Deposition of size-selected metal clusters generated by magnetron sputtering and gas condensation: a progress review

This paper presents a topical review of the production and deposition of size-selected metal clusters generated by magnetron sputtering and gas condensation. Clusters with up to 75,000 atoms can be obtained by controlling the gas pressure in the sputtering source, prior to size selection with a novel (time-of-flight) mass filter. The clusters are deposited on the model graphite substrate to study the cluster-surface interaction. Two regimes for cluster deposition have been identified at elevated impact energies: (i) 'pinning' (ca. 10 eV per atom) and (ii) implantation (ca. 100 eV per atom). Of particular importance is the pinning regime, since this allows the fabrication of monodispersed cluster arrays, which are stable against diffusion at room temperature (and above). The deposition of size-selected metal clusters represents a novel method of preparing surface nanostructures, with potential applications including model catalyst studies and the immobilization of biological molecules.     

Diffusion and clustering on the titanium (0001) surface

In this paper, we present a study of diffusion and clustering of atoms on the titanium (0001) surface using molecular dynamics and molecular statics methods. The formation energy of each defect cluster is calculated using a combination of annealing and quenching techniques. Diffusion mechanisms of clusters are elucidated by the analysis of atomic trajectories, and are confirmed by calculating energy states along the diffusion path and by fitting the diffusion coefficients to one or more Arrhenius functions. Our simulations show that the clusters diffuse by a sequence of atomic jumps; once an atom moves forward by half an atomic diameter, the others follow it. Such a correlated process involves an activation energy that is comparable to that of an adatom. Atoms in each cluster, especially those in clusters consisting of three and seven atoms, are tightly bound. However, due to high mobility, the clusters are less likely to be the critical nuclei in three-dimensional growth of thin films on the (0001) surface. Even the cluster consisting of seven atoms swaps a linear distance of 60 nm during the growth of one monolayer of thin film under typical deposition conditions and at room temperature, making it ineffective as a critical nucleus above room temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Surface nanostructures created by cluster-surface impact

This paper presents a review of the production of surface nanostructures from the controlled deposition of size-selected clusters on graphite. At high enough impact energies, the clusters can be implanted into the graphite surface to create open ‘well’ structures. Scanning tunnelling microscopy (STM) measurements, coupled with molecular dynamics (MD) simulations, for Au₇⁺, Ag₇⁺ and Si₇⁺ clusters exhibit scaling relations which reveal that the implantation depth scales linearly with the momentum of the clusters in all cases. At lower impact energies, the clusters can be pinned on the graphite surface when the impact energy exceeds a critical (threshold) value, thus allowing the fabrication of monodispersed cluster arrays which are stable at room temperature (and above). One application of such cluster arrays is to bind protein molecules; atomic force microscopy (AFM) measurements in buffer solution demonstrate dispersed arrays of both chaperonin and horseradish peroxidase molecules on graphite substrates decorated with size-selected Au_N⁺ clusters.     

Nanostructured surfaces from size-selected clusters

The deposition of ionized beams of size-selected atomic clusters onto well-defined substrates represents a new method of preparing nanostructured surfaces, with lateral feature sizes in the range 1-10 nm. 'Pinning' of the incident clusters prevents the diffusion of the clusters on the surface, and thus preserves the gas-phase cluster size, even at room temperature and above. At the same time, advances in diblock copolymer techniques allow the preparation of ordered two-dimensional arrays of clusters. Here we discuss the creation and applications of these nanostructured surfaces, ranging from the fabrication of semiconductor nanostructures to the immobilization of protein molecules.     

Work function changes in the Ag deposition on TiO2 (110)

In this study, changes in the TiO₂ (110) work function where followed during Ag deposition in the sub-monolayer regime. Changes in the surface are readily detected as changes in the work function. Significant differences where found in measurements at room temperature and at liquid nitrogen temperature, which can be attributed to differences in the growth mode at those temperatures.The observed work function lowering is evidence of dipole formation by Ag clusters on the TiO₂ surface. Since the size and shape of these clusters depend on the surface temperature the differences on the work function variation should be related with the polarization induced by the shape of such clusters.     

Atom inlays performed at room temperature using atomic force microscopy

The ability to manipulate single atoms and molecules laterally for creating artificial structures on surfaces is driving us closer to the ultimate limit of two-dimensional nanoengineering. However, experiments involving this level of manipulation have been performed only at cryogenic temperatures. Scanning tunnelling microscopy has proved, so far, to be a unique tool with all the necessary capabilities for laterally pushing, pulling or sliding single atoms and molecules, and arranging them on a surface at will. Here we demonstrate, for the first time, that it is possible to perform well-controlled lateral manipulations of single atoms using near-contact atomic force microscopy even at room temperature. We report the creation of 'atom inlays', that is, artificial atomic patterns formed from a few embedded atoms in the plane of a surface. At room temperature, such atomic structures remain stable on the surface for relatively long periods of time.     

Performance of Preformed Au/Cu Nanoclusters Deposited on MgO Powders in the Catalytic Reduction of 4‐Nitrophenol in Solution

The deposition of preformed nanocluster beams onto suitable supports represents a new paradigm for the precise preparation of heterogeneous catalysts. The performance of the new materials must be validated in model catalytic reactions. It is shown that gold/copper (Au/Cu) nanoalloy clusters (nanoparticles) of variable composition, created by sputtering and gas phase condensation before deposition onto magnesium oxide powders, are highly active for the catalytic reduction of 4‐nitrophenol in solution at room temperature. Au/Cu bimetallic clusters offer decreased catalyst cost compared with pure Au and the prospect of beneficial synergistic effects. Energy‐dispersive X‐ray spectroscopy coupled with aberration‐corrected scanning transmission electron microscopy imaging confirms that the Au/Cu bimetallic clusters have an alloy structure with Au and Cu atoms randomly located. Reaction rate analysis shows that catalysts with approximately equal amounts of Au and Cu are much more active than Au‐rich or Cu‐rich clusters. Thus, the interplay between the Au and Cu atoms at the cluster surface appears to enhance the catalytic activity substantially, consistent with model density functional theory calculations of molecular binding energies. Moreover, the physically deposited clusters with Au/Cu ratio close to 1 show a 25‐fold higher activity than an Au/Cu reference sample made by chemical impregnation.     

Surface morphology and composition of films grown by size-selected Cu nanoclusters

We report the investigation of morphology and composition of copper nanocluster films deposited on Si substrates. The nanoclusters are formed in an aggregation tube at room temperature and magnetron sputtering source is used to get negatively charged Cu-clusters' beam which is subsequently mass-filtered to get size-selected cluster on the substrates as soft-landing process of deposition. For composition of the films, X-ray photoelectron spectroscopy (XPS) technique is used. For morphological changes of the films both scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses are carried out. Additionally, Energy Dispersive X-ray (EDX) spectra support the compositional and structural informations of the film. The analysis of Cu nanoclusters' films reveals that initial nucleation of Cu clusters takes place in the form of isolated islands and the arrival of subsequent Cu clusters onto Si substrates has preferential aggregation around the preceding clusters forming a mound structure.     

Size-dependent dissociation of small cobalt clusters on ultrathin NaCl films

Mass-selected anionic cobalt clusters of two different sizes (Co2 and Co13) were deposited onto ultrathin NaC1 films grown on an Au(111) substrate.Using scanning tunneling microscopy experiments and density functional theory simulations,we show that the deposited Co2 cluster dissociates and that the resulting Co atoms dope the NaCl surface by substituting Na ions.In contrast,the larger Co13 cluster does not dissociate and remains stable on top of the NaC1 film.The sizedependent fragmentation of clusters is an important aspect in the understanding of the chemical interaction between size-selected small aggregates of atoms and supporting surfaces.     

Chemical bond manipulation for nanostructure integration on wafer scale

In this paper, we have briefly summarized our activity in the area of chemical bond manipulation for the integration of nanostructures on a full wafer scale. Chemical bond manipulation involves a judicious combination of surface phenomena: reactions or diffusion, and growth process such as molecular beam epitaxy (MBE). Here, we present our results on oxidation, metallization and nitridation and their role in the formation of nanostructures. We find that oxygen changes the bonding partner from Ge to Si and this phenomenon can be controlled by controlling the annealing temperature. We have employed this phenomenon for the fabrication of novel, multiperiod Si/SiO₂/Ge layered structure which exhibits interesting light emitting properties. Further, by making use of selective diffusion of cobalt atoms through Ge layers it is possible to incorporate metallic features into Ge quantum dots. Moreover, it is possible to fabricate Si nanopillars through high temperature reaction of nitric oxide. NO molecules dissociate on the surface and nitrogen atoms thus produced form nitride islands. These islands act as protective masks for the etching of Si by the oxygen atoms, through the desorption of SiO species. Occurrence of these two simultaneous processes result in the formation of nanometre-sized Si pillars capped by silicon nitride. All these results emphasize the fact that we can extend information obtained through traditional surface science experiments for the fabrication of novel structures on a full wafer scale.     

Initial clustering after quenching in AlZnMg alloys

After solution treatment and quenching a fast increase in hardness takes place in Al-Zn-Mg alloys in the first few minutes of ageing at or near room temperature. The kinetics of this process have been studied on a number of different compostition alloys by microhardness measurements between 246 and 295 K and by resistivity measurements between 263 and 323 K. On the basis of the observed microhardness and resistivity kinetics the activation energy of the process was determined. According to this the ageing process starts with the diffusion of zinc atoms to magnesium atoms and Guinier-Preston zones develop from the so formed Mg-Zn clusters. The average Mg/Zn ratio in the clusters is 1/4. In the alloys investigated the microhardness after 15 min ageing at room temperature can be composed simply of the contributions to the hardness of pure aluminium, MgZn₄ clusters and of magnesium atoms remaining in solid solution.     

导电银浆中纳米银团簇的分子动力学研究

目的 研究纳米银团簇的运动机理及连接料对团聚的影响。方法 以环氧树脂为导电银浆连接料,分别建立不同环氧树脂含量的纳米银团簇和环氧树脂模型,模拟不同环氧树脂含量下纳米银团簇的动力学平衡状况。结果 纳米银团簇在常温下极易发生团聚,加入环氧树脂后,系统能量虽然增大,但银团簇的运动活性明显降低;环氧树脂吸附于银团簇表面,阻止了银团簇间的团聚,银团簇的团聚倾向和银原子的运动活性随环氧树脂含量的升高逐渐降低;模拟结果显示,当环氧树脂质量分数增大至33%时,环氧树脂可起到防止银团簇团聚的效果。结论 在导电银浆中,纳米银团簇在常温下易团聚且受环氧树脂的影响较大。     

Re-electrospraying splash-landed proteins and nanoparticles

FITC-albumin, Lsr-F, or fluorescent polystyrene latex particles were electrosprayed from aqueous buffer and subjected to dispersion by differential electrical mobility at atmospheric pressure. A resulting narrow size cut of singly charged molecular ions or particles was passed through a condensation growth tube collector to create a flow stream of small water droplets, each carrying a single ion or particle. The droplets were splash landed (impacted) onto a solid or liquid temperature controlled surface. Small pools of droplets containing size-selected particles, FITC-albumin, or Lsr-F were recovered, re-electrosprayed, and, when analyzed a second time by differential electrical mobility, showed increased homogeneity. Transmission electron microscopy (TEM) analysis of the size-selected Lsr-F sample corroborated the mobility observation.     

Phase maps for sputter deposited refractory metal oxide ceramic coatings: review of niobium oxide,yttrium oxide,and zirconium oxide growth

Abstract

Reactive sputter deposition is a widely used glow discharge process for growing high melting point coatings near room temperature, and metastable and multiphase structures not attainable in bulk material grown under conditions of thermodynamic equilibrium. It is therefore ideally suited for growing refractory metal oxide coatings. In this study, ‘phase maps’ are constructed for the sputter deposition of the refractory metal oxides of Nb, Y, and Zr. These diagrams interrelate process parameters, the growth environment, and metallurgical phase in the growth regime of near room substrate temperature, low surface diffusion, and sticking coefficient of unity. Phase boundaries are discussed in terms of: (i) the fractional flux of metal atoms and metal oxide molecules to the substrate; (ii) a complete oxide layer at the metal target surface; (iii) oxygen species in the plasma available for reaction at the substrate.

MST/1693     

Molecular dynamics simulations of impact of energetic silicon clusters onto crystalline silicon

Cluster deposition is a promising technique for the growth of nanometric structures and is therefore the objective of many theoretical and experimental investigations. In this work molecular dynamics calculations have been performed to study the deposition of silicon clusters onto a crystalline silicon substrate. The evolution of the structure of the deposited cluster has been studied as a function dependence of the initial direction of incidence of the cluster, of the orientation of the exposed surface, of the cluster shape and of the interatomic forces. The comparison of deposition onto (100) and (110) shows the presence of channelling effects which lead to the implant of the cluster atoms and limit their spreading on the substrate surface. Compact clusters, obtained from the minimization of the quantum mechanical energy, show an enhanced resilience against fragmentation in comparison with clusters with a crystalline lattice. A similar resistance is obtained when using an interatomic potential accounting for a coordination larger than the one of crystalline silicon. At variance with the known behavior of metallic clusters, the effects of the incidence direction are hardly perceptible.     

From oxygen adsorption to the growth of thin oxides on silicon surfaces

Oxidation of silicon surfaces at relatively low temperatures is shown to go through several activated steps, in the form of configurations inert to further uptake of oxygen. Starting from room temperature adsorption, different configurations of oxygen atoms adsorbed on and in the Si(1 1 1) and Si(0 0 1) surfaces are found, with history and/or coverage dependent energy barriers connecting them. From well below to slightly above an effective oxide coverage of a monolayer, clustering of up to three oxygen atoms around one single silicon atom has been predicted for the Si(0 0 1) surface to represent one such energy minimum; this model is confirmed here experimentally. These and other clusters are shown to agglomerate into silicon dioxide islands before coalescing into a contiguous, inert layer upon higher oxygen supplies. Another problem addressed here is the presence of molecular adsorbates in the oxidation reaction path, an issue which is still debated in the literature. For the Si(1 1 1) surface a molecular, charged oxygen species has earlier been found at temperatures up to room temperature, but not for the Si(0 0 1) surface. This is confirmed in the present experiments, and new data for this state shows that it is highly mobile until quenched at a critical oxygen coverage. It is not the initial state of oxygen on silicon, and therefore not the precursor for atomic insertion of oxygen; rather, it is found to co-exists with atomic oxygen inserted in back-bonds, at a certain, low coverage regime in which parts of the Si(1 1 1) surface are still ordered.     

Catalytic oxidation of cyclohexane by size-selected palladium clusters pinned on graphite

We report the catalytic oxidation of cyclohexane to CO and CO₂ over size-selected palladium clusters (Pd _N clusters, N = 10–120) supported on graphite as a function of cluster size. The stability of the pinned clusters (nanoparticles) under reaction conditions is investigated by scanning tunnelling microscopy measurement both before and after reaction. Temperature-programmed reaction experiments at 800 Torr show that the turnover rates (per surface Pd atom) for both CO and CO₂ increase significantly as cluster size decreases and correlate with the number of Pd perimeter atoms at the graphite interface. Under oxygen-rich conditions, the activity of the clusters increases by a factor of 3 while the product ratio CO:CO₂ rises by an order of magnitude.     

Photoelectron spectra of Yb-Si(100) heterointerfaces formed at room temperature

The laws of formation of the heterointerfaces formed upon ytterbium (Yb) deposition at room temperature (T=300 K) onto a (2×1)-reconstructed Si(100) surface were studied by methods of photoelectron spectroscopy (PES) with excitation by synchrotron radiation and low-energy electron diffraction (LEED). In the range of submonolayer surface coverages θ, the adsorption of Yb atoms leads to destruction of the surface dimers. This is accompanied by the formation of an amorphous film consisting of mixed Yb and Si atoms. For θ>1.5, a partly ordered metallic ytterbium film containing dissolved Si atoms is formed on the substrate surface. In all stages of deposition, there is charge transfer from Yb to Si atoms, which is manifested by a shift of the Si 2p core level. This shift is especially pronounced (reaching 1.5 eV) for silicon atoms dissolved in the metallic film.     

The effect of growth parameters on CrN thin films grown by molecular beam epitaxy

In this paper, we report on the controlling of the effect of growth parameters such as substrate temperature and the ratio of Cr and N atoms on phase formation, surface morphology and crystallization of CrN(001) thin films grown by plasma-assisted molecular beam epitaxy on the MgO(001) substrate. The reflection high energy electron diffraction, atomic force microscopy, X-ray diffraction and scanning tunneling microscopy are used to characterize the thin films grown under various conditions. High-quality CrN(001) thin films are achieved at a substrate temperature 430 °C with a low Cr deposition rate.     

Size and shape of nanoclusters: single-shot imaging approach

A method of single-shot imaging via aberration-corrected scanning transmission electron microscopy equipped with high angle annular dark-field detector (STEM-HAADF) has been applied to size-selected gold model catalysts (Au(25) and Au(39) ) on hydroxyapatite. Through quantitative intensity analysis, the size, in terms of number of atoms as well as 3D shape of the clusters are obtained.