Stefan Facsko

Single ion induced surface nanostructures: a comparison between slow highly charged and swift heavy ions

This topical review focuses on recent advances in the understanding of the formation of surface nanostructures, an intriguing phenomenon in ion-surface interaction due to the impact of individual ions. In many solid targets, swift heavy ions produce narrow cylindrical tracks accompanied by the formation of a surface nanostructure. More recently, a similar nanometric surface effect has been revealed for the impact of individual, very slow but highly charged ions. While swift ions transfer their large kinetic energy to the target via ionization and electronic excitation processes (electronic stopping), slow highly charged ions produce surface structures due to potential energy deposited at the top surface layers. Despite the differences in primary excitation, the similarity between the nanostructures is striking and strongly points to a common mechanism related to the energy transfer from the electronic to the lattice system of the target. A comparison of surface structures induced by swift heavy ions and slow highly charged ions provides a valuable insight to better understand the formation mechanisms.

Ion-Induced Nanoscale Ripple Patterns on Si Surfaces: Theory and Experiment

Nanopatterning of solid surfaces by low-energy ion bombardment has received considerable interest in recent years. This interest was partially motivated by promising applications of nanopatterned substrates in the production of functional surfaces. Especially nanoscale ripple patterns on Si surfaces have attracted attention both from a fundamental and an application related point of view. This paper summarizes the theoretical basics of ion-induced pattern formation and compares the predictions of various continuum models to experimental observations with special emphasis on the morphology development of Si surfaces during sub-keV ion sputtering.

Introducing artificial length scales to tailor magnetic properties

Magnetism is a collective phenomenon. Hence, a local variation on the nanoscale of material properties, which act on the magnetic properties, affects the overall magnetism in an intriguing way. Of particular importance are the length scales on which a material property changes. These might be related to the exchange length, the domain wall width, a typical roughness correlation length, or a length scale introduced by patterning of the material. Here we report on the influence of two artificially created length scales: (i) ion erosion templates that serve as a source of a predefined surface morphology (ripple structure) and hence allow for the investigation of roughness phenomena. It is demonstrated that the ripple wave length can be easily tuned over a wide range (25–175 nm) by varying the primary ion erosion energy. The effect of this ripple morphology on the induced uniaxial magnetic anisotropy in soft magnetic Permalloy films is studied. Only below a ripple wavelength threshold (≈60 nm) is a significant induced magnetic anisotropy found. Above this threshold the corrugated Permalloy film acts as a flat film. This cross-over is discussed in the frame of dipolar interactions giving rise to the induced anisotropies. (ii) Ion implantation through a lithographically defined mask, which is used for a magnetic property patterning on various length scales. The resulting magnetic properties are neither present in non-implanted nor in homogeneously implanted films. Here new insight is gained by the comparison of different stripe patterning widths ranging from 1 to 10 μm. In addition, the appearance of more complicated magnetic domain structures, i.e. spin-flop domain configurations and head-on domain walls, during hard axis magnetization reversal is demonstrated. In both cases the magnetic properties, the magnetization reversal process as well as the magnetic domain configurations depend sensitively on the artificially introduced length scale.

Highly anisotropic effective dielectric functions of silver nanoparticle arrays

Variable-angle and Mueller matrix spectroscopic ellipsometry are used to determine the effective dielectric tensors of random and aligned silver nanoparticles and nanorods thin films. Randomly arranged particles are uniaxially anisotropic while aligned particles are biaxially anisotropic, with the anisotropy predominantly at the plasmonic resonances. The strong resonances in nanorod arrays result in the real part of the effective in-plane permittivities being opposite in sign over a significant range in the visible, suggesting the potential to design materials that display tunable negative-refraction. A structural tilt in the particle arrays results in monoclinic dielectric properties.

Self-organized metallic nanoparticle and nanowire arrays from ion-sputtered silicon templates

We demonstrate a production method for self-organized arrays of metal nanoparticles and aligned nanowires. Ion beam-sputtered Si∕SiO2 substrates are used as templates for metallic vapor deposition, forming aligned arrays of 5–20nm silver and cobalt nanoparticles with a period of 35nm. The 20nm diameter cobalt nanowires with lengths in excess of a micrometer are produced under appropriate conditions. All processing steps can be integrated into a single vacuum chamber and performed in a matter of minutes at mild temperatures. This inherently scalable technique can be extended to a range of substrate materials, array patterns, and nanoparticle materials.

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.

Nanopatterning of Si surfaces by normal incident ion erosion: Influence of iron incorporation on surface morphology evolution

The surface morphology of Si(100) induced by 1200 eV Ar+ ion bombardment at normal incidence with and without Fe incorporation is presented. The formation of nanodot patterns is observed only when the stationary Fe areal density in the surface is above a threshold value of 8×1014 cm-2. This result is interpreted in terms of an additional surface instability due to locally nonuniform sputtering in connection with the presence of a Fe rich amorphous phase at the peak of the nanodots. At Fe concentrations below the threshold, smoothing dominates and pattern formation is inhibited. The transition from a k-2 to a k-4 behavior in the asymptotic power spectral density function supports the conclusion that under these conditions ballistic smoothing and ion-enhanced viscous flow are the two dominant mechanisms of surface relaxation.

Printing nearly-discrete magnetic patterns using chemical disorder induced ferromagnetism

Ferromagnetism in certain alloys consisting of magnetic and nonmagnetic species can be activated by the presence of chemical disorder. This phenomenon is linked to an increase in the number of nearest-neighbor magnetic atoms and local variations in the electronic band structure due to the existence of disorder sites. An approach to induce disorder is through exposure of the chemically ordered alloy to energetic ions; collision cascades formed by the ions knock atoms from their ordered sites and the concomitant vacancies are filled randomly via thermal diffusion of atoms at room temperature. The ordered structure thereby undergoes a transition into a metastable solid solution. Here we demonstrate the patterning of highly resolved magnetic structures by taking advantage of the large increase in the saturation magnetization of Fe60Al40 alloy triggered by subtle atomic displacements. The sigmoidal characteristic and sensitive dependence of the induced magnetization on the atomic displacements manifests a sub-50 nm patterning resolution. Patterning of magnetic regions in the form of stripes separated by ∼ 40 nm wide spacers was performed, wherein the magnet/spacer/magnet structure exhibits reprogrammable parallel (↑/spacer/↑) and antiparallel (↑/spacer/↓) magnetization configurations in zero field. Materials in which the magnetic behavior can be tuned via ion-induced phase transitions may allow the fabrication of novel spin-transport and memory devices using existing lateral patterning tools.

Anisotropic surface enhanced Raman scattering in nanoparticle and nanowire arrays

Silver nanoparticles and nanowires self-aligned on pre-patterned rippled substrate are presented as active surface enhanced Raman scattering (SERS) substrates. The reported inter-particle gap of 5 nm and array periodicity of 35 nm are much lower than current lithographic limits. The observed anisotropy in SERS and surface plasmon resonance in such arrays is attributed to different plasmonic field enhancement along and across the chains of nanoparticles not due to shape anisotropy. For nanoparticle arrays higher SERS intensity is found along the particle chain, but for nanowire arrays higher SERS intensity is found for excitation across the wires. Higher intensity across nanowire arrays supports the argument that the SERS phenomenon is due to electromagnetic field enhancement (hot-junctions) caused by localized surface plasmon resonance across the nanowires having a 35 nm gap. The effect of inter-particle gap, ordering, and aspect ratio on field enhancement is demonstrated. Higher SERS intensity is observed in aligned elongated nanoparticles compared to aligned spherical, non-ordered nanoparticles, or aligned nanowires. Aligned silver nanowires enhance Raman scattering more strongly than aligned gold nanowires.

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.