Desorption of nanoclusters from gold nanodispersed layers by 72 keV Au400 ions: Experiment and molecular dynamics simulation

The interaction of 72 keV Au₄₀₀ ions (with a diameter of approximately 2 nm) with nanodispersed gold targets has been studied. These interactions are dominated by elastic collisions. The gold nanodispersed target with 2-12 nm nanoislets was bombarded with a fluence of 1.7 × 10¹² ions/cm². The desorbed nanoclusters were collected on carbon foils supported by TEM-grids. Intact 29 nm gold nanoclusters were found on the collectors. The desorption yield (normalized to the total cross-section of the projectile-cluster interaction) was estimated to be 0.62 nanocluster/projectile. Preliminary estimates were made using molecular dynamic simulations for comparison with the experimental results.     

Desorption of gold nanoclusters from gold nanodispersed targets by 200 keV Au5 polyatomic ions in the elastic stopping mode: Experiment and molecular-dynamics simulation

Au nanoislet targets (∅ 2-60 nm) were bombarded by 200 keV polyatomic ions (40 keV/atom), which deposit their energy mainly in the nuclear stopping mode: ∑(dE/dx)_n = 30 keV/nm and ∑(dE/dx)_e = 2 keV/nm. The matter desorbed in the form of nanoclusters was registered by TEM. The total transfer of matter was determined by neutron-activation analysis. The total yield of the ejected gold reached high values of up to 2.6 × 10⁴ atoms per Au₅ ion. The major part (2 × 10⁴ atoms per ion Au₅) of the emission is in the form of nanoclusters. The results are compared with the data of similar experiments with 1 MeV Au₅ (200 keV/atom) and other projectiles. The analysis of the experimental data and the comparison to molecular-dynamics simulation results of the desorption process show that the desorption of Au nanoislets is induced by their melting, build-up of pressure and thermal expansion.     

Modification of poly(methyl methacrylate) by keV Ar deposition

Classical molecular dynamics simulations are used to examine 1 keV Ar atom bombardment on the surface of poly(methyl methacrylate) (PMMA), which induces sputtering and chemical modifications to the surface. The simulations are carried out at various surface temperatures that range from 200 to 600 K. The results indicate that different fragments of PMMA, as characterized by their mass, are preferentially sputtered from the surface at the various temperatures considered. In addition, the simulations predict that small fragments are produced by the high energy deposition process. However, larger sized fragments are generated when the surface temperature is close to the glass transition temperature of PMMA. The atomic-scale processes by which these occur are elucidated by the simulations.     

Effect of spin polarization of Ni(1 1 0) surface on Auger neutralization for grazing scattering of He ions

The survival of ions during grazing scattering of keV He⁺ ions from a clean Ni(1 1 0) surface is studied as function of target temperature. We observe ion fractions in the scattered beams of typically 10⁻³ which show a slight increase with temperature of the target surface. From computer simulations of projectile trajectories we attribute this enhancement for ion fractions to effects of thermal vibrations of lattice atoms on the survival of ions in their initial charge state. Based on concepts of Auger neutralization, we discuss the role of the spin polarization of target electrons on charge transfer. We do not find corresponding signatures in our data and conclude that in the present case of Ni(1 1 0) the spin polarization has to be small.     

Sputtering of Langmuir-Blodgett multilayers by keV C60 projectiles as seen by computer simulations

Fundamental processes induced in a thick organic system composed of long, well-organized linear molecules by an impact of 5-20 keV C₆₀ are investigated. The organic system is represented by Langmuir-Blodgett multilayers formed from bariated molecules of arachidic acid. The thickness of the system varies between 2 and 16 nm. Coarse-grained molecular dynamics computer simulations are applied to investigate the energy transfer pathways and sputtering yields as a function of the kinetic energy of the projectile and the thickness of the organic overlayer.The results indicate that an impact of keV C₆₀ projectiles leads to significant ejection of organic material. The efficiency of desorption increases with the kinetic energy of the projectile for a given layer thickness. For a constant primary kinetic energy, the sputtering yield goes through a maximum and finally saturates as the LB layer becomes thicker. Such behaviour is caused by a competition between signal enhancement due to increasing number of organic molecules and signal decrease due to lowering of the amount of the primary energy being backreflected into the organic overlayer by the receding organic/metal interface as the layer is getting thicker. When the sample thickness becomes much larger than the penetration depth of the projectile, the sputtering yield is independent of thickness. The deposited energy is channelled by an open and ordered molecular structure, which leads to abnormally long projectile penetration and ion-induced damage.     

Effect of adsorbates on the formation of doubly excited He atoms during impact of He ions on a Ni(1 1 0) surface

The formation of doubly excited states of He atoms during impact of He²⁺ ions with projectile energies of 60-1000 eV under near-grazing angles of incidence of 5°-20° on clean and adsorbate-covered Ni(1 1 0) surfaces is studied by means of Auger electron spectroscopy. Pronounced dependencies of electron spectra from autoionization of atoms in doubly excited 2s², 2s2p and 2p² configurations on the coverage of the target surface with adsorbates are observed. These are directly related to work function changes, which are studied for the controlled adsorption of oxygen. Changes of the electron spectra on the target temperature are found for adsorbate-covered surfaces only, which puts into question recent interpretations of similar electron spectra in terms of a high local electron spin polarization of Ni(1 1 0) by an alternative interpretation based on thermal desorption or dissolution into bulk of surface contaminations. The formation of doubly excited states is studied for the oxygen p(2 × l) and p(3 × l) superstructures on Ni(1 1 0) in order to provide well-defined experimental data for theoretical investigations.     

Alkali ion scattering from Ag(0 0 1) and Ag thin films at low and hyperthermal energies

We have investigated the scattering of K⁺ and Cs⁺ ions from a single crystal Ag(0 0 1) surface and from a Ag-Si(1 0 0) Schottky diode structure. For the K⁺ ions, incident energies of 25 eV to 1 keV were used to obtain energy-resolved spectra of scattered ions at θ_i = θ_f = 45°. These results are compared to the classical trajectory simulation safari and show features indicative of light atom-surface scattering where sequential binary collisions can describe the observed energy loss spectra. Energy-resolved spectra obtained for Cs⁺ ions at incident energies of 75 eV and 200 eV also show features consistent with binary collisions. However, for this heavy atom-surface scattering system, the dominant trajectory type involves at least two surface atoms, as large angular deflections are not classically allowed for any single scattering event. In addition, a significant deviation from the classical double-collision prediction is observed for incident energies around 100 eV, and molecular dynamics studies are proposed to investigate the role of collective lattice effects. Data are also presented for the scattering of K⁺ ions from a Schottky diode structure, which is a prototype device for the development of active targets to probe energy loss at a surface.     

Energy loss straggling in carbon foils by 1-2 MeV/atom lithium and carbon cluster-ions

Energy loss straggling of lithium and carbon cluster-ions with 1-2 MeV/atom energies incident upon thin carbon foil were measured to investigate the difference of energy loss straggling between a single ion and cluster-ions. The results show that the enhancement of the energy loss straggling is observed at lower energies for two different cluster-ions in a 5 μg/cm² carbon foil. For a thicker sample, i.e. a 15 μg/cm² carbon foil, the ratio of cluster-ions to the one of a single atom at equivalent energies lies slightly close with the unity. As increasing the number of the cluster-ions, no enhancement of energy loss straggling was observed in this study.     

Angular spectra of rainbow scattering at glancing keV He bombardment of NiAl(1 0 0) surface with transverse energies in the range 1-10 eV

Previous simulations of glancing incidence ion-surface interaction have demonstrated that classical dynamics using the row-model have successfully reproduced multimodal azimuthal and polar spectra. These studies have also shown considerable sensitivity to the form of the interatomic potential thus making it a strong test of the validity of such potentials and even allow deduction of the ion-surface potentials. In these simulations the individual pairwise interactions between the projectile and the target atoms have been replaced by cylindrical potentials.Comparison to numerous experimental studies have confirmed the existence of rainbow scattering phenomena and successfully tested the validity of the cylindrical potential used in these simulations. The use of cylindrical potentials avoids stochastic effects due to thermal displacements and allows faster computer simulations leading to reliable angular distributions.In the present work we extend the row-model to consider scattering from binary alloys. Using He⁺ scattered at glancing incidence from NiAl surfaces, Al or Ni terminated, a faster method has been developed to easily and accurately quantize not only the maximum deflection azimuthal angle but all the singular points in the angular distribution. It has been shown that the influence of the surface termination on the rainbow angle and the inelastic losses is small.     

Sputtering of cryogenic films of hydrogen by keV ions: Thickness dependence and surface morphology

The sputtering yield induced by keV hydrogen ions measured at CERN and at Risø National Laboratory for solid H₂ and D₂ at temperatures below 4.2 K decreases with increasing film thickness from about 100 × 10¹⁵ molecules/cm². For a film thickness comparable to or larger than the ion range the data from Risø show a slight increase, whereas the yield from CERN continues to decrease up to very large film thicknesses, i.e. one order of magnitude larger than the ion range. The different behavior of the yield is discussed in terms of the probable growth modes of the films. The films produced at the Risø setup are quench-condensed films, while those produced at CERN are supposed to grow with large hydrogen aggregates on top of a thin bottom layer.     

Possible diamond-like nanoscale structures induced by slow highly-charged ions on graphite (HOPG)

The interaction between slow highly-charged ions (SHCI) of different charge states from an electron-beam ion trap and highly-oriented pyrolytic graphite (HOPG) surfaces is studied in terms of modification of electronic states at single-ion impact nanosize areas. Results are presented from AFM/STM analysis of the induced-surface topological features combined with Raman spectroscopy. I-V characteristics for a number of different impact regions were measured with STM and the results argue for possible formation of diamond-like nanoscale structures at the impact sites.     

Temperature-dependent surface modification of InSb(0 0 1) crystal by low-energy ion bombardment

Structural and compositional modification of InSb(0 0 1) single crystal surfaces induced by oblique incidence 2-5 keV Ar and Xe ion irradiation have been investigated by means of scanning tunneling and atomic force microscopies, and time-of-flight mass spectroscopy of secondary ion emission. In general, ion-induced patterns (networks of nanowires, or ripples) are angle of incidence- and fluence-dependent. Temperature dependences (from 300 to 600 K) of the RMS roughness and of the ripple wavelength have been determined for the samples bombarded with various fluences. Secondary ion emission from an InSb(0 0 1) surface exposed to 4.5 keV Ar⁺ ions has been investigated with a linear TOF spectrometer working in a static mode. Mass spectra of the sputtered In⁺, Sb⁺ and In₂⁺ secondary ions have been measured both for the non-bombarded (0 0 1) surface and for the surface previously exposed to a fluence of 10¹⁶ ions/cm². In⁺ and In₂⁺ intensities for the irradiated sample are much higher in comparison to the non-bombarded one, whereas Sb⁺ ions show a reversed tendency. This behavior suggests a significant In-enrichment at the InSb(0 0 1) surface caused by the ion bombardment.     

380 keV proton irradiation effects on photoluminescence of Eu-doped GaN

The effect of 380 keV proton irradiation on the photoluminescence (PL) properties has been investigated for undoped and Eu-doped GaN. As the proton irradiation exceeds , a drastic decrease of PL intensity of the near band-edge emission of undoped GaN was observed. On the other hand, for Eu-doped GaN, the PL emission corresponding to the ⁵D₀→⁷F₂ transition in Eu³⁺ kept the initial PL intensity after the proton irradiation up to . Present results, together with our previous report on electron irradiation results, suggest that Eu-doped GaN is a strong candidate for light emitting devices in high irradiation environment.     

Microstructural investigation of alumina implanted with 30 keV nitrogen ions

Among ceramics, alumina is being widely used as biomaterials now these days. It is being used as hip joints, tooth roots etc. Ion implantation has been employed to modify its surface without changing it bulk properties. 30 keV nitrogen with varying ion dose ranging from 5 × 10¹⁵ ions/cm² to 5 × 10¹⁷ ions/cm² is implanted in alumina. Surface morphology has been studied with optical microscope and atomic force microscope (AFM). Improvement in brittleness has been observed with the increase in ion dose. Compound formation and changes in grain size have been studied using X-Ray diffraction (XRD). AlN compound formation is also observed by Fourier transform infrared spectroscopy (FTIR). The change in the grain size is related with the nanohardness and Hall-Petch relationship is verified.     

Strain-driven (0 0 2) preferred orientation of ZnO nanoparticles in ion-implanted silica

We present an experimental and theoretical study on the structural properties of ZnO nanoparticles embedded in silica. The ZnO-SiO₂ nanocomposite was prepared by ion implanting a Zn⁺ beam in a silica slide and by annealing in oxidizing atmosphere at 800 °C. From an experimental point of view, the structural properties of the ZnO-SiO₂ nanocomposite were studied by using glancing incidence X-ray diffraction. According to the results, zinc crystalline nanoclusters with an average diameter of 13 nm are in the as-implanted sample. The annealing in oxidizing atmosphere promotes the total oxidation of the Zn nanoclusters and increases their size until to an average of 22 nm. Moreover, the formed ZnO nanocrystals have a preferential (0 0 2) crystallographic orientation. From a theoretical point of view, the preferential orientation of the ZnO nanoparticles can be explained satisfactory by the minimization of the strain energy of the nanoparticles placed in proximity of the surface of the matrix.     

Competition of terrace and step-edge sputtering under oblique-incidence ion impact on a stepped Pt(1 1 1) surface

Using molecular-dynamics simulation, we study the sputtering of a Pt(1 1 1) surface under oblique and glancing incidence 5 keV Ar ions. For incidence angles larger than a critical angle ?_c, the projectile is reflected off the surface and the sputter yield is zero. We discuss the azimuth dependence of the critical angle ?_c with the help of the surface corrugation felt by the impinging ion. If a step exists on the surface, sputtering occurs also for glancing incidence ?>?_c. We demonstrate that for realistic step densities, the total sputtering of a stepped surface may be sizable even at glancing incidence.     

Energy dependence of electron stopping powers in elemental solids over the 100 eV to 30 keV energy range

We analyzed the energy dependence of electron stopping powers (SPs) calculated for 41 elemental solids from experimental optical data for electron energies between 100 eV and 30 keV. Our analysis was performed with the Hill equation to represent a series of steps in plots of the slopes of Fano plots. The average root-mean-square difference between SPs from fits with an equation derived from the Hill equation and the calculated SPs was 1.0%. The new equation can provide SPs over a wide energy range for Monte Carlo simulations of electron transport with the continuous slowing-down approximation.     

Be AMS measurements at low energies (E < 1 MeV)

The longlived isotope ¹⁰Be is of great importance in earth sciences for dating applications, reconstruction of the solar activity or in climate research. Routine AMS measurements with BeO samples are performed on accelerators with a terminal voltage above 2 MV. Applying the degrader foil technique for boron suppression, first tests with BeO samples on the 0.6 MV ETH/PSI machine were limited by background to a ¹⁰Be/⁹Be ratio of 10⁻¹³. The background was identified as ⁹Be which reaches the detector by scattering processes. By applying an additional magnetic mass filter to the high energy mass spectrometer the background was effectively removed. A ¹⁰Be/⁹Be background ratio of <5 × 10⁻¹⁵ was achieved. The overall efficiency (detected ¹⁰Be compared to BeO⁻ injected into the accelerator) was 7-8%.     

Desorption of gold nanoclusters (2–150 nm) by 1 GeV Pb ions

Nanodispersed targets of gold (grains sized at 2–150 nm) were irradiated with 956 MeV ions of Pb⁵⁴⁺ ((dE/dx)_e in gold 87 keV/nm). Ejected gold was gathered on collectors. Desorbed nanoclusters of gold were detected by means of TEM while the total matter transfer was measured by neutron activation analysis. For all the targets a part of ejected gold presents nanoclusters in the same size range as that of the grains on the corresponding targets. Desorption of nanoclusters with the size up to 90 nm was observed for the first time for atomic primary ions in the electronic stopping regime. The yield of the desorbed nanoclusters decreases from 22 to 1.4 cluster/ion with increasing the mean grain size from 6 to 30 nm. The total matter transfer measured for the target with the grain size 6–10 nm has a great value – 5 × 10⁵ at./ion. Results are discussed.     

Optical degradation of silicone in ZnO/silicone white paint irradiated by <200 keV protons

By compared with ZnO powders and silicone resin, the optical degradation of the ZnO/silicone white paint caused by <200 keV protons was investigated, and the damage mechanism was revealed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectrum analysis (FTIR). Experimental results show that the optical degradation mainly occurs in the visible region and the absorption band shifts towards longer wavelengths with increasing proton fluence. Under the proton irradiation, the resistance to optical degradation of the ZnO/silicone white paint is less than the ZnO powders and better than the silicone resin. Such a change can be related to the damage of the silicone. The damage of the silicone binder enhances the degradation of ZnO/silicone white paint, whereas the existence of ZnO pigment relieves the degradation of the silicone binder. The major damage product changes from the cyclotrisiloxane in the silicone resin to the net Si-CO structure in the silicone binder of the ZnO/silicone white paint.