Self-diffusion Of Adatoms Research Articles

Electrocrystallization Regulation Enabled Stacked Hexagonal Platelet Growth toward Highly Reversible Zinc Anodes.

Realizing durative flattened and dendrite-free zinc (Zn) metal configuration is the key to resolving premature battery failure caused by the internal short circuit, which is highly determined by the crystal growth in the electrocrystallization process. Herein, we report that regulating the molecular structure of the inner Helmholtz plane (HIP) can effectively convert the deposition into activation control by weakening the solvated ion adsorption at the interface. The moderated electrochemical reaction kinetics lower than the adatom self-diffusion rate steers conformal stratiform Zn growth and dominant Zn (0001) texture, achieving crystallographic optimization. Through in situ mediation of electrolyte engineering, orientational plating and stripping behaviors at edge-sites and tailored solvation structure immensely improve the utilization efficiency and total charge passed of Zn metal, even under extreme conditions, including high areal capacity (3 mAh cm-2 ) and wide temperature range (-40-60 °C).

Self-Diffusion of Surface Defects at Copper–Water Interfaces

Solid–liquid interfaces play an important role in many fields like electrochemistry, corrosion, and heterogeneous catalysis. For understanding the related processes, detailed insights into the elementary steps at the atomic level are mandatory. Here we unravel the properties of prototypical surface-defects like adatoms and vacancies at a number of copper–water interfaces including the low-index Cu(111), Cu(100), and Cu(110), as well as the stepped Cu(211) and Cu(311) surfaces. Using a first-principles quality neural network potential constructed from density functional theory reference data in combination with molecular dynamics and metadynamics simulations, we investigate the defect diffusion mechanisms and the associated free energy barriers. Further, the solvent structure and the mobility of the interfacial water molecules close to the defects are analyzed and compared to the defect-free surfaces. We find that, like at the copper–vacuum interface, hopping mechanisms are preferred compared to exchange mechanisms, while the associated barriers for hopping are reduced in the presence of liquid water. The water structure close to adatoms and vacancies exhibits pronounced local features and differs strongly from the structure at the ideal low-index surfaces. Moreover, in particular at Cu(111) the adatoms are very mobile and hopping events along the surface are more frequent than the exchange of coordinating water molecules in their local environment. Consequently, adatom self-diffusion processes at Cu(111) involve entities of adatoms and their associated solvation shells.

Long jumps contribution to the adatom diffusion process near the step edge: The case of Ag/Cu(110)

In this work, the diffusion of a single Ag adatom on a low-index Cu surface (110) in the presence of a step edge is studied using the embedded-atom method (EAM). Molecular static simulation is carried out in order to calculate the activation energy of different diffusion processes. Our findings are in a good agreement with results existing in the literature indicating that adatom diffusion via jump process is more favored than the other mechanisms. The activation energy corresponding to diffusing via hopping is found to be 0.25 eV (at 0 K). On the other hand, the activation barrier calculated by molecular dynamics (MD) simulation for a large range of temperature (310–500 K) is found to be around 0.25 eV for both upper and lower position leading to a good agreement between static and dynamic calculations. The prefactor for Ag adatom self-diffusion via hopping on Cu(110) surface near the step edge is examined. The results show that the prefactors are 2.7 and 3.6 × 104 cm2 s−1 for the upper and lower position, respectively. This is in line with the value of 10−3 cm2 s−1 that is generally adopted. We also found that long jumps occur frequently in this system and their contribution cannot be neglected.

Direct Observation of the Transition Point Between Quasi‐Spherical and Cubic Nanoparticles in a Two‐Step Seed‐Mediated Growth Method

An aqueous seed-mediated growth method is adapted to explore the shape transformation of quasi-spherical Au seeds to nanocubes in a direct and continuous manner. Quenching the growth process at varied reaction-duration times and exploring the intermediate products by high-resolution transmission electron microscopy and UV/vis spectroscopy shows an abrupt cuboctahedral-to-nanocube transition at 25-27 nm without any change in the nanoparticle size. The size of the obtained nanocubes remains constant (25-27 nm) until most (>90%) of the cuboctahedral nanoparticles are transformed to nanocubes. At this point, the (25-27 nm) nanocubes initiate further continuous and homogeneous growth until they reach 50-nm Au cubes. These observations are ascribed to a scenario in which the kinetically controlled growth mode of the nanoparticle is significantly affected by the surface self-diffusion of metal adatoms, especially when the adatom's self-diffusion distance is comparable with the nanoparticle's size.

Origin of quasi-constant pre-exponential factors for adatom diffusion on Cu and Ag surfaces

Many-body interaction potentials from the embedded atom method with two functionals and electronic structure calculations based on density functional theory and the plane-wave pseudopotential method are used to calculate the pre-exponential factors for self-diffusion of adatoms via hopping on Cu(100) and Ag(100) surfaces with and without steps. The pre-exponential factors are found to be in the range of ${10}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}∕\mathrm{s}$ for all investigated processes regardless of whether substrate vibrational dynamics are included or omitted. When substrate dynamics are ignored, compensation effects between stiffening and softening of the vibrational frequencies of the diffusing atom are responsible for this quasi-constant pre-exponential. When these dynamics are included, subtle cancellations in the vibrational free energy make the local contribution of the diffusing atom the dominant one.

Upward self-diffusion of adatoms and small clusters on facets of fcc metal (110) surfaces

We study upward self-diffusion of adatoms and small clusters on faceted huts formed in fcc metal (110) homoepitaxy using molecular dynamics simulations with interatomic potentials described by the embedded-atom method. Our results show that both individual adatoms and small clusters on the (111)-sloped and (100)-sloped facets of the huts on Al and Cu(110) can readily diffuse upward and cross the convex edges of the huts, but with different atomistic mechanisms. An adatom crosses the convex edges via a simple concerted exchange or indirect place exchange mechanism. In contrast, the upward diffusion and convex-edge crossing of small clusters is realized by their dissociation at or near the edges of the hut after one or two atoms belonging to the cluster are incorporated into the edges. The present simulation studies reveal that there truly exists efficient upward mass transport on faceted islands in fcc metal (110) homoepitaxy, and such upward diffusion processes are expected to play an important role in defining the kinetic faceting instability in these growth systems.

Fast isotropic adatom diffusion on Ge(105) dot facets

We investigate adatom self-diffusion on the rebonded step (RS) reconstructed Ge(105) surface. Activation energies for several paths are computed ab initio by applying the nudged elastic band method and used as input parameters for kinetic Monte Carlo simulations. We show that the RS reconstruction strongly influences adatom kinetics, which turns out to be fast at typical experimental temperatures, along both [010] and f5 01g directions. The influence of strain on diffusion is also investigated. Our results are shown to be relevant for a better understanding of the growth modality of Ge pyramids on Si(001).

Self-Diffusion Mechanisms of Adatom on Al (001), (011) and(111) Surfaces

Using the first-principle molecular dynamical calculations, wehave studied the adatom self-diffusion mechanisms on fcc Al (001),(011) and (111) surfaces. On each surface, there are severalmechanisms, among which there is one favour mechanism with the minimumbarrier energy. The atomic exchange mechanism along the [100] directionon the (001) surface, the long bridge hopping mechanism along the[1̄10] direction on the (011) surface, and the bridge hopping mechanismalong the [112̄] direction on the (111) surface are the favourmechanisms. The activation energy profiles for various self-diffusionmechanisms are studied in details.

Effect of adsorbate interactions on adatom self-diffusion on Cu(111) and Ni(111) surfaces

The effect of adsorbate interactions on self-diffusion on Cu(111) and Ni(111) surfaces is examined by calculating activation barriers and prefactors for diffusion of an adatom in the presence of a second adatom. Additionally, molecular dynamics simulations are performed. The calculations for both systems reveal that the main impact on the mobility of an adatom caused by the presence of a second adatom comes from modifications of the activation barriers. Prefactors are essentially the same as for single atom diffusion beyond adatom separations of next-nearest-neighbor sites.

Self-diffusion on fcc (100) metal surfaces: Comparison of different approximations

Based on our ab initio calculations, we discuss the basic adatom self-diffusion mechanisms on Al(100) and Cu(100) surfaces. In the case of Al(100) we show that the local density approximation and the generalized gradient approximation yield similar results, provided that the lattice constant is the same. We analyze these results in terms of the diffusion barriers as well as the electronic structures. We also show that the diffusion barrier for the exchange process is significantly lowered if one takes the atomic relaxations fully into account by using the nudged elastic band algorithm for the search for the transition state.

Adatom self-diffusion on Pt (100) surface by an ad-dimer migrating

We study the self-diffusion of adatom on Pt(100) surface by molecular dynamics. The metal is modeled by the surface embedded-atom potential. Besides the conventional concerted exchange mechanism, a new exchange mechanism in which an ad-dimer diffuses by a concerted motion of three atoms is observed. The importance is that it could become a dominant diffusion mechanism in some cases.

Jumps and concerted moves in Cu, Ag, and Au(110) adatom self-diffusion

We present a molecular-dynamics simulation of self-diffusion on the ~110! surfaces of Cu, Ag, and Au. The metals are modeled by semiempirical potentials developed in the framework of the second-moment approximation to the tight-binding model. The energy barriers for the relevant diffusion processes are calculated by quenched molecular dynamics and compared with the available data in literature, obtaining a good agreement. The occurrence of long jumps is investigated in detail, showing that the three metals behave quite differently with this respect: long jumps are practically absent in Au and frequent in Cu. The effect of the specific features of the potential-energy surface and of the energy dissipation to the substrate on the probability of long jumps is investigated. The Arrhenius behavior of the jump rate is discussed, and deviations are found at high temperatures. Concerning correlated jump-exchange processes and double exchanges, we find that they are common in Cu even at rather low temperatures, whereas they are never observed in Au, Ag showing an intermediate behavior. @S0163-1829~99!01808-1#

Exchange mechanism for adatom diffusion on metal fcc(100) surfaces

We study the self-diffusion of adatoms on metal fcc(100) surfaces by molecular dynamics with Pt and Ag as examples. The metals are modeled by surface embedded-atom potentials. Besides the well-known exchange diffusion mechanism, another one that appears as the diffusion of an ad-dimer is also observed, and it can be explained by the ``concerted motion'' model owing to the strong bonds formed in the transition state. The important point is that this mechanism could appear frequently even at room temperature and makes adatoms more movable on fcc(100) surfaces.

Dynamics of Adatom Self-Diffusion and Island Morphology Evolution at a Cu(100) Surface

Molecular dynamics simulation of adatom self-diffusion and island morphology evolution at a Cu(100) surface is performed with the aim of a better understanding of the dynamical process. The simulation is at first carried out for a surface without adatoms or islands, and the vibrational spectra of the surface layers at different temperatures are calculated. We then add adatoms and islands to the surface, and perform simulations for systems with different surface structures. Based on the simulation results for a single adatom, we discuss a resonance mechanism possibly by which the adatom diffusion is activated by the surface vibration. The simulation results of an ad-dimer show a dancing motion of the dimer when it is about to dissociate. Finally we study the morphology of an island at finite temperatures. Our simulation results suggest that the exchange diffusion mechanism plays an important role on reshaping the island.

SELF-DIFFUSION OF ADATOMS ON Ag,Pt,Au(110) SURFACES

Self-diffusion of adatoms on Ag,Pt,Au(110) surfaces is investigated with the surface embed-ded atom method(SEAM) and molecular dynamics.By quenched molecular dynamics,the static energy variation and the pathway of the atoms for hopping and exchange mechanisms are obtained.The self-diffusion characteristics of these metals are analyzed,and the results show that the diffusion is isotropic on Pt(110) surface but anisotropic on Au,which are in accordance with the observation in experiments.

Self-diffusion of adatoms, dimers, and vacancies on Cu(100)

We use ab initio static relaxation methods and semi-empirical molecular-dynamics simulations to investigate the energetics and dynamics of the diffusion of adatoms, dimers, and vacancies on Cu(100). It is found that the dynamical energy barriers for diffusion are well approximated by the static, 0 K barriers and that prefactors do not depend sensitively on the species undergoing diffusion. The ab initio barriers are observed to be significantly lower when calculated within the generalized-gradient approximation (GGA) rather than in the local-density approximation (LDA). Our calculations predict that surface diffusion should proceed primarily via the diffusion of vacancies. Adatoms are found to migrate most easily via a jump mechanism. This is the case, also, of dimers, even though the corresponding barrier is slightly larger than it is for adatoms. We observe, further, that dimers diffuse more readily than they can dissociate. Our results are discussed in the context of recent submonolayer growth experiments of Cu(100).

Molecular dynamics study of the vibrational and transport properties of copper adatoms on the (111) copper surface; comparison with the (001) face

The vibrational properties and the self-diffusion process of single adatoms on the Cu(111) surface have been studied and compared with the corresponding ones of the Cu(001) face, using molecular dynamics simulation based on a n-body potential. From the calculated phonon spectral densities it comes out that the adatoms modify the surface phonon spectrum and introduce new vibrational modes. The frequencies of these new suggest, in the in-plane directions, looser coupling of the adatoms with the surface atoms of the (111) than those of the (001) face and stronger coupling in the perpendicular direction. This is in agreement with the temperature dependence of the relaxed positions of both surface atoms and adatoms. The effect is more pronounced for the (001) face, indicating that this surface presents anharmonic behavior at high temperatures. In addition, the mean-square-displacements of the surface atoms in the normal direction, are greater in the case of the (001) face. The vibrational amplitudes of the adatoms on the (111) surface are smaller than those of the surface atoms, for a wide temperature range. The adatom self-diffusion on the (111) surface is a thermally activated process, exhibiting Arrhenius behavior at two distinct temperature regions. The detailed analysis of the diffusion process revealed a new hopping mechanism.

Adatom self-diffusion processes on (001) copper surface by molecular dynamics

Abstract Using molecular dynamics and an n -body potential adapted to copper we have studied the self-diffusion of Cu adatoms on Cu(001) surface. The simulations covered the temperature range between 700 and 1100 K. Besides the simple hopping and the exchange mechanism, the detailed trajectory analysis revealed multiple hopping events and complicated multi-particle exchange processes, involving several atoms that do not necessarily belong to the same nearest-neighbor row. These processes exhibit Arrhenius behavior from which we derived the migration energies associated with each process. It is found that the hopping mechanism requires an energy of 0.43 eV, in very good agreement with available experimental data, while the energy associated with the exchange mechanism is 0.70 eV. These results are in qualitative agreement with recent ab-initio calculations. In addition, we found that all mechanisms, even the most complicated, require about the same migration energy with the simple exchange and that for temperatures above 900 K they contribute almost equally to the total diffusion. Furthermore, the activation barriers for the hopping and the exchange mechanism deduced from energy minimizations, at T = 0 K, compare well with the simulation values.

ATOMIC LEVEL STUDIES OF THE DYNAMICS AND DYNAMICAL BEHAVIOR OF SOLID SURFACES

Some of our recent results on the dynamics and dynamical behavior of atoms at metal surfaces are presented. Long range charge density oscillations due to chemisorbed impurities have been observed with the FIM. Two adatoms can interact with each other through mutual perturbation of the surface. This interaction is very weak, and exhibits a small but long range oscillatory tail. Surface self diffusion of adatoms can occur at about 1/10 to 1/5 the melting point of metals. This diffusion occurs by either atomic hopping or by atomic replacement-displacement. Diffusion of ledge atoms along a lattice step can occur at a similar temperature. At about 1/5 the melting point of a metal, dissociation of step edge atoms to terraces can occur and lattice steps can also start to roughen. Energetics of the atomic processes involved can be studied from their temperature dependences of their rates of occurrence and also from the densities of surface defects created. Recent progress in theoretical calculations makes it possible to compare them directly with experimental result, thus gaining insights into the dynamics in the dynamical behavior of metal surfaces.

Construction and evaluation of embedding functions

Non-self-consistent density functional theories require specification of the embedding energy for an atom in a reference system. We combine the embedding energies determined from linear muffin tin orbital (LMTO) calculations of the bulk cohesive energy curves with those determined from the experimental diatomic binding curve. These new embedding functions contain information about the variation of binding with both coordination and separation between atomic centers. These are shown to be superior to embedding functions determined solely from bulk cohesive energy curves through tests on structures and energies of small metal clusters, self-diffusion of adatoms on metal surfaces, and scattering of metal atoms from metal surfaces.