Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

Metal halide perovskites represent a flourishing area of research, driven by both their potential application in photo-voltaics and optoelectronics, and for the fundamental science underpinning their unique optoelectronic properties. The advent of colloidal methods for the synthesis of halide perovskite nanocrystals has brought to the attention inter-esting aspects of this new type of materials, above all their defect-tolerance. This review aims to provide an updated survey of this fast-moving field, with a main focus on their colloidal synthesis. We examine the chemistry and the ca-pability of different colloidal synthetic routes to control the shape, size and optical properties of the resulting nano-crystals. We also provide an up to date overview of their post-synthesis transformations, and summarize the various so-lution processes aimed at fabricating halide perovskite-based nanocomposites. We then review the fundamental optical properties of halide perovskite nanocrystals, by focusing on their linear optical properties, on the effects of quantum confinement and, then, on the current knowledge of their exciton binding energies. We also discuss the emergence of non-linear phenomena such as multiphoton absorption, biexcitons and carrier multiplication. At last, we provide an outlook in the field, with the most cogent open questions and possible future directions.

Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications and Their Optical Properties

Metal halide perovskites represent a flourishing area of research, which is driven by both their potential application in photovoltaics and optoelectronics and by the fundamental science behind their unique optoelectronic properties. The emergence of new colloidal methods for the synthesis of halide perovskite nanocrystals, as well as the interesting characteristics of this new type of material, has attracted the attention of many researchers. This review aims to provide an up-to-date survey of this fast-moving field and will mainly focus on the different colloidal synthesis approaches that have been developed. We will examine the chemistry and the capability of different colloidal synthetic routes with regard to controlling the shape, size, and optical properties of the resulting nanocrystals. We will also provide an up-to-date overview of their postsynthesis transformations, and summarize the various solution processes that are aimed at fabricating halide perovskite-based nanocomposites. Furthermore, we...

Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties.

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

State of the Art and Prospects for Halide Perovskite Nanocrystals.

Perovskites form an important and enormous class of inorganic compounds. Recently, perovskite materials have attracted extensive research interest owing to their excellent optoelectronic properties. Deep insights into the relationships between the crystal structure, electronic structure and properties play an important role in the development of new functional materials and high-performance devices. In this review, after a brief introduction, we first discuss the crystal structure and crystal chemistry of perovskites according to their three classes: standard perovskites, low-dimensional perovskites and perovskite-like halides. Next, the electronic structure and luminescence from different physical origins are presented. Then, we present a survey on the design, synthesis and luminescence properties of different perovskites, including halide perovskites, oxide perovskites, and lanthanide- or transition metal-doped perovskites, also including dimension-different perovskites (3D, 2D, 1D and quantum dots). We also summarize the strategies for improving the photoluminescence quantum yield (PLQY) and chemical stability, including by surface passivation, encapsulation and doping. Finally, we review their applications and give a brief outlook.

Luminescent perovskites: recent advances in theory and experiments

No‐Interference Reading for Optical Information Storage and Ultra‐Multiple Anti‐Counterfeiting Applications by Designing Targeted Recombination in Charge Carrier Trapping Phosphors

All-inorganic metal halide perovskites of the formulation ABX3 (where A is Cs+, B is commonly Pb2+, and X is a halide, X = Cl, Br, I) have been studied intensively for their unique properties. Most of the current studies focus on halogen exchange to modify the luminescence band gap. Herein we demonstrate a new avenue for changing the band gap of halide perovskites by designing mixed-monovalent cation perovskite-based colloidal quantum dot materials. We have synthesized monodisperse colloidal quantum dots of all-inorganic rubidium-cesium lead halide perovskites (APbBr3, A = mixed monovalent cation systems Rb/Cs) using inexpensive commercial precursors. Through the compositional modulation, the band gap and emission spectra are readily tunable over the visible spectral range of 474-532 nm. The photoluminescence (PL) of RbxCs1-xPbBr3 nanocrystals is characterized with excellent (NTCS color standard) wide color gamut coverage, which is similar to the cesium lead halide perovskites (CsPbX3, X = mixed halide systems Cl/Br), and narrow emission line-widths of 27-34 nm. Furthermore, simulated lattice models and band structures are used to explain the band gap variations.

Rb+ cations enable the change of luminescence properties in perovskite (RbxCs1-xPbBr3) quantum dots.

Broadband near-infrared emission enhancement in K2Ga2Sn6O16:Cr3+ phosphor by electron-lattice coupling regulation

Lithium doping induced self-crystallization of CsPbBr3 nanocrystal glass with improved quantum yield and stability

Cr3+-Doped phosphors are attracting attention for broadband near-infrared (NIR) applications. However, some mechanisms are still unclear due to the entangled crystallographic site occupancy and changeable valence of chromium in the host lattice. In this study, a particular gallium SrGa4O7 (SGO) was employed as a host for chromium doping. The characteristic broadband NIR emission of Cr3+ ions in octahedral sites centered in the range of 650 to 1200 nm with a full width at half maximum (FWHM) of 140 nm and an internal quantum efficiency of more than 46% was observed, although there were only four and seven coordination sites in the SGO lattice. The distorted octahedron in the SGO lattice was generated due to the transition from Ga–O tetrahedron to the octahedron with the doping of Cr3+ ions, and it was regulated by the cation regulation of Ca2+ substituted for Sr2+ ions in the lattice. In particular, the characteristic Cr4+ ion emission between 1200 and 1700 nm with an FWHM at ∼215 nm was observed in phosphors for the first time. This study provides new cogitation for the Cr3+ ion site occupation in the host lattice and a new idea to develop broadband NIR emission phosphors.

Disentangling site occupancy, cation regulation, and oxidation state regulation of the broadband near infrared emission in a chromium-doped SrGa4O7 phosphor

Zhangwen Long

Papers

No‐Interference Reading for Optical Information Storage and Ultra‐Multiple Anti‐Counterfeiting Applications by Designing Targeted Recombination in Charge Carrier Trapping Phosphors

Rb+ cations enable the change of luminescence properties in perovskite (RbxCs1-xPbBr3) quantum dots.

Broadband near-infrared emission enhancement in K2Ga2Sn6O16:Cr3+ phosphor by electron-lattice coupling regulation

Lithium doping induced self-crystallization of CsPbBr3 nanocrystal glass with improved quantum yield and stability

Disentangling site occupancy, cation regulation, and oxidation state regulation of the broadband near infrared emission in a chromium-doped SrGa4O7 phosphor