Search

Simultaneously achieving both broad absorption and sharp emission in the near-infrared (NIR) is challenging. Coupling of an efficient absorber such as lead halide perovskites to lanthanide emissive species is a promising way to meet the demands for visible-to-NIR spectral conversion. However, lead-based perovskite sensitizers suffer from relatively narrow absorption in the visible range, poor stability, and toxicity. Herein, we introduce a downshifting configuration based on lead-free cesium manganese bromide nanocrystals acting as broad visible absorbers coupled to sharp emission in the NIR-I and NIR-II spectral regions. To achieve this, we synthesized CsMnBr3 and Cs3MnBr5 nanocrystals and attempted to dope them with a series of lanthanides, achieving success only with CsMnBr3. The correlation of the lanthanide emission to the CsMnBr3 visible absorption was confirmed with steady-state excitation spectra and time-resolved photoluminescence measurements, whereas the mechanism of downconversion from the CsMnBr3 matrix to the lanthanides was understood by density functional theory calculations. This study shows that lead-free metal halides with an appropriate phase are effective sensitizers for lanthanides and offer a route to efficient downshifting applications.

Colloidal chemistry grants access to a wealth of materials through simple and mild reactions. However, even few elements can combine in a variety of stoichiometries and structures, potentially resulting in impurities or even wrong products. Similar issues have been long addressed in organic chemistry by using reaction-directing groups, that are added to a substrate to promote a specific product and are later removed. Inspired by such approach, we demonstrate the use of CsPbCl3 perovskite nanocrystals to drive the phase-selective synthesis of two yet unexplored lead sulfochlorides: Pb3S2Cl2 and Pb4S3Cl2. When homogeneously nucleated in solution, lead sulfochlorides form Pb3S2Cl2 nanocrystals. Conversely, the presence of CsPbCl3 triggers the formation of Pb4S3Cl2/CsPbCl3 epitaxial heterostructures. The phase selectivity is guaranteed by the continuity of the cationic subnetwork across the interface, a condition not met in a hypothetical Pb3S2Cl2/CsPbCl3 heterostructure. The perovskite domain is then etched, delivering phase-pure Pb4S3Cl2 nanocrystals that could not be synthesized directly.

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.

We elucidate here the nature of the emissive states in Rb7Sb3Cl16 nanocrystals (NCs) for which we report a synthesis. These NCs have a crystal structure comprising both isolated [SbCl6]3- octahedra and isolated [Sb2Cl10]4- dimers of octahedra. The optical properties of Rb7Sb3Cl16 NCs are compared with those of Sb-doped Rb3InCl6 NCs, the latter containing only isolated [SbCl6]3- octahedra. The remarkably similar behaviors of the two systems indicate that the optical emission in both materials originates from the isolated octahedra. Density functional theory calculations suggest that the [SbCl6]3- octahedra are optically active in emission because the local arrangement of the Rb+ ions around the octahedra limits the elongation of the Sb-Cl bonds upon excitation, helping to confine the self-trapped exciton in them. Conversely, in the [Sb2Cl10]4- dimers the constraining effect of the surrounding Rb+ ions is less marked and the Sb-Cl bonds fully break upon photoexcitation, opening up an efficient nonradiative channel for the self-trapped excitons.

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.

We report here the synthesis of undoped and Cu-doped Cs2ZnCl4 nanocrystals (NCs) in which we could tune the concentration of Cu from 0.7 to 7.5%. Cs2ZnCl4 has a wide band gap (4.8 eV), and its crystal structure is composed of isolated ZnCl4 tetrahedra surrounded by Cs+ cations. According to our electron paramagnetic resonance analysis, in 0.7 and 2.1% Cu-doped NCs the Cu ions were present in the +1 oxidation state only, while in NCs at higher Cu concentrations we could detect Cu(II) ions (isovalently substituting the Zn(II) ions). The undoped Cs2ZnCl4 NCs were non emissive, while the Cu-doped samples had a bright intragap photoluminescence (PL) at similar to 2.6 eV mediated by band-edge absorption. Interestingly, the PL quantum yield was maximum (similar to 55%) for the samples with a low Cu concentration ([Cu] <= 2.1%), and it systematically decreased when further increasing the concentration of Cu, reaching 15% for the NCs with the highest doping level ([Cu] = 7.5%). The same (similar to 2.55 eV) emission band was detected under X-ray excitation. Our density functional theory calculations indicated that the PL emission could be ascribed only to Cu(I) ions: these ions promote the formation of trapped excitons, through which an efficient emission takes place. Overall, these Cu-doped Cs2ZnCl4 NCs, with their high photo- and radio-luminescence emission in the blue spectral region that is free from reabsorption, are particularly suitable for applications in ionizing radiation detection.

We devised a hot-injection synthesis to prepare colloidal double-perovskite Cs2NaBiCl6 nanocrystals (NCs). We also examined the effects of replacing Na+ with Ag+ cations by preparing and characterizing Cs2Na1–xAgxBiCl6 alloy NCs with x ranging from 0 to 1. Whereas Cs2NaBiCl6 NCs were not emissive, Cs2Na1–xAgxBiCl6 NCs featured a broad photoluminescence band at ∼690 nm, Stokes-shifted from the respective absorption by ≥1.5 eV. The emission efficiency was maximized for low Ag+ amounts, reaching ∼3% for the Cs2Na0.95Ag0.05BiCl6 composition. Density functional theory calculations coupled with spectroscopic investigations revealed that Cs2Na1–xAgxBiCl6 NCs are characterized by a complex photophysics stemming from the interplay of (i) radiative recombination via trapped excitons localized in spatially connected AgCl6–BiCl6 octahedra; (ii) surface traps, located on undercoordinated surface Bi centers, behaving as phonon-assisted nonradiative decay channels; and (iii) a thermal equilibrium between trapping and detrapping processes. These results offer insights into developing double-perovskite NCs with enhanced optoelectronic efficiency.

We report the composition-dependent optical properties of Bi-doped Cs2Ag1-xNaxInCl6 nanocrystals (NCs) having a double perovskite crystal structure. Their photoluminescence (PL) was characterized by a large Stokes shift, and the PL quantum yield increased with the amount of Na up to ∼22% for the Cs2Ag0.4Na0.6InCl6 stoichiometry. The presence of Bi3+ dopants was crucial to achieve high PL quantum yields (PLQYs) as nondoped NC systems were not emissive. Density functional theory calculations revealed that the substitution of Ag+ with Na+ leads to localization of AgCl6 energy levels above the valence band maximum, whereas doping with Bi3+ creates BiCl6 states below the conduction band minimum. As such, the PL emission stems from trapped emission between states localized in the BiCl6 and AgCl6 octahedra, respectively. Our findings indicated that both the partial replacement of Ag+ with Na+ ions and doping with Bi3+ cations are essential in order to optimize the PL emission of these systems.

Electron-energy-loss spectroscopy is used to map composition and electronic states in epitaxial La2/3Ca1/3MnO3 LCMO films of various thicknesses grown on SrTiO3 001 and 110 substrates. For relatively thick films 20 nm , epitaxial tensile strain in 001 films promotes a compositional La/Ca gradient across the film thickness, being the interface La rich, while the relaxed 110 films are chemically homogeneous. In contrast, much thinner 001 and 110 LCMO films display a different La/Ca distribution, being La rich at the free surface. The observed distinct thickness-dependent composition gradient behavior reflects a balance between strain-induced elastic energy minimization and kinetic effects during growth.