Achieving quantum confinement by bottom-up growth of nanowires has so far been limited to the ability of obtaining stable metal droplets of radii around 10 nm or less. This is within reach for gold-assisted growth. Because of the necessity to maintain the group III droplets during growth, direct synthesis of quantum sized structures becomes much more challenging for self-assisted III−V nanowires. In this work, we elucidate and solve the challenges that involve the synthesis of galliumassisted quantum-sized GaAs nanowires. We demonstrate the existence of two stable contact angles for the gallium droplet on top of GaAs nanowires. Contact angle around 130°fosters a continuous increase in the nanowire radius, while 90°allows for the stable growth of ultrathin tops. The experimental results are fully consistent with our model that explains the observed morphological evolution under the two different scenarios. We provide a generalized theory of self-assisted III−V nanowires that describes simultaneously the droplet shape relaxation and the NW radius evolution. Bistability of the contact angle described here should be the general phenomenon that pertains for any vapor−liquid−solid nanowires and significantly refines our picture of how nanowires grow. Overall, our results suggest a new path for obtaining ultrathin one-dimensional III−V nanostructures for studying lateral confinement of carriers. KEYWORDS: III−V semiconductors on silicon, nanowires, nanoneedles, regular arrays, self-assisted growth, droplet size engineering, crystal structure, growth modeling F undamental physical properties and applications of semiconductor nanowires (NWs) may benefit from quantum confinement due to size-dependent modulation of the density of states as well as modification of the photon energy for the allowed optical transitions.1 In GaAs, quantum confinement occurs for a size below 25 nm.2,3 In addition to quantum confinement, the optical properties can be engineered by modifying the NW shape. For example, progressively tapered nanowires have exhibited adiabatic outcoupling in the emission of quantum dots, thereby enhancing their brightness. 4 A similar design may improve the light extraction or absorption in different optoelectronic devices including light emitting diodes, solar cells, and optical biosensors.5−11 Quantum confinement in the core of radial NW heterostructures allows for the fabrication of high quality NW-based quantum dots, which are a perfect platform for delicate quantum transport experiments. 12,13 It is admittedly challenging to obtain very thin (quantumconfined) III−V NWs directly by the vapor−liquid−solid (VLS) growth technique. There are several reasons for that (see ref 14 for a review), including the Gibbs−Thomson effect of elevation of chemical potential due to the curvature of the droplet surface and difficulties in obtaining very small droplets on the substrate surface. Thin tips of VLS GaAs NWs with a stable radius down to 5 nm have previously been grown by hydride vapor phase epitaxy 15,16 (a te...

Topological qubits based on Majorana Fermions have the potential to revolutionize the emerging field of quantum computing by making information processing significantly more robust to decoherence. Nanowires are a promising medium for hosting these kinds of qubits, though branched nanowires are needed to perform qubit manipulations. Here we report a gold-free templated growth of III-V nanowires by molecular beam epitaxy using an approach that enables patternable and highly regular branched nanowire arrays on a far greater scale than what has been reported thus far. Our approach relies on the lattice-mismatched growth of InAs on top of defect-free GaAs nanomembranes yielding laterally oriented, low-defect InAs and InGaAs nanowires whose shapes are determined by surface and strain energy minimization. By controlling nanomembrane width and growth time, we demonstrate the formation of compositionally graded nanowires with cross-sections less than 50 nm. Scaling the nanowires below 20 nm leads to the formation of homogeneous InGaAs nanowires, which exhibit phase-coherent, quasi-1D quantum transport as shown by magnetoconductance measurements. These results are an important advance toward scalable topological quantum computing.

We report a highly versatile and one-pot microwave route to the mass production of three-dimensional graphene-carbon nanotube-iron oxide nanostructures for the efficient removal of arsenic from contaminated water. The unique three-dimensional nanostructure shows that carbon nanotubes are vertically standing on graphene sheets and iron oxide nanoparticles are decorated on both the graphene and the carbon nanotubes. The material with iron oxide nanoparticles shows excellent absorption for arsenic removal from contaminated water, due to its high surface-to-volume ratio and open pore network of the graphene-carbon nanotube-iron oxide three-dimensional nanostructures.

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Reproducible integration of III-V semiconductors on silicon can open new path toward CMOS compatible optoelectronics and novel design schemes in next generation solar cells. Ordered arrays of nanowires could accomplish this task, provided they are obtained in high yield and uniformity. In this work, we provide understanding on the physical factors affecting size uniformity in ordered GaAs arrays grown on silicon. We show that the length and diameter distributions in the initial stage of growth are not much influenced by the Poissonian fluctuation-induced broadening, but rather are determined by the long incubation stage. We also show that the size distributions are consistent with the double exponential shapes typical for macroscopic nucleation with a large critical length after which the nanowires grow irreversibly. The size uniformity is dramatically improved by increasing the As flux, suggesting a new path for obtaining highly uniform arrays of GaAs nanowires on silicon.