We propose here a new colloidal approach for the synthesis of both all-inorganic and hybrid organic-inorganic lead halide perovskite nanocrystals (NCs). The main limitation the protocols that are currently in use, such as the hot-injection and the ligand assisted reprecipitation routes, is that they employ PbX2 (X= Cl, Br, or I) salts as both lead and halide precursors. This imposes restrictions on being able to precisely tune the amount of reaction species and, consequently, on being able to regulate the composition of the final NCs. In order to overcome this issue, we show here that benzoyl halides can be efficiently used as halide sources to be injected in a solution of metal cations (mainly in the form of metal carboxylates) for the synthesis of APbX3 NCs (in which A= Cs+, CH3NH3+ or CH(NH2)2+). In this way, it is possible to independently tune the amount of both cations and halide precursors in the synthesis. The APbX3 NCs that were prepared with our protocol show excellent optical properties, such as high photoluminescence quantum yields, low amplified spontaneous emission thresholds, and enhanced stability in air. It is noteworthy that CsPbI3 NCs, which crystallize in the cubic α phase, are stable in air for weeks without any post-synthesis treatment. The improved properties of our CsPbX3 perovskite NCs can be ascribed to the formation of lead halide terminated surfaces, in which Cs cations are replaced by alkylammonium ions.

Lead halide perovskite nanocrystals are an emerging class of materials that have gained wide interest due to their facile color tuning and high photoluminescence quantum yield; however, the lack of techniques to translate the high performance of nanocrystals into solid films restricts the successful exploitation of such materials in optoelectronic applications. Here, we report a heat-up and large scale synthesis of quantum-confined, blue-emitting CsPbBr3 nanoplatelets (NPLs) that self-assemble into stacked lamellar structures. Spin-coated films fabricated from these NPLs show stable blue emission with a photoluminescence quantum yield (PLQY) of 25%. The morphology and the optoelectronic properties of such films can be dramatically modified by UV-light irradiation under ambient conditions at high power, which transforms the self-assembled stacks of NPLs into much larger structures, such as square-shaped disks and nanobelts. The emission from the transformed thin films is in the green with a record PLQY of 65%, and under femtosecond-pulsed excitation they manifest amplified spontaneous emission with a sharp linewidth of 4 nm at full-width-half-maximum. The transformed films show stable photocurrent with responsivity of up to 15 mA/W and response times of some tens of milliseconds, and are robust under treatment with different solvents. We exploit their insolubility in ethanol to fabricate green emitting, all solution-processed light emitting diodes with external quantum efficiency of 1.1% and luminance of 590 Cd/m2.

The enhancement of the photo-physical response of fluorophores is a crucial factor for photonic and optoelectronic technologies that involve fluorophores as gain media. Recent advances in the development of an extreme light propagation regime, called Epsilon-Near-Zero (ENZ), provide a promising approach in this respect. In this work, we design metal/dielectric nanocavities to be resonant with the absorption and emission bands of the employed fluorophores. Using CsPbBr3 perovskite nanocrystal films as light emitters, we study the spontaneous emission and decay rate enhancement induced by a specifically tailored Double Epsilon-Near-Zero (double-ENZ) structure . We experimentally demonstrate the existence of two ENZ wavelengths, by directly measuring their dielectric permittivity via ellipsometric analysis. The double-ENZ nature of this plasmonic nanocavity has been exploited to achieve both Surface Plasmon Enhanced Absorption (SPEA) and Surface Plasmon Coupled Emission (SPCE), inducing a significant enhancement of both the spontaneous emission and the decay rate of the perovskite nanocrystal film that is placed on top of the nanocavity. Finally, we discuss the possibility of tailoring the two ENZ wavelengths of this structure within the visible spectrum simply by finely designing the thickness of the two dielectric layers, which enables resonance matching with a broad variety of dyes. Our device design is appealing for many practical applications, ranging from sensing to low threshold amplified spontaneous emission, since we achieve a strong PL enhancement with structures that allow for straightforward fluorophore deposition on a planar surface that keeps the fluorophores exposed and accessible.

Hyperbolic Metamaterials are artificially engineered materials whose optical properties can be specifically tailored to manifest an extremely high level of anisotropy. Due to this remarkable anisotropy they represent a unique opportunity to realize effective bulk meta-structure with extraordinary optical properties in the visible range. A simultaneous dielectric singularity in the in plane permittivity, with respect to the propagation direction, has to lead to a complete sign inversion of the same permittivity for that specific visible frequency. Such a drastic phase change has been theoretically highlighted in the past as the major challenge to be overcome in order to unlock many remarkable optical properties not present artificial optical systems. In this paper we experimentally demonstrate the realization of a metal-dielectric multilayer structure showing an inversion point of coexisting anisotropies at a specified wavelength in the visible range, rising from the particular design and fabrication process. Theoretical models and numerical simulations are in very good agreement with experimental data. Ellipsometrical experiments and optical modeling demonstrate the drastic type I/type II transition. Supercollimation effect has been achieved at the inversion point of the coexisting extreme anisotropies, whereas at the epsilon near zero and pole frequency the perfect lens behavior has been observed.

The replacement of plastic with eco-friendly and biodegradable materials is one of the most stringent environmental challenges. In this respect, cellulose outstands as a biodegradable polymer. However, a significant challenge is to obtain biodegradable materials for high-end photonics that are robust in humid environments. Here, we demonstrate the fabrication of high quality micro- and nanoscale photonic and plasmonic structures via replica molding using pure cellulose and a blended version with non-edible agro-wastes. Both materials are biodegradable in soil and seawater according to the ISO 17556 standard. The pure cellulose films are transparent in the VIS-NIR spectrum having a refractive index similar to glass. The microstructured photonic crystals show high-quality diffractive properties that are maintained under extended exposure to water. Nanostructuring the cellulose transforms it to a biodegradable metasurface manifesting bright structural colours. A subsequent deposition of Ag endowed the metasurface with plasmonic properties used to produce plasmonic colours and for surface-enhanced Raman scattering (SERS).