1,941 research outputs found
Stereoelectronic effects on the binding of neutral Lewis bases to CdSe nanocrystals
Using P-31 nuclear magnetic resonance (NMR) spectroscopy, we monitor the competition between tri-nbutylphosphine (Bu3P) and various amine and phosphine ligands for the surface of chloride terminated CdSe nanocrystals. Distinct P-31 NMR signals for free and bound phosphine ligands allow the surface ligand coverage to be measured in phosphine solution. Ligands with a small steric profile achieve higher surface coverages (Bu3P = 0.5 nm(-2), Me2P-n-octyl = 2.0 nm(-2), NH2Bu = >3 nm(-2)) and have greater relative binding affinity for the nanocrystal (binding affinity: Me3P > Me2P -n-octyl similar to Me2P -n-octadecyl > Et3P > Bu3P). Among phosphines, only Bu 3 P and Me2P-n-octyl support a colloidal dispersion, allowing a relative surface binding affinity (K-rel) to be estimated in that case (K-rel = 3.1). The affinity of the amine ligands is measured by the extent to which they displace Bu3P from the nanocrystals (K-rel: H2NBu similar to N-n-butylimidazole > 4-ethylpyridine > Bu3P similar to HNBu2 > Me2NBu > Bu3N). The affinity for the CdSe surface is greatest among soft, basic donors and depends on the number of each ligand that bind. Sterically unencumbered ligands such as imidazole, pyridine, and n-alkylamines can therefore outcompete stronger donors such as alkylphosphines. The influence of repulsive interactions between ligands on the binding affinity is a consequence of the high atom density of binary semiconductor surfaces. The observed behavior is distinct from the self-assembly of straight-chain surfactants on gold and silver where the ligands are commensurate with the underlying lattice and attractive interactions between aliphatic chains strengthen the binding
Kinetics and Mechanism of Methane, Methanol, and Dimethyl Ether C−H Activation with Electrophilic Platinum Complexes
The relative rates of C−H activation of methane, methanol, and dimethyl ether by [(N-N)PtMe(TFE-d_3)]+ ((N-N) = ArN═C(Me)−C(Me)═NAr; Ar = 3,5-di-tert-butylphenyl, TFE-d_3 = CF_3CD_2OD) (2(TFE)) were determined. Methane activation kinetics were conducted by reacting 2(TFE)-^(13)C with 300−1000 psi of methane in single-crystal sapphire NMR tubes; clean second-order behavior was obtained (k = 1.6 ± 0.4 × 10^(-3) M^(-1) s^(-1) at 330 K; k = 2.7 ± 0.2 × 10^(-4) M^(-1) s^(-1) at 313 K). Addition of methanol to solutions of 2(TFE) rapidly establishes equilibrium between methanol (2(MeOD)) and trifluoroethanol (2(TFE)) adducts, with methanol binding preferentially (K_(eq) = 0.0042 ± 0.0006). C−H activation gives [(N-N)Pt(CH_2OD)(MeOD)]^+ (4), which is unstable and reacts with [(RO)B(C_6F_5)_3]^- to generate a pentafluorophenyl platinum complex. Analysis of kinetics data for reaction of 2 with methanol yields k = 2.0 ± 0.2 × 10^(-3) M^(-1) s^(-1) at 330 K, with a small kinetic isotope effect (k_H/k_D = 1.4 ± 0.1). Reaction of dimethyl ether with 2(TFE) proceeds similarly (K_(eq) = 0.023 ± 0.002, 313 K; k = 5.5 ± 0.5 × 10^(-4) M^(-1) s^(-1), k_H/k_D = 1.5 ± 0.1); the product obtained is a novel bis(alkylidene)-bridged platinum dimer, [(diimine)Pt(μ-CH_2)(μ-(CH(OCH_3))Pt(diimine)]^(2+) (5). Displacement of TFE by a C−H bond appears to be the rate-determining step for all three substrates; comparison of the second-order rate constants (k(_(methane))/k(_(methanol)) = 1/1.3, 330 K; k(_(methane))/k(_(dimethyl ether)) = 1/2.0, 313 K) shows that this step is relatively unselective for the C−H bonds of methane, methanol, or dimethyl ether. This low selectivity agrees with previous estimates for oxidations with aqueous tetrachloroplatinate(II)/hexachloroplatinate(IV), suggesting a similar rate-determining step for those reactions
Kinetic control over CdS nanocrystal nucleation using a library of thiocarbonates, thiocarbamates, and thioureas
We report a family of substituted thiocarbonates, thiocarbamates, and thioureas and their reaction with cadmium oleate at 180-240 degrees C to form zincblende CdS nanocrystals (d = 2.25.9 nm). To monitor the kinetics of CdS formation with UV-vis spectroscopy, the size dependence of the extinction coefficient for lambda(max)(1S(e)-1S(1/2h)) is determined. The precursor conversion reactivity spans 5 orders of magnitude depending on the precursor structure (2 degrees-thioureas > 3 degrees-thioureas >= 2 degrees-thiocarbamates > 2 degrees-thiocarbonates > 4 degrees-thioureas >= 3 degrees-thiocarbamates). The concentration of nanocrystals formed during nucleation increases when more reactive precursors are used, allowing the final size to be controlled by the precursor structure. H-1 NMR spectroscopy is used to monitor the reaction of di-p-tolyl thiocarbonate and cadmium oleate where di-p-tolyl carbonate and oleic anhydride coproducts can be identified. These coproducts further decompose into p-tolyl oleate and p-cresol. The spectral features of CdS nanocrystals produced from thiocarbonates are exceptionally narrow (95-161 meV fwhm) as compared to those made from thioureas (137-174 meV fwhm) under otherwise identical conditions, indicating that particular precursors nucleate narrower size distributions than others
Direct Observation of Dynamic Symmetry Breaking above Room Temperature in Methylammonium Lead Iodide Perovskite
Lead halide perovskites such as methylammonium lead triiodide (MAPI) have
outstanding optical and electronic properties for photovoltaic applications,
yet a full understanding of how this solution processable material works so
well is currently missing. Previous research has revealed that MAPI possesses
multiple forms of static disorder regardless of preparation method, which is
surprising in light of its excellent performance. Using high energy resolution
inelastic X-ray (HERIX) scattering, we measure phonon dispersions in MAPI and
find direct evidence for another form of disorder in single crystals: large
amplitude anharmonic zone-edge rotational instabilities of the PbI_6 octahedra
that persist to room temperature and above, left over from structural phase
transitions that take place tens to hundreds of degrees below. Phonon
calculations show that the orientations of the methylammonium couple strongly
and cooperatively to these modes. The result is a non-centrosymmetric,
instantaneous local structure, which we observe in atomic pair distribution
function (PDF) measurements. This local symmetry breaking is unobservable by
Bragg diffraction, but can explain key material properties such as the
structural phase sequence, ultra low thermal transport, and large minority
charge carrier lifetimes despite moderate carrier mobility.Comment: 30 pages, 11 figure
Synthesis of phosphonic acid ligands for nanocrystal surface functionalization and solution processed memristors
Here, we synthesized 2-ethylhexyl, 2-hexyldecyl, 2-[2-(2-methoxyethoxy)ethoxy]ethyl, oleyl, and n-octadecyl phosphonic acid and used them to functionalize CdSe and HfO2 nanocrystals. In contrast to branched carboxylic acids, postsynthetic surface functionalization of CdSe and HfO2 nanocrystals was readily achieved with branched phosphonic acids. Phosphonic acid capped HfO2 nanocrystals were subsequently evaluated as memristors using conductive atomic force microscopy. We found that 2-ethylhexyl phosphonic acid is a superior ligand, combining a high colloidal stability with a compact ligand shell that results in a record-low operating voltage that is promising for application in flexible electronics
Sticky central limit theorems on open books
Given a probability distribution on an open book (a metric space obtained by
gluing a disjoint union of copies of a half-space along their boundary
hyperplanes), we define a precise concept of when the Fr\'{e}chet mean
(barycenter) is sticky. This nonclassical phenomenon is quantified by a law of
large numbers (LLN) stating that the empirical mean eventually almost surely
lies on the (codimension and hence measure ) spine that is the glued
hyperplane, and a central limit theorem (CLT) stating that the limiting
distribution is Gaussian and supported on the spine. We also state versions of
the LLN and CLT for the cases where the mean is nonsticky (i.e., not lying on
the spine) and partly sticky (i.e., is, on the spine but not sticky).Comment: Published in at http://dx.doi.org/10.1214/12-AAP899 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Continuous Nucleation and Size Dependent Growth Kinetics of Indium Phosphide Nanocrystals
Aminophosphines derived from N,N′-disubstituted ethylenediamines (R–N(H)CH2CH2N(H)–R; R = ortho-tolyl, phenyl, benzyl, iso-propyl, and n-octyl) were used to adjust the kinetics of InP nanocrystal formation by more than 1 order of magnitude. Ultraviolet–visible absorption and 31P nuclear magnetic resonance measurements demonstrate that the rate of nanocrystal formation is limited by the precursor reactivity. At low temperature (180 °C), crystal nucleation is concurrent with growth throughout the reaction, rather than occurring in a burst at early times. The low temperature produces a narrow range of small sizes (d = 4.2–4.9 nm) regardless of the precursor used. Higher temperatures (up to 270 °C) promote growth to larger sizes (d ≤ 7.8 nm), shorten the nucleation period, and create conditions where the final size is controlled by the precursor conversion reactivity. The temperature dependence is proposed to arise from growth kinetics that slow as the nanocrystal size increases, a novel surface attachment limited size distribution-focusing mechanism. Such a mechanism supports a narrow size distribution without separating the nucleation and growth phases
Flight Simulator Evaluation of Enhanced Propulsion Control Modes for Emergency Operation
This paper describes piloted evaluation of enhanced propulsion control modes for emergency operation of aircraft. Fast Response and Overthrust modes were implemented to assess their ability to help avoid or mitigate potentially catastrophic situations, both on the ground and in flight. Tests were conducted to determine the reduction in takeoff distance achievable using the Overthrust mode. Also, improvements in Dutch roll damping, enabled by using yaw rate feedback to the engines to replace the function of a stuck rudder, were investigated. Finally, pilot workload and ability to handle the impaired aircraft on approach and landing were studied. The results showed that improvement in all aspects is possible with these enhanced propulsion control modes, but the way in which they are initiated and incorporated is important for pilot comfort and perceived benefit
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