Vanishing Schottky Barriers in Blue Phosphorene/MXene Heterojunctions

An appropriate electrode material is crucial for two-dimensional (2D) semiconductors, where a vanishing Schottky barrier is ideal but is a great challenge. Blue phosphorene (BlueP) is a promising 2D semiconductor for electronic and optoelectronic applications. Here, we report that Zr-, Hf-, and Nb-based 2D transition metal carbides (MXenes) are ideal electrode materials for BlueP based on extensive investigations of the electronic properties and interfacial Schottky barrier characteristics of BlueP/MXene heterojunctions by first-principles calculations. Our results show that the strong interaction between BlueP and bare MXenes destroys the semiconducting character of BlueP, and thus bare MXenes are not ideal contact electrodes. With the surface functionalization of MXene, the intrinsic electronic feature of BlueP is well preserved in the BlueP/surface-engineered MXene heterojunctions. Furthermore, the interfacial Schottky barriers of the heterojunctions are affected by the terminal surface groups on MXenes, and vanishing Schottky barriers are achieved in some MXenes with the formula Zr<sub><i>n</i>+1</sub>C<sub>n</sub>F<sub>2</sub>, Hf<sub><i>n</i>+1</sub>C<sub><i>n</i></sub>F<sub>2</sub>, Zr<sub><i>n</i>+1</sub>C<sub>n</sub>(OH)<sub>2</sub>, Hf<sub><i>n</i>+1</sub>C<sub><i>n</i></sub>(OH)<sub>2</sub>, and Nb<sub><i>n</i>+1</sub>C<sub><i>n</i></sub>(OH)<sub>2</sub>. Finally, we demonstrate that the work functions of MXenes and the interface dipole induced by charge rearrangement are two underlying factors to determine the magnitude of Schottky barriers. This work provides fundamentals for selecting ideal electrode material for BlueP and is also beneficial for optimizing electrodes for other 2D semiconductors.