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.

We introduce selective area grown hybrid InAs/Al nanowires based on molecular beam epitaxy, allowing arbitrary semiconductor-superconductor networks containing loops and branches. Transport reveals a hard induced gap and unpoisoned 2e-periodic Coulomb blockade, with temperature dependent 1e features in agreement with theory. Coulomb peak spacing in parallel magnetic field displays overshoot, indicating an oscillating discrete near-zero subgap state consistent with device length. Finally, we investigate a loop network, finding strong spin-orbit coupling and a coherence length of several microns. These results demonstrate the potential of this platform for scalable topological networks among other applications.Majorana zero modes (MZMs) at the ends of onedimensional topological superconductors are expected to exhibit non-Abelian braiding statistics [1, 2], providing naturally fault-tolerant qubits [3, 4]. Proposed realizations of braiding [5, 6], interference-based topological qubits [7-9] and topological quantum computing architectures [10] require scalable nanowire networks. While relatively simple branched or looped wires can be realized by specialized growth methods [11,12] or by etchand gate-confined two-dimensional hybrid heterostructures [13][14][15][16], selective area growth [17] enables deterministic patterning of arbitrarily complex structures. This allows complex continuous patterns of superconductorsemiconductor hybrids and topological networks.Following initial theoretical proposals [18,19], a number of experiments have reported signatures of Majorana zero modes (MZMs) in hybrid semiconductorsuperconductor nanowires [11], including zero-bias conductance peaks [15,16,[20][21][22][23][24][25][26] and Coulomb blockade peak spacing oscillations [27,28]. To date, experiments have used individual vapor-liquid-solid (VLS) nanowires [20][21][22][23][24][25] or gate-confined two-dimensional heterostructures [15,16]. Within these approaches, constructing complex topological devices and networks containing branches and loops [5-10] is a challenge. Recently, branched and looped VLS growth has been developed toward this goal [12,29].In this Letter, we investigate a novel approach to the growth of semiconductor-superconductor hybrids that allows deterministic on-chip patterning of topological superconducting networks based on SAG. We characterize key physical properties required for building Majorana networks, including a hard superconducting gap, induced in the semiconductor, phase-coherence length of several microns, strong spin-orbit coupling, and Coulomb block-ade peak motion compatible with interacting Majoranas. Overall, these properties show great promise for SAGbased topological networks.Selective area growth was realized on a semi-insulating (001) InP substrate. PECVD grown SiO x was patterned using electron beam lithography and wet etching. InAs wires with triangular cross-sections were grown by molecular beam epitaxy (MBE). The Al was grown in-situ by MBE using angled deposition covering one of the facet...

Bottom-up approaches for producing bulk nanomaterials have traditionally lacked control over the crystallographic alignment of nanograins. This limitation has prevented nanocrystal-based nanomaterials from achieving optimized performances in numerous applications. Here we demonstrate the production of nanostructured Bi SbTe alloys with controlled stoichiometry and crystallographic texture through proper selection of the starting building blocks and the adjustment of the nanocrystal-to-nanomaterial consolidation process. In particular, we hot pressed disk-shaped Bi SbTe nanocrystals and tellurium nanowires using multiple pressure and release steps at a temperature above the tellurium melting point. We explain the formation of the textured nanomaterials though a solution-reprecipitation mechanism under a uniaxial pressure. Additionally, we further demonstrate these alloys to reach unprecedented thermoelectric figures of merit, up to ZT = 1.96 at 420 K, with an average value of ZT = 1.77 for the record material in the temperature range 320-500 K, thus potentially allowing up to 60% higher energy conversion efficiencies than commercial materials.

Copper-based chalcogenides that comprise abundant, low-cost, and environmental friendly elements are excellent materials for a number of energy conversion applications, including photovoltaics, photocatalysis, and thermoelectrics (TE). In such applications, the use of solution-processed nanocrystal (NC) to produce thin films or bulk nanomaterials has associated several potential advantages, such as high material yield and throughput, and composition control with unmatched spatial resolution and cost. Here we report on the production of Cu3SbSe4 (CASe) NCs with tuned amounts of Sn and Bi dopants. After proper ligand removal, as monitored by nuclear magnetic resonance and infrared spectroscopies, these NCs were used to produce dense CASe bulk nanomaterials for solid state TE energy conversion. By adjusting the amount of extrinsic dopants, dimensionless TE figures of merit (ZT) up to 1.26 at 673 K were reached. Such high ZT values are related to an optimized carrier concentration by Sn doping, a minimized lattice thermal conductivity due to efficient phonon scattering at point defects and grain boundaries, and to an increase of the Seebeck coefficient obtained by a modification of the electronic band structure with the Bi doping. Nanomaterials were further employed to fabricate ring-shaped TE generators to be coupled to hot pipes and which provided 20 mV and 1 mW per TE element when exposed to a 160 °C temperature gradient. The simple design and good thermal contact associated with the ring geometry and the potential low cost of the material solution processing may allow the fabrication of TE generators with short payback times.Peer ReviewedPostprint (author's final draft