Novel Janus Co3GeXTe (X= S, Se) monolayers with high structural stability: First-principles predictions

This study attempts to construct two-dimensional (2D) Janus Co${}_3$Ge$X$Te ($X=$ S and Se) monolayers from the original Co${}_3$GeTe${}_2$ based on first-principles predictions for new magnetic materials. The optimized Co${}_3$GeTe${}_2$, Co${}_3$GeSTe, and Co${}_3$GeSeTe configurations show hexagonal structures with honeycomb lattices from Co and Te atoms. Then their stabilities are investigated to evaluate the feasibility of synthesizing the Co${}_3$Ge$X$Te materials by experiments. From the phonon dispersion spectra, all three monolayers expose eighteen positive phonon modes without any imaginary frequency. This implies that the Co${}_3$Ge$X$Te structures are dynamically stable. Only small total energy fluctuations and no structure fracture/reconstruction are observed after the ab initio molecular dynamics tests, revealing the high thermal stability of the Co${}_3$Ge$X$Te systems. Besides, the Co${}_3$Ge$X$Te monolayers have high negative $E_{\mathrm{coh}}$ of about $-5$ eV/atom and the $C_{11}$, $C_{12}$, and $C_{66}$ elastic constants obey the condition of Born and Huang for mechanical stability. According to the Poisson’s ratio and Young’s modulus polar diagrams, the isotropic elastic properties are found in all three Co${}_3$GeTe${}_2$, Co${}_3$GeSTe, and Co${}_3$GeSeTe monolayers. The obtained evidence indicates the good stabilities of the Co${}_3$Ge$X$Te structures for experimental synthesis. Moreover, we utilize different approaches for the Co${}_3$Ge$X$Te band structure calculations to explore the electronic properties. The results show metallic behaviors of the Co${}_3$Ge$X$Te materials for both spin-up and spin-down configurations. The differences of spin-up and spin-down configurations in the projected density of states demonstrate that the Co${}_3$Ge$X$Te are magnetic materials. Hence, our findings offer new Janus Co${}_3$Ge$X$Te magnetic materials and stimulate further studies for electronic and magnetic applications of these Co${}_3$Ge$X$Te materials.