Growth kinetics of heterostructured GaP-GaAs nanowires

We have studied the vapor-liquid-solid (VLS) growth dynamics of GaP and GaAs in heterostructured GaP-GaAs nanowires. The wires containing multiple GaP-GaAs junctions were grown by the use of metal-organic vapor phase-epitaxy (MOVPE) on SiO(2), and the lengths of the individual sections were obtained from transmission electron microscopy. The growth kinetics has been studied as a function of temperature and the partial pressures of the precursors. We found that the growth of the GaAs sections is limited by the arsine (AsH(3)) as well as the trimethylgallium (Ga(CH(3))(3)) partial pressures, whereas the growth of GaP is a temperature-activated, phosphine(PH(3))-limited process with an activation energy of 115 +/- 6 kJ/mol. The PH(3) kinetics obeys the Hinshelwood-Langmuir mechanism, indicating that the dissociation reaction of adsorbed PH(3) into PH(2) and H on the catalytic gold surface is the rate-limiting step for the growth of GaP. In addition, we have studied the competitive thin layer growth on the sidewalls of the nanowires. Although the rate of this process is 2 orders of magnitude lower than the growth rate of the VLS mechanism, it competes with VLS growth and results in tapered nanowires at elevated temperatures.     

Linear scaling density fitting

Two modifications of the resolution of the identity (RI)/density fitting (DF) approximations are presented. First, we apply linear scaling and J-engine techniques to speed up traditional DF. Second, we develop an algorithm that produces local, accurate fits with effort that scales linearly with system size. The fits produced are continuous, differentiable, well-defined, and do not require preset fitting domains. This metric-independent technique for producing a priori local fits is shown to be accurate and robust even for large systems. Timings are presented for linear scaling RI/DF calculations on large one-, two-, and three-dimensional carbon systems.     

Quantum computing based on vibrational eigenstates: pulse area theorem analysis

In a recent paper [D. Babikov, J. Chem. Phys. 121, 7577 (2004)], quantum optimal control theory was applied to analyze the accuracy of quantum gates in a quantum computer based on molecular vibrational eigenstates. The effects of the anharmonicity parameter of the molecule, the target time of the pulse, and the penalty function on the accuracy of the qubit transformations were investigated. We demonstrate that the effects of all the molecular and laser-pulse parameters can be explained utilizing the analytical pulse area theorem, which originates from the standard two-level model. Moreover, by analyzing the difference between the optimal control theory results and those obtained using the pulse area theorem, it is shown that extremely high quantum gate fidelity can be achieved for a qubit system based on vibrational eigenstates.     

Stabilizing and destabilizing effects of phenylalanine --> F5-phenylalanine mutations on the folding of a small protein

We report a systematic evaluation of phenylalanine-to-pentafluorophenylalanine (Phe --> F5-Phe) mutants for the 35-residue chicken villin headpiece subdomain (c-VHP), the hydrophobic core of which features a cluster of three Phe side chains (residues 6, 10, and 17). Phe --> F5-Phe mutations are interesting because aryl-perfluoroaryl interactions of optimal geometry are intrinsically more favorable than aryl-aryl interactions and because perfluoroaryl units are more hydrophobic than are analogous aryl units. One mutant, Phe-10 --> F5-Phe, provides enhanced tertiary structural stability relative to the native sequence. The other six mutants analyzed caused a decrease in stability.     

Asymmetric aziridine synthesis by aza-Darzens reaction of N-diphenylphosphinylimines with chiral enolates. Part 1: Formation of cis-aziridines

The aza-Darzens (‘ADZ’) reactions of N-diphenylphosphinyl (‘N-Dpp’) imines with chiral enolates derived from oxazolidinones and camphorsultam have been studied. Whilst oxazolidinone enolates reacted poorly in terms of aziridination, the use of the chiral enolate derived from both antipodes of N-bromoacetyl 2,10-camphorsultam, 2R-(5) and 2S-(5), with N-diphenylphosphinyl aryl and tert-butylimines proceeded in generally good yield to give, respectively, (2′R,3′R)- or (2′S,3′S)-cis-N-diphenylphosphinyl aziridinoyl sultams of high de.     

Formation of a ripple phase in nanotubular dimyristoylphosphatidylcholine bilayers confined inside nanoporous aluminum oxide substrates observed by DSC

Phase properties of substrate-supported nanotubular dimyristoylphosphatidylcholine (DMPC) bilayers confined within nanoporous channels of anodic aluminum oxide were characterized by DSC and compared with unsupported vesicles. In addition to the main phase transition, all samples exhibited a pretransition with a characteristic midpoint hysteresis between heating and cooling scans. The pretransition indicates that nanotubular bilayers could exist in a ripple phase, whereas hysteresis points to a similarity in the phase transition mechanisms. Observance of the ripple phase in lipid nanotubes is an indication of fully hydrated and only slightly perturbed bilayer surface.     

Self-assembled monolayers of aromatic thiols stabilized by parallel-displaced pi-pi stacking interactions

Parallel-displaced pi-pi stacking interactions have been known to be the dominant force in stabilizing the double helical structure of DNA and the tertiary structure of proteins. However, little is known about their roles in self-assembled monolayers of other large pi molecules such as aromatic thiols. Here we report on a systematic study of the self-assembled monolayers of four kinds of anthracene-based thiols, 9-mercaptoanthracene (MA), (4-mercaptophenyl) (9-anthryl) acetylene (MPAA), (4-mercaptophenyl) (10-nitro-9-anthryl) acetylene (MPNAA), and (4-mercaptophenyl) (10-carboxyl-9-anthryl) acetylene (MPCAA) on Au(111), in which a spacer and different functional groups (NO2 and COOH) are intentionally designed to introduce and thus allow the investigation of various intermolecular interactions, in addition to pi-pi interactions in the base molecules. We find that all molecules form long-range-ordered monolayers and, more interestingly, that these assembled monolayers exhibit essentially the same fundamental packing structure. On the basis of high-resolution scanning tunneling microscopy observations, we propose the space-filling models for the observed superstructures and demonstrate that all superstructures can be understood in terms of the parallel-displaced pi-pi stacking interactions, despite the presence of competing dipole-dipole and H-bonding interactions associated with these specially designed functional groups.     

Crystallization on surfaces of well-defined topography

Single crystals of calcite with regular patterned surfaces comprising close-packed arrays of hemispherical cavities or domes were produced by crystallization on colloidal monolayers or PDMS replicas of these monolayers, respectively. Perfect replication of the substrate topography was achieved for all colloidal particles, irrespective of their size and surface chemistry when the substrate geometry permitted unrestricted ion flow to the growing crystal. This work demonstrates that crystallization within a mould provides a very general route to producing single crystals with curved surfaces and unusual morphologies and that such patterning can be applied from the micro- to the nanoscale.     

Nature identification and morphology characterization of cation-exchange membrane fouling during conventional electrodialysis

The aim of this work was to study the effect of a concentrate solution pH value and of the composition in calcium, carbonate, and protein of a diluate solution to be treated by conventional electrodialysis on the fouling of cation-exchange membranes (CEM). It appeared that after demineralization of solutions containing CaCl(2) and CaCl(2)+Na(2)CO(3) using a concentrate solution maintained at a pH of 12, mineral fouling appeared on both sides of the CEM. The nature of the deposits was identified as calcium hydroxide and/or carbonate on both surfaces. The mineral fouling presented an aggregation-like crystal following a carnation-like pattern of aggregates of small rhombohedral crystals with CaCl(2) added alone, while CaCl(2)+Na(2)CO(3) yielded a smoother spherical crystal. Protein fouling was detected only on the CEM surface in contact with the diluate after demineralization of a solution containing CaCl(2)+Na(2)CO(3) using a concentrate pH value of 2.     

Anomalous small angle x-ray scattering determination of ion distribution around a polyelectrolyte biopolymer in salt solution

The distribution of counterions in solutions of high molecular mass hyaluronic acid, in near-physiological conditions where mono- and divalent ions are simultaneously present, is studied by small angle neutron scattering and anomalous small angle x-ray scattering. The solutions contain either sodium or rubidium chloride together with varying concentrations of calcium or strontium chloride. The effects of monovalent-divalent ion exchange dominate the amplitude and the form of the counterion cloud. In the absence of divalent ions, the shape of the anomalous scattering signal from the monovalent ions is consistent with the distribution calculated from the Poisson-Boltzmann equation, as found by other workers. In mixtures of monovalent and divalent ions, however, as the divalent ion concentration increases, both the diameter and the amplitude of the monovalent ion cloud decrease. The divalent counterions always occupy the immediate neighborhood of the charged polyanion. Above a given concentration their anomalous scattering signal saturates. Even in a large excess of divalent ions, ion exchange is incomplete.