Adatom-substrate Interaction Research Articles

Domain Matching Strategy for Orientation Control of van der Waals Epitaxial Perovskite Thin Films.

The advantages of van der Waals epitaxy have attracted great interest because they can meet the requirements that conventional epitaxy struggles to satisfy. The weak adatom-substrate interaction without directional covalent bonding drastically relaxes the lattice matching limitation. However, the weak adatom-substrate interaction also leads to ineffectiveness in directing the crystal growth structure, limiting it to one orientation in epitaxial growth. In this work, we propose a domain matching strategy to guide the perovskite-type crystal epitaxial growth on 2D substrates, and we have demonstrated selective deposition of highly (001)-, (110)-, and (111)-oriented epitaxial Fe4N thin films on mica substrates using applicable transition structure design. Our work makes it possible to achieve and control different orientations of van der Waals epitaxy on the same substrate.

Phase transition of nanoscale Au atom chains on NiAl(110)

Recently, 1D Au single atom chains (SAC) were artificially constructed on the NiAl(110) surface in experiment, which has stimulated various experimental and theoretical investigations on its potential applications such as in microelectronics and catalysis. Here, we reexamine their relative stabilities and formation mechanism on NiAl(110) using first-principles calculations. We reveal that in the range of N=2∼6, the formation of SAC is energetically more stable than that of 1D Au bi-atom chains (BAC), mainly due to the electronic quantum confinement effects. However, beyond N=6, BAC are found to be energetically favored over SAC due to the energy gain of Au-Au bonding. The present findings reveal the importance of the competition between electronic quantum confinement, the adatom-adatom interactions, and adatom-substrate interactions in formation of the desirable 1D nanostructures.

Electronics of Ba adsorbed on Ge(001)

Electronic structures of experimentally verified on-top and incorporated configurations for the Ba/Ge(001) adsorption system is discussed. It is shown that the BaGe interaction in both phases is predominantly ionic in nature. It is also demonstrated that this adatom-substrate interaction governs the formation of the extended on-top and the incorporated phases. In the latter the Ba adatoms interact with the second layer Ge atoms resulting in the formation of the unoccupied dangling bonds on the remaining surface Ge dimers.

Substrate-determined exchange interactions between an STM tip and an adatom

By performing ab initio calculations, we reveal an unexpected behavior of the exchange interactions between the Fe STM tip and the Fe adatom on Cu(001) and on a ${\mathrm{Cu}}_{2}\mathrm{N}$ monolayer on Cu(001) surfaces [denoted as ${\mathrm{Cu}}_{2}\text{N/Cu}(001)$]. A tip-adatom distance-dependent antiferromagnetic-ferromagnetic transition and antiferromagnetic exchange interactions between the tip and the adatom are found for these two junctions, respectively. We demonstrate that the different exchange interactions in these systems are determined by the competition between the tip-adatom and the adatom-substrate interactions. Based on transport calculations, we found that the spin polarization and magnetoresistance in the junction on the Cu(001) system and on the ${\mathrm{Cu}}_{2}\text{N/Cu}(001)$ system depend on the tip-adatom distance.

Adatom Interactions on GaN(0001) Surface I: Coverage-Dependent Adsorption

The adsorption of an adatom on the surface of a crystal is affected by the surface coverage due to adatom–adatom and adatom–substrate interactions. We study the dependence of adsorption energy of Ga and N adatoms on wurtzite GaN(0001) as a function of decreasing surface coverage from 0.25 to 0.04 monolayers through first-principles calculations. The adsorption energies of Ga and N adatoms on the flat, clean GaN(0001) substrate do not converge with decreasing coverage. Further, it appears that the Ga and N adatoms significantly distort the substrate lattice from the flat configuration. We found that these distortions increase with increase in system size (or, equivalently, with reducing coverage). This observation is counterintuitive since it is expected that lattice distortions should decrease with decreasing surface coverage. We separate the different contributions to the adsorption energy and identify the part of adsorption energy that arises due to lattice distortions. This contribution appears to be m...

Versatile van der Waals epitaxy-like growth of crystal films using two-dimensional nanosheets as a seed layer: orientation tuning of SrTiO3 films along three important axes on glass substrates

One of the basic requirements for attaining a good epitaxy is a close structural matching between a substrate and a growing crystal epilayer. This restrictive requirement causes a major obstacle for its wide application to a range of functional crystal films in electronic, magnetic or optical devices. One approach for overcoming this problem is the so-called van der Waals epitaxy (VDWE) method, which can effectively implement the epitaxy of various crystals on cleaved faces of layered materials having no dangling bonds. The weak adatom–substrate interaction without directional covalent bonding plays a crucial role in the initial stage of VDWE, which drastically relaxes the lattice matching limitation. However, the method requires special materials for use as a substrate, thereby meaning that its applicability is limited. In this study, the concept is extended to the two-dimensional (2D) lattice of inorganic nanosheets, which are molecularly thin 2D crystals produced via artificial exfoliation of layered metal oxides. The nanosheets can neatly cover the surface of conventional substrates such as glass via a facile solution-based process. Similar to the above-mentioned cleaved faces of layered materials, such substrates can promote VDWE-like crystal growth because of their dangling bond-free nature. Based on this principle, we have demonstrated a selective deposition of highly textured (100), (110) and (111) SrTiO3 films, a fundamentally important archetype of functional crystals, on glass substrates covered with single-layer nanosheets with suitable 2D periodicities as a trigger for VDWE-like film growth. The rich varieties of nanosheet structures and their facile deposition onto almost any kinds of substrates provide a significant advantage, expanding potential applications for a range of devices based on functional crystal films.

First-principles theoretical investigation of monoatomic and dimer Mn adsorption on noble metal (111) surfaces

A theoretical investigation of the adsorption of Mn single atoms and dimers on the (111) surface of Cu, Ag, and Au, within the framework of the density functional theory, is presented. First, the bulk and the clean (111) surface electronic structures are calculated, with results that agree well with previous reports. To understand the adatom-substrate interaction, also the electronic characteristics of the free Mn dimer are determined. Then, the electronic structure of the Mn adatom, chemisorbed on four different surface geometries, is analyzed for the three noble metals. It is found that the most stable geometry, in all three cases, Cu, Ag, and Au, occurs when the Mn atom is chemisorbed on threefold coordinated sites. For the dimer, the lowest-energy configuration corresponds to the molecule lying parallel to the surface. In the three noble metals, the geometry corresponds to both atoms chemisorbed in threefold coordinated sites, but with different local symmetry. It is also found that the magnetic configuration with the lowest energy corresponds to the antiferromagnetic arrangement of Mn atoms, with individual magnetic moments close to 5${\ensuremath{\mu}}_{B}$. The ferromagnetic and antiferromagnetic solutions, in the case of a Ag substrate, are close in energy. It is also found that in this case the Mn${}_{2}$ molecule is chemisorbed with very similar energy on various geometries. To study the dynamical motion of the dimer components, we calculated the potential energy barriers for the Mn motion in the various surfaces. In contrast to Cu and Au, this leads to the conclusion that on Ag the Mn dimer moves relatively freely.

Strong metal adatom–substrate interaction of Gd and Fe with graphene

Graphene is a unique 2D system of confined electrons with an unusual electronic structureof two inverted Dirac cones touching at a single point, with high electron mobilityand promising microelectronics applications. The clean system has been studiedextensively, but metal adsorption studies in controlled experiments have beenlimited; such experiments are important to grow uniform metallic films, metalcontacts, carrier doping, etc. Two non-free-electron-like metals (rare earth Gd andtransition metal Fe) were grown epitaxially on graphene as a function of temperatureT andcoverage θ. By measuring the nucleated island density and its variation with growth conditions, informationabout the metal–graphene interaction (terrace diffusion, detachment energy) is extracted. Thenucleated island densities at room temperature (RT) are stable and do not coarsen, at least up to400 °C, which shows an unusually strong metal–graphene bond; mostlikely it is a result of C atom rebonding from the pure graphenesp2 C–C configuration to one of lower energy.

Structures and stability of Ag clusters on Ag(1 1 1) and Ni(1 1 1) surfaces

The structures of the lowest total energy for small Ag N clusters with N = 2–20, which are grown on Ag(1 1 1) and Ni(1 1 1) surfaces, have been determined using a combination of the embedded-atom method and the basin-hopping algorithm. It is found that the particularly stable Ag clusters with N < 18 have similar geometries on both surfaces when comparing clusters of the same size. On the other hand, the geometries of the less stable Ag clusters in the same size range differ for the two surfaces. From N ⩾ 18 , the sizes of the particularly stable structures are different for the two different substrates. Due to the large size mismatch of the two types of atoms it is energetically unfavorable for Ag to form a pseudomorphic monolayer structures on Ni(1 1 1) and there is considerable strain produced at the interface. The effect of this strain and the increased adatom–substrate interactions lead to irregular and elongated structures of the adsorbed Ag clusters.

Temperature-dependent activation energy for silicon desorption processes

A theoretical analysis is presented of the temperature dependence of the activation energy for thermal desorption of silicon from textured tantalum tape. The curves derived from experimental data for the initial stages of desorption are in qualitative agreement with calculation results, which makes it possible to estimate lateral interaction forces and the adatom-substrate interaction energy as a function of surface coverage and to predict structural changes in the film.

Nature of adsorption on TiC(111) investigated with density-functional calculations

Extensive density-functional calculations are performed for chemisorption of atoms in the three first periods (H, B, C, N, O, F, Al, Si, P, S, and Cl) on the polar TiC(111) surface. Calculations are also performed for O on TiC(001), for full O(1x1) monolayer on TiC(111), as well as for bulk TiC and for the clean TiC(111) and (001) surfaces. Detailed results concerning atomic structures, energetics, and electronic structures are presented. For the bulk and the clean surfaces, previous results are confirmed. In addition, detailed results are given on the presence of C-C bonds in the bulk and at the surface, as well as on the presence of a Ti-based surface resonance (TiSR) at the Fermi level and of C-based surface resonances (CSR's) in the lower part of the surface upper valence band. For the adsorption, adsorption energies E-ads and relaxed geometries are presented, showing great variations characterized by pyramid-shaped E-ads trends within each period. An extraordinarily strong chemisorption is found for the O atom, 8.8 eV/adatom. On the basis of the calculated electronic structures, a concerted-coupling model for the chemisorption is proposed, in which two different types of adatom-substrate interactions work together to provide the obtained strong chemisorption: (i) adatom-TiSR and (ii) adatom-CSR's. This model is used to successfully describe the essential features of the calculated E-ads trends. The fundamental nature of this model, based on the Newns-Anderson model, should make it apt for general application to transition-metal carbides and nitrides and for predictive purposes in technological applications, such as cutting-tool multilayer coatings and MAX phases.

The dynamics of H2 and N2 sorption in carbon nanotubes

Single-walled and multi-walled carbon nanotubes (SWNT and MWNT) were prepared by catalytic decomposition of methane and acetylene, respectively. The resulting catalyst/carbon composites were subjected to chemical treatments to obtain samples which were enriched in nanotubes. Isotherms and frequency response (FR) spectra of N2 and H2 were recorded at 195 and 77K, respectively, in the 1–800Pa pressure range. The adsorption isotherms for N2 were found to obey Henry’s law, while the isotherms of H2 adsorption indicated some energetic heterogeneity of the sorption sites. Stronger adatom–substrate interaction was detected for those samples which had been subjected to oxidative treatment. The FR measurements showed that treatment of MWNT in KMnO4 solution changed the rate-controlling mechanism of mass transport from one of sorption to one of diffusion. Results substantiated that the surface functional groups, generated by the oxidative treatment, have significant influence on both the equilibrium and the dynamic N2 and H2 sorption properties of the carbon nanotubes.

Structures and magic numbers of adatom clusters on metal fcc (001) surfaces

With a genetic algorithm, the lowest-energy structures of adatom clusters on a series of metal fcc (001) surfaces are determined. The atomic interactions are modeled by the realistic model potentials including embedded-atom method potential, surface-embedded-atom method potential, and Rosato-Guillop\'e-Legrand potential. The results show that the adatom clusters of sizes $n=6,9,\dots{},36$ have the same structures on the different surfaces. Their special stability indicates that they are magic number clusters. For clusters of other sizes, the structures are generally different on the different surfaces. The change of the cluster structure with surfaces can be interpreted in terms of the relative interaction range and the compensation effect from the adatom-substrate interaction. When the interaction range becomes long and/or the compensation effect becomes strong, the shape of the lowest-energy structure tends to change from square to rectangle or even to one-dimensional chain.

Synthesis and patterning of nanostructures of (almost) anything on anything

A necessary and very challenging condition for progress in nanoscience is being able to synthesize the requisite material. While there has been substantial success for some materials, one would like to develop protocols to fabricate structures of a wide variety of materials on an equally wide range of supports, preferably with size selection and the ability to pattern on large areas at low cost. The paper by Kerner and Asscher in this issue [1] is significant because it shows how at least some of those conditions can be satisfied in a novel and creative way. Building on the techniques of surface science, many researchers have focused on physical vapor deposition and the subsequent assembly of 1D wires, 2D islands, and 3D clusters. The problem, of course, is that Nature provides rather stringent rules that prevent the spontaneous assembly of 3D structures for all but a few materials. In particular, 3D growth will occur only if the adatoms interact only weakly with the surface but strongly with one another, with details dictated by surface and interface energies. A technique was developed a few years ago that by-passed these rules, termed buffer-layer-assistedgrowth (BLAG) [2–4], and the paper by Kerner and Asscher combines this technique with laser induced thermal desorption to form patterns in a novel and potentially important way. The idea behind buffer layer assisted growth is remarkably simple, as depicted in Fig. 1. The buffer layer replaces adatom–substrate interactions with very weak adatom–buffer interactions so that balling-up occurs out of contact with the substrate. The buffer must subsequently be removed so that the clusters are delivered in the ultimate of soft landings to the substrate where wetting is frustrated by kinetics. While there

Laminar growth of ultrathin metal films on metal oxides: Co on hydroxylated alpha-Al2O3(0001).

Metals deposited in vacuum on metal oxides such as alumina normally grow as three-dimensional clusters because of weak adatom-substrate interactions. This tendency hinders our ability to form interfaces of ultrathin, laminar metal films on oxides for use in microelectronics and other technologies where nanostructural control is desired. We present experimental and theoretical results showing that room temperature Co deposition on fully hydroxylated clean sapphire (alpha-Al2O3) produces a surface chemical reaction that leads to laminar growth, despite a large mismatch in lattice constants. This process should be applicable to a wide range of metals and metal oxides.

Structure of clusters on embedded-atom-method metal fcc (111) surfaces

With a genetic algorithm, the structure of adatom clusters is optimized and studied systematically on different metal fcc (111) surfaces modeled by the embedded atom method. It is found that the various lowest-energy structures on the different surfaces can be explained in terms of the relative interaction range and the adatom-substrate interaction. The longer the relative interaction range becomes, the sharper or more extended the shape of the lowest-energy structure tends to be, in which the number of next-nearest-neighbor bonds decreases. When the compensation effect from the adatom-substrate interaction becomes strong, the configuration with the smaller number of nearest-neighbor bonds could be the lowest-energy structure; it also induces the shape of the lowest-energy structure as being sharp or extended. In this case, another interesting phenomenon is that the plane of the cluster is obviously bent.

The Growth of Transition Metals on H-Passivated Si(111) Substrates

The growth of Co and Ag layers on wet-processed H-passivated Si(111) substrates by molecular beam epitaxy (MBE) has been studied using high resolution scanning tunneling microscopy (STM) with regard to possible applications of the layers in magnetoelectronic devices. Roughness and intermixing at interfaces as functions of deposition temperature and layer thickness are key parameters for the performance of such devices. The initial growth of Co and Ag and the influence of Ag atoms on the Si(111) surface reconstructions provide insight into adatom–substrate interactions.

On the Character of Atomic Adlayers Physiadsorbed on an Incommensurate Substrate

We examine the behavior of incommensurate atomic adlayers deposited on a heterogeneous substrate using static and Monte-Carlo techniques. In this case the use of periodic boundary conditions to simulate an infinite system, from a finite sample, is complicated because two different symmetries must be satisfied. This has been solved and results are presented. Results for Ar on graphite show that the periodic modulation of the adatom-substrate interaction is responsible for significant changes in the behavior of the system.

Diffusion of Na atoms on a Cu(001) surface

We study the equilibrium properties and the diffusive motion of Na atoms adsorbed on a Cu(001) surface at finite coverages via a model Hamiltonian. Our model Hamiltonian starts with an adatom-substrate interaction potential based on the previous analysis of the zerocoverage He scattering data. The dipole-dipole interaction between the alkali adatoms at finite coverages is then included. We use Monte Carlo simulations to study the ordered structure of the system at low temperatures and molecular-dynamics simulations for evaluating the dynamical correlation functions relevant for both the tracer and collective diffusive constants.

Computer Simulation of Nucleation on Patterned Surfaces

AbstractWe have studied qualitatively the effect of patterned substrate surface on adatom nucleation using Kinetic Monte Carlo simulations and solid-on-solid model. The inhomogeneity in the activation energies of adatom diffusion was incorporated into the simulations using two models: a) periodic variation of the adatom–substrate interaction energy as a function of the adatom lateral position and b) adding a direction dependent and periodically varying diffusion barrier energy to the adatom diffusion activation energy. The effect of the patterned surface was clearly manifested as a confinement of nucleated islands and, consequently, narrowing the island size distribution. The effect was strong in a narrow temperature range.