Black phosphorus sheets are great but they are pretty hard to fabricate and they degrade very quickly when they come in contact with air (in a matter of hours). There have been various attempts to solve this, some of them successful (as far as we can tell from lab experiments, at least), but all solutions are in pretty early stages, and most of them seem pretty difficult to implement on an industrial scale. Basically, exposing the damn thing to anything but inert gases for more than a couple of hours destroys it. That's not too feasible today.