Emerging Two-Dimensional Materials for Device Applications

Two-dimensional (2D) materials, such as Graphene, h-BN and MoS₂, are promising candidates in a number of advanced device applications, owing to their exceptional electrical, optical and mechanical properties. Scalable growth of high quality 2D materials is crucial for their adoption in technological applications the same way the arrival of high-quality silicon single crystals was to the semiconductor industry. While CVD growth of wafer-scale monolayer graphene and TMDs has been demonstrated, considerable challenges still remain. In this talk, we first report a CVD method to grow fluorine rich 2D polymer (2DP-F) on varies substrates for low-k dielectrics in 2D TMDs based devices. Using precursors with low vaporize temperature, a uniform and ultra-flat 2DP-F film with controlled thickness can be deposited on silicon oxide wafer, glass, sapphire and mica substrates. Dielectric properties and relevant mechanical properties were carefully characterized. These 2DP-F films were then used as the substrate for transition metal dichalcogenides (TMDs) based devices, and significant improvements in device performances were found, possibly owing to the ultra-smooth and dangling bond free surface of 2DP-F that can significantly reduce the interfacial scattering of carriers.<br/><br/>Next, we show that atomically thin Cs₃Bi₂I₉, an all-inorganic lead-free perovskite derivative with strong optical activity can be successfully synthesized by vapor growth method. Reducing the dimensionality of such perovskites could utilize the materials’ advantages for solid-state information devices like valleytronics. By breaking the inversion symmetry, 2D Cs₃Bi₂I₉ flakes with odd-layer number exhibited persistent, optically addressable valley polarization. This study potentially opens generalizable CVD method for growing a broad range of 2D perovskites towards cost-effective and energy-efficient integrated device applications. Finally, we report a two-step vapor phase growth process for the creation of high-quality vdW heterostructures based on perovskites and TMDCs, such as 2D Cs₃Bi₂I₉/MoSe₂, with a large lattice mismatch. Supported by experimental and theoretical investigations, we discover that the Cs₃Bi₂I₉/MoSe₂ vdW heterostructure possesses hybrid band alignments consisting of type-I and type-II heterojunctions because of the existence of defect energy levels in Cs₃Bi₂I₉. More importantly, we demonstrate that the type-II heterojunction in the Cs₃Bi₂I₉/MoSe₂ vdW heterostructure not only shows a higher interlayer exciton density, but also exhibits a longer interlayer exciton lifetime than traditional 2D TMDCs based type-II heterostructures. Such vdW heterostructures provide promising platforms for exploring novel physics and cutting-edge optoelectronics and valleytronics applications.