Cluster Deposition Research Articles

Growth Mechanism of Cluster-Assembled Surfaces: From Submonolayer to Thin-Film Regime

Nanostructured films obtained by the assembling of preformed atomic clusters are of strategic importance for a wide variety of applications. The deposition of clusters produced in the gas phase onto a substrate offers the possibility to control and engineer the structural and functional properties of the cluster-assembled films. To date the microscopic mechanisms underlying the growth and structuring of cluster-assembled films are poorly understood, and in particular the transition from the sub-monolayer to the thin film regime is experimentally unexplored. Here we report the systematic characterization by Atomic Force Microscopy of the evolution of the structural properties of cluster-assembled films deposited by Supersonic Cluster Beam Deposition. As a paradigm of nanostructured systems, we have focused our attention on cluster-assembled zirconia films, investigating the influence of the building blocks dimensions on the growth mechanisms and on the roughening of the thin films, following the growth process from the early stages of the sub-monolayer to the thin film regime. Our results demonstrate that the growth dynamics in the sub-monolayer regime determines different morphological properties of the cluster-assembled thin film. The evolution of roughness with the number of deposited clusters reproduces exactly the growth exponent of the ballistic deposition in the 2+1 model, from the sub-monolayer to the thin film regime.

High-selectivity palladium catalysts for the partial hydrogenation of alkynes by gas-phase cluster deposition onto oxide powders

The selective hydrogenation of alkynes is an important reaction in the synthesis of fine and bulk chemicals. We show that the synthesis of metal nanoparticles in the gas phase, followed by depositi...

Exchange and magnetic order in bulk and nanostructured Fe5Si3

The Curie temperature of bulk and nanostructured Fe5Si3 is investigated using experiments, density-functional simulations, and many-body model calculations. The bulk intermetallic, which crystallizes in the hexagonal D88 structure, exhibits several intriguing features: it does not exist as a room-temperature equilibrium phase, is close to the onset of ferromagnetism, and exhibits two crystallographically very different Fe sites. The samples, produced by rapid quenching (bulk) and cluster deposition (nanoparticulate thin films), have Curie temperatures of about 400 K. Interatomic exchange constants are calculated using the Kohn-Korringa-Rostoker (KKR) method and used to solve the multisublattice mean-field problem for the system. The Vienna ab initio simulation package (VASP) is employed to study the dependence of the Fe moment on the thermally induced spin misalignment, and a model calculation yields an estimate for quantum-spin-liquid corrections. The theory includes Heisenberg exchange but overestimates the Curie temperature, and a discussion is given regarding additional approaches to handle weakly ferromagnetic multisublattice intermetallic compounds.

The nanocoherer: an electrically and mechanically resettable resistive switching device based on gold clusters assembled on paper

We report the realization of a resettable resistive switching device based on a nanostructured film fabricated by supersonic cluster beam deposition of gold clusters on plain paper substrates. Through the application of suitable voltage ramps, we obtain, in the same device, either a complex pattern of resistive switchings, or reproducible and stable switchings between low resistance and high resistance states, with an amplitude up to five orders of magnitude. Our device retains a state of internal resistance following the history of the applied voltage similar to that reported for memristors. The two different switching regimes in the same device are both stable, the transition between them is reversible, and it can be controlled by applying voltage ramps or by mechanical deformation of the substrate. The device behavior can be related to the formation, growth and breaking of junctions between the loosely aggregated gold clusters forming the nanostructured films. The fact that our cluster-assembled device is mechanically resettable suggests that it can be considered as the analog of the coherer: a switching device based on metallic powders used for the first radio communication system.

Computational screening of MOF-supported transition metal catalysts for activity and selectivity in ethylene dimerization

Deposition of small metal-oxide clusters on the Zr-based nodes of a metal-organic framework has been demonstrated to provide access to a variety of single-site catalysts. Well-defined catalytic active sites are amenable to detailed computational studies of potential catalytic pathways, and they invite screening a wide range of metals to assess their expected activity. Here we report the application of density functional theory to a variety of transition metals (in particular TiIV, VII, VIV, CrII, CrIII, MnII, MnIV, FeII, FeIII, NiII, CoII, CoIII, CuII, CuIII, PdII, MoII, and WII) supported on NU-1000 inorganometallic nodes to evaluate their activity for ethylene dimerization. We found that the rate-determining step varies between different catalysts, which illustrates the importance of considering more than a single step when comparing catalytic cycles across a variety of metals. Our calculations are consistent with the known good activity of supported NiII for ethylene dimerization, and they predict that CrII and PdII are also potentially useful catalysts for this process. We also screen modifications to the organic linker of NiII-NU-1000 by considering the addition of Me, iPr, tBu, CF3 and NH2 groups to study the influence of sterically demanding and, the case of CF3 and NH2, respectively, electron-donating and -withdrawing, substituents on the activity for ethylene dimerization; we predict no improvements in activity or selectivity (for 1-butene) with such substitutions.

AFM-induced deposition of metallic nanoarrays for photonic devices

Experiments on forming periodic nanostructures with relief that repeats the trajectory of the AFM probe are performed, and the parameters affecting their dimensions, height, width, shape, profile, and so on are studied. The induced deposition of silver clusters on the surface of a p-type silicon wafer an external field is demonstrated for the first time. The possibility is demonstrated of using such structures as hybrid schemes for photoelectric converters; as active elements for the amplification, generation, and control systems in laser and optoelectronic devices for fiber optics; and for creating photonic crystals (elements) based on periodic structures.

PBE Modeling of Flocculation of Microalgae: Investigating the Overshoot in Mean Size Profiles

Microalgae is considered as a viable feedstock to biomass gasification. After synthesis in water medium, microalgae are separated and dried to a suitable degree to be fed to the gasification process. In order to achieve an efficient separation, a flocculation process is employed, in which microalgae primary particles aggregate and form larger clusters. Although flocculation is a well-established process, there are still some unknown issues related to it, that are worth further research. Experiments show that the mean size of clusters during flocculation goes through a maximum and then decreases with time. We refer to this pattern in the mean size profile as the overshoot. Studying this phenomenon is crucial since the size of clusters has a significant effect on the overall efficiency of the separation of microalgae from water. In this work, we aim at investigating the mechanisms behind the overshoot.The flocculation process is modeled as an aggregation-breakup system by using population balance equations (PBEs). The primary results show that the aggregation and breakup alone cannot lead to the overshoot in the mean size profile. Thus, we suggested three mechanisms that can lead to the overshoot: deposition of large clusters (DLC), restructuring of clusters (RC), and primary particle aggregation (PPA). These mechanisms were examined with numerical simulations and it was revealed that all three lead to the overshoot.

Surface growth by cluster particles: Effects of diffusion and cluster’s shape

We use Monte Carlo simulation to study the spatiotemporal behavior of thin films generated by low kinetic energy cluster particles. To this end, two models are employed to investigate the effects of clusters’ shapes and their diffusion ability on the surface. In the first model, clusters are considered as a string of particles with unit height and non-unit length. The growth process in this model is implemented based on Random Deposition with Surface Relaxation (RDSR) model where particles can diffuse on the surface. The second model studies deposition of porous clusters with different shapes according to Random Deposition (RD) model. According to previous studies, the deposition of unit size particles based on RD model leads to a random growth. Using the RDSR model the phenomenon is observed to be under the Edward-Wilkinson universality class. Our results for both models reveal that using cluster particles with non-unit size gives rise to a porous medium which changes the universality class of models to Kardar-Parizi-Zhang (KPZ) by scaling exponents β=0.3 and α=0.5.

Enhanced microwave-absorption performance of FeCoB/Polyimide-Graphene composite by electric field modulation

Graphene was incorporated into polyimide resin to obtain Polyimide-Graphene polymer sheet via in-situ polymerization method. FeCoB nanoparticles, generated by a high energetic cluster deposition system, were then deposited on the Polyimide-Graphene sheets by electric field assisted deposition technology to form FeCoB/Polyimide-Graphene ternary composites. An electric field of about 5–30 kV was applied on the sample platform when the composites were manufactured. The strong magnetic properties of the composites were revealed by the measurement of hysteresis loops at room temperature. Then the laminated FeCoB/Polyimide(-Graphene) composites were used to perform reflection loss scan. It is found that the addition of a small amount of graphene promote the improvement of electromagnetic properties by increasing dielectric loss. And the cover of FeCoB films can dramatically enhance the microwave absorption capacity by enriching interfacial polarization effect and electromagnetic match. The results prove that the electric field assisted deposition technique is a very attractive avenue to enhance the microwave absorption performance of the ternary composite expressed by tuning their dielectric and magnetic properties.

Helium Droplet-Mediated Deposition and Aggregation of Nanoscale Silver Clusters on Carbon Surfaces

We present experiments and calculations of the deposition and aggregation of silver clusters embedded in helium droplets onto an amorphous carbon surface at room temperature. Calculations were also performed for deposition onto a graphene surface. They involve potentials for the interaction of carbon atoms with silver and helium atoms, provided by ab initio calculations. The numerical simulations were performed for few-nanometer-sized silver clusters including up to 5000 Ag atoms and He droplets with up to 105 4He atoms. The fluid nature of the 4He droplet is accounted for by the renormalization of the He–He interaction potential. The numerical results are consistent with deposition experiments with an average number of 3000 Ag atoms per 4He droplet and indicate that the aggregation of the silver clusters on the carbon surface is mediated by secondary droplet impacts. They also reveal nontrivial dynamics of the Ag clusters within the carrier droplet, showing a tendency to drift toward the droplet surface....

Response Characteristics of Hydrogen Sensors Based on PMMA-Membrane-Coated Palladium Nanoparticle Films.

Coating a polymeric membrane for gas separation is a feasible approach to fabricate gas sensors with selectivity. In this study, poly(methyl methacrylate)-(PMMA-)membrane-coated palladium (Pd) nanoparticle (NP) films were fabricated for high-performance hydrogen (H2) gas sensing by carrying out gas-phase cluster deposition and PMMA spin coating. No changes were induced by the PMMA spin coating in the electrical transport and H2-sensing mechanisms of the Pd NP films. Measurements of H2 sensing demonstrated that the devices were capable of detecting H2 gas within the concentration range 0-10% at room temperature and showed high selectivity to H2 due to the filtration effect of the PMMA membrane layer. Despite the presence of the PMMA matrix, the lower detection limit of the sensor is less than 50 ppm. A series of PMMA membrane layers with different thicknesses were spin coated onto the surface of Pd NP films for the selective filtration of H2. It was found that the device sensing kinetics were strongly affected by the thickness of the PMMA layer, with the devices with thicker PMMA membrane layers showing a slower response to H2 gas. Three mechanisms slowing down the sensing kinetics of the devices were demonstrated to be present: diffusion of H2 gas in the PMMA matrix, nucleation and growth of the β phase in the α phase matrix of Pd hydride, and stress relaxation at the interface between Pd NPs and the PMMA matrix. The retardation effect caused by these three mechanisms on the sensing kinetics relied on the phase region of Pd hydride during the sensing reaction. Two simple strategies, minimizing the thickness of the PMMA membrane layer and reducing the size of the Pd NPs, were proposed to compensate for retardation of the sensing response.

Bridging Zirconia Nodes within a Metal-Organic Framework via Catalytic Ni-Hydroxo Clusters to Form Heterobimetallic Nanowires.

Metal-organic frameworks (MOFs), with their well-ordered pore networks and tunable surface chemistries, offer a versatile platform for preparing well-defined nanostructures wherein functionality such as catalysis can be incorporated. Notably, atomic layer deposition (ALD) in MOFs has recently emerged as a versatile approach to functionalize MOF surfaces with a wide variety of catalytic metal-oxo species. Understanding the structure of newly deposited species and how they are tethered within the MOF is critical to understanding how these components couple to govern the active material properties. By combining local and long-range structure probes, including X-ray absorption spectroscopy, pair distribution function analysis, and difference envelope density analysis, with electron microscopy imaging and computational modeling, we resolve the precise atomic structure of metal-oxo species deposited in the MOF NU-1000 through ALD. These analyses demonstrate that deposition of NiOxHy clusters occurs selectively within the smallest pores of NU-1000, between the zirconia nodes, serving to connect these nodes along the c-direction to yield heterobimetallic metal-oxo nanowires. This bridging motif perturbs the NU-1000 framework structure, drawing the zirconia nodes closer together, and also underlies the sintering resistance of these clusters during the hydrogenation of light olefins.

Copper Metallization of Gold Nanostructure Activated Polypyrrole by Electroless Deposition

We report on the study of novel polymeric electrodes made by the selective, electroless plating, of Copper (Cu) films on gold (Au) nanostructure-modified polypyrrole (Ppy). The main aim of the work is to define the effect of Au nanoparticle seed preparation method on its catalytic properties for Cu electroless deposition. The Au nanostructures were produced by two different techniques, namely, cluster beam deposition from a magnetron sputtering/gas condensation source (with precise size and composition control) and electrochemical deposition. To carry out the research, polypyrrole films were first synthesized by electro-polymerization on Au (200nm)/SiO2/Si substrates, followed by modification of the polymer surface by either electroplating of Au nanoparticles or Au923 clusters deposition from a magnetron cluster source. The gold nanoparticle modified electrodes were subjected to copper electroless deposition for different time intervals. The morphology, growth and nucleation kinetics of the resulting copper film were studied by environmental scanning electron microscopy-Energy dispersive X-ray spectroscopy (ESEM-EDS), atomic force microscopy (AFM) and X-ray fluorescence (XRF). Although both nanoparticle type showed similar incubation time, a faster catalytic response, once deposition had been initiated, was observed when the polypyrrole film was modified with Au923 clusters. The incubation time was independent of cluster size and type. This could be explained by a simple model assuming that the incubation time depends on similar parameters for both nanoparticle types, such as metal-metal (Au-Cu) binding energy, crystallographic misfit (Au- Cu) and lateral growth of the copper film. Further discussion is presented in this paper in attempt to explain the different growth rate of Cu film catalysed by the Au923 clusters and electroplated Au nanoparticles.

Massively parallel computation of accurate densities for N-body dark matter simulations using the phase-space-element method

This paper presents an accurate density computation approach for large dark matter simulations, based on a recently introduced phase-space tessellation technique and designed for massively parallel, heterogeneous cluster architectures. We discuss a memory efficient construction of an oct-tree structure to sample the mass densities with locally adaptive resolution, according to the features of the underlying tetrahedral tessellation. We propose an efficient GPU implementation for the computationally intensive operation of intersecting the tetrahedra with the cubical cells of the deposit grid, that achieves a speedup of almost an order of magnitude compared to an optimized CPU version. We discuss two dynamic load balancing schemes - the first exchanges particle data between cluster nodes and deposits all tetrahedra for each block of the grid structure on single nodes, whereas the second approach uses global reduction operations to obtain the total masses. We demonstrate the scalability of our algorithms for up to 256 GPUs and TB-sized simulation snapshots, resulting in tessellations with over 400 billion tetrahedra.

Electrostatic simulation of a complete cluster deposition apparatus.

A complete electrostatic model of a cluster deposition apparatus is presented using SIMION. It consists of fifteen different ion optical components including a quadrupole mass filter and a quadrupole ion deflector. The accuracy of the model was tested by comparing calculated cationic cluster transmissions with experimental ion currents by varying the electrostatic potential of different components. Considering the negatively charged particles produced by the magnetron cluster source as a charged background with a density of 5⋅10-7 cm-3, the influence of the first components on cluster transmission is well reproduced in comparison to the experimental results. This background was included by increasing the charge of the clusters from zero to an elementary charge using a sigmoidal function. The inflection point of this function was found to depend on the first components' electrostatic potential but in good approximation, not on later ones. All of the calculated transmissions represent the experimental data quite well; therefore, the simulation is validated and helps us to understand the influence of the electrostatic components on cluster transmission and improve the target efficiency. Furthermore, this understanding opens the possibility for a global optimization scheme to be employed in the ion optics' geometries.

Synthesis of layered platelets by self-assembly of rhenium-based clusters directed by long-chain amines

Self-assembly of nanomaterials by wet chemistry methods is a suitable approach for the preparation of engineered structures with novel functionalities. In this work, we study the ability of long-chain amines to direct the growth of a layered nanomaterial, using [RexSeyClz] clusters as building blocks. The amines link to the clusters as ligands during the synthesis, directing the self-assembly due to their amphiphilic properties, which produces a platelet-shaped 2D material with sizes up to several μm in diameter and thicknesses in the range of 60–80 nm. This is, to the best of our knowledge, the first report on a one-step mild chemistry method for the preparation of 2D structures composed of alternate layers of self-assembled amines and sub-nm clusters of a rhenium chalcogenide. Furthermore, these materials can be used as a suitable source of clusters which then, conveniently released by a simple acid/base reaction, have been successfully incorporated to the surface of graphene. The simple clusters deposition method developed here offers a promising route towards the preparation of hybrid clusters/2D materials with outstanding properties arising from quantum confinement effects combined with high surface areas and the enormous compositional variety of 2D materials and clusters. These hybrids are expected to play a key role in the development of active materials for applications ranging from highly efficient energy storage systems, more active catalysts and upper-sensitivity gas sensors.

Water-soluble single source precursors for homo- and hetero-metallic nanoparticle catalysts supported on nanocarbons

Homo- and hetero-metallic catalysts were prepared in water from a single metal source and in a single step, and were tested in cinnamaldehyde selective hydrogenation. Two new water-soluble clusters, namely [Ru5C(CO)14(TPPTS)1] and [Ru5PtC(CO)15(TPPTS)1], were synthesized by the addition of phosphine TPPTS to starting homo- and hetero-metallic clusters soluble in organic solvents. They were then impregnated onto nanocarbons (CNF and CNT) in water at different pH values. Cluster deposition occurred via two adsorption modes and was the most successful when attractive electrostatic interactions took place. Thermal activation of these samples led to heterogeneous catalysts with a small mean particle size between 2 and 4nm. The home-made catalysts showed increased conversions and selectivities (up to 73%) towards the thermodynamically unfavored product cinnamyl alcohol compared to a commercial catalyst. The selectivity was in general higher with CNF support, with less oxygenated function and with bigger particles. The selectivity increased slightly with the addition of platinum to ruthenium but the activity was enhanced tremendously (up to 57% at t=30min).

Tetragonal-Like Phase in Core–Shell Iron Iron-Oxide Nanoclusters

Two sizes of iron/iron-oxide (Fe/Fe-oxide) nanoclusters (NCs) of 10 and 35 nm diameters were prepared using a cluster deposition technique. Both these NCs displayed X-ray diffraction (XRD) peaks due to body-centered cubic (BCC) Fe0 and magnetite-like phase. 57Fe Mössbauer spectroscopy measurements confirmed (a) the core–shell nature of the NCs, (b) the Fe-oxide shell to be nanocrystalline and partially oxidized beyond magnetite, and (c) the Fe-oxide spins are significantly canted. In addition to the BCC Fe and magnetite-like phases, a phase similar to tetragonal σ-Fe-Cr (8% Cr) was clearly evident in the larger NC, based on XRD. The origin of the tetragonal-like phase in the larger NC could be due to significant distortion of the Fe0 core lattice planes; subtle peaks due to this phase were also apparent in the smaller NC. Unambiguous evidence for the presence of such a phase was not clear from X-ray photoelectron spectroscopy, vibrating sample magnetometry, X-ray magnetic circular dichroism, or transmission electron microscopy. 57Fe Mössbauer spectroscopy, however, provided support for a non-BCC Fe0 phase in the strongest sixth and the first resonance lines of the Fe0 sextet. To our knowledge, this is the first report of a tetragonal-like phase in the Fe/Fe-oxide core–shell systems.

Copper Cluster Size Effect in Methanol Synthesis from CO2

Size-selected Cun catalysts (n = 3, 4, 20) were synthesized on Al2O3 thin films using mass-selected cluster deposition. A systematic study of size and support effects was carried out for CO2 hydrogenation at atmospheric pressure using a combination of in situ grazing incidence X-ray absorption spectroscopy, catalytic activity measurement, and first-principles calculations. The catalytic activity for methanol synthesis is found to strongly vary as a function of the cluster size; the Cu4/Al2O3 catalyst shows the highest turnover rate for CH3OH production. With only one atom less than Cu4, Cu3 showed less than 50% activity. Density functional theory calculations predict that the activities of the gas-phase Cu clusters increase as the cluster size decreases; however, the stronger charge transfer interaction with Al2O3 support for Cu3 than for Cu4 leads to remarkably reduced binding strength between the adsorbed intermediates and supported Cu3, which subsequently results in a less favorable energetic pathway t...

Stabilization of copper catalysts for hydrogenation of dimethyl oxalate by deposition of Ag clusters on Cu nanoparticles

Novel Ag/Cu/SiO2 catalysts have been fabricated by galvanic deposition of Ag clusters on Cu nanoparticles for the first time. Although the Ag clusters in Ag/Cu/SiO2 covered about 20 to 30% of the Cu active surface, the turnover frequency is remarkably enhanced, resulting in a great improvement in the final DMO hydrogenation activity. More importantly, the obtained Ag/Cu/SiO2 catalyst achieves a lasting catalytic activity up to 1100h due to the strong interaction between Cu and Ag, which is among the best records for Cu-based catalysts.