Unusually high thermal conductivity in suspended monolayer MoSi₂N₄

Two-dimensional semiconductors with high thermal conductivity and charge carrier mobility are of great importance for next-generation electronic and optoelectronic devices. However, constrained by the long-held Slack's criteria, the reported two-dimensional semiconductors such as monolayers of MoS₂, WS₂, MoSe₂, WSe₂ and black phosphorus suffer from much lower thermal conductivity than silicon (~142 W·m^-1·K^-1) because of the complex crystal structure, large average atomic mass and relatively weak chemical bonds. Despite the more complex crystal structure, the recently emerging monolayer MoSi₂N₄ semiconductor has been predicted to have high thermal conductivity and charge carrier mobility simultaneously. In this work, using a noncontact optothermal Raman technique, we experimentally measure a high thermal conductivity of ~173 W·m^-1·K^-1 at room temperature for suspended monolayer MoSi₂N₄ grown by chemical vapor deposition. First-principles calculations reveal that such unusually high thermal conductivity benefits from the high Debye temperature and small Grüneisen parameter of MoSi₂N₄, both of which are strongly dependent on the high Young's modulus induced by the outmost Si-N bilayers. Our study not only establishes monolayer MoSi₂N₄ as a benchmark 2D semiconductor for next-generation electronic and optoelectronic devices, but also provides an insight into the design of 2D materials for efficient heat conduction.