A novel crystal configuration of the Janus SMoSe monolayer has been synthesized and characterized. This structure is formed by sulfurizing monolayer MoSe₂, where the top layer of selenium atoms is substituted by sulfur atoms while the bottom layer remains intact. The unique structure of Janus SMoSe was systematically investigated using Raman, photoluminescence, and X-ray photoelectron spectroscopy, and confirmed by transmission electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory (DFT) calculations were performed to understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which correlate well with experimental results. The Janus monolayer exhibits high basal plane hydrogen evolution reaction (HER) activity, and DFT calculations suggest that this activity arises from the synergistic effect of intrinsic defects and structural strain in the Janus structure. The Janus SMoSe structure consists of three layers of atoms: sulfur, molybdenum, and selenium from top to bottom. Unlike its randomly alloyed counterpart, the Janus SMoSe is highly asymmetric along the c-axis direction, potentially generating an intrinsic electric field and novel physical properties such as Zeeman-type spin splitting. The Janus SMoSe was reproducibly obtained by controlled sulfurization of monolayer MoSe₂. The monolayer MoSe₂ was first grown by chemical vapor deposition (CVD), and sulfurization of the top layer was achieved through a controlled substitutional reaction with vaporized sulfur. The sulfurization process was kept for 30 minutes before the furnace was cooled to room temperature. The Janus SMoSe triangular flake was observed to be identical to MoSe₂ under an optical microscope. Atomic force microscopy (AFM) confirmed the flake thickness was less than 1 nm, indicating it remained in the monolayer configuration. Raman spectroscopy showed distinct peaks for the Janus SMoSe, indicating a unique structure. X-ray photoelectron spectroscopy confirmed the presence of sulfur, selenium, and molybdenum in the material. DFT simulations were used to predict the bandgap of the Janus SMoSe, which was consistent with experimental results from photoluminescence spectroscopy. The Janus SMoSe was found to have an indirect band gap, with the conduction band minimum at the K point and the valence band minimum at the Γ point. The Janus SMoSe monolayer exhibited high basal plane HER activity, with DFT calculations suggesting that the activity arises from the synergistic effect of intrinsic defects and structural strain. The HER efficiency of the Janus SMoSe was found to be higher than that of MoS₂ and MoSe₂, with SeMoS showing the highest efficiency. The results suggest that the Janus SMoSe has a unique structure that enhances its catalytic activity for the hydrogen evolution reaction.A novel crystal configuration of the Janus SMoSe monolayer has been synthesized and characterized. This structure is formed by sulfurizing monolayer MoSe₂, where the top layer of selenium atoms is substituted by sulfur atoms while the bottom layer remains intact. The unique structure of Janus SMoSe was systematically investigated using Raman, photoluminescence, and X-ray photoelectron spectroscopy, and confirmed by transmission electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory (DFT) calculations were performed to understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which correlate well with experimental results. The Janus monolayer exhibits high basal plane hydrogen evolution reaction (HER) activity, and DFT calculations suggest that this activity arises from the synergistic effect of intrinsic defects and structural strain in the Janus structure. The Janus SMoSe structure consists of three layers of atoms: sulfur, molybdenum, and selenium from top to bottom. Unlike its randomly alloyed counterpart, the Janus SMoSe is highly asymmetric along the c-axis direction, potentially generating an intrinsic electric field and novel physical properties such as Zeeman-type spin splitting. The Janus SMoSe was reproducibly obtained by controlled sulfurization of monolayer MoSe₂. The monolayer MoSe₂ was first grown by chemical vapor deposition (CVD), and sulfurization of the top layer was achieved through a controlled substitutional reaction with vaporized sulfur. The sulfurization process was kept for 30 minutes before the furnace was cooled to room temperature. The Janus SMoSe triangular flake was observed to be identical to MoSe₂ under an optical microscope. Atomic force microscopy (AFM) confirmed the flake thickness was less than 1 nm, indicating it remained in the monolayer configuration. Raman spectroscopy showed distinct peaks for the Janus SMoSe, indicating a unique structure. X-ray photoelectron spectroscopy confirmed the presence of sulfur, selenium, and molybdenum in the material. DFT simulations were used to predict the bandgap of the Janus SMoSe, which was consistent with experimental results from photoluminescence spectroscopy. The Janus SMoSe was found to have an indirect band gap, with the conduction band minimum at the K point and the valence band minimum at the Γ point. The Janus SMoSe monolayer exhibited high basal plane HER activity, with DFT calculations suggesting that the activity arises from the synergistic effect of intrinsic defects and structural strain. The HER efficiency of the Janus SMoSe was found to be higher than that of MoS₂ and MoSe₂, with SeMoS showing the highest efficiency. The results suggest that the Janus SMoSe has a unique structure that enhances its catalytic activity for the hydrogen evolution reaction.