Strain-induced semiconductor to metal transition in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>M</mml:mi><mml:msub><mml:mi>A</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi>Z</mml:mi></mml:mrow><mml:msub><mml:mrow/><mml:mn>4</mml:mn></mml:msub></mml:math> bilayers (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>M</mml:mi><mml:mo>=</mml:mo><mml:mi mathvariant="normal">Ti</mml:mi><mml:mo>,</mml:mo><mml:mi mathvariant="normal">Cr</mml:mi><mml:mo>,</mml:mo><mml:mi mathvariant="normal">Mo</mml:mi></mml:math>; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>A</mml:mi><mml:mo>=</mml:mo><mml:mi mathvariant="normal">Si</mml:mi></mml:math>; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Z</mml:mi><mml:mo>=</mml:mo><mml:mi mathvariant="normal">N</mml:mi><mml:mo>,</mml:mo><mml:mi mathvariant="normal">P</mml:mi></mml:math>)
Hongxia Zhong, Wenqi Xiong, Pengfei Lv, Jin Yu, Shengjun Yuan
Abstract
Very recently, a new type of two-dimensional layered material, ${\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$, was fabricated that is semiconducting with weak interlayer interaction, high strength, and excellent stability. We systematically investigate the effect of vertical strain on the electronic structure of ${MA}_{2}{Z}_{4}$ ($M=\text{Ti/Cr/Mo}, A=\mathrm{Si}, Z=\text{N/P}$) bilayers. Taking bilayer ${\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$ as an example, our first-principles calculations show that its indirect band gap decreases monotonically as the vertical compressive strain increases. Under a critical strain around 22%, it undergoes a transition from semiconductor to metal. We attribute this to the opposite energy shift of states in different layers, which originates from the built-in electric field induced by the asymmetric charge transfer between two inner sublayers near the interface. Similar semiconductor to metal transitions are observed in other strained ${MA}_{2}{Z}_{4}$ bilayers, and the estimated critical pressures to realize such transitions are within the same order as semiconducting transition metal dichalcogenides. The semiconductor to metal transitions in the family of ${MA}_{2}{Z}_{4}$ bilayers present interesting possibilities for strain-induced engineering of their electronic properties.