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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.

Chiral hybrid lead halide perovskites show interesting chiral optoelectronic properties. The extent of chirality is often estimated by their circular dichroism (CD) response. Here, we show that the CD data depend strongly on film morphology. Four of the six chiral hybrid lead halide films prepared, 2D (R- and S-MBA)2PbI4 and 1D (R- and S-MBA)PbI3 (MBA: methylbenzylammonium), form homogenous non-textured films and show an isotropic CD signal. In contrast, the other two samples, 1D (R- and S-MBA)PbBr3, form textured films, showing uncorrelated CD signals from different parts of the film. Therefore, the role of film morphology needs to be verified before designing and comparing the chiroptic and chiral optoelectronic properties of hybrid perovskites. [ABSTRACT FROM AUTHOR]

Optically active defects often play a crucial role in governing the light emission as well as the electronic properties of materials. Moreover, defect-mediated states in the midgap region can trap electrons, thus opening a path for the recombination of electrons and holes in lower energy states that may require phonons in the process. Considering this, we have probed electron–phonon interaction in halide perovskite systems with the introduction of defects and investigated the thermal effect on this interaction. Here, we report Raman spectroscopic study of the thermal evolution of electron–phonon coupling, which is tunable with the crystal growth conditions, in the halide perovskite systems Cs2AgInCl6 and Cs2NaInCl6. The signature of electron–phonon coupling is observed as a Fano anomaly in the lowest frequency phonon mode (51 cm−1), which evolves with temperature. In addition, we observe a broad band in the photoluminescence (PL) measurements for the defect-mediated systems, which is otherwise absent in defect-free halide perovskite. The simultaneous observation of the Fano anomaly in the Raman spectrum and the emergence of the PL band suggests the defect-mediated midgap states and the consequent existence of electron–phonon coupling in the double perovskite. [ABSTRACT FROM AUTHOR]

Crown ether anchored organic‐inorganic hybrid halides have been recently reported as interesting luminescent materials in the visible region of electromagnetic spectrum. Is it possible to develop such crown ether anchored hybrid materials for near infrared emission? Motivated by this question, we designed a new hybrid material, namely, [(18‐Crown‐6)K][MoOCl4(H2O)]. 18‐Crown‐6 ether bound with K+ form the cationic part [(18‐Crown‐6)K]+. The K+ of [(18‐Crown‐6)K]+ electrostatically interacts with Cl− of the anionic part [MoOCl4(H2O)]−, forming the hybrid crystal [(18‐Crown‐6)K][MoOCl4(H2O)]. It crystallizes in orthorhombic crystal system with Pnma space group. The Mo(V) possesses one d‐electron (d1) in C4v ${{C}_{4v}}$ point group symmetry in the [MoOCl4(H2O)]− polyhedra. This electronic configuration leads to multiple spin‐allowed d-d ${d-d}$ transitions along with a ligand to metal charge transfer (LMCT) resulting into multiple optical absorption bands in the near UV‐visible‐near infrared (NIR) region. The lowest energy d-d ${d-d}$ transition via 2E (dxz1 ${{d}_{xz}^{1}}$ , dyz ${{d}_{yz}}$)/2E (dxz,dyz1 ${{d}_{xz},{d}_{yz}^{1}}$)→ ${\to }$ 2B2 (dxy1 ${{d}_{xy}^{1}}$) leads to NIR PL with peak at 952 nm, but with a poor intensity at room temperature. [ABSTRACT FROM AUTHOR]

A phosphor emitting both white light and broad near‐infrared (NIR) radiation can simultaneously provide visual inspection and early signs of rotting of food products. The broad NIR emission is absorbed by the vibrational overtones of water molecules present in food items, providing the non‐invasive image contrast to assess the food freshness. Here we design a phosphor, namely, Cr3+‐Bi3+‐codoped Cs2Ag0.6Na0.4InCl6, that simultaneously emit warm white light and broad NIR (1000 nm) radiation with quantum yield 27 %. This dual emitter is designed by combining the features of s2‐electron (Bi3+) and d3‐electron (Cr3+) doping in a weak crystal field of the halide perovskite host. 6s2→6s16p1 ${6{s}^{2}\to 6{s}^{1}6{p}^{1}}$ excitation of Bi3+, using a commercial 370 nm ultraviolet light‐emitting‐diodes (UV‐LED), yields both the emissions. A fraction of the excited Bi3+ dopants emit the warm white light, and the other fraction transfers its energy non‐radiatively to Cr3+. Then the Cr3+ de‐excites emitting broad NIR emission. Temperature dependent (6.4–300 K) photoluminescence in combination with Tanabe‐Sugano diagram show that the Cr3+ experiences a weak crystal field (Dq/B ${{D}_{q}/B}$ =2.2), yielding the 4T2→4A2 ${{{\rm \ }}^{4}{{\rm T}}_{2}\to {{\rm \ }}^{4}{{\rm A}}_{2}}$ NIR emission. As a proof of concept, we fabricated a panel containing 122 phosphor‐converted LEDs, demonstrating its capability to inspect food products. [ABSTRACT FROM AUTHOR]

Lead halide perovskites are extraordinary optoelectronic materials, but there are issues related to their toxicity and instability. To overcome these issues, various lead-free perovskites are being explored. Metal halide double perovskites, for example, Cs2AgSbCl6, in which two Pb2+ in CsPbCl3 (or Cs2Pb2Cl6) are replaced with one Ag+ and one Sb3+, provide both charge balanced and stable 3D perovskite structures. Synthesis of such double perovskites with different compositions, sizes, and solution processabilities still remains a challenge. The present communication describes synthesis and characterization of colloidal Cs2AgSb1−xBixCl6 alloy nanocrystals with 0 ≤ x ≤ 1. These nanocrystals exhibit an elpasolite structure where the lattice parameters vary systematically with the composition "x." The nanocrystals are cubic in shape with an edge-length of ∼10 nm. UV-visible absorption spectra also change systematically with composition. The lowest energy absorption peak ∼3.4 eV becomes sharper along with a red-shift with increasing Bi content. The alloying can influence the optical absorption by both modifying the intrinsic electronic band structure and changing the concentration of antisite disorders. For intermediate compositions (x = 0.22, 0.36, and 0.70), photoluminescence with a peak at 2.74 eV is observed. [ABSTRACT FROM AUTHOR]

Cs2NaInCl6 double perovskite is stable, environmentally benign and easy to prepare. But it has a wide band gap (5.1 eV), and therefore, does not show optical and optoelectronic properties in the visible and short‐wave infrared (SWIR) region. Here we introduce such functionalities in Cs2NaInCl6 by codoping Sb3+ (s‐electron doping) and Er3+ (f‐electron doping) ions. Sb3+ doping introduces optically allowed 5s2→ 5s15p1 electronic absorption at the sub‐band gap level, which then emits blue photoluminescence with ≈93 % quantum yield. But f‐f electronic absorption of Er3+ is parity forbidden. Codoping Sb3+–Er3+, leads to transfer of excitation energy from Sb3+ to Er3+, yielding SWIR emission at 1540 nm. Temperature (6 to 300 K) dependent photoluminescence measurements elucidate the excitation and emission mechanism. A phosphor converted light emitting diode (pc‐LED) fabricated by using the codoped sample emits stable blue and SWIR radiation over prolonged (84 hours) operation at 5.1 V. [ABSTRACT FROM AUTHOR]

Two‐dimensional (2D) layered hybrid perovskites like phenylethylammonium lead bromide [(PEA)2PbBr4] show interesting optoelectronic properties and improved moisture stability owing to the repeating quantum‐well structure and hydrophobic organic cations, respectively. Doping metal ions in such perovskites leads to exciton‐dopant interactions, imparting new optical properties. Here we report synthesis and optical properties of Yb3+‐doped (PEA)2PbBr4. Crystals show plate‐like morphology with lateral size of few micrometers. Also, ∼3.5 nm thin nanoplatelets are obtained after sonicating the samples. The host absorbs light via excitonic (and band‐band) transition at 407 nm. A part of the generated excitons recombines by emitting light at 414 nm along with host‐related non‐radiative recombination. Another significant part of the excitons recombines by non‐radiatively transferring the energy to the f‐electrons of Yb3+ dopant ions. The Yb3+ ions then de‐excite via f‐f (2F5/2→2F7/2) transition emitting near‐infrared (NIR) radiation with a peak at 997 nm. Such NIR‐emitting semiconductors with improved performances are desired for NIR light‐emitting diode applications. [ABSTRACT FROM AUTHOR]