Obviating Ligand Exchange Preserves the Intact Surface of HgTe Colloidal Quantum Dots and Enhances Performance of Short Wavelength Infrared Photodetectors.

Abstract

A large volume, scalable synthesis procedure of HgTe quantum dots (QDs) capped initially with short-chain conductive ligands ensures ligand exchange-free and simple device fabrication. An effective n- or p-type self-doping of HgTe QDs is achieved by varying cation-anion ratio, as well as shifting the Fermi level position by introducing single- or double-cyclic thiol ligands, that is, 2-furanmethanethiol (FMT) or 2,5-dimercapto-3,4-thiadiasole (DMTD) in the synthesis. This allows for preserving the intact surface of the HgTe QDs, thus ensuring a one order of magnitude reduced surface trap density compared with HgTe subjected to solid-state ligand exchange. The charge carrier diffusion length can be extended from 50 to 90 nm when the device active area consists of a bi-layer of cation-rich HgTe QDs capped with DMTD and FMT, respectively. As a result, the responsivity under 1340 nm illumination is boosted to 1 AW^-1 at zero bias and up to 40 AW^-1 under -1 V bias at room temperature. Due to high noise current density, the specific detectivity of these photodetectors reaches up to 10¹⁰ Jones at room temperature and under an inert atmosphere. Meanwhile, high photoconductive gain ensures a rise in the external quantum efficiency of up to 1000% under reverse bias.