First-principle Study On 2D Ferroelectric Material LaOBiS₂

Ferroelectric materials have spontaneous electric polarization and the polarization direction can be reversed by applying an external electric field.They have great application potential in many fields such as information storage,field effect devices,and sensor devices.When a traditional ferroelectric material perovskite compound is turned into ferroelectric thin film by surface epitaxial growth technology,its ferroelectricity will disappear at a critical thickness due to effect of the depolarization field.On the contrary,the ferroelectricity remains robust on two-dimensional materials due to the intrinsic low dimension nature and thus attract lots of attention in recent years.In this thesis,we are going to investigate a group of novel two-dimensional ferroelectric materials TL-LaOBiS₂.The main results including:We discovered a new two-dimensional ferroelectric material TL-LaOBiS₂ with a direct band gap of about 1 eV,ultrahigh anisotropic carrier mobility,and significant optical properties.TL-LaOBiS₂ have three distinctive distorted phases due to the lattice mismatch of the middle La O layer and ambilateral BiS₂ layers,which contributes to interlayer anti-ferroelectricity（AFE）,ferroelectricity（FE）,and orthogonal polarization configurations.The polarization of ferroelectric TL-LaOBiS₂ is about 17.4 μC/cm²,and the Curie temperature is about 420K,which implies potential applications in room temperature ferroelectric memory devices.Moreover,the ferroelectric performance can be further enhanced by applying external strain.In addition,an AFE to FE transition barrier of only 6.5 me V is found in TL-LaOBiS₂,which indicates a possible phase transition under an external electric field.The studies are extended to similar compounds like TL-LaOBiSe₂,TL-LaOSbS₂,TL-LaOSbSe₂,and TL-SrFBiS₂ which also have sizable electric polarizations due to their lattice mismatch.As a new type of two-dimensional ferroelectric material,the two-dimensional ferroelectric material TL-LaOBiS₂ and its analogues have great potential in future applications of ferroelectric devices.