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Understanding the electronic transport properties of layered, van der Waals transition metal halides (TMHs) and chalcogenides is a highly active research topic today. Of particular interest is the evolution of those properties with changing thickness as the 2D limit is approached. Here, we present the electrical conductivity of exfoliated single crystals of the TMH, cluster magnet, Nb

Understanding the electronic transport properties of layered, van der Waals transition metal halides (TMHs) and chalcogenides is a highly active research topic today. Of particular interest is the evolution of those properties with changing thickness as the 2D limit is approached. Here, we present the electrical conductivity of exfoliated single crystals of the TMH, cluster magnet, Nb3Cl8, over a wide range of thicknesses both with and without hexagonal boron nitride (hBN) encapsulation. The conductivity is found to increase by more than three orders of magnitude when the thickness is decreased from 280 {\mu}m to 5 nm, at 300 K. At low temperatures and below ~50 nm, the conductance becomes thickness independent, implying surface conduction is dominating. Temperature dependent conductivity measurements indicate Nb3Cl8 is an insulator, however the effective activation energy decreases from a bulk value of 310 meV to 140 meV by 5nm. X-ray photoelectron spectroscopy (XPS) shows mild surface oxidation in devices without hBN capping, however, no significant difference in transport is observed when compared to the capped devices, implying the thickness dependent transport behavior is intrinsic to the material. A conduction mechanism comprised of a higher conductivity surface channel in parallel with a lower conductivity interlayer channel is discussed., Comment: 17 pages, 3 figures

WTe

A flexible, nondestructive, and cost-effective replication technique for nanostructures is presented. The advantages of the process are: 1) it allows for tailoring structural parameters of the replica (e.g., line width) nearly independent of the structural geometry of the master; 2) it allows for replication of high-aspect-ratio structures also in polymer materials from solution (especially noncurable polymers) such as polystyrene and polymethylmethacrylate; 3) it includes an easy separation process, thus preserving the master for repeated use. Linear grating replicas with line widths ranging from 88 to 300 nm are obtained using a single nanostructured master. Nanofibers and complex nanopatterned replicas are achievable. The presented technique and its flexibility show that atomic layer deposition is a unique tool for the preparation of high-efficiency polarizer diffractive optics, photonics, electronics, and catalysts.

Ordered nanofiber arrays are a promising material platform for artificial adhesive structures, tissue engineering, wound dressing, sensor arrays, and self-cleaning surfaces. Their production via self-ordered porous alumina hard templates serving as shape-defining molds is well-established. However, their release requires the destruction of the hard templates, the fabrication of which is costly and time-consuming, by wet-chemical etching steps with acids or bases. We report the nondestructive mechanical extraction of arrays of cross-linked polyacrylate nanofibers from thus recyclable self-ordered nanoporous alumina hard templates. Silica replicas of the latter were synthesized using the extricated nanofiber arrays as secondary molds that could be mechanically detached from the molded material. The approach reported here, which can be combined with microstructuring, may pave the way for the high-throughput production of both functional nanofiber arrays and ordered nanoporous membranes consisting of a broad range of material systems.

We performed systematic adsorption studies using self-ordered nanoporous anodic aluminum oxide (AAO) in an extended range of mean pore diameters and with different pore topologies. These matrices were characterized by straight cylindrical pores having a narrow pore size distribution and no interconnections. Pronounced hysteresis loops between adsorption and desorption cycles were observed even in the case of pores closed at one end. These results are in contrast with macroscopic theoretical models and detailed numerical simulations of the adsorption in a single pore. Extensive measurements involving adsorption isotherms, reversal curves, and subloops carried out in closed-bottom pores suggest that the pores do not desorb independently from one another.

A detailed knowledge of the electronic properties of individual dislocations is necessary for next generation nanodevices. Dislocations are fundamental crystal defects controlling the growth of different nanostructures (nanowires) or appear during device processing. We present a method to record electric properties of single dislocations in thin silicon layers. Results of measurements on single screw dislocations are shown for the first time. Assuming a cross-section area of the dislocation core of about 1 nm2, the current density through a single dislocation is J = 3.8 × 1012 A/cm2 corresponding to a resistivity of ρ ≅ 1 × 10−8 Ω cm. This is about eight orders of magnitude lower than the surrounding silicon matrix. The reason of the supermetallic behavior is the high strain in the cores of the dissociated dislocations modifying the local band structure resulting in high conductive carrier channels along defect cores.

WTe$_2$ is one of a series of recently discovered high mobility semimetals, some of whose properties are characteristic of topological Dirac or Weyl metals. One of its most interesting properties is the unsaturated giant magnetoresistance that it exhibits at low temperatures. An important question is the degree to which this property can be ascribed to a conventional semi-metallic model in which a highly compensated, high mobility metal exhibits large magnetoresistance. Here we show that the longitudinal thermopower (Seebeck effect) of semi-metallic WTe$_2$ exfoliated flakes exhibits periodic sign changes about zero with increasing magnetic field that indicates distinct electron and hole Landau levels and nearly fully compensated electron and hole carrier densities. However, inconsistent with a conventional semi-metallic picture, we find a rapid enhancement of the Nernst effect at low temperatures that is nonlinear in magnetic field, which is consistent with Weyl points in proximity to the Fermi energy. Hence, we demonstrate the role played by the Weyl character of WTe$_2$ in its transport properties., 16 pages, 3 figures