We report the growth and characterization of ternary AlxGa1- xAs nanowires by metalorganic chemical vapor deposition as a function of temperature and V/III ratio. Transmission electron microscopy and energy dispersive X-ray spectroscopy show that, at high temperatures and high V/III ratios, the nanowires form a core-shell structure with higher Al composition in the nanowire core than in the shell. We develop a growth model that takes into account diffusion of reactants and decomposition rates at the nanowire catalyst and stem to describe the compositional difference and the shell growth rate. Utilizing this model, we have successfully grown compositionally uniform Al0.16Ga0.84As nanowires. The ability to rationally tune the composition of ternary alloy nanowires broadens the application range of nanowires by enabling more complex nanowire heterostructures.

We demonstrate an organic/inorganic solar cell architecture based on a blend of poly(3-hexylthiophene) (P3HT) and narrow bandgap GaAs nanowires. The measured increase of device photocurrent with increased nanowire loading is correlated with structural ordering within the active layer that enhances charge transport. Coating the GaAs nanowires with TiO(x) shells passivates nanowire surface states and further improves the photovoltaic performance. We find that the P3HT/nanowire cells yield power conversion efficiencies of 2.36% under white LED illumination for devices containing 50 wt % of TiO(x)-coated GaAs nanowires. Our results constitute important progress for the use of nanowires in large area solution processed hybrid photovoltaic cells and provide insight into the role of structural ordering in the device performance.

We report the controlled growth of vertically aligned GaAs/AlGaAs core-shell nanowires. By optimizing the shell deposition temperature and catalyst density we maintain high temperature stability and achieve defect-free epitaxial AlGaAs shell deposition with high aluminum incorporation. Energy dispersive x-ray analysis determines the shell composition to be Al0.9Ga0.1As and measures the uniformity of the shell thickness. Lattice-resolved high-angle annular dark-field scanning transmission electron microscopy images confirm the core-shell interface to be defect-free, epitaxial, and atomically sharp. The ability to realize GaAs/AlGaAs core-shell nanowires with precise control over the morphology and composition is essential to the development of nanowire-based high mobility electronics.

The deposition of n-GaAs shells is explored as a method of n-type doping in GaAs nanowires grown by the Au-mediated metal-organic chemical vapor deposition. Core-shell GaAs/n-GaAs nanowires exhibit an unintended rectifying behavior that is attributed to the Au diffusion during the shell deposition based on studies using energy dispersive X-ray spectroscopy, current-voltage, capacitance-voltage, and Kelvin probe force measurements. Removing the gold prior to n-type shell deposition results in the realization of n-type GaAs nanowires without rectification. We directly correlate the presence of gold impurities to nanowire electrical properties and provide an insight into the role of seed particles on the properties of nanowires and nanowire heterostructures.

The behavior of metals at high pressure is of great importance to the fields of shock physics, geophysics, astrophysics, and nuclear materials. We study here bulk crystalline aluminum from firstprinciples at pressures up to 2500 GPa -soon within reach of laser-based experimental facilities. Our simulations use density-functional theory and density-functional perturbation theory in the localdensity and generalized-gradient approximations. Notably, the two different exchange-correlation functionals predict very similar results for the f cc → hcp, f cc → bcc, and hcp → bcc transition pressures, around 175 GPa, 275 GPa, and 380 GPa respectively. In addition, our results indicate that core overlaps become noticeable only beyond pressures of 1200 GPa. From the phonon dispersions of the fcc phase at increasing pressure, we predict a softening of the lowest transverse acoustic vibrational mode along the [110] direction, which corresponds to a Born instability of the fcc phase around 725 GPa.First-principles calculations have proved useful to the fields of geophysics, 1 astrophysics, 2 and nuclear materials. 3 Aluminum, being cubic close-packed and having no d-shell electrons, is a prototype for theoretical predictions and understanding the high-pressure behavior of simple metals. 4 Currently the National Ignition Facility 5 at LLNL is expected to achieve shockless compression 6 of metals up to 2000 GPa. This new facility may provide rapid advancements to high-pressure physics and could partner very successfully with theoretical studies.The equation of state (EOS) and phase stability of aluminum were first studied from first-principles in the early 1980s. 7,8,9 In all cases the predicted phase sequence was f cc → hcp → bcc, but predictions differed greatly in the transition pressures. Several other calculations within the local-density approximation (LDA) 10 or the generalized-gradient approximation (GGA) 11,12 have since then been performed, with a predicted static (i.e. without the phonon contribution) f cc → hcp transition pressure of 205 ± 20 GPa 10 in LDA and 170 GPa 11 and 192 GPa 12 in GGA. These discrepancies are more notable for the hcp → bcc transition pressure: 565 ± 60 GPa 10 in LDA versus 360 GPa 11 in GGA, leaving significant uncertainties open. Theoretical work on the vibrational properties of aluminum also suggests for the f cc → hcp transition a transition pressure higher than the static one. 11,12 Elastic properties 13,14 and the absolute strength under tension 15 have also been calculated; the latter results are of particular interest as they demonstrate the important role vibrational modes play in determining mechanical stability and suggest that shear failure modes are inherent in aluminum.Experimentally, the equation of state of aluminum at high pressures was studied by shock-compression 16 at pressures above the predicted maximum for the f cc → hcp phase boundary, 10 but a transition was not observed. However recent diamond anvil cell experiments observed a f cc → hcp transition at 217 ± 10 GPa 17 ...