Low Energy Implantation into Transition-Metal Dichalcogenide Monolayers to Form Janus Structures

YC Lin and CZ Liu and YL Yu and E Zarkadoula and M Yoon and AA Puretzky and LB Liang and XR Kong and YY Gu and A Strasser and HM Meyer and M Lorenz and MF Chisholm and IN Ivanov and CM Rouleau and G Duscher and K Xiao and DB Geohegan, ACS NANO, 14, 3896-3906 (2020).

DOI: 10.1021/acsnano.9b10196

Atomically thin two-dimensional (2D) materials face significant energy barriers for synthesis and processing into functional metastable phases such as Janus structures. Here, the controllable implantation of hyperthermal species from pulsed laser deposition (PLD) plasmas is introduced as a top-down method to compositionally engineer 2D monolayers. The kinetic energies of Se clusters impinging on suspended monolayer WS2 crystals were controlled in the <10 eV/atom range with in situ plasma diagnostics to determine the thresholds for selective top layer replacement of sulfur by selenium for the formation of high quality WSSe Janus monolayers at low (300 degrees C) temperatures and bottom layer replacement for complete conversion to WSe2. Atomic- resolution electron microscopy and spectroscopy in tilted geometry confirm the WSSe Janus monolayer. Molecular dynamics simulations reveal that Se clusters implant to form disordered metastable alloy regions, which then recrystallize to form highly ordered structures, demonstrating low-energy implantation by PLD for the synthesis of 2D Janus layers and alloys of variable composition.

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