Direct evidence for projectile charge-state dependent crater formation due to fast ions

Latent ion tracks were finally observed in diamond

Injecting high-energy heavy ions in the electronic stopping regime into solids can create cylindrical damage zones called latent ion tracks. Although these tracks form in many materials, none have ever been observed in diamond, even when irradiated with high-energy GeV uranium ions. Here we report the first observation of ion track formation in diamond irradiated with 2-9 MeV C60 fullerene ions. Depending on the ion energy, the mean track length (diameter) changed from 17 (3.2) nm to 52 (7.1) nm. High resolution scanning transmission electron microscopy (HR-STEM) indicated the amorphization in the tracks, in which π-bonding signal from graphite was detected by the electron energy loss spectroscopy (EELS). Since the melting transition is not induced in diamond at atmospheric pressure, conventional inelastic thermal spike calculations cannot be applied. Two-temperature molecular dynamics simulations succeeded in the reproduction of both the track formation under MeV C60 irradiations and the no-track formation under GeV monoatomic ion irradiations.

Ion beam processing of DNA origami nanostructures

DNA origami nanostructures are emerging as a bottom-up nanopatterning approach. Direct combination of this approach with top-down nanotechnology, such as ion beams, has not been considered because of the soft nature of the DNA material. Here we demonstrate that the shape of 2D DNA origami nanostructures deposited on Si substrates is well preserved upon irradiation by ion beams, modeling ion implantation, lithography, and sputtering conditions. Structural changes in 2D DNA origami nanostructures deposited on Si are analyzed using AFM imaging. The observed effects on DNA origami include structure height decrease or increase upon fast heavy ion irradiation in vacuum and in air, respectively. Slow- and medium-energy heavy ion irradiation results in the cutting of the nanostructures or crater formation with ion-induced damage in the 10 nm range around the primary ion track. In all these cases, the designed shape of the 2D origami nanostructure remains unperturbed. Present stability and nature of damages on DNA origami nanostructures enable fusion of DNA origami advantages such as shape and positioning control into novel ion beam nanofabrication approaches.

Cratering Induced by Slow Highly Charged Ions on Ultrathin PMMA Films

Highly charged ions are a well-known tool for the nanostructuring of surfaces. We report on the thickness dependence of nanostructures produced by single 260 keV Xe38+ ions on ultrathin poly(methyl methacrylate) (PMMA) films (1 nm to 60 nm) deposited onto Si substrates. The nanostructures induced by slow highly charged ions are rimless craters with a diameter of around 15 nm, which are roughly independent of the thickness of the films down to layers of about 2 nm. The crater depth and thus the overall crater volume are, however, thickness-dependent, decreasing in size in films thinner than ~25 nm. Our findings indicate that although the potential energy of the highly charged ions is the predominant source of deposited energy, the depth of the excited material contributing to crater formation is much larger than the neutralization depth of the ions, which occurs in the first nanometer of the solid at the projectile velocity employed here. This suggests synergism between kinetic and potential-driven processes in nanostructure formation in PMMA.

Charge State Effect of High Energy Ions on Material Modification in the Electronic Stopping Region

It has been observed that modifications of non-metallic solids such as sputtering and surface morphology are induced by electronic excitation under high-energy ion impact and that these modifications depend on the charge of incident ions (charge-state effect or incident-charge effect). A simple model is described, consisting of an approximation to the mean-charge-evolution by saturation curves and the charge-dependent electronic stopping power, for the evaluation of the relative yield (e.g., electronic sputtering yield) of the non-equilibrium charge incidence over that of the equilibrium-charge incidence. It is found that the present model reasonably explains the charge state effect on the film thickness dependence of lattice disordering of WO3. On the other hand, the model appears to be inadequate to explain the charge-state effect on the electronic sputtering of WO3 and LiF. Brief descriptions are given for the charge-state effect on the electronic sputtering of SiO2, UO2 and UF4, and surface morphology modification of poly-methyl-methacrylate (PMMA), mica and tetrahedral amorphous carbon (ta-C).

Impact Features Induced by Single Fast Ions of Different Charge-State on Muscovite Mica

The influence of the charge state q on surface modifications induced by the impact of individual fast, heavy ions on muscovite mica was investigated. Beams of 593 MeV 197Auq+ with well-defined initial charge states over a relatively broad range of values (30 to 51) and at different irradiation geometries were used. At normal incidence, the impact features are rounded protrusions (hillocks) with ≳20 nm in diameter. At grazing angles, besides the hillocks, craters and elongated tails (up to 350 nm-long) extending along the direction of ion penetration are produced. It is shown that the impact features at normal incidence depend strongly on the initial charge state of the projectiles. This dependence is very weak at grazing angles as the ion reaches the equilibrium charge state closer to the surface. At normal ion incidence, the hillock volume scales with q3.3 ± 0.6. This dependence stems largely from the increase in the hillock height, as a weak dependence of the diameter was observed.

Negative ion formation by neutral hydrogen atom grazing scattering from a LiF(100) surface

H⁻ conversion during H⁰ grazing scattering on a LiF(100) surface, includes H⁻ affinity electron loss to a surface image state.

High resolution AFM studies of irradiated mica-following the traces of swift heavy ions under grazing incidence

High resolution AFM imaging of swift heavy ion irradiated muscovite mica under grazing incidence provides detailed insight into the created nanostructure features. Swift heavy ions under grazing incidence form a complex track structure along the surface, which consists of a double track of nanohillocks at the impact site accompanied by a single, several 100 nm long protrusion. Detailed track studies by varying the irradiation parameters, i.e. the angle of incidence (0.2°-2°) and the kinetic energy of the impinging ions (23, 55, 75, 95 MeV) are presented. Moreover, the track formation in dependence of the sample temperature (between room temperature and 600 °C) and of the chemical composition (muscovite mica and fluorphlogopite mica) is studied.

Formation of swift heavy ion tracks on a rutile TiO2 (001) surface

Nanostructuring of surfaces and two-dimensional materials using swift heavy ions offers some unique possibilities owing to the deposition of a large amount of energy localized within a nanoscale volume surrounding the ion trajectory. To fully exploit this feature, the morphology of nanostructures formed after ion impact has to be known in detail. In the present work the response of a rutile TiO2 (001) surface to grazing-incidence swift heavy ion irradiation is investigated. Surface ion tracks with the well known intermittent inner structure were successfully produced using 23 MeV I ions. Samples irradiated with different ion fluences were investigated using atomic force microscopy and grazing-incidence small-angle X-ray scattering. With these two complementary approaches, a detailed description of the swift heavy ion impact sites, i.e. the ion tracks on the surface, can be obtained even for the case of multiple ion track overlap. In addition to the structural investigation of surface ion tracks, the change in stoichiometry of the rutile TiO2 (001) surface during swift heavy ion irradiation was monitored using in situ time-of-flight elastic recoil detection analysis, and a preferential loss of oxygen was found.

Confinement effects of ion tracks in ultrathin polymer films

We show direct experimental evidence that radiation effects produced by single MeV heavy ions on a polymer surface are weakened when the length of the ion track in the material is confined into layers of a few tens of nanometers. Deviation from the bulk (thick film) behavior of ion-induced craters starts at a critical thickness as large as ∼40  nm, due to suppression of long-range additive effects of excited atoms along the track. Good agreement was found between the experimental results, molecular dynamic simulations, and an analytical model.

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.

Charge state dependent energy deposition by ion impact

We report on a measurement of craters in thin dielectric films formed by Xe(Q+) (26 ≤ Q ≤ 44) projectiles. Tunnel junction devices with ion-irradiated barriers were used to amplify the effect of charge-dependent cratering through the exponential dependence of tunneling conductance on barrier thickness. Electrical conductance of a crater σ(c)(Q) increased by 4 orders of magnitude (7.9 × 10(-4) μS to 6.1 μS) as Q increased, corresponding to crater depths ranging from 2 to 11 Å. By employing a heated spike model, we determine that the energy required to produce the craters spans from 8 to 25 keV over the investigated charge states. Considering energy from preequilibrium nuclear and electronic stopping as well as neutralization, we find that at least (27 ± 2)% of available projectile neutralization energy is deposited into the thin film during impact.