanisms and provide insightful guidance for the precisely controlled synthesis of MTPs for practical applications.

Strain Dependence of Metal Anode Surface Properties

Dendrite growth poses a significant problem in the design of modern batteries as it can lead to capacity loss and short-circuiting. Recently, it has been proposed that self-diffusion barriers might be used as a descriptor for the occurrence of dendrite growth in batteries. As surface strain effects can modify dendritic growth, we present first-principles DFT calculations of the dependence of metal self-diffusion barriers on applied surface strain for a number of metals that are used as charge carriers in batteries. Overall, we find a rather small strain dependence of the barriers. We mainly attribute this to cancellation effects in the strain dependence of the initial and the transition states in diffusion.

Cooperative particle rearrangements facilitate the self-organized growth of colloidal crystal arrays on strain-relief patterns

Strain-relief pattern formation in heteroepitaxy is well understood for particles with long-range attraction and is a routinely exploited organizational principle for atoms and molecules. However, for particles with short-range attraction such as colloids and nanoparticles, which form brittle assemblies, the mechanism(s) of strain-relief is not known. Here, we found that for colloids with short-range attraction, monolayer films on substrates with square symmetry could accommodate large compressive misfit strains through locally dewetted hexagonally ordered stripes. Unexpectedly, over a window of compressive strains, cooperative particle rearrangements first resulted in a periodic strain-relief pattern, which then guided the growth of laterally ordered defect-free colloidal crystals. Particle-resolved imaging of monomer dynamics on strained substrates also helped uncover cooperative kinetic pathways for surface transport. These processes, which substantially influenced the film morphology, have remained unobserved in atomic heteroepitaxy studies hitherto. Leaning on our findings, we developed a heteroepitaxy approach for fabricating hierarchically ordered surface structures.

Influence of electric fields on metal self-diffusion barriers and its consequences on dendrite growth in batteries

Based on the results of periodic density functional theory calculations, we have recently proposed that the height of self-diffusion barriers can serve as a descriptor for dendrite growth in batteries [M. Jäckle et al., Energy Environ. Sci. 11, 3400 (2018)]. However, in the determination of the self-diffusion barriers, the electrochemical environment has not been taken into account. Still, due to the presence of electrical double layers at electrode/electrolyte interfaces, strong electric fields can be present close to the interfacial region. In a first step toward including the electrochemical environment, we have calculated barriers for terrace-diffusion on lithium, magnesium, and silver surfaces and across-step self-diffusion on lithium in the presence of electric fields. Whereas the electric field effect is more pronounced on a stepped surface than on flat terraces, overall we find a negligible influence of electric fields on self-diffusion barriers which we explain by the good screening properties of metals.

Atomic-scale diffusion rates during growth of thin metal films on weakly-interacting substrates

We use a combined experimental and theoretical approach to study the rates of surface diffusion processes that govern early stages of thin Ag and Cu film morphological evolution on weakly-interacting amorphous carbon substrates. Films are deposited by magnetron sputtering, at temperatures T_S between 298 and 413 K, and vapor arrival rates F in the range 0.08 to 5.38 monolayers/s. By employing in situ and real-time sheet-resistance and wafer-curvature measurements, we determine the nominal film thickness Θ at percolation (Θ_perc) and continuous film formation (Θ_cont) transition. Subsequently, we use the scaling behavior of Θ_perc and Θ_cont as a function of F and T_s, to estimate, experimentally, the temperature-dependent diffusivity on the substrate surface, from which we calculate Ag and Cu surface migration energy barriers \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{E}}}_{{\boldsymbol{D}}}^{{\bf{\exp }}}$$\end{document}EDexp and attempt frequencies \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{\nu }}}_{{\bf{0}}}^{{\bf{\exp }}}$$\end{document}ν0exp. By critically comparing \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{E}}}_{{\boldsymbol{D}}}^{{\bf{\exp }}}$$\end{document}EDexp and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{\nu }}}_{{\bf{0}}}^{{\bf{\exp }}}$$\end{document}ν0exp with literature data, as well as with results from our ab initio molecular dynamics simulations for single Ag and Cu adatom diffusion on graphite surfaces, we suggest that: (i) \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{E}}}_{{\boldsymbol{D}}}^{{\bf{\exp }}}$$\end{document}EDexp and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{\nu }}}_{{\bf{0}}}^{{\bf{\exp }}}$$\end{document}ν0exp correspond to diffusion of multiatomic clusters, rather than to diffusion of monomers; and (ii) the mean size of mobile clusters during Ag growth is larger compared to that of Cu. The overall results of this work pave the way for studying growth dynamics in a wide range of technologically-relevant weakly-interacting film/substrate systems—including metals on 2D materials and oxides—which are building blocks in next-generation nanoelectronic, optoelectronic, and catalytic devices.

Effect of Steel Substrates on the Formation and Deuterium Permeation Resistance of Aluminide Coatings

The effect of steel substrates on the formation and deuterium permeation resistance of aluminide coatings for tritium permeation barrier applications was investigated. It was found that the average thickness and crystal structure of electrodeposited Al coatings varied with Cr steel substrates. After aluminization in vacuum, significant differences existed in the composition and thickness of (Fe,Cr)2Al5-type aluminide layers formed on different Cr steel substrates, in which the alloying element Cr from steel substrates was involved. Thereafter, the scale microstructural feature and thickness also changed with the Cr steel substrate after selective oxidation in an Ar gas stream. Consequently, the deuterium permeation resistance of the formed aluminide tritium permeation barriers (TPBs) differed by steel substrates, indicating that the substrate effect existed in aluminide TPB coatings and thus could have a significant influence on the formation and eventual performance of aluminide TPBs.

Engineering Stable Surface Oxygen Vacancies on ZrO2 by Hydrogen-Etching Technology: An Efficient Support of Gold Catalysts for Water-Gas Shift Reaction

The surface structure of supports is crucial to fabricate efficient supported catalysts for water-gas shift (WGS). Here, hardly reducible ZrO₂ was etched with hydrogen (H), aiming to modify surface structures with sufficient stable oxygen vacancies. After deposition of gold species, the obtained khaki ZrO₂-H notably improved WGS catalytic activities and stabilities in comparison to the traditional white ZrO₂. The characterization results and quantitative analysis indicate that sufficient surface oxygen vacancies of ZrO₂-H support give rise to more metallic Au⁰ species and higher microstrain, which all boost WGS catalytic activities. Furthermore, optoelectronic properties were successfully used to correlate with their WGS thermocatalytic activities, and then a modified electron flow process was proposed to understand the WGS pathway. For one thing, the introduction of surface oxygen vacancies narrowed the band gap of ZrO₂ and decreased the Ohmic barrier, which facilitated the flow of "hot-electron". For another thing, the conduction band electrons can be easily trapped by oxygen vacancies of ZrO₂ supports, and then these trapped electrons immediately take part in reduction of H₂O to H₂. Thus, the electron recombination was suppressed and the WGS catalytic activity was improved. It is worth extending H₂-etching technology to improve other thermocatalytic reactions.

Ultrafast Heat Flow in Heterostructures of Au Nanoclusters on Thin Films: Atomic Disorder Induced by Hot Electrons

We study the ultrafast structural dynamics, in response to electronic excitations, in heterostructures composed of size-selected Au nanoclusters on thin-film substrates with the use of femtosecond electron diffraction. Various forms of atomic motion, such as thermal vibrations, thermal expansion, and lattice disordering, manifest as distinct and quantifiable reciprocal-space observables. In photoexcited supported nanoclusters, thermal equilibration proceeds through intrinsic heat flow between their electrons and their lattice and extrinsic heat flow between the nanoclusters and their substrate. For an in-depth understanding of this process, we have extended the two-temperature model to the case of 0D/2D heterostructures and used it to describe energy flow among the various subsystems, to quantify interfacial coupling constants and to elucidate the role of the optical and thermal substrate properties. When lattice heating of Au nanoclusters is dominated by intrinsic heat flow, a reversible disordering of atomic positions occurs, which is absent when heat is injected as hot substrate phonons. The present analysis indicates that hot electrons can distort the lattice of nanoclusters, even if the lattice temperature is below the equilibrium threshold for surface premelting. Based on simple considerations, the effect is interpreted as activation of surface diffusion due to modifications of the potential energy surface at high electronic temperatures. We discuss the implications of such a process in structural changes during surface chemical reactions.

Challenges in bimetallic multilayer structure formation: Pt growth on Cu monolayers on Ru(0001)

In a joint experimental and theoretical study, we investigated the formation and morphology of PtCu/Ru(0001) bimetallic surfaces grown at room and higher temperatures under UHV conditions. We obtained the PtCu/Ru(0001) surfaces by deposition of Pt atoms on a previously created Cu/Ru(0001) structure which includes only one Cu monolayer. Bimetallic surfaces prepared at different Pt coverages are investigated using STM imaging, revealing the existence of reconstruction lines and Cu islands. Although primarily created Cu islands continue growing in size by increasing Pt coverage, a continuous formation of new Cu islands is observed. This leads to an atypical exponential increase of the island density as well as to an atypical behavior of the average number of atoms per island for low Pt coverages. Although coalescence of the islands is observed for high Pt coverages, the island density remains almost constant in that regime. In order to understand the trends observed in the experiments, we study the stability of these surfaces, atom adsorption, and adatom diffusion using periodic density functional theory calculations. On the basis of the experimental observations and the first-principles calculations, we suggest a model that includes exchange of Pt adatoms with Cu surface atoms, Pt and Cu adatom diffusion, and attractive (repulsive) interactions between Cu (Pt) adatoms with substitutional Pt surface atoms, which explains the main trends in island formation and growth observed in the experiment.

Knowledge Extraction from Atomically Resolved Images

Tremendous strides in experimental capabilities of scanning transmission electron microscopy and scanning tunneling microscopy (STM) over the past 30 years made atomically resolved imaging routine. However, consistent integration and use of atomically resolved data with generative models is unavailable, so information on local thermodynamics and other microscopic driving forces encoded in the observed atomic configurations remains hidden. Here, we present a framework based on statistical distance minimization to consistently utilize the information available from atomic configurations obtained from an atomically resolved image and extract meaningful physical interaction parameters. We illustrate the applicability of the framework on an STM image of a FeSe_xTe_1-x superconductor, with the segregation of the chalcogen atoms investigated using a nonideal interacting solid solution model. This universal method makes full use of the microscopic degrees of freedom sampled in an atomically resolved image and can be extended via Bayesian inference toward unbiased model selection with uncertainty quantification.

Valley splitting in the transition-metal dichalcogenide monolayer via atom adsorption

In this letter we study the valley degeneracy splitting of the transition-metal dichalcogenide monolayer by first-principles calculations. The local magnetic moments are introduced into the system when the transition-metal atoms are adsorbed on the monolayer surface. The Zeeman effect arising from the local magnetic moment at transition-metal atom sites lifts the valley degeneracy. Anomalous charge, spin and valley Hall effects can be accessed due to valley splitting when we can only excite carriers of one valley. The valley splitting depends on the direction of magnetization and thus can be tuned continuously by an external magnetic field. This tunable valley splitting offers a practical avenue for exploring device paradigms based on the spin and valley degrees of freedom.

Limits of size scalability of diffusion and growth: Atoms versus molecules versus colloids

Understanding fundamental growth processes is key to the control of nonequilibrium structure formation for a wide range of materials on all length scales, from atomic to molecular and even colloidal systems. While atomic systems are relatively well studied, molecular and colloidal growth are currently moving more into the focus. This poses the question to what extent growth laws are size scalable between different material systems. We study this question by analyzing the potential energy landscape and performing kinetic Monte Carlo simulations for three representative systems. While submonolayer (island) growth is found to be essentially scalable, we find marked differences when moving into the third (vertical) dimension.

Organometallic Bonding in an Ullmann-Type On-Surface Chemical Reaction Studied by High-Resolution Atomic Force Microscopy

The on-surface Ullmann-type chemical reaction synthesizes polymers by linking carbons of adjacent molecules on solid surfaces. Although an organometallic compound is recently identified as the reaction intermediate, little is known about the detailed structure of the bonded organometallic species and its influence on the molecule and the reaction. Herein atomic force microscopy at low temperature is used to study the reaction with 3,9-diiododinaphtho[2,3-b:2',3'-d]thiophene (I-DNT-VW), which is polymerized on Ag(111) in vacuum. Thermally sublimated I-DNT-VW picks up a Ag surface atom, forming a CAg bond at one end after removing an iodine. The CAg bond is usually short-lived, and a CAgC organometallic bond immediately forms with an adjacent molecule. The existence of the bonded Ag atoms strongly affects the bending angle and adsorption height of the molecular unit. Density functional theory calculations reveal the bending mechanism, which reveals that charge from the terminus of the molecule is transferred via the Ag atom into the organometallic bond and strengths the local adsorption to the substrate. Such deformations vanish when the Ag atoms are removed by annealing and CC bonds are established.

Tailoring the Catalytic Properties of Metal Nanoparticles via Support Interactions

The development of new catalysts for energy technology and environmental remediation requires a thorough knowledge of how the physical and chemical properties of a catalyst affect its reactivity. For supported metal nanoparticles (NPs), such properties can include the particle size, shape, composition, and chemical state, but a critical parameter which must not be overlooked is the role of the NP support. Here, we highlight the key mechanisms behind support-induced enhancement in the catalytic properties of metal NPs. These include support-induced changes in the NP morphology, stability, electronic structure, and chemical state, as well as changes in the support due to the NPs. Utilizing the support-dependent phenomena described in this Perspective may allow significant breakthroughs in the design and tailoring of the catalytic activity and selectivity of metal nanoparticles.

Tailoring Kinetics on a Topological Insulator Surface by Defect-Induced Strain: Pb Mobility on Bi2Te3

Heteroepitaxial structures based on Bi2Te3-type topological insulators (TIs) exhibit exotic quantum phenomena. For optimal characterization of these phenomena, it is desirable to control the interface structure during film growth on such TIs. In this process, adatom mobility is a key factor. We demonstrate that Pb mobility on the Bi2Te3(111) surface can be modified by the engineering local strain, ε, which is induced around the point-like defects intrinsically forming in the Bi2Te3(111) thin film grown on a Si(111)-7 × 7 substrate. Scanning tunneling microscopy observations of Pb adatom and cluster distributions and first-principles density functional theory (DFT) analyses of the adsorption energy and diffusion barrier Ed of Pb adatom on Bi2Te3(111) surface show a significant influence of ε. Surprisingly, Ed reveals a cusp-like dependence on ε due to a bifurcation in the position of the stable adsorption site at the critical tensile strain εc ≈ 0.8%. This constitutes a very different strain-dependence of diffusivity from all previous studies focusing on conventional metal or semiconductor surfaces. Kinetic Monte Carlo simulations of Pb deposition, diffusion, and irreversible aggregation incorporating the DFT results reveal adatom and cluster distributions compatible with our experimental observations.

Strain-induced growth instability and nanoscale surface patterning in perovskite thin films

Despite extensive studies on the effects of epitaxial strain on the evolution of the lattice and properties of materials, considerably less work has explored the impact of strain on growth dynamics. In this work, we demonstrate a growth-mode transition from 2D-step flow to self-organized, nanoscale 3D-island formation in PbZr0.2Ti0.8O3/SrRuO3/SrTiO3 (001) heterostructures as the kinetics of the growth process respond to the evolution of strain. With increasing heterostructure thickness and misfit dislocation formation at the buried interface, a periodic, modulated strain field is generated that alters the adatom binding energy and, in turn, leads to a kinetic instability that drives a transition from 2D growth to ordered, 3D-island formation. The results suggest that the periodically varying binding energy can lead to inhomogeneous adsorption kinetics causing preferential growth at certain sites. This, in conjunction with the presence of an Ehrlich-Schwoebel barrier, gives rise to long-range, periodically-ordered arrays of so-called "wedding cake" 3D nanostructures which self-assemble along the [100] and [010].

Mechanical work makes important contributions to surface chemistry at steps

The effect of mechanical strain on the binding energy of adsorbates to late transition metals is believed to be entirely controlled by electronic factors, with tensile stress inducing stronger binding. Here we show, via computation, that mechanical strain of late transition metals can modify binding at stepped surfaces opposite to well-established trends on flat surfaces. The mechanism driving the trend is mechanical, arising from the relaxation of stored mechanical energy. The mechanical energy change can be larger than, and of opposite sign than, the energy changes due to electronic effects and leads to a violation of trends predicted by the widely accepted electronic ‘d-band’ model. This trend has a direct impact on catalytic activity, which is demonstrated here for methanation, where biaxial tension is predicted to shift the activity of nickel significantly, reaching the peak of the volcano plot and comparable to cobalt and ruthenium.

Surface strain affects the performance of catalysts. Here, the authors present computational evidence that mechanical strain of late transition metals can modify binding energies at stepped surfaces through a mechanical energy contribution yielding chemical trends unique from the established d-band model.

Enhanced atom mobility on the surface of a metastable film

A remarkable enhancement of atomic diffusion is highlighted by scanning tunneling microscopy performed on ultrathin metastable body-centered tetragonal Co films grown on Fe(001). The films follow a nearly perfect layer-by-layer growth mode with a saturation island density strongly dependent on the layer on which the nucleation occurs, indicating a lowering of the diffusion barrier. Density functional theory calculations reveal that this phenomenon is driven by the increasing capability of the film to accommodate large deformations as the thickness approaches the limit at which a structural transition occurs. These results disclose the possibility of tuning surface diffusion dynamics and controlling cluster nucleation and self-organization.

Entropy-driven crystal formation on highly strained substrates

In heteroepitaxy, lattice mismatch between the deposited material and the underlying surface strongly affects nucleation and growth processes. The effect of mismatch is well studied in atoms with growth kinetics typically dominated by bond formation with interaction lengths on the order of one lattice spacing. In contrast, less is understood about how mismatch affects crystallization of larger particles, such as globular proteins and nanoparticles, where interparticle interaction energies are often comparable to thermal fluctuations and are short ranged, extending only a fraction of the particle size. Here, using colloidal experiments and simulations, we find particles with short-range attractive interactions form crystals on isotropically strained lattices with spacings significantly larger than the interaction length scale. By measuring the free-energy cost of dimer formation on monolayers of increasing uniaxial strain, we show the underlying mismatched substrate mediates an entropy-driven attractive interaction extending well beyond the interaction length scale. Remarkably, because this interaction arises from thermal fluctuations, lowering temperature causes such substrate-mediated attractive crystals to dissolve. Such counterintuitive results underscore the crucial role of entropy in heteroepitaxy in this technologically important regime. Ultimately, this entropic component of lattice mismatched crystal growth could be used to develop unique methods for heterogeneous nucleation and growth of single crystals for applications ranging from protein crystallization to controlling the assembly of nanoparticles into ordered, functional superstructures. In particular, the construction of substrates with spatially modulated strain profiles would exploit this effect to direct self-assembly, whereby nucleation sites and resulting crystal morphology can be controlled directly through modifications of the substrate.

Quantitative measurement of the surface self-diffusion on Au nanoparticles by aberration-corrected transmission electron microscopy

We present a method that allows for a quantitative measurement of the surface self-diffusion on nanostructures, such as nanoparticles, at the atomic scale using aberration-corrected high-resolution transmission electron microscopy (HRTEM). The diffusion coefficient can be estimated by measuring the fluctuation of the atom column occupation at the surface of Au nanoparticles, which is directly observable in temporal sequences of HRTEM images. Both a Au icosahedron and a truncated Au octahedron are investigated, and their diffusion coefficients are found to be in the same order of magnitude, D = 10(-17) to 10(-16) cm(2)/s. It is to be assumed that the measured surface diffusion is affected by the imaging electron beam. This assumption is supported by the observed instability of a (5 × 1) surface reconstruction on a {100} Au facet.

Anomalous decay of electronically stabilized lead mesas on Ni(111)

With their low surface free energy, lead films tend to wet surfaces. However, quantum size effects (QSE) often lead to islands with distinct preferred heights. We study thin lead films on Ni(111) using low energy electron microscopy and selected area low energy electron diffraction. Indeed, the grown lead mesas show distinct evidence for QSE's. At about 526 K metastable mesas reshape into hemispheres within milliseconds, driven by a huge reduction in interfacial free energy. The underlying diffusion rate is many orders of magnitude faster than expected for lead on bulk lead.

Effects of tensile strain on Ag(111) epitaxial growth by kinetic Monte Carlo simulations

The effects of surface strain on epitaxial growth are studied using the kinetic Monte Carlo (KMC) method. The strain dependences of the activation energy barrier and the attempt frequency of each elementary process are evaluated by the embedded atom method interatomic potential. KMC simulations of homoepitaxial growth on a Ag(111) surface with equibiaxial tensile strain are carried out and influences of the surface strain on the nucleation of islands and the surface morphology are investigated. The island density increases due to reduction of the adatom diffusion on the terrace. The averaged coordination number of atoms constituting islands decreases and the island shape is more dendritic. The tensile surface strain leads to an increase in the surface roughness at an early stage of the growth, but at high coverage the roughness is adversely lower for the strained surface.

Unusual cluster shapes and directional bonding of an fcc metal: Pt/Pt(111)

Small clusters of Pt adatoms grown on Pt(111) exhibit a preference for the formation of linear chains, which cannot be explained by simple diffusion-limited aggregation. Density functional theory calculations show that short chains are energetically favorable to more compact configurations due to strong directional bonding by d(z)(2)-like orbitals, explaining the stability of the chains. The formation of the chains is governed by substrate distortions, leading to funneling towards the chain ends.

Atomic-scale geometry and electronic structure of catalytically important pd/au alloys

Pd/Au bimetallic alloys catalyze many important reactions ranging from the synthesis of vinyl acetate and hydrogen peroxide to the oxidation of carbon monoxide and trimerization of acetylene. It is known that the atomic-scale geometry of these alloys can dramatically affect both their reactivity and selectivity. However, there is a distinct lack of experimental characterization and quantification of ligand and ensemble effects in this system. Low-temperature, ultrahigh vacuum scanning tunneling microscopy is used to investigate the atomic-scale geometry of Pd/Au111 near-surface alloys and to spectroscopically probe their local electronic structure. The results reveal that the herringbone reconstruction of Au111 provides entry sites for the incorporation of Pd atoms in the Au surface and that the degree of mixing is dictated by the surface temperature. At catalytically relevant temperatures the distribution of low coverages of Pd in the alloy is random, except for a lack of nearest neighbor pairs in both the surface and subsurface sites. Scanning tunneling spectroscopy is used to examine the electronic structure of the individual Pd atoms in surface and subsurface sites. This work reveals that in both surface and subsurface locations, Pd atoms display a very similar electronic structure to the surrounding Au atoms. However, individual surface and subsurface Pd atoms are depleted of charge in a very narrow region at the band edge of the Au surface state. dI/dV images of the phenomena demonstrate the spatial extent of this electronic perturbation.

Energetics and atomic relaxations of Cu nanowires: the effect of local strain and cross-sectional area

We have calculated the activation energies for several single atom and vacancy diffusion processes on Cu nanowires with the axial orientation of [Formula: see text], using the nudged elastic band technique based on the interaction potential obtained from the embedded atom method. It is shown that the dimer-initiated local strain and its relief at the transition state have a significant effect on the characteristics of self-surface diffusion mechanisms on nanowires. Contrary to the case for cylindrical multishell-type Cu nanowires, the vacancy formation energy for rectangular nanowires is maximum in the core region and is nearly zero at the corner of the nanowire. In addition, the activation energy barriers for the vacancy diffusion processes taking place in the core region are found to be higher than those occurring near the corner of the nanowire. Our calculations further show that the vacancy diffusion processes taking place near the corner of the wire are dictated by the lower coordination of the surrounding atoms. From the structural investigation of nanowires, we have also established that multilayer relaxations for rectangular nanowires with smaller cross-sectional area cannot be defined.

Atom movement on a dislocated surface

The standard picture of growth at a screw dislocation assumes that the movement of adatoms on a dislocation loop is the same as on an ideal plane. We have examined this proposition by investigating the movement of a single tungsten adatom on a W(110) plane intersected by a screw dislocation. Surprisingly enough, adatom movement was entirely different than on a normal (110) plane: the overall diffusivity was higher, and the mobility varied with the location of the adatom relative to the dislocation core. This study demonstrates that surface transport is strongly affected in the vicinity of dislocations.

Fabrication of a well-ordered nanohole array stable at room temperature

We report on the fabrication of a new type of nanotemplate surface consisting of a hexagonally well-ordered array of one monolayer deep holes with a tunable size of about 4 nm (2) and a fixed spacing of 7 nm. The nanohole array fabrication is based on the strain-relief trigonal network formed in the 2 monolayer Ag on Pt(111) system. Removing about 0.1 ML of the Ag top layer of this surface structure, for example, by He- or Ar-ion sputtering, leads to the formation of nanoholes at specific domains of the trigonal network, which are stable at room temperature.

An atomic resolution scanning tunneling microscope that applies external tensile stress and strain in an ultrahigh vacuum

We have developed an ultrahigh vacuum scanning tunneling microscope with an in situ external stress application capability in order to determine the effects of stress and strain on surface atomistic structures. It is necessary to understand these effects because controlling them will be a key technology that will very likely be used in future nanometer-scale fabrication processes. We used our microscope to demonstrate atomic resolution imaging under external tensile stress and strain on the surfaces of wafers of Si(111) and Si(001). We also successfully observed domain redistribution induced by applying uniaxial stress at an elevated temperature on the surface of a wafer of vicinal Si(100). We confirmed that domains for which an applied tensile stress is directed along the dimer bond become less stable and shrink. This suggests that it may be feasible to fabricate single domain surfaces in a process that controls surface stress and strain.

Synchronous relaxation algorithm for parallel kinetic Monte Carlo simulations of thin film growth

We present an optimistic synchronous relaxation algorithm for parallel kinetic Monte Carlo (KMC) simulations of thin film growth. This algorithm is based on spatial decomposition of the KMC lattice and it employs two measures aimed at improving the parallel efficiency: dynamic global updating and domain boundary shifting. We utilize this algorithm to simulate two different growth models, which represent the growth of Ag on Ag(111) and the heteroepitaxial growth of Ag on Pt(111). We show that these simulations can achieve good efficiency-especially for large domain sizes with a moderate number of processors. We find that domain boundary shifting can improve efficiency-especially for simulations of growth in the AgPt(111) system, where the potential-energy surface topology creates areas of rapid, localized motion. We analyze the origins of parallel efficiency in these simulations.

Nature, strength, and consequences of indirect adsorbate interactions on metals

Atoms and molecules adsorbed on metals affect each other indirectly even over considerable distances. Via systematic density-functional calculations, we establish the nature and strength of such interactions, and explain for what adsorbate systems they critically affect important materials properties. This is verified in kinetic Monte Carlo simulations of epitaxial growth, which help rationalize a number of recent experimental reports on anomalously low diffusion prefactors.