Monitoring the effect of asymmetrical vertical strain on Janus single layers of MoSSe via vibrational spectrum

The strain regulated physical properties of PbI2/g-C3N4for potential optoelectronic device

The van der Waals (vdW) heterostructures of Z-scheme PbI2/g-C3N4with an indirect bandgap have gained much attention in recent years due to their unique properties and potential applications in various fields. However, the optoelectronic characteristics and strain-modulated effects are not yet fully understood. By considering this, six stacking models of PbI2/g-C3N4are proposed and the stablest structure is selected for further investigation. The uniaxial and biaxial strains (-10%-10%) regulated band arrangement, charge distribution, optical absorption in the framework of density functional theory are systematically explored. The compressive uniaxial strain of -8.55% changes the band type from II→I, and the biaxial strains of -7.12%, -5.25%, 8.91% change the band type in a way of II→I→II→I, acting like the 'band-pass filter'. The uniaxial strains except -10% compressive strain, and the -6%, -4%, 2%, 4%, 10% biaxial strains will enhance the light absorption of PbI2/g-C3N4. The exerted strains on PbI2/g-C3N4generate different power conversion efficiency (ηPCE) values ranging from 3.64% to 25.61%, and the maximumηPCEis generated by -6% biaxial strain. The results of this study will pave the way for the development of new electronic and optoelectronic materials with customized properties in photocatalytic field and optoelectronic devices.

Effects of Strain and Electric Field on Molecular Doping in MoSSe

Recently, synthesized Janus MoSSe monolayers have attracted tremendous attention in science and technology due to their novel properties and promising applications. In this work, we investigate their molecular adsorption-induced structural and electronic properties and tunable doping effects under biaxial strain and external electric field by first-principles calculations. We find an effective n-type or p-type doping in the MoSSe monolayer caused by noncovalent tetrathiafulvalene (TTF) or tetracyanoquinodimethane (TCNQ) molecular adsorption. Moreover, the concentration of doping carrier with respect to the S or Se side also exhibits Janus characteristics because of the electronegativity difference between S and Se atoms and the intrinsic dipole moment in the MoSSe monolayer. In particular, this n-type or p-type molecular doping effect can be flexibly tuned by biaxial strain or under external electric field. By analyzing the valence band maximum (VBM) and conduction band minimum (CBM) in the band structure of MoSSe/TTF under strain, the strain-tunable band gap of MoSSe and the n-type molecular doping effect is revealed. Further explanation of charge transfer between TTF or TCNQ and the MoSSe monolayer by an equivalent capacitor model shows that the superimposition of external electric field and molecular adsorption-induced internal electric field plays a crucial role in achieving a controllable doping concentration in the MoSSe monolayer.

Excitonic Dynamics in Janus MoSSe and WSSe Monolayers

We report here details of steady-state and time-resolved spectroscopy of excitonic dynamics for Janus transition metal dichalcogenide monolayers, including MoSSe and WSSe, which were synthesized by low-energy implantation of Se into transition metal disulfides. Absorbance and photoluminescence spectroscopic measurements determined the room-temperature exciton resonances for MoSSe and WSSe monolayers. Transient absorption measurements revealed that the excitons in Janus structures form faster than those in pristine transition metal dichalcogenides by about 30% due to their enhanced electron-phonon interaction by the built-in dipole moment. By combining steady-state photoluminescence quantum yield and time-resolved transient absorption measurements, we find that the exciton radiative recombination lifetime in Janus structures is significantly longer than in their pristine samples, supporting the predicted spatial separation of the electron and hole wave functions due to the built-in dipole moment. These results provide fundamental insight in the optical properties of Janus transition metal dichalcogenides.