Co‐sputtering of A Thin Film Broadband Absorber Based on Self‐Organized Plasmonic Cu Nanoparticles

The efficient conversion of solar energy to heat is a prime challenge for solar thermal absorbers, and various material classes and device concepts are discussed. One exciting class of solar thermal absorbers are plasmonic broadband absorbers that rely on light absorption thanks to plasmonic resonances sustained in metallic nanoparticles. This work focuses on Cu/Al$_2$O$_3$ plasmonic absorbers, which consist of a thin film stack of a metallic Cu-mirror, a dielectric Al$_2$O$_3$ spacer, and an Al$_2$O$_3$/Cu-nanoparticle nanocomposite. This work explores two preparation routes for the Al$_2$O$_3$/Cu-nanoparticle nanocomposite, which rely on the self-organization of Cu nanoparticles from sputtered atoms, either in the gas phase (i.e., via gas aggregation source) or on the thin film surface (i.e., via simultaneous co-sputtering). While in either case, Cu-Al$_2$O$_3$-Al$_2$O$_3$/Cu absorbers with a low reflectivity over a broad wavelength regime are obtained, the simultaneous co-sputtering approach enabled better control over the film roughness and showed excellent agreement with dedicated simulations of the optical properties of the plasmonic absorber using a multi-scale modeling approach. Upon variation of the thickness and filling factor of the Al$_2$O$_3$/Cu nanocomposite layer, the optical properties of the plasmonic absorbers are tailored, reaching an integrated reflectance down to 0.17 (from 250 to 1600 nm)

A freestanding photoswitchable aero-polymer with an incorporated bridged azobenzene: 3D structure, photoinduced motion, biocompatibility and potential application as photomechanical cell scaffold

Photoswitchable polymers are of great interest for a variety of applications such as optical data storage, functional membranes and photoactuators. The latter are typically fabricated by wet-chemical approaches including gels, liquid-crystalline elastomers and supramolecular polymers. In this work we demonstrate the fabrication of a new freestanding photoswitchable aeropolymer-structure via solvent-free, single-step initiated chemical vapor deposition (iCVD) using tetrapodal zinc oxide (t-ZnO) as sacrificial substrate material. On the molecular scale, the copolymer is composed of 2-hydroxyethyl methacrylate (HEMA) and a specifically synthesized diazocine (a bridged azobenzene) as photoswitchable cross-linking unit. iCVD enables in this connection a combination of both comonomers while preserving their chemical functionalities as well as the individual structure of the t-ZnO templates without pore clogging. After post-reactional etching and drying, a hollow polymer network with nanoscopic thin walls remains maintaining the substrate characteristic tetrapodal structure, which we coined aero-photoswitch. We identify and differentiate specific properties of the fabricated structures, originating from the switchable copolymer and the highly porous tetrapodal conformation, for a comprehensive description of the overall aero-structures. These aero-photoswitchable polymers provide unique properties due to their extremely delicate yet stable morphology and their efficient transformation of molecular photoisomerization to motion on the macroscopic scale upon illumination with blue light. In addition, we investigate their biocompatibility as well as successful cell attachment and proliferation. These new photoswitchable actuators turn out to be highly promising smart materials for future research on photoswitchable scaffolds