Mid-infrared response of PbS colloidal quantum dot solids

Abstract

Lead sulfide (PbS) colloidal quantum dots (CQDs) are promising candidates for infrared photodetectors (PDs). However, solution-processed PbS CQDs with spectra ranging up to the mid-infrared range are completely absent because long-chain organic ligands capped on large size CQDs provide insufficient steric hindrance to separate the adjacent particles for dispersion in solvents, limiting their application in infrared PDs. Herein we report a strategy that addresses the extension of the response spectra of PbS CQDs by incorporating PbNO₂ impurities. This is achieved by photo-carrier transition from the valence band (VB) of PbS CQDs to the unoccupied in-gap states (IGSs) near the conduction band. The device exhibits an obvious response with responsivity of 0.11 A W⁻¹ and a detectivity of 5.38 × 10⁸ Jones under the illumination of 3000 nm light rays at room temperature. Once the temperature decreased to 78 K, the device exhibited a response with a responsivity of 0.03 A W⁻¹ and a detectivity of 3.77 × 10⁸ Jones. The X-ray photoelectron spectroscopy results indicate that these IGSs are formed by using potassium nitrite (KNO₂) to remove the oleate ligands and induce oxidation impurities on the CQD surface. Additionally, detailed characterization is carried out to investigate the mechanism of the infrared response of PbS CQDs. We are optimistic that this strategy could provide an alternative avenue toward the use of CQDs in optoelectronic devices with a broad response range.