Quantum dot breakthrough promises a world of cheap sensors

These infrared lasers are currently manufactured through a method called molecular epitaxy, a well-established and effective practice, but plagued by heavy costs and labor.

“A cost-effective and easy-to-use method to make infrared light with quantum dots could be very useful,” said Xingyu Shen, a graduate student at the University of Chicago. Shen and graduate student Ananth Kamath are the other authors of the new study published in the journal Nature Photonics. 

The study was conducted utilizing resources from the University of Chicago Research Science and Engineering Center and the Pritzker Nanofabrication Facility.

Cascading through energy levels.

Drawing from their extensive experience in quantum dots and infrared technology, the research team sought to achieve a "cascade" technique previously unattainable with colloidal quantum dots.

This "cascade" technique involves running an electrical current through a device, causing millions of electrons to travel across it. 

When the device's architecture is finely tuned, the electrons fall through a series of discrete energy levels, akin to descending a cascade of waterfalls. At each level, electrons emit energy as light.

They created a dense "ink" comprising trillions of nanocrystals and spread it onto a surface.

When an electric current was sent through this surface, the team observed an unexpectedly positive outcome. 

“We thought it would be likely to work, but we were really surprised by how well it worked,” enthused Guyot-Sionnest. “Right away, from the first time we tried it, we saw light.”

“Right now the performance for these dots is close to existing commercial infrared light sources, and we have reason to believe we could significantly improve that,” he told University of Chicago News.

An ocean of possibilities.

The implications of this new method are far-reaching. 

The newfound ability to create infrared light using colloidal quantum dots could usher in a new era of cost-effective infrared lasers and sensors, significantly reducing manufacturing and operational costs.

Guyot-Sionnest remarked that this discovery stood as a prime example of the potential of quantum dots. “Many other applications could be achieved with other materials, but this architecture really only works because of the quantum mechanics,” he added.

“I think it’s pushing the field forward in a really interesting way.”

Study Abstract

Efficient infrared light sources are needed for machine vision and molecular sensing. In the visible, electroluminescence from colloidal quantum dots is highly efficient, wavelength tunable and cost effective, which motivates using the same approach in the infrared. Despite the promising performances of colloidal quantum dots light-emitting diodes in the near-infrared, mid-infrared devices show quantum efficiencies of about 0.1% due to the much weaker emission. Moreover, these devices relied exclusively on the interband transition, restricting the possible materials. Here we show electroluminescence at 5 µm using the intraband transition between 1Se and 1Pe states within the conduction band of core–shell HgSe–CdSe colloidal quantum dots. The 4.5% quantum efficiency approaches that of commercial epitaxial cascade quantum well light-emitting diodes. The high emission efficiency and the electrical characteristics support a similar cascade process where the electrons, driven by the bias across the device, repeatedly tunnel into 1Pe and relax to 1Se as they hop from quantum dot to quantum dot.