Gray track formation in KTiOPO4 by swift ion irradiation

Potassium titanyl phosphate single crystals were irradiated with 48 MeV lithium ions at fluences varying from 5×1012 to 1016 ions/cm2. The defects created in the crystal have been characterized using x-ray rocking curve measurements, optical transmittance, and photoluminescence spectroscopy. From x-ray rocking curve studies, the full width at half maximum for the irradiated samples was observed to increase, indicating lattice strain caused by the energetic ions. Optical transparency of these samples was found to decrease upon irradiation. The irradiated samples exhibited a broadband luminescence in the 700–900 nm region, for fluences above 5×1013 ions/cm2. The results indicate that ion-beam-induced optical effects in KTiOPO4 single crystals are very similar to the ones obtained for crystals with “gray tracks,” which are attributed to the electronic transitions in the Ti3+ levels.

Recrystallization of amorphous nano-tracks and uniform layers generated by swift-ion-beam irradiation in lithium niobate.

The thermal annealing of amorphous tracks of nanometer-size diameter generated in lithium niobate (LiNbO3) by Bromine ions at 45 MeV, i.e., in the electronic stopping regime, has been investigated by RBS/C spectrometry in the temperature range from 250°C to 350°C. Relatively low fluences have been used (<1012 cm−2) to produce isolated tracks. However, the possible effect of track overlapping has been investigated by varying the fluence between 3×1011 cm−2 and 1012 cm−2. The annealing process follows a two-step kinetics. In a first stage (I) the track radius decreases linearly with the annealing time. It obeys an Arrhenius-type dependence on annealing temperature with activation energy around 1.5 eV. The second stage (II) operates after the track radius has decreased down to around 2.5 nm and shows a much lower radial velocity. The data for stage I appear consistent with a solid-phase epitaxial process that yields a constant recrystallization rate at the amorphous-crystalline boundary. HRTEM has been used to monitor the existence and the size of the annealed isolated tracks in the second stage. On the other hand, the thermal annealing of homogeneous (buried) amorphous layers has been investigated within the same temperature range, on samples irradiated with Fluorine at 20 MeV and fluences of ∼1014 cm−2. Optical techniques are very suitable for this case and have been used to monitor the recrystallization of the layers. The annealing process induces a displacement of the crystalline-amorphous boundary that is also linear with annealing time, and the recrystallization rates are consistent with those measured for tracks. The comparison of these data with those previously obtained for the heavily damaged (amorphous) layers produced by elastic nuclear collisions is summarily discussed.

Stopping power dependence of nitrogen sputtering yields in copper nitride films under swift-ion irradiation: Exciton model approach

Nitrogen sputtering yields as high as 104 atoms/ion, are obtained by irradiating N-rich-Cu3N films (N concentration: 33 ± 2 at.%) with Cu ions at energies in the range 10?42 MeV. The kinetics of N sputtering as a function of ion fluence is determined at several energies (stopping powers) for films deposited on both, glass and silicon substrates. The kinetic curves show that the amount of nitrogen release strongly increases with rising irradiation fluence up to reaching a saturation level at a low remaining nitrogen fraction (5?10%), in which no further nitrogen reduction is observed. The sputtering rate for nitrogen depletion is found to be independent of the substrate and to linearly increase with electronic stopping power (Se). A stopping power (Sth) threshold of ?3.5 keV/nm for nitrogen depletion has been estimated from extrapolation of the data. Experimental kinetic data have been analyzed within a bulk molecular recombination model. The microscopic mechanisms of the nitrogen depletion process are discussed in terms of a non-radiative exciton decay model. In particular, the estimated threshold is related to a minimum exciton density which is required to achieve efficient sputtering rates.

Refractive index changes in amorphous SiO2 (silica) by swift ion irradiation

The refractive index changes induced by swift ion-beam irradiation in silica have been measured either by spectroscopic ellipsometry or through the effective indices of the optical modes propagating through the irradiated structure. The optical response has been analyzed by considering an effective homogeneous medium to simulate the nanostructured irradiated system consisting of cylindrical tracks, associated to the ion impacts, embedded into a virgin material. The role of both, irradiation fluence and stopping power, has been investigated. Above a certain electronic stopping power threshold (∼2.5 keV/nm), every ion impact creates an axial region around the trajectory with a fixed refractive index (around n = 1.475) corresponding to a certain structural phase that is independent of stopping power. The results have been compared with previous data measured by means of infrared spectroscopy and small-angle X-ray scattering; possible mechanisms and theoretical models are discussed.

Étude théorique des mécanismes de transfert d'énergie suivant le passage d'un ion rapide sans un matériau

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

Crack mechanical failure in lithium niobate crystal under ion irradiation; novel simulation by extended finite elements

Swift heavy ion irradiation (ions with mass heavier than 15 and energy exceeding MeV/amu) transfer their energy mainly to the electronic system with small momentum transfer per collision. Therefore, they produce linear regions (columnar nano-tracks) around the straight ion trajectory, with marked modifications with respect to the virgin material, e.g., phase transition, amorphization, compaction, changes in physical or chemical properties. In the case of crystalline materials the most distinctive feature of swift heavy ion irradiation is the production of amorphous tracks embedded in the crystal. Lithium niobate is a relevant optical material that presents birefringence due to its anysotropic trigonal structure. The amorphous phase is certainly isotropic. In addition, its refractive index exhibits high contrast with those of the crystalline phase. This allows one to fabricate waveguides by swift ion irradiation with important technological relevance. From the mechanical point of view, the inclusion of an amorphous nano-track (with a density 15% lower than that of the crystal) leads to the generation of important stress/strain fields around the track. Eventually these fields are the origin of crack formation with fatal consequences for the integrity of the samples and the viability of the method for nano-track formation. For certain crystal cuts (X and Y), these fields are clearly anisotropic due to the crystal anisotropy. We have used finite element methods to calculate the stress/strain fields that appear around the ion-generated amorphous nano-tracks for a variety of ion energies and doses. A very remarkable feature for X cut-samples is that the maximum shear stress appears on preferential planes that form +/-45º with respect to the crystallographic planes. This leads to the generation of oriented surface cracks when the dose increases. The growth of the cracks along the anisotropic crystal has been studied by means of novel extended finite element methods, which include cracks as discontinuities. In this way we can study how the length and depth of a crack evolves as function of the ion dose. In this work we will show how the simulations compare with experiments and their application in materials modification by ion irradiation.

Crack mechanical failure in ceramic materials under ion irradiation: case of lithium niobate crystal

Swift heavy ion irradiation (ions with mass heavier than 15 and energy exceeding MeV/amu) transfer their energy mainly to the electronic system with small momentum transfer per collision. Therefore, they produce linear regions (columnar nano-tracks) around the straight ion trajectory, with marked modifications with respect to the virgin material, e.g., phase transition, amorphization, compaction, changes in physical or chemical properties. In the case of crystalline materials the most distinctive feature of swift heavy ion irradiation is the production of amorphous tracks embedded in the crystal. Lithium niobate is a relevant optical material that presents birefringence due to its anysotropic trigonal structure. The amorphous phase is certainly isotropic. In addition, its refractive index exhibits high contrast with those of the crystalline phase. This allows one to fabricate waveguides by swift ion irradiation with important technological relevance. From the mechanical point of view, the inclusion of an amorphous nano-track (with a density 15% lower than that of the crystal) leads to the generation of important stress/strain fields around the track. Eventually these fields are the origin of crack formation with fatal consequences for the integrity of the samples and the viability of the method for nano-track formation. For certain crystal cuts (X and Y), these fields are clearly anisotropic due to the crystal anisotropy. We have used finite element methods to calculate the stress/strain fields that appear around the ion- generated amorphous nano-tracks for a variety of ion energies and doses. A very remarkable feature for X cut-samples is that the maximum shear stress appears on preferential planes that form +/-45º with respect to the crystallographic planes. This leads to the generation of oriented surface cracks when the dose increases. The growth of the cracks along the anisotropic crystal has been studied by means of novel extended finite element methods, which include cracks as discontinuities. In this way we can study how the length and depth of a crack evolves as function of the ion dose. In this work we will show how the simulations compare with experiments and their application in materials modification by ion irradiation.

Tailoring the Optical Properties of Silica Irradiated with Swift Heavy Ions

Irradiation with swift heavy ions (SHI), roughly defined as those having atomic masses larger than 15 and energies exceeding 1 MeV/amu, may lead to significant modification of the irradiated material in a nanometric region around the (straight) ion trajectory (i.e., latent tracks). In the case of amorphous silica it has been reported that SHI irradiation originates nano-tracks of either higher density than the virgin material (for low electronic stopping powers, Se < 7 keV/nm) [1] or having a low-density core and a dense shell (Se > 12 keV/nm) [2]. The intermediate region has not been studied in detail but we will show in this work that essentially no changes in density occur in this zone. An interesting effect of the compaction is that the refractive index is increased with respect to that of the surroundings. In the first Se region it is clear that track overlapping leads to continuous amorphous layers that present a significant contrast with respect to the pristine substrate and this has been used to produce optical waveguides. The optical effects of intermediate and high stopping powers, on the other hand, are largely unknown so far. In this work we have studied theoretically (molecular dynamics and optical simulations) and experimentally (irradiation with SHI and optical characterization) the dependence of the macroscopic optical properties (i.e., the refractive index of the effective medium, n_EMA) on the electronic stopping power of the incoming ions. Our results show that the refractive index of the irradiated silica is not increased in the intermediate region, as expected; however, the core-shell tracks of the high-Se region produce a quite effective enhancement of n_EMA that could prove attractive for the fabrication of optical waveguides at ultralow fluences (as low as 1E11 cm^-2). 1. J. Manzano, J. Olivares, F. Agulló-López, M. L. Crespillo, A. Moroño, and E. Hodgson, "Optical waveguides obtained by swift-ion irradiation on silica (a-SiO2)," Nucl. Instrum. Meth. B 268, 3147-3150 (2010). 2. P. Kluth, C. S. Schnohr, O. H. Pakarinen, F. Djurabekova, D. J. Sprouster, R. Giulian, M. C. Ridgway, A. P. Byrne, C. Trautmann, D. J. Cookson, K. Nordlund, and M. Toulemonde, "Fine structure in swift heavy ion tracks in amorphous SiO2," Phys. Rev. Lett. 101, 175503 (2008).

Swift heavy ion induced structural, iono and photoluminescence properties of beta-CaSiO3:Dy3+ nanophosphor

CaSiO3:Dy3+ (1-5 mol%) nanophosphors have been prepared by a low temperature solution combustion method. The structural and luminescence (ionoluminescence; IL and photoluminescence; PL) studies have been carried out for pristine and ion irradiated samples. The XRD patterns of pristine sample show a prominent peak at (320) for the monoclinic structure of beta-CaSiO3. Upon ion irradiation, the intensity of the prominent peak is decreased at the fluence of 7.81 x 10(12) ions cm(-2) and at higher fluence of 15.62 x 10(12) ions cm(-2), the prominent peak completely vanishes. The decrease in peak intensity might be due to the stress induced point defects. On-line IL and in situ PL studies have been carried out on pelletized samples bombarded with 100 MeV Si7+ ions with fluences in the range (7.81-15.62) x 10(12) ions cm(-2). The characteristic emission peaks at 481,574, 664 and 754 nm recorded in both IL and PL are attributed to the luminescence centers activated by Dy3+ ions. It is found that IL and PL emissions intensity decreases with increase in Si7+ ion fluence. The decrease in intensity can be due to the destruction of Si-O-Si and O-Si-O type species present on the surface of the sample. FTIR studies also confirm the Si-O-Si and O-Si-O type species observed to be sensitive for swift heavy ion (SHI) irradiated samples. (C) 2012 Elsevier B.V. All rights reserved.