Mirror-finished polished molybdenum (Mo) samples were irradiated with 100eV He+ ions as a function of ion fluence (using a constant flux of 7.2×1020ionsm−2s−1) at normal incidence and at 923K. Mo surface deterioration and nanoscopic fiber-form filament (“Mo fuzz”) growth evolution were monitored by using field emission (FE) scanning electron (SEM) and atomic force (AFM) microscopy studies. Those studies confirm a reasonably clean and flat surface, up to several micrometer scales along with a few mechanical-polishing-induced scratches. However, He+ ion irradiation deteriorates the surface significantly even at 2.1×1023ionsm−2 fluence (about 5min. irradiation time) and leads to evolution of homogeneously populated ∼75-nm-long Mo nanograins having ∼8nm intergrain width. The primary stages of Mo fuzz growth, i.e., elongated half-cylindrical ∼70nm nanoplatelets, and encapsulated bubbles of 20–45nm in diameter and preferably within the grain boundaries of sub-micron-sized grains, were observed after 1.3×1024ionsm−2 fluence irradiation. Additionally, a sequential enhancement in the sharpness, density, and protrusions of Mo fuzz at the surface with ion fluence was also observed. Fluence- and flux-dependent studies have also been performed at 1223K target temperature (beyond the temperature window for Mo fuzz formation). At a constant fluence of 2.6×1024ionsm−2, 7.2×1020ionsm−2s−1 flux generates a homogeneous layered and stacked nanodiscs of ∼70nm diameter. On the other hand, 1.2×1021ionsm−2s−1 flux generates a combination of randomly patched netlike nanomatrix networked structure, mostly with ∼105nm nanostructure wall width, various-shaped pores, and self-organized nano arrays. While the observed netlike nanomatrix network structures for 8.6×1024ionsm−2 fluence (at a constant flux of 1.2×1021ionsm−2s−1) is quite similar to those for 2.6×1024ionsm−2 fluence, the nanostructure wall width extends up to ∼45nm more and has a quite different nanostructured surface. Ex-situ X-ray photoelectron spectroscopy studies show a sequential reduction in at.% of Mo 3d doublets with fluence, leading to the complete depletion of 2.6×1024ionsm−2 at 973K. For 2.6×1024ionsm−2 fluence irradiation at 973K, only MoO3 3d doublets were observed. However, the Mo 3d doublets reappear at 1273K irradiation, where a variety of nanostructures were observed with relatively much lower density than that of Mo fuzz. As in the microscopy studies, the reflectivity measurements also show a sequential reduction with ion fluence, leading to almost zero reflectivity value for fully grown fuzzy structures. The study is significant in the understanding of fuzz formation on high-Z refractory metals for fusion applications; in addition, the observed MoO3 fuzz has potential application in solar power concentration technology and in water splitting for hydrogen production.
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