T.J. Novakowski

Nb2O5 Nanostructure Evolution on Nb Surfaces via Low Energy He+ Ion Irradiation

We propose low-energy, broad-beam He+ ion irradiation as a novel processing technique for the generation of Nb2O5 surface nanostructures due to its relative simplicity and scalability in a commercial setting. Since there have been relatively few studies involving the interaction of high-fluence, low-energy He+ ion irradiation and Nb (or its oxidized states), this systematic study explores both effects of fluence and sample temperature during irradiation on resulting surface morphology. Detailed normal and cross-sectional scanning electron microscopy (SEM) studies reveal subsurface He bubble formation and elucidate potential driving mechanisms for nanostructure evolution. A combination of specular optical reflectivity and X-ray photoelectron spectroscopy (XPS) is also used to gain additional information on roughness and stoichiometry of irradiated surfaces. Our investigations show significant surface modification for all tested irradiation conditions; the resulting surface structure size and geometry have a strong dependence on both sample temperature during irradiation and total ion fluence. Optical reflectivity measurements on irradiated surfaces demonstrate increased surface roughening with increasing ion fluence, and XPS shows higher oxidation levels for samples irradiated at lower temperatures, suggesting larger surface roughness and porosity. Overall, it was found that low-energy He+ ion irradiation is an efficient processing technique for nanostructure formation, and surface structures are highly tunable by adjusting ion fluence and Nb2O5 sample temperature during irradiation. These findings may have excellent potential applications for solar energy conversion through improved efficiency due to effective light absorption.

Effect of high-flux, low-energy He + ion irradiation on Ta as a plasma-facing material

The goal of this work is to assess Ta as a potential plasma-facing material for future fusion reactors in terms of its response to high-flux, low-energy He+ ion irradiation. Ta samples were irradiated with 100 eV He+ ions at various fluences up to 3.5 × 1025 ions m−2 while simultaneously heated at constant temperatures in the range 823–1223 K. SEM studies show that irradiated Ta surfaces undergo significant morphology changes that have a strong dependence on both ion fluence and sample temperature. Optical reflectivity complements SEM and demonstrates a vertical growth of surface structures with increasing fluence. Ex situ XPS and XRD both show significant oxidation of the irradiated Ta surfaces, giving further qualitative information on the extent of surface modification. Overall, these irradiation-induced structures on Ta are similar to early-stage “fuzz” structures observed in W. However, Ta exhibits a higher fluence threshold for structure formation. While Ta may have less desirable bulk properties (e.g., thermal conductivity) when compared to W, its higher resilience to He+ ion-induced surface modification suggests that surface thermal and mechanical properties may not degrade as quickly in extreme fusion environments; this quality may be a redeeming factor for Ta as a plasma-facing material.

Silicon nanocone formation via low-energy helium ion sputtering

In this study, the effect of low-energy (100 eV) He+ ion irradiation on Si surface morphology is explored. Si (100) and (111) samples were irradiated with 100 eV He+ ions at an elevated sample temperature of 600 °C and to fluences in the range 5.0 × 1019–2.0 × 1020 ions cm–2. Through a combination of high ion flux and high sample temperature, it was found that continued He+ ion irradiation facilitates the formation of homogeneously populated, high aspect ratio silicon nanocones (NCs) (∼50–100 nm base and ∼200–400 nm height). The resulting surface morphology is shown to have excellent antireflective properties, suggesting potential application toward enhanced light absorption in photovoltaic and other optical applications. Furthermore, similar irradiations at reduced sample temperature show comparable structuring mechanisms but with smaller cone diameter. These results indicate that NC size and number density (and related wavelength-dependent reflectivity properties) may be tailored by carefully tuning ion irradiation conditions. Utilizing very low-energy He+ ions as the irradiating species, these studies also demonstrate an added benefit to limiting metallic surface contamination through reduced probability of sputtering in-vacuum components.In this study, the effect of low-energy (100 eV) He+ ion irradiation on Si surface morphology is explored. Si (100) and (111) samples were irradiated with 100 eV He+ ions at an elevated sample temperature of 600 °C and to fluences in the range 5.0 × 1019–2.0 × 1020 ions cm–2. Through a combination of high ion flux and high sample temperature, it was found that continued He+ ion irradiation facilitates the formation of homogeneously populated, high aspect ratio silicon nanocones (NCs) (∼50–100 nm base and ∼200–400 nm height). The resulting surface morphology is shown to have excellent antireflective properties, suggesting potential application toward enhanced light absorption in photovoltaic and other optical applications. Furthermore, similar irradiations at reduced sample temperature show comparable structuring mechanisms but with smaller cone diameter. These results indicate that NC size and number density (and related wavelength-dependent reflectivity properties) may be tailored by carefully tuning ion...