Tailoring molybdenum nanostructure evolution by low-energy Heup>+up> ion irradiation

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• 作者：J.K. Tripathiup> ; up>up>jtripat@purdue.edu" class="auth_mail" title="E-mail the corresponding authorup>up> ; up>up>tripathijk@gmail.com" class="auth_mail" title="E-mail the corresponding authorup> ; T.J. Novakowski ; A. Hassanein
• 关键词：Plasma-facing materials ; Molybdenum ; Fuzz formation ; Ion irradiation ; Atomic force microscopy ; X-ray photoelectron spectroscopy ; Optical reflectivity
• 刊名：Applied Surface Science
• 出版年：2015
• 出版时间：30 October 2015
• 年：2015
• 卷：353
• 期：Complete
• 页码：1070-1081
• 全文大小：6208 K

文摘

Mirror-finished polished molybdenum (Mo) samples were irradiated with 100 eV Heup>+up> ions as a function of ion fluence (using a constant flux of 7.2 × 10up>20up> ions mup>−2up> sup>−1up>) at normal incidence and at 923 K. 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, Heup>+up> ion irradiation deteriorates the surface significantly even at 2.1 × 10up>23up> ions mup>−2up> fluence (about 5 min. irradiation time) and leads to evolution of homogeneously populated ∼75-nm-long Mo nanograins having ∼8 nm intergrain width. The primary stages of Mo fuzz growth, i.e., elongated half-cylindrical ∼70 nm nanoplatelets, and encapsulated bubbles of 20–45 nm in diameter and preferably within the grain boundaries of sub-micron-sized grains, were observed after 1.3 × 10up>24up> ions mup>−2up> 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 1223 K target temperature (beyond the temperature window for Mo fuzz formation). At a constant fluence of 2.6 × 10up>24up> ions mup>−2up>, 7.2 × 10up>20up> ions mup>−2up> sup>−1up> flux generates a homogeneous layered and stacked nanodiscs of ∼70 nm diameter. On the other hand, 1.2 × 10up>21up> ions mup>−2up> sup>−1up> flux generates a combination of randomly patched netlike nanomatrix networked structure, mostly with ∼105 nm nanostructure wall width, various-shaped pores, and self-organized nano arrays. While the observed netlike nanomatrix network structures for 8.6 × 10up>24up> ions mup>−2up> fluence (at a constant flux of 1.2 × 10up>21up> ions mup>−2up> sup>−1up>) is quite similar to those for 2.6 × 10up>24up> ions mup>−2up> fluence, the nanostructure wall width extends up to ∼45 nm 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 × 10up>24up> ions mup>−2up> at 973 K. For 2.6 × 10up>24up> ions mup>−2up> fluence irradiation at 973 K, only MoO₃ 3d doublets were observed. However, the Mo 3d doublets reappear at 1273 K 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.