New Kind of Colloidal Quantum Dot Photodetectors

Optical sensing in the mid to long infrared is becoming of utmost importance in different fields since it is proving to be an excellent tool for environ­mental monitoring, gas sensing, thermal imaging as well as food quality control or the pharma­ceutical industry, to name a few. The amount of infor­mation hidden within this very rich spectral window opens new possi­bilities for multi or even hyper­spectral imaging. Even though there are techno­logies that can address these challenges, they are very complex and expensive.

Quantum dots coated on a transparent substrate with gold contacts for mid-infrared detection. (Source: ICFO)

Even though there is a strong market need in bringing such func­tionalities to the consumer market, this would require a technology that is low-cost, CMOS compatible and does not impose severe regulatory concerns. PbS Colloidal Quantum Dots have emerged as a cost-compe­titive and high performance photo­detector tech­nology, compatible with CMOS technology, which has demons­trated recently to be successful in the short-wave infrared. However, so far, there has been a fundamental limit: such quantum dots have relied on interband absorp­tion of light and as a result there is a lower energy limit that this tech­nology can operate: the bandgap of the material.

Now, ICFO researchers Iñigo Ramiro, Onur Ozdemir, Sotirios Christodoulou, Shuchi Gupta, Mariona Dalmases, Iacopo Torre, led by Gerasimos Konstan­tatos, report the develop­ment of a colloidal quantum dot photodetector that is capable of detecting light in the long infrared range, from 5 to 10 microns, using PbS CQDs that, for the first time, are made with mercury-free material. In their experiment, the researchers used a technique to elec­tronically dope the quantum dots robustly and permanently.

This heavy doping approach allowed them to enable a new regime for transitions of electrons: instead of relying on transitions across the bandgap of the material, they found a way to faci­litate tran­sitions amongst higher excited states, known as inter­subband (or intraband) transitions. By achieving this, they were able to excite electrons by absorbing photons with photon energies much lower than before in the mid and long wave infrared. They also demons­trated that the spectral coverage of such detectors can be tuned by changing the size of the dots, that is, the larger the quantum dots, the farther the absorption in the infrared.

The results of this study have reported a novel and unique material platform, based on heavily doped PbS CQDs covering a broad range of light, which could address and solve the challenges that the field of photo­detector tech­nologies is facing nowadays. This newly disco­vered property of light absorption in the long infrared together with a low-cost and maturing CQD tech­nology may bring about a revo­lution to extreme broadband as well as multispectral CMOS compatible photo­detectors. (Source: ICFO)

Reference: I. Ramiro et al.: Mid- and Long-Wave Infrared Optoelectronics via Intraband Transitions in PbS Colloidal Quantum Dots, Nano Lett., online 14 January 2020; DOI: 10.1021/acs.nanolett.9b04130

Link: Functional Optoelectronic Nanomaterials, ICFO Institut de Ciències Fotòniques, Castelldefels, Barcelona, Spain


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