i do supposedly "nano" research as an undergrad and here's what i have to say:
most nanotech is based on supporting other industries. the actual market for nanoscale materials is tiny, the big market is in devices and that's beyond plain physics and chemistry.
picking a niche is hard, but i'll just say this: I don't think MechE teaches anything important for nanotech, that you wouldn't get already with a physics degree + 1 year of general chemistry and 2 biology classes.