Charge Transfer in Janus 2D Semiconductors and Janus Heterostructures Fabricated by Low-Energy Selenium Implantation

Janus transition metal dichalcogenide (TMD) monolayer is relatively new in the family of two-dimensional (2D) materials. In 2D Janus TMDs, such as MSSe (M= W, Mo), one M atomic layer is sandwiched by S and Se atomic layers. This asymmetry perpendicular to the basal plane of 2D Janus TMD results in a built-in dipole moment pointing from Se to S layers. Many theoretical studies have predicted many exciting aspects of 2D Janus TMDs, including electronic band structures, spin properties, piezoelectric, and photocarrier transfer. First, the preparation and properties of 2D WSSe and MoSSe prepared by selenium implantation will be discussed.[1] The plasma plumes of pulsed laser deposition offer controllable hyperthermal kinetic energies to synthesize metastable 2D materials that conventional thin film techniques cannot achieve. Using the plasma plumes, we replace the top S atoms in 2D WS₂ and MoS₂ with Se atoms and convert them to Janus structures. Second, transient absorption measurements reveal that the excitons in 2D Janus TMDs form faster than those in their pristine counterparts by about 30% due to their enhanced electron−phonon interaction by the newly introduced built-in dipole moment.[2] Finally, we study the charge transfer properties of bilayer heterostructures formed by Janus and regular TMDs with photoluminescence and time-resolved pump-probe measurements.[3] The results from three heterostructures with atomic layer sequences of S–W–Se/S–W–S, Se–W–S/S–W–S, and S–W–Se/Se–W–Se reveal that charge transfer from regular to Janus TMDs is ultrafast and independent of the direction of the built-in electrical field (Janus field). On the other hand, the charge transfer from Janus to regular TMDs is directional and controlled by the Janus field: When the current direction is along the field, the charge transfer is ultrafast and efficient, while the field blocks the charge transfer with an opposite charge current direction. The understanding of ultrafast and directional charge transfer between Janus and regular TMDs shows that the Janus structures can be used to make 2D heterostructures with efficient and directional charge transfer properties.<br/><br/>Reference:<br/>[1] <u>Y.-C. Lin</u>, C. Liu, Y. Yu, E. Zarkadoula, M. Yoon, A. A. Puretzky, L. Liang, Y. Gu, A. M. Strasser, G. Duscher, M. F. Chisholm, I. Ivanov, C. Rouleau, H. Meyer III, K. Xiao, D. B. Geohegan, “Low-energy implantation into transition metal dichalcogenides for Janus structures” <b>ACS Nano</b> (2020), 14, 3896-3906<br/>[2] T. Cheng, <u>Y.-C. Lin</u>^†, Y. Yu, P. Valencia-Acuna, A. A. Puretzky, C. Liu, I. N. Ivanov, G. Duscher, D. B. Geohegan, Z. Ni^†, H. Zhao^† “Excitonic Dynamic in Janus MoSSe and WSSe Monolayers” <b>Nano Letters</b> (2021), 21, 931-937 († Corresponding author)<br/>[3] T. Cheng, <u>Y.-C. Lin</u>^†, N. Rafizadeh, D. B. Geohegan, Z. Ni^†, K. Xiao, H. Zhao^†, “Janus Monolayers for Ultrafast and Directional Charge Transfer in Transition Metal Dichalcogenide Heterostructures” <b>ACS Nano</b> (2022), 16, 4197-4205 († Corresponding author)