Making tracks: electronic excitation roles in forming swift heavy ion tracks

Ion Tracks and Nanohillocks Created in Natural Zirconia Irradiated with Swift Heavy Ions

Natural monoclinic zirconia (baddeleyite) was irradiated with 340 MeV Au ions, and the irradiation-induced nanostructures (i.e., ion tracks and nanohillocks) were observed using transmission electron microscopy. The diameter of the nanohillocks was approximately 10 nm, which was similar to the maximum molten region size calculated using the analytical thermal spike model. Ion tracks were imaged as strained regions that maintained their crystalline structure. The cross-sections of most of the ion tracks were imaged as rectangular contrasts as large as 10 nm. These results strongly indicated that the molten region was recrystallized anisotropically, reflecting the lattice structure. Furthermore, low-density track cores were formed in the center of the ion tracks. The formation of low-density track cores can be attributed to the ejection of molten matter toward the surface. A comparison of the ion tracks in the synthetic zirconia nanoparticles and those in larger natural zirconia samples showed that the interface between the strained track contrast and the matrix was less clear in the former than in the latter. These findings suggest that the recrystallization process was affected by the size of the irradiated samples.

Insights into nanoparticle shape transformation by energetic ions

Shape modification of embedded nanoparticles can be achieved by means of swift heavy ion irradiation. During irradiation, the particles elongate and align with the direction of the ion beam, presumably due to nanometer-scale phase transitions induced by individual ion impacts. However, the details of this transformation are not fully understood. The shape of metal nanoparticles embedded in dielectric matrices defines the non-linear optical properties of the composite material. Therefore, understanding the transformation process better is beneficial for producing materials with the desired optical properties. We study the elongation mechanism of gold nanoparticles using atomistic simulations. Here we focus on long-timescale processes and adhesion between the nanoparticle and the matrix. Without the necessity of ad-hoc assumptions used earlier, our simulations show that, due to adhesion with the oxide, the nanoparticles can grow in aspect ratio while in the molten state even after silicon dioxide solidifies. Moreover, they demonstrate the active role of the matrix: Only explicit simulations of ion impacts around the embedded nanoparticle provide the mechanism for continuous elongation up to experimental values of aspect ratio. Experimental transmission electron microscopy micrographs of nanoparticles after high-fluence irradiation support the simulations. The elongated nanoparticles in experiments and their interface structures with silica, as characterized by the micrographs, are consistent with the simulations. These findings bring ion beam technology forward as a precise tool for shaping embedded nanostructures for various optical applications.

Swift Heavy Ion-Induced Reactivity and Surface Modifications in Indium Thin Films

As an energetic ion traverses a target material, it loses its energy through the processes of electronic energy loss (S _e) and nuclear energy loss (S _n). Controlled swift heavy ion (SHI) irradiation on solid targets produces its effects through both of these mechanisms, as a consequence of which modifications occur in the structure, surface morphology, and magnetic and optical properties, apart from ion implantation and ion-induced reactivity. A systematic investigation of these effects can be useful in developing standard protocols for creating desired effects in materials using specific ion beams. In this study, indium films of thickness 25 nm were deposited on silicon substrates and were subjected to 100 MeV O⁷⁺ and 100 MeV Si⁷⁺ ion irradiation, with the fluences varying from 1 × 10¹¹ to 1 × 10¹³ ions/cm². The pristine and SHI-irradiated films were then characterized using glancing incidence X-ray diffraction (GIXRD), Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The motive was to identify the effects of irradiation with different ion species having large variations in electronic and nuclear energy losses. While the RBS results suggest that sputtering is extremely low and that there are no major changes in the film composition due to ion beam-induced mixing, the GIXRD analysis indicates that increasing the ion fluence reduces the crystallinity of the film for both the ions. Ion beam irradiation with O⁷⁺ ions, however, results in beam-induced reactivity, as the GIXRD scan shows characteristic peaks from indium oxide (In₂O₃), which become the predominant peaks at the highest fluence used here. Si⁷⁺ ion irradiation results in a narrowing of the particle size distribution on the surface, with no evidence of reactivity. SEM results indicate fusion and fragmentation of grains with the increase in the ion fluences, and AFM images reveal an increase in the surface roughness of a few percent when irradiated with both 100 MeV O⁷⁺ and 100 MeV Si⁷⁺ ions.

Surface nanostructures on Nb-doped SrTiO3irradiated with swift heavy ions at grazing incidence

A single crystal of SrTiO₃doped with 0.5 wt% niobium (Nb-STO) was irradiated with 200 MeV Au³²⁺ions at grazing incidence to characterize the irradiation-induced hillock chains. Exactly the same hillock chains are observed by using atomic force microscopy (AFM) and scanning electron microscopy (SEM) to study the relation between irradiation-induced change of surface topography and corresponding material property changes. As expected, multiple hillocks as high as 5-6 nm are imaged by AFM observation in tapping mode. It is also found that the regions in between the adjacent hillocks are not depressed, and in many cases they are slightly elevated. Line-like contrasts along the ion paths are found in both AFM phase images and SEM images, indicating the formation of continuous ion tracks in addition to multiple hillocks. Validity of preexisting models for explaining the hillock chain formation is discussed based on the present results. In order to obtain new insights related to the ion track formation, cross-sectional transmission electron microscopy (TEM) observation was performed. The ion tracks in the near-surface region are found to be relatively large, whereas buried ion tracks in the deeper region are relatively small. The results suggest that recrystallization plays an important role in the formation of small ion tracks in the deep region, whereas formation of large ion tracks in the near-surface region is likely due to the absence of recrystallization. TEM images also show shape deformation of ion tracks in the near-surface region, suggesting that material transport towards the surface is the reason for the absence of recrystallization.

High-Energy Heavy Ion Irradiation of Al2O3, MgO and CaF2

High-energy heavy ion irradiation can produce permanent damage in the target material if the density of deposited energy surpasses a material-dependent threshold value. It is known that this threshold can be lowered in the vicinity of the surface or in the presence of defects. In the present study, we established threshold values for Al₂O₃, MgO and CaF₂ under the above-mentioned conditions, and found those values to be much lower than expected. By means of atomic force microscopy and Rutherford backscattering spectrometry in channelling mode, we present evidence that ion beams with values of 3 MeV O and 5 MeV Si, despite the low density of deposited energy along the ion trajectory, can modify the structure of investigated materials. The obtained results should be relevant for radiation hardness studies because, during high-energy ion irradiation, unexpected damage build-up can occur under similar conditions.

Modification of SiO2, ZnO, Fe2O3 and TiN Films by Electronic Excitation under High Energy Ion Impact

It has been known that the modification of non-metallic solid materials (oxides, nitrides, etc.), e.g., the formation of tracks, sputtering representing atomic displacement near the surface and lattice disordering are induced by electronic excitation under high-energy ion impact. We have investigated lattice disordering by the X-ray diffraction (XRD) of SiO2, ZnO, Fe2O3 and TiN films and have also measured the sputtering yields of TiN for a comparison of lattice disordering with sputtering. We find that both the degradation of the XRD intensity per unit ion fluence and the sputtering yields follow the power-law of the electronic stopping power and that these exponents are larger than unity. The exponents for the XRD degradation and sputtering are found to be comparable. These results imply that similar mechanisms are responsible for the lattice disordering and electronic sputtering. A mechanism of electron–lattice coupling, i.e., the energy transfer from the electronic system into the lattice, is discussed based on a crude estimation of atomic displacement due to Coulomb repulsion during the short neutralization time (~fs) in the ionized region. The bandgap scheme or exciton model is examined.

Evidence for the formation of two types of oxygen interstitials in neutron-irradiated α-Al2O3 single crystals

Due to unique optical/mechanical properties and significant resistance to harsh radiation environments, corundum (α-Al₂O₃) is considered as a promising candidate material for windows and diagnostics in forthcoming fusion reactors. However, its properties are affected by radiation-induced (predominantly, by fast neutrons) structural defects. In this paper, we analyze thermal stability and recombination kinetics of primary Frenkel defects in anion sublattice − the F-type electronic centers and complementary oxygen interstitials in fast-neutron-irradiated corundum single crystals. Combining precisely measured thermal annealing kinetics for four types of primary radiation defects (neutral and charged Frenkel pairs) and the advanced model of chemical reactions, we have demonstrated for the first time a co-existence of the two types of interstitial defects – neutral O atoms and negatively charged O^- ions (with attributed optical absorption bands peaked at energies of 6.5 eV and 5.6 eV, respectively). From detailed analysis of interrelated kinetics of four oxygen-related defects, we extracted their diffusion parameters (interstitials serve as mobile recombination partners) required for the future prediction of secondary defect-induced reactions and, eventually, material radiation tolerance.

Ion tracks in silicon formed by much lower energy deposition than the track formation threshold

Damaged regions of cylindrical shapes called ion tracks, typically in nano-meters wide and tens micro-meters long, are formed along the ion trajectories in many insulators, when high energy ions in the electronic stopping regime are injected. In most cases, the ion tracks were assumed as consequences of dense electronic energy deposition from the high energy ions, except some cases where the synergy effect with the nuclear energy deposition plays an important role. In crystalline Si (c-Si), no tracks have been observed with any monomer ions up to GeV. Tracks are formed in c-Si under 40 MeV fullerene (C₆₀) cluster ion irradiation, which provides much higher energy deposition than monomer ions. The track diameter decreases with decreasing the ion energy until they disappear at an extrapolated value of ~ 17 MeV. However, here we report the track formation of 10 nm in diameter under C₆₀ ion irradiation of 6 MeV, i.e., much lower than the extrapolated threshold. The diameters of 10 nm were comparable to those under 40 MeV C₆₀ irradiation. Furthermore, the tracks formed by 6 MeV C₆₀ irradiation consisted of damaged crystalline, while those formed by 40 MeV C₆₀ irradiation were amorphous. The track formation was observed down to 1 MeV and probably lower with decreasing the track diameters. The track lengths were much shorter than those expected from the drop of S_e below the threshold. These track formations at such low energies cannot be explained by the conventional purely electronic energy deposition mechanism, indicating another origin, e.g., the synergy effect between the electronic and nuclear energy depositions, or dual transitions of transient melting and boiling.

Comprehensive Understanding of Hillocks and Ion Tracks in Ceramics Irradiated with Swift Heavy Ions

Amorphizable ceramics (LiNbO3, ZrSiO4, and Gd3Ga5O12) were irradiated with 200 MeV Au ions at an oblique incidence angle, and the as-irradiated samples were observed by transmission electron microscopy (TEM). Ion tracks in amorphizable ceramics are confirmed to be homogenous along the ion paths. Magnified TEM images show the formation of bell-shaped hillocks. The ion track diameter and hillock diameter are similar for all the amorphizable ceramics, while there is a tendency for the hillocks to be slightly bigger than the ion tracks. For SrTiO3 (STO) and 0.5 wt% niobium-doped STO (Nb-STO), whose hillock formation has not been fully explored, 200 MeV Au ion irradiation and TEM observation were also performed. The ion track diameters in these materials are found to be markedly smaller than the hillock diameters. The ion tracks in these materials exhibit inhomogeneity, which is similar to that reported for non-amorphizable ceramics. On the other hand, the hillocks appear to be amorphous, and the amorphous feature is in contrast to the crystalline feature of hillocks observed in non-amorphizable ceramics. No marked difference is recognized between the nanostructures in STO and those in Nb-STO. The material dependence of the nanostructure formation is explained in terms of the intricate recrystallization process.

Atomic, electronic and magnetic structure of an oxygen interstitial in neutron-irradiated Al2O3 single crystals

A single radiation-induced superoxide ion \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$O_{2}^{ - }$$\end{document}O2- has been observed for the first time in metal oxides. This structural defect has been revealed in fast-neutron-irradiated (6.9×10¹⁸n/cm²) corundum (α-Al₂O₃) single crystals using the EPR method. Based on the angular dependence of the EPR lines at the magnetic field rotation in different planes and the determined g tensor components, it is shown that this hole-type \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$O_{2}^{ - }$$\end{document}O2- center (i) incorporates one regular and one interstitial oxygen atoms being stabilized by a trapped hole (S = 1/2), (ii) occupies one oxygen site in the (0001) plane being oriented along the a axis, and (iii) does not contain any other imperfection/defect in its immediate vicinity. The thermal stepwise annealing (observed via the EPR signal and corresponding optical absorption bands) of the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$O_{2}^{ - }$$\end{document}O2- centers, caused by their destruction with release of a mobile ion (tentatively the oxygen ion with the formal charge −1), occurs at 500–750 K, simultaneously with the partial decay of single F-type centers (mostly with the EPR-active F⁺ centers). The obtained experimental results are in line with the superoxide defect configurations obtained via density functional theory (DFT) calculations employing the hybrid B3PW exchange-correlation functional. In particular, the DFT calculations confirm the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$O_{2}^{ - }$$\end{document}O2- center spin S = 1/2, its orientation along the a axis. The \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$O_{2}^{ - }$$\end{document}O2- center is characterized by a short O–O bond length of 1.34 Å and different atomic charges and magnetic moments of the two oxygens. We emphasize the important role of atomic charges and magnetic moments analysis in order to identify the ground state configuration.

Analysis of nanoscale fluid inclusions in geomaterials by atom probe tomography: Experiments and numerical simulations

The spatial correlation between defects in crystalline materials and trace element segregation plays a fundamental role in determining the physical and mechanical properties of a material, which is particularly important in naturally deformed materials. Herein, we combine electron backscatter diffraction, electron channelling contrast imaging, scanning transmission electron microscopy and atom probe tomography on a naturally occurring metal sulphide in an attempt to document mechanisms of element segregation in a brittle-dominated deformation regime. Within APT reconstructions, features with a high point density comprising O-rich discs stacked over As-rich spherules are observed. The combined microscopy data allow us to interpret these as nanoscale fluid inclusions. Our observations are confirmed by simulated APT experiments of core-shell particles with a core exhibiting a very low evaporation field and the shell emulating a segregated layer at the inclusion interface. Our data has significant trans-disciplinary implications to the geosciences, the material sciences, and analytical microscopy.

Hot-Electron-Mediated Ion Diffusion in Semiconductors for Ion-Beam Nanostructuring

Ion-beam-based techniques are widely utilized to synthesize, modify, and characterize materials at the nanoscale, with applications from the semiconductor industry to medicine. Interactions of the beam with the target are fundamentally interesting, as they trigger multilength and time-scale processes that need to be quantitatively understood to achieve nanoscale precision. Here we demonstrate for magnesium oxide, as a testbed semiconductor material, that in a kinetic-energy regime in which electronic effects are usually neglected, a proton beam efficiently excites oxygen-vacancy-related electrons. We quantitatively describe the excited-electron distribution and the emerging ion dynamics using first-principles techniques. Contrary to the common picture of charging the defect, we discover that most of the excited electrons remain locally near the oxygen vacancy. Using these results, we bridge time scales from ultrafast electron dynamics directly after impact to ion diffusion over migration barriers in semiconductors and discover a diffusion mechanism that is mediated by hot electrons. Our quantitative simulations predict that this mechanism strongly depends on the projectile-ion velocity, suggesting the possibility of using it for precise sample manipulation via nanoscale diffusion enhancement in semiconductors with a deep, neutral, intrinsic defect.

Monitoring Ion Track Formation Using In Situ RBS/c, ToF-ERDA, and HR-PIXE

The aim of this work is to investigate the feasibility of ion beam analysis techniques for monitoring swift heavy ion track formation. First, the use of the in situ Rutherford backscattering spectrometry in channeling mode to observe damage build-up in quartz SiO₂ after MeV heavy ion irradiation is demonstrated. Second, new results of the in situ grazing incidence time-of-flight elastic recoil detection analysis used for monitoring the surface elemental composition during ion tracks formation in various materials are presented. Ion tracks were found on SrTiO₃, quartz SiO₂, a-SiO₂, and muscovite mica surfaces by atomic force microscopy, but in contrast to our previous studies on GaN and TiO₂, surface stoichiometry remained unchanged. Third, the usability of high resolution particle induced X-ray spectroscopy for observation of electronic dynamics during early stages of ion track formation is shown.

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.

Ionization-induced annealing of pre-existing defects in silicon carbide

A long-standing objective in materials research is to effectively heal fabrication defects or to remove pre-existing or environmentally induced damage in materials. Silicon carbide (SiC) is a fascinating wide-band gap semiconductor for high-temperature, high-power and high-frequency applications. Its high corrosion and radiation resistance makes it a key refractory/structural material with great potential for extremely harsh radiation environments. Here we show that the energy transferred to the electron system of SiC by energetic ions via inelastic ionization can effectively anneal pre-existing defects and restore the structural order. The threshold determined for this recovery process reveals that it can be activated by 750 and 850 keV Si and C self-ions, respectively. The results conveyed here can contribute to SiC-based device fabrication by providing a room-temperature approach to repair atomic lattice structures, and to SiC performance prediction as either a functional material for device applications or a structural material for high-radiation environments.

Silicon carbide possesses myriad properties which render it an ideal material for implementation in harsh radiation environments. Here, the authors show that damaged SiC can be repaired through ionization-induced healing, which has broader implications for the design of future radiation-tolerant materials.

Atomistic simulation of ion track formation in UO2

The atomistic simulation of track formation due to the moving of swift heavy ion is performed for uranium dioxide. The two-temperature atomistic model with an explicit account of electron pressure and electron thermal conductivity is used. This two-temperature model describes a ionic subsystem by means of molecular dynamics while the electron subsystem is considered in the continuum approach. The various mechanisms of track formation are examined. It is shown that the mechanism of surface track formation differs from the mechanism of track formation in the bulk. The threshold values of the stopping power for track formation are estimated.

Multi-scale simulation of structural heterogeneity of swift-heavy ion tracks in complex oxides

Tracks formed by swift-heavy ion irradiation, 2.2 GeV Au, of isometric Gd2Ti2O7 pyrochlore and orthorhombic Gd2TiO5 were modeled using the thermal-spike model combined with a molecular-dynamics simulation. The thermal-spike model was used to calculate the energy dissipation over time and space. Using the time, space, and energy profile generated from the thermal-spike model, the molecular-dynamics simulations were performed to model the atomic-scale evolution of the tracks. The advantage of the combination of these two methods, which uses the output from the continuum model as an input for the atomistic model, is that it provides a means of simulating the coupling of the electronic and atomic subsystems and provides simultaneously atomic-scale detail of the track structure and morphology. The simulated internal structure of the track consists of an amorphous core and a shell of disordered, but still periodic, domains. For Gd2Ti2O7, the shell region has a disordered pyrochlore with a defect fluorite structure and is relatively thick and heterogeneous with different degrees of disordering. For Gd2TiO5, the disordered region is relatively small as compared with Gd2Ti2O7. In the simulation, 'facets', which are surfaces with definite crystallographic orientations, are apparent around the amorphous core and more evident in Gd2TiO5 along [010] than [001], suggesting an orientational dependence of the radiation response. These results show that track formation is controlled by the coupling of several complex processes, involving different degrees of amorphization, disordering, and dynamic annealing. Each of the processes depends on the mass and energy of the energetic ion, the properties of the material, and its crystallographic orientation with respect to the incident ion beam.

Swift heavy ion damage to sodium chloride: synergy between excitation and thermal spikes

Systematic data on the effect of irradiation with swift ions (Zn at 735 MeV and Xe at 929 MeV) on NaCl single crystals have been analysed in terms of a synergetic two-spike approach (thermal and excitation spikes). The coupling of the two spikes, simultaneously generated by the irradiation, contributes to the operation of a non-radiative exciton decay model as proposed for purely ionization damage. Using this scheme, we have accounted for the π-emission yield of self-trapped excitons and its temperature dependence under ion-beam irradiation. Moreover, the initial production rates of F-centre growth have also been reasonably simulated for irradiation at low temperatures ( < 100 K), where colour centre annealing and aggregation can be neglected.

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.

Atomistic simulation of track formation by energetic recoils in zircon

We have performed classical molecular dynamics simulations of fission track formation in zircon. We simulated the passage of a swift heavy ion through crystalline zircon using cylindrical thermal spikes with energy deposition (dE/dx) of 2.5-12.8 keV nm( - 1) and a radius of 3 nm. At a low dE/dx of 2.55 keV nm( - 1), the structural damage recovered almost completely and a damage track was not produced. At higher values of dE/dx, tracks were observed and the radius of the track increased with increasing dE/dx. Our structural analysis shows amorphization in the core of the track and phase separation into Si-rich regions near the center of the track and Zr-rich regions near the periphery. These simulations establish a threshold dE/dx for fission track formation in zircon that is relevant to thermochronology and nuclear waste immobilization.