First-principles study of stability, electronic and optical properties of two-dimensional Nb<sub>2</sub>SiTe<sub>4</sub>-based materials
Authors: Luo Xiong , Meng Wei-Wei , Chen Guo-Xu-Jia , Guan Xiao-Xi , Jia Shuang-Feng , Zheng He , Wang Jian-Bo ,   School of Physics and Technology, Center for Electron Microscopy, MOE Key Laboratory of Artificial Micro- and Nano-structures, and Institute for Advanced Studies, Wuhan University, Wuhan 430072, China,   Suzhou Institute of Wuhan University, Suzhou 215123, China,   Wuhan University Shenzhen Research Institute, Shenzhen 518057, China
Published: November 2020

Two-dimensional (2D) niobium silicon telluride (Nb<sub>2</sub>SiTe<sub>4</sub>) was recently proposed as a promising candidate in infrared detector, photoelectric conversion, polarized optical sensor and ferroelastic switching application due to its narrow bandgap, long-term air stability, high carrier mobility, etc. However, the in-plane strains and interfacial defects induced by the lattice misfits between functional layers are harmful to 2D heterojunction nanodevice performance, making the crystal-lattice regulation and strain engineering necessary to achieve lattice matching and strain-controllable interface. Here, using first-principles calculations and elemental substitutions, i.e., replacing cations (anions) with elements in the same group of periodic table, we identify three new and stable single-layer <i>A</i><sub>2</sub><i>BX</i><sub>4</sub> analogues (Nb<sub>2</sub>SiSe<sub>4</sub>, Nb<sub>2</sub>SnTe<sub>4</sub> and Ta<sub>2</sub>GeTe<sub>4</sub>) as appealing candidates in manipulating the lattice parameters of Nb<sub>2</sub>SiTe<sub>4</sub>. The controllable lattice parameters are 6.04 Å ≤ <i>a</i> ≤ 6.81 Å and 7.74 Å ≤ <i>b</i> ≤ 8.15 Å. Among them, Ta<sub>2</sub>GeTe<sub>4</sub> exhibits similar lattice parameters to Nb<sub>2</sub>SiTe<sub>4</sub> but smaller bandgap, yielding better response in far-infrared region. Strain engineering shows that the external biaxial tensile stress narrows the bandgaps of <i>A</i><sub>2</sub><i>BX</i><sub>4</sub> due to the downshifting in energy of conduction band minimum (CBM). External biaxial compressive stress induces valance band maximum (VBM) orbital inversion for Nb<sub>2</sub>SiTe<sub>4</sub>, Nb<sub>2</sub>GeTe<sub>4</sub> and Ta<sub>2</sub>GeTe<sub>4</sub>, which pushes up VBM and discontinues the trend of corresponding bandgap increase. In this case, the bandgap change depends on the competition between energy upshifts of both CBM and VBM. In the Nb<sub>2</sub>SiSe<sub>4</sub> and Nb<sub>2</sub>SnTe<sub>4</sub> cases, the d-p antibonding coupling in valance band is so strong that no valance band inversion appears while the bandgap increases by ~0.3 eV under −5% compressive strain. Regarding Nb<sub>2</sub>SiTe<sub>4</sub>, Nb<sub>2</sub>GeTe<sub>4</sub> and Ta<sub>2</sub>GeTe<sub>4</sub>, their bandgaps can hardly change under −5% compressive strain, indicating that the energy upshift in VBM equals that in CBM. Such a valance band inversion is attributed to Te outmost <i>p</i> orbital overlapping, which introduces more dispersive VBM and smaller effective mass of hole. Our findings suggest that Nb<sub>2</sub>SiTe<sub>4</sub> can be alloyed with Nb<sub>2</sub>SiSe<sub>4</sub>, Nb<sub>2</sub>SnTe<sub>4</sub> and Ta<sub>2</sub>GeTe<sub>4</sub> to achieve controllable device lattice matching while maintaining its superior properties at the same time. The use of external biaxial compressive stress can promote the hole diffusion and improve the device performance.