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

We propose and analyze a mechanism for inducing spin Hall currents in ordinary (1H phase) monolayer transition metal dichalcogenides (TMDs) due to the nonlinear process of optical rectification. The photo-induced spin current is proportional to the light intensity, and originates from the intrinsic spin-orbit coupling in TMDs. The spin current spectrum is strongly influenced by electron-hole interactions, i.e. excitonic effects, analogous to the optical absorption. Remarkably, excitons change the temperature dependence of the induced spin current, to the point that the current direction can even be reversed by varying the temperature. This peculiar excitonic behavior is shown to emerge from the relative strength of two distinct mechanisms contributing to the optical response, i.e. a purely interband part and a mixed inter/intraband contribution. Furthermore, we investigate the valley dichrosim of second-harmonic charge and spin currents, and demonstrate different valley polarization of $s$ and $p$ excitons that stem from their distinct angular momenta. Our findings pave the path to the generation of dc spin currents in ordinary TMDs without external static electric or magnetic fields.

Nonlinear excitonic spin Hall effect in monolayer transition metal dichalcogenides

Janus MoSSe monolayers were recently synthesised by replacing S by Se on one side of MoS$_2$ (or vice versa for MoSe$_2$). Due to the different electronegativity of S and Se these structures carry a finite out-of-plane dipole moment. As we show here by means of density functional theory (DFT) calculations, this intrinsic dipole leads to the formation of built-in electric fields when the monolayers are stacked to form $N$-layer structures. For sufficiently thin structures ($N 4$. Based on band structure calculations and the Mott-Wannier exciton model, we compute the energies of intra- and interlayer excitons as a function of film thickness suggesting that the Janus multilayer films are ideally suited for achieving ultrafast charge separation over atomic length scales without chemical doping or applied electric fields. Finally, we explore a number of other potentially synthesisable 2D Janus structures with different band gaps and internal dipole moments. Our results open new opportunities for ultrathin opto-electronic components such as tunnel diodes, photo-detectors, or solar cells.

Efficient Charge Separation in 2D Janus van der Waals Structures with Build-in Electric Fields and Intrinsic p-n Doping.

We demonstrate the possibility of engineering the optical properties of transition metal dichalcogenide heterobilayers when one of the constitutive layers has a Janus structure. We investigate different MoS2@Janus layer combinations using first-principles methods including excitons and exciton-phonon coupling. The direction of the intrinsic electric field from the Janus layer modifies the electronic band alignments and, consequently, the energy separation between dark interlayer exciton states and bright in-plane excitons. We find that in-plane lattice vibrations strongly couple the two states, so that exciton-phonon scattering may be a viable generation mechanism for interlayer excitons upon light absorption. In particular, in the case of MoS2@WSSe, the energy separation of the low-lying interlayer exciton from the in-plane exciton is resonant with the transverse optical phonon modes (40 meV). We thus identify this heterobilayer as a prime candidate for efficient generation of charge-separated electron-hole pairs.

Intrinsic Control of Interlayer Exciton Generation in Van der Waals Materials via Janus Layers.

Mechanical stability and electronic and optical properties of AA and AA$$^{''}$$ Janus 1T PdXY (X, Y = S, Se, Te) bilayers

Stacking sequence of bilayer van der Waals transition metal dichalcogenides determines their electronic and related optical excitations. When the Janus monolayer structure has been taken to construct bilayer TMDs, it would introduce another degree of freedom, the out-of-plane intrinsic dipole moment, to tune the electronic and optical properties. Here we reveal that the electronic band structures and interlayer excitons can be dramatically tuned via the stacking sequence of the bilayer MoSSe with the different intrinsic dipole orientations. Moreover, the lowest energy interlayer excitons exhibit diverse spatial extensions, and the corresponding radiative lifetimes can be tailored within the range of ∼10−8 to ∼10−2 seconds at room temperature, by means of optimizing the dipole orientation and stacking sequence, and when the dipole moment keeps the same orientation for the constituent layer, it will slower the radiative recombination. Our findings shed a light on the applications of the interlayer excitons in Janus MoSSe on optoelectronics.

Stacking-tailoring quasiparticle energies and interlayer excitons in bilayer Janus MoSSe

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Stacking-tailoring quasiparticle energies and interlayer excitons in bilayer Janus MoSSe