Effect of Crystal Orientation on Self-Assembly Nanocones Formed on Tungsten Surface Induced by Helium Ion Irradiation and Annealing.

The self-assembly nanocone structures on the surface of polycrystalline tungsten were created by He⁺ ion irradiation and then annealing, and the resulting topography and morphology were characterized using atomic force microscopy and scanning electron microscopy. The cross-sectional samples of the self-assembly nanocones were prepared using an in situ-focused ion beam and then observed using transmission electron microscopy. The self-assembly nanocones were induced by the combined effect of He⁺ ion irradiation, the annealing process and the chromium impurity. The distribution characteristics, density and morphology of the nanocones exhibited a distinct difference relating to the crystal orientations. The highest density of the nanocones was observed on the grain surface with a (1 1 1) orientation, with the opposite for that with a (0 0 1) orientation and a medium value on the (1 0 1)-oriented grain. The size of the self-assembly nanocones increased with increasing the annealing time which met a power-law relationship. Irradiation-induced defects acted as the nucleation locations of the protrusions which attracted the migration of the tiny amount of chromium atoms. Under the action of temperature, the protrusions finally evolved into the nanocones.