Janus monolayers of transition metal dichalcogenides

Abstract

Structural symmetry-breaking plays a crucial role in determining the electronic band structures of two-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with electric fields on AB-stacked bilayers or stacked van der Waals heterostructures. In contrast, transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics. Apart from their in-plane inversion asymmetry, an additional degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external electric fields or, as theoretically proposed, with an asymmetric out-of-plane structural configuration. Here, we report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS monolayer, we fully replace the top-layer S with Se atoms. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements.

Enhanced electron–phonon scattering in Janus MoSSe

“…The crystal structure and the corresponding electric dipole moments of monolayer Janus MoSSe are illustrated in figure 1. The relaxed lattice parameter is a=b=3.25 Å in conjunction with a 2.43 Å S-Mo bond length and a 2.54 Å Se-Mo bond length, which agree well with the results in [10]. Owing to the different electronegativity, the electrons locating around S and Se atoms are less than Mo atoms, and thus the dipole moments direction is from S and Se atoms to Mo atoms.…”

“…Besides, different MoS 2 based morphologies have been successfully synthesized, such as MoS 2 nanorods [6], nanopetals [7], nanopowder [8] and nanotubes [9]. Recently, a new monolayer 2D TMDs Janus MoSSe, with out-of-plane asymmetry, has been successfully fabricated [10,11]. The out-of-plane asymmetry introduce a intrinsic vertical electric field which is absent in symmetry MX 2 .…”

Janus transition metal dichalcogenides: a superior platform for photocatalytic water splitting

“…Due to the progress in experimental equipment and techniques, the Janus structure (a sandwich structure with the central metallic layer and two marginal layers of different chalcogen atoms) can be fabricated using the modified Chemical Vapor Deposition (CVD) method. As shown in figure 2(a), Zhang and co-workers synthesized Janus SeMoS using the following steps: the top-layer S atoms of the synthesized MoS 2 SL is firstly replaced by H atoms with a remote hydrogen plasma (Janus HMoS SL formed); then, the H atoms of HMoS SL are replaced by Se atoms with a thermal selenization (Janus SeMoS SL formed) [19]. Soon after, as displayed in figure 2(b), Lou and co-workers also independently fabricated the SMoSe SL using the sulfurization method.…”

“…Based on the experimental progress, novel electronic properties of Janus SLs have been found, such as a huge piezoelectric effect, robust Rashba spin splitting, a second-harmonic generation (SHG) response [19], and high basal plane HER catalytic activity [20]. They can result in the interesting applications in gas sensors, piezoelectric devices, thermal electric devices, solar cells, ion batteries, and so on.…”

Characterization of atomic defects on the photoluminescence in two‐dimensional materials using transmission electron microscope

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