Jamie C. Wang

Bright Light‐Emitting Diodes Based on Organometal Halide Perovskite Nanoplatelets

Bright light-emitting diodes based on solution-processable organometal halide perovskite nanoplatelets are demonstrated. The nanoplatelets created using a facile one-pot synthesis exhibit narrow-band emissions at 529 nm and quantum yield up to 85%. Using these nanoparticles as emitters, efficient electroluminescence is achieved with a brightness of 10 590 cd m(-2) . These ligand-capped nanoplatelets appear to be quite stable in moisture, allowing out-of-glovebox device fabrication.

Enhanced Optical and Electrical Properties of Polymer‐Assisted All‐Inorganic Perovskites for Light‐Emitting Diodes

Highly bright light-emitting diodes based on solution-processed all-inorganic perovskite thin film are demonstrated. The cesium lead bromide (CsPbBr3 ) created using a new poly(ethylene oxide)-additive spin-coating method exhibits photoluminescence quantum yield up to 60% and excellent uniformity of electrical current distribution. Using the smooth CsPbBr3 films as emitting layers, green perovskite-based light-emitting diodes (PeLEDs) exhibit electroluminescent brightness and efficiency above 53 000 cd m-2 and 4%: a new benchmark of device performance for all-inorganic PeLEDs.

One-dimensional organic lead halide perovskites with efficient bluish white-light emission.

Organic-inorganic hybrid metal halide perovskites, an emerging class of solution processable photoactive materials, welcome a new member with a one-dimensional structure. Herein we report the synthesis, crystal structure and photophysical properties of one-dimensional organic lead bromide perovskites, C4N2H14PbBr4, in which the edge sharing octahedral lead bromide chains [PbBr4 2−]∞ are surrounded by the organic cations C4N2H14 2+ to form the bulk assembly of core-shell quantum wires. This unique one-dimensional structure enables strong quantum confinement with the formation of self-trapped excited states that give efficient bluish white-light emissions with photoluminescence quantum efficiencies of approximately 20% for the bulk single crystals and 12% for the microscale crystals. This work verifies once again that one-dimensional systems are favourable for exciton self-trapping to produce highly efficient below-gap broadband luminescence, and opens up a new route towards superior light emitters based on bulk quantum materials.

Characterization of berkelium(III) dipicolinate and borate compounds in solution and the solid state

Bonding to berkelium A geographical theme prevailed in the recent naming of the heaviest chemical elements. The choices brought to mind berkelium (Bk) and californium (Cf), the names chosen for elements 97 and 98 over half a century ago. Silver et al. now revisit the chemistry of Bk, which has proven fiercely challenging to study over the years on account of its vigorous radioactive decay. Synthetic crystallized Bk borate and dipicolinate compounds structurally resembled Cf analogs in the solid state but manifested distinct electronic and magnetic characteristics stemming from spin-orbit coupling effects. Science, this issue p. 888 Experiments and theory probe the coordination chemistry of a highly radioactive heavy element. INTRODUCTION Developing the chemistry of late actinides is hindered by the lack of availability of isotopes, the need for specialized research facilities, and the nuclear instability of the elements. Berkelium represents one of the last elements that can be prepared on a milligram scale in nuclear reactors. However, its only available isotope, 249Bk, has a half-life of only 320 days, which has greatly curtailed the expansion of its chemistry and fundamental exploration of how large relativistic and spin-orbit coupling effects alter its electronic structure. Furthermore, data gathered from Bk(III) in aqueous media suggest that its coordination may be different from that of earlier actinides. However, a single-crystal structure of a berkelium compound has remained elusive, leaving unanswered whether these structural changes occur in the solid state. RATIONALE This work focuses on characterizing two distinct berkelium compounds on the milligram scale. In particular, the goal was to obtain crystals of these compounds that could be used in structure determinations and physical property measurements. Two compounds were selected: a coordination complex of dipicolinate and a borate. Dipicolinate complexation occurs with most other lanthanides and actinides in the +3 oxidation state, facilitating comparisons across the series to discern periodic trends. In the borate family, the structural frameworks are hypersensitive to the nature of the bonding at the metal center and are rearranged accordingly. Modeling the experimental data using a variety of computational techniques allows us to deconvolute the role of covalent bonding and spin-orbit coupling in determining the electronic properties of berkelium. RESULTS Experiments with milligram quantities of 249Bk were choreographed for 6 months before the arrival of the isotope because the total quantity used in the studies was 13 mg, which corresponds to a specific activity of 21 Ci. Although this isotope is a low-energy β emitter, it decays to 249Cf at a rate of about 1.2% per week, and the latter produces hard γ radiation that represents a serious external hazard. In addition, the samples described in this work undergo about 1012 decays per second. This rapid decomposition necessitated the development of techniques for swiftly preparing and encapsulating samples and for collecting all structural and spectroscopic data within 24 hours of crystal formation. After this preparation, the single-crystal structures of Bk(III)tris(dipicolinate) and Bk(III) borate were determined. The latter compound has the same topology as that of californium(III) (Cf) and contains an eight-coordinate BkO8 unit. This reduction in coordination number is consistent with previous solution-phase x-ray absorption measurements and indicates that a drop in coordination number in the actinide series from nine to eight begins at berkelium. The magnetic and optical properties of these samples were also measured. The red luminescence from Bk(III) was similar in nature to that of curium(III) and is primarily based on an f-f transition. The ingrowth of the broad green luminescence from Cf(III), which is caused by a ligand-to-metal charge transfer, was shown to be distinct in nature from that originating from Bk(III). Ligand-field, density functional theory, and wave-function calculations were used to understand the spectroscopic features and revealed that the single largest contributor to the unexpected electronic properties of Bk(III) is spin-orbit coupling. This effect mixes the first excited state with the ground state and causes a large deviation from a pure Russell-Saunders state. The reduction in the measured magnetic moment for these samples from that calculated for an f8 electron configuration is primarily attributable to this multiconfigurational ground state. CONCLUSION The crystallographic data indicate that Bk(III) shares more structural similarities with Cf(III) than with Cm(III). However, ligand-field effects are more similar between Bk(III) and Cm(III). Terbium (Tb), in the lanthanide series, represents the closest analog of Bk because the trivalent cations possess 4f8 and 5f8 configurations, respectively. Spin-orbit coupling in Bk(III) creates mixing of the first excited state (5G6) with the ground state. In contrast, the ground state of the Tb(III)tris(dipicolinate) contains negligible contributions of this type. An overall conclusion from this study is that spin-orbit coupling plays a large role in determining the ground state of late actinide compounds. Crystal structure of a berkelium coordination compound. The central Bk(III) ion is coordinated by three monoprotonated dipicolinate ligands in tridentate O,N,O fashion. Bk, yellow; C, gray; N, blue; O, red; H, white. Berkelium is positioned at a crucial location in the actinide series between the inherently stable half-filled 5f7 configuration of curium and the abrupt transition in chemical behavior created by the onset of a metastable divalent state that starts at californium. However, the mere 320-day half-life of berkelium’s only available isotope, 249Bk, has hindered in-depth studies of the element’s coordination chemistry. Herein, we report the synthesis and detailed solid-state and solution-phase characterization of a berkelium coordination complex, Bk(III)tris(dipicolinate), as well as a chemically distinct Bk(III) borate material for comparison. We demonstrate that berkelium’s complexation is analogous to that of californium. However, from a range of spectroscopic techniques and quantum mechanical calculations, it is clear that spin-orbit coupling contributes significantly to berkelium’s multiconfigurational ground state.

Low Dimensional Organic Tin Bromide Perovskites and Their Photoinduced Structural Transformation

Hybrid organic-inorganic metal halide perovskites possess exceptional structural tunability, with three- (3D), two- (2D), one- (1D), and zero-dimensional (0D) structures on the molecular level all possible. While remarkable progress has been realized in perovskite research in recent years, the focus has been mainly on 3D and 2D structures, with 1D and 0D structures significantly underexplored. The synthesis and characterization of a series of low-dimensional organic tin bromide perovskites with 1D and 0D structures is reported. Using the same organic and inorganic components, but at different ratios and reaction conditions, both 1D (C4 N2 H14 )SnBr4 and 0D (C4 N2 H14 Br)4 SnBr6 can be prepared in high yields. Moreover, photoinduced structural transformation from 1D to 0D was investigated experimentally and theoretically in which photodissociation of 1D metal halide chains followed by structural reorganization leads to the formation of a more thermodynamically stable 0D structure.

Inhibiting Interfacial Recombination Events in Dye-Sensitized Solar Cells using Self-Assembled Bilayers

The rate and efficiency of electron transfer events at the semiconductor-dye-electrolyte interface is of critical importance to the overall performance of dye-sensitized solar cells (DSSCs) and dye-sensitized photoelectrosynthesis cells. In this work, we introduce self-assembled bilayers composed of a metal oxide electrode, bridging molecules, linking ions, and dye as an effective strategy to manipulate interfacial electron transfer events at the photoanode of DSSCs. Spectroelectrochemical measurements including current-voltage, incident photon-to-current efficiency, and electrochemical impedance spectroscopy are used to quantify interfacial electron transfer and transport events with respect to the length of the bridging molecules. The general trend in increased lifetime and diffusion length in TiO2 as well as an increase in open circuit voltage with bridge length indicate that the bilayer is an effective strategy in inhibiting the TiO2(e(-)) to redox mediator recombination events. However, the increased separation between the dye and the semiconductor also reduces the electron injection rate resulting in a decrease in photocurrent as the bridge length increases. The observed enhancement in open circuit voltages are far outweighed by the significant decrease in photocurrent and thus overall device performance decreases with increasing bridge length.

Spontaneous Partitioning of Californium from Curium: Curious Cases from the Crystallization of Curium Coordination Complexes

The reaction of (248)CmCl3 with excess 2,6-pyridinedicarboxylic acid (DPA) under mild solvothermal conditions results in crystallization of the tris-chelate complex Cm(HDPA)3 · H2O. Approximately half of the curium remains in solution at the end of this process, and evaporation of the mother liquor results in crystallization of the bis-chelate complex [Cm(HDPA)(H2DPA)(H2O)2Cl]Cl·2H2O. (248)Cm is the daughter of the α decay of (252)Cf and is extracted in high purity from this parent. However, trace amounts of (249,250,251)Cf are still present in all samples of (248)Cm. During the crystallization of Cm(HDPA)3 · H2O and [Cm(HDPA)(H2DPA)(H2O)2Cl]Cl · 2H2O, californium(III) spontaneously separates itself from the curium complexes and is found doped within crystals of DPA in the form of Cf(HDPA)3. These results add to the growing body of evidence that the chemistry of californium is fundamentally different from that of earlier actinides.

Energy and Electron Transfer Cascade in Self-Assembled Bilayer Dye-Sensitized Solar Cells

Current high efficiency dye-sensitized solar cells (DSSCs) rely on the incorporation of multiple chromophores, via either codeposition or preformed assemblies, as a means of increasing broad band light absorption. These strategies have some inherent limitations including decreased total light absorption by each of the dyes, low surface loadings, and complex synthetic procedures. In this report, we introduce an alternative strategy, self-assembled bilayers, as a simple, stepwise method of incorporating two complementary chromophores into a DSSC. The bilayer devices exhibit a 10% increase in Jsc, Voc, and η over the monolayer devices due to increased incident photon-to-electron conversion efficiency across the entire visible spectrum and slowed recombination losses at the interface. Directional energy and electron transfer toward the metal oxide surface are key steps in the bilayer photon-to-current generation process. These results are important as they open the door to a new architecture for harnessing broadband light in dye-sensitized devices.

Electronic Structure and Properties of Berkelium Iodates

The reaction of 249Bk(OH)4 with iodate under hydrothermal conditions results in the formation of Bk(IO3)3 as the major product with trace amounts of Bk(IO3)4 also crystallizing from the reaction mixture. The structure of Bk(IO3)3 consists of nine-coordinate BkIII cations that are bridged by iodate anions to yield layers that are isomorphous with those found for AmIII, CfIII, and with lanthanides that possess similar ionic radii. Bk(IO3)4 was expected to adopt the same structure as M(IO3)4 (M = Ce, Np, Pu), but instead parallels the structural chemistry of the smaller ZrIV cation. BkIII-O and BkIV-O bond lengths are shorter than anticipated and provide further support for a postcurium break in the actinide series. Photoluminescence and absorption spectra collected from single crystals of Bk(IO3)4 show evidence for doping with BkIII in these crystals. In addition to luminescence from BkIII in the Bk(IO3)4 crystals, a broad-band absorption feature is initially present that is similar to features observed in systems with intervalence charge transfer. However, the high-specific activity of 249Bk (t1/2 = 320 d) causes oxidation of BkIII and only BkIV is present after a few days with concomitant loss of both the BkIII luminescence and the broadband feature. The electronic structure of Bk(IO3)3 and Bk(IO3)4 were examined using a range of computational methods that include density functional theory both on clusters and on periodic structures, relativistic ab initio wave function calculations that incorporate spin-orbit coupling (CASSCF), and by a full-model Hamiltonian with spin-orbit coupling and Slater-Condon parameters (CONDON). Some of these methods provide evidence for an asymmetric ground state present in BkIV that does not strictly adhere to Russel-Saunders coupling and Hunds Rule even though it possesses a half-filled 5f 7 shell. Multiple factors contribute to the asymmetry that include 5f electrons being present in microstates that are not solely spin up, spin-orbit coupling induced mixing of low-lying excited states with the ground state, and covalency in the BkIV-O bonds that distributes the 5f electrons onto the ligands. These factors are absent or diminished in other f7 ions such as GdIII or CmIII.

Self-Assembled Bilayers on Nanocrystalline Metal Oxides: Exploring the Non-Innocent Nature of the Linking Ions

Self-assembled bilayers on nanocrystalline metal oxide films are an increasingly popular strategy for modulating electron and energy transfer at dye-semiconductor interfaces. A majority of the work to date has relied on ZrII and ZnIV linking ions to assemble the films. In this report, we demonstrate that several different cations (CdII, CuII, FeII, LaIII, MnII, and SnIV) are not only effective in generating the bilayer assemblies but also have a profound influence on the stability and photophysical properties of the films. Bilayer films with ZrIV ions exhibited the highest photostability on both TiO2 and ZrO2. Despite the metal ions having a minimal influence on the absorption/emission energies and oxidation potentials of the dye, bilayers composed of CuII, FeII, and MnII exhibit significant excited-state quenching. The excited-state quenching decreases the electron injection yield but also, for CuII and MnII bilayers, significantly slows the back electron transfer kinetics.