S. E. Donnelly

Helium ion bombardment of thin aluminium films

A study is presented of the effect of 5 keV helium ion bombardment on thin (about 2000 A) aluminium films using proton backscattering, scanning and transmission electron microscopy and α particle energy loss spectroscopy. Measurements of helium content after irradiation using proton backscattering indicate low below-saturation retention for both room temperature and low temperature implantations (19% and 24% respectively). Electron microscopy examination of the films reveals a severe deformation in the form of coarse and fine-scale wrinkling whose amplitude increases with increasing helium dose. This deformation does not appear to be the result of bubble swelling. An attempt has been made to quantify the wrinkling by measuring the energy loss spectrum of α particles transmitted through irradiated films and the combination of these measurements with a simple sinusoidal deformation model indicates an increase in film area of up to 20%.

Engineering self-organising helium bubble lattices in tungsten

The self-organisation of void and gas bubbles in solids into superlattices is an intriguing nanoscale phenomenon. Despite the discovery of these lattices 45 years ago, the atomistics behind the ordering mechanisms responsible for the formation of these nanostructures are yet to be fully elucidated. Here we report on the direct observation via transmission electron microscopy of the formation of bubble lattices under He ion bombardment. By careful control of the irradiation conditions, it has been possible to engineer the bubble size and spacing of the superlattice leading to important conclusions about the significance of vacancy supply in determining the physical characteristics of the system. Furthermore, no bubble lattice alignment was observed in the <111> directions pointing to a key driving mechanism for the formation of these ordered nanostructures being the two-dimensional diffusion of self-interstitial atoms.

An in-situ TEM investigation of He bubble evolution in SiC

This paper presents work using the capabilities of two TEM with in-situ ion irradiation facilities: Microscope and Ion Accelerator for Materials Investigation (MIAMI) at the University of Huddersfield and Joint Accelerators for Nano-science and Nuclear Simulation JANNuS at Centre de Spectrometrie Nucleaire et de Spectrometrie de Masse (CSNSM), Orsay, France, to study the nucleation and growth of He bubbles in silicon carbide (SiC) and to carry out an investigation into bubble behaviour at high temperatures and under displacing irradiation. Preliminary results on bubble nucleation and growth during He irradiation of SiC are presented together with results from a simultaneous anneal and high-energy heavy-ion irradiation of samples containing He bubbles. The displacing irradiation is observed to impede He bubble growth resulting in smaller bubbles than those obtained from an anneal alone. A tentative interpretation of these observations is presented.

Effects of crystallographic and geometric orientation on ion beam sputtering of gold nanorods

Nanostructures may be exposed to irradiation during their manufacture, their engineering and whilst in-service. The consequences of such bombardment can be vastly different from those seen in the bulk. In this paper, we combine transmission electron microscopy with in situ ion irradiation with complementary computer modelling techniques to explore the physics governing the effects of 1.7 MeV Au ions on gold nanorods. Phenomena surrounding the sputtering and associated morphological changes caused by the ion irradiation have been explored. In both the experiments and the simulations, large variations in the sputter yields from individual nanorods were observed. These sputter yields have been shown to correlate with the strength of channelling directions close to the direction in which the ion beam was incident. Craters decorated by ejecta blankets were found to form due to cluster emission thus explaining the high sputter yields.

TEM with in situ Ion Irradiation of Nuclear Materials under In-Service Conditions

Nuclear materials are subjected to extreme conditions including elevated temperatures, large numbers of atomic displacements and the introduction of gas atoms. The exact conditions faced by a material in a nuclear reactor will depend on the type of reactor, the operating conditions, elemental composition and location within the core. Atomic displacements can be measured in Displacements Per Atom (DPA) where at 1 DPA each atom has, on average, been displaced from a lattice site once. Atomic displacements are caused by the energetic products of induced nuclear reactions and of spontaneous events such as (n,α) or (n,p) decay. Such transmutation reactions can also introduce light-gas atoms of He and H of which He poses the substantially-greater problem [1]. Generally, He is insoluble in structural reactor materials, diffuses rapidly and accumulates in regions of low electron-density such as grain boundaries and vacancies [2]. The combination of temperature, atomic displacements and He accumulation leads to a complex evolution of the microstructure which can have important implications for the performance of nuclear reactor components over their operational lifetime.

Ion implantation in nanodiamonds: size effect and energy dependence

Nanoparticles are ubiquitous in nature and are increasingly important for technology. They are subject to bombardment by ionizing radiation in a diverse range of environments. In particular, nanodiamonds represent a variety of nanoparticles of significant fundamental and applied interest. Here we present a combined experimental and computational study of the behaviour of nanodiamonds under irradiation by xenon ions. Unexpectedly, we observed a pronounced size effect on the radiation resistance of the nanodiamonds: particles larger than 8 nm behave similarly to macroscopic diamond (i.e. characterized by high radiation resistance) whereas smaller particles can be completely destroyed by a single impact from an ion in a defined energy range. This latter observation is explained by extreme heating of the nanodiamonds by the penetrating ion. The obtained results are not limited to nanodiamonds, making them of interest for several fields, putting constraints on processes for the controlled modification of nanodiamonds, on the survival of dust in astrophysical environments, and on the behaviour of actinides released from nuclear waste into the environment.

Rapid and damage-free outgassing of implanted helium from amorphous silicon oxycarbide

Damage caused by implanted helium (He) is a major concern for material performance in future nuclear reactors. We use a combination of experiments and modeling to demonstrate that amorphous silicon oxycarbide (SiOC) is immune to He-induced damage. By contrast with other solids, where implanted He becomes immobilized in nanometer-scale precipitates, He in SiOC remains in solution and outgasses from the material via atomic-scale diffusion without damaging its free surfaces. Furthermore, the behavior of He in SiOC is not sensitive to the exact concentration of carbon and hydrogen in this material, indicating that the composition of SiOC may be tuned to optimize other properties without compromising resistance to implanted He.

Atom-by-Atom STEM Investigation of Defect Engineering in Graphene

1 SuperSTEM Laboratory, STFC Daresbury Campus, Keckwick Lane, Daresbury WA4 4AD, U.K. 2 Faculty of Physics, University of Vienna, Strulhofgasse 4, A-1090 Vienna, Austria 3 Department of Physics, University of Helsinki, P.O. Box 43, FI-00014, Helsinki, Finland 4 School of Computing and Engineering, University of Huddersfield, Huddersfield HD1 3DH, U.K. 5 School of Materials, University of Manchester, Manchester M13 3PL, U.K. 6 Department of Physics and Energy, University of Limerick, Limerick, Ireland