We demonstrate that white coatings consisting of silicone embedded with randomly distributed microbubbles provide highly efficient daytime radiative cooling with inexpensive materials and fabrication processes. In our material system, sunlight is strongly scattered with minimal absorption, and heat is effectively removed through mid-infrared (IR) radiation. In our previous study, solid microsphere-based coatings outperformed commercial solar-rejection white paint in cooling efficiency, but their mechanical robustness needed improvement for practical applications. The material system in our work substantially enhances the mechanical robustness while providing superior cooling performance to commercial solar-rejection paint. For ease of processing, we use nonoptimized structures with reduced optical scattering strength. Strong solar rejection is yet achieved by increasing coating thickness. This strategy is desirable for practical rooftop applications where coating thickness is of minor importance in comparison to cooling performance and materials cost. In addition, silicone is stable in extraterrestrial environments and efficiently radiates heat over broad mid-IR spectrum. These material properties of our silicone coatings promise great potential for radiative cooling in space applications.
Growing a lattice-mismatched, dislocation-free epitaxial film on Si has been a challenge for many years. Herein, we exploit nanoscale heterojunction engineering to grow high-quality Ge epilayer on Si. A 1.2-nm-thick chemical SiO<sub>2</sub> film is produced on Si in a H<sub>2</sub>O<sub>2</sub> and H<sub>2</sub>SO<sub>4</sub> solution. When the chemically oxidized Si substrate is exposed to Ge molecular beam, relatively uniform-size nanoscale seed pads form in the oxide layer and "touchdown" on the underlying Si substrate. Although the touchdown location is random, the seed pad growth is self-limiting to 7 nm in size. Upon continued exposure, Ge selectively grows on the seed pads rather than on SiO<sub>2</sub>, and the seeds coalesce to form an epilayer. The Ge epilayer is characterized by high-resolution, cross-sectional as well as plan-view transmission electron microscopy, Raman spectroscopy, and etch-pit density (EPD). The cross-sectional TEM images reveal that the Ge epilayer is free of dislocation network and that the epilayer is fully relaxed at 2 nm from the heterojunction. The Raman shift of Ge optical phonon mode exactly matches that of relaxed bulk Ge, further supporting that the epilayer is fully relaxed. The cross-sectional TEM images, however, show that stacking faults exist near the Ge-SiO<sub>2</sub> interface. A small fraction (~4x10<sup>-3</sup>%) of these stacking faults propagate to the epilayer surface. The plan-view TEM sampling provides an estimate on the density of stacking faults (SF) at approximately 10<sup>6</sup> cm<sup>-2</sup> and threading dislocations (TD) far below 10<sup>6</sup> cm<sup>-2</sup>. The SF and TD propagating to the surface form etch pits, when immersed in a solution containing HF, HNO<sub>3</sub>, glacial acetic acid, and I<sub>2</sub>. The total EPD, as a statistically more reliable estimate on SF and TD than the plan-view TEM, is consistently less than 2x10<sup>6</sup> cm<sup>-2</sup>, where SFs constitute 99 %, and TDs constitute 1 %. That is, the TD density is ~10<sup>5</sup> cm<sup>-2</sup> as a conservative upper bound. The reduction of strain density near the Ge-Si heterojunction, leading to high quality Ge epilayer, is attributed to (1) a high density (~10<sup>11</sup> cm<sup>-2</sup>) of nanoscale Ge seed pads interspaced by 2- to 12-nm-wide SiO<sub>2</sub> patches and (2) the SiO<sub>2</sub> patches serving as artificially introduced dislocation centers. Burgers circuit around each SiO<sub>2</sub> patch results in b = (1/2). We have also determined that the surface mechanism responsible for the selective growth of Ge on Si over SiO<sub>2</sub> is the high desorption rate of Ge adspecies based on their low desorption activation energy of 42 ± 3 kJ/mol.