Probing Charge Carrier Dynamics to Unveil the Role of Surface Ligands in HgTe Narrow Band Gap Nanocrystals

摘要

Colloidal nano crystals are an interesting platform for the design of low cost optoelectronic devices especially in the infrared range of wavelengths. Mercury chalcogenides have reached high maturity to address wavelengths above the telecom range (1.5 mu m). However, no screening of the surface chemistry influence has been conducted yet. In this paper, we systematically probe the influence of a series of ligands, Cl-, SCN-, 1,2-ethanedithiol, 1,4-benzenedithiol, 1-octanethiol, 1-butanethiol, As2S3, and S2-, On the photoconductive properties of HgTe nanocrystal thin films. A high bandwidth, large dynamic transient photocurrent setup is used to determine the photocarrier dynamics. Two regimes are clearly identified. At the early stage (few nanoseconds) a fast decay of the photocurrent is resulting from recombination and trapping. Then transport enters in a multiple trapping regime where carriers present a continuously decreasing effective value of their mobility. The power law dependence of the conductance can be used to estimate the trap carrier density and determine the value of the Urbach energy (35-50 meV). We demonstrate that a proper choice of ligand is necessary for a trade-off between the material performance (mu tau product) and the quality of the surface passivation (to keep a low Urbach energy).