Inelastic Thermal Spike Research Articles

Predictive modeling of synergistic effects in nanoscale ion track formation.

Molecular dynamics techniques in combination with the inelastic thermal spike model are used to study the coupled effects of the inelastic energy loss due to 21 MeV Ni ion irradiation with pre-existing defects in SrTiO3. We determine the dependence on pre-existing defect concentration of nanoscale track formation occurring from the synergy between the inelastic energy loss and the pre-existing atomic defects. We show that the size of nanoscale ion tracks can be controlled by the concentration of pre-existing disorder. This work identifies a major gap in fundamental understanding on the role of defects in electronic energy dissipation and electron-lattice coupling.

The inelastic thermal spike model applied to metal/insulator interfaces

AbstractRecently, inter‐diffusion induced by 200 MeV Xe ion irradiation in metal‐insulator interfaces (Bi/Al2O3and Au/Al2O3) was investigated experimentally via in‐situ RBS measurements using 16 MeV C ions. In this work, the experimental results were tried to be explained by the inelastic thermal spike model (ITSM). Although these results explained qualitatively the inter‐diffusion, the experimentally observed diffusion length (∼800 nm) at a fluence value of 7 x 1013cm‐2was much larger than the estimated maximum value within the ITSM (∼80 nm). It was found that the Bi concentration profiles measured via in‐situ RBS could not be fitted by simple diffusional ones. We suggested that the main responsible of the huge atomic diffusion exhibited by Bi in Al2O3could be the directional motion of Bi atoms in molten Al2O3triggered off by the pressure due to liquid‐vapor phase transition of Bi near the interface. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Raman spectrum study of graphite irradiated by swift heavy ions

Highly oriented pyrolytic graphites are irradiated with 40.5-MeV and 67.7-MeV 112Sn-ions in a wide range of fluences: 1 × 1011 ions/cm2−1 × 1014 ions/cm2. Raman spectra in the region between 1200 cm−1 and 3500 cm−1 show that the disorder induced by Sn-ions increases with ion fluence increasing. However, for the same fluence, the amount of disorder is greater for 40.5-MeV Sn-ions than that observed for 67.7-MeV Sn-ions, even though the latter has a slightly higher value for electronic energy loss. This is explained by the ion velocity effect. Importantly, ∼ 3-cm−1 frequency shift toward lower wavenumber for the D band and ∼ 6-cm−1 shift toward lower wavenumber for the 2D band are observed at a fluence of 1 × 1014 ions/cm2, which is consistent with the scenario of radiation-induced strain. The strain formation is interpreted in the context of inelastic thermal spike model, and the change of the 2D band shape at high ion fluence is explained by the accumulation of stacking faults of the graphene layers activated by radiation-induced strain around ion tracks. Moreover, the hexagonal structure around the ion tracks is observed by scanning tunneling microscopy, which confirms that the strains near the ion tracks locally cause electronic decoupling of neighboring graphene layers.

A study on the consequence of swift heavy ion irradiation of Zn-silica nanocomposite thin films: electronic sputtering.

Zn–silica nanocomposite thin films with varying Zn metal content, deposited by atom beam sputtering technique were subjected to 100 MeV Ag ion irradiation. Rutherford backscattering spectrometry reveals the loss of Zn with irradiation, which is observed to be greater from thin films with lower Zn content. The sputtered species collected on carbon-coated transmission electron microscopy (TEM) grids consist of Zn nanoparticles of sizes comparable to those present in the nanocomposite thin film. The process of size-dependent electronic sputtering of Zn is explained on the basis of an inelastic thermal spike model. The possibility of direct cluster emission is explained by pressure spike built inside the track, initiated by a temperature spike.

Energy deposition by heavy ions: additivity of kinetic and potential energy contributions in hillock formation on CaF2.

Modification of surface and bulk properties of solids by irradiation with ion beams is a widely used technique with many applications in material science. In this study, we show that nano-hillocks on CaF2 crystal surfaces can be formed by individual impact of medium energy (3 and 5 MeV) highly charged ions (Xe22+ to Xe30+) as well as swift (kinetic energies between 12 and 58 MeV) heavy xenon ions. For very slow highly charged ions the appearance of hillocks is known to be linked to a threshold in potential energy (Ep) while for swift heavy ions a minimum electronic energy loss per unit length (Se) is necessary. With our results we bridge the gap between these two extreme cases and demonstrate, that with increasing energy deposition via Se the Ep-threshold for hillock production can be lowered substantially. Surprisingly, both mechanisms of energy deposition in the target surface seem to contribute in an additive way, which can be visualized in a phase diagram. We show that the inelastic thermal spike model, originally developed to describe such material modifications for swift heavy ions, can be extended to the case where both kinetic and potential energies are deposited into the surface.

Purification/annealing of graphene with 100-MeV Ag ion irradiation

Studies on interaction of graphene with radiation are important because of nanolithographic processes in graphene-based electronic devices and for space applications. Since the electronic properties of graphene are highly sensitive to the defects and number of layers in graphene sample, it is desirable to develop tools to engineer these two parameters. We report swift heavy ion (SHI) irradiation-induced annealing and purification effects in graphene films, similar to that observed in our studies on fullerenes and carbon nanotubes (CNTs). Raman studies after irradiation with 100-MeV Ag ions (fluences from 3 × 1010 to 1 × 1014 ions/cm2) show that the disorder parameter α, defined by ID/IG ratio, decreases at lower fluences but increases at higher fluences beyond 1 × 1012 ions/cm2. This indicates that SHI induces annealing effects at lower fluences. We also observe that the number of graphene layers is reduced at fluences higher than 1 × 1013 ions/cm2. Using inelastic thermal spike model calculations, we estimate a radius of 2.6 nm for ion track core surrounded by a halo extending up to 11.6 nm. The transient temperature above the melting point in the track core results in damage, whereas lower temperature in the track halo is responsible for annealing. The results suggest that SHI irradiation fluence may be used as one of the tools for defect annealing and manipulation of the number of graphene layers.PACS60.80.x; 81.05.ue

Structural analysis of simulated swift heavy ion tracks in quartz

Swift heavy ions (SHI), of specific kinetic energies in the excess of 1MeV/u, can create cylindrical regions of structural transformation in SiO2 targets, also known as SHI tracks. Recent measurements of the track cross-sections in α-quartz show significant and consistent discrepancies across different experimental techniques used. In particular, the track radii obtained from channelling experiments based on the Rutherford Backscattering Spectrometry (RBS-c) method increase monotonically with the electronic stopping power, whereas the track radii obtained from the Small Angle X-ray scattering (SAXS) saturate past a certain stopping power threshold. We perform a systematic study of the structure of the α-quartz tracks obtained from the molecular dynamics (MD) simulations incorporating a time-dependent energy deposition based on the inelastic thermal spike model, which allows us to discuss the possible origins of these experimental discrepancies.

Structural and optical characterization of SiO2 under irradiation with swift heavy ions

SiO2 single crystals irradiated with 600keV, 4 and 5MeV Kr ions using the 320kV High Voltage Experimental Platform (IMP, Lanzhou) were investigated with infrared, fluorescence spectroscopes and TEM. In the low-energy regime, single ion tracks are well separated and the damage process is dominated by the formation of simple color centers such as F+ and F2 centers. For the high energy ions, the energy density in ion tracks produced at a high stopping power is larger, and consequently the defect concentration increases significantly. At higher defect densities, the distance between single defects is smaller facilitating the aggregation of individual electron center to defect cluster and defect annihilation by recombination of hole and electron centers. The latter process determines the limited efficiency of color-center creation under heavy-ion irradiation. Using the unified thermal spike model, a combination of the elastic collision spike model and the inelastic thermal spike model based on electronic energy losses, it is possible to fully describe the experimental data, which clearly demonstrate a synergy between the nuclear energy loss and the electronic energy loss processes.

On the threshold of damage formation in aluminum oxide via electronic excitations

This work is aimed to determine the threshold of dense ionization induced damage formation and their morphology in sapphire single crystals irradiated with 1.2MeV/amu Xe ions. Cross-sectional TEM examination of r-oriented Al2O3 specimens irradiated to fluences of 2×1012 and 2×1013cm−2 has revealed discontinuous ion tracks visible from the irradiated surface up to a depth of 7.6±0.1μm. According to the SRIM code calculation, the threshold electronic stopping power for track formation in Al2O3 is within the range 9.8÷10.5keV/nm. This value agrees with those predicted by both inelastic and analytical thermal spike models.

Comment on ``Dense and nanometric electronic excitations induced by swift heavy ions in an ionic CaF2crystal: Evidence for two thresholds of damage creation''

Systematic experiments on irradiation damage in CaF${}_{2}$ were reported in a recent paper [M. Toulemonde et al., Phys. Rev. B 85, 054112 (2012)]. The measurements were performed in a broad range of ion energies and energy deposition and the inelastic thermal spike model was applied for the analysis. Here we demonstrate that contrary to what is stated in the paper, the experimental results do not prove that there is a velocity effect in CaF${}_{2}$ similar to that observed previously in Y${}_{3}$Fe${}_{5}$O${}_{12}$ and other track-forming insulators. We also show that the authors' results do not support their hypothesis that tracks are formed by quenching of a boiling phase.

Color center creation in SiO2 under irradiation with swift heavy ions: Dependence on energy loss and fluence

SiO2 single crystals irradiated with 600keV, 2MeV, 4MeV and 95MeV Ar ions using 320kV high voltage Experimental platform (IMP, Lanzhou) and the HIRFL-SFC (Heavy Ion Research Facility in Lanzhou) facility in Lanzhou were investigated by Infrared spectra and fluorescence spectroscopes. PL spectra peaks of silicon dioxide irradiated with 600keV, 2MeV, 4MeV and 95MeV Ar ions were located at 445nm (F2 color center), 570nm (F4 color center) and 650nm (F2+ and F3+ color centers) emission band etc. Using the unified thermal spike model, a combination of the elastic collision spike model and the inelastic thermal spike model based on electronic energy losses derived from the reciprocity approach, it is possible to fully describe the experimental data, which clearly demonstrate a synergy between the nuclear energy loss and the electronic energy loss processes.

Hybrid continuum–atomistic modelling of swift heavy ion radiation damage in germanium

The response of germanium to swift heavy ion irradiation is simulated using a hybrid continuum–atomistic approach. The continuum part of the model, which characterises the electronic excitations is an extension of the inelastic thermal spike based on an approximation to the Boltzmann transport equation; while the atomistic part is represented with molecular dynamics. This integrated method can realistically account for the non-equilibrium carrier dynamics in band-gap materials under irradiation, unlike earlier developments based on the two-temperature approach. The model is used to obtain temporal and spatial evolution of carrier density, electronic temperature and lattice temperature for germanium irradiated with carbon cluster ions. Good agreement with experimental data of amorphised latent track radii for different stopping powers is obtained by fitting a constant value for the electron–phonon coupling strength – the only parameter treated as free in the model.

SAXS investigations of the morphology of swift heavy ion tracks in α-quartz

The morphology of swift heavy ion tracks in crystalline α-quartz was investigated using small angle x-ray scattering (SAXS), molecular dynamics (MD) simulations and transmission electron microscopy. Tracks were generated by irradiation with heavy ions with energies between 27 MeV and 2.2 GeV. The analysis of the SAXS data indicates a density change of the tracks of ∼2 ± 1% compared to the surrounding quartz matrix for all irradiation conditions. The track radii only show a weak dependence on the electronic energy loss at values above 17 keV nm−1, in contrast to values previously reported from Rutherford backscattering spectrometry measurements and expectations from the inelastic thermal spike model. The MD simulations are in good agreement at low energy losses, yet predict larger radii than SAXS at high ion energies. The observed discrepancies are discussed with respect to the formation of a defective halo around an amorphous track core, the existence of high stresses and/or the possible presence of a boiling phase in quartz predicted by the inelastic thermal spike model.

Cooperative effect of electronic and nuclear stopping on ion irradiation damage in silica

Radiation damage by ions is conventionally believed to be produced either by displacement cascades or electronic energy deposition acting separately. There is, however, a range of ion energies where both processes are significant and can contribute to irradiation damage. The combination of two computational methods, namely binary collision approximation and molecular dynamics, the latter with input from the inelastic thermal spike model, makes it possible to examine the simultaneous contribution of both energy deposition mechanisms on the structural damage in the irradiated structure. We study the effect in amorphous SiO2 irradiated by Au ions with energies ranging between 0.6 and 76.5 MeV. We find that in the intermediate energy regime, the local heating due to electronic excitations gives a significant contribution to the displacement cascade damage.

Atomistic simulations of MeV ion irradiation of silica

We used molecular dynamics simulations to study 2.3MeV Au ion irradiation of silica. In this energy regime, the energy loss of the ion is divided almost equally between electronic and nuclear energy loss. The inelastic thermal spike model was used to model the electron–phonon interactions due to the high electronic energy loss. Binary collision approximation calculations provided input for the recoil energies due to MeV ions. We performed simulations of the damage due to the separate damage mechanisms as well as together, and found that the inelastic thermal spike is needed to accurately simulate the irradiation damage from MeV ions.

Molecular dynamics simulations of swift heavy ion induced defect recovery in SiC

Swift heavy ions induce a high density of electronic excitations that can cause the formation of amorphous ion tracks in insulators. No ion tracks have been observed in the semiconductor SiC, but recent experimental work suggests that irradiation damaged SiC can undergo defect recovery under swift heavy ion irradiation. It is believed that local heating of the lattice due to the electronic energy deposition can anneal, and thereby recover, some of the disordered structure. We simulate the local heating due to the ions by the inelastic thermal spike model and perform molecular dynamics simulations of different model damage states to study the defect recovery on an atomistic level. We find significant recovery of point defects and a disordered layer, as well as recrystallization at the amorphous-to-crystalline interface of an amorphous layer. The simulation results support the swift heavy ion annealing hypothesis.

A study on the formation of Ag nanoparticles on the surface and catcher by ion beam irradiation of Ag thin films

Irradiation of Ag thin films with 100 MeV Ag ions leads to the formation of Ag nanoparticles on the surface as well as on the catcher, due to electronic energy loss mediated sputtering of Ag. The experimentally determined sputter yield of Ag is found to be three orders of magnitude higher than the values expected for bulk Ag, which is explained on the basis of the inelastic thermal spike model. The confinement of energy in the nanoparticles having size smaller than the electron mean free path (λ) and higher surface coverage area results in a higher sputtering yield. Transmission electron microscopy was performed to study the size distribution of nanoparticles on the catcher. The variation of sputtered particle yield with the number of constituent atoms follows an inverse power law with the value of exponent (δ) ∼ 0.33, at a fluence of 1 × 1013 ions cm−2. With increase in fluence up to 1 × 1014 ions cm−2, an additional value of exponent of δ ≈ 1 arises. The size of Ag nanoparticles is decreased with increased fluence due to ion-induced sputtering. The irradiated sample is found to have partially embedded nanoparticles showing localized surface plasmon resonance.

Dense and nanometric electronic excitations induced by swift heavy ions in an ionic CaF2crystal: Evidence for two thresholds of damage creation

CaF${}_{2}$ crystals as representatives of the class of ionic nonamorphizable insulators were irradiated with many different swift heavy ions of energy above 0.5 MeV/u providing a broad range of electronic energy losses (${S}_{e}$). Beam-induced modifications were characterized by Channeling Rutherford Backscattering Spectrometry (C-RBS) and x-ray diffraction (XRD), complemented by transmission electron microscopy (TEM). Results from C-RBS give evidence of significant damage appearing above a ${S}_{e}$ threshold of 5 $\ifmmode\pm\else\textpm\fi{}$ 2 keV/nm. A second critical ${S}_{e}$ appears around 18 $\ifmmode\pm\else\textpm\fi{}$ 3 keV/nm; below this value the damage as function of ion fluence saturates at 20$%$, while above this the damage saturation level increases with ${S}_{e}$, reaching $\ensuremath{\sim}$60$%$ for ions of ${S}_{e}$ $=$ 30 keV/nm. XRD measurements also show effects indicating two threshold values. Above 5 keV/nm, the widths of the XRD reflection peaks increase due to the formation of nanograins, as seen by TEM, while a significant decrease of the peak areas only occurs above 18 keV/nm. The track radii deduced from C-RBS measurements are in agreement with those extracted from the fluence evolution of the widths of the XRD peaks. Moreover, track radii deduced from the peak area analysis are slightly smaller but in agreement with previous track observations by high resolution electron microscopy. Calculations based on the inelastic thermal spike model suggest that the lower threshold at 5 keV/nm is linked to the quenching of the molten phase, whereas the threshold at 18 keV/nm can be interpreted as quenching of the boiling phase. The results of CaF${}_{2}$ are compared with other nonamorphizable materials such as LiF and UO${}_{2}$.

Modification of Fe-B based metallic glasses using swift heavy ions

We report on small-angle x-ray scattering (SAXS) measurements of amorphous Fe80B20, Fe85B15, Fe81B13.5Si3.5C2, and Fe40Ni40B20 metallic alloys irradiated with 11.1 MeV/u 132 Xe, 152 Sm, 197 Au, and 8.2 MeV/u 238 U ions. SAXS experiments are nondestructive and give evidence for ion track formation including quantitative information about the size of the track radius. The measurements also indicate a cylindrical track structure with a sharp transition to the undamaged surrounding matrix material. Results are compared with calculations using an inelastic thermal spike model to deduce the critical energy loss for the track formation threshold. The damage recovery of ion tracks produced in Fe80B20 by 11.1 MeV/u 197 Au ions was studied by

Damage creation threshold of Al2O3 under swift heavy ion irradiation

Among insulators, sapphire (Al2O3) is less sensitive than any amorphizable oxides regarding the irradiation with heavy ions in the electronic energy loss regime. Earlier experimental results indicated a critical energy loss for damage creation of around of 20keV/nm, while calculations based on the inelastic thermal spike model predict a value of ∼10keV/nm. In order to clarify this discrepancy, additional irradiation experiments were performed with different ions between Ni and U covering electronic energy losses between 10 and 43keV/nm and kinetic energies in the range from 1 to 11MeV/u. Irradiation effects were characterized by atomic force microscopy (hillock formation), profilometry (swelling), and channeling Rutherford backscattering (near-surface disorder). Combining the results from these measurements, an electronic energy loss threshold of 9.5±1.5keV/nm is extracted, indicating that the earlier threshold was overestimated and confirming the predictive power of the inelastic thermal spike model. Threshold predictions based on thermal spike model calculations are given for lower and higher beam energies.