Unconventional zigzag indium phosphide single-crystalline and twinned nanowires

Unconventional zigzag indium phosphide (InP) single-crystalline and twinned nanowires were produced via thermal evaporation of indium phosphide in the presence of zinc selenide. The structure and morphology of the as-synthesized products were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Studies found that two type of nanowires exist in the products, namely, the periodic-rhombus-decorated single-crystalline InP (type I) nanowires and jagged twinned InP (type II) nanowires. Both of them have preferential 111 growth directions. The optical properties were also investigated at room temperature, and they show that the nanowires display a strong emission at approximately 750 nm, which is quite different from that observed in all previous reports related to the InP nanostructures.     

Morphology-controlled synthesis of ZnO nanostructures by a simple round-to-round metal vapor deposition route

One-dimensional ZnO nanostructures with different morphologies have been successfully synthesized through a simple round-to-round metal vapor deposition route at 550 degrees C with a zinc powder covered indium film as the source material. The structures and morphologies of the products were characterized in detail by using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Studies found that the morphology of the products can be easily tuned from one experimental round to another. Possible growth mechanisms for the formation of one-dimensional ZnO nanostructures with different morphologies are discussed. Photoluminescence studies show that there are sharp UV emission and broad defect-related green emissions for the products obtained in all experimental rounds. Relative intensity of the UV emission to defect-related emissions gradually increased from one experimental round to another.     

Nanosecond relaxation dynamics of protein GB1 identified by the time-dependent red shift in the fluorescence of tryptophan and 5-fluorotryptophan

The B1 domain of Streptococcal protein G (GB1) is a small, thermostable protein containing a single tryptophan residue. We recorded time-resolved fluorescence of the wild-type GB1 and its 5-fluorotryptophan (5FTrp) variant at more than 30 emission wavelengths between 300 and 470 nm. The time-resolved emission spectra reveal no signs of heterogeneity, but show a time-dependent red shift characteristic of microscopic dielectric relaxation. This is true for both 5FTrp and unmodified Trp in GB1. The time-dependent red shifts in the fluorescence of 5FTrp and unmodified Trp are essentially identical, confirming that the shift is caused by the relaxation of the protein matrix rather than by the fluorophore itself. The total amplitude (but not the rate) of the time-dependent red shift depends on the fluorophore, specifically, on the magnitude of the vector difference between its excited state and ground state electric dipole moments; for 5FTrp this is estimated to be about 88% of that for the unmodified Trp. The decay of the excited state fluorophore population is not monoexponential for either fluorophore; however, the deviation from the monoexponential decay law is larger in the case of unmodified Trp. The relaxation dynamics of GB1 was found to be considerably faster than that of other proteins studied previously, consistent with the small size, tightly packed core, and high thermodynamic stability of GB1.     

Carbon-coated single-crystalline zinc sulfide nanowires

Single-crystalline ZnS nanowires coated with graphitic carbon shells were synthesized by thermal evaporation of a mixture of ZnS and SnS powders in a graphite crucible. As-synthesized ZnS/C nanostructures were characterized using X-ray diffraction, scanning electron microscope, and transmission electron microscopy equipped with an energy-dispersive X-ray spectrometer. The ZnS core nanowires were formed by a Sn-catalytic vapor-liquid-solid process and grew along the [210] directions. Photoluminescence spectrum reveals that the carbon-coated ZnS nanowires have a strong emission band centered at 586 nm and a shoulder band at 645 nm.     

Full counting statistics for a single-electron transistor: nonequilibrium effects at intermediate conductance

We evaluate the current distribution for a single-electron transistor with intermediate strength tunnel conductance. Using the Schwinger-Keldysh approach and the drone (Majorana) fermion representation, we account for the renormalization of system parameters. Nonequilibrium effects induce a lifetime broadening of the charge-state levels, which suppress large current fluctuations.     

Imaging the dynamics of gas phase reactions

Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied--most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.     

Formation of the surface NO during N2O interaction at low temperature with iron-containing ZSM-5

Interaction of N2O at low temperatures (473-603 K) with Fe-ZSM-5 zeolites (Fe, 0.01-2.1 wt %) activated by steaming and/or thermal treatment in He at 1323 K was studied by the transient response method and temperature-programmed desorption (TPD). Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) of NO adsorbed at room temperature as a probe molecule indicated heterogeneity of surface Fe(II) sites. The most intensive bands were found at 1878 and 1891 cm(-1), characteristic of two types mononitrosyl species assigned to Fe2+(NO) involved in bi- and oligonuclear species. Fast loading of atomic oxygen from N2O on the surface and slower formation of adsorbed NO species were observed. The initial rate of adsorbed NO formation was linearly dependent on the concentration of active Fe sites assigned to bi- and oligonuclear species, evolving oxygen in the TPD at around 630-670 K. The maximal coverage of a zeolite surface by NO was estimated from the TPD of NO at approximately 700 K. This allowed the simulation of the dynamics of the adsorbed NO formation at 523 K, which was consistent with the experiments. The adsorbed NO facilitated the atomic oxygen recombination/desorption, the rate determining step during N2O decomposition to O2 and N2, taking place at temperatures > or =563 K.     

Intense photoluminescence from mixed solutions of C70 and palladium octaethylporphyrin: A supramolecular heavy atom effect

Unusually intense near-infrared (near-IR) photoluminescence has been observed from mixed solutions of C70 and palladium octaethylporphyrin (PdOEP). The novel emission has a spectrum similar to C70 phosphorescence and an intensity that is approximately 20 times greater than that of C70 fluorescence. The emitting species is identified as a noncovalently bound, short-lived triplet exciplex of C70 with PdOEP. The emission is essentially C70 phosphorescence intensified by spin-orbit coupling from the Pd atom in the nearby metalloporphyrin. This supramolecular heavy atom effect increases the C70 emissive quantum yield to approximately 1 x 10(-2) in degassed hexane solution at room temperature. The radiative rate constant is enhanced by a factor of 10(5), to approximately 7 x 10(4) s(-1), which is a value that exceeds the phosphorescence rate constant of PdOEP. Comparative studies in a rigid poly(methyl methacrylate) (PMMA) matrix show that the excited state of the static C70-PdOEP complex decays in approximately 150 ns. A Job's plot analysis shows that the complex has a 1:1 stoichiometry. It forms dynamically in solution and is relatively weakly bound, with an estimated equilibrium constant near 100 M(-1). Qualitatively similar supramolecular heavy atom effects were also observed for complexes of PdOEP with C60 and fullerene derivatives.