posted on 2020-03-17, 20:45authored byYu-Chuan Lin, Chenze Liu, Yiling Yu, Eva Zarkadoula, Mina Yoon, Alexander A. Puretzky, Liangbo Liang, Xiangru Kong, Yiyi Gu, Alex Strasser, Harry M. Meyer, Matthias Lorenz, Matthew F. Chisholm, Ilia N. Ivanov, Christopher M. Rouleau, Gerd Duscher, Kai Xiao, David B. Geohegan
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
°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.