Bottom-up evolution of consolidated porous diamond layer by hot-filament-CVD

Synthesis of the porous chemical vapor deposition (CVD) diamond layer, which might enable many noble applications, usually employed the template growth or post-processing, which accompanied the undesirable complexity and cost issues. In a small number of the porous diamond layer synthesis efforts, majority of them employed the plasma-assisted CVD (PA-CVD), while the hot-filament CVD (HF-CVD) was well known for its outstanding scalability and cost-effectiveness. Here we demonstrate a post-processing/template-free approach for the porous diamond layer synthesis by HF-CVD. Intriguingly, we found that the seeding the substrate by detonation nanodiamond (DND) and the relatively low substrate temperature were key enabling factors. The consolidated layer was consisted of quasi-spherical nanodiamond particles. Such observations suggested a precipitation-relevant growth mechanism, which we disproved here, and we clarified an alternative bottom-up growth mechanism. We also demonstrated a preliminary result of applying the porous diamond layer as a waveguide in the attenuated-total-reflection (ATR)-type waveguide mode resonance sensor.