Spectrally Selective Inorganic-Based Multilayer Emitter for Daytime Radiative Cooling.

Daytime radiative coolers are used to pump excess heat from a target object into a cold exterior space without energy consumption. Radiative coolers have become attractive cooling options. In this study, a daytime radiative cooler was designed to have a selective emissive property of electromagnetic waves in the atmospheric transparency window of 8-13 µm and preserve low solar absorption for enhancing radiative cooling performance. The proposed daytime radiative cooler has a simple multilayer structure of inorganic materials, namely, Al2O3, Si3N4, and SiO2, and exhibits high emission in the 8-13 µm region. Through a particle swarm optimization method, which is based on an evolutionary algorithm, the stacking sequence and thickness of each layer were optimized to maximize emissions in the 8-13 µm region and minimize the cooling temperature. The average value of emissivity of the fabricated inorganic radiative cooler in the 8-13 µm range was 87%, and its average absorptivity in the solar spectral region (0.3-2.5 µm) was 5.2%. The fabricated inorganic radiative cooler was experimentally applied for daytime radiative cooling. The inorganic radiative cooler can reduce the temperature by up to 8.2 °C compared to the inner ambient temperature during the daytime under direct sunlight.

Generation of highly integrated multiple vivid colours using a three-dimensional broadband perfect absorber.

The colour printing technology based on interactions between geometric structures and light has various advantages over the pigment-based colour technology in terms of nontoxicity and ultrasmall pixel size. The asymmetric Fabry-Perot (F-P) cavity absorber is the simplest light-interacting structure, which can easily represent and control the colour by the thickness of the dielectric layer. However, for practical applications, an advanced manufacturing technique for the simultaneous generation of multiple reflective colours is required. In this study, we demonstrate F-P cavity absorbers with micropixels by overcoming the difficulties of multi-level pattern fabrication using a nanoimprinting approach. Our asymmetric F-P cavity absorber exhibited a high absorption (approximately 99%) in a wide visible light range upon the incorporation of lossy metallic materials, yielding vivid colours. A high-resolution image of eight different reflective colours was obtained by a one-step process. This demonstrates the potential of this technology for device applications such as high-resolution colour displays and colour patterns used for security functions.

Design of a Broadband Solar Thermal Absorber Using a Deep Neural Network and Experimental Demonstration of Its Performance.

In using nanostructures to design solar thermal absorbers, computational methods, such as rigorous coupled-wave analysis and the finite-difference time-domain method, are often employed to simulate light-structure interactions in the solar spectrum. However, those methods require heavy computational resources and CPU time. In this study, using a state-of-the-art modeling technique, i.e., deep learning, we demonstrate significant reduction of computational costs during the optimization processes. To minimize the number of samples obtained by actual simulation, only regulated amounts are prepared and used as a data set to train the deep neural network (DNN) model. Convergence of the constructed DNN model is carefully examined. Moreover, several analyses utilizing an evolutionary algorithm, which require a remarkable number of performance calculations, are performed using the trained DNN model. We show that deep learning effectively reduces the actual simulation counts compared to the case of a design process without a neural network model. Finally, the proposed solar thermal absorber is fabricated and its absorption performance is characterized.

Customizable 3D-printed architecture with ZnO-based hierarchical structures for enhanced photocatalytic performance.

ZnO-based hierarchical structures including nanoparticles (NPs), nanorods (NRs) and nanoflowers (NFs) on a 3D-printed backbone were effectively fabricated via the combination of the fused deposition modelling (FDM) 3D-printing technique and hydrothermal reaction. The photocatalytic performance of the ZnO-based hierarchical structures on the 3D-backbone was verified via the degradation of the organic pollutant methylene blue, which was monitored by UV-vis spectroscopy. The new photocatalytic architectures used in this investigation give an effective approach and wide applicability to overcome the limitation of photocatalysts such as secondary removal photocatalyst processes.

Uniformly embedded silver nanomesh as highly bendable transparent conducting electrode.

Ag-nanomesh-based highly bendable conducting electrodes are developed using a combination of metal nanotransfer printing and embossing for the 6-inch wafer scale. Two Ag nanomeshes, including pitch sizes of 7.5 and 10 µm, are used to obtain highly transparent (approximately 85% transmittance at a wavelength of 550 nm) and electrically conducting properties (below 10 Ω sq(-1)). The Ag nanomeshes are also distinguished according to the fabrication process, which is called transferred or embedded Ag nanomesh on polyethylene terephthalate (PET) substrate, in order to compare their stability against bending stress. Then the enhancement of bending stability when the Ag nanomesh is embedded in the PET substrate is confirmed.