Cluster Projectiles Research Articles

Reducing the Matrix Effect in Organic Cluster SIMS Using Dynamic Reactive Ionization.

Dynamic reactive ionization (DRI) utilizes a reactive molecule, HCl, which is doped into an Ar cluster projectile and activated to produce protons at the bombardment site on the cold sample surface with the presence of water. The methodology has been shown to enhance the ionization of protonated molecular ions and to reduce salt suppression in complex biomatrices. In this study, we further examine the possibility of obtaining improved quantitation with DRI during depth profiling of thin films. Using a trehalose film as a model system, we are able to define optimal DRI conditions for depth profiling. Next, the strategy is applied to a multilayer system consisting of the polymer antioxidants Irganox 1098 and 1010. These binary mixtures have demonstrated large matrix effects, making quantitative SIMS measurement not feasible. Systematic comparisons of depth profiling of this multilayer film between directly using GCIB, and under DRI conditions, show that the latter enhances protonated ions for both components by 4- to ~15-fold, resulting in uniform depth profiling in positive ion mode and almost no matrix effect in negative ion mode. The methodology offers a new strategy to tackle the matrix effect and should lead to improved quantitative measurement using SIMS. Graphical Abstract ᅟ.

CO2 Cluster Ion Beam, an Alternative Projectile for Secondary Ion Mass Spectrometry.

The emergence of argon-based gas cluster ion beams for SIMS experiments opens new possibilities for molecular depth profiling and 3D chemical imaging. These beams generally leave less surface chemical damage and yield mass spectra with reduced fragmentation compared with smaller cluster projectiles. For nanoscale bioimaging applications, however, limited sensitivity due to low ionization probability and technical challenges of beam focusing remain problematic. The use of gas cluster ion beams based upon systems other than argon offer an opportunity to resolve these difficulties. Here we report on the prospects of employing CO2 as a simple alternative to argon. Ionization efficiency, chemical damage, sputter rate, and beam focus are investigated on model compounds using a series of CO2 and Ar cluster projectiles (cluster size 1000-5000) with the same mass. The results show that the two projectiles are very similar in each of these aspects. Computer simulations comparing the impact of Ar2000 and (CO2)2000 on an organic target also confirm that the CO2 molecules in the cluster projectile remain intact, acting as a single particle of m/z 44. The imaging resolution employing CO2 cluster projectiles is improved by more than a factor of two. The advantage of CO2 versus Ar is also related to the increased stability which, in addition, facilitates the operation of the gas cluster ion beams (GCIB) system at lower backing pressure. Graphical Abstract ᅟ.

XPS depth profiling of organic photodetectors with the gas cluster ion beam

In this work, the authors study active layers of organic photodetector devices containing phenyl-C61-butyric acid methyl ester and Poly[(4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5-b′)dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene-)-2-6-diyl)] (PBDTTT-C). Thin films are examined by x-ray photoemission spectroscopy coupled with argon gas cluster ion sputtering. The use of massive cluster projectiles instead of monoatomic ions has the advantage of not destroying the chemical structure of organic materials under study. The authors show how simulated aging influences the chemistry of these blends and how these alterations extend from sample surface into the bulk of the film. The authors identify several possible processes resulting from aging, including C=O bond breakage and PBDTTT-C diffusion.

Computer modeling of angular emission from Ag(100) and Mo(100) surfaces due to Arn cluster bombardment

Molecular dynamics computer simulations are employed to investigate the effect of projectile size and surface morphology on the angular emission stimulated by impact of Ar gas cluster projectiles. Argon clusters of sizes n = 10–1000 and kinetic energies of 10 and 20 keV Arn aimed at normal incidence are used to sputter Ag(100) and Mo(100) samples. The total sputtering yield is larger for Ag(100) than for Mo(100). The ratio of sputtering yields is inversely proportional to the ratio of sublimation energies of these solids for projectiles between Ar20 and Ar250. In both systems, the angular distributions are sensitive to both the projectile size and the surface roughness. The maximum of angular spectra shifts from direction normal to the surface toward off-normal direction with the increase in the projectile size. An opposite trend is observed with the increase in the surface roughness. Formation of a cloud composed of projectile atoms and the enhanced lateral material relocation caused by projectile lateral expansion upon impact are the main factors responsible for promoting off-normal ejection. On the other hand, material ejection from randomly inclined surface areas and the influence of nearby topography are found to be responsible for enhancing ejection along the surface normal for rough surfaces.

Secondary ion emission from a GaAs single crystal upon bombardment with Bi m + cluster ions

Secondary-ion mass spectra and energy distributions upon bombarding a gallium arsenide single crystal using Bim+(m = 1–5) cluster ions with energies of 2–12 keV are investigated. The gallium cluster ion yield grew nonadditively with the number of atoms in the cluster projectiles. A quasi-thermal component found in the energy spectra of secondary Ga+ and Ga2+ ions is indicative of the occurrence of the thermal spike mode upon cluster ion bombardment. The quasi-thermal component in the yield of atomic Ga+ ions upon bombardment with Bi2+–Bi5+–ions is 35–75%.

Physical basis of energy per cluster atom in the universal concept of sputtering

The interpretation of the variables, scaled by the number of projectile cluster atoms n, in the universal relation of the sputtering yield Y versus incident energy E, that is, Y/n vs E/n, is not necessarily obvious. Following on previous works, the objective of this study is to elucidate the physical basis of the energy per atom variable E/n. The authors employ molecular dynamics simulations of Arn cluster bombardment of Ag(111) metal samples for this study. The authors find that the energy per cluster atom quantity E/n is responsible for the fraction of the initial energy that is deposited in the solid, rather than energy per cluster mass E/m. The results show that even though there is an average loss of the energy for a cluster, each cluster atom loses a different fraction of its initial energy, thus yielding a distribution of energy loss by individual atoms. The analysis of these distributions indicates that the energy deposition process is more effective for clusters with higher E/n when compared to the clusters with lower E/n. This conclusion is supported by a visual analysis of the cluster bombardment event. The cluster atoms that lose most of their initial energy are those which split off from the cluster and penetrate into the bulk of the solid. Conversely, the atoms of the clusters with low E/n keep together during the interaction with the solid, and eventually reflect into the vacuum taking away a portion of the initial kinetic energy. In addition, the simulations indicate that the clusters of different sizes have the same distribution of energy loss for individual atoms if they have the same E/n, in other words, if the initial energy E is proportional to the cluster size n.

Hunting shot - evolution of manufacturing technology

Hunting shot are 1.2-10 mm diameter balls, usually made of lead alloys, forming a cluster projectile used in smoothbore hunting shotguns. Shot may also be used in pistol and revolver ammunition, in which it can constitute structural element of the projectile. Shot pellets may also be made of other materials and have other shapes. The aim of this paper is to aggregate information on the topic available from a number of different sources. It is hoped that such information will be useful for forensic ballistics experts. Historical development of pellets and their manufacturing technology from the 15th century is presented.

Dynamic Reactive Ionization with Cluster Secondary Ion Mass Spectrometry

Gas cluster ion beams (GCIB) have been tuned to enhance secondary ion yields by doping small gas molecules such as CH4, CO2, and O2 into an Ar cluster projectile, Arn + (n = 1000–10,000) to form a mixed cluster. The ‘tailored beam’ has the potential to expand the application of secondary ion mass spectrometry for two- and three-dimensional molecular specific imaging. Here, we examine the possibility of further enhancing the ionization by doping HCl into the Ar cluster. Water deposited on the target surface facilitates the dissociation of HCl. This concerted effect, occurring only at the impact site of the cluster, arises since the HCl is chemically induced to ionize to H+ and Cl– , allowing improved protonation of neutral molecular species. This hypothesis is confirmed by depth profiling through a trehalose thin film exposed to D2O vapor, resulting in ~20-fold increase in protonated molecules. The results show that it is possible to dynamically maintain optimum ionization conditions during depth profiling by proper adjustment of the water vapor pressure. H–D exchange in the trehalose molecule M was monitored upon deposition of D2O on the target surface, leading to the observation of [Mn* + H]+ or [Mn* + D]+ ions, where n = 1–8 hydrogen atoms in the trehalose molecule M have been replaced by deuterium. In general, we discuss the role of surface chemistry and dynamic reactive ionization of organic molecules in increasing the secondary ion yield.

Seduction of Finding Universality in Sputtering Yields Due to Cluster Bombardment of Solids.

Universal descriptions are appealing because they simplify the description of different (but similar) physical systems, allow the determination of general properties, and have practical applications. Recently, the concept of universality has been applied to the dependence of the sputtering (ejection) yield due to energetic cluster bombardment versus the energy of the incident cluster. It was observed that the spread in data points can be reduced if the yield Y and initial projectile cluster kinetic energy E are expressed in quantities scaled by the number of cluster atoms n, that is, Y/n versus E/n. The convergence of the data points is, however, not perfect, especially when the results for molecular and atomic solids are compared. In addition, the physics underlying the apparent universal dependence in not fully understood. For the study presented in this Account, we performed molecular dynamics simulations of Arn cluster bombardment of molecular (benzene, octane, and β-carotene) and atomic (Ag) solids in order to address the physical basis of the apparent universal dependence. We have demonstrated that the convergence of the data points between molecular and atomic solids can be improved if the binding energy of the solid U0 is included and the dependence is presented as Y/(E/U0) versus (E/U0)/n. As a material property, the quantity U0 is defined per the basic unit of material, which is an atom for atomic solids and a molecule for molecular solids. Analogously, the quantity Y is given in atoms and molecules, respectively. The simulations show that, for almost 3 orders of magnitude variation of (E/U0)/n, there are obvious similarities in the ejection mechanisms between the molecular and atomic solids, thus supporting the concept of universality. For large (E/U0)/n values, the mechanism of ejection is the fluid flow from a cone-shaped volume. This regime of (E/U0)/n is generally accessed experimentally by clusters with hundreds of atoms and results in the largest yields. For molecular systems, a large fraction of the total energy E is consumed by internal excitation and molecular fragmentation, which are energy loss channels not present in atomic solids. For small (E/U0)/n values, the cluster deforms the surface and the ejection occurs from a ring-shaped ridge of the forming crater rim. This regime of (E/U0)/n is generally accessed experimentally by clusters with thousands of atoms and results in the smallest yields. For the molecular systems, there is little or no molecular fragmentation. The simulations indicate, however, that the representation which includes U0 as the only material property cannot be completely universal, because there are other material properties which influence the sputtering efficiency. Furthermore, neither the Y/n nor Y/(E/U0) representation includes the energy loss physics associated with molecular fragmentation in the high (E/U0)/n regime. The analysis of the universal concept implies for practical applications that if the objective of the experiment is large material removal, then the high energy per cluster atom regime is applicable. If the objective is little or no molecular fragmentation in organic materials, then the low energy per atom regime is appropriate.

Computer simulations of material ejection during C60 and Arm bombardment of octane and β-carotene

Molecular dynamics (MD) computer simulations are used to investigate material ejection and fragment formation during keV C60 and Arm (m=60, 101, 205, 366, 872 and 2953) bombardment of organic solids composed from octane and β-carotene molecules at 0° and 45° impact angle. Both systems are found to sputter efficiently. For the octane system, material removal occurs predominantly by ejection of intact molecules, while fragment emission is a significant ejection channel for β-carotene. A difference in the molecular dimensions is proposed to explain this observation. It has been shown that the dependence of the sputtering yield Y on the primary kinetic energy E and the cluster size n can be expressed in a simplified form if represented in reduced units. A linear and nonlinear dependence of the Y/n on the E/n are identified and the position of the transition point from the linear to nonlinear regions depends on the size of the cluster projectile. The impact angle has a minor influence on the shape of the simplified representation.

Chemistry and sputtering induced by fullerene and argon clusters in carbon‐based materials

Classical MD computer simulations are used as a tool for the theoretical investigation of the processes initiated in carbon‐based materials by the irradiation with C60 and Arn clusters. This paper shows the comparison of data regarding the chemical reactions occurring in polystyrene and fullerite bombarded by C60 and Arn projectiles with the initial energy of 2.5 and 10 keV. Its main focus is to elucidate the influence of material type on the bond breaking as new bond formation processes induced by impinging cluster projectiles. Copyright © 2014 John Wiley & Sons, Ltd.

Cluster knockout reactions

Cluster knockout reactions are expected to reveal the amount of clustering (such as that of α, d and even of heavier clusters such as12C, 16O etc.) in the target nucleus. In simple terms, incident medium high-energy nuclear projectile interacts strongly with the cluster (present in the target nucleus) as if it were existing as a free entity. Theoretically, the relatively softer interactions of the two outgoing particles with the residual nucleus lead to optical distortions and are treated in terms of distorted wave (DW) formalism. The long-range projectile–cluster interaction is accounted for, in terms of the finite range (FR) direct reaction formalism, as against the more commonly adopted zero-range (ZR) distorted wave impulse approximation (DWIA) formalism. Comparison of the DWIA calculations with the observed data provide information about the momentum distribution and the clustering spectroscopic factor of the target nucleus. Interesting results and some recent advancements in the area of (α, 2 α) reactions and heavy cluster knockout reactions are discussed. Importance of the finite-range vertex and the final-state interactions are brought out.

Incomplete fusion reactions at low energies in13C+169Tm system

Aiming to investigate the incomplete fusion processes at low projectile energies, experiments have been carried out for the 13 C + 169 Tm system at ≈ 4-7 MeV/A. Excitation functions for several heavy residues likely to be populated via complete and incomplete fusion processes have been measured using heavy recoil residue catcher technique followed by γ - ray spectroscopy. The measured cross-sections for the complete fusion (xn and pxn ) channels are compared with the statistical model code PACE4, consistently using the same set of parameters. The complete fusion channels are found to be consistent with the model calculations. However, the cross-sections for all the measured α-emitting channels are found to be significantly enhanced over the calculations. Analysis of data indicate a significant fraction of incomplete fusion even at energies as low as 17% above barrier. The present results are discussed in light of the Morgenstern’s systematics. Incomplete fusion strength function is found to be relatively large for alpha cluster projectile i.e. for 12 C as compared to one neutron excess 13 C projectile.

Molecular dynamics simulations of cluster impacts on solid targets: implantation, surface modification, and sputtering

The collisions of cluster projectiles on solid targets were studied using molecular dynamics (MD) simulations. The penetration range and damage induced by small boron and carbon cluster implantations are compared with those induced by monomers with the same energy per atom. The simulations indicated enhanced penetration depth and the formation of dense track damage at the surface region. In addition, large argon and fluorine clusters (up to 1 million atoms) have shown effects such as crater formation and low-damage surface etching of silicon. The MD simulations revealed that cluster penetration and crater formation depends not on the total incident energy, but on the incident energy per atom, and that the damage threshold for the argon cluster is several eV/atom. In the impact of a very large and slow fluorine cluster on silicon, an enhancement of chemical reactivity at the near surface region was observed because of the high density of fluorine molecules depositing kinetic energy.

Sputtering and reflection under cluster bombardment of solids

Using molecular-dynamics simulation, we study the sputtering induced by energetic impacts of projectile clusters containing N=100 and 1000 atoms. Both a metallic target and a van-der-Waals-bonded material are studied. We focus on self-bombardment at perpendicular incidence. The total emission yield is composed of the sputter yield of the target material and the reflection yield of reflected projectile cluster atoms. The dependence of these yields on the impact energy shows a slow transition from reflection- to sputter-dominated emission. Similarities and differences between the emission yield of metals and of van-der-Waals-bonded materials are discussed.

Secondary ion emission from insulin film bombarded with methane and noble gas cluster ion beams

Recent advances in large cluster projectiles for secondary ion mass spectrometry (SIMS) allow the intact ions of some protein molecules to be detected without a matrix. However, detailed mechanisms of soft-sputtering and ionization of biomolecules remain unknown. Herein we investigate the secondary ion emission from insulin films under argon, krypton, and methane cluster ion bombardment. The intact insulin ion intensity significantly decreases for (CH4)1500+ ion bombardment compared with Ar1500+ ion bombardment at the same energy range of 3.3eV/atom (or molecule), even though collisions with energetic methane clusters should generate numerous protons on the surface, which would enhance the ionization probability through proton attachment. In contrast, the intact ion intensity is almost the same for Ar2500+ and Kr2500+ cluster ion bombardment at the same energy range of 2eV/atom. These observations suggest that detailed mechanisms for the ionization and sputtering by gas cluster ions should be investigated to enhance the intact ion intensity.

Application of the surface ionization for the detection of secondary particles in the secondary-ion mass spectrometry (SIMS)

A method for postionization in the course of ion sputtering that is based on surface ionization of the sputtered particles is developed. The estimations show that the method allows a significant increase in the sensitivity of the secondary-ion mass spectrometry for several elements. Nonadditive increase in the sputtering coefficient of indium is experimentally studied using the surface-ionization method of postionization when the number of atoms in projectile clusters Bim+ (m = 1–7) increases at energies 2–10 keV. Such a scheme for the detection of neutral particles can be used in alternative methods for the surface analysis, in particular, laser evaporation of surface and electron-stimulated desorption.

Sputtering of Benzene Sample by Large Ne, Ar and Kr Clusters - Molecular Dynamics Computer Simulations

Molecular dynamics simulations are employed to probe the role of an impact angle on emission e ciency of organic molecules sputtered from benzene crystal bombarded by 15 keV Ne2953, Ar2953, and Kr2953 clusters. It is found that both the cluster type and the angle of incidence have signi cant e ect on the emission e ciency. The shape of the impact angle dependence does not resemble the dependence characteristic for medium size clusters (C60, Ar366), where sputtering yield only moderately increases with the impact angle, has a shallow maximum around 40◦ and then decreases. On the contrary, for the large projectiles (Ne2953, Ar2953, and Kr2953) the emission e ciency steeply increases with the impact angle, has a pronounced maximum around 55◦ followed by rapid signal decay. It has been found that the sputtering yield is the most sensitive to the impact angle change for Kr cluster projectiles, while change of the impact angle of Ne projectile has the smallest e ect on the e ciency of material ejection.

Computer simulation of cluster impact induced electronic excitation of solids

We present a computational study of electronic excitation upon bombardment of a metal surface with cluster projectiles. Our model employs a molecular dynamics (MD) simulation to calculate the particle dynamics following the projectile impact. Kinetic excitation is implemented via two mechanisms describing the electronic energy loss of moving particles: autoionization in close binary collisions and a velocity proportional friction force resulting from direct atom–electron collisions. Two different friction models are compared with respect to the predicted sputter yields after single atom and cluster bombardment. We find that a density dependent friction coefficient leads to a significant reduction of the total energy transferred to the electronic sub-system as compared to the Lindhard friction model, thereby strongly enhancing the predicted sputter yield under cluster bombardment conditions. In contrast, the yield predicted for monoatomic projectile bombardment remains practically unchanged.

Sputtering of Al nanoclusters by 1–13 keV monatomic or polyatomic ions studied by Molecular Dynamics simulations

Molecular Dynamics (MD) simulations were employed to study sputtering of freestanding and supported spherical Al nanoclusters with 2–10nm diameters under bombardment by Al1 and Al13 projectiles with energies of 1–13keV at normal and oblique incidence. Both monatomic and clustered yields of secondary emission are found to be larger than those for (111) flat surface of the bulk Al (at equal irradiation conditions). In some events, target nanocluster receives a backward momentum and therefore its major part (more than 1/2 of the mass) is ejected due to produced secondary emission mainly towards the substrate direction. This “recoil effect” is found more pronounced under the impact of cluster projectiles and its probability decreases with increase of the target cluster size. A restricted number of MD simulations were performed to verify whether this “recoil effect” is strong enough to desorb a 4nm Al nanocluster off an Al (111) substrate. Desorption was observed under oblique Al13 impact within the impact parameter range of 0.6–0.9.