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.

Janus Monolayer Transition Metal Dichalcogenides

Magnetism in two-dimensional (2D) van der Waals (vdW) materials has recently emerged as one of the most promising areas in condensed matter research, with many exciting emerging properties and significant potential for applications ranging from topological magnonics to low-power spintronics, quantum computing, and optical communications. In the brief time after their discovery, 2D magnets have blossomed into a rich area for investigation, where fundamental concepts in magnetism are challenged by the behavior of spins that can develop at the single layer limit. However, much effort is still needed in multiple fronts before 2D magnets can be routinely used for practical implementations. In this comprehensive review, prominent authors with expertise in complementary fields of 2D magnetism (i.e., synthesis, device engineering, magneto-optics, imaging, transport, mechanics, spin excitations, and theory and simulations) have joined together to provide a genome of current knowledge and a guideline for future developments in 2D magnetic materials research.

/pdf/the-magnetic-genome-of-two-dimensional-van-der-waals-1hvraszg.pdf

The Magnetic Genome of Two-Dimensional van der Waals Materials

Structural symmetry breaking in two-dimensional materials can lead to superior physical properties and introduce an additional degree of piezoelectricity. In the present paper, we propose three structural phases ($1H, 1T$, and $1{T}^{\ensuremath{'}}$) of Janus $\mathrm{W}X\mathrm{O}$ ($X=\mathrm{S}$, Se, and Te) monolayers and investigate their vibrational, thermal, elastic, piezoelectric, and electronic properties by using first-principles methods. Phonon spectra analysis reveals that while the $1H$ phase is dynamically stable, the $1T$ phase exhibits imaginary frequencies and transforms to the distorted $1{T}^{\ensuremath{'}}$ phase. Ab initio molecular dynamics simulations confirm that $1H$- and $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ monolayers are thermally stable even at high temperatures without any significant structural deformations. Different from binary systems, additional Raman active modes appear upon the formation of Janus monolayers. Although the mechanical properties of $1H\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ are found to be isotropic, they are orientation dependent for $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$. It is also shown that $1H\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ monolayers are indirect band-gap semiconductors and the band gap narrows down the chalcogen group. Except $1{T}^{\ensuremath{'}}$-WSO, $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ monolayers have a narrow band gap correlated with the Peierls distortion. The effect of spin-orbit coupling on the band structure is also examined for both phases and the alteration in the band gap is estimated. The versatile mechanical and electronic properties of Janus $\mathrm{W}X\mathrm{O}$ monolayers together with their large piezoelectric response imply that these systems are interesting for several nanoelectronic applications.

Janus two-dimensional transition metal dichalcogenide oxides: First-principles investigation of W X O monolayers with X = S , Se, and Te

Two-dimensional CdO/PtSSe heterojunctions used for Z-scheme photocatalytic water-splitting

Structural symmetry plays a crucial role in the electronic band structure and properties of two-dimensional materials. In contrast to graphene, monolayer transition metal dichalcogenides exhibit intrinsic in-plane asymmetry with suitable direct bandgaps and distinctive optical properties. Efforts have been devoted to breaking their out-of-plane mirror symmetry by applying external electric fields, vertical stacking, or functionalization. The successful fabrication of Janus transition metal dichalcogenides offers a synthetic strategy to breaking the vertical mirror symmetry, leading to a variety of novel properties, such as vertical piezoelectricity, Rashba spin splitting, and excellent exciton properties. Here, we discuss the universal fabrication approaches and unique properties of Janus transition metal dichalcogenides and further present a brief perspective on their potential applications and challenges.

https://aip.scitation.org/doi/pdf/10.1063/5.0095203

Janus two-dimensional transition metal dichalcogenides

Janus single-layer transition metal dichalcogenides, in which the two chalcogen layers have a different chemical nature, push chemical composition control beyond what is usually achievable with van der Waals heterostructures. Here, we report such a Janus compound, SPtSe, which is predicted to exhibit strong Rashba spin-orbit coupling. We synthetized it by conversion of a single-layer of PtSe 2 on Pt(111) via sulfurization under H 2 S atmosphere. Our in situ and operando structural analysis with grazing incidence synchrotron X-ray diffraction reveals the process by which the Janus alloy forms. The crystalline long-range order of the as-grown PtSe 2 monolayer is first lost due to thermal annealing. A subsequent recrystallization in presence of a source of sulfur yields a highly ordered SPtSe alloy, which is isostructural to the pristine PtSe 2. The chemical composition is resolved, layer-by-layer, using angleresolved X-ray photoelectron spectroscopy, demonstrating that Se-by-S substitution occurs selectively in the topmost chalcogen layer.

/pdf/synthesis-of-epitaxial-monolayer-janus-sptse-ja78674vce.pdf

Synthesis of epitaxial monolayer Janus SPtSe

Janus single-layer transition metal dichalcogenides, in which the two chalcogen layers have a different chemical nature, push chemical composition control beyond what is usually achievable with van der Waals heterostructures. Here we report such a novel Janus compound, SPtSe, which is predicted to exhibit strong Rashba spin-orbit coupling. We synthetized it by conversion of a single-layer of PtSe$_2$ on Pt(111) via sulphurization under H$_2$S atmosphere. Our in situ and operando structural analysis with grazing incidence synchrotron X-ray diffraction reveals the process by which the Janus alloy forms. The crystalline long-range order of the as-grown PtSe$_2$ monolayer is first lost due to thermal annealing. A subsequent recrystallization in presence of a source of sulphur yields a highly ordered SPtSe alloy, which is iso-structural to the pristine PtSe$_2$. The chemical composition is resolved, layer-by-layer, using angle-resolved X-ray photoelectron spectroscopy, demonstrating that Se-by-S substitution occurs selectively in the topmost chalcogen layer.

/pdf/synthesis-of-epitaxial-monolayer-janus-sptse-3ixn6t8y4l.pdf

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Synthesis of epitaxial monolayer Janus SPtSe

Synthesis of epitaxial monolayer Janus SPtSe