Nanotechnology is here to stay, it's just a fleeting soup of biology, chemistry and physics coming together. Some would say that it is physics maturing and becoming really useful to humanity. Other would say that nanotechnology is more dangerous than strangelets from the LHC.
I for one will take the master in nanotechnology. But it will be coupled with a lot of physics. Now I don't know what angle you will do nano from, but I suppose it's from at least a physical chemistry-angle. biotech-people doing nano makes me sick.
Then you probably will know that you will have to wade through most of the really gritty physics there is, a phd-course in condensed matter physics, probably one in non-relativistic quantum mechanics, and if you are really good, probably some QFT too.
But I wouldn't recommend nanotechnology if you are not inclined to either one of these two, computational physics or experimental physics.
What kind of field you should go into? hmm, tricky one. I would say it depends on what country you are going to work aso in. Like in some parts of europe it would be smarter to take a biophysics-nano-msc, because they have a large industry which are a bit needy for biophysicsts.
While other countries with a large steel-industry for example would be in need of material-nanotechnologists.
It's all about competetive awareness.