Atom-thin transition metal dichalcogenides (TMDs) have emerged as fascinating materials and key structures for electrocatalysis. So far, their edges, dopant heteroatoms and defects have been intensively explored as active sites for the hydrogen evolution reaction (HER) to split water. However, grain boundaries (GBs), a key type of defects in TMDs, have been overlooked due to their low density and large structural variations. Here, we demonstrate the synthesis of wafer-size atom-thin TMD films with an ultra-high-density of GBs, up to ~1012 cm−2. We propose a climb and drive 0D/2D interaction to explain the underlying growth mechanism. The electrocatalytic activity of the nanograin film is comprehensively examined by micro-electrochemical measurements, showing an excellent hydrogen-evolution performance (onset potential: −25 mV and Tafel slope: 54 mV dec−1), thus indicating an intrinsically high activation of the TMD GBs.

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.

III-V semiconductor nanowires have shown great potential in various quantum transport experiments. However, realizing a scalable high-quality nanowire-based platform that could lead to quantum information applications has been challenging. Here, we study the potential of selective area growth by molecular beam epitaxy of InAs nanowire networks grown on GaAs-based buffer layers. The buffered geometry allows for substantial elastic strain relaxation and a strong enhancement of field effect mobility. We show that the networks possess strong spin-orbit interaction and long phase coherence lengths with a temperature dependence indicating ballistic transport. With these findings, and the compatibility of the growth method with hybrid epitaxy, we conclude that the material platform fulfills the requirements for a wide range of quantum experiments and applications.Material science plays a key role in quantum computing research. Long quantum state lifetimes -the fundamental prerequisite for realizing quantum computers -rely on the ability to produce materials with high purity and structural quality. Together with the requirements of scalability and reproducibility, these properties are what mainly defines the challenges of material science in quantum computing today. Proposals for topological quantum computing, 1-3 which are based on hybrid semiconductor-superconductor nanowire (NW) networks, are being pursued by numerous research groups and have ignited intense research efforts on hybrid epitaxy. 4-8 NW scalability is tightly related to the semiconductor growth approach. Top-down lithography has been used to define NWs in two-dimensional layers 5,9 and a variety of methods have been pursued for alignment and positioning of bottom-up vapor-liquid-solid (VLS) grown NWs, such as dielectrophoresis techniques, 10 nanoscale combing 11 and magnetic aligning of NWs. 12 Despite of these developments, large-scale synthesis of bottom-up grown high-mobility NW networks that are compatible with epitaxial interwire connections and semiconductor/superconductor epitaxy has still not been realized. To realize the epitaxial connections, a lot of effort has been put into the growth of branched NWs via the VLS method. 8,13-15 A scalable approach has been developed in Ref. [16,17] using template assisted growth of inplane NW networks. 18 Nonetheless, this approach is not yet compatible with superconductor epitaxy. An alternative scalable approach is to use lithographically defined openings in a mask on a crystalline substrate. This method is referred to as selective area growth (SAG) and until recently has mainly been used in conjunction with metal organic chemical vapour deposition 19,20 , metal organic vapour phase epitaxy 21,22 , chemical beam epitaxy and metal organic molecular beam epitaxy (chemical beam epitaxy). [23][24][25][26] In contrast to molecular beam epitaxy (MBE), the dissociation kinetics of the chemical precursors in these methods enhance the growth selectivity on masked substrates by expanding the growth parameter window, ...

In the present work, we demonstrate crystallographically textured n-type BiTeSe nanomaterials with exceptional thermoelectric figures of merit produced by consolidating disk-shaped BiTeSe colloidal nanocrystals (NCs). Crystallographic texture was achieved by hot pressing the asymmetric NCs in the presence of an excess of tellurium. During the hot press, tellurium acted both as lubricant to facilitate the rotation of NCs lying close to normal to the pressure axis and as solvent to dissolve the NCs approximately aligned with the pressing direction, which afterward recrystallize with a preferential orientation. NC-based BiTeSe nanomaterials showed very high electrical conductivities associated with large charge carrier concentrations, n. We hypothesize that such large n resulted from the presence of an excess of tellurium during processing, which introduced a high density of donor Te antisites. Additionally, the presence in between grains of traces of elemental Te, a narrow band gap semiconductor with a work function well below BiTeSe , might further contribute to increase n through spillover of electrons, while at the same time blocking phonon propagation and hole transport through the nanomaterial. NC-based BiTeSe nanomaterials were characterized by very low thermal conductivities in the pressing direction, which resulted in ZT values up to 1.31 at 438 K in this direction. This corresponds to a ca. 40% ZT enhancement from commercial ingots. Additionally, high ZT values were extended over wider temperature ranges due to reduced bipolar contribution to the Seebeck coefficient and the thermal conductivity. Average ZT values up to 1.15 over a wide temperature range, 320 to 500 K, were measured, which corresponds to a ca. 50% increase over commercial materials in the same temperature range. Contrary to most previous works, highest ZT values were obtained in the pressing direction, corresponding to the c crystallographic axis, due to the predominance of the thermal conductivity reduction over the electrical conductivity difference when comparing the two crystal directions.

Selective-area growth is a promising technique for enabling of the fabrication of the scalable III–V nanowire networks required to test proposals for Majorana-based quantum computing devices. However, the contours of the growth parameter window resulting in selective growth remain undefined. Herein, we present a set of experimental techniques that unambiguously establish the parameter space window resulting in selective III–V nanowire networks growth by molecular beam epitaxy. Selectivity maps are constructed for both GaAs and InAs compounds based on in situ characterization of growth kinetics on GaAs(001) substrates, where the difference in group III adatom desorption rates between the III–V surface and the amorphous mask area is identified as the primary mechanism governing selectivity. The broad applicability of this method is demonstrated by the successful realization of high-quality InAs and GaAs nanowire networks on GaAs, InP, and InAs substrates of both (001) and (111)B orientations as well as homoepitaxial InSb nanowire networks. Finally, phase coherence in Aharonov–Bohm ring experiments validates the potential of these crystals for nanoelectronics and quantum transport applications. This work should enable faster and better nanoscale crystal engineering over a range of compound semiconductors for improved device performance.