A Janus monolayer of transition metal dichalcogenides (TMDCs) was synthesized by replacing the top-layer sulfur atoms in a MoS₂ monolayer with selenium atoms using a hydrogen plasma and subsequent thermal selenization. This process breaks the out-of-plane structural symmetry, resulting in a Janus MoSSe monolayer with distinct electronic and optical properties. The Janus structure was confirmed using scanning transmission electron microscopy, energy-dependent X-ray photoelectron spectroscopy, and second harmonic generation measurements. The monolayer exhibits a vertical dipole, leading to an intrinsic piezoelectric response, which was verified by piezoresponse force microscopy. The optical gap of the Janus MoSSe monolayer was found to be 1.68 eV, close to the average of MoS₂ and MoSe₂. The monolayer also shows a strong Rashba spin-orbit interaction, making it a promising platform for spintronics. The synthesis method involves chemical vapor deposition, hydrogen plasma treatment, and thermal selenization, with careful control of temperature and plasma power to achieve the desired structural and electronic properties. The study demonstrates a synthetic strategy to create asymmetric TMDC monolayers with out-of-plane symmetry breaking, offering new opportunities for optoelectronics and nanoelectromechanical devices. The results highlight the potential of Janus monolayers in studying light-matter interactions and their applications in advanced electronic and spintronic devices.A Janus monolayer of transition metal dichalcogenides (TMDCs) was synthesized by replacing the top-layer sulfur atoms in a MoS₂ monolayer with selenium atoms using a hydrogen plasma and subsequent thermal selenization. This process breaks the out-of-plane structural symmetry, resulting in a Janus MoSSe monolayer with distinct electronic and optical properties. The Janus structure was confirmed using scanning transmission electron microscopy, energy-dependent X-ray photoelectron spectroscopy, and second harmonic generation measurements. The monolayer exhibits a vertical dipole, leading to an intrinsic piezoelectric response, which was verified by piezoresponse force microscopy. The optical gap of the Janus MoSSe monolayer was found to be 1.68 eV, close to the average of MoS₂ and MoSe₂. The monolayer also shows a strong Rashba spin-orbit interaction, making it a promising platform for spintronics. The synthesis method involves chemical vapor deposition, hydrogen plasma treatment, and thermal selenization, with careful control of temperature and plasma power to achieve the desired structural and electronic properties. The study demonstrates a synthetic strategy to create asymmetric TMDC monolayers with out-of-plane symmetry breaking, offering new opportunities for optoelectronics and nanoelectromechanical devices. The results highlight the potential of Janus monolayers in studying light-matter interactions and their applications in advanced electronic and spintronic devices.