Dependence of Photoresponsivity and On/Off Ratio on Quantum Dot Density in Quantum Dot Sensitized MoS 2 Photodetector.

Non-radiative energy transfer (NRET) from quantum dots (QDs) to monolayer MoS(2)has been shown to greatly enhance the photoresponsivity of the MoS(2)photodetector, lifting the limitations imposed by monolayer absorption thickness. Studies were often performed on a photodetector with a channel length of only a few mu m and an active area of a few mu m(2). Here, we demonstrate a QD sensitized monolayer MoS(2)photodetector with a large channel length of 40 mu m and an active area of 0.13 mm(2). The QD sensitizing coating greatly enhances photoresponsivity by 14-fold at 1.3 mu W illumination power, as compared with a plain monolayer MoS(2)photodetector without QD coating. The photoresponsivity enhancement increases as QD coating density increases. However, QD coating also causes dark current to increase due to charge doping from QD on MoS2. At low QD density, the increase of photocurrent is much larger than the increase of dark current, resulting in a significant enhancement of the signal on/off ratio. As QD density increases, the increase of photocurrent becomes slower than the increase of dark current. As a result, photoresponsivity increases, but the on/off ratio decreases. This inverse dependence on QD density is an important factor to consider in the QD sensitized photodetector design.