Mirror-finished polished molybdenum (Mo) samples were irradiated with 100 eV He+ ions as a function of ion fluence (using a constant flux of 7.2 x 10(20) ions m(-2) s(-1)) 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, He+ ion irradiation deteriorates the surface significantly even at 2.1 x 10(23) ions m(-2) fluence (about 5 min. irradiation time) and leads to evolution of homogeneously populated similar to 75-nm-long Mo nanograins having similar to 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 x 10(24) ions m(-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 x 10(24) ions m(-2), 7.2 x 10(20) ions m(-2) s(-1) flux generates a homogeneous layered and stacked nanodiscs of similar to 70 nm diameter. On the other hand, 1.2 x 10(21) ions m(-2) s(-1) flux generates a combination of randomly patched netlike nanomatrix networked structure, mostly with similar to 105 nm nano structure wall width, various-shaped pores, and self-organized nano arrays. While the observed netlike nanomatrix network structures for 8.6 x 10(24) ions m(-2) fluence (at a constant flux of 1.2 x 10(21) ions m(-2) s(-1)) is quite similar to those for 2.6 x 10(24) ions m(-2) fluence, the nanostructure wall width extends up to similar to 45 nm more and has a quite different nano structured 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 x 10(24) ions m(-2) at 973 K. For 2.6 x 10(24) ions m(-2) 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. (C) 2015 Elsevier B.V. All rights reserved.