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

Co-doping of tellurium with bismuth enhances stability and photoluminescence quantum yield of Cs2AgInCl6 double perovskite nanocrystals

The co-doping of double perovskites is a useful approach in terms of improving their stability and photoluminescence quantum yield. Herein, Bi³⁺ and Te⁴⁺ cations have been co-doped into Cs₂AgInCl₆ nanocrystals. Doping with Te⁴⁺ cations promotes radiative recombination of self-trapped excitons due to increased defect formation energies of silver and indium vacancies, according to experimental and theoretical results. When used in excess, the TeO₂ precursor would generate residual TeO₂, Te₂O₃Cl₂, R₂TeO, or all three of them, which confined undesired chlorine ions on oxygen vacancies to counteract the pull from the Cs₂AgInCl₆ host, resulting in improved coordination with surface oleic acid ligands. As a result, 1% Bi and 8% Te co-doped Cs₂AgInCl₆ nanocrystals reach a high photoluminescence quantum yield of 34% and show an improved stability, maintaining over 70% of their original emission intensity after storage for more than 1 month. These findings are important in the context of producing high-performance properly doped double perovskite nanocrystals for optoelectronic applications.

Unveiling Local Electronic Structure of Lanthanide-Doped Cs2 NaInCl6 Double Perovskites for Realizing Efficient Near-Infrared Luminescence

Lanthanide ion (Ln³⁺ )-doped halide double perovskites (DPs) have evoked tremendous interest due to their unique optical properties. However, Ln³⁺ ions in these DPs still suffer from weak emissions due to their parity-forbidden 4f-4f electronic transitions. Herein, the local electronic structure of Ln³⁺ -doped Cs₂ NaInCl₆ DPs is unveiled. Benefiting from the localized electrons of [YbCl₆ ]^3- octahedron in Cs₂ NaInCl₆ DPs, an efficient strategy of Cl^- -Yb³⁺ charge transfer sensitization is proposed to obtain intense near-infrared (NIR) luminescence of Ln³⁺ . NIR photoluminescence (PL) quantum yield (QY) up to 39.4% of Yb³⁺ in Cs₂ NaInCl₆ is achieved, which is more than three orders of magnitude higher than that (0.1%) in the well-established Cs₂ AgInCl₆ via conventional self-trapped excitons sensitization. Density functional theory calculation and Bader charge analysis indicate that the [YbCl₆ ]^3- octahedron is strongly localized in Cs₂ NaInCl₆ :Yb³⁺ , which facilitates the Cl^- -Yb³⁺ charge transfer process. The Cl^- -Yb³⁺ charge transfer sensitization mechanism in Cs₂ NaInCl₆ :Yb³⁺ is further verified by temperature-dependent steady-state and transient PL spectra. Furthermore, efficient NIR emission of Er³⁺ with the NIR PLQY of 7.9% via the Cl^- -Yb³⁺ charge transfer sensitization is realized. These findings provide fundamental insights into the optical manipulation of Ln³⁺ -doped halide DPs, thus laying a foundation for the future design of efficient NIR-emitting DPs.

Studies on the photoelectronic properties of a manganese (Mn)-doped lead-free double perovskite

Taking Cs₂NaBiCl₆, Cs₂AgInCl₆ and Cs₂AgBiCl₆ as examples of lead-free double perovskites (DPs), we study the photoluminescence (PL) properties of Mn-doped DPs. The electron localization function (ELF) reveals the more ionic nature of the Na-Cl bond in Cs₂NaBiCl₆ than that of the Ag-Cl bond in Cs₂AgBiCl₆. Bader charge calculations confirm the nominal +2 valence state of Mn ions in both DPs. Mn²⁺ ions introduce two defect levels in the band gap of the Cs₂NaBiCl₆ host, accounting for the d-d transition (⁴T₁-⁶A₁ transition) of Mn²⁺ and thus the subsequent orange PL. The changes of the crystal field and their influences on the emission energy of Mn²⁺ ions in different DPs are evaluated by calculating the Racah parameters (B and C) and the crystal field strength (Dq) obtained from energies of the terms of d⁵ in the Cs₂NaBiCl₆:Mn²⁺ and Cs₂AgInCl₆:Mn²⁺ systems. The results show that Dq in Cs₂AgInCl₆:Mn²⁺ is stronger than that in Cs₂NaBiCl₆:Mn²⁺. The analyses on bonding interactions of the Mn-Cl bond via ELF and the integrated projected pCOHP also confirm the stronger ionic bonding interactions and thus the boost of the crystal field strength in the Cs₂AgInCl₆:Mn²⁺ system, which results in the blue-shift of the Mn²⁺ introduced PL peak from Cs₂AgInCl₆ to Cs₂NaBiCl₆. Our results provide a new strategy to modulate the emission wavelengths, i.e., tuning the crystal field.

Recent progress and future prospects on halide perovskite nanocrystals for optoelectronics and beyond

As an emerging new class of semiconductor nanomaterials, halide perovskite (ABX₃, X = Cl, Br, or I) nanocrystals (NCs) are attracting increasing attention owing to their great potential in optoelectronics and beyond. This field has experienced rapid breakthroughs over the past few years. In this comprehensive review, halide perovskite NCs that are either freestanding or embedded in a matrix (e.g., perovskites, metal-organic frameworks, glass) will be discussed. We will summarize recent progress on the synthesis and post-synthesis methods of halide perovskite NCs. Characterizations of halide perovskite NCs by using a variety of techniques will be present. Tremendous efforts to tailor the optical and electronic properties of halide perovskite NCs in terms of manipulating their size, surface, and component will be highlighted. Physical insights gained on the unique optical and charge-carrier transport properties will be provided. Importantly, the growing potential of halide perovskite NCs for advancing optoelectronic applications and beyond including light-emitting devices (LEDs), solar cells, scintillators and X-ray imaging, lasers, thin-film transistors (TFTs), artificial synapses, and light communication will be extensively discussed, along with prospecting their development in the future.

Direct Electron Transfer Enables Highly Efficient Dual Emission Modes of Mn2+-Doped Cs2Na1-xAgxBiCl6 Double Perovskites

Double perovskites with bright emission, low toxicity, and excellent stability have drawn considerable attention. Herein, we report the hydrothermal synthesis of Mn²⁺-doped Cs₂Na_1-xAg_xBiCl₆ double perovskites that exhibit dual emission modes. Introducing Ag⁺ ions to Cs₂NaBiCl₆ samples enables a bright self-trapped exciton (STE) emission in orange-red color, whereas Mn²⁺ dopants induce a yellow-orange emission. Importantly, Mn²⁺ doping into Cs₂Na_1-xAg_xBiCl₆ double perovskites with an indirect bandgap enables a high photoluminescence quantum yield of 49.52 ± 2%. Density functional theory calculations reveal that bringing Ag⁺ ions into Cs₂NaBiCl₆ can localize wave function to the [AgCl₆]^5- octahedron and convert dark transitions to bright STE transitions. Moreover, the 3d orbitals of Mn²⁺ dopants hybridize with Bi-6p and Cl-3p orbitals at the conduction band minimum, resulting in direct electron transfer from the host to Mn²⁺ and a significant increase in photoluminescence efficiency. These results shed light on the optical physical process of Mn²⁺-doped systems, providing useful information for further improvement of the photoluminescence efficiency of double perovskites.

Cu+ → Mn2+ Energy Transfer in Cu, Mn Coalloyed Cs3ZnCl5 Colloidal Nanocrystals

In this work, we report the hot-injection synthesis of Cs₃ZnCl₅ colloidal nanocrystals (NCs) with tunable amounts of Cu⁺ and Mn²⁺ substituent cations. All the samples had a rodlike morphology, with a diameter of ∼14 nm and a length of ∼30-100 nm. Alloying did not alter the crystal structure of the host Cs₃ZnCl₅ NCs, and Cu ions were mainly introduced in the oxidation state +1 according to X-ray photoelectron and electron paramagnetic resonance spectroscopies. The spectroscopic analysis of unalloyed, Cu-alloyed, Mn-alloyed, and Cu, Mn coalloyed NCs indicated that (i) the Cs₃ZnCl₅ NCs have a large band gap of ∼5.35 eV; (ii) Cu(I) aliovalent alloying leads to an absorption shoulder/peak at ∼4.8 eV and cyan photoluminescence (PL) peaked at 2.50 eV; (iii) Mn(II) isovalent alloying leads to weak Mn PL, which intensifies remarkably in the coalloyed samples, prompted by an energy transfer (ET) process between the Cu and Mn centers, favored by the overlap between the lowest (⁶A₁ → ⁴T₁) transition for tetrahedrally coordinated Mn²⁺ and the PL profile from Cu(I) species in the Cs₃ZnCl₅ NCs. The efficiency of this ET process reaches a value of 61% for the sample with the highest extent of Mn alloying. The PL quantum yield (QY) values in these Cu, Mn coalloyed NCs are lower at higher Mn contents. The analysis of the Mn PL dynamics in these samples indicates that this PL drop stems from inter-Mn exciton migration, which increases the likelihood of trapping in defect sites, in agreement with previous studies.

The making and breaking of perovskite photochromism through doping

Photochromic single crystals of high transparency are considered as basic building blocks for many advanced applications, such as three-dimensional optical storage and volumetric displays. In this work, a transparent crystal of Cs₂AgInCl₆ perovskite was grown in a hydrothermal reactor. A high transmittance-contrast degree up to 30% was achieved by photochromism activation via a Mn²⁺-doping strategy. Intriguingly, both coloration and decoloration could be triggered by light at appropriate wavelengths, 365 nm and 520 nm for example. Unlike heat-induced decoloration, the optical route represents a much faster tool to erase information. As a caveat from photochromism, the photoluminescence quantum yield (PL QY) was significantly decreased down to 1.7%. In an attempt to boost the quantum yield, sodium ions were introduced as a co-dopant. Surprisingly, the co-doping strategy not only boosted the PL QY to 33%, but also deactivated the photochromism. Our work expanded the library of photochromic materials by introducing a new perovskite single crystal, representing a paradigm for the making and breaking of photochromism.

Humidity Sensing Applications of Lead-Free Halide Perovskite Nanomaterials

Over the past decade, perovskite-based nanomaterials have gained notoriety within the scientific community and have been used for a variety of viable applications. The unique structural properties of these materials, namely good direct bandgap, low density of defects, large absorption coefficient, high sensitivity, long charge carrier lifetime, good selectivity, acceptable stability at room temperature, and good diffusion length have prompted researchers to explore their potential applications in photovoltaics, light-emitting devices, transistors, sensors, and other areas. Perovskite-based devices have shown very excellent sensing performances to numerous chemical and biological compounds in both solid and liquid mediums. When used in sensing devices, Perovskite nanomaterials are for the most part able to detect O₂, NO₂, CO₂, H₂O, and other smaller molecules. This review article looks at the use of lead-free halide perovskite materials for humidity sensing. A complete description of the underlying mechanisms and charge transport characteristics that are necessary for a thorough comprehension of the sensing performance will be provided. An overview of considerations and potential recommendations for the creation of new lead-free perovskite nanostructure-based sensors is presented.

All-Inorganic Manganese-Based CsMnCl3 Nanocrystals for X-Ray Imaging

Lead-based halide perovskite nanomaterials with excellent optical properties have aroused great attention in the fields of solar cells, light-emitting diodes, lasing, X-ray imaging, etc. However, the toxicity of lead prompts researchers to develop alternatives to cut down the usage of lead. Herein, all-inorganic manganese-based perovskite derivatives, CsMnCl₃ nanocrystals (NCs), with uniform size and morphology have been synthesized successfully via a modified hot-injection method. These NCs have a direct bandgap of 4.08 eV and a broadband emission centered at 660 nm. Through introducing modicum lead (1%) into the CsMnCl₃ NCs, the photoluminescence intensity greatly improves, and the quantum yield (PLQY) increases from 0.7% to 21%. Furthermore, the CsMnCl₃ :1%Pb NCs feature high-efficiency of X-ray absorption and radioluminescence, which make these NCs promising candidates for X-ray imaging.

Post-doping induced morphology evolution boosts Mn2+ luminescence in the Cs2NaBiCl6:Mn2+ phosphor

As we know, defects caused in the synthetic process of metal halide perovskite are the most difficult to overcome, and greatly limit their photoelectric performances. Herein, a post-doped strategy was utilized to achieve an interesting morphology evolution from a standard octahedron to a snowflake-like sheet during the Mn²⁺-doped Cs₂NaBiCl₆ process, which realizes the obvious photoluminescence quantum efficiency (PLQY) enhancement of the Cs₂NaBiCl₆:Mn²⁺ phosphor. This surprising evolution is ascribed to the morphology collapse and reconstruction induced by Mn²⁺ exchange. The obtained phosphor exhibits enhanced 31.56% PLQY, which is two-fold higher than that synthesized by the traditional co-precipitation method, with broad emission spectrum and good PL color stability at 150 °C. Combined with the Cs₂SnCl₆ : 1mol%Bi³⁺ phosphor to fabricate the phosphor-converted light-emitting diode, bright white light emission with Ra = 88, CCT = 4320 K, CIE (0.36, 0.33) and a good application potential in high-resolution PL imaging agents was obtained. This work provides a possible effective strategy to improve the PL performance for impurity-doped lead-free metal halide perovskite.

Co-Doping of Cerium and Bismuth into Lead-Free Double Perovskite Cs2AgInCl6 Nanocrystals Results in Improved Photoluminescence Efficiency

Lead-free double cation metal halide perovskites have recently attracted considerable attention, with continuing research efforts focusing on the improvement of their stability and photoluminescence quantum yield (PL QY). In this study, Ce3+ has been co-doped together with Bi3+ into lead-free double perovskite Cs2AgInCl6 nanocrystals (NCs) in order to improve their crystallinity and PL QY. Both uncoordinated chloride ions and silver vacancies could be eliminated using this co-doping strategy, and the resulting Ce3+,Bi3+-co-doped Cs2AgInCl6 NCs showed adjustable PL emission peaks in the range of 589 to 577 nm by varying the doping amount of Ce3+ with a fixed feeding ratio of bismuth precursor set at 1%. Cs2AgInCl6 NCs doped with 1% Bi alone reached a PL QY of 10% for the PL peak centered at 591 nm, while those co-doped with 1% Bi and 2% Ce together achieved the highest PL QY of 26% for the PL peak centered at 580 nm. The use of Ce3+ as a dopant promoted the localization of self-trapped excitons to prevent PL quenching, although the ion's 5d excited state may potentially provide an energetically favorable indirect route for the radiative relaxation process. This also resulted in a blue shift of the PL maximum and increased the exciton binding energy, thus promoting the radiative recombination of self-trapped excitons.

Tunable Dual Emission in Bi3+/Te4+-Doped Cs2HfCl6 Double Perovskites for White Light-Emitting Diode Applications

Multicolor-emission-based single-phase white light derived from different luminescence centers is an effective way to manipulate the optical properties of halide perovskites. In this work, we developed a codoping strategy to incorporate Bi³⁺ and Te⁴⁺ emission centers into all-inorganic lead-free Cs₂HfCl₆ perovskite by a hydrothermal method. The as-prepared Bi³⁺/Te⁴⁺-doped Cs₂HfCl₆ microcrystals show bright blue (Bi³⁺), yellow (Te⁴⁺), and warm-white emissions (Bi³⁺/Te⁴⁺), respectively. The broad efficient dual emission in Bi³⁺/Te⁴⁺ co-doped Cs₂HfCl₆ is assigned to the typical ³P₁ → ¹S₀ transition emission from Bi³⁺ originating from [Bi_Hf + V_Cl] and self-trapped excitons (STEs) from Te⁴⁺. Moreover, the concentration-optimized Cs₂HfCl₆:Te⁴⁺ shows excellent antiwater stability and high photoluminescence quantum yield (PLQY) of ∼70%. Meanwhile, a white light-emitting diode (WLED) fabricated using Bi³⁺/Te⁴⁺ co-doped Cs₂HfCl₆ is close to warm white with a color rendering index (CRI) of 75.4, CIE color coordinate of (0.370, 0.393), and a correlated color temperature (CCT) of 4380 K. These results suggest that Bi³⁺/Te⁴⁺ co-doped all-inorganic lead-free Cs₂HfCl₆ is a potential single-phase white light-emitting phosphor candidate for solid-state lightings.

Theoretical exploration of mechanical, electronic structure and optical properties of aluminium based double halide perovskite

The mechanical, electronic structure and optical properties of aluminium based double halide perovskite were calculated by density functional theory. The formation energy and elastic constant confirm the stability of the cubic perovskite materials. The materials are all ductile and suitable for flexible photovoltaic and optoelectronic devices. The band gap values vary from 0.773 eV to 3.430 eV, exactly corresponding to the range of ideal band gap values for good photoresponse. The band structure analysis shows that all the materials possess small effective mass, which indicates a good transport of carriers. And these materials have a broad energy range of optical absorption for utilization and a detector of photons. Moreover, less expensive K₂AgAlBr₆ were investigated for comparison with materials containing a cesium element, and according to the results, is also a candidate for photoelectronic devices due to the similar properties.

M₂AgAlX₆ (M = Cs, Rb and K, X = Cl, Br and I) is a stable vacancy ordered double halide perovskite direct band gap semiconductor material with good absorption of near-ultraviolet and short-wavelength visible light.

Facile Melting-Crystallization Synthesis of Cs2NaxAg1-xInCl6: Bi Double Perovskites for White Light-Emitting Diodes

Lead-free double perovskites (DPs) have outstanding luminescent properties, which make them excellent candidates for wide use in optoelectronics. Herein, a solvent-free melting-crystallization technique, which can produce kilogram-scale DP microcrystals (DP-MCs) in one batch, is invented to synthesize the Cs₂Na_xAg_1-xInCl₆: Bi (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) DP-MCs. The structure and composition analysis confirmed the products are pure Cs₂Na_xAg_1-xInCl₆ DP-MCs. Affected by Jahn-Teller distortion of AgCl₆ octahedra, self-trapped excitons appear in the excited state, resulting in the broadband emission (400-850 nm) of Cs₂Ag_1-xNa_xInCl₆: Bi DP-MCs. The enhancement of the photoluminescence quantum yield can be realized by introducing Na⁺ to break the parity-forbidden transition in the Cs₂AgInCl₆ DP. Optimized Cs₂Na_0.4Ag_0.6InCl₆: Bi DP-MC phosphors combined with commercial blue and green phosphors were coated on ultraviolet chips (365 nm) to fabricate white light-emitting diodes (WLEDs) from warm white (2930 K) to cold white (6957 K). An ultrahigh color rendering index of 97.1 and a CCT of 5548 K as well as Commission Internationale de l'Eclairage color coordinates of (0.331, 0.339) have been demonstrated. This kilogram-scale synthesis technique could stimulate the industrial development of WLEDs for general lighting based on DP-MC phosphors.

Electrical and chemical properties of vacancy-ordered lead free layered double perovskite nanoparticles

In this work we synthesized vacancy-ordered lead-free layered double perovskite (LDP) nanoparticles. This structure consists of two layers of trivalent metal halide octahedra [B(III)X₆]^3- separated by a layer of divalent metal [B(II)X₆]^4- (B is a divalent or trivalent metal). The chemical formula of this structure is based on A₄B(II)B(III)₂X₁₂ where A is Cs, B(III) is Bi, X is Cl and B(II) is a different ratio between Mn²⁺ and Cd²⁺. Well-defined colloidal nanoplates of Cs₄Cd_xMn_1-xBi₂Cl₁₂ were successfully synthesized. These nanoplates show photoluminescence (PL) in the orange to red region that can be tuned by changing the Cd/Mn ratio. High resolution scanning transmission electron microscopy (HR-STEM) and atomic resolution elemental analysis were performed on these lead free LDP nanoplates revealing two different particle compositions that can be controlled by the Cd/Mn ratio. Ultraviolet Photoelectron Spectroscopy (UPS) and scanning tunneling spectroscopy (STS) reveal the band gap structure of these LDP nanoplates. Density functional theory (DFT) calculations show the existence of [MnCl₆]^4- in-gap states. While the absorption occurs from the valence band maximum (VBM) to the conduction band minimum (CBM), the emission may occur from the CBM to an in-gap band maximum (IGM), which could explain the PL in the orange to red region of these nanoplates. This work provides a detailed picture of the chemical and electronic properties of LDP nanoparticles.

Boosting the Self-Trapped Exciton Emission in Alloyed Cs2 (Ag/Na)InCl6 Double Perovskite via Cu+ Doping

Fundamental understanding of the effect of doping on the optical properties of 3D double perovskites (DPs) especially the dynamics of self-trapped excitons (STEs) is of vital importance for their optoelectronic applications. Herein, a unique strategy via Cu⁺ doping to achieve efficient STE emission in the alloyed lead-free Cs₂ (Ag/Na)InCl₆ DPs is reported. A small amount (1.0 mol%) of Cu⁺ doping results in boosted STE emission in the crystals, with photoluminescence (PL) quantum yield increasing from 19.0% to 62.6% and excitation band shifting from 310 to 365 nm. Temperature-dependent PL and femtosecond transient absorption spectroscopies reveal that the remarkable PL enhancement originates from the increased radiative recombination rate and density of STEs, as a result of symmetry breakdown of the STE wavefunction at the octahedral Ag⁺ site. These findings provide deep insights into the STE dynamics in Cu⁺ -doped Cs₂ (Ag/Na)InCl₆ , thereby laying a foundation for the future design of new lead-free DPs with efficient STE emission.

Germanium Halides Serving as Ideal Precursors: Designing a More Effective and Less Toxic Route to High-Optoelectronic-Quality Metal Halide Perovskite Nanocrystals

The three-precursors approach has proven to be advantageous for obtaining high-quality metal halide perovskite nanocrystals (PNCs). However, the current halide precursors of choice are mainly limited to those highly toxic organohalides, being unfavorable for large-scale and sustainable use. Moreover, most of the resulting PNCs still suffer from low quality in terms of photoluminescence quantum yield (PLQY). Herein we present all-inorganic germanium salts, GeX₄ (X = Cl, Br, I), serving as robust and less hazardous alternatives that are capable of ensuring improved material properties for both Pb-based and Pb-free PNCs. Importantly, unlike most of the other inorganic halide sources, the GeX₄ compound does not deliver the Ge element into the final compositions, whereas the PLQY and phase stability of the resulting nanocrystals are significantly improved. Theoretical calculations suggest that Ge halide precursors provide favorable conditions in both dielectric environment and thermodynamics, which jointly contribute to the formation of size-confined defect-suppressed nanoparticles.

Revealing Weak Dimensional Confinement Effects in Excitonic Silver/Bismuth Double Perovskites

Lead-free perovskites are attracting increasing interest as nontoxic materials for advanced optoelectronic applications. Here, we report on a family of silver/bismuth bromide double perovskites with lower dimensionality obtained by incorporating phenethylammonium (PEA) as an organic spacer, leading to the realization of two-dimensional double perovskites in the form of (PEA)₄AgBiBr₈ (n = 1) and the first reported (PEA)₂CsAgBiBr₇ (n = 2). In contrast to the situation prevailing in lead halide perovskites, we find a rather weak influence of electronic and dielectric confinement on the photophysics of the lead-free double perovskites, with both the 3D Cs₂AgBiBr₆ and the 2D n = 1 and n = 2 materials being dominated by strong excitonic effects. The large measured Stokes shift is explained by the inherent soft character of the double-perovskite lattices, rather than by the often-invoked band to band indirect recombination. We discuss the implications of these results for the use of double perovskites in light-emitting applications.

Moisture-Assisted near-UV Emission Enhancement of Lead-Free Cs4CuIn2Cl12 Double Perovskite Nanocrystals

Lead-based halide perovskite nanocrystals (NCs) are recognized as emerging emissive materials with superior photoluminescence (PL) properties. However, the toxicity of lead and the swift chemical decomposition under atmospheric moisture severely hinder their commercialization process. Herein, we report the first colloidal synthesis of lead-free Cs₄CuIn₂Cl₁₂ layered double perovskite NCs via a facile moisture-assisted hot-injection method stemming from relatively nontoxic precursors. Although moisture is typically detrimental to NC synthesis, we demonstrate that the presence of water molecules in Cs₄CuIn₂Cl₁₂ synthesis enhances the PL quantum yield (mainly in the near-UV range), induces a morphological transformation from 3D nanocubes to 2D nanoplatelets, and converts the dark transitions to radiative transitions for the observed self-trapped exciton relaxation. This work paves the way for further studies on the moisture-assisted synthesis of novel lead-free halide perovskite NCs for a wide range of applications.

Red-emissive nanocrystals of Cs4MnxCd1-xSb2Cl12 layered perovskites

Layered double perovskites are currently being investigated as emerging halide-based materials for optoelectronic applications. Herein, we present the synthesis of Cs₄Mn_xCd_1-xSb₂Cl₁₂ (0 ≤ x ≤ 1) nanocrystals (NCs). X-ray powder diffraction evidences the retention of the same crystal structure for all the inspected compositions; transmission electron microscopy revealed monodisperse particles with a mean size between 10.7 nm and 12.7 nm. The absorption spectra are mostly determined by transitions related to Sb³⁺, whereas Mn²⁺ induced a red emission in the 625-650 nm range. The photoluminescence emission intensity and position vary with the Mn²⁺ content and reach the maximum for the composition with x = 0.12. Finally, we demonstrate that the photoluminescence quantum yield of the latter NCs was increased from 0.3% to 3.9% through a post-synthesis treatment with ammonium thiocyanate. The present work expands the knowledge of colloidal layered double perovskite nanocrystals, stimulating future investigations of this emerging class of materials.

Rare-Earth Doping in Nanostructured Inorganic Materials

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

Bi3+ and Sm3+ co-doped Cs2AgInCl6 perovskite microcrystals with co-enhancement of fluorescence emission

Bi3+–Sm3+ co-doped Cs2AgInCl6 MCs show a combination of luminescence in the visible range.

Lead-free all-inorganic halide double perovskite materials for optoelectronic applications: progress, performance and design

The geometrical and electronic structures of all-inorganic halide double perovskites and their applications in optoelectronic devices are reviewed. Novel design methods are desirable to develop this type of perovskite with superior performance.

Lead-free Double Perovskite Cs2 AgIn0.9 Bi0.1 Cl6 Quantum Dots for White Light-Emitting Diodes

Perovskite-based optoelectronic devices have attracted considerable attention owing to their excellent device performances and facile solution processing. However, the toxicity and intrinsic instability of lead-based perovskites have limited their commercial development. Moreover, the provision of an efficient white emission from a single perovskite layer is challenging. Here, novel electrically excited white light-emitting diodes (WLEDs) based on lead-free double perovskite Cs2 AgIn0.9 Bi0.1 Cl6 quantum dots (QDs) without any phosphor are fabricated for the first time. Density functional theory calculations are carried out to clarify the mechanism of absorption and recombination in Cs2 AgIn0.9 Bi0.1 Cl6 with Bi-doping breaking the parity-forbidden transition of the direct bandgap. Microzone optical and electronic characterizations reveal that the broadband emission of Cs2 AgIn0.9 Bi0.1 Cl6 QDs originates from self-trapped excitons, and luminescent properties are unchanged after the film deposition. The QD-WLED exhibits excellent Commission Internationale de L'Eclairage color coordinates, correlated color temperature and relatively high color rendering index of (0.32, 0.32), 6432 K, and 94.5, respectively. The maximum luminance of 158 cd m-2 is achieved by triphenylphosphine oxide passivation, and this lead-free QD-WLED exhibits a superior stability in ambient air with a long T50 ≈48.53 min. Therefore, lead-free perovskite Cs2 AgIn0.9 Bi0.1 Cl6 QDs are promising candidates for use in WLEDs in the future.

Bulk Mn2+ Doped 1D Hybrid Lead Halide Perovskite with Highly Efficient, Tunable and Stable Broadband Light Emissions

Mn²⁺ doped colloidal three-dimensional (3D) lead halide perovskite nanocrystal (PNC) has attracted intensive research attention; however, the low exciton binding energy and fatal optical instability of 3D PNC seriously hinder the optoelectronic application. Therefore, it remains significant to explore new stable host perovskite with strongly bound exciton to realize more desirable luminescent property. In this work, we utilized bulk one-dimensional (1D) hybrid perovskite of [AEP]PbBr₅  ⋅ H₂ O (AEP=N-aminoethylpiperazine) as structural platform to rationally optimize the luminescent property by a controllable Mn²⁺ doping strategy. Significantly, the series of Mn²⁺ -doped 1D [AEP]PbBr₅  ⋅ H₂ O show enhanced energy transfer efficiency from the strongly bound excitons of host material to 3d electrons of Mn²⁺ ions, resulting in tunable broadband light emissions from weak yellow to strong red spectral range with highest photoluminescence quantum yield up to 28.41 %. More importantly, these Mn²⁺ -doped 1D perovskites display ultrahigh structural and optical stabilities in humid atmosphere, water and high temperature exceeding the conventional 3D PNC. Combined highly efficient, tunable and stable broadband light emissions enable Mn²⁺ -doped 1D perovskite as excellent down-converting phosphor showcasing the potential application in white light emitting diode. This work not only provides a profound understanding of low-dimensional perovskites but also opens a new way to rationally design high-performance broadband light emitting perovskites for solid-state lighting and displaying devices.

Recent Advances in Pyroelectric Materials and Applications

As one important subclass of piezoelectric materials, pyroelectric materials have caused increasing attention owing to the unique pyroelectric effect induced by spontaneous polarization, showing broad promising application prospects due to various electrical responses induced by time-dependent temperature variation. This review systematically introduces the pyroelectric effect and evaluation of pyroelectric materials and follows by analyzing and concluding the novel properties corresponding to four kinds of main pyroelectric materials. The emphasis of this review focuses on several significant and practical applications of pyroelectric materials in thermal energy harvesting from the external environment, pyroelectric sensing, and imaging, even some electrochemical applications including hydrogen generation, wastewater treatment, sterilization, and disinfection. Finally, the development direction of pyroelectric materials, potential challenges and opportunities in the future are all discussed and proposed. Through systematical conclusion and analysis of the latest research progress in the recent two decades, this review may provide significant guide and inspiration in the development of pyroelectric materials.

Room-temperature synthesis of blue-emissive zero-dimensional cesium indium halide quantum dots for temperature-stable down-conversion white light-emitting diodes with a half-lifetime of 186 h

Recently, the newly-emerging lead halide perovskite quantum dots (QDs) have attracted intensive research interest for lighting and display applications owing to their remarkable optoelectronic properties. It is regrettable that the toxicity and instability of lead largely hinder their practical applications. Here, zero-dimensional (0D) cesium indium halide (Cs₃InX₆) QDs were synthesized for the first time using a modified ligand-assisted reprecipitation method, and the emission wavelength can be tuned facilely via an anion exchange reaction. Typically, the Cs₃InBr₆ QDs showed broadband blue emission with a high photoluminescence (PL) quantum yield of 46%. First-principles calculations were performed and temperature-dependent PL was studied to investigate the emission mechanisms of the Cs₃InBr₆ QDs; the 0D nature of Cs₃InBr₆ enhances the localization of excitons, resulting in a large exciton binding energy. It is worth noting that the strong electron-phonon coupling of Cs₃InBr₆ indicates that the broadband emission comes from self-trapped exciton emission. Moreover, the Cs₃InX₆ QDs exhibit excellent stability against moisture, ultraviolet light and heat degradation, significantly better than for conventional lead halide perovskites. Subsequently, the white light-emitting diodes (WLEDs) prepared using blue-emissive Cs₃InBr₆ QD powder used as the phosphor showed an excellent working stability with a record half-life (T₅₀) of 186 h. Even if the operating temperature is as high as 106.9 °C, the LED can still operate well and reach a T₅₀ of 50 h. These results highlight the huge advantages and application potential of 0D Cs₃InX₆ QDs as an environmentally friendly emitter in the field of solid-state lighting.

Solution-Grown Chloride Perovskite Crystal of Red Afterglow

We report the growth of a halide-based double perovskite, Cs₂ Na_x Ag_1-x InCl₆ :y%Mn, via a facile hydrothermal reaction at 180 °C. Through a co-doping strategy of both Na⁺ and Mn²⁺ , the as-prepared crystals exhibited a red afterglow featuring a high color purity (ca. 100 %) and a long duration time (>5400 s), three orders of magnitude longer than those solution-processed organic afterglow crystals. The energy transfer (ET) process between self-trapped excitons (STE) and activators was investigated through time-resolved spectroscopy, which suggested an ET efficiency up to 41 %. Importantly, the nominal concentration of dopants, especially in the case of Na⁺ , was found a useful tool to control both energy level and number distribution of traps. Cryogenic afterglow measurements suggested that the afterglow phenomenon was likely governed by thermal-activated exciton diffusion and electron tunneling process.

A multi-responsive indium-viologen hybrid with ultrafast-response photochromism and electrochromism

A novel 0D organic-inorganic metal halide hybrid (C₁₃H₁₆N₂O₂)₂InCl₆·Cl (1) has been obtained by integrating the mono-viologen derivative with InCl₃. Compound 1 exhibits reversible and ultrafast UV/sunlight/X-ray induced photochromic properties, as well as excellent electrochromic performance, which is the first example of an indium-based organic-inorganic chromic hybrid.

Bright Triplet Self-Trapped Excitons to Dopant Energy Transfer in Halide Double-Perovskite Nanocrystals

For inorganic semiconductor nanostructure, excitons in the triplet states are known as the "dark exciton" with poor emitting properties, because of the spin-forbidden transition. Herein, we report a design principle to boost triplet excitons photoluminescence (PL) in all-inorganic lead-free double-perovskite nanocrystals (NCs). Our experimental data reveal that singlet self-trapped excitons (STEs) experience fast intersystem crossing (80 ps) to triplet states. These triplet STEs give bright green color emission with unity PL quantum yield (PLQY). Furthermore, efficient energy transfer from triplet STEs to dopants (Mn²⁺) can be achieved, which leads to white-light emitting with 87% PLQY in both colloidal and solid thin film NCs. These findings illustrate a fundamental principle to design efficient white-light emitting inorganic phosphors, propelling the development of illumination-related applications.

Bandgap Modulation of Cs2AgInX6 (X = Cl and Br) Double Perovskite Nano- and Microcrystals via Cu2+ Doping

Recently, the double perovskite Cs₂AgInCl₆, which has high stability and low toxicity, has been proposed as a potential alternative to Pb-based perovskites. However, the calculated parity-allowed transition bandgap of Cs₂AgInCl₆ is 4.25 eV; this wide bandgap makes it difficult to use as an efficient solar absorber. In this study, we explored the effect of Cu doping on the optical properties of Cs₂AgInCl₆ double perovskite nano- and microcrystals (MCs), particularly in its changes of absorption profile from the ultraviolet (UV) to near-infrared (NIR) region. Undoped Cs₂AgInCl₆ showed the expected wide bandgap absorbance, but the Cu-doped sample showed a new sharp absorption peak at 419 nm and broad absorption bands near 930 nm, indicating bandgap reduction. Electron paramagnetic resonance (EPR) spectroscopy demonstrated that this bandgap reduction effect was due to the Cu doping in the double perovskite and confirmed that the Cu²⁺ paramagnetic centers were located on the surface of the nanocrystals (NCs) and at the center of the perovskite octahedrons (g_∥ > g_⊥ > g_e). Finally, we synthesized Cu-doped Cs₂AgInCl₆ MCs and observed results similar to those of the NCs, showing that the application range could be expanded to multidimensions.

Uncovering the Role of Trioctylphosphine on Colloidal and Emission Stability of Sb-Alloyed Cs2NaInCl6 Double Perovskite Nanocrystals

Doping and compositional tuning of Cs₂AInCl₆ (A = Ag, Na) double perovskite nanocrystals (PNCs) is considered a promising strategy toward the development of light-emitting sources for applications in solution-processed optoelectronic devices. Oleic acid and oleylamine are by far the most often used surface capping ligands for PNCs. However, the undesirable desorption of these ligands due to proton-exchange reaction during isolation and purification processing results in colloidal and structural instabilities. Thus, the improvement of colloidal and optical stability of PNCs represents one of the greatest challenges in the field. Here, we report a trioctylphosphine-mediated synthesis and purification method toward Sb-alloyed Cs₂NaInCl₆ PNCs with excellent stability and optical features. Nuclear magnetic resonance spectroscopy enabled one to explain the role of trioctylphosphine and to reveal the reaction mechanism during crystal nucleation and growth. Under the optimized reaction conditions, in situ-generated trioctylphosphonium chloride and benzoyl trioctylphosphonium chloride serve as highly reactive halide sources, while benzoyl trioctylphosphonium and oleylammonium cations together with the oleate anion serve as surface capping ligands, which are bound strongly to the PNC surface. The tightly bound ionic pair of oleylammonium oleate and benzoyl trioctylphosphonium chloride/oleate ligands allows one to obtain monodispersed bright-blue-emitting PNCs with high photoluminescence quantum yields exceeding 50% at an optimum Sb content (0.5%), which also exhibit long-term colloidal stability. The approach based on dual cationic ligand passivation of double PNCs opens the doors for applications in other systems with a potential to achieve higher stability along with superior optical properties.

Ultrafast Dynamics of Self-Trapped Excitons in Lead-Free Perovskite Nanocrystals

Lead-free halide perovskite nanocrystals (NCs) have received increasing attention owing to their low toxicity and high stability. Localized charge distribution and strong carrier-phonon coupling in lead-free perovskite NCs facilitates the formation of self-trapped excitons (STEs), which typically give a broadband photoluminescence (PL) emission with a large Stokes shift. In this Perspective, we highlight how PL modulations can give rise to an efficient white-light emission by understanding and tuning the ultrafast dynamics of STEs in lead-free perovskite NCs. We then present the exciton energy transfer mediated by STEs to provide an efficient thermally activated delayed fluorescence and dopant PL. We also illustrate promising directions for future applications based on STEs. We hope that this Perspective can provide a new viewpoint for researchers to understand the ultrafast dynamics of STEs and promote lead-free perovskite NCs for optoelectronic applications.

State of the Art and Prospects for Halide Perovskite Nanocrystals

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.

Luminescence Change from Orange to Blue for Zero-Dimensional Cs2 InCl5 (H2 O) Metal Halides in Water and a New Post-doping Method

Zero-dimensional metal halides have attracted much attention due to their attractive photoelectric properties. Here, we propose a new strategy of synthesizing metal halides crystals by recrystallization in water. The as-synthesized Cs₂ InCl₅ (H₂ O)-orange crystals are dissolved and recrystallized in water (Cs₂ InCl₅ (H₂ O)-blue), with its photoluminescence (PL) changing from orange to blue, both of which are derived from self-trapping excitons (STEs). The time-resolved photoluminescence (TRPL) spectrum of Cs₂ InCl₅ (H₂ O)-blue shows that it has an ultralong lifetime up to milliseconds (τ=52.98 ms), which is expected to be applied in biological sensors. The photoluminescence quantum yield (PLQY) increases from 2.25% to 11.61% in the self-assembly process. By using a post-doping method, the PL of crystals turns into red when we introduce Mn²⁺ as dopant while there is no obvious change upon using a traditional solvent-thermal method. Recrystallization in water and post-doping provide a new perspective for the synthesis and doping of metal halides.

Sb-Doped Metal Halide Nanocrystals: A 0D versus 3D Comparison

We synthesize colloidal nanocrystals (NCs) of Rb₃InCl₆, composed of isolated metal halide octahedra ("0D"), and of Cs₂NaInCl₆ and Cs₂KInCl₆ double perovskites, where all octahedra share corners and are interconnected ("3D"), with the aim to elucidate and compare their optical features once doped with Sb³⁺ ions. Our optical and computational analyses evidence that the photoluminescence quantum yield (PLQY) of all these systems is consistently lower than that of the corresponding bulk materials due to the presence of deep surface traps from under-coordinated halide ions. Also, Sb-doped "0D" Rb₃InCl₆ NCs exhibit a higher PLQY than Sb-doped "3D" Cs₂NaInCl₆ and Cs₂KInCl₆ NCs, most likely because excitons responsible for the PL emission migrate to the surface faster in 3D NCs than in 0D NCs. We also observe that all these systems feature a large Stokes shift (varying from system to system), a feature that should be of interest for applications in photon management and scintillation technologies. Scintillation properties are evaluated via radioluminescence experiments, and re-absorption-free waveguiding performance in large-area plastic scintillators is assessed using Monte Carlo ray-tracing simulations.

Recent Advances in Ligand Design and Engineering in Lead Halide Perovskite Nanocrystals

Lead halide perovskite (LHP) nanocrystals (NCs) have recently garnered enhanced development efforts from research disciplines owing to their superior optical and optoelectronic properties. These materials, however, are unlike conventional quantum dots, because they possess strong ionic character, labile ligand coverage, and overall stability issues. As a result, the system as a whole is highly dynamic and can be affected by slight changes of particle surface environment. Specifically, the surface ligand shell of LHP NCs has proven to play imperative roles throughout the lifetime of a LHP NC. Recent advances in engineering and understanding the roles of surface ligand shells from initial synthesis, through postsynthetic processing and device integration, finally to application performances of colloidal LHP NCs are covered here.