Nanostructure Evolution on Nb Surfaces via Low-Energy He+

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

Theodore Joseph Novakowski, Jitendra Kumar Tripathi, and Ahmed Hassanein ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12502 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 4, 2016

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ACS Applied Materials & Interfaces

Nb2O5 Nanostructure Evolution on Nb Surfaces via Low Energy He+ Ion Irradiation Theodore Joseph Novakowski*, Jitendra Kumar Tripathi, and Ahmed Hassanein Center for Materials Under eXtreme Environment (CMUXE), School of Nuclear Engineering, Purdue University, West Lafayette, IN 47907, USA *Corresponding author: [email protected] Abstract In this study, 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 sub-surface 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; 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 1 ACS Paragon Plus Environment


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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.

Keywords: Niobium pentoxide, Nanostructures, Ion irradiation, Surface analysis, Fuzz formation, Solar energy conversion.

Introduction

Metal-oxide nanostructures are of growing interest in various scientific fields due to their wide range of technological applications.1 As the most stable and abundant niobiumoxide, Nb2O5 (an n-type semiconductor with a band gap of about 3.4 eV)2 has found applications in gas sensing,3 chemical catalysts,4,5 electrochromics,6 solar energy conversion,7-9 field-emission displays,10 supercapacitors,11

bio-applications,12-14 and

microelectronics.15 While increasing effective surface area benefits most of these applications, studies involving the formation of metal-oxide nanostructures for bioapplications also emphasize the ability to finely tune and control nanostructure size and/or geometry.16-18 Consequently, recent studies have investigated the use of electrosynthesis and anodization,13,19-21 phase transformation,22 sol-gel processing,12,23,24 and even growth through thermal oxidation10 to efficiently synthesize such Nb2O5 nanostructures in a controlled manner. As one of the more promising fabrication methods,1 the sol-gel process is able to control nano-scale features for a wide range of materials.25 However, sol-gel processing for Nb2O5 often involves the use of harsh

2 ACS Paragon Plus Environment

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reactants and could present safety hazards toxicity issues in an industrial setting. As an alternative, bottom-up approach to the synthesis of Nb2O5 nanostructures, recent studies have proposed the use of broad-beam ion processing.26 Utilizing low-energy, chemically inert helium ions (He+), this one-step fabrication method would also preserve contact between different crystallites in heterogeneous systems and avoid electrical conductivity limitations. Due to the rich variety of nanostructure geometries that may be formed by ion beam irradiation, the number of potential technological applications for these nanostructured Nb2O5 surfaces surmounts the scope of a single research article. Instead, the analysis and discussion here will focus solely on a single application: solar energy conversion. In particular, highly structured metal (and metal-oxide) surfaces are relevant to solar-thermal applications,27 where solar light is converted to heat. For this application, the structured substrate must absorb light over a wide range of incident light (ideally, the entire wavelength spectrum of solar light). While ion beam processing is a relatively new approach for metal-oxide nanostructure formation, studies involving the interaction of low-energy He ions and metal surfaces date back to the early 1970’s in the context of nuclear fusion studies.28 For Nb, He+ ion irradiation at sufficiently high doses was determined to cause the formation of subsurface bubbles,29-31 surface blisters,32 and even nano-scale surface protrusions.33 While these studies involved irradiation of bulk Nb, rather than Nb2O5, these sample surfaces were assumed to be naturally oxidized due to lack of explicit sample cleaning or preparation. Furthermore, our recent preliminary studies26 on stoichiometric Nb2O5 under similar irradiation conditions suggest congruent trends in surface structure formation with little discretion toward surface oxidation state. More recently, several studies have



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investigated the response of high-flux, low-energy He+ ion irradiation in other refractory metals such as tungsten (W),34-39 molybdenum (Mo),39-42 tantalum (Ta),43,44 and vanadium (V)45 due to their relevance as plasma-facing materials in future nuclear fusion devices.46 In all studies, substantial doses of He+ ions were found to have dramatic effects on surface morphology. In W, for example, high doses of low-energy He+ ion irradiation under certain conditions were found to cause bubble formation,37 surface blistering,34 and fine nano-scale surface protrusions that are commonly referred to as “fuzz”.38 This “fuzz” is essentially a conglomerated mass of inter-connecting surface tendrils resulting from accumulation, growth, and rupture of sub-surface He bubbles. While the formation of W fuzz is generally concerning for the operation of nuclear fusion reactors,47-49 exposing these high-surface-area fuzzy structures allows for rapid oxidation and efficient production of nanostructured W-oxide surfaces. Due to a developing interest in refractory metal-oxide nanostructures, these fusion-relevant fuzzy W surfaces have been applied toward alternative applications such as solar light absorption,27 solar water splitting,50 and photocatalysts51 with promising results. For Nb2O5, on the other hand, fully developed fuzz has not yet been demonstrated experimentally. However, our preliminary investigations indicate that Nb2O5 fuzz formation should be possible under certain irradiation conditions.26 In this work, we systematically study the response of Nb2O5 surfaces (naturally oxidized Nb) to high-flux, low-energy He+ ion irradiation, considering this irradiation process as a potential route to Nb2O5 nanostructure formation.

Results and Discussion


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Electron backscatter diffraction (EBSD) was used to characterize pre-irradiated surfaces and generate the Nb grain map and corresponding inverse pole figure (IPF) shown in Figure 1. The grain map shows noticeable grain deformation and evidence of banding, both of which are expected in cold-rolled refractory metals.52 Average grain size, calculated from an intercept line method applied to the grain map in Figure 1, was ~1.2 µm. The discrete IPF in Figure 1 shows a preferential grain orientation in the (111) direction; some preferential orientation is also expected from the cold-rolling process.52 One rudimentary yet crucial result from this analysis is that the present Nb substrate does not exhibit ultrafine or nanocrystalline grain sizes. While this result is not unexpected, some understanding of grain size distribution is customarily considered useful for the study of radiation-induced morphology changes. As the grain size of a material decreases, the grain boundary density increases; these grain boundaries are known to act as sinks for interstitials and vacancies and may help to improve the radiation tolerance of materials.53,54 In fact, previous studies specifically involving He+ ion irradiation in W have experimentally demonstrated an improved radiation tolerance for samples exhibiting nanocrystalline or ultrafine grain sizes.55,56 Therefore, understanding material grain size distribution is necessary to justly decouple grain boundary effects from intrinsic material properties in the analysis of irradiated materials. Our EBSD data shows Nb grain sizes on the order of microns, suggesting that grain boundaries would have a relatively insignificant contribution to radiation tolerance. This result is advantageous for this study since our goal is to encourage surface structure formation rather than mitigate it. After adequate surface pre-characterization, Nb samples were allowed to naturally oxidize in ambient atmospheric conditions. X-ray photoelectron spectroscopy (XPS)



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confirmed that Nb surfaces were heavily oxidized in the Nb2O5 state before irradiation processing. To study the effect of both He+ ion fluence and sample temperature during irradiation on resulting surface morphology, Nb2O5 surfaces were irradiated with 100 eV He+ ions at several temperatures in the range 823 – 1223 K and to various total He+ ion fluences in the range 4.3 × 1024 – 1.7 × 1025 ions m-2. Figure 2 shows scanning electron microscopy (SEM) micrographs for each temperature and fluence combination, effectively building a parameter map for surface structure evolution. Looking at the lowest ion fluence (Figure 2 (a) - (c)), sample temperature has a clear and dramatic effect on surface structure evolution. All these low-fluence samples show damage primarily in the form of surface pores; average pore size increases substantially with increasing sample temperature from ~40 nm (823 K) to ~200 nm (1223 K). These results confirm that surface features are highly tunable with sample temperature during irradiation. While surface pores at lower sample temperatures are smaller, they are relatively abundant and actually lead to an overall higher surface porosity, as discussed in our previous report.26 Additionally, Figures 2 (a) and (b) show somewhat uneven surfaces, demonstrating some material protrusion from the surface plane. For higher fluences, Nb2O5 surfaces irradiated at 823 K exhibit similar surface structures to their lower-fluence counterparts. At the highest fluence of 1.7 × 1025 ions m-2 (Figure 2 (g)), the network of surface pores gives way to a more pronounced and protruding surface structure. These fine, nano-tendril structures are indicative of “fuzz” formation found in other materials such as W. This is for the first time, to our knowledge, that fully-developed fuzz formation has been observed in Nb or its oxidized state; although, our preliminary studies suggested that fuzz would form at this temperature for sufficiently high ion fluences.26 For samples irradiated


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at 1023 K, higher ion fluences also facilitate a transition from surface pores to tendril structures. However, tendrils observed at this temperature (Figure 2 (h)) are noticeably thicker in diameter (~50 vs. 20 nm) than the tendrils observed for at lower temperatures (Figure 2 (g)). For the highest sample temperature of 1223 K, higher ion fluences (Figure 2 (f) and (i)) actually promote the formation of nano-pillars rather than fuzz-like structures. These nano-pillars are distinguished from fuzz by their primarily out-of-plane orientation. Fuzz tends to be a tangled mass of tendrils that may only have vaguely preferred orientations; however, nano-pillars have a clear orientation directly out of the surface plane. To get a more detailed sense of surface structure formation, selected samples were milled via focused ion beam (FIB) to produce the cross-sectional SEM micrographs in Figures 3 and 4. Figure 3 shows micrographs for Nb2O5 surfaces irradiated at 1223 K to total fluences of 4.3 × 1024 and 1.7 × 1025 ions m-2, respectively. These samples were chosen to demonstrate the formation of nano-pillars at this temperature. The low-fluence crosssection in Figure 3 (a) shows some surface roughening, presumed to result from the rupture of sub-surface He bubbles. Notably, this micrograph also displays several subsurface bubbles or voids from the irradiation process. It is theorized that these observed voids are merely remnant cavities from where He bubbles once resided. This sample’s higher-fluence counterpart, displayed in Figure 3 (b), also shows several bubbles/voids and significant surface structuring. Here, the formation of ~1 µm out-of-plane nanopillars is clearly evident. Conversely, Figure 4 shows a cross section of an Nb2O5 surface irradiated at 1023 K to a high fluence of 1.7 × 1025 ions m-2. Here, the out-of-plane surface protrusion of nano-tendrils is less consistent; individual tendrils are



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interconnected and have more chaotic growth directions. While this cross-section also shows sub surface voids or bubbles, they are relatively small and abundant compared to those found in Figure 3. This agrees with surface morphology trends observed in the topdown SEM micrographs in Figure 2; a higher quantity of smaller surface pores is observed at lower irradiation temperatures. While the formation of He+ ion-induced surface morphology changes in metals is still not completely understood, the normal and cross-sectional micrographs in Figures 2, 3, and 4 and the corresponding analyses offer valuable insights to surface morphology evolution mechanisms. He is an inert gas and virtually insoluble in metals.57 It is generally agreed upon that fuzz ultimately results from the accumulation, growth, and rupture of subsurface He bubbles.58 Due to the relatively low energy of He+ ions typically involved in fuzz formation experiments, sputtering is usually perceived to have little effect on the formation process. In fact, fuzz is known to occur even when the energy of He+ ions are well below the material’s sputtering and displacement thresholds, further evidencing He bubble rupturing as the main influence for surface structure formation.58 Its insolubility, combined with its relatively small size, allows He to diffuse rapidly through heated refractory metal substrates until trapped at either an intrinsic or irradiation-induced defects such as vacancies. Simulations suggest that the formation of He-vacancy defects would effectively immobilize sub-surface He.59 These immobile He-vacancy complexes may then further trap mobile sub-surface He atoms, leading the growth of these complexes into larger clusters or bubbles.60 As He+ ion fluence increases, bubbles may become large enough to coalesce with neighboring bubbles and effectively increase the mean bubble size, leading to dramatically enlarged surface pores upon bubble


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rupture.61,62 As observed in our cross-sectional SEM studies (Figures 3 and 4), higher sample temperatures promote He diffusion and help to facilitate enhanced bubble coalescence, producing larger sub-surface bubbles. Upon rupturing, these larger bubbles would naturally produce larger surface pores, as evidenced by Figure 1. In fact, our previous studies with other refractory metals clearly suggest similar trends between increasing surface feature size with increasing temperature during irradiation.26,41,43,45 While bubble rupture provides a source of roughening and material migration to the surface, the mechanisms involved in the protrusion of fine tendril structures is still not entirely understood. Surface pores are typically observed as a precursor to the formation of fuzz,63 suggesting that sub-surface bubbles play a crucial role in surface structure formation. Additionally, it has been theoretically demonstrated that sub-surface He bubbles provide an efficient route for self-interstitials and dislocation loops to glide to the sample surface and further contribute to surface roughening and mass migration.64 Other studies have suggested that thermal and stress gradients within the irradiated material may also help to accelerate sub-surface He bubbles to the surface and lead to enhanced and more pronounced surface roughening.65 On the other hand, sufficiently high sample temperatures may also promote surface smoothening via enhanced surface diffusion.61,66 Ultimately, the interplay between these complex, temperature-dependent mechanisms are likely responsible for the dramatic, temperature-dependent trends in irradiation-induced surface morphology changes. However, verification through detailed modeling attempts of these combined phenomena are outside the scope of the present study and will be performed in future investigations.



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To further monitor and characterize Nb2O5 surface morphology changes, a combination of specular optical reflectivity and XPS was used post-irradiation. Figure 5 shows optical reflectivity for virgin and irradiated surfaces both as a function of incident light wavelength and as a function of irradiation fluence and temperature. An initial inspection of the wavelength-dependent results (Figure 5 (a)) shows a significant reduction in optical reflectivity at all incident light wavelengths for all irradiated samples. The physical description for this phenomenon is relatively straightforward: He+ ion irradiation causes surface roughening; this roughening, in turn, enhances both diffuse scattering of light from the surface and absorption of light into the sample. An enhancement of either of these mechanisms would contribute to a reduction in reflected light signal and would effectively decrease measured reflectivity values. Unsurprisingly, optical reflectivity decreases for all incident light wavelengths as He+ ion fluence increases, demonstrating continued surface roughening on multiple length scales. To more concisely show optical reflectivity differences as a function of sample temperature during irradiation and total ion fluence, Figure 5 (b) plots reflectivity values for a single light wavelength of 670 nm. It is worth noting that samples irradiated at 1223 K have optical reflectivity values that are consistently lower than samples irradiated at 823 K, even though samples irradiated at 823 K have higher reported surface porosity.26 This effect can be attributed to both the periodicity and size of surface features observed in each sample. In principle, highly periodic surface structures should have improved optical absorption properties; however, randomized and chaotic surface features are often shown to outperform their periodic counterparts in terms of light absorption.67 In fact, previous studies have shown extraordinarily low optical reflectivity in W due to the inherent randomness of surface


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tendrils.68 Therefore, the surface porosity of a given sample is not strictly indicative of its optical performance. Furthermore, while a wide wavelength range was used for these optical reflectivity studies (200 – 1100 nm), the lowest light wavelength of 200 nm is roughly an order of magnitude larger than the features observed at 823 K (~20 nm). Since this wavelength is many times larger than observed surface features, it is expected that a fraction of incident light may be reflected back at normal incidence; structures that are much smaller than a given light wavelength are relatively reflective for that wavelength.27 Therefore, while optical reflectivity is useful for showing fluence-dependent evolution of surface structures, trends observed between structures of largely varying sizes may not be conclusive. In addition to information on surface roughening, optical reflectivity results also demonstrate this material’s potential application toward solar energy conversion. Particularly for solar-thermal applications, the objective is to convert incoming light into heat. An efficient solar-thermal material would thus have high optical absorptivity properties over a wide range of light wavelengths, ideally over the entire spectrum of incident solar light. While reflectivity data was only collected for the range 200 – 1100 nm, it should be noted that ~80% of the spectral irradiance of solar light lies in this range of wavelengths.69 Reflectivity data presented in Figure 5 (a) show that an Nb2O5 surface irradiated at 1223 K to a total fluence of 1.7 × 1025 ions m-2 has near-zero reflectivity values over this wavelength. While low reflectivity is not strictly indicative of high absorption (due to the contribution of a diffuse scattering component), this data certainly suggests very high light absorption over this wavelength range. Future investigations will



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focus on separating light absorption and diffuse scattering components via angle-resolved reflectivity to elucidate each component’s contribution toward decreased reflectivity. Complementary to optical reflectivity, previous studies have demonstrated that ex situ XPS may be used to indirectly gain qualitative information on surface roughness and porosity.26,41-43,45 As surface roughness increases, the number of sites available for oxidation also increases. If a structured sample is then left to oxidize in ambient conditions before XPS is performed, the measurements should reveal a slightly larger contribution from Nb-oxide state(s) rather than pure Nb for the structured sample. It should be noted here that oxidation only took place on sample surfaces; the underlying pure Nb substrate should then have an expectedly small contribution to XPS signal. Therefore, the extent of surface oxidation (measured by XPS) should indicate the extent of surface roughening or surface porosity. Figure 6 shows survey XPS spectra for an irradiated sample (1.7 × 1025 ions m-2, 1023 K) as well as a virgin, polished sample both before and after an Ar+ sputter cleaning process to remove the native oxide layer. The sputter cleaning process was performed in situ to the XPS system such that the substrate would not oxidize before XPS measurements were taken; this was primarily done to resolve XPS peak positions of pure Nb. For all spectra, only oxygen (O) and carbon (C) peaks are observed in addition to Nb. This demonstrates that no surface impurities were introduced during sample preparation or irradiation; both O and C impurities are expected from storing samples in ambient conditions and transferring samples ex situ between irradiation and XPS facilities. It should also be noted that the sputter-cleaned sample shows only pure Nb peaks. Figure 7 shows high resolution XPS spectra of the Nb 3d doublet region for multiple samples: a virgin sample both before and after sputter


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cleaning, samples irradiated at 823 K to fluences of 4.3 × 1024 and 1.7 × 1025 ions m-2, and samples irradiated at 1223 K to fluences of 4.3 × 1024 and 1.7 × 1025 ions m-2. Each region spectrum was deconvoluted into several peaks using commercial CasaXPS peak fitting software.70 Deconvolution, at most, produced 4 peaks for each spectrum: 2 corresponding to a pure Nb 3d doublet and 2 corresponding to an Nb2O5 3d doublet; this pent-oxide stoichiometry was determined by XPS peak shifts between doublets (~5 eV), which is consistent with the expected shift for Nb2O5.71 Important parameters (binding energy and full-width half-maximum (FWHM)) for each deconvoluted peak can be found in Table 1. Additionally, Table 2 lists the calculated atomic concentration of Nb and Nb2O5 (taken as the ratio of integrated counts between pure and oxidized XPS components) for each measured sample. The virgin Nb2O5 sample (Figure 7 (a) and corresponding values in Table 2) shows that the unirradiated surface is heavily oxidized in the Nb2O5 state before irradiation. Some contribution from pure (non-oxidized) Nb is also observed and originates from the underlying Nb substrate. After the Ar+ sputter cleaning process, only pure Nb peaks are observed since all oxide has been removed from the sample surface. This data is useful for resolving the native positions of pure Nb 3d doublet peaks to perform accurate XPS peak deconvolution. XPS data for irradiated samples (Figure 7 (b), (c), (e), and (f) and corresponding values in Table 2) show slight differences in oxidation behavior, dependent on irradiation conditions. While all samples are primarily oxidized in the Nb2O5 state, the calculated atomic concentration of this state varies somewhat. For each temperature, the extent of oxidation increases with increasing fluence. This suggests that the sample surfaces are becoming rougher and more porous as the irradiation process continues. As a function of sample temperature during irradiation,



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surface oxidation actually increases with decreasing sample temperature. This finding suggests that surface porosity also increases with decreasing sample temperature (at least for the tested range of sample temperatures) and aligns well with our preliminary investigations.26 Temperature-dependent XPS trends also agree well with temperaturedependent trends observed in our SEM studies (Figure 2), despite conflicting results from optical reflectivity.

Conclusions In summary, we propose low-energy, broad-beam He+ ion irradiation as a novel approach to the formation of Nb2O5 surface nanostructures. Naturally oxidized Nb2O5 surfaces were irradiated at multiple fluence-temperature combinations in the ranges 4.3 × 1024 – 1.7 × 1025 ions m-2 and 823 – 1223 K, respectively, to simultaneously study effects of ion fluence and sample temperature during irradiation. For all irradiation conditions, SEM reveals significant surface modification with features being tunable by both ion fluence and sample temperature. At each sample temperature considered in this study, increasing ion fluence facilitates an out-of-plane growth of surface nanostructures, demonstrated by both SEM and optical reflectivity studies. At the highest tested fluence of 1.7 × 1025 ions m-2, an irradiation temperature of 823 K produces a “fuzzy” and highly porous surface. Higher sample temperatures during irradiation generate similar tendril-like structures with larger feature sizes (1023 K) and even surface nano-pillars (1223 K). Crosssectional SEM reveals evidence of sub-surface He bubbles; relevant discussions on how these bubbles impact resulting surface morphology have been made. Specular optical reflectivity and XPS were used to further characterize irradiated surfaces and monitor


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surface roughness changes. While samples irradiated at higher temperatures have generally lower optical reflectivity values (implying larger surface roughness), these results may be clouded by varying surface feature sizes and undefined feature periodicity. Regardless, surfaces irradiated at 1223 K to a high fluence of 1.7 × 1025 ions m-2 exhibit near-zero reflectivity values, suggesting very high light absorption over a wide range of incident light wavelengths. Therefore, these surfaces may be promising for solar-thermal energy conversion. XPS shows higher oxidation levels for lower temperature samples and implies higher surface porosity with decreasing temperature. These findings agree with current SEM studies and preliminary investigations.26 Overall, the relative simplicity and scalability of these irradiation experiments suggest legitimate potential for broad-beam ion processing as a potential method to produce tunable nanostructures on Nb2O5 surfaces.

Methods Ion irradiation experiments and surface characterizations were performed at the UHFI (Ultra-High Flux Irradiation) and IMPACT (Interaction of Materials with Particles And Components Testing) facilities. More details about these facilities can be found in in our previous studies.72,73 0.5 mm-thick Nb sheets (cold-rolled and annealed) were cut into 10 mm × 10 mm samples and mechanically polished to a mirror-like finish. For EBSD, one extra sample underwent an additional vibratory polishing step with a Pace GIGA900 vibrator polisher and colloidal silica solution to produce an ultra-smooth surface. EBSD was performed with an FEI XL40 model field emission (FE) SEM. Before irradiating, each sample was allowed to naturally oxidize in ambient conditions to form sufficiently



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thick Nb2O5 surface layers; XPS was used to confirm surface stoichiometry. Irradiation experiments were performed in an ultra-high vacuum chamber with a base pressure of ~10−7 Torr (~10-5 Pa). Each sample was irradiated with 100 eV He+ ions, produced by a gridless End-Hall type ion source, the EH-400LE (Kaufmann & Robinson, Inc.), and simultaneously heated via resistive heater. To determine both the temperature- and fluence-dependent characteristics of irradiated sample surface morphology, samples were heated to constant temperatures in the range 823 – 1223 K and to total fluences in the range 4.3 × 1024 – 1.7 × 1025 ions m-2. It should be noted that both the temperature and the ion flux (1.2 × 1021 ions m-2 s-1) remained constant for the entirety of each irradiation experiment. To ensure that the sample temperature remained constant, despite additional surface heating from the ion beam, the resistive heater was equipped with a thermocouple-based feedback mechanism: a thermocouple imbedded in the back of the heater (touching the back of the sample) is fed into a PID controller that adjusts heater filament current to compensate for any temperature changes. Surface temperatures during irradiation were also independently verified with radiation pyrometers (Ircon Modline 5 infrared sensor). Post-irradiation, resulting sample surface morphologies were primarily characterized by SEM using a Hitachi S-4800 FE-SEM. Additional cross-sectional images were obtained by FIB milling of selected samples; these images were obtained by a separate SEM with FIB capabilities: the FEI Nova 200 NanoLab DualBeam SEM/FIB. A combination of specular optical reflectivity and high-resolution XPS were used to further characterize changes in surface roughness and stoichiometry/oxidation levels, respectively. For optical reflectivity, a combination of deuterium and halogen light was used to acquire reflectivity values over a wide range of light wavelengths: 200 – 1100


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nm. Incident light was transmitted to the sample surface by an optical fiber probe (Ocean Optics); light reflected back from the sample at normal incidence was collected by the same probe. This probe consists of 6 illumination fibers positioned around one central collection fiber. Due to implied geometrical constraints, the full-angle field of view is 25°. Before each measurement, the optical reflectivity system was calibrated with a Spectralon reflectance standard with ~100% optical reflectivity. In principle, 100% reflectivity implies a perfectly reflective surface; 0% reflectivity suggests that all incident light is either absorbed by the sample or diffusely scattered at large, off-incidence angles. X-rays for XPS were produced by an Mg-Kα source and emitted photoelectrons were collected and analyzed with an Omicron Argus hemispherical analyzer. Recorded XPS spectra were fitted with commercial CasaXPS peak fitting software.70 This deconvolution process consisted of fitting each spectrum with several mixed Gaussian-Lorentzian peaks (70% Gaussian; 30% Lorentzian). To resolve the position of pure (non-oxidized) Nb peaks, one sample was sputter-cleaned in situ to the XPS system. This sputter cleaning process consisted of a broad-beam 1 keV Ar+ ion irradiation. The ion flux for sputter cleaning was ~1021 ions m-2 s-1 and the irradiation lasted 50 minutes, producing a total Ar+ ion fluence of ~3 × 1023 ions m-2. As an estimate of sputtered thickness, TRIM/SRIM software74,75 was used to calculate sputtering yields of Nb and O from Nb2O5 under 1 keV Ar+ ion irradiation. These sputtering yields, taking into account steady-state surface stoichiometry changes resulting from preferential sputtering and following relevant calculations outlined in 76, provide an estimate of ~8 µm of removed material.

Additional information about competing financial interests:



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The authors declare no competing financial interests.

Acknowledgements: This research was partially supported by the National Science Foundation, PIRE project (Grant Number:1243490-OISE).


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Table 1. Binding energies (BE) and full-width half-maxima (FWHM) for deconvoluted XPS 3d region peaks shown in Figure 2.

Nb 3d5/2

Nb 3d3/2

Nb2O5 3d3/2

Nb2O5 3d5/2

BE

FWHM

BE

FWHM

BE

FWHM

BE

FWHM

Sample

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

Virgin, sputter-cleaned

205.58

2.50

202.89

1.68

Virgin

206.06

2.40

203.44

1.92

211.04

2.14

208.33

1.92

823 K, 4.3e24 ions m-2

205.30

1.83

202.79

1.65

210.44

2.14

207.71

1.89

1023 K, 4.3e24 ions m-2

205.35

1.93

202.78

1.73

210.49

2.11

207.76

1.90

1223 K, 4.3e24 ions m-2

205.59

1.92

203.12

1.82

210.70

2.12

207.96

1.91

823 K, 1.7e25 ions m-2

205.44

1.10

202.80

1.73

210.56

2.10

207.84

1.87

1023 K, 1.7e25 ions m-2

205.39

1.07

202.74

1.53

210.52

2.09

207.80

1.85

1223 K, 1.7e25 ions m-2

205.41

1.76

203.20

1.94

210.52

2.13

207.77

1.94



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Table 2. Atomic concentration contributions from Nb and Nb2O5 for each sample, calculated from XPS measurements.

Total Atomic Concentration (%) Sample

Nb

Nb2O5

Virgin, sputter-cleaned

100

0

Virgin

12

88

823 K, 4.3e24 ions m-2

8

92

1023 K, 4.3e24 ions m-2

11

89

1223 K, 4.3e24 ions m-2

14

86

823 K, 1.7e25 ions m-2

3

97

1023 K, 1.7e25 ions m-2

3

97

1223 K, 1.7e25 ions m-2

11

89


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Figure 1. (Left) EBSD grain orientation map and (right) IPF color legend and discrete IPF plot of the same region.



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Figure 2. SEM micrographs of Nb2O5 surfaces irradiated at (a) 4.3 × 1024 ions m-2, 823 K; (b) 4.3 × 1024 ions m-2, 1023 K; (c) 4.3 × 1024 ions m-2, 1223 K; (d) 8.6 × 1024 ions m2

, 823 K; (e) 8.6 × 1024 ions m-2, 1023 K; (f) 8.6 × 1024 ions m-2, 1223 K; (g) 1.7 × 1025

ions m-2, 823 K; (h) 1.7 × 1025 ions m-2, 1023 K; and (i) 1.7 × 1025 ions m-2, 1223 K.


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Figure 3. Cross-sectional SEM micrographs of Nb2O5 surfaces irradiated at 1223 K to total fluences of (a) 4.3 × 1024 ions m-2 and (b) 1.7 × 1025 ions m-2. Micrographs reveal evolution of He+ ion-induced surface nano-tendrils and sub-surface bubbles/voids.



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Figure 4. Cross-sectional SEM micrograph of an Nb2O5 surface irradiated at 1023 K to a total fluence of 1.7 × 1025 ions m-2, showing the formation of chaotic surface nanostructures and sub-surface bubbles/voids.


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Figure 5. (a) Specular optical reflectivity over a wide range of incident light wavelengths for several irradiated Nb2O5 surfaces, referenced against a virgin, mirror-polished surface and (b) sample reflectivity as a function of He+ ion fluence and irradiation temperature for 670 nm light (line to guide the eye).



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Figure 6. Survey XPS spectra for an irradiated (1.7×1025 ions m-2, 1023 K) Nb2O5 surface and a virgin, oxidized Nb surface both before and after a 1 keV Ar+ sputter cleaning process.


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Figure 7. XPS region spectra for various Nb2O5 surfaces: (a) virgin, before sputter cleaning; (b) irradiated at 823 K, 4.3 × 1024 ions m-2; (c) irradiated at 1223 K, 4.3 × 1024 ions m-2; (d) virgin, after sputter cleaning; (e) irradiated at 823 K, 1.7 × 1025 ions m-2; and (f) irradiated at 1223 K, 1.7 × 1025 ions m-2.



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Nanostructure Evolution on Nb Surfaces via Low-Energy He+

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