The disappearance and return of nanoparticles upon low energy ion irradiation.

Ion irradiation of bulk and thin film materials is tightly connected to well described effects such as sputtering or/and ion beam mixing. However, when a nanoparticle is ion irradiated and the ion range is comparable to the nanoparticle size, these effects are to be reconsidered essentially. This study investigates the morphology changes of silver nanoparticles on top of silicon substrates, being irradiated with Ga+ions in an energy range from 1 to 30 keV. The hemispherical shaped nanoparticles become conical due to an enhanced and curvature-dependent sputtering, before they finally disappear. The sputter yield and morphology changes can be well described by 3D Monte Carlo TRI3DYN simulations. However, the combination of sputtering, ion beam mixing, ion beam induced diffusion, and Ostwald ripening at ion energies lower than 8 keV results in the reappearance of new particles. These newly formed nanoparticles appear in various structures depending on the material and ion energy.

Nanoscale modification of one-dimensional single-crystalline cuprous oxide.

In this work we report for the first time a method to modify the surface of Cu2O nanowires in a controllable way and physically weld them into a network form, which contributes to higher electrical conductivity as well as a strong water-repelling nature. We have used state-of-the-art theoretical calculations to support our experimental observations. We demonstrate how varying the irradiation fluence can modulate the surface and decorate the nanowire with a uniform distribution of Cu8O nanocrystals due to preferential sputtering. While several well studied joining techniques are available for carbon and metal-based nanowires, the same information for ceramic nanowires is scarce at present. The current study sheds light into this and a state-of-the-art 3D simulation technique predicts most of the modifications including surface modulation, oxygen depletion and welding. The welded network shows higher electrical conductivity than the unwelded assembly. With Cu2O being of p-type the current ion beam joining technique shows a novel path for fabricating p-i-n junctions or solar cell devices through bottom-up approach. Furthermore, we have explored the response of this network to moisture. Our calculation based on density functional theory predicts the hydrophilic nature of individual copper oxide nanowires both before and after irradiation. However, the network shows a strong water-repelling nature, which has been explained quantitatively using the Cassie-Baxter model.

Site-controlled formation of single Si nanocrystals in a buried SiO2 matrix using ion beam mixing.

For future nanoelectronic devices - such as room-temperature single electron transistors - the site-controlled formation of single Si nanocrystals (NCs) is a crucial prerequisite. Here, we report an approach to fabricate single Si NCs via medium-energy Si+ or Ne+ ion beam mixing of Si into a buried SiO2 layer followed by thermally activated phase separation. Binary collision approximation and kinetic Monte Carlo methods are conducted to gain atomistic insight into the influence of relevant experimental parameters on the Si NC formation process. Energy-filtered transmission electron microscopy is performed to obtain quantitative values on the Si NC size and distribution in dependence of the layer stack geometry, ion fluence and thermal budget. Employing a focused Ne+ beam from a helium ion microscope, we demonstrate site-controlled self-assembly of single Si NCs. Line irradiation with a fluence of 3000 Ne+/nm2 and a line width of 4 nm leads to the formation of a chain of Si NCs, and a single NC with 2.2 nm diameter is subsequently isolated and visualized in a few nanometer thin lamella prepared by a focused ion beam (FIB). The Si NC is centered between the SiO2 layers and perpendicular to the incident Ne+ beam.

Dynamics of nanoparticle morphology under low energy ion irradiation.

If nanostructures are irradiated with energetic ions, the mechanism of sputtering becomes important when the ion range matches about the size of the nanoparticle. Gold nanoparticles with diameters of ∼50 nm on top of silicon substrates with a native oxide layer were irradiated by gallium ions with energies ranging from 1 to 30 keV in a focused ion beam system. High resolution in situ scanning electron microscopy imaging permits detailed insights in the dynamics of the morphology change and sputter yield. Compared to bulk-like structures or thin films, a pronounced shaping and enhanced sputtering in the nanostructures occurs, which enables a specific shaping of these structures using ion beams. This effect depends on the ratio of nanoparticle size and ion energy. In the investigated energy regime, the sputter yield increases at increasing ion energy and shows a distinct dependence on the nanoparticle size. The experimental findings are directly compared to Monte Carlo simulations obtained from iradina and TRI3DYN, where the latter takes into account dynamic morphological and compositional changes of the target.

Shaping and compositional modification of zinc oxide nanowires under energetic manganese ion irradiation.

For ZnO nanowires of 150 to 200 nm diameter standing on a flat substrate, the development of the surface contour/morphology and the local elemental composition under 175 keV Mn irradiation has been investigated both experimentally and by means of three-dimensional dynamic Monte Carlo computer simulation. The simulation results reveal a complex interplay of sputter erosion, implant incorporation, resputtering and atomic mixing, which is discussed in detail. The sputter-induced thinning of the wire is in good quantitative agreement with the experimental results obtained from pre- and post-irradiation scanning electron microscopy. The experiments also confirm the predicted sharpening of the tip, neck formation at the bottom interface, and ultimately the detachment of the nanowires from the substrate at high ion fluence. Additional good agreement with experimental results from nano-x-ray fluorescence is also obtained for the continuously increasing Mn/Zn atomic ratio within the nanowires as a function of ion fluence. The simulation yields a great deal of additional information that has not been accessible in the experiments. From this, preferential sputtering of O compared with Zn is deduced. A significant contamination of the wires with substrate material arises from ion mixing at the wire/substrate interface, rather than from redeposition of sputtered substrate atoms. Surprising hollow profiles are observed. Their formation is attributed to a special mechanism of collisional transport which is characteristic of the irradiation of nanowires at a suitable combination of wire diameter and ion energy.

Optical properties of silver nanowire arrays with 35 nm periodicity.

We present highly ordered Ag nanowire arrays with 35nm periodicity grown on patterned templates. The optical properties measured using generalized ellipsometry exhibit strong anisotropy. Dielectric functions are calculated by fitting the Jones matrix elements with a biaxial layer model, accounting for both metallic behavior and localized surface plasmon resonances. The amplitude and wavelength maximum of the plasmon resonance perpendicular to the wires increase with increasing wire width and thickness. The dielectric coefficients of 10-mm-wide nanowires show a transition behavior from insulating in UV to metallic above 550nm. Their potential application as polarization-dependent plasmonic-scattering transparent conductive electrodes is discussed.

The morphology of amorphous SiO(2) surfaces during low energy ion sputtering.

The morphology of different amorphous or amorphized SiO(2) surfaces, including thermally grown films, fused silica, and single crystalline quartz, during low energy ion sputtering has been investigated by means of atomic force microscopy. For all three materials, the formation of periodic ripple patterns oriented normal to the direction of the ion beam is observed at intermediate incident angles. At near-normal incidence, the SiO(2) surfaces remain flat, whereas a rotation of the ripple patterns is observed at grazing incidence. At intermediate angles, the patterns on the different surfaces exhibit wavelength coarsening of different strengths, which can be attributed to different amounts of near-surface mass transport by the surface-confined ion-enhanced viscous flow. In the framework of the recent hydrodynamic model of ion erosion, the observed differences in ripple coarsening are consistent with this interpretation and indicate that the surface energies of thermally grown SiO(2) and amorphized quartz are lower and higher than that of fused silica, respectively.

Simultaneous formation of two ripple modes on ion sputtered silicon.

The amorphized surface of Si(100) sputtered with low energy ions at moderate temperature was found to develop two perpendicular ripple patterns overlaying each other. The evolution of these patterns was studied over a wide range of fluence. Coarsening of both ripple modes was observed, showing a similar time dependence with a coarsening exponent of n approximately 0.08. In the high fluence regime, the surface enters a steady state with both ripple modes still present.

Anomalous ion accelerated bulk diffusion of interstitial nitrogen.

Interstitial N diffusion under low energy (approximately 700 eV) Ar+ bombardment at 673 K in ion beam nitrided austenitic stainless steel is investigated. Ar+ ion bombardment increases the N mobility in depths far beyond the ion penetration depth, resulting in an increased broadening of the N depth profile as a function of Ar+ flux. This effect cannot be explained by any established mechanism of radiation-enhanced diffusion. An explanation based on quasiparticle-enhanced mobility is proposed.