FA_0.8MA_0.2Sn_<i>x</i>Pb_1–<i>x</i>I₃ Hybrid Perovskite Solid Solution: Toward Environmentally Friendly, Stable, and Near-IR Absorbing Materials

We report the first investigation addressing the synthesis and characterization of the FA_0.8­MA_0.2­Sn_<i>x</i>Pb_1–<i>x</i>­I₃ solid solution showing a complete solubility of Sn on the Pb-site leading to cubic single-phase materials. The explored composition shows excellent phase stability and absorbance in the near-IR spectral region

Optical and Structural Property Tuning in Physical Vapor Deposited Bismuth Halides Cs₃Bi₂(I_1–<i>x</i>Br_<i>x</i>)₉ (0 ≤ <i>x</i> ≤ 1)

Crystalline films of lead-free all-inorganic Cs3Bi2X9 (X = Br, I) perovskites have been deposited by radio frequency (RF)-magnetron sputtering providing high-quality, single-phase films as confirmed by structural, morphological, and optical property characterization. Progressive tuning of crystal structure characteristics and optical absorbance has been achieved in mixed Br/I phases Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1), highlighting a shift of the band gap from about 2.0 eV for Cs3Bi2I9 to 2.64 eV for Cs3Bi2Br9. X-ray diffraction and Raman scattering allowed defining the range of alloyed compositions where single-phase compositions are found. Finally, preliminary photocatalytic activity tests on the degradation of methylene blue provided solid data indicating the future possible exploitation of Bi-based perovskite derivative materials as active photocatalysts

Table_1_Highly Tunable Emission by Halide Engineering in Lead-Free Perovskite-Derivative Nanocrystals: The Cs2SnX6 (X = Cl, Br, Br/I, I) System.DOCX

Nanocrystals of Cs2SnX6 (X = Cl, Br, Br0.5I0.5, and I) have been prepared by a simple, optimized, hot-injection method, reporting for the first time the synthesis of Cs2SnCl6, Cs2SnBr6, and mixed Cs2Sn(I0.5Br0.5)6 nanocrystalline samples. They all show a cubic crystal structure with a linear scaling of lattice parameter by changing the halide size. The prepared nanocrystals have spherical shape with average size from 3 to 6 nm depending on the nature of the halide and span an emission range from 444 nm (Cs2SnCl6) to 790 nm (Cs2SnI6) with a further modulation provided by mixed Br/I systems.</p

Exploring the Limits of Three-Dimensional Perovskites: The Case of FAPb_1–<i>x</i>Sn_<i>x</i>Br₃

Metal halide perovskites based on formamidinium (FA) show interesting properties compared to methylammonium (MA) compounds. The larger molecular size of FA compared to that of MA leads, however, to a structural instability in FAPbI3, placing FA at the border of tolerance factor for three-dimensional (3D) lead iodide perovskites. Because of the huge interest in mixed Sn/Pb perovskites, here we investigate FAPb1–xSnxBr3 perovskites in a full compositional range (0 ≤ x ≤ 1). We find a non-monotonic band gap evolution with increasing Sn content, which, through first-principles computational analyses, we relate to a distorted structure that dynamically averages to a cubic phase, as determined by X-ray diffraction. The large FA cation induces an instantaneous structure made by partly decoupled SnBr3 units, which leads to the observed band gap opening. FASnBr3 thus likely represents a limit compound of 3D perovskites. Intermediate FAPb1–xSnxBr3 compositions maintain a band gap of ∼1.8 eV up to 85% Sn content, making them interesting candidates for applications in tandem devices

A Hybrid Plasmonic−Photonic Nanodevice for Label-Free Detection of a Few Molecules

Noble metal nanowaveguides supporting plasmon polariton modes are able to localize the optical fields at nanometer level for high sensitivity biochemical sensing devices. Here we report on the design and fabrication of a novel photonic−plasmonic device which demonstrates label-free detection capabilities on single inorganic nanoparticles and on monolayers of organic compounds. In any case, we determine the Raman scattering signal enhancement and the device detection limits that reach a number of molecules between 10 and 250. The device can be straightforwardly integrated in a scanning probe apparatus with the possibility to match topographic and label-free spectroscopic information in a wide range of geometries

All-Polymer Microcavities for the Fluorescence Radiative Rate Modification of a Diketopyrrolopyrrole Derivative

Controlling the radiative rate of emitters with macromolecular photonic structures promises flexible devices with enhanced performances that are easy to scale up. For instance, radiative rate enhancement empowers low-threshold lasers, while rate suppression affects recombination in photovoltaic and photochemical processes. However, claims of the Purcell effect with polymer structures are controversial, as the low dielectric contrast typical of suitable polymers is commonly not enough to provide the necessary confinement. Here we show all-polymer planar microcavities with photonic band gaps tuned to the photoluminescence of a diketopyrrolopyrrole derivative, which allows a change in the fluorescence lifetime. Radiative and nonradiative rates were disentangled systematically by measuring the external quantum efficiencies and comparing the planar microcavities with a series of references designed to exclude any extrinsic effects. For the first time, this analysis shows unambiguously the dye radiative emission rate variations obtained with macromolecular dielectric mirrors. When different waveguides, chemical environments, and effective refractive index effects in the structure were accounted for, the change in the radiative lifetime was assigned to the Purcell effect. This was possible through the exploitation of photonic structures made of polyvinylcarbazole as a high-index material and the perfluorinated Aquivion as a low-index one, which produced the largest dielectric contrast ever obtained in planar polymer cavities. This characteristic induces the high confinement of the radiation electric field within the cavity layer, causing a record intensity enhancement and steering the radiative rate. Current limits and requirements to achieve the full control of radiative rates with polymer planar microcavities are also addressed

Mild Sol–Gel Conditions and High Dielectric Contrast: A Facile Processing toward Large-Scale Hybrid Photonic Crystals for Sensing and Photocatalysis

Solution processing of highly performing photonic crystals has been a towering ambition for making them technologically relevant in applications requiring mass and large-area production. It would indeed represent a paradigm changer for the fabrication of sensors and for light management nanostructures meant for photonics and advanced photocatalytic systems. On the other hand, solution-processed structures often suffer from low dielectric contrast and poor optical quality or require complex deposition procedures due to the intrinsic properties of components treatable from solution. This work reports on a low-temperature sol–gel route between the alkoxides of Si and Ti and poly­(acrylic acid), leading to stable polymer–inorganic hybrid materials with tunable refractive index and, in the case of titania hybrid, photoactive properties. Alternating thin films of the two hybrids allows planar photonic crystals with high optical quality and dielectric contrast as large as 0.64. Moreover, low-temperature treatments also allow coupling the titania hybrids with several temperature-sensitive materials including dielectric and semiconducting polymers to fabricate photonic structures. These findings open new perspectives in several fields; preliminary results demonstrate that the hybrid structures are suitable for sensing and the enhancement of the catalytic activity of photoactive media and light emission control

Label-Free Vapor Selectivity in Poly(<i>p</i>‑Phenylene Oxide) Photonic Crystal Sensors

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(<i>p</i>-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

High Quality Factor in Solution-Processed Inorganic Microcavities Embedding CsPbBr₃ Perovskite Nanocrystals

Optical microcavities grant manipulation over light–matter interactions and light propagation, enabling the fabrication of foundational optical and optoelectronic components. However, the materials used for high-performing systems, mostly bulk inorganics, are typically costly, and their processing is hardly scalable. In this work, we present an alternative way to fabricate planar optical resonators via solely solution processing while approaching the performances of conventional systems. Here, we couple fully solution-processed high dielectric contrast inorganic Bragg mirrors obtained by sol–gel deposition with the remarkable photoluminescence properties of CsPbBr3 perovskite nanocrystals. The approach yields microcavities with a quality factor of ∼220, which is a record value for solution-processed inorganic structures, and a strong emission redistribution resulting in a 3-fold directional intensity enhancement

3D Hollow Nanostructures as Building Blocks for Multifunctional Plasmonics

We present an advanced and robust technology to realize 3D hollow plasmonic nanostructures which are tunable in size, shape, and layout. The presented architectures offer new and unconventional properties such as the realization of 3D plasmonic hollow nanocavities with high electric field confinement and enhancement, finely structured extinction profiles, and broad band optical absorption. The 3D nature of the devices can overcome intrinsic difficulties related to conventional architectures in a wide range of multidisciplinary applications