First-principle study of sulfur vacancy and O₂ adsorption on the electronic and optical properties of ferroelectric CuInP₂S₆ monolayer

Sulfur vacancy in MoS₂ has been found to have an important influence on the performance of optoelectronic devices. Here, we study the effect of sulfur vacancy and O₂ adsorption on the electronic and optical properties in the two-dimensional ferroelectric CuInP₂S₆. It is revealed that a defect state appears at the top of valence band with the presence of sulfur vacancy. However, when O₂ is chemisorbed at sulfur vacancy, the defect state disappears. The variation of charge state and charge transfer are calculated and discussed. Although the ferroelectricity is greatly suppressed with the presence of sulfur vacancy, the ferroelectric state can be recovered when the O₂ is adsorbed. Within the framework of GW + BSE method, the optical absorption edge of CuInP₂S₆ monolayer exhibits a red-shift for the presence of sulfur vacancy and further O₂ adsorption gives rise to a blue-shift of the spectrum. Our findings have shown an effective way to improve the functionality of two-dimensional ferroelectrics via defect engineering. (paper)