A surface atomic-ligand exchange method is applied the first time in the construction of photodetectors (PDs) based on PbS quantum dots (QDs) for ultrasensitivity. The device thus produces a high photosensitivity to visible and near-infrared light with a photoresponsivity up to 7.5 x 10(3) A W-1 and a high stability in air. In particular, these PbS-QD-based PDs show the capability of following a pulse light with a frequency up to 100 kHz well at a relatively fast response time/recovery time of approximate to 4/40 mu s, much faster than most previous QD-based PDs. The short response time is attributed to modification for the surface of the PbS-QDs by cetyltrimethylammonium bromide treatment, which effectively improves the contact between the QDs and the Au electrodes, leading to extracting a high carrier mobility (approximate to 0.142 cm(2) V-1 s(-1)). These findings show the great potential of PbS-QDs as high-speed nano-photodetectors, and, more importantly, demonstrate the importance of the surface atomic-ligand exchange method in the construction of QD-based devices.