Charge Transfer-Stabilized SiP Monolayer: An Unexpected Two-Dimensional Honeycomb on Au(111)

Exploring two-dimensional (2D) honeycomb structures beyond naturally layered materials is increasingly attracting interest, yet discoveries remain limited. Traditional methods often prioritize thermodynamic and dynamic stability, overlooking many inherently unstable materials such as those deviating from electron counting rules. Here, we challenge these traditional limitations by using the Si-P system as a case study. Combining ab initio evolutionary simulations with available scanning tunneling microscopy data, we show that an unexpected 2D SiP honeycomb monolayer can be successfully synthesized on a Au(111) substrate. This synthesis suggests that charge transfer can enable the creation of numerous previously inconceivable materials. Notably, this system exhibits diverse quantum phenomena, including flat bands near the Fermi level and fractional corner charges. These features present an alternative to moiré systems for exploring correlated electron physics and offer an intriguing platform for studying higher-order topology. Our findings pave the way for discovering a fascinating category of hidden 2D materials.