Using first-principles calculation, we have studied the adsorption effect of small gas molecules (H₂O, CO₂, CH₄, SO₂, H₂S, and NH₃) on GeP₃ monolayer. To determine the most stable adsorption site, five adsorption sites (center, Ge, P, bridge GeP, and bridge PP) were considered in the paper. Through calculations of adsorption energy, adsorption distance, and charge transfer, we preliminarily determined that H₂O, CO₂, and CH₄ were physically adsorbed on GeP₃ via weak van der Waals force. However, SO₂, H₂S, and NH₃ were chemically adsorbed on GeP₃ with new covalent bonds formed, as concluded by calculations of electron localization function and charge density difference. Gas molecule adsorption can cause significant changes in the band gap of single-layer GeP₃, indicating that pristine GeP₃ monolayer is sensitive to these gases. In addition, the adsorption energy of the H₂O, CO₂, and CH₄ adsorbed on GeP₃ can be tuned effectively by employing an external electric field. Our theoretical studies reveal that GeP₃ monolayer is a promising gas-sensitive material used in nanometer devices.                                                                                                    

We have investigated the electronic and optical properties of monolayer SnP₃ adsorbed by small gas molecules by using first-principle calculations based on density functional theory. Six initial adsorption sites were selected to explore the most sensitive site to investigate the optimal adsorption positions of CH₄, H₂O, NH₃, NO, NO₂, and SO₂ adsorbed on the monolayer SnP₃. Aiming at the optimal adsorption configurations, the adsorption energy, adsorption distance, and charge densities are computed to investigate the adsorption types. Our results reveal that monolayer SnP₃ is sensitive to NO and SO₂ via strong physical adsorption, and NO₂ via chemical adsorption, thereby forming a new bond between O and P atoms. Analysis of the optical property shows that the adsorption of NO, NO₂, and SO₂ can remarkably influence the dielectric function and adsorption coefficient of monolayer SnP₃. Our theoretical studies indicate that monolayer SnP₃ is a good gas-sensitive material used for NO, NO₂, and SO₂ detection.