Anomalous Behavior of 2D Janus Excitonic Layers under Extreme Pressures

Abstract

Newly discovered 2D Janus transition metal dichalcogenides layers have gained much attention from theory perspective owing to their unique atomic structure and exotic materials properties, but little to no experimental data are available on these materials. Here, our experimental and theoretical studies establish the vibrational and optical behavior of 2D Janus S-W-Se and S-Mo-Se monolayers under high pressures for the first time. CVDgrown classical TMD monolayers were first transferred onto vdW mica substrates and converted to 2D Janus sheets by surface plasma technique, and then integrated into 500µm size diamond anvil cell for high-pressure studies. Our results show that 2D Janus layers do not undergo phase transition up to 15 GPa, and in this pressure regime, their vibrational modes exhibit a non-monotonic response to the applied pressures (d/dP). Interestingly, these 2D Janus monolayers exhibit unique blue-shift in PL upon compression which is in contrast to many other traditional semiconductor materials. Overall theoretical simulations offer in-depth insights and reveal that the overall optical response is a result of competition between the ab-plane (blue-shift) and c-axis (red-shift) compression. Overall findings shed the very first light on how 2D Janus monolayers respond under extreme pressures and expand the fundamental understanding of these materials.Results mark the first high pressure studies on 2D Janus monolayers. Pressure studies show that 2D Janus monolayers S-Mo-Se and S-W-Se do not undergo phase transition up to 15GPa, but their vibrational modes exhibit a non-monotonic response to pressure. Janus S-W-Se monolayer undergo anomalous blue-shifting behavior and direct to indirect bandgap transition.

Room‐Temperature Synthesis of 2D Janus Crystals and their Heterostructures

“…The observed Raman spectra also differ significantly from 2D alloy WSSe [25] and MoSSe [26,27] (see Figure S3, Supporting Information). Moreover, our calculated vibrational dispersion, together with the published predictions for WSSe and MoSSe [7,8,14,28] match reasonably well with our experimental datasets with a deviation of around 1.1-2.8%, as shown in Figures S4 and S5, Supporting Information, and Table 1. These observations again validate that the as-synthesized layers have Janus structure instead of random alloying.…”

High‐Throughput Computational Characterization of 2D Compositionally Complex Transition‐Metal Chalcogenide Alloys

“…However, the underestimated band gaps will be compensated for by the absence of SOC in the calculations. The net effect is that, for example, the PBE band gaps of WSSe and MoSSe agree well with the experimental band gaps [98]. We use a Monkhorst-Pack [99] 15  15  1 k-point grid.…”

“…The PBE‐calculated optical band gaps of WSSe and MoSSe agreed well with the experimental band gaps. [ 54,55 ] A Monkhorst–Pack [ 56 ] 15 × 15 × 1 k ‐point grid was used. The in‐plane lattice constants and atomic coordinates of all systems were fully optimized using a quasi‐Newton algorithm with the force convergence criterion of 0.01 eV Å −1 .…”

2D Materials and Heterostructures at Extreme Pressure

“…Their vibration modes are lack of monotonic response to pressure. [162] Furthermore, pressure-dependent phonon dynamics of black phosphorus are also discussed experimentally and theoretically (Figure 5c,d). [33] It is concluded that: 1) FWHM of first-order Raman modes attain a minimum at ≈1.1 GPa and this is associated with the anomalies of electron-phonon coupling at electronic topological transition; In detail, through first-principle calculations, a phase transition from a semiconductor to topological insulator occurs at a low-pressure range.…”