We have grown nearly freestanding single-layer 1T'-WTe2 on graphitized 6H-SiC(0001) by using molecular beam epitaxy (MBE), and characterized its electronic structure with scanning tunneling microscopy / spectroscopy (STM/STS). The existence of topological edge states at the periphery of single-layer WTe2 islands were confirmed. Surprisingly, a bulk band gap at the Fermi level and the insulating behaviors were also found in the single-layer WTe2 at low temperature, which is likely associated with an incommensurate charge order transition. The realization of two-dimensional topological insulators (2D TIs) in single-layer transition metal dichalcogenide (TMD) provides a promising platform for further exploration of the 2D TIs' physics and related applications.

Interfacial electron engineering between noble metal and transition metal carbide is identified as a powerful strategy to improve the intrinsic activity of electrocatalytic oxygen reduction reaction (ORR). However, this short-range effect and the huge structural differences make it a significant challenge to obtain the desired electrocatalyst with atomically thin noble metal layers. Here, we demonstrated the combinatorial strategies to fabricate the heterostructure electrocatalyst of Mo 2 C-coupled Pd atomic layers (AL-Pd/ Mo 2 C) by precise control of metal−organic framework confinement and covalent interaction. Both atomic characterizations and density functional theory calculations uncovered that the strong electron effect imposed on Pd atomic layers has intensively regulated the electronic structures and d-band center and then optimized the reaction kinetics. Remarkably, AL-Pd/Mo 2 C showed the highest ORR electrochemical activity and stability, which delivered a mass activity of 2.055 A mg Pd −1 at 0.9 V, which is 22.1, 36.1, and 80.3 times higher than Pt/C, Pd/C, and Pd nanoparticles, respectively. The present work has developed a novel approach for atomically noble metal catalysts and provides new insights into interfacial electron regulation.

The two-dimensional topological insulators host a full gap in the bulk band, induced by spin–orbit coupling (SOC) effect, together with the topologically protected gapless edge states. However, it is usually challenging to suppress the bulk conductance and thus to realize the quantum spin Hall (QSH) effect. In this study, we find a mechanism to effectively suppress the bulk conductance. By using the quasiparticle interference technique with scanning tunneling spectroscopy, we demonstrate that the QSH candidate single-layer 1T’-WTe2 has a semimetal bulk band structure with no full SOC-induced gap. Surprisingly, in this two-dimensional system, we find the electron–electron interactions open a Coulomb gap which is always pinned at the Fermi energy (EF). The opening of the Coulomb gap can efficiently diminish the bulk state at the EF and supports the observation of the quantized conduction of topological edge states.