Casimir-Lifshitz quantum state of superhydrophobic black-silicon surfaces manufactured by a metal-assisted hierarchical nano-microtexturing process

We investigated superhydrophobic Si nanosurfaces similar to the lotus leaf by performing a hierarchical nanotexturing process on micropyramidal Si surfaces. The process was carried out using a metal-assisted chemical etching process based upon the deposition of Ag nanoparticles. The hierarchical micro-nanosurfaces showed a superhydrophobic character with contact angles of approximately 134∼150°. The photon tunnelling also provides a strong light absorption as a black Si. The surface-light emission from broad and sharp photoluminescence was observed in the wavelength ranges of 414.7∼440 and 509∼516.2 nm. The field-induced tunnelling current on nanosurface shows the formation of quantum surface states. From the analyses of Casimir-Lifshitz quantum state of a photon in vacuum, the superhydrophobic behaviour of water droplet is closely related to the nanosurface and the nanoporous cavity shows the absorption of terahertz energy. Si nanosurface shows the broadband absorption in the spectral range of 800∼900 cm^-1 corresponding to the energy range of 99.2∼111.6 meV with 24∼27 THz.