photonic states (PDOS), [2][3][4] which helps explaining the photoluminescence (PL) changes of materials when embedded in the PhC structure.Traditionally, PhCs have been made carving bulky inorganic dielectrics or sem iconductors. These structures are still a hot topic in photonics for optical fibers, light emitting diodes (LEDs), sensors, photo voltaic devices, lasers, discrete and integrated optical components, lightening, and quantum computing. [6] However, new solution processable photoactive materials (e.g., organic semiconductors, quantum dots, hybrid perovskites, and selfassem bling supramolecular systems) stimulated the development of PhCs grown with organic and colloidal materials by solution and melt processes. [7] Among PhCs, distributed Bragg reflectors (DBRs) are cur rently the most interesting owing to their planar structure which generates a simple optical response that represents a playground to understand deep physical concepts, as described in Sections 2.4 and 4. DBRs are indeed made of alternated thin films of different dielectric materials. This simplicity makes them the only PhCs that can take advantage of large area growths (see Section 3.1). [8] Such fabrication methods are unconceivable with bulky inorganic DBRs and might reduce processing costs and enhance customizability, also on industrial scale, thus adding unprecedented market opportunities. Figure 1 illustrates the evolution and fabrications of solution and melt processed DBRs. The top of the Figure displays three wellknown natural DBRs belonging to animal and plant reigns: the mother of pearl, [9] the Panamanian Tortoise beetle exoskel eton, [10] and the Pollia Condensata skin, [11] whose growth is driven by the thermodynamics of spinodal phase separation. [12] The interest in these natural structures leads to their emulation until the development of solutionbased fabrication methods for flexible synthetic DBRs made of polymers and inorganic nanoparticles (Figure 1). At the base of Figure 1, we also show some applications arose only in the last decades. From left to right: the use of DBRs in functional architecture, in enhance ment of photon absorption for photovoltaic cells and modules, emission control, lasing, and sensing. [7c,13] In this paper we will first briefly review the properties of DBRs and then focus on the reasons that guided the research toward new solutionbased and largearea fabrications, describing current processes used both at the laboratory and largearea scales. We will then review the main applications of these structures comparing the performances of mesoporous inorganic and polymer devices. An overview on the properties and applications of polymer and inorganic planar 1D photonic crystals fabricated from solution is provided here. In the last decades, photonic crystals became technologically relevant for light management, photovoltaics, sensing, and lasing. Such structures are traditionally produced by lithographic and vacuum techniques, but the need to reduce costs and to scale-up the fabrication have lea...

We report on the formation of ordered arrays of micrometric holes on the surface of polystyrene (PS) films cast from volatile solvents in the presence of humidity at different temperatures. The formation mechanism is investigated for PS having different molecular weights, polydispersities, and carboxylic terminations. Among the chosen materials, a highly regular honeycomb microstructured morphology is obtained on the surface of films prepared with dicarboxy-terminated PS with = 100,000. Experiments and observations on film formation indicate that polar groups are playing a fundamental role in this process. Tuning the surface tension by means of polar terminations allows the film morphology to be modified and in particular the preparation of two- or three-dimensional microstructured films. Finally, we show how these structures can be replicated by soft lithography and then used in the fields of photonic crystals and organic electronics.

The lack of sensors for low cost, extensive, and continuous detection of vapor pollutants is a serious concern for health and safety in industrialized urban areas. Colorimetric sensors, such as distributed Bragg reflectors made of polymers, could achieve this task thanks to their low cost and easy signal transduction but are typically affected by low vapor permeability and lack of selectivity without chemical labeling. Here we demonstrate all-polymer Bragg multilayers for label-free selective detection of organic volatile compounds. The system exploits the ability of amorphous poly(p-phenylene oxide), PPO, to uptake large amount of guest molecules and to form cocrystalline phases with distinct optical properties. Bragg stacks embedding PPO active layers show selective colorimetric response to vapors of carbon tetrachloride and aromatic homologues, which can be revealed by the naked eye.