Large‐Area Structure‐Selective Synthesis of Symmetry‐Broken MoSe₂ and Their Broadband Nonlinear Optical Response

Abstract Transition metal dichalcogenides (TMDCs) have various electronic and optical properties depending on their structure, so they can be used as a fascinating material in various applications including photonics, electronics, optoelectronics, and valleytronics. In particular, spiral TMDCs grown through the formation of screw dislocations exhibit novel electronic and optical properties different from layer‐by‐layer TMDCs. However, large‐area structure‐selective synthesis of TMDCs remains challenging. Here, this work reports for the first time the large‐area structure‐selective synthesis of monolayer MoSe₂ and spiral MoSe₂ using a flux‐controlled chemical vapor deposition method. Under a low MoSe₂ flux condition, monolayer MoSe₂ is synthesized, whereas thick spiral MoSe₂ is synthesized under a high flux condition. Under a medium flux condition, both monolayer and spiral MoSe₂ are synthesized. In addition, through the nonlinear optical (NLO) signal analysis of monolayer MoSe₂ and spiral MoSe₂, the giant enhancement of NLO signals induced by the combined effect of breaking inversion symmetry and the excitonic resonance effects in the synthesized MoSe₂ is confirmed. Monolayer MoSe₂ and spiral MoSe₂ synthesized using this method are expected to be used as advanced optical materials for novel electronics, optoelectronics, and NLO applications.