Do they really have to be planted on research facilities?
Did not know that ... I always thought tech stealing would still be like in DL, where you came to steal a tech once you had all info on the civ. Either way, I do not recommend you to use spies to steal tech. That is far too expensive and you will need all of your spies to defend yourself against the other civs, spying at you.
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You need to have spies planted in DA in order to have the chance to steal a tech. It is unlike DL in that once you hit Advanced, you no longer passively steal techs. The spies don't need to be planted on a research building, they just need to be planted.
Also, you don't NEED spies to defend yourself if you sacrifice 1 tile for a CEC.