Can this be placed directly under shingles?

No, radiant barrier foil must have an air space on at least one side in order for it to work. In the case of a shingled roof, the foil would be sandwiched between asphalt shingles and the roofing deck/underlayment. In this set up, there is no air space, so the aluminum radiant barrier layer will not be effective in any way.

The only way to make this work is to have an air space (ie. a double-deck system or installed below the decking, draped between the rafter cavities).