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Lead‐free metal halide double perovskites have emerged as promising scintillators owing to their superior optoelectronic properties, low‐cost and solution processability. However, it is still challenging to develop high‐performance flexible X‐ray scintillators based on the B(I)‐site alkali metal modulation in lead‐free double perovskite nanocrystals (NCs). Herein, a series of novel Cs2B(I)GdCl6 (B: Li, Na, K) of double perovskite A2B(I)M(III)X6 NCs structures are successfully synthesized, the optical and scintillator properties of which are significantly sensitive to the B(I)‐site alkali metals with the doping of Sb3+. They showed efficient self‐trapped exciton (STE) emission with high photoluminescence quantum yield (PLQY) and exhibited the highest X‐ray detection limit (86 nGyair s−1) and excellent spatial resolutions (>15 lp mm−1). The femtosecond transient absorption measurement and theoretical analysis further revealed that the B(I)‐site alkali metal fundamentally balanced the exciton–phonon coupling with appropriate STEs formation energy barrier and the electron localization and thus improved the optical and scintillator properties. This B(I)‐site alkali metal engineering offers a strategy to develop bright luminescent double perovskite NCs for X‐ray detection and imaging devices. [ABSTRACT FROM AUTHOR]

Vacancy‐ordered perovskite nanocrystals (NCs) have recently attracted much interest owing to their unique structure and optical properties; however, regulating self‐trapped excitons (STEs) in vacancy‐ordered perovskite NCs to exhibit tunable multicolor emission is still challenging because of the highly localized charge distribution and strong carrier‐phonon coupling. Herein, the first colloidal synthesis of lead‐free vacancy‐ordered Cs2HfCl6 perovskite NCs by a hot‐injection method using hafnium acetylacetonate as metal precursors is reported. The passivation strategy of sub‐bandgap trap states inside Cs2HfCl6 NCs is proposed by doping Sb3+. X‐ray photoelectron spectroscopy (XPS) measurements verify that the passivation of sub‐bandgap trap states may be attributed to the enhanced chemical interaction between the Hf4+ cations and Cl– anions. Simultaneously, the introduction of Sb3+ ions yields a bright orange emission. Employing lanthanide acetylacetonate compounds, four lanthanide ions including Pr3+, Eu3+, Tb3+, and Ho3+ are doped into the Cs2HfCl6 NCs, which achieves tunable multicolor emissions (from blue to green to pink) through the energy transfer from STEs to lanthanide ions. This investigation not only develops a novel vacancy‐ordered perovskite NCs, but also provides effective strategies for tuning the optical properties of lead‐free vacancy‐ordered perovskite NCs. [ABSTRACT FROM AUTHOR]

Lead‐free halide perovskites and their variants have attracted great attention due to their non‐toxicity and good photophysical properties. Here, a series of undoped and antimony‐doped all‐inorganic, 0D, indium‐based perovskite variant nanocrystals (NCs): (RbxCs1−x)3InCl6 and (RbxCs1−x)3InCl6:Sb (0 ≤ x ≤ 1) NCs, is reported. Among them, Sb‐doped NCs exhibit green emission. Mixing Rb‐Cs NCs show the higher photoluminescence (PL) quantum efficiency (PLQE) and better stability than unalloyed NCs (x = 0 or 1). The effect of crystal size on PL property of (Rb0.75Cs0.25)3InCl6:Sb is analyzed in detail. More interestingly, In‐based perovskite variant NCs containing coordinated water are successfully synthesized by a two‐step method (variable temperature hot injection and saturated steam post treatment). The best PLQE of Sb‐doped Cs2InCl5(H2O) NCs reaches 95.5%, which is a new PLQE record for yellow emissive lead‐free perovskite and variant NCs. In addition, the emission mechanism is further clarified by temperature‐dependent PL and femtosecond transient absorption techniques. [ABSTRACT FROM AUTHOR]

Lead‐free halide perovskite nanocrystals (NCs) with the unique optical properties are of interest for broad applications. There are a few studies on near‐infrared (NIR) emission in perovskite NCs. Here, all‐inorganic rare‐earth‐based Cs2NaEr1−xBxCl6 (B: In, Sb, or Bi; x = 0, 0.13, and 0.5) double perovskite (DP) NCs have been successfully sythesized. The Cs2NaErCl6 NCs show good air stability and emit a sharp NIR photoluminescence (PL) at telecommunication wavelength of 1543 nm, originating from 4I13/2→4I15/2 transition of the Er3+, and its average lifetime is 35.7 µs. Especially for Cs2NaEr0.5Sb0.5Cl6 NCs, NIR PL can be enhanced 23‐fold with the lifetime being 119.1 µs. Moreover, femtosecond transient absorption measurements prove that a long‐lifetime trap state promotes NIR emitting in the mixed Sb/Er DP NCs. [ABSTRACT FROM AUTHOR]

For display applications, it is highly desirable to obtain tunable red/green/blue emission. However, lead‐free perovskite nanocrystals (NCs) generally exhibit broadband emission with poor color purity. Herein, we developed a unique phase transition strategy to engineer the emission color of lead‐free cesium manganese bromides NCs and we can achieve a tunable red/green/blue emission with high color purity in these NCs. Such phase transition can be triggered by isopropanol: from one dimensional (1D) CsMnBr3 NCs (red‐color emission) to zero dimensional (0D) Cs3MnBr5 NCs (green‐color emission). Furthermore, in a humid environment both 1D CsMnBr3 NCs and 0D Cs3MnBr5 NCs can be transformed into 0D Cs2MnBr4⋅2 H2O NCs (blue‐color emission). Cs2MnBr4⋅2 H2O NCs could inversely transform into the mixture of CsMnBr3 and Cs3MnBr5 phase during the thermal annealing dehydration step. Our work highlights the tunable optical properties in single component NCs via phase engineering and provides a new avenue for future endeavors in light‐emitting devices. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) is generally observed in solid‐state organic molecules or metal‐organic complexes. However, TADF in all‐inorganic colloidal nanocrystals (NCs) is rare. Herein, we report the first colloidal synthesis of an air‐stable all‐inorganic lead‐free Cs2ZrCl6 perovskite NCs. The Cs2ZrCl6 NCs exhibit long‐lived triplet excited state (138.2 μs), and feature high photoluminescence (PL) quantum efficiency (QY=60.37 %) due to TADF mechanism. The emission color can be easily tuned from blue to green by synthesizing the mixed‐halide Cs2ZrBrxCl6−x (0≤x≤1.5) NCs. Femtosecond transient absorption and temperature dependent PL measurements are performed to clarify the emission mechanism. In addition, Bi3+ ions are successfully doped into Cs2ZrCl6 NCs, which further extends the PL properties. This work not only develops a new lead‐free halide perovskite NCs for potential optoelectronic applications, but also offers unique strategies for developing new inorganic phosphors. [ABSTRACT FROM AUTHOR]

Lead‐free halide perovskite nanocrystals (NCs) have drawn wide attention for solving the problem of lead perovskites toxicity and instability. Herein, we synthesize the direct band gap double perovskites undoped and Ag‐doped Cs2NaInCl6 NCs by variable temperature hot injection. The Cs2NaInCl6 NCs have little photoluminescence because of dark self‐trapped excitons (STEs). The dark STEs can be converted into bright STEs by doping with Ag+ to produce a bright yellow emission, with the highest photoluminescence quantum efficiency of 31.1 %. The dark STEs has been directly detected experimentally by ultrafast transient absorption (TA) techniques. The dynamics mechanism is further studied. In addition, the Ag‐doped NCs show better stability than the undoped ones. This result provides a new way to enhance the optical properties of lead‐free perovskites NCs for high‐performance light emitters. [ABSTRACT FROM AUTHOR]

Low‐dimensional metal halides have recently attracted extensive attention owing to their unique structure and photoelectric properties. Herein, we report the colloidal synthesis of all‐inorganic low‐dimensional cesium copper halide nanocrystals (NCs) by adopting a hot‐injection approach. Using the same reactants and ligands, but different reaction temperatures, both 1D CsCu2I3 nanorods and 0D Cs3Cu2I5 NCs can be prepared. Density functional theory indicates that the reduced dimensionality in 1D CsCu2I3 compared to 0D Cs3Cu2I5 makes the excitons more localized, which accounts for the strong emission of 0D Cs3Cu2I5 NCs. Subsequent optical characterization reveals that the highly luminescent, strongly Stokes‐shifted broadband emission of 0D Cs3Cu2I5 NCs arises from the self‐trapped excitons. Our findings not only present a method to control the synthesis of low‐dimensional cesium copper halide nanocrystals but also highlight the potential of 0D Cs3Cu2I5 NCs in optoelectronics. [ABSTRACT FROM AUTHOR]

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Abstract: Lead‐free perovskite nanocrystals (NCs) were obtained mainly by substituting a Pb2+ cation with a divalent cation or substituting three Pb2+ cations with two trivalent cations. The substitution of two Pb2+ cations with one monovalent Ag+ and one trivalent Bi3+ cations was used to synthesize Cs2AgBiX6 (X=Cl, Br, I) double perovskite NCs. Using femtosecond transient absorption spectroscopy, the charge carrier relaxation mechanism was elucidated in the double perovskite NCs. The Cs2AgBiBr6 NCs exhibit ultrafast hot‐carrier cooling (<1 ps), which competes with the carrier trapping processes (mainly originate from the surface defects). Notably, the photoluminescence can be increased by 100 times with surfactant (oleic acid) added to passivate the defects in Cs2AgBiCl6 NCs. These results suggest that the double perovskite NCs could be potential materials for optoelectronic applications by better controlling the surface defects. [ABSTRACT FROM AUTHOR]

Lead-based perovskite nanocrystals (NCs) have outstanding optical properties and cheap synthesis conferring them a tremendous potential in the field of optoelectronic devices. However, two critical problems are still unresolved and hindering their commercial applications: one is the fact of being lead-based and the other is the poor stability. Lead-free all-inorganic perovskite Cs3Bi2X9 (X=Cl, Br, I) NCs are synthesized with emission wavelength ranging from 400 to 560 nm synthesized by a facile room temperature reaction. The ligand-free Cs3Bi2Br9 NCs exhibit blue emission with photoluminescence quantum efficiency (PLQE) about 0.2 %. The PLQE can be increased to 4.5 % when extra surfactant (oleic acid) is added during the synthesis processes. This improvement stems from passivation of the fast trapping process (2-20 ps). Notably, the trap states can also be passivated under humid conditions, and the NCs exhibited high stability towards air exposure exceeding 30 days. [ABSTRACT FROM AUTHOR]

Intrinsic broadband photoluminescence (PL) of self-trapped excitons (STEs) are systematically studied in lead-free double perovskite nanocrystals (NCs). It is clarified that bandgap (direct/indirect) has important influence on the PL properties of STEs: indirect bandgap NCs exhibit strong exciton-phonon coupling which results in non-radiative STEs, while direct bandgap NCs exhibit moderate exciton-phonon coupling, inducing bright STE PL. Furthermore, by alloying K+ and Li+ ions in Cs 2 AgInCl 6 NCs, the NCs exhibit broadband white-light emission. Charge-carrier dynamics study indicates that the efficient white-light emission originates from the further suppressed non-radiative processes of the STEs in the direct bandgap structure. This work may deepen the understanding of STEs and guide the design of high-performance lead-free perovskites. [ABSTRACT FROM AUTHOR]