Direct tracking of lithium ions with time and spatial resolution can provide an important diagnostic tool for understanding mechanisms in lithium ion batteries. A fluorescent indicator of lithium ions, 2-(2-hydroxyphenyl)naphthoxazole, was synthesized and used for real-time tracking of lithium ions via widefield fluorescence microscopy. The fluorophore can be excited with visible light and was shown to enable quantitative determination of the lithium ion diffusion constant in a microfluidic model system for a plasticized polymer electrolyte lithium battery. The use of widefield fluorescence microscopy for in situ tracking of lithium ions in batteries is discussed.Keywords: fluorescence microscopy; in situ imaging; lithium ion batteries; lithium ion sensing; microfluidics; widefield imaging;

Hybrid organic–inorganic perovskites demonstrate desirable photophysical behaviors and promising applications from efficient photovoltaics to lasing, but the fundamental nature of excited state species is still under debate. We collected time-resolved photoluminescence of single-crystal nanoplates of methylammonium lead iodide perovskite (MAPbI3) with excitation over a range of fluences and repetition rates to provide a more complete photophysical picture. A fundamentally different way of simulating the photophysics is developed that relies on unnormalized decays, global analysis over a large array of conditions, and inclusion of steady-state behavior; these details are critical to capturing observed behaviors. These additional constraints require inclusion of spatially correlated pairs along with free carriers and traps, demonstrating the importance of our comprehensive analysis. Modeling geminate and nongeminate pathways shows that geminate processes are dominant at high carrier densities and early times and that geminate recombination is catalyzed by free holes. Our combination of data and simulation provides a detailed picture of perovskite photophysics across multiple excitation regimes that was not previously available.

The action of molecular catalysts comprises multiple microscopic kinetic steps whose nature is of central importance in determining catalyst activity and selectivity. Single-molecule microscopy enables the direct examination of these steps, including elucidation of molecule-to-molecule variability. Such molecular diversity is particularly important for the behavior of molecular catalysts supported at surfaces. We present the first combined investigation of the initiation dynamics of an operational palladium cross-coupling catalyst at the bulk and single-molecule levels, including under turnover conditions. Base-initiated kinetics reveal highly heterogeneous behavior indicative of diverse catalyst population. Unexpectedly, this distribution becomes more heterogeneous at increasing base concentration. We model this behavior with a two-step saturation mechanism and identify specific microscopic steps where chemical variability must exist in order to yield observed behavior. Critically, we reveal how structural diversity at a surface translates into heterogeneity in catalyst behavior, while demonstrating how single-molecule experiments can contribute to understanding of molecular catalysts.

The nature of silica surfaces is relevant to many chemical systems, including heterogeneous catalysis and chromatographies utilizing functionalized-silica stationary phases. Surface linkages must be robust to achieve wide and reliable applicability. However, silyl ether–silica support linkages are known to be susceptible to detachment when exposed to basic conditions. We use single-molecule spectroscopy to examine the rate of surface linkage failure upon exposure to base at a variety of deposition conditions. Kinetic analysis elucidates the role of thermal annealing and addition of blocking layers in increasing stability. Critically, it was found that successful surface modification strategies alter the rate at which base molecules approach the silica surface as opposed to reducing surface linkage reactivity. Our results also demonstrate that the innate structural diversity of the silica surface is likely the cause of observed heterogeneity in surface-linkage disruption kinetics.

Organic–inorganic lead iodide perovskites are efficient materials for photovoltaics and light-emitting diodes. We report carrier decay dynamics of nanorods of mixed cation formamidinium and methylammonium lead iodide perovskites [HC(NH2)2]1–x[CH3NH3]xPbI3 (FA1–xMAxPbI3) synthesized through a simple solution method. The structure and FA/MA composition ratio of the single-crystal FA1–xMAxPbI3 nanorods are fully characterized, which shows that the mixed cation FA1–xMAxPbI3 nanorods are stabilized in the perovskite structure. The photoluminescence (PL) emission from FA1–xMAxPbI3 nanorods continuously shifts from 821 to 782 nm as the MA ratio (x) increases from 0 to 1 and is shown to be inhomogeneously broadened. Time-resolved PL from individual FA1–xMAxPbI3 nanorods demonstrates that lifetimes of mixed cation FA1–xMAxPbI3 nanorods are longer than those of the pure FAPbI3 or MAPbI3 nanorods, and the FA0.4MA0.6PbI3 displays the longest average PL lifetime of about 2 μs. These results suggest that mixed cation FA1–xMAxPbI3 perovskites are promising for high-efficiency photovoltaics and other optoelectronic applications.

A series of surface-supported molecular palladium catalysts were synthesized using a dendrimeric attachment motif to incorporate multiple BODIPY fluorophores for single-molecule fluorescence microscopy. An unusual fluorescence intensity scaling law was observed, whereby the addition of multiple fluorophores did not result in a substantial increase in single-molecule brightness. Possible quenching mechanisms are discussed, and simulations of photophysical population dynamics are used to identify singlet–triplet annihilation as the likely origin of the scaling law. This work is a conspicuous example of how the availability of different photophysical kinetic pathways can have substantial influence on molecular design rules, with implications for light-harvesting strategies.

A powerful new paradigm for single-particle microscopy on nonluminescent targets is reported using ultrahigh-quality factor optical microresonators as the critical detecting element. The approach is photothermal in nature as the microresonators are used to detect heat dissipated from individual photoexcited nano-objects. The method potentially satisfies an outstanding need for single-particle microscopy on nonluminescent objects of increasingly smaller absorption cross section. Simultaneously, our approach couples the sensitivity of label-free detection using optical microresonators with a means of deriving chemical information on the target species, a significant benefit. As a demonstration, individual nonphotoluminescent multiwalled carbon nanotubes are spatially mapped, and the per-atom absorption cross section is determined. Finite-element simulations are employed to model the relevant thermal processes and elucidate the sensing mechanism. Finally, a direct pathway to the extension of this new technique to molecules is laid out, leading to a potent new method of performing measurements on individual molecules.Keywords: carbon nanotube; optical microresonators; photothermal; single-particle imaging; single-particle spectroscopy;

•New attachment chemistries for supported Pd-PEPPSI complexes implemented.•Polystyrene bead, poly(norbornene), and silica supports investigated.•Soluble palladium species responsible for catalytic activity in Suzuki coupling.•Interplay between catalyst stability and surface-support chemistry highlighted.Secure surface-molecule linkages are critical to designing recyclable surface-supported molecular catalysts. A series of N-heterocyclic carbene (NHC)-functionalized Pd-PEPPSI complexes are tethered using different linker chemistries to polymer and silica supports and examined for their reactivity in Suzuki-Miyaura cross-coupling reactions. Attachment chemistries investigated include tetrazine-norbornene cycloaddition click chemistry onto a polystyrene bead, incorporation via ring-opening metathesis polymerization (ROMP) into a poly(norbornene) polymer, and immobilization onto silica gel via silyl ether linkage. Palladium black formation suggests that the catalysts become detached from the support, and soluble palladium particles are the catalytically active species. These results help probe the manner in which the pyridine ligand of the PEPPSI complex helps to stabilize the catalysts and ultimately, how the catalysts fail. Further, we highlight the interplay between catalyst stability, recyclability, and reliable surface linkages.Download full-size image