Cluster Substrate Interaction Research Articles

First-Principles Study of Irn (n = 3-5) Clusters Adsorbed on Graphene and Hexagonal Boron Nitride: Structural and Magnetic Properties.

Magnetic clusters have attracted great attention and interest due to their novel electronic properties, and they have potential applications in nanoscale information storage devices and spintronics. The interaction between magnetic clusters and substrates is still one of the challenging research focuses. Here, by using the density functional theory (DFT), we study the structural stability and magnetic properties of iridium clusters (Irn, n = 3–5) adsorbed on two-dimensional (2D) substrates, such as graphene and hexagonal boron nitride (hBN). We find that the most favorable configurations of free Irn clusters change when adsorbed on 2D substrates. In the meantime, the magnetic moments of the most stable Irn reduce to 53% (graphene) and 23.6% (hBN) compared with those of the free−standing ones. Interestingly, about 12-times enlargement on the magnetic anisotropy energy can be found on hBN substrates. These theoretical results indicate that the cluster–substrate interaction has vital effects on the properties of Irn clusters.

Immobilized trimeric metal clusters: A family of the smallest catalysts for selective CO2 reduction toward multi-carbon products

Abstract Using clean electricity to convert carbon dioxide (CO2), an earth abundant carbon feedstock, to high-energy fuels is a fascinating energy strategy for a sustainable future. However, high yield of multi-carbon products remains a grand challenge. Herein, we show that trimeric metal clusters anchored on nitrogen doped porous carbon materials possess remarkable activity and selectivity for C2–C3 hydrocarbons and alcohols. By comprehensive ab initio calculations on 3d, 4d and 5d transition metal trimers, we for the first time illuminate their specialty for catalyzing this tough reaction. These spatially confined triatomic metal centers are capable of simultaneous fixation of plural CO2 molecules, provide exclusive reaction channels that sterically and electronically facilitate C–C coupling, and have tunable selectivity by mediating the cluster-substrate interaction. The catalytic mechanism and structure-activity relationship are uncovered for these sub-nano clusters with robust stability. These results provide important knowledge for atomically precise design of novel catalysts for direct conversion of CO2 to high-energy fuels and high-value chemicals.

シリコン基板に担持された単原子層単一サイズ白金クラスターの特異的触媒活性とクラスター・担体間相互作用

シリコン基板に担持された単原子層単一サイズ白金クラスターの特異的触媒活性とクラスター・担体間相互作用

On the structural and electronic properties of hexanuclear vanadium oxide clusters V6On−/0 (n = 12–15): Is V6O12 cluster planar or cage-like?

Density functional theory (DFT) calculations are carried out to investigate the structural and electronic properties of a series of hexanuclear vanadium oxide clusters V6On−/0 (n=12–15). Generalized Koopmans’ theorem is applied to predict the vertical detachment energies (VDEs) and simulate the photoelectron spectra (PES) for V6On− (n=12–15) clusters. Extensive DFT calculations are performed in search of the lowest-energy structures for both the anions and neutrals. All of these clusters appear to prefer the polyhedral cage structures, in contrast to the planar star-like structures observed in prior model surface studies for the V6O12 cluster. Molecular orbitals are performed to analyze the chemical bonding in the hexanuclear vanadium oxide clusters and provide insights into the sequential oxidation of V6On− (n=12–15) clusters. The V6On− (n=12–15) clusters possess well-defined V5+ and V3+ sites, and may serve as molecular models for surface defects. Electron spin density analyses show that the unpaired electrons in V6On− (n=12–14) clusters are primarily localized on the V3+ sites rather than on the V5+ sites. The difference gas phase versus model surface structures of V6O12 hints the critical roles of cluster–substrate interactions in stabilizing the planar V6O12 cluster on model surfaces.

Au Clusters on TiO2(110) (1 × 1) and (1 × 2) Surfaces Examined by Polarization-Dependent Total Reflection Fluorescence XAFS

The structure of Au clusters on the TiO2(110) surface and the cluster-substrate interaction were investigated by polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS). The TiO2(110) (1 × 1) and (1 × 2) surfaces were compared in terms of substrate structural effects on Au cluster structure. We could not find evidence for the direct Au-substrate bonding and observed that the substrate morphology did not significantly affect the size and structure of Au clusters. Finally, we precisely determined that the Au clusters favored a geometry with icosahedral Au55 symmetry. We discuss the possible interaction mechanism between Au and TiO2(110) that leads to these results.

The effect of deposition velocity and cluster size on thin film growth by Cu cluster deposition

The deposition of Cu clusters on a Si (001) substrate was studied using molecular dynamics simulations. EAM, SW and LJ potentials were used to describe the interaction between cluster atoms, substrate atoms, and the cluster–substrate interaction, respectively. Three cluster sizes comprised of 13, 55 and 147atoms were deposited with various initial velocities (100, 500, 1000, 1500 and 3000m/s). Results indicated that the degree of epitaxy and film flatness generally increased with the deposition velocity in a certain window. We found that Cu13 cluster at 1500m/s, Cu55 at 1000m/s and Cu147 at 500m/s produced more flat epitaxial layers that matched well with the substrate surface. The flat morphology was much desired for the secondary cluster deposition. The epitaxial degree of a small cluster was higher than that of a large cluster at lower and higher velocities, while at moderate velocities a large cluster showed higher degree of epitaxy. The effect of cluster velocity on the height of mass center of the cluster was also identified.

The effects of substrate size and temperature on the deposition of Cu clusters on a Si substrate

The deposition of a Cu13 cluster on a Si (001) surface was studied by molecular dynamics simulations. Embedded atom method, Stillinger-Weber, and Lennar-Jones potentials were used to describe the interaction between cluster atoms, substrate atoms, and the cluster-substrate interaction. Quantitative characteristic parameters, such as kinetic energy of the cluster and the substrate, the degree of epitaxy, and the mean height of mass center of the cluster, were calculated to study the effect of substrate size and substrate temperature on cluster deposition. The substrate temperature was found to affect the degree of epitaxy at different substrate sizes. When the size ratio of the substrate and cluster is relatively small or large, the epitaxial degree was higher at 800 K than at 300 K. If the size of the substrate matches that of the cluster, the substrate temperature appeared to have minimum effect. For a given temperature, the substrate size was found to have no obvious effect on the degree of epitaxy or the mean height of mass center of the cluster. This observation indicated that simulations using even a small system can still give reliable results and qualitative description of the deposition phenomena. We also found that the cluster atoms prefer to diffuse along the [110] direction on the Si (001) surface.

Adsorption of Pt and Bimetallic PtAu Clusters on the Partially Reduced Rutile (110) TiO2 Surface: A First-Principles Study

An extensive study of the adsorption of small Ptn (n = 1–8) and bimetallic Pt2Aum (m = 1–5) clusters on the partially reduced rutile (110) TiO2 surface has been performed via total energy pseudopotential calculations based on density functional theory. Structures, energetics, and electronic properties of adsorbed Ptn and Pt2Aum clusters have been determined. The surface oxygen vacancy site has been found to be the nucleation center for the growth of Pt clusters. These small Pt clusters strongly interact with the partially reduced surface and prefer to form planar structures for n = 1–6 since the cluster–substrate interaction governs the cluster growth at low Pt coverage. We found a planar-to-three-dimensional structural transition at n = 7 for the formation of Ptn clusters on the reduced TiO2 surface. GGA+U calculations have also been performed to get a reasonable description of the reduced oxide surface. We observed significant band gap narrowing upon surface–Ptn cluster interaction which leads to the formation of gap localized Pt states. In the case of bimetallic Pt–Au clusters, Aum clusters have been grown on the Pt2–TiO2 surface. The previously adsorbed Pt dimer at the vacancy site of the reduced surface acts as a clustering center for Au atoms. The presence of the Pt dimer remarkably enhances the binding energy and limits the migration of Au atoms on the titania surface. The charge state of both individual atoms and clusters has been obtained from the Bader charge analysis, and it has been found that charge transfer among the Pt atoms of Ptn clusters and the metal oxide surface is stronger compared to that of Au clusters and the Pt2–TiO2 system.

Interaction of Au16 Nanocluster with Defects in Supporting Graphite: A Density-Functional Study

Soft-landed adsorption of Au16 on bilayered graphene is investigated using density functional theory. The orientation of the Au16 cluster and number of neighboring surface vacancies affect the overall structural and electronic properties of the cluster. The results of the PBE, vdW-DF, and vdW-DF2 exchange-correlation functionals are compared for the cluster-substrate interaction for systems with and without defects. In the presence of defects size two and greater, an Au atom adsorbs into the topmost graphene layer; this strongly influences the binding energy (>3 eV), while inducing substantial bending in the carbon plane and altering electronic properties of the system. Though the Td-symmetry and electronegative properties of the Au16 structure change in the presence of greater neighboring defects, elements of the cagelike starting structure remain throughout. The electron localization function shows that the in-plane Au–C bonds are of delocalized (metallic) nature and there is a local charge transfer to ...

Gold nanoparticles for applications in energy and environment: synthesis and characterization

Nanometre-size gold displays exceptional physical and chemical properties that depend not only on their sizes but also on their shapes. Here we investigate the growth of size-selected gold nanoclusters and their shapes characterized using both TEM and STEM. It is found that the cluster-substrate interaction influences the height of small nanoclusters Au80, as indicated by comparison between MgO and amorphous carbon substrate, but this effect is overcome by the dynamical landing process at high deposition energy. Moreover, large gold nanoclusters, Au8860 have asymmetric structures with aspect ratio between 1 and 2. The simulation shows that the icosahedral shape observed is significantly lower in energy than the alternatives and is expected to be stable.

Cobalt nanoclusters on metal-supported Xe monolayers: Influence of the substrate on cluster formation kinetics and magnetism

The growth dynamics of submonolayer coverages of cobalt during buffer-layer assisted growth on Ag(111) and Pt(111) substrates is investigated by variable temperature scanning tunneling microscopy in the temperature range between 80 and 150 K. It is found that attractive cluster-substrate interactions can govern the cluster formation on the Xe buffer layer if the Xe layer is sufficiently thin. The interpretation of the microscopy results is supported by x-ray magnetic circular dichroism data which monitor the effect of cluster-substrate interactions on the formation of magnetic moments and magnetic anisotropy of Co nanocluster during the different stages of growth. Ab initio calculations show that the cluster magnetism is controlled by the interface anisotropy, leading to perpendicular magnetization for Co on Pt(111). Limits of and new potential for nanocluster fabrication by buffer-layer-assisted growth are discussed.

MELTING POINT AND LATTICE PARAMETER SHIFT IN SUPPORTED METAL NANOCLUSTERS

The dependencies of the melting point and the lattice parameter of supported metal nanoclusters as functions of clusters height are theoretically investigated in the framework of the uniform approach. The vacancy mechanism describing the melting point and the lattice parameter shifts in nanoclusters with decrease in their sizes is proposed. It is shown that under the high vacuum conditions (p < 10-7 torr ) the essential role in clusters melting point and lattice parameter shifts is played by van der Waals forces of cluster–substrate interaction. The proposed model satisfactorily accounts for the experimental data.

Control of electron–phonon dynamics by quantum confinement in isolated Y 2S iO 5:Pr 3+ nanocrystal

The fluorescence spectrum and decay of isolated Y 2SiO 5:Pr 3+nanocrystals at the glass-air interface were investigated using the confocal fluorescence microscopy and time-resolved techniques. Intense fluorescence from the upper crystal field components of the split 1D 2 term of Pr 3+ was observed. The fluorescence decay is characterized by the non-exponential law t - 1 / 2 which is caused by the back-to-back energy exchange between Pr 3+ electronic states and long-lived nanocrystal phonons. The observed effect is very sensitive to the cluster–substrate interaction and opens a new spectroscopic way to explore the excited states of J-manifolds of RE ions directly.

Stimulation of vapor nucleation on perfect and imperfect hexagonal lattice surfaces

Monte Carlo simulations of water vapor nucleation on a perfect crystal surface and on a surface with defects are performed. Mass exchange with the vapor phase is modeled by using an open ensemble. Cluster-substrate interaction is described in terms of conventional atom-atom potentials. The Hamiltonian of the system includes expressions for electrostatic, polarization, exchange, and dispersion interactions. The Gibbs free energy and work of adsorption are calculated by Monte Carlo simulation in the bicanoĭnical ensemble. The microscopic structure of nuclei is analyzed in terms of pair correlation functions. Periodic boundary conditions are used to simulate an infinite substrate surface. Molecule-substrate and molecule-molecule long-range electrostatic interactions are calculated by summing the Fourier harmonics of the electrostatic potential. Dispersion interactions are calculated by direct summation over layers of unit cells. Nucleation on a surface with matching structure follows a layer-by-layer mechanism. The work of adsorption per molecule of a monolayer on the substrate surface has a maximum as a function of nucleus size. The steady rate of nucleation of islands of supercritical size is evaluated. The work of adsorption per molecule for layer-by-layer film growth is an oscillating function of cluster size. As a function of layer number, it has a minimum depending on the vapor pressure. The electric field generated by a microscopic surface protrusion destroys the layered structure of the condensate and eliminates free-energy nucleation barriers. However, point lattice defects do not stimulate explosive nucleation.

Influence of Temperature on the Interaction between Pd Clusters and theTiO2(110) Surface

The behavior of a Pd nanocluster on the rutile TiO2 (110) surface has been analyzed by extensive first principles molecular dynamics simulations between 100 K and 1073 K. Calculations predict a steep change in the morphological and electronic cluster structure around 800 K in excellent agreement with previous experimental evidence. At low temperature, the cluster geometry is mainly controlled by the substrate structure; however, upon the transition temperature, the cluster-substrate interaction decreases appreciably, and the cluster adopts a geometry more stable in vacuum and becomes metallic. These results illustrate at an atomistic level the influence of temperature on the geometrical and electronic properties of oxide-supported clusters.

Mapping the femtosecond dynamics of supported clusters with nanometer resolution

In this paper we present a combined STM, SEM and time-resolved PEEM study of silver clusters on a nano-patterned HOPG-substrate, exhibiting areas of different defect type and defect densities. The areas show small but distinct differences in the femtosecond dynamics associated with electronic excitations in the clusters. We assign these differences to variations in the cluster size distribution and variations in the cluster-substrate interaction as governed by the bonding to the different defect types.

Noncollinear magnetism of Cr and Mn nanoclusters on Ni(111): Changing the magnetic configuration atom by atom

The Korringa-Kohn-Rostoker Green-function method for noncollinear magnetic structures was applied on Mn and Cr nanoclusters deposited on the Ni(111) surface. We consider various dimers, trimers, and tetramers. We obtain collinear and noncollinear magnetic solutions, brought about by the competition of antiferromagnetic interactions. It is found that the triangular geometry of the Ni(111) substrate, together with the intracluster antiferromagnetic interactions, is the main cause of the noncollinear states, which are secondarily affected by the cluster-substrate exchange interactions. The stabilization energy of the noncollinear, compared to the collinear, states is calculated to be typically of the order of $100\phantom{\rule{0.3em}{0ex}}\mathrm{meV}∕\text{atom}$, while multiple local-energy minima are found, corresponding to different noncollinear states, differing typically by $1--10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}∕\text{atom}$. Open structures exhibit sizable total moments, while compact clusters tend to have very small total moments, resulting from the complex frustration mechanisms in these systems.

Magnetic moments, exchange coupling, and crossover temperatures of Co clusters onPt(111) and Au(111)

The influence of cluster size and of cluster–substrate interaction on the magnetic propertiesof Co clusters of 1–10 atoms on Pt(111) and Au(111) is studied by fully relativistic ab initiocalculations. The focus is on systematic trends of the spin and orbital magnetic moments,the exchange coupling, and the crossover temperature. The spin magnetic moments of Coclusters are larger for the Pt substrate than for the Au substrate, while the reverse is truefor the orbital magnetic moments. The local magnetic moments of Co atoms generallyincrease if the number of Co neighbours decreases. The exchange coupling constantsJij depend on the cluster size and on the location of respective atoms. The crossovertemperature increases monotonically with cluster size and is larger for clusters on Pt thanfor clusters on Au.

Comparison between the classical theory predictions and molecular simulation results for heterogeneous nucleation of argon

We have performed Monte Carlo simulations of homogeneous and heterogeneous nucleations of Lennard-Jones argon clusters. The simulation results were interpreted using the major concept posing a difference between the homogeneous and heterogeneous classical nucleation theories-the contact parameter. Our results show that the multiplication concept of the classical heterogeneous nucleation theory describes the cluster-substrate interaction surprisingly well even for small molecular clusters. However, in the case of argon nucleating on a rigid monolayer of fcc(111) substrate at T=60 K, the argon-substrate atom interaction being approximately one-third as strong as the argon-argon interaction, the use of the classical theory concept results in an underestimation of the heterogeneous nucleation rate by two to three orders of magnitude even for large clusters. The main contribution to this discrepancy is induced by the failure of the classical theory of homogeneous nucleation to predict the energy involved in bringing one molecule from the vapor to the cluster for clusters containing less than approximately 15 molecules.

Structure of small Au n, Ag n, and Cu n clusters ( n=2−4) on rutile TiO 2(110): A density functional theory study

We examine the structure of small Au n , Ag n , and Cu n ( n=2−4) clusters on rutile TiO 2(110) surfaces using density functional theory calculations. Based on the comparison of supported and gas-phase clusters, we also discuss the effect of the cluster–substrate interaction on the atomic structure of small Au, Ag, and Cu clusters grown on TiO 2(110).