Ar Cluster Research Articles

Angular distributions of particles sputtered from multicomponent targets with gas cluster ions

The experimental angular distributions of atoms sputtered from polycrystalline W, Cd and Ni based alloys with 10keV Ar cluster ions are presented. RBS was used to analyze a material deposited on a collector. It has been found that the mechanism of sputtering, connected with elastic properties of materials, has a significant influence on the angular distributions of sputtered components. The effect of non-stoichiometric sputtering at different emission angles has been found for the alloys under cluster ion bombardment. Substantial smoothing of the surface relief was observed for all targets irradiated with cluster ions.

Computer study of the removal of Cu from the graphene surface using Ar clusters

The method of molecular dynamics has been used to study the bombardment of a copper film on supported graphene by Ar13 clusters with kinetic energies of 5, 10, 20 and 30eV and different angles of incidence. It is obtained that the cluster energy should be in the interval 20–30eV for effective graphene cleaning. There is no cleaning effect at vertical incidence (θ=0°) of Ar13 clusters. The bombardments at 45° and 90° incident angles are the most effective ones at a moderate and big amount of deposited copper respectively.

Dual beam depth profiling of organic materials: assessment of capabilities and limitations

With the advent of large argon cluster beams, organic materials can be sputtered without the accumulation of radiation damage. The dual beam mode with a Bi cluster analysis beam is successfully applied to depth profiling and 3D analysis of organic materials providing both high‐depth resolution and high‐lateral resolution. For the analysis of very small sample volumes, it is desirable that a rather large fraction of the material is consumed by the analysis beam and contributes to the analytical signal. However, at higher analysis beam sputter rates, the radiation damage by the Bi clusters can become quite severe leading to a molecular ion signal decay with depth. In this paper, we investigate the conditions for the optimum use of the sample material in the dual beam mode using the Irganox delta layer structure introduced by NPL (National Physical Laboratory (UK)). A model that describes the dual beam mode and allows calculating the intact area fraction in the steady state is presented. Yield volumes and damage volumes are determined for Bi3+ and Bi5++ as well as the useful sample fraction that contributes to the molecular ion signal. We will compare the results to the depth profiling with C60 and Ar clusters in a single DC beam mode. Copyright © 2014 John Wiley & Sons, Ltd.

Ar Cluster Sputtering of Polymers: Effects of Cluster Size and Molecular Weights

Ar Cluster Sputtering of Polymers: Effects of Cluster Size and Molecular Weights

Ar cluster ion beam sputtering of InSb investigated by ToF‐SIMS

Ar gas cluster ion beam (GCIB) sputtering of InSb was investigated, and the sputtering rate and depth resolution of time‐of‐flight secondary ion mass spectrometry sputter depth profiling were examined. The practical sputtering rate for InSb using 20 keV Ar2500+ irradiation was confirmed at the incident angles of 0°, 40°, and 60°, and the sputtering rate was too low to perform sputtering at 80°. The preferential sputtering and surface roughness observed under GCIB irradiation was lower than that observed under 3 keV Ar+ irradiation. Characteristic ripple formation was confirmed under 20 keV Ar2500+ irradiation and had a strong influence on the depth resolution. The highest depth resolution was obtained at the incident angle of 0° because no ripple formation occurred and the level of surface roughness was low among the irradiation of GCIB at incident angles of 0°, 40°, and 60°. The present results revealed that GCIB sputtering is an effective method for sputter depth profiling of InSb. Copyright © 2014 John Wiley & Sons, Ltd.

Re-print of “Sputtering of polymers by keV clusters: Microscopic views of the molecular dynamics”

This article reviews the results of molecular dynamics simulations of cluster sputtering of hydrocarbon polymers obtained in the last few years and expand them with unpublished data of Ar cluster bombardment. The targets are molecular solids of linear hydrocarbons, polyethylene and polystyrene, including a polyethylene substrate decorated with adsorbed globular macromolecules. The projectiles are (hydro)carbon and Arn clusters, from small to massive, as well as Bin and Au400 clusters. The study focuses on the dynamics of cratering and sputtering, using a coarse-grained representation of the samples, on the study of molecular fragmentation, crosslinking and free H formation, using a fully atomistic model, and on the conditions of desorption of macromolecules by massive clusters. The results explain the similarities and differences between several cluster types and sizes and, to a large extent, the sputtering yields of Arn clusters and their ‘universal’ dependence on the scaled cluster energy, as observed in the experiments. They also demonstrate the reduction of sample fragmentation and crosslinking when going to larger clusters and the incidence angle dependence of intact macromolecule emission. Recent experimental validations obtained in our laboratory are also introduced and comparisons with data obtained by other groups are discussed in order to present a more complete picture of the physics of cluster bombardment of organic solids and polymers.

Fundamental aspects of Arn+ SIMS profiling of common organic semiconductors

We investigate the capabilities and limitations of performing (quantitative) in‐depth composition analysis of polymer:fullerene blends, using time‐of‐flight SIMS with large Arn+ clusters. These blends constitute an important class of organic solar cell materials, whose performance and reliability critically depend on the blend's physicochemical properties at the nanoscale. We investigate the effect of Ar cluster size n, kinetic energy E and E/n on the secondary ion and sputter yields and evaluate the Ar beam‐induced damage in these layers. On the basis of this, the ideal experimental conditions are derived that allow us to investigate matrix effects and variations in sputter yields as function of polymer:fullerene compositions. We demonstrate that time‐of‐flight SIMS is capable to retrieve quantitative in‐depth information in common polythiophene, polycarbazole, and fullerene blends. Copyright © 2014 John Wiley & Sons, Ltd.

Composition of Ar–Kr, Kr–Xe, and N2–Ar Clusters Produced by Supersonic Expansion of Gas Mixtures

Size and composition of clusters produced by adiabatic expansion of binary gas mixtures (Ar-Kr, Kr-Xe, and N2-Ar) with various component concentrations are studied by using electron-diffraction technique. The resulting homogeneous and heterogeneous clusters are shown to have compositions substantially different from those of the primary gas mixtures and dependent on cluster size. We have found that the key parameters needed for an analysis of cluster composition are the critical cluster radius and the heavier component concentration in the gas mixture that can be used to establish the regions of existence of homogeneous and heterogeneous clusters. These critical values determine the coefficient of the enrichment of clusters with the heavier component with respect to its concentration in the primary gas mixture. Theoretical relations are obtained which provide a good quantitative description of the experimental results and can be expected to be also valid for finding the composition of binary clusters of other van der Waals systems.

Deformation and clustering in excited states of 42Ca

The coexistence of various low-lying deformed states in |$^{42}$|Ca and the |$\alpha $|–|$^{38}$|Ar correlations in those deformed states have been investigated using deformed-basis antisymmetrized molecular dynamics. Wave functions of the low-lying states are obtained via parity and angular momentum projections and the generator coordinate method (GCM). Basis wave functions of the GCM calculation are obtained via energy variations with constraints on the quadrupole deformation parameter |$\beta $| and the distance between |$\alpha $| and the |$^{38}$|Ar clusters. The rotational bands built on the |$J^\pi = 0_2^+$| (1.84 MeV) state as well as the |$J^\pi = 0_3^+$| (3.30 MeV) state are both reproduced. The coexistence of two additional |$K^\pi = 0^+$| rotational bands is predicted; one band is shown to be built on the |$J^\pi = 0_3^+$| state. Members of the ground-state band and the rotational band built on the |$J^\pi = 0_3^+$| state contain |$\alpha $|–|$^{38}$|Ar cluster structure components.

Sputtering of polymers by keV clusters: Microscopic views of the molecular dynamics

This article reviews the results of molecular dynamics simulations of cluster sputtering of hydrocarbon polymers obtained in the last few years and expand them with unpublished data of Ar cluster bombardment. The targets are molecular solids of linear hydrocarbons, polyethylene and polystyrene, including a polyethylene substrate decorated with adsorbed globular macromolecules. The projectiles are (hydro)carbon and Arn clusters, from small to massive, as well as Bin and Au400 clusters. The study focuses on the dynamics of cratering and sputtering, using a coarse-grained representation of the samples, on the study of molecular fragmentation, crosslinking and free H formation, using a fully atomistic model, and on the conditions of desorption of macromolecules by massive clusters. The results explain the similarities and differences between several cluster types and sizes and, to a large extent, the sputtering yields of Arn clusters and their ‘universal’ dependence on the scaled cluster energy, as observed in the experiments. They also demonstrate the reduction of sample fragmentation and crosslinking when going to larger clusters and the incidence angle dependence of intact macromolecule emission. Recent experimental validations obtained in our laboratory are also introduced and comparisons with data obtained by other groups are discussed in order to present a more complete picture of the physics of cluster bombardment of organic solids and polymers.

Molecular imaging of biological tissue using gas cluster ions.

An Ar n+ (n = 1-6000) gas cluster ion source has been utilized to map the chemical distribution of lipids in a mouse brain tissue section. We also show that the signal from high mass species can be further enhanced by doping a small amount of CH4 into the Ar cluster to enhance the ionization of several biologically important molecules. Coupled with secondary ion mass spectrometry instrumentation which utilizes a continuous Ar cluster ion projectile, maximum spatial resolution and maximum mass resolution can be achieved at the same time. With this arrangement, it is possible to achieve chemically resolved molecular ion images at the 4-µm resolution level. The focused Ar n+/[Ar x (CH4) y ]+ beams (4-10 µm) have been applied to the study of untreated mouse brain tissue. A high signal level of molecular ions and salt adducts, mainly from various phosphocholine lipids, has been seen and directly used to map the chemical distribution. The signal intensity obtained using the pure Ar cluster source, the CH4-doped cluster source and C60 is also presented.

Novel SIMS system with focused massive cluster ion source for mass imaging spectrometry with high lateral resolution

Recent developments in cluster ion beams for secondary ion mass spectrometry (SIMS) have enabled the realization of molecular depth profiling and mass imaging of organic and biological materials. Massive Ar cluster beams present reduced surface damage and fragmented ion generation and are suitable as primary beams for SIMS. We recently obtained a finely focused massive 1.2-µm-diameter cluster ion beam and combined it with an orthogonal acceleration time-of-flight mass spectrometer. A mesh pattern of a phospholipid thin film was clearly reproduced in the mass images of molecular ions with a measurement time of 100 s.

Electron capture at low velocity in the collision of Ar17+ ions with atoms, clusters and solids

Absolute x-ray emission has been measured when Ar17+ ions at 255 keV collide either with atomic gaseous targets of Ar or N2, with Ar clusters or with thin carbon foils. Preliminary results show a strong decrease of the x-ray signal with the backing pressure of the cluster supersonic jet compared to ion-atom interaction. This effect is a clear signature of slow highly charged ions interacting with clusters. Additionally, high resolution x-ray technique gives us access to the charge state distribution of the emitting ions.

Size dependent transition to solid hydrogen and argon clusters probed via spectroscopy of PTCDA embedded in helium nanodroplets.

Complexes made of either Ar(N) or (H2)N clusters (N = 1-170) and a single PTCDA molecule (3,4,9,10-perylene-tetracarboxylic-dianhydride) are assembled inside helium droplets and spectroscopically studied via laser-induced fluorescence spectroscopy. The frequency shift and line-broadening are analyzed as a function of N and of the pick-up order of the PTCDA and cluster material in order to track liquid or solid properties of the clusters. For argon, the solid phase is observed for N > 10 above which the pick-up order dramatically influences the localization of the chromophore with respect to the Ar cluster. If the droplets are doped first with Ar, the chromophore remains on the surface of a solid cluster whereas for the reversed pick-up order the molecule is surrounded by an argon shell. At N < 10 wetting and the formation of the first solvation shell are observed. For para-hydrogen, a transition to the solid is observed at N ~ 20-25, confirming previous theoretical predictions on the existence of a liquid-like phase at such small sizes, even below the bulk hydrogen freezing temperature.

Ar cluster sputtering of polymers: effects of cluster size and molecular weights

This fundamental contribution on time‐of‐flight secondary ion mass spectrometry polymer depth‐profiling by massive argon clusters is focused on the investigation of the influence of the Ar cluster size and the molecular weight of the investigated polymers on the efficiency of the sputtering process. For this purpose, 100‐nm thin films of polymethylmethacrylate and polystyrene, with a range of molecular weights (103–1.5 × 105 amu) spin‐coated onto Si wafers, are sputtered by 10 keV Arn+ cluster ions with selected sizes (1500, 3000 and 5000 atoms/cluster) employing Bi5+ as an analysis source. The experiments show that the sputtering yield volume (nm3/primary ion), Y, decreases with the increase of the molecular weight, Mw, for a selected cluster size, and at constant molecular weight, Y decreases with the increase in cluster size. The experimental results are in good agreement with molecular dynamics simulations and previous studies for high molecular weight PS. Furthermore, the trend of Y versus Mw seems to be related to the variation of the glass transition temperature (Tg) of the investigated polymers. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Ab initio study of the structures and electronic states of small neutral and ionic DABCO – Arn clusters

In the present theoretical work, we investigated the stationary points (minima and transition states) on the ground state potential energy surfaces of neutral and ionic 1,4-diazabicyclo[2.2.2]octane (DABCO)--Ar(n)⁰,⁺¹ (n = 1-4) clusters. As established in our systematic work on DABCO--Ar cluster (Mathivon et al., J Chem Phys 139:164306, 2013), the (R)MP2/aug-cc-pVDZ level is accurate enough for validating the prediction of stable forms. For n = 1 and 2, further computations at the MP2/aug-cc-pVTZ level confirm these assumptions. We show that some of the already known isomers of these heteroclusters derived using lower levels of theory are not realistic. More interestingly, our work reveals that DABCO is subject to slight deformations when binding to a small number of Ar atoms. Moreover, we computed the potential energy surfaces of the lowest singlet electronic states of DABCO--Ar(n)(n = 1-3) and of DABCO⁺--Ar(n)(n = 1-3), and the transition moments for the Sp(p = 1-3) ← S0 neutral transitions. These electronic states are found to be Rydberg in nature. The shape of their potentials is mainly repulsive with slight stabilization in the S2 potentials. Finally, the effects of microsolvation of DABCO in Ar clusters in ground and electronic excited states are discussed. The photophysical and photochemical dynamics of these electronic states may be complex.

A generalized recipe to construct elementary or multi-step reaction paths via a stochastic formulation: Application to the conformational change in noble gas clusters

• We demonstrate a general strategy to map out reaction paths irrespective of the number of kinetic steps involved. • The objective function proposed does not need the information of gradient norm and eigenvalue of Hessian matrix explicitly. • A stochastic optimizer Simulated Annealing is used in searching reaction path. • The strategy is applied in mapping out the path for conformational changes in pure Ar clusters and Ar N Xe mixed clusters. In this paper we demonstrate a general strategy to map out reaction paths irrespective of the number of kinetic steps required to bring about the change. i.e., whether the transformation takes place in a single step or in multiple steps with the appearance of intermediates. The objective function proposed is unique and works equally well for a concerted or a multiple step pathway. As the objective function proposed does not explicitly involves the calculation of the gradient of the potential energy function or the eigenvalues of the Hessian Matrix during the iterative process, the calculation is computationally economical. To map out the reaction path, we cast the entire problem as one of optimization and the solution is done with the use of the stochastic optimizer Simulated Annealing. The formalism is tested on Argon clusters ( Ar N ) and Argon clusters singly doped with Xenon ( Ar N - 1 Xe ). The size of the systems for which the method is applied ranges from N = 7 - 25 , where N is the total number of atoms in the cluster. We also test the results obtained by us by comparing with an established gradient only method. Moreover to demonstrate that our strategy can overcome the standard problems of drag method, we apply our strategy to a two dimensional LEPS + harmonic oscillator Potential to locate the TS, in which standard drag method has been seen to encounter problems.

Study of argon/hydrogen mixed cluster in supersonic gas jet

The average mixed cluster sizes in different mixing proportions of Ar-CH4 mixed cluster and Ar-H2 mixed cluster in supersonic gas jet are studied by Rayleigh scattering method. It is found that Ar-CH4 mixed cluster could form easily when the mixed Ar and CH4 gas are used in gas jet, and the maximum cluster size is achieved when the content of Ar is 50%. The maximum cluster size of Ar-CH4 mixed cluster is larger than that of either Ar cluster or CH4 cluster. Being different from pure hydrogen cluster which only forms at liquid nitrogen temperature, Ar-H2 mixed cluster can form at room temperature. So this is the first time we have obtained hydrogen cluster at room temperature. Ar-H2 mixed cluster starts to form at H2 content value higher than 40% and it reaches maximum size when the content of H2 is 60%. Hydrogen (deuterium) mixed clusters introduce heavier Ar element on the basis of hydrogen (deuterium) clusters. It will further accelerate the deuterium ions to higher energy in deuterium cluster laser fusion experiments, so we can obtain higher neutron yield and fusion efficiency.

Caging of Cl atoms from photodissociation of CF2Cl2in clusters

We investigate the photodissociation dynamics of freon CF2Cl2 by velocity map imaging at 193 nm. The Cl fragment images are recorded in various expansion regimes corresponding to isolated molecules and molecules in rare gas clusters. The molecular kinetic energy distributions are dominated by a peak at Ekin(Cl) ≈ 0.97 eV corresponding to the direct C-Cl bond dissociation but they also reveal features at lower kinetic energies. Possible mechanisms leading to these slow Cl atoms are discussed. The photodissociation in clusters is investigated in two regimes: (i) small Ar clusters with the CF2Cl2 molecule embedded in approximately one solvation Ar layer; (ii) large Xen, n¯ ≈ 100-500, clusters with embedded CF2Cl2 molecules. In the former clusters we observe caging yielding the Cl fragments with zero kinetic energy and direct exit resulting in fragments with the kinetic energies corresponding to the fragments from isolated molecules. In the latter case (ii) only the caged Cl fragments are observed.

Reduced Preferential Sputtering of TiO&lt;sub&gt;2&lt;/sub&gt; using Massive Argon Clusters

In this study, we demonstrate low damage etching of titania for depth profile analysis using large Ar cluster ions. Sample damage caused by impinging ion beams is discussed and a comparison is made between monatomic Ar+ ions and Arn+ cluster ions with respect to surface damage. Monatomic ions of beam energy 5 kV cause considerable changes to the structure of amorphous titania resulting in the preferential removal of oxygen and damage to the lattice structure. Titanium is reduced from the +4 oxidation state to +3 and +2 oxidation state during this preferential removal. In contrast, the use of clusters greatly diminishes this damage and also reduces the incorporation of the impinging ion into the sample surface. Here we discuss the use of Arn+ clusters as a new methodology for reducing the damaging effects caused by ion bombardment.