ACS Nano 8, 11707–11714 (2014)

Interband optical transitions between valence and conduction bands in colloidal quantum dots are well studied and have been put to good use, but intraband transitions between states within the same band have not been explored. Zhiyou Deng and colleagues at the University of Chicago in the USA have now synthesized monodispersed HgSe colloidal quantum dots with air-stable doping of the lowest conduction band quantum state and observed an intraband transition. The transition, which covers the region between 3 μm and 5 μm, is especially interesting for mid-infrared source and sensing applications. Exposure to sulphide ions enabled switching from the mid-infrared intraband transition to a near-infrared interband transition. The team noted that a larger intraband photoluminescence efficiency could be attained with longer intraband lifetimes by reducing the energy lost to vibrational absorption. The photocurrent response at the intraband transition was investigated at room temperature and at 80 K and the results suggest the applicability of the colloidal quantum dots for infrared photodetector devices. A responsivity of 5×10−4 A W−1 with a 1.1 μA dark current was achieved using a 10 V bias, but the team emphasized that a lower resistance structure should result in an order of magnitude improvement.