Ultrasensitive all-2D MoS2 phototransistors enabled by an out-of-plane MoS2 PN homojunction

Two-dimensional transition metal dichalcogenide-based photodetectors have demonstrated potential for the next generation of 2-dimensional optoelectronics. However, to date, their sensitivity has not been superior to that of other technologies. Here we report an ultrasensitive two-dimensional photodetector employing an in-plane phototransistor with an out-of-plane vertical MoS2 p-n junction as a sensitizing scheme. The vertical built-in field is introduced for the first time in the transport channel of MoS2 phototransistors by facile chemical surface doping, which separates the photo-excited carriers efficiently and produces a photoconductive gain of > 10(5) electrons per photon, external quantum efficiency greater than 10%, responsivity of 7 x 10(4)AW(-1), and a time response on the order of tens of ms. This taken together with a very low noise power density yields a record sensitivity with specific detectivity D* of 3.5 x 10(14) Jones in the visible and a broadband response up to 1000 nm.