flexible qd material between fingers

Researchers in Barcelona have made a broadband infrared light emitter by using a visible LED to illuminate a stack of colloidal quantum dots assembled on a flexible plastic substrate. [Credit: ICFO]

Infrared spectrometers integrated into smartphones and other portable devices could potentially enable a range of new mobile applications, from environmental monitoring to chemical analysis of foods. But unlike their counterparts at visible wavelengths, the light-emitting diodes (LEDs) that could enable such measurements remain a work in progress.

Researchers in Spain have now shown how this hurdle might be overcome by making a broadband emitter of the short-wave infrared light (SWIR) that sits between 1 μm and 2.5 μm (Adv. Mater., doi: 10.1002/adma.202003830). They did so using layers of different-sized colloidal quantum dots powered either by visible light or electricity, arguing that these demonstrations pave the way to cheap, compact on-chip spectrometers.

The promise of SWIR

Radiation at SWIR wavelengths, like that in the near-infrared, is absorbed by vibrations of molecules containing bonds between carbon and hydrogen as well as oxygen and hydrogen atoms. It therefore acts as a sensitive detector of various organic and biological materials of interest to numerous industries, including food production and agriculture as well as medicine and pharmaceuticals.

Currently, most sources of broadband SWIR light are incandescent. These rely on heating to generate black-body radiation, which makes them cheap but bulky and inefficient. The heat they give off would also be hard to shed in very compact devices.

A quantum boost

In the latest work, Santanu Pradhan, Mariona Dalmases and Gerasimos Konstantatos at the Institute of Photonic Sciences (ICFO) in Barcelona made solid-state thin films comprising three layers of lead sulfide quantum dots. In each layer, they put one of three kinds of light-emitting dot—having peak emissions at either 1.25 μm, 1.4 μm or 1.55 μm—as well as a fourth, smaller type of dot designed to absorb light at 0.7 μm. They laid the layers on a flexible plastic substrate and then glued the substrate to a commercial LED with an output at about 0.6 μm.

The idea was that visible photons from the LED would be absorbed by the smaller dots in each layer, generating charge carriers that would lead to emission from the larger dots. And that is what they found, recording a spectrum that spanned 1050–1650 nm while achieving a quantum efficiency of 5.4% (the number of infrared photons generated as a fraction of the number of injected charges). This compares to about 2% for a thermal source.

In addition, the researchers also powered their device electrically. They point out that unlike the materials being developed for LEDs in the near-infrared—phosphor doped with transition metal and rare Earth elements—their lead sulfide quantum dots are conductive. By hooking their light emitter to a DC power supply in their lab, they generated a spectrum starting at about 1200 nm and reached a quantum efficiency of 5.2%.

Spectral signatures

The researchers then put their device to the test, powering it optically and using it to illuminate samples whose transmittance was recorded by a CCD camera. They found that their homemade spectrometer produced distinct spectra for different substances, showing it could clearly distinguish water's absorption from that of two other solvents. They were also able to pick out animal milk from various types of plant-based milk, as well as identify different kinds of plastic—something which could come in handy for recyclers.

The ICFO team says that its technology is compatible with complementary metal-oxide–semiconductor fabrication, making it suited for high-volume manufacturing. Indeed, Konstantatos estimates that an optically pumped device might appear on the market in as little as one to two years. But he says that the electrically powered version requires further R&D to drive the quantum dots efficiently at higher radiance levels.