Hard nanotech (the kind usually envisioned in sci-fi) may be physically impossible, and at the very least is extremely difficult. The type of nanotech that is more feasible is 1.) top-down lithography (ie chips), and 2.) bottom up cellular biology, or some combinations thereof.
Biological cells are already near optimal nanotech robots in both practical storage density and computational energy efficiency (landauer limit). Even a superintelligence, no matter how clever, will not be able to design nanobots that are vastly more generally capable than biological cells. Robots are fundamentally limited by energy efficiency and storage density and biology is already operating at the physical limits for those key constraints. So plausible bottom up nanotech just looks like more 'boring' advanced biotech.