You don’t need to be PhD holders from an American university in order to apply for the green card in NIW category. In principle, you can file I-140 while on a visitor visa if you elect consular processing abroad (you can do that while being outside of the U.S., too). However, you should not file I-485 while on B2, because one has to wait for 90 days after entering U.S. on a nonimmigrant visa before filing I-485 (otherwise you may/would create a conflict of intents), and the visitor’s visa has a similar duration constraint.