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Owing to the small energy differences between its polymorphs, MoTe₂ can access a full spectrum of electronic states from the 2H semiconducting state to the 1T' semimetallic state and from the T_d Weyl semimetallic state to the superconducting state in the 1T' and T_d phase at low temperature. Thus, it is a model system for phase transformation studies as well as quantum phenomena such as the quantum spin Hall effect and topological superconductivity. Careful studies of MoTe₂ and its potential applications require large-area MoTe₂ thin films with high crystallinity and thickness control. Here, we present cm²-scale synthesis of 2H-MoTe₂ thinmore » films with layer control and large grains that span several microns. Layer control is achieved by controlling the initial thickness of the precursor MoO_x thin films, which are deposited on sapphire substrates by atomic layer deposition and subsequently tellurized. Despite the van der Waals epitaxy, the precursor–substrate interface is found to critically determine the uniformity in thickness and grain size of the resulting MoTe₂ films: MoTe₂ grown on sapphire show uniform films while MoTe₂ grown on amorphous SiO₂ substrates form islands. Furthermore, this synthesis strategy decouples the layer control from the variabilities of growth conditions for robust growth results and is applicable to growing other transition-metal dichalcogenides with layer control.« less

Quantum transport properties in monolayer graphene are sensitive to structural modifications. In this work we find that the introduction of a hexagonal lattice of antidots has a wide impact on weak localization and Shubnikov-de Haas (SdH) oscillation of graphene. The antidot lattice reduces both phase coherence and intervalley scattering length. Remarkably, even with softened intervalley scattering, i.e., the phase-breaking time is shorter than intervalley scattering time, coherence between time reversed states remains adequate to retain weak localization, an offbeat and rarely reported occurrence. Whereas SdH oscillation is boosted by the antidot lattice, the amplitude of the SdH signal rises rapidlymore » with the increasing antidot radius. But both effective mass and carrier density are reduced in a larger antidot lattice. A bandgap of ~10 meV is opened. The antidot lattice is an effective dopant-free way to manipulate electronic properties in graphene.« less

Due to the increased surface to volume ratios, topological nanomaterials can enhance contributions from the topological surface states in transport measurements, which is critical for device applications that exploit the topological properties. It is particularly important for topological semimetals in which bulk carriers are unavoidable to make them into nanostructures to reveal the nature of the topological surface states, such as the Fermi arcs or nodal lines. Here, we report the nanostructure synthesis of the recently discovered triple-point topological metal MoP by direct conversion of MoO₃ nanostructures and study their transport properties. We observe that the initial size of themore » MoO₃ templates critically determines the crystalline quality of the resulting MoP nanostructures: large MoO₃ flakes lead to porous MoP flakes, while narrow MoO₃ nanowires lead to MoP nanowires without pores. The size-dependent porosity observed in MoP nanostructures is attributed to the volume change during the conversion reaction and nanoscale confinement effects. For MoO₃ nanowires with diameters less than 10 nm, the resulting MoP nanowires are single crystalline. The resistivity values of MoP nanostructures are higher than the reported values of MoP bulk crystals owing to the porous nature. However, despite the high porosity present in MoP flakes, the residual resistance ratio is ~2 and the temperature-dependent resistivity curves do not show any strong surface or grain-boundary scattering. Demonstration of the facile synthesis of MoP nanostructures provides opportunities for careful investigations of the surface states in transport measurements and exploration of future electronic devices, including nanoscale interconnects.« less

Carbon nanostructures with the spherical voids exhibit interesting temperature and magnetic field dependent transport properties. By increasing the void size, the structures are tuned from metallic to insulating; in addition, the magnetoresistance (MR) is enhanced. Here, our investigation in the magnetic fields (B) up to 18 T at temperatures (T) from 250 mK to 20 K shows that at high temperatures (T > 2 K), the MR crosses over from quadratic to a non-saturating linear dependence with increasing magnetic field. Furthermore, all MR data in this temperature regime collapse onto a single curve as a universal function of B/T, followingmore » Kohler's rule. Remarkably, the MR also exhibits orientation insensitivity, i.e., it displays a response independent of the direction on the magnetic field.« less