Synthesis of Near to Mid Infrared Mercury Telluride Quantum Dots

Abstract

Colloidal semiconductor quantum dots (QDs) are solution processable, and their bandgaps are tunable by material composition and size control. They are of particular interest for optoelectronic devices such as photodetectors, solar cells and light emitting diodes. Among II-VI semiconductor materials, mercury telluride (HgTe) has attracted attention for applications in the near-infrared (NIR) and mid-infrared (MIR) because of its zero bandgap in the bulk, which means that a full range of IR wavelengths can be accessed via size tuning and quantum confinement. Nowadays, the most common HgTe QD syntheses use either aqueous or organic solvent-based techniques. HgTe QD growth in aprotic solvents such as dimethyl sulfoxide (DMSO) or dimethyl formamide (DMF) are less common, and the kinetics of the growth process for HgTe QDs is still largely unknown.

In this thesis, we developed an aprotic solvent based synthesis of HgTe QDs, using mercury salt and H2Te as precursors. Computer-controlled synthesis of HgTe QDs in DMSO was done at room temperature. We investigated the influence of the reaction temperature on the kinetics of HgTe QD synthesis and the solubility improvement of H2Te gas at higher reaction temperature using DMF and diphenylether (DPE) as organic additives. The emission wavelength of HgTe QDs could readily attain up to 3000 nm but lower synthetic yields were obtained at higher reaction temperature due to the H₂Te gas decomposition. Hence, a certain amount of DMF or DPE additive was used to improve the synthetic yields and this could extend the emission wavelength to 4000-5000 nm. However, DMSO-based method still obtained narrower size distribution, and strong dips were appeared in the PL emission spectra due to C-H overtone and combination bands (OTCs).

To address these issues, we then demonstrated HgTe QDs grown with long IR emission wavelength in DMF under basic condition with different sets of synthetic conditions including single-step and multiple-step syntheses. Base was used to stabilize the H₂Te gas in solution, also strengthening the function of ligand. Multiple-step HgTe QDs synthesis was modified from a single-step one, consisting of nucleation and enlargement stage. Concentrated, relatively small HgTe QDs were grown as seed solution in the first synthesis stage, followed by the second synthesis stage which enlarged the seed QDs. This approach could avoid the broad size distribution and keep the optical band edge as sharp as possible.

Finally, we also discuss some emerging application of HgTe QDs. This sub-section will introduce poly(3-hexylthiophene) (P3HT): HgTe QD hybrid phototransistor to achieve high sensitivity and fast photodetection up to 2400 nm wavelength range at room temperature. HgTe QDs were incorporated into active layer of the phototransistor which improved the gate-voltage tuning, 15 times faster photo-response, and close to one order of magnitude reduction in the noise level, which is favorably comparable to commercial IR photodetectors. We conclude that HgTe QDs could be applied for low-cost high-performance MIR photodetection.

Date of Award	11 Sept 2020
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Andrey ROGACH (Supervisor)