2D materials provide extraordinary materials platform with many interesting physical properties distinct from their bulk counterpart and offer unprecedented opportunities for ultrathin electronic and optical applications. Here I will present our recent theoretical works on novel 2D multiferroics and 2D topological materials as well as a few device concepts. First, I will highlight our discoveries on 2D topological materials including binary and ternary transition metal dichalcogenides etc. Our works show that electric field, elastic strain, and van der Waals stacking can be utilized to introduce topological phase transition [1,2]. In particular, the electric field induced topological phase transition offers a possibility for realizing topological field effect transistor, which will be fundamentally different from conventional transistors. In the second example, I will introduce the discovery of 2D multiferroics in semiconducting group IV monochalcogenide monolayers [3]. These 2D materials exhibit giant spontaneous ferroelastic lattice strain and ferroelectric polarization. Both are strongly coupled, accompanied with large anisotropic 2D excitonic responses and nonlinear optical responses [4]. Our theoretical findings indicate that these multiferroic 2D materials possess low domain wall energy and migration barrier, suggesting great opportunities for multifunctional opto-electronic-mechanical devices such as 2D ferroelectric and ferroelastic memory and 2D ferroelectric excitonic photovoltaics. Finally, relevant experimental progresses as well as some critical challenges and opportunities in 2D materials and their nanostructures will be discussed.

1. Xiaofeng Qian, Junwei Liu, Liang Fu, and Ju Li. Quantum spin Hall effect in two-dimensional transition metal dichalcogenides. Science 346, 1344-1347 (2014).

2. Junwei Liu, Hua Wang, Chen Fang, Liang Fu, and Xiaofeng Qian. van der Waals Stacking-Induced Topological Phase Transition in Layered Ternary Transition Metal Chalcogenides. Nano Letters 17, 467-475 (2017).

3. Hua Wang and Xiaofeng Qian. Two-dimensional multiferroics in monolayer group IV monochalcogenides. 2D Materials 4, 015042 (2017).

4. Hua Wang and Xiaofeng Qian. Giant Optical Second Harmonic Generation in Two-Dimensional Multiferroics. Nano Letters 17, 5027-5034 (2017).