1 Introduction Global energy demand is predicted to exceed 30 TW by 2050, about double the present value [1]. This predicament, known as the TeraWatt challenge, and concern over anthropogenic climate change, resource availability ("peak energy") and energy security have all increased the interest in renewable energy (hydroelectric, wind, solar, geothermal and biomass) [2]. One of the most promising renewable energy technologies is solar photovoltaics (PV) which convert sunlight directly into electrical energy. Although the resource potential of PV is enormous, it currently constitutes a small fraction (<1%) of global energy supply [2]. One of the main factors limiting the widespread adoption of PV is its low energy density, low efficiency, and relatively high cost in comparison to other energy technologies. This means that current PV technology can only compete in areas of high insolation or by government incentives such as feed-in tariff programs.One of the most relevant metrics for PV devices is the power conversion efficiency (PCE); that is, the efficiency with which sunlight can be converted to electrical power. A significant effort in PV research today aims to improve PCE while simultaneously reducing (or, at least, not significantly impacting) production cost. The vast majority of

Vertically oriented and ordered GaAs nanowire arrays have been grown by the self-assisted mechanism using substrates prepared with nano-patterned oxide templates. Patterned Ga-assisted GaAs nanowire growth on (111) silicon by molecular beam epitaxy showed that the axial and radial growth rates increased with increasing interhole spacing. A model is described which accounts for the correlation of the final length and diameter with pattern pitch. The model considers that growth material is supplied by a secondary flux of both gallium and arsenic adatoms desorbing from the oxide surface between the nanowires which subsequently impinge on the liquid droplet and nanowire sidewalls. We show that shading of the incident and scattered flux by neighboring nanowires in the array can strongly affect the axial and radial growth rates, leading to significant differences in final nanowire morphologies.

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Periodic arrays of self-catalyzed GaAs nanowires (NWs) were grown on Si substrates by gas source molecular beam epitaxy (GS-MBE) using patterned oxide templates. The various challenges of the patterning process that result in undesired outcomes are described, such as pattern transfer by wet/dry etching, oxide thickness variations, and native oxide re-growth. Transmission electron microscopy (TEM) results are used to illustrate each case. In particular, we show that a linearly increasing length-radius distribution, analogous to that observed for unpatterned self-catalyzed growth on substrates with thin oxides, may be obtained even when using patterned oxide masks due to an unintended residual layer of oxide, as confirmed by TEM analysis. We explain how a linear length-radius dependence can result from the individual NWs beginning their growth at different times, accompanied by significant radial growth. The spread in obtained NW dimensions was decreased by improving the patterning method.

Dimerized antiferromagnetic spin-1 2 ladders are known to exhibit a quantum critical phase transition in the ground state, the existence or absence of which is dependent on the dimerization pattern of the ladder. The gapped phases cannot be distinguished by the conventional Landau long-range order parameter. However, they possess a nonlocal (hidden) string-order parameter, which is nonzero in one phase and vanishes in the other. We use an exact diagonalization technique to calculate ground-state energies, energy gaps, and string-order parameters of dimerized two-and three-leg Heisenberg ladders, as well as a critical scaling analysis to yield estimates of the critical exponents ν and β.