Development of Synthetic Methods to Grow Long-Wavelength Infrared-Emitting HgTe Quantum Dots in Dimethylformamide

Most successful syntheses of long-wavelength infrared (IR)-absorbing/emitting Hg-chalcogenide quantum dots (QDs) use either aqueous- or organic-solvent-based methods. Accounts of IR QD growth in aprotic solvents such as dimethyl sulfoxide (DMSO) or dimethylformamide (DMF) are much less common, and yet, producing QDs in such solvents can be useful from the perspective of further reactions to conjugate or incorporate the QDs with other materials since such solvents can be a useful meeting ground for both organic and ionic solutes. Here, we start by demonstrating long-wavelength infrared emission (across the wide spectral range of up to 4500 nm) in HgTe QDs grown in DMF under basic conditions. While many existing synthetic approaches use soluble chalcogen precursors in adduct, salt, or organo-chalcogenide forms, we have opted to take the approach of using slow addition of gaseous H₂Te generated under programmed control, which allowed us to investigate the growth kinetics to manipulate the different competing processes and to obtain larger HgTe QDs with the best size distributions on a repeatable and controlled basis. We demonstrate how the nucleation process of HgTe QDs can be carried out analogously to the way that it occurs in classic hot injection syntheses but in our case at a far lower (subambient) temperature, owing to the use of a much more labile Te precursor. We also demonstrate the use of a two-stage, seeded QD growth process, which allows the synthesis conditions for the initial nucleation step and the subsequent enlargement stage to be decoupled; in other words, the QD concentration during the enlargement phase need not be forced to be excessively large by the choice of nucleation conditions. This approach should eventually be extendable to making >5000 nm emitting HgTe QD-based materials, and the use of aprotic solvents will offer compatibility with other nanomaterial chemistries, e.g., oxide glass formers, etc., for the synthesis of composites. By comparing the emission spectra of HgTe QDs grown in DMF with the spectra of those grown in DMSO, we show that polaron-mediated coupling to ligands and solvents, previously seen when using DMSO, can be substantially suppressed using our new synthetic method.