Artificial manipulation of electronic degrees of freedom is the key point to realize modern electronic devices. Both charge and spin of electron have been widely studied and applied to logic circuits and information storage devices. Valley, the unique degree of freedom of crystal electrons, has also attracted great attention of the researchers in the past decade. The valleytronics progress benefits from the tremendous improvements of the two-dimensional atomic layer material growth technologies and in-depth explorations of valley properties. Valleytronic materials, represented by two-dimensional transition metal dichalcogenides, have become an excellent platform for the research and design of new electronic devices due to their special optical responses and distinctive electronic transport properties. The valley devices have the advantages of fast operation, low energy consumption, less information loss, high integration and long transmission distance.
In this review, we first introduce the basic concepts and properties of the energy valley, such as the valley Hall effect and the valley circular dichroism. Second, we describe the crystal structures and energy band diagrams of the two-dimensional transition metal dichalcogenides. Third, the progress in artificial manipulation of the valley effects is summarized. Some approaches which can break the inversion symmetry and therefore induce the valley degree of freedom are introduced. Fourth, we discuss the methods of realizing valley polarization. Fifth, the developments of valleytronic devices in recent years are reviewed. Finally, a summary and an outlook are given.
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