Controlling the properties of semiconductor/metal interfaces is a powerful method for designing functionality and improving the performance of electrical devices. Recently semiconductor/superconductor hybrids have appeared as an important example where the atomic scale uniformity of the interface plays a key role in determining the quality of the induced superconducting gap. Here we present epitaxial growth of semiconductor-metal core-shell nanowires by molecular beam epitaxy, a method that provides a conceptually new route to controlled electrical contacting of nanostructures and the design of devices for specialized applications such as topological and gate-controlled superconducting electronics. Our materials of choice, InAs/Al grown with epitaxially matched single-plane interfaces, and alternative semiconductor/metal combinations allowing epitaxial interface matching in nanowires are discussed. We formulate the grain growth kinetics of the metal phase in general terms of continuum parameters and bicrystal symmetries. The method realizes the ultimate limit of uniform interfaces and seems to solve the soft-gap problem in superconducting hybrid structures.

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Au free GaAs nanowires with zinc blende structure, free of twin planes and with remarkable aspect ratios, have been grown on (111) Si substrates by molecular beam epitaxy. Nanowires with diameters down to 20 nm are obtained using a thin native oxide layer on the Si substrates. We discuss how the structural phase distribution along the wire length is controlled by the effective V/III ratio and temperature at the growth interface and explain how to obtain a pure twin plane free zinc blende structure.

Atoms on the edge: The atomic edge structure of industrial‐style MoS2 nanocatalysts was imaged using single‐atom sensitive electron microscopy (see picture). The observed industrial‐style edge terminations match predictions of model catalyst studies and thus address the so‐called “materials gap” in catalysis.

Nanowire (NW) crystal growth via the vapour-liquid-solid mechanism is a complex dynamic process involving interactions between many atoms of various thermodynamic states. With increasing speed over the last few decades many works have reported on various aspects of the growth mechanisms, both experimentally and theoretically. We will here propose a general continuum formalism for growth kinetics based on thermodynamic parameters and transition state kinetics. We use the formalism together with key elements of recent research to present a more overall treatment of III-V NW growth, which can serve as a basis to model and understand the dynamical mechanisms in terms of the basic control parameters, temperature and pressures/beam fluxes. Self-catalysed GaAs NW growth on Si substrates by molecular beam epitaxy is used as a model system.

The III-V nanowire structure (zinc blende or wurtzite) grown by the vapor-liquid-solid process is shown to be highly dependent on the parameters which shape the droplet at the top of the nanowire. Under conditions that the droplet volume does not exceed a certain value, it is demonstrated that when the nucleation of the solid starts at the solid-liquid-vapor triple line, a relatively large droplet volume and low wetting angle favor the formation of the wurtzite structure. We show that the effective V/III flux ratio is the primary parameter controlling the structure.

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The quality of interfaces and surfaces is crucial for the performance of nanoscale devices. A pertinent example is the close tie between current progress in gate-tunable and topological superconductivity using semiconductor/superconductor electronic devices and the hard proximity-induced superconducting gap obtained from epitaxial indium arsenide/aluminium heterostructures. Fabrication of devices requires selective etch processes; these only exist for InAs/Al hybrids, which precludes the use of other, potentially better material combinations in functional devices. We present a crystal growth platform based on three-dimensional structuring of growth substrates for synthesising semiconductor nanowires with in-situ patterned superconductor shells, which enables independent choice of material by eliminating etching. We realise and characterise all the most frequently used architectures in superconducting hybrid devices, finding increased yield and electrostatic stability compared to etched devices, along with evidence of ballistic superconductivity. In addition to aluminium, we present hybrid devices based on tantalum, niobium and vanadium.One dimensional semiconductor (SE) nanowires (NWs) proximity coupled to superconductors (SU) have attracted considerable attention from the condensed matter community since the prediction 1,2 and observation of Majorana zero-modes 3-5 , which have been proposed as a basis for topologically protected quantum information processors 6,7 . To ensure topological protection, methods for growing disorder-free 'hard-gap' SE/SU epitaxial hybrids were developed 8-10 . These materials utilise bottom-up crystal growth of InAs nanowires with uniform epitaxial aluminium coatings, an approach which has been extended to high mobility two-dimensional systems 11,12 and selective area grown networks 13,14 . The success of epitaxial InAs/Al hybrids lies in the ability to realise important device classes such as normal metal spectroscopic devices, 5,9,11,12 Josephson Junctions 15-18 for gate-controlled transmon qubits 19,20 , and superconducting Majorana islands 21-23 , using top-down processing to selectively remove the Al. A limitation of this method is that relying on post-process etching inherently limits materials choice. For instance, despite strong incentives to utilise technologically important superconductors such as Nb 24 and NbTiN 25 -which exhibit higher transition temperatures, critical magnetic fields and superconducting energy gaps -selectively removing Nb from InAs remains an unsolved problem. Similarly, InSb is an attractive semiconductor due to its high mobility, g-factor and strong spin-orbit coupling 25-28 . Yet, selectively removing even aluminum from InSb without damage is impossible with known methods. Thus, most potential improvements in epitaxial SE/SU technology are predicated on developing a materials-independent method for device fabrication. An attractive approach to eliminate etching is to employ an in-situ 'shadow approach' to mask specific segments along the NW from supe...