photo of hand holding small sensor chip

The ICFO team’s mid-infrared photodetector consisted of colloidal quantum dots, coated on a transparent substrate with gold contacts. [Credit: ICFO]

A research team in Spain has developed a low-cost colloidal quantum dot photodetector capable of sensing in the long-wave infrared (IR), with the potential to replace currently available, more expensive commercial solutions (Nano Lett., doi: 10.1021/acs.nanolett.9b04130). The new technology fills an existing gap in the photodetection spectrum and, according to the researchers, may prove useful for applications like environmental monitoring, food inspection, and gas analysis.

Finding a cheaper alternative

Up to this point, photodetector technology has been rather fragmented, with silicon photodetectors readily available for the visible/near-IR range and InGaAs photodetectors for the short-wave IR region. While devices do exist for the mid- and long-wave IR—for example, CdHgTe or more exotic detectors—they tend to be costly, complex to manufacture, and not CMOS-compatible.

To get to a less costly alternative, Gerasimos Konstantatos of ICFO–The Institute of Photonic Sciences and his colleagues thought about creating a mid- and long-wave IR photodetector with colloidal quantum dots (QDs). In particular, they decided to focus on lead chalcogenide (PbS) colloidal QDs, since earlier studies with the material in solution reported steady-state intraband absorption within the conduction band.

“PbS quantum dots have demonstrated their potential to compete with InGaAs in the short-wave infrared with very good performance and extremely low cost. Moreover, they are also compatible with CMOS electronics,” said Konstantatos, the new study’s senior author. “So we wanted to expand the spectral reach of this material platform towards the mid- and long-wave infrared so that we finally have a single material platform for everything that is cheap and CMOS-compatible.”

Exploiting intraband transitions

The researchers found an innovative way to get around the fundamental limitation in the band gap engineering of colloidal QDs. Traditionally, spectral coverage is determined by the bulk band gap of the semiconductor compound. Instead, Konstantatos and his colleagues exploited intraband transitions rather than interband transitions, meaning that photons with energy lower than the band gap could be used to excite electrons to higher energy states.

After colloidal synthesis of the QDs, the team performed a ligand exchange procedure that substitutes exposed sulfur atoms with iodine atoms. This step is followed by infilling and capping them with alumina, resulting in heavily doped quantum dots.

Expanding the spectral coverage

Experiments demonstrated intraband absorption and photodetection in the 5-to-9-µm range, which breaks the band gap limit of PbS colloidal QDs for the first time. The results also revealed that the intraband transition redshifts with increasing dot size. The researchers hopes that further development of the technology will lead to cost-efficient multispectral imaging systems in the infrared.

“To our knowledge, this has been the first time that robust electronic heavy doping has been reached in this material that has allowed the realization of intraband PbS quantum dot photodetectors,” Konstantatos said. “What we report is a proof-of-concept. The performance is still inferior to currently available technologies, and we are working intensively now to make this also a competitive technology.”