Structural and chemical evolution of Au-silica core-shell nanoparticles during 20 keV helium ion irradiation: a comparison between experiment and simulation.

Au-silica core-shell nanoparticles have been irradiated with 20 keV He+ ions up to a maximum fluence of 4.7 × 1017 ions/cm2. The nanoscale structural and crystallographic evolution induced by He+ ion irradiation was followed at various stages using Transmission Electron Microscopy (TEM). During irradiation satellite Au clusters are formed around the main Au core, which remained crystalline even after the maximum He+ ion fluence. The spherical silica shell deformed into a hemisphere due to He+ ion irradiation. Three dimensional Monte-Carlo simulations, based on the binary collision approximation, have been performed on stacked infinite layers and an individual particle. The stacked layers results show that the He+ beam interacts with most of the nanoparticle and Au migrates in the direction of beam incidence agreeing with experimental findings. The individual particle results match the experiment in terms of the volume which is sputtered away however additional mechanisms, not included in the simulations, are present in the experiment during the satellite formation and silica shell deformation. These results show the ability for 20 keV He+ ions to be used for the modification of nanostructures. Furthermore, these results contribute to a quantitative understanding of the dynamic evolution of materials observed using microscopy techniques based on He+ ions.

Time-of-flight secondary ion mass spectrometry in the helium ion microscope.

A helium ion microscope, known for high resolution imaging and modification with helium or neon ions, has been equipped with a time-of-flight spectrometer for compositional analysis. Here we report on its design, implementation and show first results of this powerful add-on. Our design considerations were based on the results of detailed ion collision cascade simulations that focus on the physically achievable resolution for various detection limits. Different secondary ion extraction geometries and spectrometer types are considered and compared with respect to the demands and limitations of the microscope. As a result the development and evaluation of a secondary ion extraction optics and time-of-flight spectrometer that allows the parallel measurement of all secondary ion masses is reported. First experimental results demonstrate an excellent mass resolution as well as high-resolution secondary ion imaging capabilities with sub-8 nm lateral resolution. The combination of high resolution secondary electron images and mass-separated sputtered ion distributions have a high potential to answer open questions in microbiology, cell biology, earth sciences and materials research.

Nanometer scale elemental analysis in the helium ion microscope using time of flight spectrometry.

Time of flight backscattering spectrometry (ToF-BS) was successfully implemented in a helium ion microscope (HIM). Its integration introduces the ability to perform laterally resolved elemental analysis as well as elemental depth profiling on the nm scale. A lateral resolution of ≤54nm and a time resolution of Δt≤17ns(Δt/t≤5.4%) are achieved. By using the energy of the backscattered particles for contrast generation, we introduce a new imaging method to the HIM allowing direct elemental mapping as well as local spectrometry. In addition laterally resolved time of flight secondary ion mass spectrometry (ToF-SIMS) can be performed with the same setup. Time of flight is implemented by pulsing the primary ion beam. This is achieved in a cost effective and minimal invasive way that does not influence the high resolution capabilities of the microscope when operating in standard secondary electron (SE) imaging mode. This technique can thus be easily adapted to existing devices. The particular implementation of ToF-BS and ToF-SIMS techniques are described, results are presented and advantages, difficulties and limitations of this new techniques are discussed.

A high resolution ionoluminescence study of defect creation and interaction.

Helium ion microscopy has been used to investigate the ionoluminescence of NaCl. A 35 keV, sub-nanometer He(+) ion beam was used to generate ionoluminescence. The interaction of ionizing radiation with alkali halides leads to the formation of various crystal defects, in particular so-called color-centers. Their subsequent recombination with charge carriers leads to the emission of visible light. Broad peaks at 2.46 eV and 3.05 eV were measured. We have also investigated the dynamics of defect creation as a function of the beam scanning parameters (current and pixel spacing). The resolution and detection capabilities of ionoluminescence in helium ion microscopy are sensitive to both sample properties and scanning parameters.

Modified energetics and growth kinetics on H-terminated GaAs (110).

Atomic hydrogen modification of the surface energy of GaAs (110) epilayers, grown at high temperatures from molecular beams of Ga and As4, has been investigated by friction force microscopy (FFM). The reduction of the friction force observed with longer exposures to the H beam has been correlated with the lowering of the surface energy originated by the progressive de-relaxation of the GaAs (110) surface occurring upon H chemisorption. Our results indicate that the H-terminated GaAs (110) epilayers are more stable than the As-stabilized ones, with the minimum surface energy value of 31 meV∕Å(2) measured for the fully hydrogenated surface. A significant reduction of the Ga diffusion length on the H-terminated surface irrespective of H coverage has been calculated from the FFM data, consistent with the layer-by-layer growth mode and the greater As incorporation coefficient determined from real-time reflection high-energy electron diffraction studies. Arsenic incorporation through direct dissociative chemisorption of single As4 molecules mediated by H on the GaAs (110) surface has been proposed as the most likely explanation for the changes in surface kinetics observed.

Structure, stresses and stress relaxation of TiN/Ag nanocomposite films.

Morphology, structure and thermal behavior of magnetron sputtered TiN/Ag nanocomposite thin films deposited at 150 degrees C with an Ag content in the range of 7 to 45 at% were characterized. The films were thermally cycled and the relationship between Ag content, film structure and stress development is analyzed. The results indicate that the residual stresses in as-deposited films and the behavior during heating are determined by the film structure and the plastic deformation of the Ag phase. The increasing plastic deformation with increasing Ag content causes significant changes in the stress-temperature behavior. While films with low Ag content show a plateau in compressive stress from deposition temperature up to 280 degrees C followed by stress relaxation, films with higher Ag content exhibit a zero stress level from deposition temperature up to the maximum annealing temperature. During cooling, all films exhibit linear thermo-elastic behavior, where the slope of the stress-temperature curves also depends on the Ag content.

Growth kinetics, structure, and morphology of para-quaterphenyl thin films on gold(111).

The adsorption, desorption, and growth kinetics as well as the thin film morphology and crystal structure of p-quaterphenyl (4P) grown under ultrahigh vacuum conditions on single crystalline Au(111) have been investigated. Thermal desorption spectroscopy (TDS) reveals two distinct first-order peaks attributed to monolayer desorption followed by a zero-order multilayer desorption. The saturation coverage of the full 4P monolayer has been quantitatively measured with a quartz microbalance to be 8 x 10(13) molecules/cm2. Using low energy electron diffraction the structures of the 0.5 and 1 ML (monolayer) adsorbates have been studied, showing highly regular arrangements of the 4P molecules, which are affected by the (111) surface structure. At the transition from 0.5 to 1 ML a structural compression of the overlayer has been observed. The behavior of thicker 4P films has been investigated by combined TDS-XPS (XPS-x-ray photoelectron spectroscopy). A temperature-induced recrystallization process at about 270 K has been observed for a 7 nm thick 4P film grown at 93 K, corresponding to a transition from a disordered layerlike growth to a crystalline island growth. Ex situ optical microscopy and atomic-force microscopy investigations have revealed needle-shaped 4P islands. Applying x-ray diffraction the crystalline order and epitaxial relationship of the 4P films with 30 nm and 200 nm mean thicknesses have been determined.