Xingxing Zhang

Advanced Materials, Volume 30, Issue 39, September 26, 2018.

Planar alignment of suspended graphene sheets is realized with a near‐perfect order parameter and high optical anisotropy by using a rotating magnetic field produced by a pair of small NdFeB magnets, and can be further patterned and immobilized by photolithography. The arbitrary orientational and spatial control has enabled a wide range of device applications of graphene and related materials.

Abstract

Planar alignment of disc‐like nanomaterials is required to transfer their superior anisotropic properties from microscopic individual structures to macroscopic collective assemblies. However, such alignment by electrical or magnetic field is challenging due to their additional degrees of orientational freedom compared to that of rod‐like nanostructures. Here, the realization of planar alignment of suspended graphene sheets using a rotating magnetic field produced by a pair of small NdFeB magnets and subsequent demonstration of high optical anisotropy and potential novel device applications is reported. Compared to partially aligned sheets with a static magnetic field, planar aligned graphene suspensions exhibit a near‐perfect order parameter, much higher birefringence and anisotropic absorption/transmission. A unique feature of discotic nanomaterial assemblies is that the observed order parameter and optical property can vary from isotropic to partial and complete alignment depending on the experimental configuration. By immobilizing and patterning aligned graphene in a UV‐curable polymer resin, we further demonstrated an all‐graphene permanent display, which exhibits wide‐angle, high dark‐bright contrast in either transmission or reflection mode without any polarizing optics. The ability to control and pattern graphene orientation in all three dimensions opens up new exploration and broad device applications of graphene.

We study the temperature-dependent photoluminescence of monolayer and bilayer molybdenum ditelluride in the temperature range between 5 K and room temperature. We disentangle the effects of interactions of excitons with acoustic and optical phonons and show that molybdenum ditelluride excitons have an unusually small coupling with acoustical phonons. This observation, together with the large luminescence yield which can be obtained from the bilayer, puts forward molybdenum ditelluride as a robust and bright light source in the near infrared range. The scaling of luminescence wavelength and linewidth of the molybdenum ditelluride bilayer differs from the observations in the monolayer by effects that can be traced to symmetry and wellwidth. This suggests a similar band alignment of mono- and bilayer, in contrast to other transition metal dichalcogenides.

We studied the nonlinear electric response in WTe 2 and MoTe 2 monolayers. When the inversion symmetry is breaking but the the time-reversal symmetry is preserved, a second-order Hall effect called the nonlinear anomalous Hall effect (NLAHE) emerges owing to the nonzero Berry curvature on the nonequilibrium Fermi surface. We reveal a strong NLAHE with a Hall-voltage that is quadratic with respect to the longitudinal current. The optimal current direction is normal to the mirror plane in these two-dimensional (2D) materials. The NLAHE can be sensitively tuned by an out-of-plane electric field, which induces a transition from a topological insulator to a normal insulator. Crossing the critical transition point, the magnitude of the NLAHE increases, and its sign is reversed. Our work paves the way to discover exotic nonlinear phenomena in inversion-symmetry-breaking 2D materials.

Although many possible two-dimensional (2D) topological insulators (TIs) have been predicted in recent years, there is still lack of experimentally realizable 2D TI. Through first-principles and tight-binding simulations, we found an effective way to stabilize the robust quantum spin Hall state with a large nontrivial gap of 227 meV in 2D honeycomb HgTe monolayer by the Al 2 O 3 (0 0 0 1) substrate. The band topology originates from the band inversion between the s -like and p -like orbitals that are contributed completely by the Hg and Te atoms, so the quantized edge states are restricted within the honeycomb HgTe monolayer. Meanwhile, the strong interaction between HgTe and Al 2 O 3 (0 0 0 1) ensures high stability of the atomic structure. Therefore, the TI states may be realized in HgTe/Al 2 O 3 (0 0 0 1) at high temperature.

MoTe 2 is a Weyl semimetal, which exhibits unique non-saturating magnetoresistance and strongly reinforced superconductivity under pressure. Here, we demonstrate that a novel mesoscopic superconductivity at ambient pressure arises on the surface of MoTe 2 with a critical temperature up to 5 K significantly exceeding the bulk T c   =  0.1 K. We measured the derivatives of I – V curves for hetero-contacts of MoTe 2 with Ag or Cu, homo-contacts of MoTe 2 as well as ‘soft’ point contacts (PCs). Large number of these hetero-contacts exhibit a dV / dI dependence, which is characteristic for Andreev reflection. It allows us to determine the superconducting gap Δ. The average gap values are 2Δ  =  1.30  ±  0.15 meV with a 2Δ/ k B T c ratio of 3.7  ±  0.4, which slightly exceeds the standard BCS value of 3.52. Furthermore, the temperature dependence of the gap follows a BCS...