  Very recently, a new type of two-dimensional layered material MoSi2N4 has
been fabricated, which is semiconducting with weak interlayer interaction, high
strength, and excellent stability. We systematically investigate theoretically
the effect of vertical strain on the electronic structure of MA2Z4 (M=Ti/Cr/Mo,
A=Si, Z=N/P) bilayers. Taking bilayer MoSi2N4 as an example, our first
principle 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 MA2Z4 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 observed in the family of MA2Z4 bilayers present interesting
possibilities for strain-induced engineering of their electronic properties.
