Formation of stable polonium monolayers with tunable semiconducting properties driven by strong quantum size effects

Elemental two-dimensional (2D) materials have attracted extraordinary interest compared with other 2D materials over the past few years. Fifteen elements from group IIIA to VIA have been discussed experimentally or theoretically for the formation of 2D monolayers, and the remaining few elements still need to be identified. Here, using first-principles calculations within density functional theory (DFT) and ab initio molecular dynamics simulations (AIMDs), we demonstrated that polonium can form stable 2D monolayers (MLs) with a 1T-MoS2-like structure. The band structure calculations revealed that polonium monolayers possess strong semiconducting properties with a band gap of similar to 0.9 eV, and such semiconducting properties can well sustain up to a thickness of 4 MLs with a bandgap of similar to 0.1 eV. We also found that polonium monolayers can be achieved through a spontaneous phase transition of ultrathin films with magic thicknesses, resulting in a weaker van der Waals interaction of similar to 32 meV angstrom(-2) between each three atomic layers. Also, the underlying physics comes from layered Peierls-like distortion driven by strong quantum size effects. Based on these intriguing findings, a suitable substrate on which the polonium monolayer can be grown through an epitaxial growth technique is proposed for further experiments. Our work not only extends completely the puzzle of elemental 2D monolayer materials from group IIIA to VIA, but also presents a new formation mechanism of 2D materials beyond the database of bulk materials with layered van der Waals interactions.

Automated summary

This study demonstrates that polonium can form stable 2D monolayers with strong semiconducting properties. The monolayers can be achieved through a spontaneous phase transition of ultrathin films and exhibit unique van der Waals interactions. These findings contribute to the understanding of the formation mechanism of 2D materials.