From nanoscopic to macroscopic photo-driven motion in azobenzene-containing materials

AbstractThe illumination of azobenzene molecules with UV/visible light efficiently converts the molecules between trans and cis isomerization states. Isomerization is accompanied by a large photo-induced molecular motion, which is able to significantly affect the physical and chemical properties of the materials in which they are incorporated. In some material systems, the nanoscopic structural movement of the isomerizing azobenzene molecules can be even propagated at macroscopic spatial scales. Reversible large-scale superficial photo-patterning and mechanical photo-actuation are efficiently achieved in azobenzene-containing glassy materials and liquid crystalline elastomers, respectively. This review covers several aspects related to the phenomenology and the applications of the light-driven macroscopic effects observed in these two classes of azomaterials, highlighting many of the possibilities they offer in different fields of science, like photonics, biology, surface engineering and robotics

Varifocal diffractive lenses for multi-depth microscope imaging

Flat optical elements enable the realization of ultra-thin devices able to either reproduce or overcome the functionalities of standard bulky components. The fabrication of these elements involves the structuration of material surfaces on the light wavelength scale, whose geometry has to be carefully designed to achieve the desired optical functionality. In addition to the limits imposed by lithographic design-performance compromises, their optical behavior cannot be accurately tuned afterward, making them difficult to integrate in dynamic optical systems. Here we show the realization of fully reconfigurable flat varifocal diffractive lens, which can be in-place realized, erased and reshaped directly on the surface of an azopolymer film by an all-optical holographic process. Integrating the lens in the same optical system used as standard refractive microscope, results in a hybrid microscope capable of multi-depth object imaging. Our approach demonstrates that reshapable flat optics can be a valid choice to integrate, or even substitute, modern optical systems for advanced functionalities

Anomalous evolution of broadband optical absorption reveals dynamic solid state reorganization during eumelanin build-up in thin films

The origin of eumelanin optical properties remains a formidable conundrum preventing a detailed understanding of the complex photo-protective role of these widespread natural pigments and the rational design of innovative bioinspired materials for optoelectronic applications. Here we report the unusual kinetic and thickness-dependent evolution of the optical properties of black eumelanin polymers generated by spontaneous aerial polymerization of 5,6-dihydroxyindole (DHI) thin films (0.1-1 μm), consistent with peculiar solid state reorganization mechanisms governing broadband absorption. The complete reversal of eumelanin UV-visible transmittance spectrum curvature on passing from 0.2 to 0.5 μm thick films, the marked increase in visible extinction coefficients with increasing film thickness and the higher UV extinction coefficients in slowly vs. rapidly generated polymers concur to support distinct dynamic regimes of solid-state molecular reorganization at the nanoscale level and to do affect the development of broadband visible absorption. Solid state control of molecular reorganization disclosed herein may delineate new rational strategies for tuning optical properties in eumelanin thin films for optoelectronic applications

Ab initio Study of Anchoring Groups for CuGaO2 Delafossite-Based p-Type Dye Sensitized Solar Cells

Here we report the first theoretical characterization of the interface between the CuGaO2 delafossite oxide and the carboxylic (–COOH) and phosphonic acid (–PO3H2) anchoring groups. The promising use of delafossites as effective alternative to nickel oxide in p-type DSSC is still limited by practical difficulties in sensitizing the delafossite surface. Thus, this work provides atomistic insights on the structure and energetics of all the possible interactions between the anchoring functional groups and the CuGaO2 surface species, including the effects of the Mg doping and of the solvent medium. Our results highlight the presence of a strong selectivity toward the monodentate binding mode on surface Ga atoms for both the carboxylic and phosphonic acid groups. Since the binding modes have a strong influence on the hole injection thermodynamics, these findings have direct implications for further development of delafossite based p-type DSSCs