Colloidal HgTe Nanocrystals with Widely Tunable Narrow Band Gap Energies:  From Telecommunications to Molecular Vibrations

Abstract

A convenient, aqueous-based synthesis of stable HgTe nanocrystals with widely size-tunable room temperature emission between wavelengths of 1.2 to 3.7 mum is demonstrated. By the choice of the thiols, applied as stabilizers, we optimized the growth dynamics, the luminescence quantum yields (up to 40%), and a ligand-exchange procedure, required to transfer the nanocrystals from water to nonpolar organic solvents. The latter is greatly improved and facilitated by the use of mercaptoethylamine as initial stabilizer. The possibility to tune the HgTe nanocrystal sizes from 3 to 12 nm and to control their surface functionalities (hydrophobic and hydrophilic) makes them very promising for the development of infrared optical devices, emitting in the wavelength region between the telecommunications and the molecular vibrations.

Infrared Emitting HgTe Quantum Dots and Their Waveguide and Optoelectronic Devices

“…In this respect they make the point that as far as the bandgap energy is concerned, for shapes that are not markedly anisotropic, the same bandgap energy should be obtained for an equivalent QD volume irrespective of the shape. The calculations show good agreement with the optical data of Kovalenko et al [27] and Lhuillier et al [108] (Figure 17b) and confirm that in this size range excitonic effects are rather weaker than in the wider bandgap II-VI materials such as CdTe and CdSe, and the lead chalcogenides such as PbS and PbSe.…”

“…The same approach as used by Gaponik et al [26] for CdTe or Kovalenko et al [27] could also be used to switch from short chain water soluble ligands to organic solvent compatible longer chain alkanethiols, dodecanethiol being a popular and the least disagreeable choice. Thus HgTe QDs could also be incorporated into spin coated polymer thin films from which IR transmitting waveguides could be fabricated, but much of this work went unreported in the literature at the time as industrial research which might have been patentable had we succeeded in making optical amplifiers.…”

“…They cite a comprehensive set of modelling band parameters and calculate the short wavelength (Maxwell Garnett regime) absorption cross-section per Hg atom within the QDs as 2.6 ± 0.4 × 10 −17 cm 2 /atom, whilst the QD size dependent integrated absorption cross-section per dot at the band edge is around 1.5 × 10 −15 cm For comparison, Kovalenko et al [27] give a PL peak energy/size relationship (where the PL peak is Stokes shifted relative to the absorption edge) as…”

Inkjet‐Printed Nanocrystal Photodetectors Operating up to 3 μm Wavelengths

“…After synthesis the initial thiol shell is partly replaced by dodecanethiol, a hydrophobic thiol, via a ligand exchange-procedure [19] to make the nanocrystals soluble in organic solvents (details are given in Ref. [20]). For inkjetprinting a 2 wt % HgTe NC/chlorobenzen solution was found to have a suitable viscosity and surface tension [21] for ejection from the used piezo-driven print head.…”

“…At least the photoluminescence intensity is reported to decrease dramatically, when the emission wavelength becomes larger than 2 lm. This is observed for lead-chalcogenides [27] as well as for the HgTe nanocrystals, [20] which makes the development of nanocrystal based devices operating at wavelength longer than 2 lm a challenge. Nevertheless, we have obtained operation of nanocrystal based photodetectors up to 3 lm wavelength.…”