Supramolecular Approach for Fine-Tuning of the Bright Luminescence from Zero-Dimensional Antimony(III) Halides

Halides of ns2 metal ions have recently regained broad research interest as bright narrowband and broadband emitters. Sb(III) is particularly appealing for its oxidative stability (compared to Ge2+ and Sn2+) and low toxicity (compared to Pb2+). Square pyramidal SbX5 anion had thus far been the most common structural motif for realizing high luminescence efficiency, typically when cocrystallized with an organic cation. Luminescent hybrid organic–inorganic halides with octahedral coordination of Sb(III) remain understudied, whereas fully inorganic compounds show very limited structural engineerability. We show that the host–guest complexation of alkali metal cations with crown ethers fosters the formation of zero-dimensional Sb(III) halides and allows for adjusting the coordination number (5 or 6). The obtained compounds exhibit bright photoluminescence with quantum yields of up to 89% originating from self-trapped excitons, with emission energies, Stokes shifts, and luminescence lifetimes finely-adjustable by structural engineering. A combination of environmental stability and strong, intrinsic temperature-dependence of the luminescence lifetimes in the nanosecond-to-microsecond range nominate these compounds as highly potent luminophores for remote thermometry and thermography owing to their sensitivity range of 200–450 K and high specific sensitivities of 0.04 °C–1

Solid-State NMR and NQR Spectroscopy of Lead-Halide Perovskite Materials

Two- and three-dimensional lead-halide perovskite (LHP) materials are novel semiconductors that have generated broad interest owing to their outstanding optical and electronic properties. Characterization and understanding of their atomic structure and structure–property relationships are often nontrivial as a result of the vast structural and compositional tunability of LHPs as well as the enhanced structure dynamics as compared with oxide perovskites or more conventional semiconductors. Nuclear magnetic resonance (NMR) spectroscopy contributes to this thrust through its unique capability of sampling chemical bonding element-specifically (1/2H, 13C, 14/15N, 35/37Cl, 39K, 79/81Br, 87Rb, 127I, 133Cs, and 207Pb nuclei) and locally and shedding light onto the connectivity, geometry, topology, and dynamics of bonding. NMR can therefore readily observe phase transitions, evaluate phase purity and compositional and structural disorder, and probe molecular dynamics and ionic motion in diverse forms of LHPs, in which they can be used practically, ranging from bulk single crystals (e.g., in gamma and X-ray detectors) to polycrystalline films (e.g., in photovoltaics, photodetectors, and light-emitting diodes) and colloidal nanocrystals (e.g., in liquid crystal displays and future quantum light sources). Herein we also outline the immense practical potential of nuclear quadrupolar resonance (NQR) spectroscopy for characterizing LHPs, owing to the strong quadrupole moments, good sensitivity, and high natural abundance of several halide nuclei (79/81Br and 127I) combined with the enhanced electric field gradients around these nuclei existing in LHPs as well as the instrumental simplicity. Strong quadrupole interactions, on one side, make 79/81Br and 127I NMR rather impractical but turn NQR into a high-resolution probe of the local structure around halide ions.ISSN:0002-7863ISSN:1520-512

Single crystals of caesium formamidinium lead halide perovskites: solution growth and gamma dosimetry

Formamidinium (FA)-based hybrid lead halide perovskites (FAPbX3, X=I or Br/I) have recently led to significant improvements in the performance of perovskite photovoltaics. The remaining major pitfall is the instability of α-FAPbI3, causing the phase transition from the desired three-dimensional cubic perovskite phase to a non-perovskite one-dimensional hexagonal lattice. In this work, we report the facile, inexpensive, solution-phase growth of cm-scale single crystals (SCs) of variable composition CsxFA1−xPbI3−yBry (x=0–0.1, y=0–0.6) which exhibit improved phase stability compared to the parent α-FAPbI3 compound. These SCs possess outstanding electronic quality, manifested by a high-carrier mobility–lifetime product of up to 1.2 × 10−1 cm2 V−1 and a low dark carrier density that, combined with the high absorptivity of high-energy photons by Pb and I, allows the sensitive detection of gamma radiation. With stable operation up to 30 V, these novel SCs have been used in a prototype of a gamma-counting dosimeter

Efficient Lone-Pair-Driven Luminescence: Structure–Property Relationships in Emissive 5s2 Metal Halides

Low-dimensional metal halides have been the focus of intense investigations in recent years following the success of hybrid lead halide perovskites as optoelectronic materials. In particular, the light emission of low-dimensional halides based on the 5s2 cations Sn2+ and Sb3+ has found utility in a variety of applications complementary to those of the three-dimensional halide perovskites because of its unusual properties such as broadband character and highly temperature-dependent lifetime. These properties derive from the exceptional chemistry of the 5s2 lone pair, but the terminology and explanations given for such emission vary widely, hampering efforts to build a cohesive understanding of these materials that would lead to the development of efficient optoelectronic devices. In this Perspective, we provide a structural overview of these materials with a focus on the dynamics driven by the stereoactivity of the 5s2 lone pair to identify the structural features that enable strong emission. We unite the different theoretical models that have been able to explain the success of these bright 5s2 emission centers into a cohesive framework, which is then applied to the array of compounds recently developed by our group and other researchers, demonstrating its utility and generating a holistic picture of the field from the point of view of a materials chemist. We highlight those state-of-the-art materials and applications that demonstrate the unique capabilities of these versatile emissive centers and identify promising future directions in the field of low-dimensional 5s2 metal halides

Manganese(II) in Tetrahedral Halide Environment: Factors Governing Bright Green Luminescence

Finding narrow-band light emitters for the visible spectral region remains an immense challenge. Such phosphors are in great demand for solid-state lighting and display application. In this context, green luminescence from tetrahedrally coordinated Mn(II) is an attractive research direction. While the oxide–ligand environment had been studied for decades, much less systematic efforts have been undertaken with regard to halide coordination, especially in the form of fully inorganic halide matrixes. In this study, we synthesized a series of hybrid organic–inorganic Mn(II) halides as well as a range of fully inorganic Zn halide hosts (chlorides, bromides, iodides) doped with Mn(II). In the latter, tetrahedral coordination is attained via substitutional doping owing to the tetrahedral symmetry of Zn sites. We find that the choice of the halide as well as subtle details of the crystal structure profoundly govern the photoluminescence peak positions (500–550 nm range) and emission line widths (40–60 nm) as well as radiative lifetimes (shorter for iodides) through the altered ligand-field effects and degrees of spin–orbit coupling. The photoluminescence quantum yields were as high as 70–90%. The major hurdle for the practical use of these compounds lies in their low absorption coefficients in the blue spectral regions.ISSN:0897-475

Disphenoidal Zero-Dimensional Lead, Tin, and Germanium Halides: Highly Emissive Singlet and Triplet Self-Trapped Excitons and X-ray Scintillation

Low-dimensional metal halides have been researched as optoelectronic materials for the past two decades. Zero-dimensional halides of ns2 elements (Sn, Pb, Sb) have recently gained attention as highly efficient broadband light emitters. These compounds comprise discrete metal halide centers, isolated by bulky organic cations. Herein, we report isostructural halide complexes of Ge(II), Sn(II), and Pb(II) with a 1-butyl-1-methyl-piperidinium cation (Bmpip), featuring unusual disphenoidal coordination with a highly stereoactive lone pair. Spectrally broad, bright emission from highly localized excitons, with quantum efficiencies of up to 75%, is observed in blue to red spectral regions for bromides (for Pb, Sn, and Ge, respectively) and extends into the near-infrared for Bmpip2SnI4 (peak at 730 nm). In the case of Sn(II) and Ge(II), both singlet and triplet excitonic emission bands have been observed. Furthermore, Bmpip2SnBr4 and Bmpip2PbBr4 exhibit X-ray-excited luminescence (radioluminescence) with brightness being commensurate with that of a commercial inorganic X-ray scintillator (NaI:Tl).ISSN:0002-7863ISSN:1520-512

Bright Blue and Green Luminescence of Sb(III) in Double Perovskite Cs2MInCl6 (M = Na, K) Matrices

The vast structural and compositional space of metal halides has recently become a major research focus for designing inexpensive and versatile light sources; in particular, for applications in displays, solid-state lighting, lasing, etc. Compounds with isolated ns2-metal halide centers often exhibit bright broadband emission that stems from self-trapped excitons (STEs). The Sb(III) halides are attractive STE emitters due to their low toxicity and oxidative stability; however, coupling these features with an appropriately robust, fully inorganic material containing Sb3+ in an octahedral halide environment has proven to be a challenge. Here, we investigate Sb3+ as a dopant in a solution-grown metal halide double perovskite (DP) matrix, namely Cs2MInCl6:xSb (M = Na, K, x = 0–100%). Cs2KInCl6 is found to crystallize in the tetragonal DP phase, unlike Cs2NaInCl6 that adopts the traditional cubic DP structure. This structural difference results in distinct emission colors, as Cs2NaInCl6:xSb and Cs2KInCl6:xSb compounds exhibit broadband blue and green emissions, respectively, with photoluminescence quantum yields (PLQYs) of up to 93%. Spectroscopic and computational investigations confirm that this efficient emission originates from Sb(III)-hosted STEs. These fully inorganic DP compounds demonstrate that Sb(III) can be incorporated as a bright emissive center for stable lighting applications.ISSN:0897-475

White CsPbBr3: Characterizing the One‐Dimensional Cesium Lead Bromide Polymorph

Inorganic lead halide perovskites have gained immense scientific interest for optoelectronic applications. In this work, we present a one‐dimensional polymorph of cesium lead bromide (δ‐CsPbBr3) synthesized through a simple anion‐exchange reaction, wherein distorted edge‐sharing PbBr6 octahedra form 1D chains isolated by Cs ions. δ‐CsPbBr3 was characterized by Raman spectroscopy, X‐ray diffraction, 207Pb and 133Cs solid‐state NMR, and by optical emission and absorption spectroscopies. This non‐perovskite material irreversibly transforms into the well‐known three‐dimensional perovskite phase (γ‐CsPbBr3) upon heating to above 151 °C. The indirect bandgap was determined by absorption measurements and calculation to be 2.9 eV. δ‐CsPbBr3 exhibits broadband yellow photoluminescence with a quantum yield of 3.2 %±0.2 % at room temperature and 95 %±5 % at 77 K, and this emission is attributed to the recombination of self‐trapped excitons. This study emphasizes that the metastable δ‐CsPbBr3 may be a persistent, concomitant phase in Cs−Pb‐Br‐containing materials systems, such as those used in solar cells and LEDs, and it showcases the characterization tools used for its detection.ISSN:0018-019XISSN:1522-267

White CsPbBr 3

Inorganic lead halide perovskites have gained immense scientific interest for optoelectronic applications. In this work, we present a one‐dimensional polymorph of cesium lead bromide (δ‐CsPbBr3) synthesized through a simple anion‐exchange reaction, wherein distorted edge‐sharing PbBr6 octahedra form 1D chains isolated by Cs ions. δ‐CsPbBr3 was characterized by Raman spectroscopy, X‐ray diffraction, 207Pb and 133Cs solid‐state NMR, and by optical emission and absorption spectroscopies. This non‐perovskite material irreversibly transforms into the well‐known three‐dimensional perovskite phase (γ‐CsPbBr3) upon heating to above 151 °C. The indirect bandgap was determined by absorption measurements and calculation to be 2.9 eV. δ‐CsPbBr3 exhibits broadband yellow photoluminescence with a quantum yield of 3.2 %±0.2 % at room temperature and 95 %±5 % at 77 K, and this emission is attributed to the recombination of self‐trapped excitons. This study emphasizes that the metastable δ‐CsPbBr3 may be a persistent, concomitant phase in Cs−Pb‐Br‐containing materials systems, such as those used in solar cells and LEDs, and it showcases the characterization tools used for its detection.ISSN:0018-019XISSN:1522-267

Hybrid Metal Halides with Multiple Photoluminescence Centers

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