Heterodimensional superlattice with in-plane anomalous Hall effect.

Superlattices-a periodic stacking of two-dimensional layers of two or more materials-provide a versatile scheme for engineering materials with tailored properties1,2. Here we report an intrinsic heterodimensional superlattice consisting of alternating layers of two-dimensional vanadium disulfide (VS2) and a one-dimensional vanadium sulfide (VS) chain array, deposited directly by chemical vapour deposition. This unique superlattice features an unconventional 1T stacking with a monoclinic unit cell of VS2/VS layers identified by scanning transmission electron microscopy. An unexpected Hall effect, persisting up to 380 kelvin, is observed when the magnetic field is in-plane, a condition under which the Hall effect usually vanishes. The observation of this effect is supported by theoretical calculations, and can be attributed to an unconventional anomalous Hall effect owing to an out-of-plane Berry curvature induced by an in-plane magnetic field, which is related to the one-dimensional VS chain. Our work expands the conventional understanding of superlattices and will stimulate the synthesis of more extraordinary superstructures.

One-dimensional van der Waals heterostructures: Growth mechanism and handedness correlation revealed by nondestructive TEM.

We recently synthesized one-dimensional (1D) van der Waals heterostructures in which different atomic layers (e.g., boron nitride or molybdenum disulfide) seamlessly wrap around a single-walled carbon nanotube (SWCNT) and form a coaxial, crystalized heteronanotube. The growth process of 1D heterostructure is unconventional-different crystals need to nucleate on a highly curved surface and extend nanotubes shell by shell-so understanding the formation mechanism is of fundamental research interest. In this work, we perform a follow-up and comprehensive study on the structural details and formation mechanism of chemical vapor deposition (CVD)-synthesized 1D heterostructures. Edge structures, nucleation sites, and crystal epitaxial relationships are clearly revealed using transmission electron microscopy (TEM). This is achieved by the direct synthesis of heteronanotubes on a CVD-compatible Si/SiO2 TEM grid, which enabled a transfer-free and nondestructive access to many intrinsic structural details. In particular, we have distinguished different-shaped boron nitride nanotube (BNNT) edges, which are confirmed by electron diffraction at the same location to be strictly associated with its own chiral angle and polarity. We also demonstrate the importance of surface cleanness and isolation for the formation of perfect 1D heterostructures. Furthermore, we elucidate the handedness correlation between the SWCNT template and BNNT crystals. This work not only provides an in-depth understanding of this 1D heterostructure material group but also, in a more general perspective, serves as an interesting investigation on crystal growth on highly curved (radius of a couple of nanometers) atomic substrates.

Covalently functionalized layered MoS2 supported Pd nanoparticles as highly active oxygen reduction electrocatalysts.

Molybdenum disulfide nanosheets covalently modified with a 1,2-dithiolane derivative were used as a novel substrate for the immobilization of Pd nanoparticles (PdNPs) towards the development of a highly efficient hybrid electrocatalyst, namely PdNPs/f-MoS2, for the oxygen reduction in an alkaline medium. The newly prepared hybrid material was thoroughly characterized through complementary techniques such as Raman and IR spectroscopy, TGA, HRTEM, STEM/EELS, and EDS. The PdNPs/f-MoS2 nanohybrid exhibited excellent performance towards oxygen electroreduction with a positive onset potential of +0.066 V and a half-wave potential of -0.116 V vs. Hg/HgO, along with a high current response, which are superior to those of its graphene counterpart and comparable to those of the benchmark Pd/C product. Moreover, PdNPs/f-MoS2 was proved to be remarkably stable as chronoamperometric assays showed minimum activity loss among the tested materials, clearly outperforming the commercial catalyst. The excellent performance of PdNPs/f-MoS2 is attributable to (i) the high affinity of the catalytic PdNPs with the f-MoS2 substrate, (ii) the absence of any capping agent for the stabilization of PdNPs onto f-MoS2, and more importantly (iii) the preservation of the integrity of the MoS2 basal plane during the functionalization process. Lastly, the oxygen reduction on PdNPs/f-MoS2 proceeded through the energy efficient four-electron pathway, showing great potential for the use of layered transition metal dichalcogenides in energy conversion applications, comprising fuel cells.