Schematic of the process to prepare MoS2 two-dimensional QDs, illustrating the benefits of the reflux pre-treatment-mediated sonication process. Nanostructure and concentration of MoS2 samples prepared with combination of reflux and sonication: AFM image of MoS2, thickness distribution of MoS2 and size distribution of MoS2.
Schematic of the process to prepare MoS2 two-dimensional QDs, illustrating the benefits of the reflux pre-treatment-mediated sonication process. Nanostructure and concentration of MoS2 samples prepared with combination of reflux and sonication: AFM image of MoS2, thickness distribution of MoS2 and size distribution of MoS2.

Despite the enormous promise of two-dimensional materials, there is no simple and low-cost way of producing such materials in quantum dot form in large quantities. Until now, that is, according to a team from Rice University, Sichuan University, Fujian University of Technology, University of Cincinnati, Sanatana Dharma College, University of Central Florida, Hefei University of Technology, and Saudi Basic Industries Corporation [Liu et al., Materials Today (2018), https://doi.org/10.1016/j.mattod.2018.06.007].

The researchers have developed a simple and universal reflux pre-treatment and sonication method that produces measurable amounts of two-dimensional quantum dots (QDs) from bulk raw materials including graphene, hexagonal boron nitride (h-BN), semiconducting SnS2, and transition metal dichalcogenides (TiS2, MoS2, WSe2, NbS2).

The simple process begins by refluxing the starting bulk material in a chemical solvent for 24 hours. The resulting dispersion is then sonicated for 4 hours before being centrifuged for 30 minutes. Filtering off the liquid that separates from the solid residue, known as the supernatant, yields two-dimensional quantum dots (QDs) typically 2-7 nm wide and a monolayer (0.8-1 nm) thick.

The crucial part of the process is the reflux pre-treatment because this allows the solvent to permeate into cracks and channels between the layers of the bulk material, which are held together by weak van der Waals’ forces. The confined solvent helps force apart – or delaminate – the layers and break them up into QDs during the sonication part of the process. The solvent has to be carefully chosen to match the bulk material.

“A solvent with a surface tension components ratio best matched to the bulk material has to be found before sonication,” explains first author of the study, Yang Liu of Fujian University of Technology. “The surface tension components ratio is the ratio between the polar and dispersive parts of a material or solvent; when the value of the solvent is close to that of a two-dimensional material, it will show good immersion and insertion.”

Although the reported yield of 1.5wt% may not sound very high, this far exceeds any previous reported yields for top-down fabrication of QDs, which have been too low to measure.

“This method is universal and could be applied to various two-dimensional materials, including other transition metal dichalcogenides,” says Liu. “Moreover, the process doesn’t involve any surfactants and should be easy to industrialize.”

The researchers are now working on improving the efficiency of the process by increasing the amount of solvent confined in the channels in between the layers of the bulk materials after refluxing. The resulting two-dimensional QDs could be useful in catalysis, energy storage, bioimaging, biosensing, photovoltaics, and optical applications.