Article pubs.acs.org/JPCC
Decay Kinetics of Cluster-Beam-Deposited Metal Particles Niklas Grönhagen,† Tommi T. Jar̈ vi,‡,§ Natalie Miroslawski,† Heinz Hövel,*,† and Michael Moseler*,‡,∥,⊥ †
Technische Universität Dortmund, Experimentelle Physik I, D-44221 Dortmund, Germany Fraunhofer-Institut für Werkstoffmechanik IWM, Wöhlerstrasse 11, D-79108 Freiburg, Germany § Karlsruhe Institute of Technology, Institute of Applied MaterialsReliability of Components and Systems (IAM-ZBS), D-76131 Karlsruhe, Germany ∥ Freiburger Materialforschungszentrum, Stefan-Meier-Str. 21, D-79104 Freiburg, Germany ⊥ Albert-Ludwigs Universität, Physikalisches Institut, Hermann-Herder-Str. 3, D-79104 Freiburg, Germany ‡
S Supporting Information *
ABSTRACT: High-precision experiments and atomistic simulations are used to determine the flattening kinetics of mass-selected 55−147 atom Ag clusters deposited on Au(111). The clusters are shown to align epitaxially and decay through an exchange pathway with a range of ratelimiting barriers, from ca. 0.25 to 0.4 eV, depending on the shape of the particle. It is also shown that nonlocal effects at the Au−Ag interface lead to a dramatic reduction in the barrier of the dominant transition pathway, requiring ab initio methods for correct modeling. As a result, quantitative correspondence between experimental and simulated island heights is obtained.
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with atom and cluster beams,21−23 surface smoothing,24−27 and film formation,8,28 have arisen. Even so, even in relatively simple cases, such as the deposition of metal clusters on the surface of another metal, it is not straightforward to predict how the deposited structure will look. Exactly predicting structures arising from cluster deposition is thus a formidable challenge. While molecular dynamics simulations can be used to study the process in great detail,3,12−15,17,29 the time scale covered is mostly limited to some hundred picoseconds after the cluster impact, thermal evolution being treated approximately at best. Experimental investigation of cluster−surface systems, on the other hand, generally takes place some hours after the impact. This, together with necessarily limited experimental resolution, makes it hard to precisely determine the mechanisms involved. In the present study, we address the above problem in detail, taking the Ag−Au as our system of choice. This combination is of particular interest because of its use in nanoplasmonics, e.g., in the form of core−shell nanoparticles.30,31 We compare experiment and simulation for silver clusters impacting on an Au(111) surface, in order to determine the resulting structures and the mechanisms leading to their formation. Here, a series of well-defined cluster deposition experiments with a systematic variation of cluster size and kinetic energy is performed in
INTRODUCTION One of the most flexible and well-controlled ways to produce nanosystems at surfaces is the deposition of mass-selected clusters.1−4 A vast store of knowledge exists for the sizedependent structure of clusters in a vacuum, in the free beam. Recent examples for gold clusters are shown in refs 5−7. However, even in soft-landing conditions, with low kinetic energy for the deposition process, the interaction with the substrate changes the cluster shape significantly,8−15 unless an extremely nonreactive substrate is used.16−18 In order to use mass selected clusters for the production of devices or other applications of clusters at surfaces it will be crucial to understand the changes of the cluster geometry due to the cluster−surface interaction with atom-by-atom accuracy. For example, varying offset charges caused by small differences in size or the surroundings of the nano islands render the coupling of many single-electron devices into network structures very difficult.19 Also, in the field of catalysis, the size and shape of nanoparticles are important for their activity.20 Not only the deformation during the impact process but also long-term post deposition changes at different temperatures will be important for real-world applications. In the case of metal-on-metal deposition, surface diffusion processes are the key to understanding deposited film morphologies. For the case of metal cluster and atom deposition, a wealth of literature on surface diffusion, spanning several decades, is available. Consequently, several applications, such as surface patterning © 2012 American Chemical Society
Received: May 25, 2012 Revised: August 17, 2012 Published: August 17, 2012 19327
dx.doi.org/10.1021/jp305089d | J. Phys. Chem. C 2012, 116, 19327−19334
The Journal of Physical Chemistry C
Article
deposition. Here, clusters equilibrated at 120 K were deposited on an Au(111) surface equilibrated at 77 K. Depending on the energy deposited on the surface, the lateral surface size was either ca. 4.5 × 4.5 nm2 with a depth of 2 nm or ca. 10 × 10 nm2 with a depth of 6 nm. The larger cell was verified to be sufficient by repeating calculations with a 10 nm deep cell. Periodic boundary conditions were employed in the lateral directions, while at the bottom of the cell two layers were held fixed. Temperature control was applied for an additional two layers at the bottom and within one lattice constant at the lateral sides, using the Berendsen thermostat36 with a time constant of 100 fs. The clusters were relaxed starting from structures from refs 37 and 38 corresponding to minima from a Gupta potential. It must be emphasized, however, that, even at extremely low deposition energies, the strong metallic cluster−surface interaction completely destroys any memory of the incoming cluster structure (see, e.g., refs 12 and 15), as also demonstrated below by comparison against clusters on C60 films. For each set of deposition parameters, 50−100 individual impact events were calculated. Prior to deposition, the clusters were randomly rotated. To randomize the impact point on the surface while keeping the cluster at the center of the cell to prevent interaction with the temperature control on the sides, the substrate was translated randomly. Finally, the cluster was given a velocity toward the surface and the dynamics followed until no further evolution was apparent (up to 100 ps). Sutton−Chen potentials39,40 were used to describe the interatomic interactions, as these have been shown to yield reliable results in similar applications in the past.18 The deposition simulations were carried out using the PARCAS code. 41 For studying the detailed flattening mechanisms of the clusters, nudged elastic band (NEB) calculations42−44 were performed to find transition states. For the classical case (the Sutton−Chen potential), this was done using the LAMMPS package.45 We also performed density functional theory (DFT) computations using the Vienna ab initio simulation package46−49 (VASP, versions 4.6 and 5.2), with the projector-augmented-wave method and pseudopotentials50,51 in the Perdew−Burke−Ernzerhof (PBE) generalized gradient approximation. 52 Unless otherwise stated, we performed spin-paired calculations with an energy cutoff of 250 eV, using Gaussian smearing with a width of 0.1 eV for the partial occupancies. For each periodic direction, a k-point sampling of 4 was used, and in slab calculations, a 10−12 Å space was left in the z-direction. For NEB calculations in DFT and MD, spring constants of 5 and 10 eV/Å were used, respectively. Details on each individual calculation are given in parentheses. For analyzing partial charge distributions in DFT, we used Bader analysis.53,54 VMD was used for visualization.55
ultrahigh vacuum (UHV) and compared to corresponding molecular dynamics (MD) simulations. To bridge the gap between experiment and simulation, we will use low temperature in the experiments to suppress thermally activated processes which change cluster shape on time scales not accessible to MD. Unfortunately, as we will show here, clusters with a few ten or hundred atoms can change their structure even at liquid nitrogen temperature on the hour time scale, preventing direct comparison with results from molecular dynamics. Speeding up molecular dynamics simulations to draw conclusions to macroscopic time scales mostly requires using very high temperatures at which new processes, not present at low temperature, can be triggered. We use another approach which is thus more realistic, albeit less straightforward: Starting with the configuration of the cluster−surface system at the end of the impact process, resulting from a molecular dynamics simulation at the experimental conditions, the slow thermally induced processes are investigated by the identification of transition states and barriers for further changes in cluster shape. On the basis of known barriers, the behavior at different temperatures can be extrapolated to experimental time scales. This approach has been successfully used for several different systems, such as a metal inside carbon nanotubes32 and silver clusters on a C60 film,18 and will be employed here to obtain a complete picture of the flattening kinetics of metal clusters on a metallic surface.
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METHODS Experimental Section. The sample system consisted of a Au film that was evaporated onto a mica substrate, forming a Au(111) surface. It was transferred into ultrahigh vacuum (
