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Ytterbium doping in all-inorganic lead-halide perovskites [CsPb(Cl1−xBrx )3] generates interesting properties including quantum cutting and narrow line emission, making these materials attractive spectral down converters for solar photovoltaics. The relationship between this optical efficiency and the defect structure(s) associated with Yb3+ dopants within perovskites is not well understood. Various charge-neutral doping motifs have previously been proposed and studied computationally, including clusters involving two substitutional Yb3+ ions charge compensated by a single local Pb2+ vacancy. Near-band-edge defect states associated with such motifs are believed to play an important mechanistic role in quantum cutting itself. Here, we report the results of x-ray absorption and x-ray total-scattering measurements on ytterbium-doped CsPbCl3. XANES shows that the dopant oxidation state is exclusively Yb3+, and a combination of Yb L3 and Pb L3 extended x-ray absorption fine structure (EXAFS) shows that this Yb3+ substitutes exclusively at Pb2+ sites, where it adopts a pseudo-octahedral [YbCl6] 3− coordination environment. Shell-by-shell fits to the data show a short Yb-Cl bond distance of 2.58 Å compared to the Pb-Cl bond distance of 2.83 Å. We confirm this finding by x-ray pair distribution function analysis, which also shows evidence of additional Pb2+ vacancy formation induced by Yb3+ doping. We evaluate whether this is the primary mechanism of charge compensation using simulated EXAFS and pair distribution function data for several computed defect structures. Together, these results resolve the local dopant structures and charge-compensation mechanisms in lanthanide-doped all-inorganic lead-halide perovskites, and, as such, significantly advance the understanding of structure-function relationships in this important class of materials.

Metal-halide semiconductors exhibit attractive properties for a host of applications including photovoltaics, solid-state lighting, and photodetection. Among the remarkable recent developments is t...

${\mathrm{Yb}}^{3+}$-doped inorganic metal-halide perovskites $(\mathrm{Y}{\mathrm{b}}^{3+}:\mathrm{CsPb}{X}_{3}$, $X=\mathrm{Cl},\mathrm{Br})$ have recently been discovered to display highly efficient quantum cutting, in which the energy from individual blue or UV photons absorbed by the material is reemitted in the form of pairs of near-infrared photons by ${\mathrm{Yb}}^{3+}$ dopants. Experimental photoluminescence quantum yields approaching 200% have been reported. As the first quantum-cutting materials that combine such high-photoluminescence quantum yields with strong, broadband absorption in the visible, these materials offer unique opportunities for enhancing the efficiencies of solar technologies. Little is known about the fundamental origins of this quantum cutting, however. Here, we describe variable-temperature and time-resolved photoluminescence studies of ${\mathrm{Yb}}^{3+}:\mathrm{CsPb}{\mathrm{Cl}}_{3}$ in two disparate forms--colloidal nanocrystals and macroscopic single crystals. Both forms show very similar spectroscopic properties, demonstrating that quantum cutting is an intrinsic property of the ${\mathrm{Yb}}^{3+}:\mathrm{CsPb}{X}_{3}$ composition itself. Diverse ${\mathrm{Yb}}^{3+}$ speciation is observed in both forms by low-temperature photoluminescence spectroscopy, but remarkably, quantum cutting is dominated by the same specific ${\mathrm{Yb}}^{3+}$ species in both cases. Time-resolved photoluminescence measurements provide direct evidence of the previously hypothesized intermediate state in the quantum-cutting mechanism. This intermediate state mediates relaxation from the photogenerated excited state of the perovskite to the emissive excited state of ${\mathrm{Yb}}^{3+}$, and hence is of critical mechanistic importance. At room temperature, this intermediate state is populated within a few picoseconds and has a decay time of only \ensuremath{\sim}7 ns in both nanocrystalline and single-crystal ${\mathrm{Yb}}^{3+}:\mathrm{CsPb}{\mathrm{Cl}}_{3}$. The mechanistic implications of these observations are discussed. These results provide valuable information about characteristics of this unique quantum cutter that will aid its optimization and application in solar technologies.

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, low-cost hydrothermal synthesis of potassium lutetium fluoride (KLF) microcrystals for applications in solid-state laser refrigeration. Four crystalline phases were synthesized, namely orthorhombic K2LuF5 (Pnma), trigonal KLuF4 (P3121), orthorhombic KLu2F7 (Pna21), and cubic KLu3F10 (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:KLuF4 with cooling of 8.6 $\pm$ 2.1 K below room temperature. Reduced heating was observed with 10%Yb:K2LuF5

Contrary to multicellular organisms that display segmentation during development, communities of unicellular organisms are believed to be devoid of such sophisticated patterning. Unexpectedly, we find that the gene expression underlying the nitrogen stress response of a developing Bacillus subtilis biofilm becomes organized into a ring-like pattern. Mathematical modeling and genetic probing of the underlying circuit indicate that this patterning is generated by a clock and wavefront mechanism, similar to that driving vertebrate somitogenesis. We experimentally validated this hypothesis by showing that predicted nutrient conditions can even lead to multiple concentric rings, resembling segments. We additionally confirmed that this patterning mechanism is driven by cell-autonomous oscillations. Importantly, we show that the clock and wavefront process also spatially patterns sporulation within the biofilm. Together, these findings reveal a biofilm segmentation clock that organizes cellular differentiation in space and time, thereby challenging the paradigm that such patterning mechanisms are exclusive to plant and animal development. We acknowledge Munehiro Asally, Tolga Çağatay, and Katherine Süel for helpful discussions. K.-T.C. acknowledges support from National Institutes of Health grant T32GM127235. J.G.-O. acknowledges support from the Spanish Ministry of Science, Innovation and Universities and FEDER (Project PGC2018-101251-B-I00 and CEX2018-000792-M), and from the Generalitat de Catalunya (ICREA Academia program). G.M.S. acknowledges support for this research from National Institute of General Medical Sciences grants R01 GM121888 and R35 GM139645. G.M.S. is a Howard Hughes Medical Institute - Simons Foundation Faculty Scholar.

A two-step solution-deposition method for preparing ytterbium-doped (Yb3+) CsPb(Cl1–xBrx)3 perovskite thin films is described. Yb3+-doped CsPb(Cl1–xBrx)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–xBrx)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.

Bacteria exhibit cell-to-cell variability in their resilience to stress, for example, following antibiotic exposure. Higher resilience is typically ascribed to "dormant" non-growing cellular states. Here, by measuring membrane potential dynamics of Bacillus subtilis cells, we show that actively growing bacteria can cope with ribosome-targeting antibiotics through an alternative mechanism based on ion flux modulation. Specifically, we observed two types of cellular behavior: growth-defective cells exhibited a mathematically predicted transient increase in membrane potential (hyperpolarization), followed by cell death, whereas growing cells lacked hyperpolarization events and showed elevated survival. Using structural perturbations of the ribosome and proteomic analysis, we uncovered that stress resilience arises from magnesium influx, which prevents hyperpolarization. Thus, ion flux modulation provides a distinct mechanism to cope with ribosomal stress. These results suggest new approaches to increase the effectiveness of ribosome-targeting antibiotics and reveal an intriguing connection between ribosomes and the membrane potential, two fundamental properties of cells. This work was supported by funding from The Spanish Ministry of Economy and Competitiveness and FEDER (project FIS2015-66503-C3-1-P) (to J.G.-O.), the ICREA Academia program (to J.G.-O.), and the Maria de Maeztu Program for Units of Excellence in Research and Development (Spanish Ministry of Economy and Competitiveness, MDM-2014-0370) (to J.G.-O.), the San Diego Center for Systems Biology (NIH P50 GM085764) (to G.M.S), National Institute of General Medical Sciences (R01 GM121888) (to G.M.S), and the Howard Hughes Medical Institute-Simons Foundation Faculty Scholars program (to G.M.S.).

SummaryGenes encoding proteins in a common regulatory network are frequently located close to one another on the chromosome to facilitate co-regulation or couple gene expression to growth rate. Contrasting with these observations, here, we demonstrate a functional role for the arrangement of Bacillus subtilis sporulation network genes on opposite sides of the chromosome. We show that the arrangement of two sporulation network genes, one located close to the origin and the other close to the terminus, leads to a transient gene dosage imbalance during chromosome replication. This imbalance is detected by the sporulation network to produce cell-cycle coordinated pulses of the sporulation master regulator Spo0A∼P. This pulsed response allows cells to decide between sporulation and continued vegetative growth during each cell cycle spent in starvation. The simplicity of this coordination mechanism suggests that it may be widely applicable in a variety of gene regulatory and stress-response settings.Video Abstract

The role of electricity in biological systems was first appreciated through electrical stimulation experiments performed by Luigi Galvani in the 18th century. These pioneering experiments demonstrated that the behavior of living tissues is governed by the flow of electrochemical species-an insight that gave rise to the modern field of electrophysiology. Since then, electrophysiology has largely remained a bastion of neuroscience. However, exciting recent developments have demonstrated that even simple bacteria residing in communities use electrochemical communication to coordinate population-level behaviors. These recent works are defining the emerging field of bacterial biofilm electrophysiology. To view this SnapShot, open or download the PDF.

Bacteria within communities can interact to organize their behavior. It has been unclear whether such interactions can extend beyond a single community to coordinate the behavior of distant populations. We discovered that two Bacillus subtilis biofilm communities undergoing metabolic oscillations can become coupled through electrical signaling and synchronize their growth dynamics. Coupling increases competition by also synchronizing demand for limited nutrients. As predicted by mathematical modeling, we confirm that biofilms resolve this conflict by switching from in-phase to antiphase oscillations. This results in time-sharing behavior, where each community takes turns consuming nutrients. Time-sharing enables biofilms to counterintuitively increase growth under reduced nutrient supply. Distant biofilms can thus coordinate their behavior to resolve nutrient competition through time-sharing, a strategy used in engineered systems to allocate limited resources. This work was in part supported by the San Diego Center for Systems Biology (NIH grant P50 GM085764), the National Institute of General Medical Sciences (grant R01 GM121888 to G.M.S.), the Defense Advanced Research Projects Agency (grant HR0011-16-2-0035 to G.M.S.), the Howard Hughes Medical Institute–Simons Foundation Faculty Scholars program (to G.M.S.), a Simons Foundation Fellowship of the Helen Hay Whitney Foundation (to A.P.), a Career Award at the Scientific Interface from the Burroughs Wellcome Fund (to A.P.), the Spanish Ministry of Economy and Competitiveness and FEDER (European Regional Development Fund) (project FIS2015-66503-C3-1-P, to J.G.-O.), the Generalitat de Catalunya (project 2014SGR0947, to J.G.-O.), the ICREA (Catalan Institution for Research and Advanced Studies) Academia program (to J.G.-O.), the “Maria de Maeztu” Programme for Units of Excellence in Research and Development (Spanish Ministry of Economy and Competitiveness, grant MDM-2014-0370 to J.G.-O.), the La Caixa foundation (to R.M.-C.), and a doctoral grant from the Formacion del Profesorado Universitario program of the Ministerio de Educacion, Cultura y Deportes, Spain (to M.G.-S.)