First-principles study of stability, electronic and optical properties of two-dimensional Nb2SiTe4-based materials

Abstract

Two-dimensional (2D) niobium silicon telluride (Nb₂SiTe₄) 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>₂<i>BX</i>₄ analogues (Nb₂SiSe₄, Nb₂SnTe₄ and Ta₂GeTe₄) as appealing candidates in manipulating the lattice parameters of Nb₂SiTe₄. 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, Ta₂GeTe₄ exhibits similar lattice parameters to Nb₂SiTe₄ 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>₂<i>BX</i>₄ due to the downshifting in energy of conduction band minimum (CBM). External biaxial compressive stress induces valance band maximum (VBM) orbital inversion for Nb₂SiTe₄, Nb₂GeTe₄ and Ta₂GeTe₄, 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 Nb₂SiSe₄ and Nb₂SnTe₄ 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 Nb₂SiTe₄, Nb₂GeTe₄ and Ta₂GeTe₄, 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 Nb₂SiTe₄ can be alloyed with Nb₂SiSe₄, Nb₂SnTe₄ and Ta₂GeTe₄ 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.