Influence of helium ion irradiation on the structure and strength of diamond

Microfabrication of synthetic single crystal diamond using accelerated helium ions beams has significant potential for functional applications, such as high precision optical devices, through tailoring of the optical properties via diamond graphitization. The use of helium ion microscopes (HIM) with nano-scaled focused ion beam spot sizes also allows for precision nano-patterning of the diamond surface through post-exposure selective etching of the generated graphitic phase. Here, single-crystalline diamonds with ⟨100⟩, ⟨110⟩, ⟨111⟩ and ⟨123⟩ orientations are exposed to He ion radiation from a HIM at a range of acceleration voltages and fluences. It is observed that ⟨123⟩ orientation was notably more sensitive to ion irradiation as sp3 bonds transition to sp2 bonds at lower fluence (1015 ions/cm2) compared to other orientations. In situ uniaxial compression of SC diamond micro-pillars revealed the strength of ⟨123⟩-oriented pillars is strongly dependent on the ion fluence, and thus is tunable by ion irradiation. Notably, ⟨100⟩-oriented pillars exhibit a better damage resistance as a small strength degradation due to its higher ion channeling efficiency. The irradiation damage of energetic helium ions on the structure and strength of diamond is therefore highly orientation-dependent. Such results provide the critical knowledge for precise patterning and designing of diamond-based functional structures in nano-fabrication.