Phase transition in MoS$_{\mathrm{2}}$ monolayers through alloying

Alloying of 2D monolayers can produce materials with different properties than either component. MoS$_{\mathrm{2}}$ is known to have a stable hexagonal 1H structure, but the alternative tetragonal DT structure has been stabilized in multilayer form by Li intercalation. It is also known that the stable structure of monolayer ReS$_{\mathrm{2}}$ is DT. In this work, we use electron microscopy and density functional theory calculations to demonstrate that an H-to-T phase transition can be achieved in MoS$_{\mathrm{2}}$ by Re doping. Both the microscopy images and the calculations find that the phase transition occurs at a Re concentration of just below 50{\%}. In contrast to the phase transition by lithium intercalation which is induced by the electron doping effect, the calculations find that electron doping alone, compensated by a uniform positive background, shift the critical concentration to 75{\%}, which indicates that changes in chemical bonding facilitate the transition. At 50{\%} alloying, the energy gap of the DT material is only 0.2 eV, compared with 1.8 eV in 1H MoS$_{\mathrm{2}}$.