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Structural symmetry plays a crucial role in the electronic band structure and properties of two-dimensional materials. In contrast to graphene, monolayer transition metal dichalcogenides exhibit intrinsic in-plane asymmetry with suitable direct bandgaps and distinctive optical properties. Efforts have been devoted to breaking their out-of-plane mirror symmetry by applying external electric fields, vertical stacking, or functionalization. The successful fabrication of Janus transition metal dichalcogenides offers a synthetic strategy to breaking the vertical mirror symmetry, leading to a variety of novel properties, such as vertical piezoelectricity, Rashba spin splitting, and excellent exciton properties. Here, we discuss the universal fabrication approaches and unique properties of Janus transition metal dichalcogenides and further present a brief perspective on their potential applications and challenges.

Tunneling electroresistance (TER) effect was investigated in Pt/MgO/Pt/PbTiO3/Pt ferroelectric tunnel junctions (FTJs) theoretically. Compared to Pt/MgO/PbTiO3/Pt FTJs with the same composite barrier thickness (unit cells), FTJs with a thicker Pt interlayer (2 ∼ 3 unit cells) could provide 1 ∼ 6 orders of magnitude improvement in the TER ratio in a wide range of polarization. Resonant tunneling effect and/or enhanced asymmetry of the potential energy profile induced by the resonant-tunneling structure of MgO/Pt/PbTiO3 in Pt/MgO/Pt/PbTiO3/Pt FTJs is responsible for the TER improvement.

20-nm-thickness epitaxial BaTiO3 (BTO) thin films were prepared on the Nb-doped SrTiO3 single-crystal substrates by RF magnetron sputtering technology. The rectifying characteristic current-voltage curves were observed and was discussed using the Schottky barrier model taking into account the movement of oxygen vacancies. The leakage current mechanisms were interpreted by the Schottky emission and Poole-Frenkel emission near the Pt/BTO interface at lower bias and higher bias, respectively.