As Albert Einstein famously quipped, things get spooky at the quantum level. Matter and light behave in mysterious ways at the smallest scales, and these phenomena can be leveraged to power a suite of technologies from medical imaging to energy storage.
For example, quantum dots (QDs) are man-made nanoscale crystals that shuttle electrons and possess one-of-a-kind optical, physical, catalytic and electrical properties. Yet despite their potential, materials science experts say that methods for reliably fabricating QDs aren’t quite up to scratch.
“Synthesising QDs using methods such as laser, plasma and electron bombardment can result in low quantum yields,” said Houjuan Zhu from A*STAR’s Institute of Materials Research and Engineering (IMRE). Besides quality issues, it is also difficult to introduce defects during production—a necessary step for tuning QD properties.
“These methods are also complicated and costly, and crucially, not effective at regulating and engineering the defects in QDs to achieve a specific desired effect, limiting the novel application of QDs in various fields,” she explained.
In partnership with colleagues from the National University of Singapore, Shanxi University, Nanjing University of Posts and Telecommunications and Southwest University, Zhu’s team explored high-performing, scalable processes to synthesise QDs using a material called molybdenum sulfide (MoS2).
They designed a two-stage procedure that first uses a bottom-up approach to create MoS2 QDs through carefully controlled chemical reactions between Mo and S ions. Next up, an alkaline etching step, which generates a high number of S vacancy defects, resulting in QDs that emit a bright blue glow called photoluminescence.
“This makes them highly valuable for medical applications such as bioimaging,” said Zhu. “The increase in defect density also enhances their photocatalytic capabilities, enabling them to accelerate chemical reactions.”
The researchers hypothesised that the alkaline etching process enriches the oxygen content in the MoS2 QDs, which led to a higher electron density of active sites, subsequently creating more defects—and they turned out to be right.
“Through density functional theory calculations and simulations, we were able to prove, for the first time, that the increase in photoluminescence and photo-oxidation in QDs is driven by the controlled formation of defects,” said Zhu.
Catapulting off the study’s success, the team is now exploring novel precision engineering methods for MoS2 QDs. “We believe that this will open the door to new and exciting applications in areas such as biomedicine, catalysis, energy storage and optoelectronics,” Zhu said, adding that the team is also looking into QDs made from other materials to expand their application landscape.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).