Colloidal Synthesis of Double Perovskite Cs2AgInCl6 and Mn-Doped Cs2AgInCl6 Nanocrystals

Homo- and Heterovalent Doping-Mediated Self-Trapped Exciton Emission and Energy Transfer in Mn-Doped Cs2Na1-xAgxBiCl6 Double Perovskites

Double perovskites exhibit low toxicity, intrinsic thermodynamic stability, and small carrier effective mass. Herein, a novel doping route was adopted to incorporate Mn ions into Cs₂Na_1-xAg_xBiCl₆ double perovskites for engineering the band gap and tailoring the energy transfer. The as-prepared Cs₂Na_1-xAg_xBiCl₆ (0 < x < 1) exhibited excellent photoluminescence and a broad self-trapped exciton (STE) band from 500 to 900 nm, which exhibited an abnormal emission peak blue shift with increasing temperature. For Mn-doped Cs₂Na_1-xAg_xBiCl₆, the two photoluminescence (PL) bands from d-d transition emission of Mn ions and STEs were always observed simultaneously in the PL window. The distinct energy-transfer channel from the Mn²⁺ ion guest to the double-perovskite host resulted in the dominant Mn²⁺ emission. Our results will be helpful for further understanding the nature of the photophysics of double perovskites.

Doping and ion substitution in colloidal metal halide perovskite nanocrystals

The past decade has witnessed tremendous advances in synthesis of metal halide perovskites and their use for a rich variety of optoelectronics applications. Metal halide perovskite has the general formula ABX3, where A is a monovalent cation (which can be either organic (e.g., CH3NH3+ (MA), CH(NH2)2+ (FA)) or inorganic (e.g., Cs+)), B is a divalent metal cation (usually Pb2+), and X is a halogen anion (Cl-, Br-, I-). Particularly, the photoluminescence (PL) properties of metal halide perovskites have garnered much attention due to the recent rapid development of perovskite nanocrystals. The introduction of capping ligands enables the synthesis of colloidal perovskite nanocrystals which offer new insight into dimension-dependent physical properties compared to their bulk counterparts. It is notable that doping and ion substitution represent effective strategies for tailoring the optoelectronic properties (e.g., absorption band gap, PL emission, and quantum yield (QY)) and stabilities of perovskite nanocrystals. The doping and ion substitution processes can be performed during or after the synthesis of colloidal nanocrystals by incorporating new A', B', or X' site ions into the A, B, or X sites of ABX3 perovskites. Interestingly, both isovalent and heterovalent doping and ion substitution can be conducted on colloidal perovskite nanocrystals. In this review, the general background of perovskite nanocrystals synthesis is first introduced. The effects of A-site, B-site, and X-site ionic doping and substitution on the optoelectronic properties and stabilities of colloidal metal halide perovskite nanocrystals are then detailed. Finally, possible applications and future research directions of doped and ion-substituted colloidal perovskite nanocrystals are also discussed.

Investigation of the Mn dopant-enhanced photoluminescence performance of lead-free Cs2AgInCl6 double perovskite crystals

The lead-free double perovskite Cs2AgInCl6 is a potential candidate for LEDs, the photoluminescence performance of which is reinforced greatly by Mn doping. Here, we analyzed the geometric, electronic and photoluminescence properties of Mn-doped Cs2AgInCl6 by means of first-principle calculations. We found that in the interior of Cs2AgInCl6, the Mn dopant formed defect complexes by substituting an Ag atom and generating an Ag vacancy (MnAgVAg) owing to the charge balance and the weak distortion of the metal octahedra. The MnAgVAg defect introduced two defect bands in the forbidden gap, which was contributed predominantly by the 3d orbitals of the Mn2+ ions. The electron transition of the Mn2+ ions from the first excited state to the ground state, i.e., from 4T1 to 6A1 states, gives rise to the PL spectrum that is lower than the bandgap. Therefore, we show that the Mn dopant indeed reinforces the PL performance of Cs2AgInCl6 greatly and is beneficial for its use as an LED material.

Lead-Free Halide Perovskite Nanocrystals: Crystal Structures, Synthesis, Stabilities, and Optical Properties

In recent years, there have been rapid advances in the synthesis of lead halide perovskite nanocrystals (NCs) for use in solar cells, light emitting diodes, lasers, and photodetectors. These compounds have a set of intriguing optical, excitonic, and charge transport properties, including outstanding photoluminescence quantum yield (PLQY) and tunable optical band gap. However, the necessary inclusion of lead, a toxic element, raises a critical concern for future commercial development. To address the toxicity issue, intense recent research effort has been devoted to developing lead-free halide perovskite (LFHP) NCs. In this Review, we present a comprehensive overview of currently explored LFHP NCs with an emphasis on their crystal structures, synthesis, optical properties, and environmental stabilities (e.g., UV, heat, and moisture resistance). In addition, strategies for enhancing optical properties and stabilities of LFHP NCs as well as the state-of-the-art applications are discussed. With the perspective of their properties and current challenges, we provide an outlook for future directions in this rapidly evolving field to achieve high-quality LFHP NCs for a broader range of fundamental research and practical applications.

Sb3+ Doping-Induced Triplet Self-Trapped Excitons Emission in Lead-Free Cs2SnCl6 Nanocrystals

Doped halide perovskite nanocrystals (NCs) have opened new opportunities for the emerging optical and optoelectronic applications. Here, we describe a hot-injection synthesis of all-inorganic lead-free Cs₂SnCl₆ and Sb³⁺ doped Cs₂SnCl₆ NCs. Cs₂SnCl₆ NCs present a blue emission peak at 438 nm, whereas a new broad-band emission peak appears at 615 nm for the Sb³⁺ doped NCs. Comparative structural and spectral characterizations of Sb³⁺ doped Cs₂SnCl₆ NCs with micrometer-sized undoped and Sb³⁺ doped crystals show that the formation of broad-band orange emission is originted from triplet self-trapped excitons, attributed to the ³P_n-¹S₀ transitions (n = 0, 1, 2). Our results in Sb³⁺ doped Cs₂SnCl₆ materials provide insights into the machanisms of doping-induced emission centers, and it extends the existing knowledge of optical properties of doped halide NCs for further studies.

Charge-Carrier Dynamics of Lead-Free Halide Perovskite Nanocrystals

Lead halide perovskite nanocrystals (NCs) have been widely studied for application in optoelectronic devices due to their excellent optical properties and low-cost synthesis. However, the toxicity of lead and the poor stability of the NCs hindered their practical applications. Sn²⁺-based perovskite with low toxicity was first developed; however, the Sn²⁺-based perovskite NCs are unstable in air and oxidize easily. Recently, air-stable lead-free perovskite NCs have been developed and received increasing attention. Unfortunately, the optical and optoelectronic properties of these lead-free halide perovskite NCs are generally far worse than those of lead-perovskite NCs. Understanding the charge-carrier dynamics of semiconductors is crucial to improve their optical properties. In this Account, we mainly review our recent research progress on the study of charge-carrier dynamics in air-stable lead-free perovskite NCs. The exciton trapping followed by nonradiative recombination was the major carrier relaxation pathway and resulted in a low photoluminescence quantum efficiency (PLQE). A feasible route for passivating surface traps and tuning the self-trapped excitons from "dark" (nonradiative) to "bright" (radiative) was proposed. Through this strategy, the PLQE could be increased over 100-fold. In addition, we have compared several photophysical properties of lead-free perovskite NCs with that of lead perovskite NCs, such as charge-carrier relaxation, exciton-phonon coupling, and hot-carrier cooling. In 2017, we reported the synthesis, optical properties, and charge-carrier dynamics of Cs₃Bi₂X₉ (X: Cl, Br, I) NCs. The Cs₃Bi₂Br₉ NCs exhibited clear exciton trapping processes with time scales in the range of 2-20 ps. The fast trapping processes could be passivated via the use of surfactants (such as oleic acid), and the PLQE increased over 20-fold (from 0.2% to 4.5%). The low PLQE may be due to the reduced dimensionality of Cs₃Bi₂Br₉ (2D) compared with the 3D cubic perovskite structure of CsPbBr₃. We next reported double perovskite Cs₂AgSb_1-yBi_yX₆ (X: Br, Cl; 0 ≤ y ≤ 1) NCs, which exhibited a similar 3D cubic perovskite structure to that of the lead-perovskite NCs. The charge-carrier dynamics indicated that the sub-band-gap exciton trapping processes were dominated by ultrafast (∼1-2 ps) intrinsic self-trapping and trapping at surface defects (∼50-100 ps). While trapping at surface defects can be passivated using surfactants, the self-trapping processes is due to the giant carrier-phonon coupling effect. By designing direct band gap double perovskite NCs to tune the sub-band-gap trapping processes, bright dual-color emission was achieved. Furthermore, the violet PLQE could be improved to 36.6%, which is comparable to that in lead halide perovskite NCs. We hope this Account will deepen the understanding of the charge-carrier dynamics in lead-free perovskite NCs and guide the design of high-performance lead-free perovskites.

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 Cs₂AgSb_1-xBi_xCl₆ 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.

Broadband emission of double perovskite Cs2Na0.4Ag0.6In0.995Bi0.005Cl6:Mn2+ for single-phosphor white-light-emitting diodes

In this Letter, we report the broadband photoluminescence of lead-free double perovskite Cs₂Na_0.4Ag_0.6In_0.95Bi_0.05Cl₆:Mn²⁺. Under ultraviolet excitation, the white phosphor shows two emission peaks at 550 nm and 610 nm from self-trapped exciton and doped Mn²⁺ ions, respectively, leading to a broad emission spectrum over the whole visible spectrum suitable for lighting application. The white-light-emitting diodes exhibit high light quality with CIE coordinates (0.38, 0.42) and color rendering index of 82.6. The mechanism of luminescence of this double perovskite is also discussed in this Letter.

Tunable Color Temperatures and Efficient White Emission from Cs2 Ag1- x Nax In1- y Biy Cl6 Double Perovskite Nanocrystals

Recently, Bi-doped Cs₂ Ag_0.6 Na_0.4 InCl₆ lead-free double perovskites demonstrating efficient warm-white emission have been reported. To enable the solution processing and enrich the application fields of this promising material, here a colloidal synthesis of Cs₂ Ag_{1- x} Na_x In_{1- y} Bi_y Cl₆ nanocrystals is further developed. Different from its bulk states, the emission color temperatures of the nanocrystal can be tuned from 9759.7 to 4429.2 K by Na⁺ and Bi³⁺ incorporation. Furthermore, the newly developed nanocrystals can break the wavefunction symmetry of the self-trapped excitons by partial replacement of Ag⁺ ions with Na⁺ ions and consequently allow radiative recombination. Assisted with Bi³⁺ ions doping and ligand passivation, the photoluminescence quantum yield of the Cs₂ Ag_0.17 Na_0.83 In_0.88 Bi_0.12 Cl₆ nanocrystals is further promoted to 64%, which is the highest value for lead-free perovskite nanocrystals at present. The new colloidal nanocrystals with tunable color temperature and efficient photoluminescence are expected to greatly advance the research progress of lead-free perovskites in single-emitter-based white emitting materials and devices.

Band Gap Engineering in Cs2(NaxAg1-x)BiCl6 Double Perovskite Nanocrystals

Lead-free double perovskite materials, A₂M(I)M'(III)X₆, have recently attracted attention as environment-friendly alternatives to lead-based perovskites, APbX₃, because of both rich fundamental science and potential applications. We report band gap tuning via alloying of Cs₂AgBiCl₆ nanocrystals (NCs) with nontoxic, abundant Na. It results in a series of Cs₂Na_xAg_1-xBiCl₆ (x = 0, 0.25, 0.5, 0.75, and 1) double perovskite NCs, leading to increase in optical band gap from 3.39 eV (x = 0) to 3.82 eV (x = 1) and 30-fold increment in weak photoluminescence. The tuning of band gap has been further explored by electronic structure calculation under the framework of density functional theory (DFT). The latter confirms that the increase in band gap is due to reduction of Ag contribution near valence band maxima (VBM) on incorporation of Na ion in place of Ag. These alloyed double perovskites can have useful potential applications in optoelectronic devices.

Lead-Free Halide Perovskites and Perovskite Variants as Phosphors toward Light-Emitting Applications

Lead halide perovskites have attracted tremendous research interests in the light-emitting field because of their high defect tolerance, solution processability, tunable spectrum, and efficient emission. In terms of luminescence types, both the narrowband emission derived from free-exciton (FE) and broadband white light emission from self-trapped exciton (STE) show great advantages in light-emitting applications. Despite the fascinating characteristics, their commercialization still suffers from the presence of toxic lead (Pb) and unsatisfactory stability. In this spotlight, we mainly focus on the lead-free candidates as phosphors for possible light-emitting applications. Thanks to the chemical diversity of metal halide perovskites and perovskite variants, many excellent lead-free light-emitting materials have recently been synthesized and characterized. We first classify these materials into three types according to material structures, including (1) double perovskites A₂B(I)B(III)X₆, (2) vacancy ordered perovskites A₂B(IV)X₆, (3) miscellaneous perovskite variants or halide semiconductors, which refer to halides without clear relation to the perovskite structure. We then highlight the importance of electronic dimensionality, defect passivation, and impurity doping in developing highly efficient perovskite-based emitters. We also discuss their applications in white light-emitting diodes (W-LED). Further challenges toward practical applications and potential applications are also included in a section on outlook and future challenges.

Improved Doping and Emission Efficiencies of Mn-Doped CsPbCl3 Perovskite Nanocrystals via Nickel Chloride

It is challenging to improve the emission efficiency of Mn-doped CsPbCl₃ (Mn:CsPbCl₃) nanocrystals (NCs) because the excellent optical performances are dependent on high doping efficiency and few defects and traps. Steady-state and time-resolved photoluminescence (PL) spectroscopies were used to investigate the luminescence properties of Mn:CsPbCl₃ NCs with different Mn doping levels synthesized in the presence of nickel chloride. The doping efficiency of Mn ions in Mn:CsPbCl₃ NCs was greatly enhanced in the presence of NiCl₂, and the PL wavelength of Mn²⁺ ions was tuned from 594 to 638 nm by varying the concentration of dopant Mn from 0.11% to 15.25%. The high emission quantum yields of Mn:CsPbCl₃ NCs with orange and red emissions peaked at 600 and 620 nm in hexane were 70% and 39%, respectively. The improvement in doping and emission efficiencies of Mn²⁺ was attributed to the enhanced formation energies of the Mn doping under the Mn and Ni codoped configuration and the resulting reduction of defects and traps in Mn:CsPbCl₃ NCs with incorporation of Ni²⁺ ions.

Lead-free double perovskites Cs2InCuCl6 and (CH3NH3)2InCuCl6: electronic, optical, and electrical properties

Searching for alternatives to lead-containing metal halide perovskites, we explored the properties of indium-based inorganic double perovskites Cs₂InMX₆ with M = Cu, Ag, Au and X = Cl, Br, I, and of its organic-inorganic hybrid derivative MA₂InCuCl₆ (MA = CH₃NH₃⁺) using computation within Kohn-Sham density functional theory. Among these compounds, Cs₂InCuCl₆ and MA₂InCuCl₆ were found to be potentially promising candidates for solar cells. Calculations with different functionals provided the direct band gap of Cs₂InCuCl₆ between 1.05 and 1.73 eV. In contrast, MA₂InCuCl₆ exhibits an indirect band gap between 1.31 and 2.09 eV depending on the choice of exchange-correlation functional. Cs₂InCuCl₆ exhibits a much higher absorption coefficient than that calculated for c-Si and CdTe, common semiconductors for solar cells. Even MA₂InCuCl₆ is predicted to have a higher absorption coefficient than c-Si and CdTe across the visible spectrum despite the fact that it is an indirect band gap material. The intrinsic charge carrier mobilities for Cs₂InCuCl₆ along the L-Γ path are predicted to be comparable to those for MAPbI₃. Finally, we carried out calculations of the band edge positions for MA₂InCuCl₆ and Cs₂InCuCl₆ to offer guidance for solar cell heterojunction design and optimization. We conclude that Cs₂InCuCl₆ and MA₂InCuCl₆ are promising semiconductors for photovoltaic and optoelectronic applications.

Advances in lead-free double perovskite nanocrystals, engineering band-gaps and enhancing stability through composition tunability

In this topical review, we have focused on the recent advances made in the studies of lead-free perovskites in the bulk form and as nanocrystals. Substitution of lead in halide perovskites is essential to overcome the toxicity concerns and improve the relatively low stability of these materials. In lead-free double perovskites the unit cell is doubled and two divalent lead cations are replaced by mono and trivalent cations. The current main challenge with the double perovskite metal halides lies in overcoming their inherently indirect and disallowed optical transitions. In this review, we have discussed the recent discoveries made in the synthesis of these materials and highlighted how nanocrystals can serve as model systems to explore the schemes of cationic exchange, doping and alloying for engineering the electronic structure of double perovskites. In nanocrystals, the quantum confinement effects can modify the electronic structure and the resulting optical transition, thus increasing the absorption cross-section and emission, which are important properties for optoelectronic devices. Lastly, the enlarged surface to volume ratio in the nanocrystals adds a surface energy term that may enhance the stability of the metastable crystallographic phases. We have reviewed how the nanocrystal can provide information on phases that are inherently stable and investigated how the facile exchange reactions can help in achieving material compositions that are impossible to achieve by any other way. Finally, based on our recent synthetic experience, we have emphasized the similarities between lead-based and lead-free perovskite nanocrystals; we hope that our insight along with a summary of recent progress in this fast-growing field will help to expand the interest in lead-free perovskites towards a greener and brighter future.

Yb- and Mn-Doped Lead-Free Double Perovskite Cs2AgBiX6 (X = Cl-, Br-) Nanocrystals

Lead-free double perovskite nanocrystals (NCs) have emerged as a new category of materials that hold the potential for overcoming the instability and toxicity issues of lead-based counterparts. Doping chemistry represents a unique avenue toward tuning and optimizing the intrinsic optical and electronic properties of semiconductor materials. In this study, we report the first example of doping Yb³⁺ ions into lead-free double perovskite Cs₂AgBiX₆ (X = Cl^-, Br^-) NCs via a hot injection method. The doping of Yb³⁺ endows the double perovskite NCs with a newly emerged near-infrared emission band (sensitized from the NC hosts) in addition to their intrinsic trap-related visible photoluminescence. By controlling the Yb-doping concentration, the dual emission profiles and photon relaxation dynamics of the double perovskite NCs can be systematically tuned. Furthermore, we have successfully inserted divalent Mn²⁺ ions in Cs₂AgBiCl₆ NCs and observed emergence of dopant emission. Our work illustrates an effective and facile route toward modifying and optimizing optical properties of double perovskite Cs₂AgBiX₆ (X = Cl^-, Br^-) NCs with an indirect bandgap nature, which can broaden a range of their potential applications in optoelectronic devices.

Insights of Doping and the Photoluminescence Properties of Mn-Doped Perovskite Nanocrystals

Doping Mn²⁺ in semiconductor nanocrystals is widely known for its long-lifetime Mn d-d orange emission. While this had been extensively studied for chalcogenide nanostructures, recently this was also extended to perovskite nanocrystals. Being that CsPbCl₃ has a wide bandgap, the exciton energy transfer was found to be more efficient, but the dopant-induced photoluminescence was also obtained for layered perovskites and quantum-confined CsPbBr₃ nanocrystals. In recent years significant advances have been achieved in understanding the physical insights of doping following various approaches and optimizing the conditions for obtaining intense dopant emission. In addition, several new properties associated with these doped nanocrystals were also reported, and by modulating the compositions, the host bandgap and the dopant emission positions were also tuned. Keeping all of these developments in mind, this Perspective focuses on the insights of doping and the photoluminescence properties of Mn²⁺-doped perovskite nanocrystals. In addition, it also proposes possible future prospects of both synthesis and optical properties of these nanomaterials.

Doping Mn(II) in All-Inorganic Ruddlesden-Popper Phase of Tetragonal Cs2PbCl2I2 Perovskite Nanoplatelets

Doping Mn(II) in inorganic Ruddlesden-Popper phase Cs₂PbCl₂I₂ perovskite nanoplatelets is reported. The host nanostructures were prepared with a calculative protocol taking the exact required composition of Cs(I) and Pb(II) and injecting the preformed mixed oleylammonium chlorides and iodides at optimized reaction temperature. Reactions were optimized with various halides and their mixtures, but the stable phase of the Cs₂PbX₄ system was obtained only for the chloride-iodide mixed-halide system. Introduction of Mn(II) along with Pb(II), resulted in successful light-emitting doped nanocrystals. Measuring the photoluminescence and the decay lifetimes at room and liquid nitrogen temperatures, the variations in the excitonic, self-trapped, and Mn dopant emission properties were compared with those of the chalcogenide and perovskite nanocrystals.

Self-Trapped Excitons in All-Inorganic Halide Perovskites: Fundamentals, Status, and Potential Applications

Photoluminescence is a radiative recombination process of electron-hole pairs. Self-trapped excitons (STEs), occurring in a material with soft lattice and strong electron-phonon coupling, emit photons with broad spectrum and large Stokes shift. Recently, series halide perovskites with efficient STE emission have been reported and showed promise for solid-state lighting. In this Perspective, we present an overview of various photoluminescence phenomena with the emphasis on the mechanism and characteristics of emission derived from STEs. This is followed by the introduction of STE emission in hybrid halide perovskites. We then introduce all-inorganic STE emitters and focus in particular on the mechanism of STEs in double-perovskite Cs₂AgInCl₆ and strategies for efficiency improvement. Finally, we summarize the current photoluminescence and electroluminescence applications of STE emitters as well as the potential in luminescent solar concentrators and provide an overview of future research opportunities.

Oxide Analogs of Halide Perovskites and the New Semiconductor Ba2AgIO6

The past few years witnessed the rise of halide perovskites as prominent materials for a wide range of optoelectronic applications. However, oxide perovskites have a much longer history and are pivotal in many technological applications. As of today, a rational connection between these important materials is missing. Here, we explore this missing link and develop a novel concept of perovskite analogs, which led us to identify a new semiconductor, Ba₂AgIO₆. It exhibits an electronic band structure remarkably similar to that of our recently discovered halide double perovskite Cs₂AgInCl₆, but with a band gap in the visible range at 1.9 eV. We show that Ba₂AgIO₆ and Cs₂AgInCl₆ are analogs of the well-known transparent conductor BaSnO₃. We synthesize Ba₂AgIO₆ following a low-temperature solution process, and we perform crystallographic and optical characterizations. Ba₂AgIO₆ is a cubic oxide double perovskite with a direct low gap, opening new opportunities in perovskite-based electronics optoelectronics and energy applications.

Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties

Metal halide perovskites represent a flourishing area of research, which is driven by both their potential application in photovoltaics and optoelectronics and by the fundamental science behind their unique optoelectronic properties. The emergence of new colloidal methods for the synthesis of halide perovskite nanocrystals, as well as the interesting characteristics of this new type of material, has attracted the attention of many researchers. This review aims to provide an up-to-date survey of this fast-moving field and will mainly focus on the different colloidal synthesis approaches that have been developed. We will examine the chemistry and the capability of different colloidal synthetic routes with regard to controlling the shape, size, and optical properties of the resulting nanocrystals. We will also provide an up-to-date overview of their postsynthesis transformations, and summarize the various solution processes that are aimed at fabricating halide perovskite-based nanocomposites. Furthermore, we will review the fundamental optical properties of halide perovskite nanocrystals by focusing on their linear optical properties, on the effects of quantum confinement, and on the current knowledge of their exciton binding energies. We will also discuss the emergence of nonlinear phenomena such as multiphoton absorption, biexcitons, and carrier multiplication. Finally, we will discuss open questions and possible future directions.

Tunable dual emission in Mn2+-doped CsPbX3 (X = Cl, Br) quantum dots for high efficiency white light-emitting diodes

Doping of Mn²⁺ into semiconductor nanocrystals has been demonstrated to endow them with novel electronic, optical and magnetic functionalities. In this paper, Mn-doped CsPbX₃ (X = Br, Cl) quantum dots (QDs) were synthesized at room temperature via a facile strategy by introducing dimethyl sulfoxide (DMSO)-MnBr₂/PbX₂ composite as a precursor. The excitonic emission spectra of the as-obtained Mn-doped CsPbX₃ QDs can be tuned from 517 nm to 418 nm by adjusting the ratio of PbBr₂/PbCl₂ precisely, and the luminescence mechanism of the doped QDs is discussed in detail. Moreover, the highest photoluminescence quantum yield of the Mn²⁺ emission achieves 36.7%, which is comparable with QDs prepared by the conventional hot-injection method. Depending on the ratios of PbPb₂/PbCl₂, the energy transfer rate from the band-edge to Mn²⁺ excited state is in the range of 0.006-20.42 × 10⁷ s^-1. Furthermore, white light-emitting diodes (LEDs) were successfully fabricated by combining the as-prepared Mn-doped CsPbX₃ QDs with commercial UV GaN chips, and the high luminous efficiency of the as-prepared white LEDs was developed to 55.9 lm W^-1. This work strongly supports the fact that Mn-doped CsPbX₃ QDs are promising materials for application in lighting and displaying fields.

Lead-free silver-antimony halide double perovskite quantum dots with superior blue photoluminescence

Sb-based lead-free double perovskite Cs₂AgSbX₆ (X = Cl, Br or I) quantum dots exhibiting excellent air stability and blue emission with photoluminescence quantum yields of 31.33% were synthesized firstly using surfactant-assisted method.

Double perovskite Cs2AgInCl6:Cr3+: broadband and near-infrared luminescent materials

A Cr³⁺-doped halide double perovskite Cs₂AgInCl₆:Cr³⁺ is first reported which exhibits a broad near-infrared emission ranging from 850 to 1350 nm centered at 1010 nm with a FWHM of 180 nm.