Energetic Cluster Research Articles

Stopping of energetic cobalt clusters and formation of radiation damage in graphite

The interaction of energetic (up to 200 eV/atom) size-selected ${\text{Co}}_{n}$ clusters with HOPG is studied both experimentally and theoretically. Etching of the radiation damaged areas introduced by cluster impacts provides a measure of the depth to which the collision cascades are developed and allows a comparison of these data with the molecular dynamics simulations. Good agreement between the experimental results and modeling is obtained. It is shown that the projected range of the cluster constituents can be linearly scaled with the projected momentum (the cluster momentum divided by surface impact area). With decrease in cluster energies to ca. 10 eV/atom the transition from implantation to pinning is suggested. It is found that even after quite energetic impacts residual clusters remain intact in the shallow graphite layer. These clusters can catalyze reaction of atmospheric oxygen with damaged graphite areas under the thermal heating that leads to the formation of narrow (5--15 nm) random in shape surface channels (trenches) in the top few graphene layers. Thus, small imbedded Co nanoparticles can be used as a processing tool for graphene.

An investigation of the internal temperature dependence of Pd–Pt cluster beam deposition: A molecular dynamics study

We investigated the internal temperature dependence of the Pd 1− a Pt a cluster beam deposition in the present study via the molecular dynamics simulations of soft-landing. By analysis of the velocity distribution and diffusion coefficient of the bimetallic cluster, Pd atoms with better mobility improved the diffusibility of Pt atoms. The radial composition distribution showed that a Pt-core/Pd-shell structure of the cluster formed at high internal temperatures through migrations of the Pd atoms from inner to surface shells. In the soft-landing process, the diffusing and epitaxial behaviors of the deposited clusters mainly depended on the internal temperature because the incident energy of the cluster was very small. By depositing clusters at high internal temperatures, we obtained a thin film of good epitaxial growth as the energetic cluster impact. Furthermore, nonepitaxial configurations such as scattered nonepitaxial atoms, misoriented particles, and grain boundaries of (1 1 1) planes were produced in the growth of the cluster-assembled film. As the size of the incident cluster increased, the internal temperature of the cluster needed for better interfacial diffusion and contact epitaxy on the substrate also rose.

Correlated growth of organic material tris (8-hydroxyquinoline) aluminum (Alq3) and its relation to optical properties

We report slow correlated growth mode in energetic cluster vapor deposited organic light emissive material tris(8-hydroxyquinoline) aluminum from 5 to 100 nm. Phase modulated atomic force microscopy shows very slow grain growth with thickness, with very small phase differences within the film. Fractal dimension calculated from correlation function shows growth process above 10 nm consistent with diffusion-limited aggregation. For low thickness (5 nm), photoluminescence measurement shows the emission peak is shifted by ∼0.4 eV toward lower wavelength.

The effect of cluster density on the penetration depth of an energetic large gas cluster impinging on a silicon target

Large accelerated gas clusters are being increasingly employed in a number of different surface treatment and analysis schemes, from semiconductor doping, through surface smoothing to the probing beam for secondary ion mass spectrometry (SIMS). Although the use of such clusters is finding great new application and providing potentially new and exciting capabilities in materials processing and analysis some of the effects of key parameters in the process are less understood and inadequately studied. The gas clusters are normally created by an aerosol effect in which droplets rapidly expand after passing through a fine nozzle and cool quickly and form frozen droplets. The precise density of the frozen droplets is not well known and it is possible that the clusters could have a density ranging from a perfect solid packing solid density to that of a frozen gas. The work presented here uses molecular dynamics computer simulation to investigate the likely effect that changes in the cluster density will have on the processing properties of large argon gas clusters. In particular it is found that the penetration depth of the energy deposited by the cluster is dependent upon the cluster density.

Electrochemically stable gold nanoclusters in HOPG nanopits

Gold nanoparticles on highly oriented pyrolytic graphite (HOPG) electrodes are synthesized. They are stabilized in nanosized pits of well defined depth in the graphite surface. These pits are created by energetic cluster impact followed by etching under a well controlled oxygen atmosphere. We succeeded in the preparation of highly dispersed and stable Au electrodes with gold particles with a mean diameter smaller than 5 nm. The stability of the gold nanostructures for electrochemical applications has been tested by performing cyclic voltammetric measurements in 0.5 M H 2SO 4. While conventionally prepared sputter deposited electrodes show highly unstable structures in this size range, Au clusters stabilized in the nanosized containers are stable.

Dynamics of Molecular Impacts on Soft Materials: From Fullerenes to Organic Nanodrops

The present theoretical study explores the interaction of various energetic molecular projectiles and clusters with a model polymeric surface, with direct implications for surface analysis by mass spectrometry. The projectile sizes (up to 23 kDa) are intermediate between the polyatomic ions (SF(5), C(60)) used in secondary ion mass spectrometry and the large organic microdroplets generated, for example, in desorption electrospray ionization. The target is a model of amorphous polyethylene, already used in a previous study [Delcorte, A.; Garrison, B. J. J. Phys. Chem. C 2007, 111, 15312]. The chosen method relies on classical molecular dynamics (MD) simulations, using a coarse-grained description of polymeric samples for high energy or long time calculations (20-50 ps) and a full atomistic description for low energy or short time calculations (<1 ps). Two regions of sputtering or desorption are observed depending on the projectile energy per nucleon (i.e., effectively the velocity). The transition, occurring around 1 eV/nucleon, is identified by a change of slope in the curve of the sputtering yield per nucleon vs energy per nucleon. Beyond 1 eV/nucleon, the sputtering yield depends only on the total projectile energy and not on the projectile nuclearity. Below 1 eV/nucleon, i.e., around the sputtering threshold for small projectiles, yields are influenced by both the projectile energy and nuclearity. Deposition of intact molecular clusters is also observed at the lowest energies per nucleon. The transition in the sputtering curve is connected to a change of energy deposition mechanisms, from atomistic and mesoscopic processes to hydrodynamic flow. It also corresponds to a change in terms of fragmentation. Below 1 eV/nucleon, the projectiles are not able to induce bond scissions in the sample. This region of molecular emission with minimal fragmentation offers new analytical perspectives, out of reach of smaller molecular clusters such as fullerenes.

Energy partitioning within a micro-particle cluster due to impact with a rigid planar wall

A combined finite and discrete element method is used to examine the energetics of a two-dimensional micro-particle cluster that impacts a rigid planar wall. The method combines conservation principles with an elastic-viscoplastic and friction constitutive theory to predict thermomechanical fields within particles resulting from both particle-wall and particle-particle contact. Emphasis is placed on characterizing the temporal and spatial partitioning of cluster energy with impact angle (0◦ ≤ \({\phi}\) ≤ 80◦, where \({\phi}\) = 0◦ corresponds to normal impact). Predictions for a close-packed cluster of well-resolved particles having an average initial radius and uniform speed of 50μm and 300 m/s indicate that particles adjacent to the wall experience the largest plastic and friction work. Friction significantly affects cluster kinetic energy, but minimally affects its elastic strain energy and plastic work. Local temperature rises in excess of 900K are predicted for \({\phi}\) = 0◦, increasing to 4,400K for approximately \({\phi}\) > 60◦, with most of the cluster mass (≈98%) experiencing temperature rises less than 200K due to plastic work. These predictions highlight the importance of friction work as a heating mechanism that may induce combustion of energetic clusters. Sensitivity of the cluster response to its initial packing configuration is demonstrated.

Collapse dynamics of laser-nucleated bubble clusters in a spherical resonator.

The collective collapse of bubble clouds or clusters is important to many applications, including underwater sound propagation, shock-wave lithotripsy, and focused ultrasound therapy. Energetic collective cluster collapses appear to happen only when there is sufficient nuclei density. In order to study this phenomenon, we have implemented a laser-based method for creating clusters consisting of planar or cubic arrays of bubbles with controllable interbubble spacing. The growth and collapse of the arrays are observed using ultrahigh-speed photography and other diagnostics. Effects of interbubble spacing and nucleation phase with respect to the acoustic period are studied. [Work supported by the US Army Space and Missile Command.]

The microstructure and magnetic behavior of Co nanostructured film prepared by energetic cluster beam deposition

Cobalt nanostructured films were successfully prepared by depositing Co clusters onto a Si(100) substrate in our home-made cluster beam deposition apparatus. During preparation, the energy of the cluster beam was accelerated by applying an external electric field. The effect of the cluster beam energy on the microstructure, phase transformation and magnetic behavior was investigated for the cluster-assembled film in the case of both low energy cluster deposition and energetic cluster deposition. The scanning electron microscopy observation confirms that the films are assembled by the regular spherical nanoclusters. Compared with the low-energy Co cluster-assembled film, the magnetization of the energetic Co cluster-assembled film was improved significantly. The present work describes an effective way to prepare the magnetic nanostructured films.

Production of equal sized atomic clusters by a hot wire

A resistively heated metal wire is shown to be a source of charged atomic clusters consisting of only a few atoms. They are size classified with a differential mobility analyzer, and their relative abundance is determined as a function of size. Ag n K + clusters are obtained from wires containing silver with traces of potassium to provide the electric charge. First principles calculations reveal that the abundance observed can be fully explained by the energetic and chemical stability of the neutral cluster and K + attachment energy. K + attachment is a non-invasive way of charging, as the observed Ag n K + cluster properties are similar with respect to the pure Ag n clusters in terms of energetic and electronic stability and cluster structure. The equally sized clusters are basically available for reactivity, coalescence and deposition studies, and the method is extendable to other materials.

What do we want from computer simulation of SIMS using clusters?

Computer simulation of energetic cluster interactions with surfaces has provided much needed insight into some of the complex processes which occur and are responsible for the desirable as well as undesirable effects which make the use of clusters in SIMS both useful and challenging. Simulations have shown how cluster impacts can cause meso-scale motion of the target material which can result in the relatively gentle up-lift of large intact molecules adsorbed on the surface in contrast to the behaviour of single atom impacts which tend to create discrete motion in the surface often ejecting fragments of adsorbed molecules instead. With the insight provided from simulations experimentalists can then improve their equipment to best maximise the desired effects. The past 40 years has seen great progress in simulation techniques and computer equipment. 40 years ago simulations were performed on simple atomic systems of around 300 atoms employing only simple pair-wise interaction potentials to times of several hundred femtoseconds. Currently simulations can be performed on large organic materials employing many body potentials for millions of atoms for times of many picoseconds. These simulations, however, can take several months of computation time. Even with the degree of realism introduced with these long time simulations they are still not perfect are often not capable of being used in a completely predictive way. Computer simulation is reaching a position where by any more effort to increase its realism will make it completely intractable to solution in a reasonable time frame and yet there is an increasing demand from experimentalists for something that can help in a predictive way to help in experiment design and interpretation. This paper will discuss the problems of computer simulation and what might be possible to achieve in the short term, what is unlikely ever to be possible without a major new break through and how we might exploit the meso-scale effects in cluster impacts to simplify the simulations.

Sputtering of organic molecules by clusters, with focus on fullerenes

Energetic cluster beam bombardment of organic samples involves a number of physical and chemical effects that are interconnected in a complex manner. In this labyrinth for the scientist's mind, molecular dynamics (MD) simulation constitutes our Ariadne's thread, because it provides the time-resolved microscopic view of the desorption process that is needed to interpret experiments. By combining molecular ion yield and energy distribution measurements with MD simulations, it is possible to explore desorption and ionization from thin organic films and polymers. Recent results show that the overall dynamics and, to a large extent, the details of the cluster-induced sputtering process are well described using a simpler coarse-grain prescription instead of a full atomistic model, allowing us to speed up computations and leap towards larger systems and higher projectile energies. In this article, we first describe some of the characteristic mechanisms of energetic cluster and especially fullerene projectile bombardment of organics. Then, open issues arising from recent experimental investigations are addressed, such as the importance of specific chemical reactions and ionization processes that are outside the scope of the model. Finally, the discussion highlights cases where the predictive nature of the simulations should be used to facilitate the work of experimentalists, such as testing the properties of new cluster projectiles.

MOLECULAR-DYNAMICS STUDY OF ENERGETIC CLUSTER IMPACT ON SILICON

To investigate the dynamic deformation behavior of silicon induced by the energetic cluster impact at an atomistic level, cluster impact simulations are carried out using molecular dynamics. Clusters are emitted to the silicon (100) surface with external kinetic energies varying from 1 to 10 eV/atom. While a structural phase transformation is identified as the dominant deformation mechanism in silicon due to the increased pressure caused by the energetic cluster impact, its deformation pathway tends to change according to the kinetic energy of cluster. In the lower energy region, the initial diamond structure of silicon is first transformed into the beta-tin structure during the impact process, and then this high-pressure structure is transformed into an amorphous structure after the impact process is completed. This result is very similar to those computed in the quasi-static deformation. On the other hand, amorphous structures are directly transformed from initial diamond structure in the higher energy region. Additionally, the propagation of the shock-wave is accompanied in this deformation.

Computational Analysis of Dissipative Heating Due to Impact Between a Micro‐Particle Cluster and a Deformable Wall

Abstract: A combined finite and discrete element method is used to examine the energetics of a micro‐particle cluster that impacts a deformable planar wall. The method combines conservation principles with a penalty based, two‐dimensional (2‐D) distributed potential force algorithm, and an elastic‐viscoplastic and friction constitutive theory, to predict thermomechanical fields within the wall and cluster resulting from both particle‐wall and particle‐particle contact. Emphasis is placed on characterizing the temporal and spatial partitioning of wall and cluster energy for both normal (θ= 0°, whereθis incidence angle) and oblique (θ= 45°) impact. Predictions for an initially close‐packed cluster of well‐resolved particles, each having an initial radius and speed of 50μm and 500 m/s, indicate that particles adjacent to the wall experience significant dissipative heating due to plastic and friction work. Frictionally induced temperature rises in excess of 2,000 K are predicted for cluster mass located in the immediate vicinity of sliding contact surfaces, even for normal impact, whereas temperature rises near 200 K are predicted for significantly larger cluster mass due to plastic work. Friction is shown to significantly affect cluster kinetic energy, and to weakly affect its elastic potential energy and plastic work. This analysis highlights the importance of friction as a viable heating mechanism that may trigger combustion of energetic clusters.

Magnetic and electrical characteristics in dense Fe–Ni alloy cluster-assembled films prepared by energetic cluster deposition

Fe–Ni alloy cluster-assembled films were obtained by a plasma–gas-condensation-type cluster-deposition method. We studied the magnetic and electrical properties of these assemblies prepared on an electrically grounded substrate [bias voltage (Va) = 0 kV] and on a negatively biased substrate (Va = −20 kV). The packing density and saturation magnetization per volume, Ms, are much larger for the assemblies prepared at Va = −20 kV than those prepared at Va = 0 kV, while the magnetic coercivity, Hc, and electrical resistivity, ρ, are much lower for the assemblies prepared at Va = −20 kV than those prepared at Va = 0 kV. For Ni-rich Fe–Ni alloy cluster-assembled films obtained at Va = −20 kV, the Hc values can become smaller than 160 A/m (the precision limit of our superconducting quantum interference device magnetometer) by adjusting the initial cluster size. The magnetic and electrical properties of Fe–Ni cluster-assembled films are much improved in comparison with those of pure Fe cluster-assembled films.

Soft magnetic property and magnetic exchange correlation in high-density Fe-Co alloy cluster assemblies

Magnetically soft Fe-Co alloy cluster-assembled films were produced at room temperature by an energetic cluster deposition. Size-monodispersed Fe-Co alloy clusters whose average sizes ranged between 8 and 14 nm were obtained using a plasma-gas-condensation-type cluster deposition apparatus. Positively charged clusters in a cluster beam were accelerated electrically and deposited onto a negatively biased substrate together with neutral clusters from the same cluster source, leading to formation of high-density Fe-Co alloy cluster-assembled films. Magnetic properties were observed for these films prepared by changing the bias voltage. Based on the temperature dependence of magnetic coercivity, we estimated the effective magnetic anisotropy constant, Keff, and magnetic exchange length Lex (or Dex) as a function of the packing fraction in the Fe-Co cluster-assembled films: Keff decreases and Lex (or Dex) increases dramatically with the packing fraction.

The computer simulation of energetic cluster–solid interactions

There is a growing interest in the application of accelerated clusters and molecular species as both implantation and analysis tools. Large clusters have been in use for the deposition of films and the soft landing of biomaterials for a number of years now. More recently, the use of fullerene as a sputtering ion in SIMS has lead to a revitalisation of academic interest in the technique–as witnessed by the recent SIMS XV conference–and a host of new potential applications in the identification of molecular species and molecular imaging. The use of even larger clusters employed with electro-spraying techniques has also been developed as a new analysis technique (DESI) created. The application of giant gas clusters combining a component of ‘active’ ingredient in a large inert cluster is making a large impact in the semiconductor world to provide a new method (‘infusion doping’) for shallow junction formation in silicon and for providing a tool for surface smoothing. It is clear that there are an increasing number of applications of energetic clusters in the materials world. In this paper, computer simulation techniques are employed to investigate why clusters are of such interest. In particular the role of cluster impacts on molecular desorption will be investigated and the effects of variable energy deposition for large gas clusters in infusion doping will be highlighted.

Specificity in Transmembrane Helix-Helix Interactions Mediated by Aromatic Residues

Aromatic residues have been previously shown to mediate the self-assembly of different soluble proteins through pi-pi interactions (McGaughey, G. B., Gagne, M., and Rappe, A. K. (1998) J. Biol. Chem. 273, 15458-15463). However, their role in transmembrane (TM) assembly is not yet clear. In this study, we performed statistical analysis of the frequency of occurrence of aromatic pairs in a bacterial TM data base that provided an initial indication that the appearance of a specific aromatic pattern, Aromatic-XX-Aromatic, is not coincidental, similar to the well characterized QXXS motif. The QXXS motif was previously shown to be both critical and sufficient for stabilizing TM self-assembly. Using the ToxR system, we monitored the dimerization propensities of TM domains that contain mutations of interacting residues to aromatic amino acids and demonstrated that aromatic residues can adequately stabilize self-association. Importantly, we have provided an example of a natural TM domain, the cholera toxin secretion protein EpsM, whose TM self-assembly is mediated by an aromatic motif (WXXW). This is, in fact, the first evidence that aromatic residues are involved in the dimerization of a wild type TM domain. The association mediated by aromatic residues was found to be sensitive to the TM sequence, suggesting that aromatic residue motifs can provide a general means for specificity in TM assembly. Molecular dynamics provided a structural explanation for this backbone sequence sensitivity.

Quadratic Friction Model for Cluster Bombardment of Molecular Solids

Molecular dynamics simulations of energetic cluster bombardment (C 20 , C 60 , C 120 , C 180 ) are performed in the range of 5-20 keV on a solid benzene substrate. The goal is to study the trends exhibited by these bombardment events and to develop a simple analytical model that explains the general dynamics associated with cluster projectiles. These results indicate that the dynamics of these clusters can be described by a macroscale quadratic friction model and thus can provide an interpretation for the dependencies on cluster mass and size, as well as the observation that velocity of the cluster relative to its initial value decays exponentially with time with a decay constant that linearly depends on the initial velocity.

Magnetic Nanocrystalline Films Softened by Obliquely Accelerating Iron Nanoclusters

We have generated magnetically ultrasoft thin films composed of iron nanoclusters with diameters of about 20 nm by energetic cluster impact (ECI). The clusters were created in a sputtering-gas-aggregation cluster source with approximately 40% of the particles negatively charged. The charged particles were accelerated onto substrates held at potentials of 0, 5, and 15 kV. The substrates were tilted at an angle of 30deg with respect to the incident beam to generate elliptically shaped clusters and thereby induce shape anisotropy. Visible changes of the film structures were observed for increasing potential with the zero potential samples being black due to the porous nature of soft landed clusters. For the 5 and 15 kV samples, the films take on a metallic sheen due to the increased density. The magnetic properties of the films were measured by SQUID at room temperature. Dramatic softening of the films was observed for samples with applied potentials, in good agreement with the random anisotropy model. The coercivity at zero potential was measured to be 78 Oe. Along the easy axis, the coercivities of the 5 and 15 kV samples were 0.50 and 0.65 Oe and 2.06 and 2.46 Oe for the hard axis, respectively. The coercivities for the axis perpendicular to the substrates were 9.86 and 6.70 Oe. In this experiment, we have demonstrated a novel technique for directly controlling film morphology and thereby ability to custom tailor magnetic characteristics