Breaking Inversion Symmetry in 2D Semiconductors by Built-In Electric Fields across the van der Waals Metal-Semiconductor Interface.

As important building blocks of integrated optoelectronic chips, most multilayer and bulk 2D layered semiconductors undertake out-of-plane inversion symmetry, which limits their multifunctional applications. Here, we propose a facile interfacial engineering means of breaking such inversion symmetry by built-in electric fields across the semiconductor-metal van der Waals heterointerface. Broken inversion symmetry of 2H WS2 layers has been experimentally confirmed by strong second harmonic generation (SHG) in fabricated bulk WS2/Au heterostructures. Through correlated SHG, atomic force, and surface potential mapping measurements and density function theory simulation (DFT), we unambiguously confirm the proposed physical mechanism. These results provide more possibilities of utilizing a 2D semiconductor/metal heterostructure to construct multifunctional integrated optoelectronic chips.