Toward on-board microchip synthesis of CdSe vs. PbSe nanocrystalline quantum dots as a spectral decoy for protecting space assets
Quantum dots (QDs) were suggested a decade ago as a means of spectral decoy to divert rocket attacks from flying objects such as satellites but have not yet found industrial applications. A possible reason is that these advanced nanomaterials are unstable in the long-term under the forces exhibited on a flying object and the nanoparticles might agglomerate to form larger microparticles, which have a different, unwanted spectral footprint. This work proposes an on-board (in situ) real-time synthesis of quantum dots on a satellite as a result of an interdisciplinary innovation that combines nanomaterial chemistry, heating and mixing with batch microfluidics, and microfabrication of a miniaturized chip. We employed a microchip as the in situ reaction chamber due to its capability of fast, compact synthesis. The implementation of the space micro-reactor, on the other hand, leads to other fundamental challenges that need to be solved such as robust pump-free fluid motion and quick synthesis of quantum dots in less than 180 s which is equivalent to the typical time a rocket needs to reach the satellite from the ground. Additionally, quantum dots need to be tailored in the type of material and size to replicate the spectral signal of the to-be-protected space asset while the synthesis process needs to be robust and resilient to work reliably and automatically under the demanding conditions of a flying object. In this study, we selected cadmium selenide (CdSe) and lead selenide (PbSe) as the materials of choice. Our synthesis process aims to simplify and tailor CdSe and PbSe quantum dots to match the above demands. The produced quantum dots were characterized by their wavelength emission spectra and high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) images. Results show that both CdSe and PbSe quantum dots have been successfully synthesized within a maximum of 10 minutes. While the chosen CdSe-based synthesis was unable to reach IR-emitting sizes within several minutes, we managed to meet all other research targets. The synthesized PbSe QDs were able to reach the mock-spectrum and could produce an emission wavelength peak from 1200 to 1400 nm. This work finally provides insights into the limits of batch synthesis in a microfabricated nanodot chip, such as the need to improve reproducibility and enhance particle growth. Based on this learning, we propose a self-priming flow microchip synthesis that can capitalize on mixing as a key asset to overcome those limits.