Metal Fluorides Passivate II-VI and III-V Quantum Dots

Quantum dots (QDs) with metal fluoride surface ligands were prepared via reaction with anhydrous oleylammonium fluoride. Carboxylate terminated II-VI QDs underwent carboxylate for fluoride exchange, while InP QDs underwent photochemical acidolysis yielding oleylamine, PH3, and InF3. The final photoluminescence quantum yield (PLQY) reached 83% for InP and near unity for core-shell QDs. Core-only CdS QDs showed dramatic improvements in PLQY, but only after exposure to air. Following etching, the InP QDs were bound by oleylamine ligands that were characterized by the frequency and breadth of the corresponding ν(N-H) bands in the infrared absorption spectrum. The fluoride content (1.6-9.2 nm-2) was measured by titration with chlorotrimethylsilane and compared with the oleylamine content (2.3-5.1 nm-2) supporting the formation of densely covered surfaces. The influence of metal fluoride adsorption on the air stability of QDs is discussed.

Synthesis of graded CdS1-xSex nanoplatelet alloys and heterostructures from pairs of chalcogenoureas with tailored conversion reactivity

A mixture of N,N,N'-trisubstituted thiourea and cyclic N,N,N',N'-tetrasubstituted selenourea precursors were used to synthesize three monolayer thick CdS1-xSex nanoplatelets in a single synthetic step. The microstructure of the nanoplatelets could be tuned from homogeneous alloys, to graded alloys to core/crown heterostructures depending on the relative conversion reactivity of the sulfur and selenium precursors. UV-visible absorption and photoluminescence spectroscopy and scanning transmission electron microscopy electron energy loss spectroscopy (STEM-EELS) images demonstrate that the elemental distribution is governed by the relative precursor conversion kinetics. Slow conversion kinetics produced nanoplatelets with larger lateral dimensions, behavior that is characteristic of precursor conversion limited growth kinetics. Across a 10-fold range of reactivity, CdS nanoplatelets have 4× smaller lateral dimensions than CdSe nanoplatelets grown under identical conversion kinetics. The difference in size is consistent with a rate of CdSe growth that is 4× greater than the rate of CdS. The influence of the relative sulfide and selenide growth rates, the duration of the nucleation phase, and the solute composition on the nanoplatelet microstructure are discussed.

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Indefinite and bidirectional near-infrared nanocrystal photoswitching

Materials whose luminescence can be switched by optical stimulation drive technologies ranging from superresolution imaging^1-4, nanophotonics⁵, and optical data storage^6,7, to targeted pharmacology, optogenetics, and chemical reactivity⁸. These photoswitchable probes, including organic fluorophores and proteins, can be prone to photodegradation and often operate in the ultraviolet or visible spectral regions. Colloidal inorganic nanoparticles^6,9 can offer improved stability, but the ability to switch emission bidirectionally, particularly with near-infrared (NIR) light, has not, to our knowledge, been reported in such systems. Here, we present two-way, NIR photoswitching of avalanching nanoparticles (ANPs), showing full optical control of upconverted emission using phototriggers in the NIR-I and NIR-II spectral regions useful for subsurface imaging. Employing single-step photodarkening^10-13 and photobrightening^12,14-16, we demonstrate indefinite photoswitching of individual nanoparticles (more than 1,000 cycles over 7 h) in ambient or aqueous conditions without measurable photodegradation. Critical steps of the photoswitching mechanism are elucidated by modelling and by measuring the photon avalanche properties of single ANPs in both bright and dark states. Unlimited, reversible photoswitching of ANPs enables indefinitely rewritable two-dimensional and three-dimensional multilevel optical patterning of ANPs, as well as optical nanoscopy with sub-Å localization superresolution that allows us to distinguish individual ANPs within tightly packed clusters.

Persistent nucleation and size dependent attachment kinetics produce monodisperse PbS nanocrystals

Modern syntheses of colloidal nanocrystals yield extraordinarily narrow size distributions that are believed to result from a rapid "burst of nucleation" (La Mer, JACS, 1950, 72(11), 4847-4854) followed by diffusion limited growth and size distribution focusing (Reiss, J. Chem. Phys., 1951, 19, 482). Using a combination of in situ X-ray scattering, optical absorption, and 13C nuclear magnetic resonance (NMR) spectroscopy, we monitor the kinetics of PbS solute generation, nucleation, and crystal growth from three thiourea precursors whose conversion reactivity spans a 2-fold range. In all three cases, nucleation is found to be slow and continues during >50% of the precipitation. A population balance model based on a size dependent growth law (1/r) fits the data with a single growth rate constant (k G) across all three precursors. However, the magnitude of the k G and the lack of solvent viscosity dependence indicates that the rate limiting step is not diffusion from solution to the nanoparticle surface. Several surface reaction limited mechanisms and a ligand penetration model that fits data our experiments using a single fit parameter are proposed to explain the results.

Growth kinetics determine the polydispersity and size of PbS and PbSe nanocrystals

A library of thio- and selenourea derivatives is used to adjust the kinetics of PbE (E = S, Se) nanocrystal formation across a 1000-fold range (k_r = 10⁻¹ to 10⁻⁴ s⁻¹), at several temperatures (80–120 °C), under a standard set of conditions (Pb : E = 1.2 : 1, [Pb(oleate)₂] = 10.8 mM, [chalcogenourea] = 9.0 mM). An induction delay (t_ind) is observed prior to the onset of nanocrystal absorption during which PbE solute is observed using in situ X-ray total scattering. Density functional theory models fit to the X-ray pair distribution function (PDF) support a Pb₂(μ₂-S)₂(Pb(O₂CR)₂)₂ structure. Absorption spectra of aliquots reveal a continuous increase in the number of nanocrystals over more than half of the total reaction time at low temperatures. A strong correlation between the width of the nucleation phase and reaction temperature is observed that does not correlate with the polydispersity. These findings are antithetical to the critical concentration dependence of nucleation that underpins the La Mer hypothesis and demonstrates that the duration of the nucleation period has a minor influence on the size distribution. The results can be explained by growth kinetics that are size dependent, more rapid at high temperature, and self limiting at low temperatures.

Colloidal lead chalcogenide nanocrystals nucleate slowly throughout their synthesis rather than in a burst. There is no correlation between the temporal width of the nucleation phase and the polydispersity.

Relations between absorption, emission, and excited state chemical potentials from nanocrystal 2D spectra

For quantum-confined nanomaterials, size dispersion causes a static broadening of spectra that has been difficult to measure and invalidates all-optical methods for determining the maximum photovoltage that an excited state can generate. Using femtosecond two-dimensional (2D) spectroscopy to separate size dispersion broadening of absorption and emission spectra allows a test of single-molecule generalized Einstein relations between such spectra for colloidal PbS quantum dots. We show that 2D spectra and these relations determine the thermodynamic standard chemical potential difference between the lowest excited and ground electronic states, which gives the maximum photovoltage. Further, we find that the static line broadening from many slightly different quantum dot structures allows single-molecule generalized Einstein relations to determine the average single-molecule linewidth from Stokes' frequency shift between ensemble absorption and emission spectra.

Au/TiO2-Catalyzed Benzyl Alcohol Oxidation on Morphologically Precise Anatase Nanoparticles

Au nanoparticles (NP) on TiO₂ have been shown to be effective catalysts for selective oxidation reactions by using molecular oxygen. In this work, we have studied the influence of support morphology on the catalytic activity of Au/TiO₂ catalysts. Two TiO₂ anatase supports, a nanoplatelet-shaped material with predominantly the {001} facet exposed and a truncated bipyramidal-shaped nanoparticle with predominantly the {101} facet exposed, were prepared by using a nonaqueous solvothermal method and characterized by using DRIFTS, XPS, and TEM. Au nanoparticles were deposited on the supports by using the deposition-precipitation method, and particle sizes were determined by using STEM. Au nanoparticles were smaller on the support with the majority of the {101} facet exposed. The resulting materials were used to catalyze the aerobic oxidation of benzyl alcohol and trifluoromethylbenzyl alcohol. Support morphology impacts the catalytic activity of Au/TiO₂; reaction rates for reactions catalyzed by the predominantly {101} material were higher. Much of the increased reactivity can be explained by the presence of smaller Au particles on the predominantly {101} material, providing more Au/TiO₂ interface area, which is where catalysis occurs. The remaining modest differences between the two catalysts are likely due to geometric effects as Hammett slopes show no evidence for electronic differences between the Au particles on the different materials.

Performance of Spherical Quantum Well Down Converters in Solid State Lighting

We report the color conversion performance of amber and red emitting quantum dots (QDs) on InGaN solid-state lighting (SSL) light emitting diode (LED) packages. Spherical quantum well (SQW) architectures (CdS/CdSe_1-xS_x/CdS) were prepared using a library of thio- and selenourea synthesis reagents and high throughput synthesis robotics. CdS/CdSe_1-xS_x QDs with narrow luminescence bands were coated with thick CdS shells (thickness = 1.6-7.5 nm) to achieve photoluminescence quantum yields (PLQY) up to 88% at amber and red emission wavelengths (λ_max = 600-642 nm, FWHM < 45 nm). The photoluminescence from SQWs encapsulated in silicone and deposited on LED packages was monitored under accelerated aging conditions (oven temperature = 85 °C, relative humidity = 5-85%, blue optical power density = 3-45 W/cm²) by monitoring the red photon output over several hundred hours of continuous operation. The growth of a ZnS shell on the SQW surface increases the stability under long-term operation but also reduces the PLQY, especially of SQWs with thick CdS shells. The results illustrate that the outer ZnS shell layer is key to optimizing the PLQY and the long-term stability of QDs during operation on SSL packages.

A Brazilian regional basic diet-induced chronic malnutrition drives liver inflammation with higher ApoA-I activity in C57BL6J mice

Malnutrition is still considered endemic in many developing countries. Malnutrition-enteric infections may cause lasting deleterious effects on lipid metabolism, especially in children living in poor settings. The regional basic diet (RBD), produced to mimic the Brazilian northeastern dietary characteristics (rich in carbohydrate and low in protein) has been used in experimental malnutrition models, but few studies have explored the effect of chronic RBD on liver function, a central organ involved in cholesterol metabolism. This study aimed to investigate whether RBD leads to liver inflammatory changes and altered reverse cholesterol metabolism in C57BL6/J mice compared to the control group, receiving a standard chow diet. To evaluate liver inflammation, ionized calcium-binding adapter protein-1 (IBA-1) positive cell counting, interleukin (IL)-1β immunohistochemistry, and tumor necrosis factor (TNF)-α and IL-10 transcription levels were analyzed. In addition, we assessed reverse cholesterol transport by measuring liver apolipoprotein (Apo)E, ApoA-I, and lecithin-cholesterol acyltransferase (LCAT) by RT-PCR. Furthermore, serum alanine aminotransferase (ALT) was measured to assess liver function. RBD markedly impaired body weight gain compared with the control group (P<0.05). Higher hepatic TNF-α (P<0.0001) and IL-10 (P=0.001) mRNA levels were found in RBD-challenged mice, although without detectable non-alcoholic fatty liver disease. Marked IBA-1 immunolabeling and increased number of positive-IBA-1 cells were found in the undernourished group. No statistical difference in serum ALT was found. There was also a significant increase in ApoA mRNA expression in the undernourished group, but not ApoE and LCAT, compared with the control. Altogether our findings suggested that chronic RBD-induced malnutrition leads to liver inflammation with increased ApoA-I activity.

Precursor reaction kinetics control compositional grading and size of CdSe1-x S x nanocrystal heterostructures

We report a method to control the composition and microstructure of CdSe_1-x S _x nanocrystals by the simultaneous injection of sulfide and selenide precursors into a solution of cadmium oleate and oleic acid at 240 °C. Pairs of substituted thio- and selenoureas were selected from a library of compounds with conversion reaction reactivity exponents (k _E) spanning 1.3 × 10^-5 s^-1 to 2.0 × 10^-1 s^-1. Depending on the relative reactivity (k _Se/k _S), core/shell and alloyed architectures were obtained. Growth of a thick outer CdS shell using a syringe pump method provides gram quantities of brightly photoluminescent quantum dots (PLQY = 67 to 90%) in a single reaction vessel. Kinetics simulations predict that relative precursor reactivity ratios of less than 10 result in alloyed compositions, while larger reactivity differences lead to abrupt interfaces. CdSe_1-x S _x alloys (k _Se/k _S = 2.4) display two longitudinal optical phonon modes with composition dependent frequencies characteristic of the alloy microstructure. When one precursor is more reactive than the other, its conversion reactivity and mole fraction control the number of nuclei, the final nanocrystal size at full conversion, and the elemental composition. The utility of controlled reactivity for adjusting alloy microstructure is discussed.

Dynamic emission Stokes shift and liquid-like dielectric solvation of band edge carriers in lead-halide perovskites

Lead-halide perovskites have emerged as promising materials for photovoltaic and optoelectronic applications. Their significantly anharmonic lattice motion, in contrast to conventional harmonic semiconductors, presents a conceptual challenge in understanding the genesis of their exceptional optoelectronic properties. Here we report a strongly temperature dependent luminescence Stokes shift in the electronic spectra of both hybrid and inorganic lead-bromide perovskite single crystals. This behavior stands in stark contrast to that exhibited by more conventional crystalline semiconductors. We correlate the electronic spectra with the anti-Stokes and Stokes Raman vibrational spectra. Dielectric solvation theories, originally developed for excited molecules dissolved in polar liquids, reproduce our experimental observations. Our approach, which invokes a classical Debye-like relaxation process, captures the dielectric response originating from the incipient anharmonicity of the LO phonon at about 20 meV (160 cm⁻¹) in the lead-bromide framework. We reconcile this liquid-like model incorporating thermally-activated dielectric solvation with more standard solid-state theories of the emission Stokes shift in crystalline semiconductors.

Lead halide perovskites have unique electronic properties that depend on the crystal’s anharmonicity. Dielectric solvation theories, developed for molecules dissolved in polar liquids, are shown here to reproduce the temperature behavior of carrier solvation in the electronic spectra, implying strongly anharmonic lattice dynamics.

Size-Dependent Lattice Dynamics of Atomically Precise Cadmium Selenide Quantum Dots

Material properties depend sensitively on the atomic arrangements and atomic bonding, but these are notoriously difficult to measure in nanosized atomic clusters due to the small size of the objects and the challenge of obtaining bulk samples of identical clusters. Here, we have combined the recent ability to make gram quantities of identical semiconductor quantum-dot nanoparticles with the ability to measure lattice dynamics on small sample quantities of hydrogenated materials using high energy resolution inelastic x-ray scattering, to measure the size dependence of the phonon density of states in CdSe quantum dots. The fact that we have atomically precise structural models for these nanoparticles allows the calculation of the phonon density of states using density functional theory, providing both experimental and theoretical confirmations of the important role that the inertia of the surface capping species plays on determining the lattice dynamics.

Flexible Nanopipettes for Minimally Invasive Intracellular Electrophysiology In Vivo

Intracellular recordings in vivo remains the best technique to link single-neuron electrical properties to network function. Yet existing methods are limited in accuracy, throughput, and duration, primarily via washout, membrane damage, and movement-induced failure. Here, we introduce flexible quartz nanopipettes (inner diameters of 10-25 nm and spring constant of ∼0.08 N/m) as nanoscale analogs of traditional glass microelectrodes. Nanopipettes enable stable intracellular recordings (seal resistances of 500 to ∼800 MΩ, 5 to ∼10 cells/nanopipette, and duration of ∼1 hr) in anaesthetized and awake head-restrained mice, exhibit minimal diffusional flux, and facilitate precise recording and stimulation. When combined with quantum-dot labels and microprisms, nanopipettes enable two-photon targeted electrophysiology from both somata and dendrites, and even paired recordings from neighboring neurons, while permitting simultaneous population imaging across cortical layers. We demonstrate the versatility of this method by recording from parvalbumin-positive (Pv) interneurons while imaging seizure propagation, and we find that Pv depolarization block coincides with epileptic spread. Flexible nanopipettes present a simple method to procure stable intracellular recordings in vivo.

Platelet Aggregation and Plasma Lipoproteins in Alcoholics During Alcohol Withdrawal

Platelet aggregation, platelet lipid composition and plasma lipoprotein concentrations were measured each week in a group of seventeen alcoholics, without overt liver disease, for one month, following acute, total alcohol withdrawal. The platelets were initially hypoaggregable but, within 1-2 weeks of cessation of drinking, they became hyperaggregable and then gradually returned towards normal values. Hyperaggregability could not be explained by increases in either the cholesterol or the arachidonic acid content of the platelets. Plasma very-low-density lipoprotein cholesterol levels remained high throughout the study, but the initially raised levels of high-density lipoprotein (HDL) cholesterol fell by 26%. Low-density lipoprotein (LDL) cholesterol concentration rose by 10% after two weeks of withdrawal but then returned to about the starting level. The resulting changes in the plasma LDL-cholesterol: HDL-cholesterol ratio, which had increased by more than 50% after two weeks of abstinence, essentially paralleled the time course of enhanced platelet reactivity in all but four of the alcoholics. These findings suggest that alterations in plasma lipoprotein concentrations during acute alcohol withdrawal may be a contributory factor to the haemostatic disorders present in such patients.

Two-Dimensional Fullerene Assembly from an Exfoliated van der Waals Template

Two-dimensional (2D) materials are commonly prepared by exfoliating bulk layered van der Waals crystals. The creation of synthetic 2D materials from bottom-up methods is an important challenge as their structural flexibility will enable chemists to tune the materials properties. A 2D material was assembled using C₆₀ as a polymerizable monomer. The C₆₀ building blocks are first assembled into a layered solid using a molecular cluster as structure director. The resulting hierarchical crystal is used as a template to polymerize its C₆₀ monolayers, which can be exfoliated down to 2D crystalline nanosheets. Derived from the parent template, the 2D structure is composed of a layer of inorganic cluster, sandwiched between two monolayers of polymerized C₆₀ . The nanosheets can be transferred onto solid substrates and depolymerized by heating. Electronic absorption spectroscopy reveals an optical gap of 0.25 eV, narrower than that of the bulk parent crystalline solid.

Targeted intracellular voltage recordings from dendritic spines using quantum-dot-coated nanopipettes

Dendritic spines are the primary site of excitatory synaptic input onto neurons, and are biochemically isolated from the parent dendritic shaft by their thin neck. However, due to the lack of direct electrical recordings from spines, the influence that the neck resistance has on synaptic transmission, and the extent to which spines compartmentalize voltage, specifically excitatory postsynaptic potentials, albeit critical, remains controversial. Here, we use quantum-dot-coated nanopipette electrodes (tip diameters ∼15-30 nm) to establish the first intracellular recordings from targeted spine heads under two-photon visualization. Using simultaneous somato-spine electrical recordings, we find that back propagating action potentials fully invade spines, that excitatory postsynaptic potentials are large in the spine head (mean 26 mV) but are strongly attenuated at the soma (0.5-1 mV) and that the estimated neck resistance (mean 420 MΩ) is large enough to generate significant voltage compartmentalization. Nanopipettes can thus be used to electrically probe biological nanostructures.

Tris(2-mercaptoimidazolyl)hydroborato Cadmium Thiolate Complexes, [TmBut]CdSAr: Thiolate Exchange at Cadmium in a Sulfur-Rich Coordination Environment

A series of cadmium thiolate compounds that feature a sulfur-rich coordination environment, namely [TmBut]CdSAr, have been synthesized by the reactions of [TmBut]CdMe with ArSH (Ar = C6H4-4-F, C6H4-4-But, C6H4-4-OMe, and C6H4-3-OMe). In addition, the pyridine-2-thiolate and pyridine-2-selenolate derivatives, [TmBut]CdSPy and [TmBut]CdSePy have been obtained via the respective reactions of [TmBut]CdMe with pyridine-2-thione and pyridine-2-selone. The molecular structures of [TmBut]CdSAr and [TmBut]CdEPy (E = S or Se) have been determined by X-ray diffraction and demonstrate that, in each case, the [CdS4] motif is distorted tetrahedral and approaches a trigonal monopyramidal geometry in which the thiolate ligand adopts an equatorial position; [TmBut]CdSPy and [TmBut]CdSePy, however, exhibit an additional long-range interaction with the pyridyl nitrogen atoms. The ability of the thiolate ligands to participate in exchange was probed by 1H and 19F nuclear magnetic resonance (NMR) spectroscopic studies of the reactions of [TmBut]CdSC6H4-4-F with ArSH (Ar = C6H4-4-But or C6H4-4-OMe), which demonstrate that (i) exchange is facile and (ii) coordination of thiolate to cadmium is most favored for the p-fluorophenyl derivative. Furthermore, a two-dimensional EXSY experiment involving [TmBut]CdSC6H4-4-F and 4-fluorothiophenol demonstrates that degenerate thiolate ligand exchange is also facile on the NMR time scale.

Local Polar Fluctuations in Lead Halide Perovskite Crystals

Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low-frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in the cubic phase of both hybrid (CH_{3}NH_{3}PbBr_{3}) and all-inorganic (CsPbBr_{3}) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations indicate that head-to-head Cs motion coupled to Br face expansion, occurring on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr_{3}.

Unbalanced Hole and Electron Diffusion in Lead Bromide Perovskites

We use scanning photocurrent microscopy and time-resolved microwave conductivity to measure the diffusion of holes and electrons in a series of lead bromide perovskite single crystals, APbBr₃, with A = methylammonium (MA), formamidinium (FA), and Cs. We find that the diffusion length of holes (L_D^h+ ∼ 10-50 μm) is on average an order of magnitude longer than that of electrons (L_D^e- ∼ 1-5 μm), regardless of the A-type cation or applied bias. Furthermore, we observe a weak dependence of L_D across the A-cation series MA > FA > Cs. When considering the role of the halide, we find that the diffusion of holes in MAPbBr₃ is comparable to that in MAPbI₃, but the electron diffusion length is up to five times shorter. This study shows that the disparity between hole and electron diffusion is a ubiquitous feature of lead halide perovskites. As with organic photovoltaics, this imbalance will likely become an important consideration in the optimization of lead halide perovskite solar cells.

A Library of Selenourea Precursors to PbSe Nanocrystals with Size Distributions near the Homogeneous Limit

We report a tunable library of N,N,N'-trisubstituted selenourea precursors and their reaction with lead oleate at 60-150 °C to form carboxylate-terminated PbSe nanocrystals in quantitative yields. Single exponential conversion kinetics can be tailored over 4 orders of magnitude by adjusting the selenourea structure. The wide range of conversion reactivity allows the extent of nucleation ([nanocrystal] = 4.6-56.7 μM) and the size following complete precursor conversion (d = 1.7-6.6 nm) to be controlled. Narrow size distributions (σ = 0.5-2%) are obtained whose spectral line widths are dominated (73-83%) by the intrinsic single particle spectral broadening, as observed using spectral hole burning measurements. The intrinsic broadening decreases with increasing size (fwhm = 320-65 meV, d = 1.6-4.4 nm) that derives from exciton fine structure and exciton-phonon coupling rather than broadening caused by the size distribution.

Synthesis, structure and reactivity of [TmBut]ZnH, a monomeric terminal zinc hydride compound in a sulfur-rich coordination environment: access to a heterobimetallic compound

The zinc hydride complex, [Tm^But]ZnH, undergoes insertion of CO₂ and facile protolytic cleavage, of which the latter provides access to heterobimetallic [Tm^But]ZnMo(CO)₃Cp.

Exchange of Alkyl and Tris(2-mercapto-1-t-butylimidazolyl)hydroborato Ligands Between Zinc, Cadmium and Mercury

The tris(2-mercaptoimidazolyl)hydroborato ligand, [Tm^But ], has been used to investigate the exchange of alkyl and sulfur donor ligands between the Group 12 metals, Zn, Cd and Hg. For example, [Tm^But ]₂Zn reacts with Me₂Zn to yield [Tm^But ]ZnMe, while [Tm^But ]CdMe is obtained readily upon reaction of [Tm^But ]₂Cd with Me₂Cd. Ligand exchange is also observed between different metal centers. For example, [Tm^But ]CdMe reacts with Me₂Zn to afford [Tm^But ]ZnMe and Me₂Cd. Likewise, [Tm^But ]HgMe reacts with Me₂Zn to afford [Tm^But ]ZnMe and Me₂Hg. However, whereas the [Tm^But ] ligand transfers from mercury to zinc in the methyl system, [Tm^But ]HgMe/Me₂Zn, transfer of the [Tm^But ] ligand from zinc to mercury is observed upon treatment of [Tm^But ]₂Zn with HgI₂ to afford [Tm^But ]HgI and [Tm^But ]ZnI. These observations demonstrate that the phenomenological preference for the [Tm^But ] ligand to bind one metal rather than another is strongly influenced by the nature of the co-ligands.

Infrared Spectroscopic Study of Vibrational Modes in Methylammonium Lead Halide Perovskites

The organic cation and its interplay with the inorganic lattice underlie the exceptional optoelectronic properties of organo-metallic halide perovskites. Herein we report high-quality infrared spectroscopic measurements of methylammonium lead halide perovskite (CH3NH3Pb(I/Br/Cl)3) films and single crystals at room temperature, from which the dielectric function in the investigated spectral range is derived. Comparison with electronic structure calculations in vacuum of the free methylammonium cation allows for a detailed peak assignment. We analyze the shifts of the vibrational peak positions between the different halides and infer the extent of interaction between organic moiety and the surrounding inorganic cage. The positions of the NH3(+) stretching vibrations point to significant hydrogen bonding between the methylammonium and the halides for all three perovskites.

Synthesis and structures of cadmium carboxylate and thiocarboxylate compounds with a sulfur-rich coordination environment: carboxylate exchange kinetics involving tris(2-mercapto-1-t-butylimidazolyl)hydroborato cadmium complexes, [Tm(Bu(t))]Cd(O2CR)

A series of cadmium carboxylate compounds in a sulfur-rich environment provided by the tris(2-tert-butylmercaptoimidazolyl)hydroborato ligand, namely, [Tm(Bu(t))]CdO2CR, has been synthesized via the reactions of the cadmium methyl derivative [Tm(Bu(t))]CdMe with RCO2H. Such compounds mimic aspects of cadmium-substituted zinc enzymes and also the surface atoms of cadmium chalcogenide crystals, and have therefore been employed to model relevant ligand exchange processes. Significantly, both (1)H and (19)F NMR spectroscopy demonstrate that the exchange of carboxylate groups between [Tm(Bu(t))]Cd(κ(2)-O2CR) and the carboxylic acid RCO2H is facile on the NMR time scale, even at low temperature. Analysis of the rate of exchange as a function of concentration of RCO2H indicates that reaction occurs via an associative rather than dissociative pathway. In addition to carboxylate compounds, the thiocarboxylate derivative [Tm(Bu(t))]Cd[κ(1)-SC(O)Ph] has also been synthesized via the reaction of [Tm(Bu(t))]CdMe with thiobenzoic acid. The molecular structure of [Tm(Bu(t))]Cd[κ(1)-SC(O)Ph] has been determined by X-ray diffraction, and an interesting feature is that, in contrast to the carboxylate derivatives [Tm(Bu(t))]Cd(κ(2)-O2CR), the thiocarboxylate ligand binds in a κ(1) manner via only the sulfur atom.

Surface Structure of Aerobically Oxidized Diamond Nanocrystals

We investigate the aerobic oxidation of high-pressure, high-temperature nanodiamonds (5-50 nm dimensions) using a combination of carbon and oxygen K-edge X-ray absorption, wavelength-dependent X-ray photoelectron, and vibrational spectroscopies. Oxidation at 575 °C for 2 h eliminates graphitic carbon contamination (>98%) and produces nanocrystals with hydroxyl functionalized surfaces as well as a minor component (<5%) of carboxylic anhydrides. The low graphitic carbon content and the high crystallinity of HPHT are evident from Raman spectra acquired using visible wavelength excitation (λ_excit = 633 nm) as well as carbon K-edge X-ray absorption spectra where the signature of a core-hole exciton is observed. Both spectroscopic features are similar to those of chemical vapor deposited (CVD) diamond but differ significantly from the spectra of detonation nanodiamond. The importance of these findings to the functionalization of nanodiamond surfaces for biological labeling applications is discussed.

Molecular structures of tris(2-mercapto-1-tert-butylimidazolyl)hydroborato and tris(2-mercapto-1-adamantylimidazolyl)hydroborato sodium complexes: analysis of [Tm(R)] ligand coordination modes and conformations

The tris(mercaptoimidazolyl)hydroborato complexes, [κ(3)-S2H-Tm(Bu(t))]Na(THF)3 and [κ(3)-S2H-Tm(Ad)]Na(THF)3, which feature t-butyl and adamantyl substituents, have been synthesized via the reactions of the respective 1-R-1,3-dihydro-2H-imidazole-2-thiones with NaBH4 in THF (R = Bu(t), 1-Ad). X-ray diffraction studies indicate that the compounds are monomeric and that the [Tm(R)] ligands coordinate to the metal in a κ(3)-S2H manner via two of the sulfur donors and the hydrogen attached to boron, a combination that is unprecedented for sodium derivatives. Analysis of the tris(mercaptoimidazolyl)hydroborato compounds that are listed in the Cambridge Structural Database has allowed for the formulation of a set of criteria that enables κ(x)-S(x) and κ(x+1)-S(x)H coordination modes to be identified. Furthermore, the various κ(x)-S(x) and κ(x+1)-S(x)H coordination modes have also been analyzed with respect to the conformations of the [Tm(R)] ligands, which differ by rotation of the imidazolethione moieties about the B-N bond.

Electrical transport and grain growth in solution-cast, chloride-terminated cadmium selenide nanocrystal thin films

We report the evolution of electrical transport and grain size during the sintering of thin films spin-cast from soluble phosphine and amine-bound, chloride-terminated cadmium selenide nanocrystals. Sintering of the nanocrystals occurs in three distinct stages as the annealing temperature is increased: (1) reversible desorption of the organic ligands (≤150 °C), (2) irreversible particle fusion (200-300 °C), and (3) ripening of the grains to >5 nm domains (>200 °C). Grain growth occurs at 200 °C in films with 8 atom % Cl(-), while films with 3 atom % Cl(-) resist growth until 300 °C. Fused nanocrystalline thin films (grain size = 4.5-5.5 nm) on thermally grown silicon dioxide gate dielectrics produce field-effect transistors with electron mobilities as high as 25 cm(2)/(Vs) and on/off ratios of 10(5) with less than 0.5 V hysteresis in threshold voltage without the addition of indium.

Atomic structures and gram scale synthesis of three tetrahedral quantum dots

Luminescent semiconducting quantum dots (QDs) are central to emerging technologies that range from tissue imaging to solid-state lighting. However, existing samples are heterogeneous, which has prevented atomic-resolution determination of their structures and obscured the relationship between their atomic and electronic structures. Here we report the synthesis, isolation, and structural characterization of three cadmium selenide QDs with uniform compositions (Cd35Se20(X)30(L)30, Cd56Se35(X)42(L)42, Cd84Se56(X)56(L)56; X = O2CPh, L = H2N-C4H9). Their UV-absorption spectra show a lowest energy electronic transition that decreases in energy (3.54 eV, 3.26 eV, 3.04 eV) and sharpens as the size of the QD increases (fwhm = 207 meV, 145 meV, 115 meV). The photoluminescence spectra of all three QDs are broad with large Stokes shifts characteristic of trap-luminescence. Using a combination of single-crystal X-ray diffraction and atomic pair distribution function analysis, we determine the structures of their inorganic cores, revealing a series of pyramidal nanostuctures with cadmium terminated {111} facets. Theoretical and experimental studies on these materials will open the door to a deeper fundamental understanding of structure-property relationships in quantum-confined semiconductors.

Structure of methylammonium lead iodide within mesoporous titanium dioxide: active material in high-performance perovskite solar cells

We report the structure of methylammonium lead(II) iodide perovskite in mesoporous TiO2, as used in high-performance solar cells. Pair distribution function analysis of X-ray scattering reveals a two component nanostructure: one component with medium range crystalline order (30 atom %) and another with only local structural coherence (70 atom %). The nanostructuring correlates with a blueshift of the absorption onset and increases the photoluminescence. Our findings underscore the importance of fully characterizing and controlling the structure for improved solar cell efficiency.

Ligand exchange and the stoichiometry of metal chalcogenide nanocrystals: spectroscopic observation of facile metal-carboxylate displacement and binding

We demonstrate that metal carboxylate complexes (L-M(O2CR)2, R = oleyl, tetradecyl, M = Cd, Pb) are readily displaced from carboxylate-terminated ME nanocrystals (ME = CdSe, CdS, PbSe, PbS) by various Lewis bases (L = tri-n-butylamine, tetrahydrofuran, tetradecanol, N,N-dimethyl-n-butylamine, tri-n-butylphosphine, N,N,N',N'-tetramethylbutylene-1,4-diamine, pyridine, N,N,N',N'-tetramethylethylene-1,2-diamine, n-octylamine). The relative displacement potency is measured by (1)H NMR spectroscopy and depends most strongly on geometric factors such as sterics and chelation, although also on the hard/soft match with the cadmium ion. The results suggest that ligands displace L-M(O2CR)2 by cooperatively complexing the displaced metal ion as well as the nanocrystal. Removal of up to 90% of surface-bound Cd(O2CR)2 from CdSe and CdS nanocrystals decreases the Cd/Se ratio from 1.1 ± 0.06 to 1.0 ± 0.05, broadens the 1S(e)-2S(3/2h) absorption, and decreases the photoluminescence quantum yield (PLQY) from 10% to <1% (CdSe) and from 20% to <1% (CdS). These changes are partially reversed upon rebinding of M(O2CR)2 at room temperature (∼60%) and fully reversed at elevated temperature. A model is proposed in which electron-accepting M(O2CR)2 complexes (Z-type ligands) reversibly bind to nanocrystals, leading to a range of stoichiometries for a given core size. The results demonstrate that nanocrystals lack a single chemical formula, but are instead dynamic structures with concentration-dependent compositions. The importance of these findings to the synthesis and purification of nanocrystals as well as ligand exchange reactions is discussed.

A hot electron-hole pair breaks the symmetry of a semiconductor quantum dot

The best-understood property of semiconductor quantum dots (QDs) is the size-dependent optical transition energies due to the quantization of charge carriers near the band edges. In contrast, much less is known about the nature of hot electron-hole pairs resulting from optical excitation significantly above the bandgap. Here, we show a transient Stark effect imposed by a hot electron-hole pair on optical transitions in PbSe QDs. The hot electron-hole pair does not behave as an exciton, but more bulk-like as independent carriers, resulting in a transient and varying dipole moment which breaks the symmetry of the QD. As a result, we observe redistribution of optical transition strength to dipole forbidden transitions and the broadening of dipole-allowed transitions during the picosecond lifetime of the hot carriers. The magnitude of symmetry breaking scales with the amount of excess energy of the hot carriers, diminishes as the hot carriers cool down and disappears as the hot electron-hole pair becomes an exciton. Such a transient Stark effect should be of general significance to the understanding of QD photophysics above the bandgap.

Population Pharmacokinetics of Tigecycline in Healthy Volunteers

Tigecycline, a novel glycylcycline, possesses broad-spectrum antimicrobial activity. A structural population pharmacokinetic model for tigecycline was developed based on data pooled from 5 phase I studies. Intravenous tigecycline was administered as single (12.5-300 mg) or multiple (25-100 mg) doses every 12 hours for up to 10 days. Three-compartment models with zero-order input and first-order elimination separately described the single- or multiple-dose full-profile data. Additional models were evaluated using a subset of the phase I data mimicking the phase II/III trial sparse-sampling scheme and dosage. A 2-compartment model best described the reduced phase I data following single or multiple doses and provided reliably accurate estimates of tigecycline AUC(0-12). This modeling supported phase II/III population pharmacokinetic model development to further determine individual patient tigecycline exposures for safety and efficacy analyses.

The importance of nanocrystal precursor conversion kinetics: mechanism of the reaction between cadmium carboxylate and cadmium bis(diphenyldithiophosphinate)

We describe the synthesis of cadmium bis(diphenyldithiophosphinate) (Cd(S(2)PPh(2))(2)) from secondary phosphine sulfides and its conversion to cadmium sulfide nanocrystals. Heating Cd(S(2)PPh(2))(2) and cadmium tetradecanoate (≥4 equiv) to 240 °C results in complete conversion of Cd(S(2)PPh(2))(2) to cadmium sulfide nanocrystals with tetradecanoate surface termination. The nanocrystals have a narrow size distribution (d = 3.8-4.1 nm, σ < 10%) that is evident from the line width of the lowest energy absorption feature (λ = 412-422 nm, fwhm = 0.17 eV) and display bright photoluminescence (PLQY(band edge+trap) = 36%). Interestingly, the final diameter is insensitive to the reaction conditions, including the total concentration of precursors and initial cadmium to sulfur ratio. Monitoring the reaction with (31)P NMR, UV-visible, and infrared absorption spectroscopies shows that the production of cadmium diphenylphosphinate (Cd(O(2)PPh(2))(2)) and tetradecanoic anhydride co-products is coupled with the formation of cadmium sulfide. From these measurements we propose a balanced chemical equation for the conversion reaction and use it to optimize a synthesis that affords CdS nanocrystals in quantitative yield. In light of these results we discuss the importance of well-defined precursor reactivity to reproducible conversion kinetics and the synthesis of nanocrystals with unambiguous chemical composition.

Tuning the Surface Structure and Optical Properties of CdSe Clusters Using Coordination Chemistry

A series of nonstoichiometric CdSe clusters with lowest energy electronic absorptions between 409 - 420 nm has been prepared from cadmium 1-naphthoate, 2-naphthoate, 4-thiomethyl-1-naphthaote, and 1-naphthalene thiolate complexes and diphenylphosphine selenide (DPPSe). Pair distribution function analysis of X-ray diffraction data, ligand exchange experiments, and NMR molecular weight analyses suggest the nanocrystal core changes minimally among these clusters despite significant changes to their absorption and luminescence spectra. Photoluminescence excitation spectra obtained at 77 K reveal an energy transfer process between the surface-trapped excited state and the naphthalene-containing ligands that leads to ligand phosphorescence. A Dexter energy transfer mechanism is proposed to explain the observation of ligand phosphorescence on excitation of the cluster. These compounds demonstrate that cluster absorption and trap luminescence can be controlled with surface coordination chemistry.

Focusing nanocrystal size distributions via production control

We present a theoretical description of how continuous monomer production affects the focusing of nanocrystal size distributions in solution. We show that sufficiently high monomer production rates can drive a decrease in the polydispersity even as the average nanocrystal size increases. This is in sharp contrast to Ostwald ripening, where polydispersity increases with mean crystal size. We interpret several experimental nanocrystal studies through our model and show how production-controlled growth promises exquisite control over the size and polydispersity of functional nanocrystals.

Plasma pharmacokinetics of two consecutive doses of ferumoxytol in healthy subjects

Intravenous (IV) iron is used to treat iron-deficiency anemia in patients with chronic kidney disease (CKD). Ferumoxytol is a novel iron formulation administered rapidly as two IV boluses of 510 mg each. In this placebo-controlled, double-blind, parallel-group study, 58 healthy volunteers received ferumoxytol in two 510 mg doses administered 24 h apart. Population pharmacokinetics (PK) analysis was conducted, and a two-compartment open model with zero-order input and Michaelis-Menten elimination was found to best describe the data. The population mean estimates for volume of distribution of the central compartment (V(1)), maximal elimination rate (V(max)), and ferumoxytol concentration at which rate of metabolism would be one-half of V(max) (K(m)) were 2.71 l, 14.3 mg/h, and 77.5 mg/l, respectively. When the effect of body weight on V(1) was added in the analysis, interindividual variability was found to be reduced. A noncompartmental analysis of two simulated 510-mg ferumoxytol doses was also performed to provide clinically interpretable data on half life and exposure. Ferumoxytol given as two consecutive 510-mg doses was well tolerated.