A. M. Vredenberg

Mega-electron-volt ion beam induced anisotropic plasmon resonance of silver nanocrystals in glass

30 MeV Si ion beam irradiation of silica glass containing Ag nanocrystals causes alignment of Ag nanocrystals in arrays along the ion tracks. Optical transmission measurements show a large splitting of the surface plasmon resonance bands for polarizations longitudinal and transversal to the arrays. The splitting is in qualitative agreement with a model for near-field electromagnetic plasmon coupling within the arrays. Resonance shifts as large as 1.5 eV are observed, well into the near-infrared.

On the initial oxidation of iron: Quantification of growth kinetics by the coupled-currents approach

Polycrystalline iron was oxidized at pO2=10−4 Pa and at temperatures ranging from 300 to 500 K. Ellipsometry was used for monitoring the oxide-film thickness as a function of time. The oxidation kinetics were described quantitatively by application of the model due to Fromhold and Cook by considering coupled currents of cations and electrons. At the lower temperatures tunneling is the dominant electron transport mechanism and an excellent agreement of experimental and calculated oxidation kinetics was obtained by adopting a time-dependent difference of the work functions of the metal-oxide and the oxide-oxygen interfaces. At the higher temperatures the experimental kinetics can be described quantitatively for a film thickness up to about 3 nm. Above this thickness electron transport becomes dominated by thermal emission rather than by tunneling. To investigate the influence of the surface pretreatment on the oxidation kinetics a sample was oxidized at pO2=10−4 Pa at 330 K, both after sputter cleaning and ...

Mechanism of MeV ion induced hydrogen depletion from organic layers

In many materials, the loss of hydrogen during elastic recoil detection (ERD) analysis and nuclear reaction analysis (NRA) with MeV ions is observed. This phenomenon, which is especially pronounced in organic materials, limits the accuracy of the quantitative measurements feasible with these techniques. The MeV ion induced depletion was investigated in an ion beam analysis study, using a wide range of projectiles and energies. From the results we conclude that the evolution of hydrogen from electropolymerized porphyrin layers is molecular, which leads to a model incorporating second order kinetics. With this model, the dependence of hydrogen content on the primary ion flux is described by only the initial amount of hydrogen, the probability of ion induced bond breaking and a characteristic distance within two liberated hydrogen atoms will combine to a hydrogen molecule. When incorporating the energy deposited by the primary ion within the ion track into the model, the observed quadratic dependence of ion induced damage on the stopping power is also explained.

Luminescence quenching in erbium‐doped hydrogenated amorphous silicon

Hydrogenated amorphous silicon thin films are doped with erbium by ion implantation. Room‐temperature photoluminescence at 1.54 μm, due to an intra‐4f transition in Er4+, is observed after thermal annealing at 300–400 °C. Excitation of Er3+ is shown to be mediated by photocarriers. The Er3+ luminescence intensity is quenched by a factor of 15 as the temperature is raised from 10 K to room temperature. Codoping with oxygen (1 at. %) reduces the luminescence quenching to a factor of 7. The quenching is well correlated with a decrease in luminescence lifetime, indicating that nonradiative decay of excited Er3+ is the dominant quenching mechanism as the temperature is increased.

Ion beam-induced anisotropic plastic deformation of silicon microstructures

Amorphous silicon micropillars show anisotropic plastic shape changes upon irradiation with 30 MeV Cu ions. The transverse plastic strain rate is (2.5±0.2)×10−17 cm2/ion at 77 K, which is about one order of magnitude less than that of silica glass. In contrast, crystalline silicon pillars, irradiated under the same conditions, do not exhibit anisotropic deformation. A viscoelastic and free volume model is used to qualitatively describe the data. By irradiating partially amorphous structures a variety of silicon microshapes can be fabricated.

Ion beam shaping of Au nanoparticles in silica: Particle size and concentration dependence

Irradiation with swift heavy ions of spherical Au nanoparticles confined within a silica matrix shapes them into prolate nanorods and nanowires whose principal axes are aligned along the beam direction. In the present paper, we investigate the role that is played by the initial nanoparticle size and concentration in this so-called ion-shaping mechanism. We have produced silica films wherein Au nanoparticles with average diameters of 15, 30, and 45 nm were embedded within a single plane and have irradiated these films at 300 K at normal incidence with 18, 25, and 54 MeV Ag ions. We demonstrate the existence of both threshold and saturation fluences for the elongation effects mentioned. The values of these critical fluences depend both on the ion energy and the initial nanoparticle size. Moreover, we show that 45 nm Au particles are not deformed when irradiated with 18 MeV Ag ions, such that this value corresponds to an energy threshold for the deformation process. As far as the influence of the nanoparticle concentration on the shaping characteristics is concerned, we have found that above the critical irradiation fluence, the deformation effect becomes very sensitive to the initial concentration of the nanoparticles.

Ion engineering of embedded nanostructures: From spherical to facetted nanoparticles

We show that the high-energy ion irradiation of embedded metallic spherical nanoparticles (NPs) is not limited to their transformation into prolate nanorods or nanowires. Depending on their pristine size, the three following morphologies can be obtained: (i) nanorods, (ii) facettedlike, and (iii) almost spherical nanostructures. Planar silica films containing nearly monodisperse gold NPs (8–100 nm) were irradiated with swift heavy ions (5 GeV Pb) at room temperature for fluences up to 5×1013 cm−2. The experimental results are accounted for by considering a liquid-solid transformation of the premelted NP surface driven by the in-plane stress within the ion-deformed host matrix. This work demonstrates the interest of using ion-engineering techniques to shape embedded nanostructures into nonconventional configurations.

Ion-induced elongation of gold nanoparticles in silica by irradiation with Ag and Cu swift heavy ions: track radius and energy loss threshold.

Systematic investigations of the energy loss threshold above which the irradiation-induced elongation of spherical Au nanoparticles occurs are reported. Silica films containing Au nanoparticles with average diameters of 15-80 nm embedded within a single plane were irradiated with 12-54 MeV Ag and 10-45 MeV Cu ions at 300 K and at normal incidence. We demonstrate that the efficiency of the ion-induced nanoparticle elongation increases linearly with the electronic energy transferred per ion track length unit from the energetic ions to the silica film. Ion beam shaping occurs above a threshold value of the specific electronic energy transfer. Three relevant regions are identified with respect to the original size of the Au nanoparticles. For 15 and 30 nm diameter particles, elongation occurs for electronic stopping power larger than 3.5 keV nm(-1). For Au nanoparticles with 40-50 nm diameter an electronic stopping power above 5.5 keV nm(-1) is required for elongation to be observed. Elongation of Au nanoparticles with 80 nm diameter is observed for electronic stopping between ∼ 7-8 keV nm(-1). For all combinations of ions and energies, the ion track temperature profiles are calculated within the framework of the thermal spike model. The correlation between experimental results and simulated data indicates a thermal origin of the increase in the elongation rate with increasing the track diameter.

Characterization of the plasma in a radio-frequency magnetron sputtering system

In order to understand the fundamental mechanisms in a radio-frequency magnetron sputtering system, the main properties of the argon plasma used in the process have been measured. A complete three-dimensional map of the ion density, electron temperature, and plasma potential has been obtained using a Langmuir probe. The electron temperature as well as the ion density have been found to increase in the region of the so called race track at the cathode. Furthermore, from the plasma potential map, the time-averaged local electric field has been obtained, pointing out the race track as the region where the most intense ion bombardment takes place. Besides, only the ions produced near the race track are accelerated towards the cathode, whereas those produced in the remaining volume move towards the anode. Finally, the dependence of the plasma quantities on the incident radio-frequency power and deposition pressure has been studied. The plasma potential measured using the Langmuir probe has been found to agree ...

Determination of photoelectron attenuation lengths in thin oxide films on iron surfaces using quantitative XPS and elastic recoil detection

Thin oxide films play an important role in the corrosion of metals. Using XPS it is possible in principle to obtain information on the chemical state of near-surface atoms, the stoichiometry of the surface layer and its thickness. A problem is the quantification of XPS spectra, due to the large uncertainty in the value of both electron attenuation lengths as well as sensitivity factors. By applying the Tougaard background subtraction method and comparing only peak intensities of the Fe 2p and O 1s peaks with those of Fe, FeO and Fe2O3 reference samples, we determined the stoichiometry of the oxide layer grown on Fe(100) and Fe(110) at 200°C and room temperature to be Fe0.90±0.05O. We combined XPS with the high-energy ion-beam technique of elastic recoil detection (ERD). By comparing the absolute coverages of oxygen determined by ERD with the intensities of different XPS peaks, we were able to determine the attenuation lengths for kinetic energies of 776 eV (Fe 2p) and 957 eV (O 1s) to be 9.3×1015 and 9.9×1015 Fe0.9O molecules cm−2, respectively.