A two-step solution-deposition method for preparing ytterbium-doped (Yb3+) CsPb­(Cl1–x Br x )3 perovskite thin films is described. Yb3+-doped CsPb­(Cl1–x Br x )3 films are made that exhibit intense near-infrared photoluminescence with extremely high quantum yields reaching over 190%, stemming from efficient quantum cutting that generates two emitted near-infrared photons for each absorbed visible photon. The near-infrared Yb3+ f–f photoluminescence is largely independent of the anion content (x) in CsPb­(Cl1–x Br x )3 films with energy gaps above the quantum-cutting threshold of twice the Yb3+ f–f transition energy, but it decreases abruptly when the perovskite energy gap becomes too small to generate two Yb3+ excitations. Excitation power dependence measurements show facile saturation of the Yb3+ luminescence intensity, identifying a major challenge for future solar applications of these materials.

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Metal-halide semiconductors exhibit attractive properties for a host of applications including photovoltaics, solid-state lighting, and photodetection. Among the remarkable recent developments is the discovery of extraordinarily high photoluminescence quantum yields in Yb3+-doped inorganic lead-halide perovskites. Although all previous research and development of such quantum-cutting materials has involved solution-phase preparation, particularly as colloidal nanocrystals, such methods can introduce both processing and technical challenges that limit the scope of accessible compositions, morphologies, and scaled-up applications. Here, we demonstrate a scalable single-source vapor deposition (SSVD) method for depositing high-quality conformal thin films of complex metal-halide perovskites, including doped perovskites, over large areas at high deposition rates. Focusing on quantum-cutting Yb3+:CsPb­(Cl1–x Br x )3, we demonstrate large-area deposition of films with photoluminescence quantum yields as high as 183%, starting from single-source powders prepared mechanochemically from solid ionic precursors. We also prepare thin films of the solar absorber material (FA0.81MA0.14Cs0.05)­Pb­(Cl0.02Br0.14I0.84)3 to illustrate the generality of this SSVD method. These results demonstrate a promising approach to high-throughput vapor processing of metal-halide coatings for photonic and optoelectronic applications.

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CsPb(Cl 1−x Br x ) 3 (0 ≤ x ≤ 1) nanocrystals and thin films doped with a series of trivalent rare-earth ions (RE 3+ = Y 3+ , La 3+ , Ce 3+ , Gd 3+ , Er 3+ , Lu 3+ ) have been prepared and studied using variable-temperature and time-resolved photoluminescence spectroscopies. We demonstrate that aliovalent (trivalent) doping of this type universally generates a new and oftenemissive defect state ca. 50 meV inside the perovskite band gap, independent of the specific RE 3+ dopant identity or of the perovskite form (nanocrystals vs thin films). Chloride-to-bromide anion exchange is used to demonstrate that this near-band-edge photoluminescence shifts with changing band-gap energy to remain just below the excitonic luminescence for all compositions of CsPb(Cl 1−x Br x ) 3 (0 ≤ x ≤ 1). Computations show that this shift stems from the effect of the changing lattice dielectric constants on a shallow defect-bound exciton. Microscopic descriptions of this dopant-induced near-band-edge state and its relation to quantum cutting in Yb 3+ -doped CsPb(Cl 1−x Br x ) 3 are discussed.

A novel treatment flowchart approach for surface-enhanced Raman scattering (SERS) is used to identify both blue and yellow organic pigments in a single microscopic sample from a series of reference oil paints as well as an actual 18th century oil painting. In particular, several treatment strategies using acids and solvents are integrated into a specific flowchart designed to enable the minimally invasive identification of unknown blue (i.e., indigo, Prussian blue) and yellow organic (i.e., Reseda lake, Stil de Grain, gamboge) pigments in one sample. We demonstrate the first successful identification of a yellow lake pigment in a historic painting using SERS as well as the utility of our treatment flowchart approach for identifying pigments of varying resonance conditions, surface affinities, and treatment requirements in a single microscopic sample from a historic oil painting.

Fluoride crystals, due to their low phonon energies, are attractive hosts of trivalent lanthanide ions for applications in upconverting phosphors, quantum information science, and solid-state laser refrigeration. In this article, we report the rapid, lowcost hydrothermal synthesis of potassium lutetium fluoride (KLF) microcrystals for applications in solid-state laser refrigeration. Four crystalline phases were synthesized, namely orthorhombic K 2 LuF 5 (Pnma), trigonal KLuF 4 (P3 1 21), orthorhombic 1 KLu 2 F 7 (Pna2 1 ), and cubic KLu 3 F 10 (Fm3m), with each phase exhibiting unique microcrystalline morphologies. Luminescence spectra and emission lifetimes of the four crystalline phases were characterized based on the point-group symmetry of trivalent cations. Laser refrigeration was measured by observing both the optomechanical eigenfrequencies of microcrystals on cantilevers in vacuum, and also the Brownian dynamics of optically trapped microcrystals in water. Among all four crystalline phases, the most significant cooling was observed for 10%Yb:KLuF 4 with cooling of 8.6 ± 2.1 K below room temperature. Reduced heating was observed with 10%Yb:K 2 LuF 5

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