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

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. HgTe quantum dots synthesized in aprotic solvents are capped from the outset with short cyclic thiol ligands 2-furanmethanethiol and 2,5-dimercapto-3,4-thiadiasole, which ensure high electrical conductivity of their films. Preserving an intact HgTe surface allows for extension of the charge carrier diffusion length in the active layer, benefiting their extraction efficiency and the devices' figures-of-merit.image

Automated summary

By adjusting the cation-anion ratio and introducing different types of thiol ligands, effective n- or p-type self-doping of HgTe quantum dots was achieved, resulting in improved optoelectronic performance of the devices. The reduction in surface trap density and the increase in charge carrier diffusion length are key factors contributing to the performance enhancement.