A Broadly Applicable Strategy for Entry into Homogeneous Nickel(0) Catalysts from Air-Stable Nickel(II) Complexes

A series of air-stable nickel complexes of the form L[subscript 2]Ni(aryl) X (L = monodentate phosphine, X = Cl, Br) and LNi(aryl)X (L = bis-phosphine) have been synthesized and are presented as a library of precatalysts suitable for a wide variety of nickel-catalyzed transformations. These complexes are easily synthesized from low-cost NiCl[subscript 2]·6H[subscript 2]O or NiBr[subscript 2]·3H[subscript 2]O and the desired ligand followed by addition of 1 equiv of Grignard reagent. A selection of these complexes were characterized by single-crystal X-ray diffraction, and an analysis of their structural features is provided. A case study of their use as precatalysts for the nickel-catalyzed carbonyl-ene reaction is presented, showing superior reactivity in comparison to reactions using Ni(cod)[subscript 2]. Furthermore, as the precatalysts are all stable to air, no glovebox or inert-atmosphere techniques are required to make use of these complexes for nickel-catalyzed reactions.National Institute of General Medical Sciences (U.S.) (GM63755)National Science Foundation (U.S.). Graduate Research Fellowshi

Raman scattering as a probe of the tensile strain distribution in GaAs grown on Si(111) by molecular beam epitaxy

Epitaxial layers of GaAs grown on Si substrates, where the layer thickness greatly exceeds any critical thickness based on mismatch in lattice constant alone, have been shown to be under tensile strain for temperature at or below 300 K. This "thermal" strain arises from the difference in thermal expansion coefficients between GaAs and Si. We have performed Raman experiments on GaAs layers grown on both Si (001) and Si (111) substrates. We have observed a shift in the optical modes towards lower frequencies which is indicative of tensile strain in the GaAs layers, this is greater in the (111) growth direction than in the (001) one. In order to investigate the strain distribution as a function of distance from the GaAs/Si interface we have measured Raman spectra after successive removing of the epitaxial layer by chemical etching. We have found out that the strain decreases with increasing distance from the interface. We have developed the theory of Cerdeira et al. (1) to determine quantitatively the strain present in the heteroepitaxial layers. We have used, for the first time, polarization selection rules to separate the various components of the optical phonon modes. According to the theory we have observed that the doubly degenerate TO phonon line exhibits both a splitting and shift with strain, while only a shift is observed for the LO phonon line. In conformity with Cerdeira we have remarked that the strain dependence of the LO phonon is equal to that of the TO phonon mode observed in crossed polarization configuration

Phonon study of temperature evolution of strain in GaAs/Si(0 0 1) and GaAs/Si(1 1 1) heterostructures

The temperature evolution of strain in GaAs layers grown on Si (0 0 1) and Si (1 1 1) substrates has been investigated by Raman scattering. The Raman results show that for temperatures lower than a certain temperature (≅280 K for the (0 0 1) and ≅400 K for (1 1 1) growth directions) the phonon modes for the epilayers shift toward lower frequencies in comparison to bulk material. This is due to the “thermal” strain arising from the difference in thermal expansion coefficients between GaAs epilayer and Si. A reverse phenomenon occurs at higher temperatures indicating the presence of compressive strain due to the mismatch in lattice constants between the GaAs epilayers and the Si substrate

The e-h plasma in direct-gap GaAlas under 3-d confinement

Uniformely excited e-h plasma in direct-gap Ga1 12xAlxAs under 3-D confinement has been measured by photoluminescence at 2 K. A satisfactory fit to experiment in the whole range of densities investigated is given by a theoretical model with k-conservation and collision broadening, using a single value of density and temperature throughout the excited region. The fitted renormalized gap matches closely current theoretical values, independent of composition x