Feature: Sensor technology
Figure 3: QDs are fabricated with simple benchtop wet chemistry
Figure 4: SWIR sensors can benefit a whole range of applications
If SWIR imaging and sensing are to become widespread
in consumer electronics, engineers must look elsewhere for the solution.
Quantum dots Quantum dots are already commonly used for display purposes since they can be tuned to emit visible light. However, their ability to be tuned to SWIR wavelengths is disrupting the industry by improving the quality and quantity of infrared data that can be accessed by all types of devices. During PbS QD formulation, directly controlling the size of
the material enables the semiconductor band gap to be precisely tuned, with larger QDs sensitive to longer wavelengths of light, and smaller nanoparticles sensitive to shorter, higher-energy wavelengths – from 800nm to 2,400nm. For optoelectronic applications, in addition to broadband
tunability, QDs with a higher excitonic absorption peak and narrower full width at half maximum (FWHM) of the absorption spectrum offer superior performance. The ability to tightly control this specification is highly desirable in QD synthesis. Quantum Science is a British firm that’s an expert in infrared
QD technology. It has already demonstrated industry-leading performance in this field by creating infrared PbS-based QDs that feature FWHM significantly better than other market- available QDs. Using transmission electron microscopy to monitor the uniformity of shape and size distribution for QD materials shows that its INFIQ QDs are monodisperse (having particles of approximately the same size), with diameters of 4.9 ± 0.1nm at 1,350nm and 6.0 ± 0.1nm at 1,550nm. Our QDs also feature a high degree of crystallinity, meaning they experience a lower rate of defects and fewer electronic trap states, protecting them from performance degradation.
The QD ink formulation process SWIR-sensitive QD films are typically created by exchanging the long-chain organic ligands (organic molecules that are optimised for synthetic control and colloidal stability) present on the surface. During this process, long-chain ligands are replaced with shorter, more conductive ones, making the resulting QDs suitable for use in optoelectronic devices. By conducting the ligand exchange in solution, stable
colloidal QD inks can be created. These inks overcome long- existing and key manufacturing challenges of QD sensor production where multi-step spin coating methods are used. The spin coating process sees up to 14 thin layers of QDs
deposited sequentially, with chemical treatments applied to each layer and washing steps to replace the insulating ligands. This process is very resource greedy, and the multi-layer deposition leads to high risks of defects, with the end result being low- yield, higher-cost QDs. In contrast, QD inks allow deposition of QD coatings in a
single step, with no need for further chemical processes, and minimal defect occurrence. Additionally, it is significantly quicker and less labour intensive, enabling the mass production of high-performance optoelectronic devices and bringing SWIR sensing closer to market.
Applications Infrared PbS QD technology has proven benefits for photodetection, photovoltaics and infrared light emitters. With reported photon conversion efficiencies of over 80% and granting outstanding SWIR sensitivity, infrared light from the sun is converted into energy, making these semiconductor materials revolutionary for most electronics-based systems. For product-sorting applications, QD-equipped SWIR sensors can detect specific spectral signatures, enabling them to monitor container fill levels or check for hidden spoilage, without unnecessary contact. With SWIR light capable of peering deeper into tissue than
any other wavelength, SWIR sensors can benefit medicine with early detection of subdermal conditions. In defence and security, QD-based scanning systems can
better operate in all weather conditions because SWIR light is not scattered by dust or fog. Similarly, QD sensors can enhance vehicle light detection and ranging (LiDAR) systems, increasing their effective range to 500m. What’s more, with high-performance, lead-free QDs rapidly
approaching market readiness, we will soon find SWIR-sensitive QDs in everyday consumer devices, dramatically improving the technologies we use every day. As QD technology develops further, and as more
manufacturers learn of their benefits, this list of applications will grow, bringing the quantum revolution a lot closer.
www.electronicsworld.co.uk October 2023 23
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