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Photoelectrochemical (PEC) water splitting is a sustainable pathway for solar to hydrogen conversion. Graphitic carbon nitride (g‐C3N4) nanosheets have the suitable bandgap and band‐edge energies to act as a visible‐light photocatalyst for water splitting, but the fast recombination of photoexcited electron–hole pair limits the efficiency. Herein, N‐doped few‐layer graphene (NFG) dressed with titanium nitride (TiN) nanocrystals (TiN‐NFG) is introduced as an efficient co‐catalyst which improved the water reduction activity of g‐C3N4 by 16 times. The 2D nanocomposite of g‐C3N4:TiN‐NFG has an extended interface for efficient separation of photoexcited electron–hole pair through electron transfer from g‐C3N4 to TiN‐NFG. The metal‐like electronic structure of TiN in combination with good charge conducting capability of NFG reduces the charge transfer resistance at the electrode/electrolyte interface. Both these aspects are responsible for the enhanced PEC activity leading to a photocurrent density of −196 µA cm−2 at 0.11 V versus reversible hydrogen electrode as a photocathode for the g‐C3N4:TiN‐NFG nanocomposite. The nanocomposite is stable, low cost (free from noble metals), and the extent of enhancement in the PEC efficiency for reduction reaction is remarkable compared to prior literature. An efficient noble metal free co‐catalyst: Titanium nitride embedded few‐layer N‐doped graphene nanosheet (TiN‐NFG) is developed to boost the photoelectrochemical (PEC) reduction of water with graphitic carbon nitride (g‐C3N4). The huge enhancement in PEC activity is achieved owing to extended interface area at the 2D/2D heterojunction of composite promoting the superior separation of photoexcited charges in g‐C3N4. [ABSTRACT FROM AUTHOR]

We discuss the transport properties of CsPbBrxI3−x perovskite nanocrystal arrays as a model ensemble system of caesium lead halide-based perovskite nanocrystal arrays. While this material is very promising for the design of light emitting diodes, laser, and solar cells, very little work has been devoted to the basic understanding of their (photo)conductive properties in an ensemble system. By combining DC and time-resolved photocurrent measurements, we demonstrate fast photodetection with time response below 2 ns. The photocurrent generation in perovskite nanocrystal-based arrays is limited by fast bimolecular recombination of the material, which limits the lifetime of the photogenerated electron-hole pairs. We propose to use nanotrench electrodes as a strategy to ensure that the device size fits within the obtained diffusion length of the material in order to boost the transport efficiency and thus observe an enhancement of the photoresponse by a factor of 1000. [ABSTRACT FROM AUTHOR]

Graphitic carbon nitride (g-C 3 N 4) nanosheets are well studied for photocatalytic water splitting using solar light. However, its photocatalytic activity is restrained due to fast recombination of photo-generated electron-hole pairs. Here, we introduce Sn-doped In 2 O 3 (ITO) nanocrystals (NCs) as co-catalysts with g-C 3 N 4 nanosheets, forming g-C 3 N 4 :ITO (2 ​wt%) nanocomposites, for photoelectrochemical (PEC) reduction of water to H 2. The co-catalyst has two major impacts: (i) enhances charge transfer from g-C 3 N 4 to ITO NCs suppressing the recombination of photoexcited electron-hole pair, and (ii) reduces charge transfer resistance at electrode/electrolyte interface. Both these aspects improve PEC activity of the nanocomposites. Our g-C 3 N 4 :ITO nanocomposites photoelectrode shows a photocurrent density of -70 ​μA/cm2 for reduction of water to H 2 , whereas the pristine g-C 3 N 4 nanosheet photoelectrode shows -12 ​μA/cm2 photocurrent density at 0.11 ​V versus reversible hydrogen electrode (RHE). This (~6 times) enhancement in photocurrent density by ITO NCs co-catalyst is reasonably high compared to other co-catalysts for g-C 3 N 4 reported in prior literature. This paper is on solar light driven photoelectrochemical (PEC) reduction of water to H 2. Sn-doped In 2 O 3 (ITO) nanocrystals is introduced as co-catalyst with graphitic carbon (g-C 3 N 4) nanosheets photocatalyst. The g-C 3 N 4 :ITO nanocomposites show ~6 times higher PEC activity compared to g-C 3 N 4 nanosheets. This enhancement is because of ITO nanocrystal co-catalyst facilitating the charge separation and charge transfer processes on the nanocomposite. Image 1 • Solar light driven photoelectrochemical (PEC) reduction of water to H 2. • ITO nanocrystals is introduced as co-catalyst with g-C 3 N 4 nanosheets photocatalyst. • g-C 3 N 4 :ITO nanocomposites show superior PEC activity compared to g-C 3 N 4 nanosheets. • ITO co-catalyst facilitates charge separation and charge transfer processes. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) chiral hybrid perovskites A 2 PbI 4 (A: chiral organic ion) enable chirality controlled optoelectronic and spin-based properties. A + organic sublattice induces chirality into the semiconducting [PbI 4 ] 2- inorganic sublattice through non-covalent interactions at organic-inorganic interface. Often, the A + cations in the lattice have different orientations, leading to asymmetry in the non-covalent interactions. In a novel approach, we use different conformers of A + cations to create asymmetry in the non-covalent interactions, thereby, achieving chiral perovskites with rare helical enantiomorphic structures. We prepared (R-IdPA) 2 PbI 4 and (S-IdPA) 2 PbI 4 (IdPA: 1-iodopropan-2-ammonium) which crystallize in the helical enantiomorphic space groups P4 3 2 1 2 and P4 1 2 1 2, respectively. The gauche- and anti-conformers of IdPA + are arranged alternatively in the hybrid structure. Importantly, the anti-conformer of IdPA + ion have significantly stronger electrostatic, N-H⋅⋅⋅I hydrogen bonding, and I⋅⋅⋅I halogen bonding interactions with the [PbI 4 ] 2- sublattice, compared to the gauche-conformer. This periodic asymmetry in non-covalent interactions caused by the alternative arrangement of gauche- and anti-conformers induces chirality in the inorganic sublattice with four-fold screw axes (4 3 and 4 1 ). The enantiomers (R-/S-IdPA) 2 PbI 4 show mirror-image like circular dichroism from excitonic absorption of the inorganic sublattice. This conformer-based design of chiral hybrid perovskites in helical space groups broadens material choices for advanced optoelectronic applications., (© 2024 Wiley-VCH GmbH.)

Typical layered 2D A 2 PbX 4 (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 A 2 PbX 4 as a mixture of ions (like ionic liquids), and hypothesized that the increase in the structural entropy of fusion (ΔS fus ) will be the driving force to decrease their melting temperature. Then to increase structural ΔS fus , 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 ─NH 3 + head group is suppressed by replacing ─H with ─CH 3 , further enhancing the rigidity. Six A 2 PbX 4 crystals with high ΔS fus and low melting temperatures are prepared using this approach. For example, [I-(CH 2 ) 3 -NH 2 (CH 3 )] 2 PbI 4 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., (© 2024 Wiley‐VCH GmbH.)

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.

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., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

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

Hybrid lead halide perovskites and their derivatives are important optoelectronic materials but suffer from water instability. Combining both the optoelectronics and the water stability of such systems is a major challenge in material design today. To address this issue, we employ the well-known π-conjugation and cation-π interaction concepts in designing a hybrid lead halide perovskite derivative system. (4,4'-VDP)Pb 2 Br 6 (VDP = vinylenedipyridinium) single crystals are prepared. They have a one-dimensional (1D) arrangement of inorganic Pb-Br sublattices connected via the 4,4'-VDP organic sublattice. The π-conjugation in the 4,4'-VDP sublattice allows electronic communication between the 1D Pb-Br units, reducing the band gap and improving the photoconductivity. Importantly, N + of one 4,4'-VDP molecular ion interacts with the π-electron cloud of the adjacent one. This intermolecular cation-π interaction extends throughout the organic sublattice, making the hybrid crystal stable when stored under water for more than a year without requiring any encapsulations.

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-(CH 2 ) 3 -NH 2 (CH 3 ) + (MIPA), -CH 3 is substituted by -CH 2 I at one end, and -NH 3 + is substituted by -NH 2 (CH 3 ) + at the other end. These substitutions of two -H atoms by -I and -CH 3 reduce the rotational symmetry of MIPA at both ends, compared to an unsubstituted cation, for example, H 3 C-(CH 2 ) 3 -NH 3 + . Consequently, the dissimilar hydrogen-iodine and iodine-iodine interactions at the organic-inorganic interface of (MIPA) 2 PbI 4 2D perovskites break the local inversion symmetries of Pb-I octahedra. Owing to this non-centrosymmetry, (MIPA) 2 PbI 4 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 m 2 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 (H 3 N-(CH 2 ) 3 -NH(CH 3 ) 2 )BiI 5 shows a second harmonic generation susceptibility of 7.3 pm V -1 and a high anomalous photovoltaic open-circuit voltage of 22.6 V.

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.

Cs 2 NaInCl 6 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 Cs 2 NaInCl 6 by codoping Sb 3+ (s-electron doping) and Er 3+ (f-electron doping) ions. Sb 3+ doping introduces optically allowed 5s 2 → 5s 1 5p 1 electronic absorption at the sub-band gap level, which then emits blue photoluminescence with ≈93 % quantum yield. But f-f electronic absorption of Er 3+ is parity forbidden. Codoping Sb 3+ -Er 3+ , leads to transfer of excitation energy from Sb 3+ to Er 3+ , 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., (© 2022 Wiley-VCH GmbH.)

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) 2 PbI 4 (MBA: methylbenzylammonium) at cryogenic temperatures. The induced chirality splits the excitonic emissions below 150 K. Additionally, ( R - and S -α-MBA) 2 PbI 4 show wavelength-tunable second harmonic generation (SHG) that depends strongly on the polarization angle of the incident light.

Sb 3+ - and Bi 3+ -doped Cs 2 SnCl 6 zero-dimensional perovskites are emerging as stable and nontoxic phosphors for light emitting diodes. The outermost s-electrons (ns 2 ) of the dopants are responsible for both light absorption (ns 2 to ns 1 np 1 ) and emission (ns 1 np 1 to ns 2 ). At cryogenic temperatures, the Sb 3+ dopant shows two emission peaks, but Bi 3+ shows only one emission peak. Why? Here we address such questions, revealing the origin of luminescence in Sb 3+ - and Bi 3+ -doped Cs 2 SnCl 6 . We find that the emitting excited state ns 1 np 1 is a triplet state 3 T 1u * . The notation "*" implies spin-orbit coupling between the 3 T 1u and 1 T 1u states. After light absorption, 3 T 1u * is occupied with one electron, which then undergoes Jahn-Teller distortion yielding a relaxed excited state (RES). For the Sb 3+ dopant, the combination of Jahn-Teller distortion and spin-orbit coupling gives rise to two minima in RES 3 T 1u * , resulting in two emission peaks, whereas for the Bi 3+ dopant, the spin-orbit coupling significantly dominates over the Jahn-Teller splitting yielding a single minimum in RES 3 T 1u * and, therefore, a single emission peak.

Tin halide perovskites are potential alternatives of lead halide perovskites. However, the easy oxidation of Sn 2+ to Sn 4+ 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) 2 SnI 4 ] using temperature-dependent (6 K to 300 K) photoluminescence (PL). HFB intercalation increases the bandgap. A strong PL quenching is observed in pristine (PEA) 2 SnI 4 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., (© 2021 Wiley-VCH GmbH.)

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)Pb 2 Br 6 ], 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)Pb 2 Br 6 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., (© 2021 Wiley-VCH GmbH.)

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.

Lead-free halide perovskites have drawn wide attention as alternatives to their toxic and poorly stable lead-based counterparts. Among them, double perovskites with Cs 2 AgInCl 6 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 In 3+ . In this Minireview we describe the recent developments on Cs 2 AgInCl 6 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., (© 2020 Wiley-VCH GmbH.)

The theoretical optoelectronic properties of chalcogenide perovskites (e.g., BaZrS3) are as good as those of halide perovskites (e.g., CH3NH3PbI3). But the fabrication of optoelectronic devices is rarely reported, mainly because researchers still do not know how to prepare good quality thin films of chalcogenide perovskites. Here, we report colloidal BaZrS3 nanocrystals (NCs, 40-60 nm) and their solution processed thin film transistors. BaZrS3 NCs are first prepared using a solid-state synthesis route, and the subsequent surface modifications lead to a colloidal dispersion of NCs in both polar N-methyl-2-pyrrolidinone and non-polar chloroform solvents. The NCs exhibit good thermal (15-673 K) and aqueous stability. Colloidal BaZrS3 NCs in chloroform are then used to make field effect transistors showing ambipolar properties with a hole mobility of 0.059 cm2 V-1 s-1 and an electron mobility of 0.017 cm2 V-1 s-1. This report of solution processed chalcogenide perovskite thin films with reasonable carrier mobility and optical absorption and emission is expected to pave the way for future optoelectronic devices of chalcogenide 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.

Lead halide perovskites are seriously considered for next generation photovoltaic technology. They have a unique combination of easy synthesis, high efficiency, and cost-effective techniques. Still, the major concern is the toxicity of lead used in perovskite devices. The research community is still debating whether the amount of lead used in a solar cell really poses a danger or not. However, it is pretty clear that mitigating the lead leakage from the lead halide perovskite device is of utmost importance. In this review, we discuss new material chemistry approaches that can be applied to reduce the lead leakage/wastage from damaged lead halide perovskite solar cells. ECR (encapsulate, capture, and recycle) approaches have the potential to significantly reduce the environmental and health hazard risks of lead halide perovskite devices. Encapsulation by a self-healing material and rigid glass can help the perovskite survive the extreme conditions and avoid exposure of the perovskite layer to the external environment. Capturing of lead can also be done by an encapsulant layer that can very quickly and efficiently bind to lead, in the case that it leaks from the damaged perovskite device. Moreover, the recycling of damaged or decommissioned devices helps to avoid the lead wastage and contamination in the environment. Finally, we also discuss the potential of lead-free perovskite for optoelectronic applications., Competing Interests: The authors declare no competing financial interest., (© 2020 American Chemical Society.)

In layered hybrid perovskites, such as (BA) 2 PbI 4 (BA=C 4 H 9 NH 3 ), 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., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Bi 3+ 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 Bi 3+ and lanthanide ions. Codoping of Bi 3+ and Ln 3+ (Ln=Er and Yb) in Cs 2 AgInCl 6 double perovskite is presented. Bi 3+ -Er 3+ codoped Cs 2 AgInCl 6 shows Er 3+ f-electron emission at 1540 nm (suitable for low-loss optical communication). Bi 3+ codoping decreases the excitation (absorption) energy, such that the samples can be excited with ca. 370 nm light. At that excitation, Bi 3+ -Er 3+ codoped Cs 2 AgInCl 6 shows ca. 45 times higher emission intensity compared to the Er 3+ doped Cs 2 AgInCl 6 . Similar results are also observed in Bi 3+ -Yb 3+ 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., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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, Cs 2 AgSbCl 6 , in which two Pb 2+ in CsPbCl 3 (or Cs 2 Pb 2 Cl 6 ) are replaced with one Ag + and one Sb 3+ , 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 Cs 2 AgSb 1-x Bi x Cl 6 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.

Long term stability of the black perovskite phase of CsPbI3 nanocrystals under ambient conditions is an important challenge for their optoelectronic applications in real life. The nanocrystalline size is found to improve the stability of the black phase at room temperature. Furthermore, doping Mn is proposed to improve the stability of the black perovskite phase of CsPbI3 nanocrystals (NCs). However, the undoped and Mn-doped CsPbI3 NCs are prepared in different batches under somewhat different synthesis conditions thus obliterating the role of Mn in the stability of the black phase of CsPbI3 NCs. Here, we elucidate the effect of Mn doping on the surface and lattice energy of CsPbI3 NCs, stabilizing the black phase. For this purpose, we employ a postsynthesis doping strategy which has an advantage that the initial host remains the same for both undoped and Mn-doped samples. Uncertainties in the size/shape, surface energy, and structure through direct synthesis of undoped and Mn-doped NCs in different batches can be neglected in our postsynthesis doping strategy, allowing us to study the effect of dopants in a more controlled manner. Our postsynthesis Mn-doping in CsPbI3 NCs shows that the black phase stability under ambient conditions improves from few days for the undoped sample to nearly a month's time for the Mn-doped sample. We found that though surface passivation with a dopant precursor improves both colloidal and phase stability of black CsPbI3 NCs, it is the contraction of the lattice upon Mn-doping that mainly stabilizes the films of black phase CsPbI3 NCs. Similarly, we found that Mn-doped CsPbBr3 NCs show improved ambient stability of photoluminescence compared to the undoped sample.

Double perovskites (DPs) with a generic formula A 2 M'(I)M III X 6 (A and M are metal ions, and X=Cl, Br, I) are now being explored as potential alternatives to Pb-halide perovskites for solar cells and other optoelectronic applications. However, these DPs typically suffer from wide (≈3 eV) and/or indirect band gaps. In 2017, a new structural variety, namely layered halide DP Cs 4 CuSb 2 Cl 12 (CCSC) with bivalent Cu II ion in the place of M'(I) was reported, which exhibit a band gap of approximately 1 eV. Here, we report a mechanochemical synthesis of CCSC, its thermal and chemical stability, and magnetic response of Cu II d 9 electrons controlling the optoelectronic properties. A simple grinding of precursor salts at ambient conditions provides a stable and scalable product. CCSC is stable in water/acetone solvent mixtures (≈30 % water) and many other polar solvents unlike Pb-halide perovskites. It decomposes to Cs 3 Sb 2 Cl 9 , Cs 2 CuCl 4 , and SbCl 3 at 210 °C, but the reaction can be reversed back to produce CCSC at lower temperatures and high humidity. A long-range magnetic ordering is observed in CCSC even at room temperature. The role of such magnetic ordering in controlling the dispersion of the conduction band, and therefore, controlling the electronic and optoelectronic properties of CCSC has been discussed., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Herein we report the colloidal synthesis of Cs 3 Sb 2 I 9 and Rb 3 Sb 2 I 9 perovskite nanocrystals, and explore their potential for optoelectronic applications. Different morphologies, such as nanoplatelets and nanorods of Cs 3 Sb 2 I 9 , and spherical Rb 3 Sb 2 I 9 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 Cs 3 Sb 2 I 9 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  cm 2 was estimated. The results suggest that colloidal Cs 3 Sb 2 I 9 and Rb 3 Sb 2 I 9 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., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Optoelectronic properties of CsPbBr 3 perovskite nanocubes (NCs) depend strongly on the interaction of the organic passivating molecules with the inorganic crystal. To understand this interaction, we employed a combination of synchrotron-based X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR) spectroscopy, and first-principles density functional theory (DFT)-based calculations. Variable energy XPS elucidated the internal structure of the inorganic part in a layer-by-layer fashion, whereas NMR characterized the organic ligands. Our experimental results confirm that oleylammonium ions act as capping ligands by substituting Cs + ions from the surface of CsPbBr 3 NCs. DFT calculations shows that the substitution mechanism does not require much energy for surface reconstruction and, in contrast, stabilizes the nanocrystal by the formation of three hydrogen bonds between the -NH 3 + moiety of oleylammonium and surrounding Br - on the surface of NCs. This substitution mechanism and its origin are in stark contrast to the usual adsorption of organic ligands on the surface of typical NCs.

Colloidal all inorganic CsPbX 3 (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 CsPbBr 3 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 CsPbX 3 NCs with electron and hole transport layers would harvest hot carriers, increasing the photovoltaic and photocatalytic efficiencies.

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., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Green photoluminescence (PL) from CsPbBr3 nanocubes (∼11 nm edge-length) exhibits a high quantum yield (>80%), narrow spectral width (∼85 meV), and high reproducibility, along with a high molar extinction coefficient (3.5 × 10(6) M(-1) cm(-1)) for lowest energy excitonic absorption. In order to obtain these combinations of excellent properties for blue (PL peak maximum, λ max < 500 nm) emitting samples, CsPbBr3 nanocubes and nanoplatelets with various dimensions were prepared. Systematic increases in both the optical gap and transition probability for radiative excitonic recombination (PL lifetime 3-7 ns), have been achieved with the decreasing size of nanocubes. A high quantum yield (>80%) was also maintained, but the spectral width increased and became asymmetric for blue emitting CsPbBr3 nanocubes. Furthermore, PL was unstable and irreproducible for samples with λ max ∼ 460 nm, exhibiting multiple features in the PL. These problems arise because smaller (<7 nm) CsPbBr3 nanocubes have a tendency to form nanoplatelets and nanorods, eventually yielding inhomogeneity in the shape and size of blue-emitting nanocrystals. Reaction conditions were then modified achieving nanoplatelets, with strong quantum confinement along the thickness of the platelets, yielding blue emission. But inhomogeneity in the thickness of the nanoplatelets again broadens the PL compared to green-emitting CsPbBr3 nanocubes. Therefore, unlike high quality green emitting CsPbBr3 nanocubes, blue emitting CsPbBr3 nanocrystals of any shape need to be improved further.

Sn(4+) -doped In2 O3 (ITO) is a benchmark transparent conducting oxide material. We prepared ligand-free but colloidal ITO (8 nm, 10 % Sn(4+) ) nanocrystals (NCs) by using a post-synthesis surface-modification reaction. (CH3 )3 OBF4 removes the native oleylamine ligand from NC surfaces to give ligand-free, positively charged NCs that form a colloidal dispersion in polar solvents. Both oleylamine-capped and ligand-free ITO NCs exhibit intense absorption peaks, due to localized surface plasmon resonance (LSPR) at around λ=1950 nm. Compared with oleylamine-capped NCs, the electrical resistivity of ligand-free ITO NCs is lower by an order of magnitude (≈35 mΩ cm(-1) ). Resistivity over a wide range of temperatures can be consistently described as a composite of metallic ITO grains embedded in an insulating matrix by using a simple equivalent circuit, which provides an insight into the conduction mechanism in these systems., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Traditional CdSe-based colloidal quantum dots (cQDs) have interesting photoluminescence (PL) properties. Herein we highlight the advantages in both ensemble and single-nanocrystal PL of colloidal CsPbBr3 nanocrystals (NCs) over the traditional cQDs. An ensemble of colloidal CsPbBr3 NCs (11 nm) exhibits ca. 90 % PL quantum yield with narrow (FWHM=86 meV) spectral width. Interestingly, the spectral width of a single-NC and an ensemble are almost identical, ruling out the problem of size-distribution in PL broadening. Eliminating this problem leads to a negligible influence of self-absorption and Förster resonance energy transfer, along with batch-to-batch reproducibility of NCs exhibiting PL peaks within ±1 nm. Also, PL peak positions do not alter with measurement temperature in the range of 25 to 100 °C. Importantly, CsPbBr3 NCs exhibit suppressed PL blinking with ca. 90 % of the individual NCs remain mostly emissive (on-time >85 %), without much influence of excitation power., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Donor-Acceptor transition was previously suggested as a mechanism for luminescence in (ZnS)1-x(AgInS2)x nanocrystals. Here we show the participation of delocalized valence/conduction band in the luminescence. Two emission pathways are observed: Path-1 involves transition between a delocalized state and a localized state exhibiting higher energy and shorter lifetime (∼25 ns) and Path-2 (donor-acceptor) involves two localized defect states exhibiting lower emission energy and longer lifetime (>185 ns). Surprisingly, Path-1 dominates (82% for x = 0.33) for nanocrystals with lower x, in sharp difference with prior assignment. Luminescence peak blue shifts systematically by 0.57 eV with decreasing x because of this large contribution from Path-1. X-ray absorption fine structure (XAFS) study of (ZnS)1-x(AgInS2)x nanocrystals shows larger AgS4 tetrahedra compared with InS4 tetrahedra with Ag-S and In-S bond lengths 2.52 and 2.45 Å respectively, whereas Zn-S bond length is 2.33 Å along with the absence of second nearest-neighbor Zn-S-metal correlation.

Extensively studied Mn-doped semiconductor nanocrystals have invariably exhibited photoluminescence over a narrow energy window of width ≤150  meV in the orange-red region and a surprisingly large spectral width (≥180  meV), contrary to its presumed atomic-like origin. Carrying out emission measurements on individual single nanocrystals and supported by ab initio calculations, we show that Mn PL emission, in fact, can (i) vary over a much wider range (∼370  meV) covering the deep green--deep red region and (ii) exhibit widths substantially lower (∼60-75  meV) than reported so far, opening newer application possibilities and requiring a fundamental shift in our perception of the emission from Mn-doped semiconductor nanocrystals.

A new analogue of graphene containing boron, carbon and nitrogen (BCN) has been obtained by the reaction of high-surface-area activated charcoal with a mixture of boric acid and urea at 900 degrees C. X-ray photoelectron spectroscopy and electron energy-loss spectroscopy reveal the composition to be close to BCN. The X-ray diffraction pattern, high-resolution electron microscopy images and Raman spectrum indicate the presence of graphite-type layers with low sheet-to-sheet registry. Atomic force microscopy reveals the sample to consist of two to three layers of BCN, as in a few-layer graphene. BCN exhibits more electrical resistivity than graphene, but weaker magnetic features. BCN exhibits a surface area of 2911 m(2) g(-1), which is the highest value known for a B(x)C(y)N(z) composition. It exhibits high propensity for adsorbing CO(2) ( approximately 100 wt %) at 195 K and a hydrogen uptake of 2.6 wt % at 77 K. A first-principles pseudopotential-based DFT study shows the stable structure to consist of BN(3) and NB(3) motifs. The calculations also suggest the strongest CO(2) adsorption to occur with a binding energy of 3.7 kJ mol(-1) compared with 2.0 kJ mol(-1) on graphene.

It has been an outstanding problem that a semiconducting host in the bulk form can be doped to a large extent, while the same host in the nanocrystal form is found to resist any appreciable level of doping rather stubbornly, this problem being more acute in the wurtzite form compared to the zinc blende one. In contrast, our results based on the lattice parameter tuning in a Zn(x)Cd(1-x)S alloy nanocrystal system achieves approximately 7.5% Mn(2+) doping in a wurtzite nanocrystal, such a concentration being substantially higher compared to earlier reports even for nanocrystal hosts with the "favorable" zinc-blende structure. These results prove a consequence of local strains due to a size mismatch between the dopant and the host that can be avoided by optimizing the composition of the alloyed host. Additionally, the present approach opens up a new route to dope such nanocrystals to a macroscopic extent as required for many applications. Photophysical studies show that the quantum efficiency per Mn(2+) ion decreases exponentially with the average number of Mn(2+) ions per nanocrystal; en route, a high quantum efficiency of approximately 25% is achieved for a range of compositions.