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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]

The introduction of chirality in layered hybrid perovskites breaks the symmetry of their inorganic sub-lattices. Consequently, they show intriguing linear and non-linear optical properties. Here we explore the effect of chirality on the excitonic photoluminescence of chiral (R- and S-α-MBA)2PbI4 (MBA: methylbenzylammonium) at cryogenic temperatures. The induced chirality splits the excitonic emissions below 150 K. Additionally, (R- and S-α-MBA)2PbI4 show wavelength-tunable second harmonic generation (SHG) that depends strongly on the polarization angle of the incident light. [ABSTRACT FROM AUTHOR]

Tin halide perovskites are potential alternatives of lead halide perovskites. However, the easy oxidation of Sn2+ to Sn4+ brings in a challenge. Recently, layered two‐dimensional hybrid tin halide perovskites have been shown to partially resist the oxidation process because of the presence of hydrophobic organic molecules. Consequently, such layered hybrid perovskites are being explored for optoelectronic applications. The optical properties of layered tin halide perovskites depend on the interlayer separation and the dielectric mismatch between the organic and inorganic layers. Intercalation (insertion) of a molecular species between the layers modifies the interlayer interactions affecting the optical properties of layered hybrid perovskites. We investigated the effect of hexafluorobenzene (HFB) intercalation in phenethylammonium tin iodide [(PEA)2SnI4] using temperature‐dependent (6 K to 300 K) photoluminescence (PL). HFB intercalation increases the bandgap. A strong PL quenching is observed in pristine (PEA)2SnI4 below 150 K, probably because of the presence of non‐emissive states. HFB intercalation suppresses the influence of such non‐emissive states resulting in an increase in PL intensity at the cryogenic temperatures. Our results highlight that a simple molecular intercalation (non‐covalent interaction) into layered hybrid perovskites can significantly tailor the electronic and optical properties. [ABSTRACT FROM AUTHOR]

Optoelectronically active hybrid lead halide perovskites dissociate in water. To prevent this dissociation, here, we introduce long‐range intermolecular cation‐π interactions between A‐site cations of hybrid perovskites. An aromatic diamine like 4,4′‐trimethylenedipyridine, if protonated, can show a long‐range cation‐π stacking, and therefore, serves as our A‐site cation. Consequently, 4,4′‐trimethylenedipyridinium lead bromide [(4,4′‐TMDP)Pb2Br6], a one‐dimensional hybrid perovskite, remains completely stable after continuous water treatment for six months. Mechanistic insights about the cation‐π interactions are obtained by single‐crystal X‐ray diffraction and nuclear magnetic resonance spectroscopy. The concept of long‐range cation‐π interaction is further extended to another A‐site cation 4,4′‐ethylenedipyridinium ion (4,4′‐EDP), forming water‐stable (4,4′‐EDP)Pb2Br6 perovskite. These water‐stable perovskites are then used to fabricate white light‐emitting diode and for light up‐conversion through tunable third‐harmonic generation. Note that the achieved water stability is the intrinsic stability of perovskite composition, unlike the prior approach of encapsulating the unstable perovskite material (or device) by water‐resistant materials. The introduced cation‐π interactions can be a breakthrough strategy in designing many more compositions of water‐stable low‐dimensional hybrid perovskites. [ABSTRACT FROM AUTHOR]

Lead‐free halide perovskites have drawn wide attention as alternatives to their toxic and poorly stable lead‐based counterparts. Among them, double perovskites with Cs2AgInCl6 composition, often doped with various elements, have been in the spotlight owing to their intriguing optical properties, namely, self‐trapped exciton (STEs) emission and dopant‐induced photoluminescence. This interest has sparked different synthesis approaches towards both crystals and nanocrystals, and the exploration of many alloy compositions with mono‐ and trivalent cations other than Ag+ and In3+. In this Minireview we describe the recent developments on Cs2AgInCl6 bulk crystals and nanocrystals, their synthesis strategies, intrinsic optical properties, and tunable photoluminescence originating from different alloying and doping effects. We also discuss progress on computational studies aimed at understanding the thermodynamic stability, the role of defects, and the origin of photoluminescence in relation to the STEs and the direct band gap character. [ABSTRACT FROM AUTHOR]

Lead‐free halide perovskites have drawn wide attention as alternatives to their toxic and poorly stable lead‐based counterparts. Among them, double perovskites with Cs2AgInCl6 composition, often doped with various elements, have been in the spotlight owing to their intriguing optical properties, namely, self‐trapped exciton (STEs) emission and dopant‐induced photoluminescence. This interest has sparked different synthesis approaches towards both crystals and nanocrystals, and the exploration of many alloy compositions with mono‐ and trivalent cations other than Ag+ and In3+. In this Minireview we describe the recent developments on Cs2AgInCl6 bulk crystals and nanocrystals, their synthesis strategies, intrinsic optical properties, and tunable photoluminescence originating from different alloying and doping effects. We also discuss progress on computational studies aimed at understanding the thermodynamic stability, the role of defects, and the origin of photoluminescence in relation to the STEs and the direct band gap character. [ABSTRACT FROM AUTHOR]

Lead‐free halide perovskites have drawn wide attention as alternatives to their toxic and poorly stable lead‐based counterparts. Among them, double perovskites with Cs2AgInCl6 composition, often doped with various elements, have been in the spotlight owing to their intriguing optical properties, namely, self‐trapped exciton (STEs) emission and dopant‐induced photoluminescence. This interest has sparked different synthesis approaches towards both crystals and nanocrystals, and the exploration of many alloy compositions with mono‐ and trivalent cations other than Ag+ and In3+. In this Minireview we describe the recent developments on Cs2AgInCl6 bulk crystals and nanocrystals, their synthesis strategies, intrinsic optical properties, and tunable photoluminescence originating from different alloying and doping effects. We also discuss progress on computational studies aimed at understanding the thermodynamic stability, the role of defects, and the origin of photoluminescence in relation to the STEs and the direct band gap character. [ABSTRACT FROM AUTHOR]

Lead‐free halide perovskites have drawn wide attention as alternatives to their toxic and poorly stable lead‐based counterparts. Among them, double perovskites with Cs2AgInCl6 composition, often doped with various elements, have been in the spotlight owing to their intriguing optical properties, namely, self‐trapped exciton (STEs) emission and dopant‐induced photoluminescence. This interest has sparked different synthesis approaches towards both crystals and nanocrystals, and the exploration of many alloy compositions with mono‐ and trivalent cations other than Ag+ and In3+. In this Minireview we describe the recent developments on Cs2AgInCl6 bulk crystals and nanocrystals, their synthesis strategies, intrinsic optical properties, and tunable photoluminescence originating from different alloying and doping effects. We also discuss progress on computational studies aimed at understanding the thermodynamic stability, the role of defects, and the origin of photoluminescence in relation to the STEs and the direct band gap character. [ABSTRACT FROM AUTHOR]

Two dimensional (2D) layered hybrid lead halide perovskites are a fascinating class of semiconductors displaying a plethora of interesting optoelectronic properties with potential for application in solar cells, light emitting diodes, etc. Most of these properties can be linked to their repeating quantum well-like structures providing 2D excitons. In this perspective, we discuss how dielectric confinement of excitons originates in these layered hybrid perovskites, and then, how it can be used to tune the excitonic properties. In particular, we discuss the recent theoretical and experimental advances correlating dielectric confinement with chemical composition, excitonic binding energy, and optoelectronic property. The freedom from the restrictions of the Goldsmith tolerance factor allows the synthesis of hundreds of compositions of 2D layered hybrid perovskites by independently varying the organic and inorganic layers. We envisage that the combination of this compositional flexibility with the concepts of dielectric confinement discussed in this perspective would be a path forward for designing novel optoelectronic materials. [ABSTRACT FROM AUTHOR]

Doping impurity ions into semiconductor luminescent materials offers a unique pathway for inducing new emission centers and enabling photoluminescence (PL) tuning. Among various luminescence materials, doping Mn2+ into metal halide perovskites becomes a hot topic since Mn2+ ions demonstrate an energy transfer route from host to dopants, resulting in interesting photophysical properties. This review aims to discuss the PL properties of Mn2+ ions in halide perovskites nanocrystals or bulk crystals with different structural dimensions and local environments (MnX42– tetrahedron, MnX62– octahedron, or shortest Mn─Mn distance). In this regard, the effects of Mn2+ doping on the PL properties and their modifications are summarized. Variable ion exchange dynamics, increased emission intensity, and enhanced stability induced by Mn2+ doping are analyzed. These results also provide beneficial insights into applications of the doped luminescent halide perovskites. Finally, the present challenges in Mn2+‐doped luminescent halide perovskites are elaborated. [ABSTRACT FROM AUTHOR]

In layered hybrid perovskites, such as (BA)2PbI4 (BA=C4H9NH3), electrons and holes are considered to be confined in atomically thin two dimensional (2D) Pb–I inorganic layers. These inorganic layers are electronically isolated from each other in the third dimension by the insulating organic layers. Herein we report our experimental findings that suggest the presence of electronic interaction between the inorganic layers in some parts of the single crystals. The extent of this interaction is reversibly tuned by intercalation of organic and inorganic molecules in the layered perovskite single crystals. Consequently, optical absorption and emission properties switch reversibly with intercalation. Furthermore, increasing the distance between inorganic layers by increasing the length of the organic spacer cations systematically decreases these electronic interactions. This finding that the parts of the layered hybrid perovskites are not strictly electronically 2D is critical for understanding the electronic, optical, and optoelectronic properties of these technologically important materials. [ABSTRACT FROM AUTHOR]

Bi3+ and lanthanide ions have been codoped in metal oxides as optical sensitizers and emitters. But such codoping is not known in typical semiconductors such as Si, GaAs, and CdSe. Metal halide perovskite with coordination number 6 provides an opportunity to codope Bi3+ and lanthanide ions. Codoping of Bi3+ and Ln3+ (Ln=Er and Yb) in Cs2AgInCl6 double perovskite is presented. Bi3+‐Er3+ codoped Cs2AgInCl6 shows Er3+ f‐electron emission at 1540 nm (suitable for low‐loss optical communication). Bi3+ codoping decreases the excitation (absorption) energy, such that the samples can be excited with ca. 370 nm light. At that excitation, Bi3+‐Er3+ codoped Cs2AgInCl6 shows ca. 45 times higher emission intensity compared to the Er3+ doped Cs2AgInCl6. Similar results are also observed in Bi3+‐Yb3+ codoped sample emitting at 994 nm. A combination of temperature‐dependent (5.7 K to 423 K) photoluminescence and calculations is used to understand the optical sensitization and emission processes. [ABSTRACT FROM AUTHOR]

The results of incorporating high concentrations of manganese in wurtzite [Zn.sub.x][Cd.sub.1]S nanocrystals (NCs) are described by using a high temperature synthetic route, thus forming a stable state and explaining the difficulties in achieving such doping. Photophysical studies have shown that the quantum efficiency per [Mn.sup.+] ion has decreased exponentially with the average number of [Mn.sup.2+] ions per NC.

Lead halide perovskites are a sensational class of optoelectronic materials, but they suffer from the problems of lead toxicity and poor thermal stability of organic–inorganic hybrid systems. Various lead-free metal halide double-perovskite (DP) systems are being explored as potential lead-free perovskites for optoelectronics. In these DPs, Pb2+ are replaced by equal proportions of monovalent and trivalent metal ions. In early 2018, colloidal nanocrystals (NCs) of Cs2AgBiX6 (X = Cl, Br, I) DPs were reported and, in quick succession, multiple research teams have reported the synthesis, stabilization of unusual phases, optical properties, and photocatalytic applications of DP NCs. Here, we provide the first review of DP NCs. We first summarize the progress made in this research field, and then discuss the potential future directions of DP NCs compared with their bulk counterparts. [ABSTRACT FROM AUTHOR]