I'm trying to make nanotube models and molecules in Blender but I'd prefer if I can work with the defaulted units as nm or angstroms.
That is the official explanation of units in blender and how to change them.
The short answer is no. (Blender allows you to set a Unit to 10μm, and will display 1μm in its numerical fields).
But apart from notational convenience, and possibly unambiguous communication with others when you share files, you would gain nothing; this is so far below anything in Blender that is scale-dependent: simulations of Newtonian physics, and the some aspects of the renderers' approximations to / imitations of the behavior of light.
This scale is smaller than the wavelengths of visible light. Since there is no possible visual representation of the world at this scale, only symbolization, @JaroslavJerrynoNovotny 's suggestion seems right. In Properties > Scene > Units, you set the units to 'None', noting for yourself and others that a Blender Unit is, say, 1nm.