Low Energy Helium Implantation Research Articles

Emergent Magnetism with Continuous Control in the Ultrahigh-Conductivity Layered Oxide PdCoO2.

The current challenge to realizing continuously tunable magnetism lies in our inability to systematically change properties, such as valence, spin, and orbital degrees of freedom, as well as crystallographic geometry. Here, we demonstrate that ferromagnetism can be externally turned on with the application of low-energy helium implantation and can be subsequently erased and returned to the pristine state via annealing. This high level of continuous control is made possible by targeting magnetic metastability in the ultrahigh-conductivity, nonmagnetic layered oxide PdCoO2 where local lattice distortions generated by helium implantation induce the emergence of a net moment on the surrounding transition metal octahedral sites. These highly localized moments communicate through the itinerant metal states, which trigger the onset of percolated long-range ferromagnetism. The ability to continuously tune competing interactions enables tailoring precise magnetic and magnetotransport responses in an ultrahigh-conductivity film and will be critical to applications across spintronics.

The effect of low energy helium implantation on the structural, vibrational, and piezoelectric properties of AlN thin films

Epitaxial AlN thin films were implanted with He at 8 keV along the c-axis growth direction with fluences ranging from 1015 to 1017 atoms/cm2. We find that the He implantation generated uniaxial tensile strain along the c-axis for all fluences that increased with fluence. There was an additional (002) X-ray diffraction peak for the highest He fluences of 5 × 1016 atoms/cm2 and 1017 atoms/cm2 that we interpret in terms of He bubble formation. He implantation leads to significant nitrogen (N) site disorder as evidenced by rapid broadening of the E2 (high) Raman mode with increasing fluence due to a reduction of the phonon mean free path. The piezoelectric coefficient, d33, reduces with increasing fluence, and is ~27% of the unimplanted d33 for the highest fluence. This result can be interpreted as implantation-induced nitrogen site disorder that randomises and reduces the Al–N dipoles. Thus, we show that while He implantation induces uniaxial strain, it decreases d33 due to implantation-induced N site disorder.

Spatially dependent kinetics of helium in tungsten under fusion conditions

Tungsten is the prime candidate material for divertor applications in future nuclear reactors (e.g. ITER and DEMO). In the present work, a spatially dependent cluster dynamics model is developed to investigate and understand the microstructure evolution of tungsten under low energy helium implantation and neutron irradiation varying over bulk length scales of millimetres and irradiation time scales of hours. The diffusion of helium, helium clusters and their trapping at neutron induced defects is simulated along the tungsten monoblock depth. The temperature gradient resulting from a steady state heat load of 10 MWm−2 along the monoblock depth is considered and its influence on the evolution of defects is discussed. The trapping of helium at vacancies and the associated formation of helium-vacancy clusters is found to be pronounced in the sub-surface layers. A significant influence of helium detrapping from grain boundaries and dislocations, along with its resolution from clusters, on the helium diffusion length scales is observed. Additionally, the effect of helium cluster mobility is investigated and overall lower retention in the monoblock bulk is observed through significant release of helium at the surface.

In-Situ Helium Implantation and TEM Investigation of Radiation Tolerance to Helium Bubble Damage in Equiaxed Nanocrystalline Tungsten and Ultrafine Tungsten-TiC Alloy.

The use of ultrafine and nanocrystalline materials is a proposed pathway to mitigate irradiation damage in nuclear fusion components. Here, we examine the radiation tolerance of helium bubble formation in 85 nm (average grain size) nanocrystalline-equiaxed-grained tungsten and an ultrafine tungsten-TiC alloy under extreme low energy helium implantation at 1223 K via in-situ transmission electron microscope (TEM). Helium bubble damage evolution in terms of number density, size, and total volume contribution to grain matrices has been determined as a function of He+ implantation fluence. The outputs were compared to previously published results on severe plastically deformed (SPD) tungsten implanted under the same conditions. Large helium bubbles were formed on the grain boundaries and helium bubble damage evolution profiles are shown to differ among the different materials with less overall damage in the nanocrystalline tungsten. Compared to previous works, the results in this work indicate that the nanocrystalline tungsten should possess a fuzz formation threshold more than one order of magnitude higher than coarse-grained tungsten.

Designing Morphotropic Phase Composition in BiFeO3.

In classical morphotropic piezoelectric materials, rhombohedral and tetragonal phase variants can energetically compete to form a mixed phase regime with improved functional properties. While the discovery of morphotropic-like phases in multiferroic BiFeO3 films has broadened this definition, accessing these phase spaces is still typically accomplished through isovalent substitution or heteroepitaxial strain which do not allow for continuous modification of phase composition postsynthesis. Here, we show that it is possible to use low-energy helium implantation to tailor morphotropic phases of epitaxial BiFeO3 films postsynthesis in a continuous and iterative manner. Applying this strain doping approach to morphotropic films creates a new phase space based on internal and external lattice stress that can be seen as an analogue to temperature-composition phase diagrams of classical morphotropic ferroelectric systems.

Large-scale atomistic simulations of low-energy helium implantation into tungsten single crystals

Large-scale molecular dynamics simulations of post-implantation helium behavior in plasma-facing tungsten single crystals reveal orientation-dependent depth profiles, surface evolution patterns, and other crystallographic and diffusion-related characteristics of helium behavior in tungsten during the first microsecond. The flux of implanted helium atoms studied, Γ ≈ 4 × 1025 m−2 s−1, is about one order of magnitude larger than that expected in ITER, the experimental fusion reactor currently being constructed in France. With simulation times on the order of 1 μs, these results serve to discover the mechanisms involved in surface evolution as well as to serve as benchmarks for coarse-grained simulations such as kinetic Monte Carlo and continuum-scale drift–reaction–diffusion cluster dynamics simulations. The findings of our large-scale simulations are significant due to diminished finite-size effects and the longer times reached (corresponding to higher fluences). Specifically, our findings are drastically different from findings published previously in the literature for (001) surfaces under a helium flux of Γ ∼ 1028 m−2 s−1, which is typical of smaller size and shorter time atomistic simulations. In particular, this study highlights the atomic-scale materials processes relevant to helium entrapment and transport in metals, which have implications not only for nuclear fusion–relevant processes, but also helium-induced embrittlement in irradiated materials such as hospital equipment and fission reactor materials.

Symmetry driven control of optical properties in WO3 films

In this work, we demonstrate that the optical bandgap of WO3 films can be continuously controlled through uniaxial strain induced by low-energy helium implantation. The insertion of He into epitaxially grown and coherently strained WO3 films can be used to induce single axis out-of-plane lattice expansion of up to 2%. Ellipsometric spectroscopy reveals that the optical bandgap is reduced by about 0.18 eV per percent expansion of the out-of-plane unit cell length. Density functional theory calculations show that this response is a direct result of changes in orbital degeneracy driven by changes in the octahedral rotations and tilts.

Continuously Controlled Optical Band Gap in Oxide Semiconductor Thin Films.

The optical band gap of the prototypical semiconducting oxide SnO2 is shown to be continuously controlled through single axis lattice expansion of nanometric films induced by low-energy helium implantation. While traditional epitaxy-induced strain results in Poisson driven multidirectional lattice changes shown to only allow discrete increases in bandgap, we find that a downward shift in the band gap can be linearly dictated as a function of out-of-plane lattice expansion. Our experimental observations closely match density functional theory that demonstrates that uniaxial strain provides a fundamentally different effect on the band structure than traditional epitaxy-induced multiaxes strain effects. Charge density calculations further support these findings and provide evidence that uniaxial strain can be used to drive orbital hybridization inaccessible with traditional strain engineering techniques.

Low energy helium implantation in evaporated nickel surfaces

This paper reports data on helium (He) implantation at low energy (100--200eV) in nickel (Ni) that support a concept for He self-pumping in tokamaks, and specifically an experiment in TEXTOR. These data indicate trapping saturates at about 4 {times} 10{sup 19} He/m{sup 2} areal density for He implanted at 100 eV He in Ni at 500{degrees}C. Neither prior implantation of deuterium (D) nor D bombardment of Ni surfaces already containing implanted He significantly changed the He saturation value. A model for trapping capacity and these data were used to predict He self pumping in TEXTOR.

Helium bubbles in molybdenum investigated by positron annihilation spectroscopy

Abstract Positron lifetime and Doppler broadening measurements have been performed in homogeneously helium implanted (715 appm) molybdenum. A new stage beyond the temperature range of stability of voids has been observed. The shorter lifetime componet τ1 = 158 ± 2 ps of 60% intensity is assigned to the positron state in helium-decorated dislocation loops while the longer lifetime component τ2 = 408 ± 5 ps of 40% intensity is explained in terms of positron trapping at voids with a small percentage of helium associated with them. Dissociation of helium from loops is observed in the range 300–800 K followed by loop annealing above 900 K. A sharp reduction of τ2 around 1300 K is assigned to multiple helium occupancy of voids, resulting in their transformation into stable bubbles. Helium retention in bubbles is found stable up to the highest annealing temperature of 1700 K. The bubble parameters deduced from the positron lifetime results reveal that the pressure in the bubbles is maintained at the near-equilibrium value and that the vacancy mode of bubble growth is operative, in contrast to the reported growth of athermal bubbles in molybdenum following very low energy helium implantation where loop punching mechanism seems to control the excess pressure in bubbles. A brief comparison is also made between the observed behaviour of helium with that of hydrogen in molybdenum and the differences are discussed.

Investigation of the state of helium and radiation defects in He-irradiated nickel and copper by glancing angle X-ray diffraction

Abstract Damage caused by low energy helium implantation in nickel and copper has been investigated for the first time by glancing angle X-ray diffraction technique. The damage confined only to the near surface region, typically few thousands A, comprises of displacement type damage and implanted species. As X-ray penetration depth (for 99% absorption) can be continuously varied as a function of angle of incidence and X-ray wavelength, accurate lattice parameter measurements (back-reflection) can be used to obtain cumulative information regarding long- and short-range strain field of defects. High purity nickel and copper foils were irradiated at room temperature with 100 keV helium ions to fluence levels ranging from 1016 to 1018 ions/cm2. The results show: (a) simultaneous presence of two defect complexes of opposite strain field character in both nickel and copper, (b) mobility of vacancies is an important parameter for determining the nature and the strength of defect complex, and (c) local strains du...

He Desorption of Defects in Metals

By low energy helium implantation of a metal single crystal, defects present in the metal can be decorated with helium. During linear heating with time helium dissociation from the defects is observed. Desorption peaks emerge, characteristic for certain defects. A short review will be presented of the experimental requirements together with the results obtained so far with the technique. Helium desorption from point defects like empty vacancies and vacancies occupied by a heavy noble gas atom or a foreign metal atom is discussed. The first stages of clustering of two or three adjacent point defects, can well be detected. Desorption studies of a pulsed laser irradiated single crystal indicate the formation of vacancy clusters. The interaction of self-interstitials with some of the above mentioned point defects is described.