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Register a new userOn the threshold of ion track formation in CaF2
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There is ongoing debate regarding the mechanism of swift heavy ion track formation in CaF2. The objective of this study is to shed light on this important topic using a range of complimentary experimental techniques. Evidence of the threshold for ion track formation being below 3 keV/nm is provided by both transmission electron microscopy and Rutherford backscattering spectroscopy in the channeling mode which has direct consequences for the validity of models describing the response of CaF2 to swift heavy ion irradiation. Advances in the TEM and RBS/c analyses presented here pave the way for better understanding of the ion track formation.
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The aim of this work is to investigate feasibility of the ion beam analysis techniques for monitoring swift heavy ion track formation. First, use of the in situ Rutherford backscattering spectroscopy 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.
The formation of nano-hillocks on CaF2 crystal surfaces by individual ion impact has been studied using medium energy (3 and 5 MeV) highly charged ions (Xe19+ to Xe30+) as well as swift (kinetic energies between 12 and 58 MeV) heavy ions. For very slow highly charged ions the appearance of hillocks is known to be linked to a threshold in potential energy while for swift heavy ions a minimum electronic energy loss is necessary. With our results we bridge the gap between these two extreme cases and demonstrate, that with increasing energy deposition via electronic energy loss the potential energy threshold for hillock production can be substantially lowered. Surprisingly, both mechanisms of energy deposition in the target surface seem to contribute in an additive way, as demonstrated when plotting the results 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 case where kinetic and potential energies are deposited into the surface.
We report the structural and optical properties of molecular beam epitaxy (MBE) grown 2-dimensional (2D) material molybdenum diselenide (MoSe2) on graphite, CaF2 and epitaxial graphene. Extensive characterizations reveal that 2H- MoSe2 grows by van-der-Waals epitaxy on all 3 substrates with a preferred crystallographic orientation and a Mo:Se ratio of 1:2. Photoluminescence at room temperature (~1.56 eV) is observed in monolayer MoSe2 on both CaF2 and epitaxial graphene. The band edge absorption is very sharp, <60 meV over 3 decades. Overcoming the observed small grains by promoting mobility of Mo atoms would make MBE a powerful technique to achieve high quality 2D materials and heterostructures.
In relativistic ion collisions there are excellent opportunities to produce and investigate hyper-nuclei. We have systematically studied the formation of hypernuclear spectator residues in peripheral heavy-ion collisions with the transport DCM and UrQMD models. The hyperon capture was calculated within the potential and coalescence approaches. We demonstrate that even at the beam energies around and lower than the threshold for producing Lambda hyperons in binary nucleon-nucleon interactions a considerable amount of hypernuclei, including multi-strange ones, can be produced. This is important for preparation of new experiments on hypernuclei in the wide energy range. The uncertainties of the predictions are investigated within the models, and the comparison with the strangeness production measured in experiments is also performed.
The atomic structure of nanomaterials is often studied using transmission electron microscopy. In addition to image formation, the energetic electrons may also cause damage while impinging on the sample. In a good conductor such as graphene the damage is limited to the knock-on process caused by elastic electron-nucleus collisions. This process is determined by the kinetic energy an atom needs to be sputtered, ie, its displacement threshold energy. This is typically assumed to have a fixed value for all electron impacts on equivalent atoms within a crystal. Here we show using density functional tight-binding simulations that the displacement threshold energy is affected by the thermal perturbation of the atoms from their equilibrium positions. We show that this can be accounted for in the estimation of the displacement cross section by replacing the constant threshold value with a distribution. The improved model better describes previous precision measurements of graphene knock-on damage, and should be considered also for other low-dimensional materials.
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