Computational materials science 

In 2014 the computational materials research activities were marked by exciting results obtained together with experimental groups around the world. Working with collaborators in Australia,
we examined the experimentally observed surprising effect of controllable shape modification of nanoparticles embedded in solids by very energetic (MeV and GeV) heavy ion irradiation.
By combining transmission electron microscopy experiments and atomistic computer simulations, we showed that flow of hot molten metal and its recrystallization between ion impacts together explain the experimentally observed elongation [1].

On the other end of the energy scale used for ion irradiations, working with experimental groups in Luxemburg and at Aalto University, we examined the deposition of organic molecules with energies of a few eV on surfaces. The results showed that sputter deposition can be used to identify surface reactions between different molecular fragments [2].

As part of an increasingly active collaboration with the OIST institute in Japan, we combined experiments and simulations of the formation of Si nanoclusters during gas phase condensation.  This work showed conclusively that, given the right combinations of temperature, cooling rate and partial gas densities in the condensation phase, Si nanoclusters can crystallize already in the gas phase.  Detailed analysis in the simulations established that a sufficiently high
temperature in the gas phase is required for crystallization, and also that polycrystalline phases grow from separate crystallization nuclei [3].

As part of the large international ITER fusion reactor development process, we examined the recently discovered intriguing effect where the normally hard and very dense metal W turns into a low density fuzz when exposed to He bombardment in fusion reactor conditions. We showed,
with a new kinetic Monte Carlo algorithm developed specifically for the purpose, that the fuzz formation is caused by He bubble growth via dislocation loop punching and rupture, leading to a surface roughening [4].

Working together with groups in Germany and Austria, we found that
ion irradiation of single layers of graphene on iridium surfaces can lead to the formation of additional nanoplatelets of graphene between the initial layer and the metal. This further lead us to introduce the new concept of trapping yield, i.e. the fraction of incoming atoms trapped in the material [5].

[1] A. A. Leino, O. H. Pakarinen, F. Djurabekova, K. Nordlund, P. Kluth, and M . C. Ridgway, Swift Heavy Ion Induced Shape Transformation of Au Nanocrystals Me diated by Molten Material Recrystallization, Materials Research Letters 2, 37 (2014).

[2] C. Turgut, G. Sinha, L. Mether, J. Lahtinen, K. Nordlund, M. Belmahi, and P. Philipp, An experimental and numerical study of sub-monolayer sputter deposit ion of polystyrene fragments on silver for the Storing Matter technique, Anal. Chem. 86, 11217 (2014).

[3] J. Zhao, C. Cassidy, P. Grammatikopoulos, V. Singh, K. Aranishi, M. Sowwan, K. Nordlund, and F. Djurabekova, Crystallization of Silicon Nanoclusters with Inert Gas Temperature Control, Phys. Rev. B (2014), accepted for publication.

[4] Loop punching and bubble rupture causing surface roughening - a model for W fuzz growth,
EPL 105, 25002 (2014).

[5] C. Herbig, E.H. Åhlgren, W. Jolie, C. Busse, J. Kotakoski, A.V. Krasheninnikov & Thomas Michely, Interfacial Carbon Nanoplatelet Formation by Ion Irradiation of Graphene on Ir(111), ACS Nano 8, 12208 (2014).

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Figure 2. Molecular dynamics simulations of the mechanism by which Au nanoclusters (yellow) embedded in a silica matrix (red) is elongated by 3 impacts of swift heavy ions. From Leino et al, Mater. Res. Lett. 2, 37 (2014).