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VISible to far-IR optical tuning: passive DAYtime cooling by hierarchical structures and hybrid materials

Periodic Reporting for period 2 - VISIRday (VISible to far-IR optical tuning: passive DAYtime cooling by hierarchical structures and hybrid materials)

Reporting period: 2018-09-01 to 2020-02-29

"Cooling is a crucial technology of our modern life. A device capable of efficient sub-ambient daytime cooling without the need for an external power input would considerably lower our overall energy consumption. Nature offers a chance to off-load heat directly into the cold outer space via the so-called ""sky window"": a wavelength range from 8 – 13 µm, where our atmosphere is transparent. Inspired by examples from nature, namely the white beetle and the Saharan silver ant, this project aims to develop materials that selectively emit thermal energy into this sky window wavelength range, and scatter and reflect radiation of all other wavelengths, particularly visible and near-infrared (300 nm – 2.5 µm) irradiated by the sun.
This is a difficult task to achieve and will only be possible by addressing fundamental materials science research across multiple length scales.
The overall goal to master this challenge is to combine top-down direct write lithography with controlled bottom-up colloidal self-assembly. This allows creating hierarchically structured systems fully addressing the stringent optical properties, which cover the entire spectral range from 300 nm – 20 µm. The colloidal nano- and mesoparticles of interest will consist of novel surface phonon polariton (SPhP) supporting materials, which feature tuneable sky window absorption properties depending on their size, shape, and composition. Conjugation of hierarchical structures with such mid-infrared designed SPhP particles enables to meet the specific demands for broadband optical tuning.
We also plan to combine these novel nanophotonic materials with polymers and metallic nanostructures. Such hybrid devices could feature finely adjusted and even externally tunable optical properties, allowing for switching of the cooling capacity.
Based on this fundamental work, VISIRday strives to provide concepts for functional paints and fibers - advanced materials that enable passive daytime cooling as a new green energy technology.
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During the last reporting period, we achieved the setup and purchase of the required experimental infrastructure. We are now a team of 1 Postdoc and 2 Ph.D. students and are currently addressing the following questions:
Which synthesis route is most suitable to access well-defined materials with adjustable surface phonon polariton properties? For this, we investigate (inert) calcination routes to convert silica particles into silicon carbide objects in combination with colloidal self-assembly. Up to now, we succeeded in a low-temperature conversion strategy to yield SiC nanostructures. We were also successful in the fabrication of non-spherical colloidal particles, which are now being scaled up. We also strive to tune their surface phonon polariton properties.
Which microstructure is most effective in transmitting, absorbing, and emitting in the mid-infrared range? For this, we develop photolithographic strategies to generate microstructures in a wide variety of materials. We succeeded in the development of suitable photoresist and development strategies and are now optimizing the lift-off process to gain free-standing structures.
We are currently in the process of establishing reliable protocols to fabricate nano- and microstructured materials. Working with this material platform and our advanced analytical tools we expect to develop exciting materials and devices beyond the state-of-the-art during the coming reporting periods.
Various contributions of passive radiative cooling to buildings [by Daniela Leitner]