Entropy-Driven Reversible Melting and Recrystallization of Layered Hybrid Perovskites.

Typical layered 2D A2PbX4 (A: organic ammonium cation, X: Br, I) perovskites undergo irreversible decomposition at high temperatures. Can they be designed to melt at lower temperatures without decomposition? Which thermodynamic parameter drive the melting of layered perovskites? These questions are addressed by considering the melt of A2PbX4 as a mixture of ions (like ionic liquids), and hypothesized that the increase in the structural entropy of fusion (ΔSfus) will be the driving force to decrease their melting temperature. Then to increase structural ΔSfus, A-site cations are designed that are rigid in the solid crystal, and become flexible in the molten state. Different tail groups in the A-site cations form hydrogen-, halogen- and even covalent bonding-interactions, making the cation-layer rigid in the solid form. Additionally, the rotation of ─NH3 + head group is suppressed by replacing ─H with ─CH3, further enhancing the rigidity. Six A2PbX4 crystals with high ΔSfus and low melting temperatures are prepared using this approach. For example, [I-(CH2)3-NH2(CH3)]2PbI4 reversibly melts at 388 K (decomposition temperature 500 K), and then recrystallizes back upon cooling. Consequently, melt-pressed films are grown demonstrating the solvent- and vacuum-free perovskite films for future optoelectronic devices.

Superfluorescence from Electron-Hole Plasma at Moderate Temperatures of 175 K.

Superfluorescence, a cooperative coherent spontaneous emission, is of great importance to the understanding of many-body correlation in optical processes. Even though superfluorescence has been demonstrated in many diverse systems, it is hard to observe in electron-hole plasma (EHP) due to its rapid dephasing and hence needs strong magnetic fields or complex microcavities. Herein, we report the first experimental observation of superfluorescence from EHP up to a moderate temperature of 175 K without external stimuli in a coupled metal halide perovskite quantum dots film. The EHP exhibits macroscopic quantum coherence through spontaneous synchronization. The coherence of the excited state decays by superfluorescence, which is redshifted 40 meV from the spontaneous emission with a ∼1700 times faster decay rate and exhibits quadratic fluence dependence. Notably, the excited state population's delayed growth and abrupt decay, which are strongly influenced by the pump fluence and the Burnham-Chiao ringing, are the characteristics of the superfluorescence. Our findings will open up a new frontier for cooperative emission and light beam-based technologies.

Effect of film morphology on circular dichroism of low-dimensional chiral hybrid perovskites.

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.

Emissive Dark Excitons in Monoclinic Two-Dimensional Hybrid Lead Iodide Perovskites.

Typically, bright excitons (XB) emit light in two-dimensional (2D) layered hybrid perovskites. There are also dark excitons (XD), for which radiative recombination is spin-forbidden. Application of a magnetic field can somewhat relax the spin-rule, yielding XD emission. Can we obtain XD light emission in the absence of a magnetic field? Indeed, we observe unusually intense XD emission at ∼7 K for (Rac-MBA)2PbI4, (Rac-4-Br-MBA)2PbI4, and (R-4-Br-MBA)2PbI4 (Rac-MBA: racemic methylbenzylammonium), which crystallize in a lower symmetry monoclinic phase. For comparison, orthorhombic (R-MBA)2PbI4 does not exhibit XD emission. XD has a lower energy than XB, with energy difference ΔE. In monoclinic samples, ΔE ∼ 20 meV is large enough to suppress the thermal excitation of XD to XB, at temperatures <30 K. Consequently, XD recombines by emitting light with a long lifetime (∼205 ns). At higher temperatures, the emission switches to the spin-allowed XB (lifetime < 1 ns).

Ultrabroad Near Infrared Emitting Perovskites.

Phosphor converted light emitting diodes (pc-LEDs) have revolutionized solid-state white lighting by replacing energy-inefficient filament-based incandescent lamps. However, such a pc-LED emitting ultrabroad near-infrared (NIR) radiations still remains a challenge, primarily because of the lack of ultrabroad NIR emitting phosphors. To address this issue, we have prepared 2.5% W4+-doped and 2.8% Mo4+-doped Cs2Na0.95Ag0.05BiCl6 perovskites emitting ultrabroad NIR radiation with unprecedented spectral widths of 434 and 468 nm, respectively. Upon band-edge excitation, the soft lattice of the host exhibits broad self-trapped exciton (STE) emission covering NIR-I (680 nm), which then nonradiatively excites the dopants. The π-donor ligand Cl⁻ reduces the energy of dopant d-d transitions emitting NIR-II with a peak at ~950 nm. Vibronic coupling broadens the dopant emission. The large spin-orbit coupling and local structural distortion might possibly enhance the dopant emission intensity, leading to an overall NIR photoluminescence quantum yield ~40%. The composite of our ultrabroad NIR phosphors with biodegradable polymer polylactic acid could be processed into free-standing films and 3D printed structures. Large (170 × 170 mm2), robust, and thermally stable 3D printed pc-LED panels emit ultrabroad NIR radiation, demonstrating NIR imaging applications.

Rational Design of Non-Centrosymmetric Hybrid Halide Perovskites.

Structural non-centrosymmetry in semiconducting organic-inorganic hybrid halide perovskites can introduce functionalities like anomalous photovoltaics and nonlinear optical properties. Here we introduce a design principle to prepare Pb- and Bi-based two- and one-dimensional hybrid perovskites with polar non-centrosymmetric space groups. The design principle relies on creating dissimilar hydrogen and halogen bonding non-covalent interactions at the organic-inorganic interface. For example, in organic cations like I-(CH2)3-NH2(CH3)+ (MIPA), -CH3 is substituted by -CH2I at one end, and -NH3+ is substituted by -NH2(CH3)+ at the other end. These substitutions of two -H atoms by -I and -CH3 reduce the rotational symmetry of MIPA at both ends, compared to an unsubstituted cation, for example, H3C-(CH2)3-NH3+. Consequently, the dissimilar hydrogen-iodine and iodine-iodine interactions at the organic-inorganic interface of (MIPA)2PbI4 2D perovskites break the local inversion symmetries of Pb-I octahedra. Owing to this non-centrosymmetry, (MIPA)2PbI4 displays visible to infrared tunable nonlinear optical properties with second and third harmonic generation susceptibility values of 5.73 pm V-1 and 3.45 × 10-18 m2 V-2, respectively. Also, the single crystal shows photocurrent on shining visible light at no external bias, exhibiting anomalous photovoltaic effect arising from the structural asymmetry. The design strategy was extended to synthesize four new non-centrosymmetric hybrid perovskite compounds. Among them, one-dimensional (H3N-(CH2)3-NH(CH3)2)BiI5 shows a second harmonic generation susceptibility of 7.3 pm V-1 and a high anomalous photovoltaic open-circuit voltage of 22.6 V.

Magnetoplasmonics beyond Metals: Ultrahigh Sensing Performance in Transparent Conductive Oxide Nanocrystals.

Active modulation of the plasmonic response is at the forefront of today's research in nano-optics. For a fast and reversible modulation, external magnetic fields are among the most promising approaches. However, fundamental limitations of metals hamper the applicability of magnetoplasmonics in real-life active devices. While improved magnetic modulation is achievable using ferromagnetic or ferromagnetic-noble metal hybrid nanostructures, these suffer from severely broadened plasmonic response, ultimately decreasing their performance. Here we propose a paradigm shift in the choice of materials, demonstrating for the first time the outstanding magnetoplasmonic performance of transparent conductive oxide nanocrystals with plasmon resonance in the near-infrared. We report the highest magneto-optical response for a nonmagnetic plasmonic material employing F- and In-codoped CdO nanocrystals, due to the low carrier effective mass and the reduced plasmon line width. The performance of state-of-the-art ferromagnetic nanostructures in magnetoplasmonic refractometric sensing experiments are exceeded, challenging current best-in-class localized plasmon-based approaches.

Broad Dual Emission by Codoping Cr3+ (d→d) and Bi3+ (s→p) in Cs2 Ag0.6 Na0.4 InCl6 Double Perovskite.

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 Cs2 Ag0.6 Na0.4 InCl6 , 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. 6 s 2 → 6 s 1 6 p 1 ${6{s}^{2}\to 6{s}^{1}6{p}^{1}}$ 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 ( D q / B ${{D}_{q}/B}$ =2.2), yielding the 4 T 2 → 4 A 2 ${{{\rm \ }}^{4}{{\rm T}}_{2}\to {{\rm \ }}^{4}{{\rm A}}_{2}}$ NIR emission. As a proof of concept, we fabricated a panel containing 122 phosphor-converted LEDs, demonstrating its capability to inspect food products.

Sb3+ -Er3+ -Codoped Cs2 NaInCl6 for Emitting Blue and Short-Wave Infrared Radiation.

Cs2 NaInCl6 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 Cs2 NaInCl6 by codoping Sb3+ (s-electron doping) and Er3+ (f-electron doping) ions. Sb3+ doping introduces optically allowed 5s2 → 5s1 5p1 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.

Dielectric confinement for designing compositions and optoelectronic properties of 2D layered hybrid perovskites.

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.

Introducing Intermolecular Cation-π Interactions for Water-Stable Low Dimensional Hybrid Lead Halide Perovskites.

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)Pb2 Br6 ], 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)Pb2 Br6 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.

Lead-Free Double Perovskite Cs2 AgInCl6.

Lead-free halide perovskites have drawn wide attention as alternatives to their toxic and poorly stable lead-based counterparts. Among them, double perovskites with Cs2 AgInCl6 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 Cs2 AgInCl6 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.

Molecular Intercalation and Electronic Two Dimensionality in Layered Hybrid Perovskites.

In layered hybrid perovskites, such as (BA)2 PbI4 (BA=C4 H9 NH3 ), 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.

Bi3+ -Er3+ and Bi3+ -Yb3+ Codoped Cs2 AgInCl6 Double Perovskite Near-Infrared Emitters.

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 Cs2 AgInCl6 double perovskite is presented. Bi3+ -Er3+ codoped Cs2 AgInCl6 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 Cs2 AgInCl6 shows ca. 45 times higher emission intensity compared to the Er3+ doped Cs2 AgInCl6 . 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.

Synthesis and optical properties of colloidal Cs2AgSb1-xBixCl6 double perovskite nanocrystals.

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.

Terahertz Spectroscopic Probe of Hot Electron and Hole Transfer from Colloidal CsPbBr3 Perovskite Nanocrystals.

Colloidal all inorganic CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) have emerged to be an excellent material for applications in light emission, photovoltaics, and photocatalysis. Efficient interfacial transfer of photogenerated electrons and holes are essential for a good photovoltaic and photocatalytic material. Using time-resolved terahertz spectroscopy, we have measured the kinetics of photogenerated electron and hole transfer processes in CsPbBr3 NCs in the presence of benzoquinone and phenothiazine molecules as electron and hole acceptors, respectively. Efficient hot electron/hole transfer with a sub-300 fs time scale is the major channel of carrier transfer thus overcomes the problem related to Auger recombination. A secondary transfer of thermalized carriers also takes place with time scales of 20-50 ps for electrons and 137-166 ps for holes. This work suggests that suitable interfaces of CsPbX3 NCs with electron and hole transport layers would harvest hot carriers, increasing the photovoltaic and photocatalytic efficiencies.

Terahertz Conductivity within Colloidal CsPbBr3 Perovskite Nanocrystals: Remarkably High Carrier Mobilities and Large Diffusion Lengths.

Colloidal CsPbBr3 perovskite nanocrystals (NCs) have emerged as an excellent light emitting material in last one year. Using time domain and time-resolved THz spectroscopy and density functional theory based calculations, we establish 3-fold free carrier recombination mechanism, namely, nonradiative Auger, bimolecular electron-hole recombination, and inefficient trap-assisted recombination in 11 nm sized colloidal CsPbBr3 NCs. Our results confirm a negligible influence of surface defects in trapping charge carriers, which in turn results into desirable intrinsic transport properties, from the perspective of device applications, such as remarkably high carrier mobility (∼4500 cm(2) V(-1) s(-1)), large diffusion length (>9.2 µm), and high luminescence quantum yield (80%). Despite being solution processed and possessing a large surface to volume ratio, this combination of high carrier mobility and diffusion length, along with nearly ideal photoluminescence quantum yield, is unique compared to any other colloidal quantum dot system.

Luminescence, Plasmonic, and Magnetic Properties of Doped Semiconductor Nanocrystals.

Introducing a few atoms of impurities or dopants in semiconductor nanocrystals can drastically alter the existing properties or even introduce new properties. For example, mid-gap states created by doping tremendously affect photocatalytic activities and surface controlled redox reactions, generate new emission centers, show thermometric optical switching, make FRET donors by enhancing the excited state lifetime, and also create localized surface plasmon resonance induced low energy absorption. In addition, researchers have more recently started focusing their attention on doped nanocrystals as an important and alternative material for solar energy conversion to meet the current demand for renewable energy. Moreover, the electrical and magnetic properties of the host are also strongly altered on doping. These beneficial dopant-induced changes suggest that doped nanocrystals with proper selections of dopant-host pairs may be helpful for generating designer materials for a wide range of current technological needs. How properties relate to the doping of a variety of semiconductor nanocrystals are summarized in this Review.

Colloidal Synthesis and Photophysics of M3 Sb2 I9 (M=Cs and Rb) Nanocrystals: Lead-Free Perovskites.

Herein we report the colloidal synthesis of Cs3 Sb2 I9 and Rb3 Sb2 I9 perovskite nanocrystals, and explore their potential for optoelectronic applications. Different morphologies, such as nanoplatelets and nanorods of Cs3 Sb2 I9 , and spherical Rb3 Sb2 I9 nanocrystals were prepared. All these samples show band-edge emissions in the yellow-red region. Exciton many-body interactions studied by femtosecond transient absorption spectroscopy of Cs3 Sb2 I9 nanorods reveals characteristic second-derivative-type spectral features, suggesting red-shifted excitons by as much as 79 meV. A high absorption cross-section of ca. 10-15  cm2 was estimated. The results suggest that colloidal Cs3 Sb2 I9 and Rb3 Sb2 I9 nanocrystals are potential candidates for optical and optoelectronic applications in the visible region, though a better control of defect chemistry is required for efficient applications.