Ultra-High Mechanical Flexibility of 2D Silicon Telluride*

Silicon telluride (Si₂Te₃) is a two-dimensional material with a unique variable structure where the silicon atoms form Si-Si dimers to fill the “metal” sites between the Te layers. The Si-Si dimers have four possible orientations: three in-plane and one out-of-the plane directions. The structural variability of Si₂Te₃ allows unusual properties, especially mechanical properties. First-principles density functional theory calculations are performed to determine the critical strain of monolayer Si₂Te₃. The results show that Si₂Te₃ can sustain a critical uniaxial tensile strain up to 38% with a breaking stress of 8.63 N/m along the direction of Si dimer, making Si₂Te₃ the most flexible 2D material reported. Because of the high flexibility, a large strain can be used to tune the band structure, and the bandgap can be reduced by up to 1.5 eV. With increasing strain, the bandgap undergoes an unusual indirect-direct-indirect-direct transition. We also find that the uniaxial strain can effectively control the orientation of Si dimers, which may be beneficial for certain applications.