Janus Monolayer Transition Metal Dichalcogenides (1704.06389v1)

Abstract: A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

• The paper introduces a novel Janus SMoSe monolayer synthesized through controlled sulfurization, advancing insights into asymmetric 2D TMD structures.
• It employs comprehensive Raman, PL, XPS, TEM, and ToF-SIMS analyses along with DFT simulations to validate distinct vibrational and electronic properties.
• High basal plane catalytic activity for hydrogen evolution demonstrates the material’s potential in sustainable energy and device engineering.

Analyzing Janus Monolayer Transition Metal Dichalcogenides

The study of Janus monolayer transition metal dichalcogenides (TMDs) presents a significant advancement in the exploration of asymmetric atomic structures in two-dimensional materials. Building off the current understanding of TMDs, the authors have synthesized a novel Janus structure, SMoSe, through a controlled sulfurization process, offering new insights and applications in the field of materials science.

Synthesis and Characterization

The authors detail a robust methodology for fabricating the Janus SMoSe monolayer by selectively substituting sulfur atoms into monolayer MoSe. This sulfurization occurs at atmospheric pressure and specific thermal conditions, ensuring that only the top layer of selenium atoms is replaced. The process is characterized using Raman spectroscopy, PL spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and time-of-flight secondary ion mass spectrometry, all confirming the successful synthesis of a uniform tri-layer atomic structure of S-Mo-Se.

Theoretical Insights via DFT Simulations

The paper incorporates density functional theory (DFT) simulations to further interpret the material’s vibrational and electronic properties. The calculated Raman modes align well with experimental observations, affirming the existence of distinct vibrational modes due to the lack of symmetry in the Janus structure. Additionally, the electronic band structure reveals the emergence of an indirect band gap semiconductor behavior, diverging significantly from symmetric TMD monolayers like MoS$_2$ and MoSe$_2$.

Catalytic Properties and Practical Implications

The Janus SMoSe shows high basal plane catalytic activity for hydrogen evolution reactions (HER). Intriguingly, the higher catalytic activity compared to symmetric TMDs potentially arises from intrinsic defects and the inherent strain of the Janus structure. This finding suggests that Janus monolayers could serve as efficient catalysts in energy-related applications, such as clean hydrogen production.

Future Directions and Impact

The research indicates promise for the use of asymmetric two-dimensional materials in various applications. The distinct photoluminescent properties and bandgap characteristics of the Janus structure are expected to have broader implications in electronic and optoelectronic device engineering. Further, the observed catalytic efficiency underscores potential utility in sustainable energy technologies. Ongoing investigations into defect density and structural optimization could enhance device performance and material stability, offering numerous paths for future research.

In conclusion, this study enriches the understanding of TMDs by introducing Janus SMoSe, a new structural variant, and elucidating its innovative properties and potential applications across scientific and technological domains. Through meticulous synthesis, robust characterization, and insightful theoretical analyses, the paper advances the discourse on two-dimensional materials, marking a strategic step in materials research.