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.
The failure to achieve stable Ohmic contacts in two-dimensional material devices currently limits their promised performance and integration. Here we demonstrate that a phase transformation in a region of a layered semiconductor, PdSe 2 , can form a contiguous metallic Pd 17 Se 15 phase, leading to the formation of seamless Ohmic contacts for field-effect transistors. This phase transition is driven by defects created by exposure to an argon plasma. Cross-sectional scanning transmission electron microscopy is combined with theoretical calculations to elucidate how plasma-induced Se vacancies mediate the phase transformation. The resulting Pd 17 Se 15 phase is stable and shares the same native chemical bonds with the original PdSe 2 phase, thereby forming an atomically sharp Pd 17 Se 15 /PdSe 2 interface. These Pd 17 Se 15 contacts exhibit a low contact resistance of ∼0.75 kΩ μm and Schottky barrier height of ∼3.3 meV, enabling nearly a 20-fold increase of carrier mobility in PdSe 2 transistors compared to that of traditional Ti/Au contacts. This finding opens new possibilities in the development of better electrical contacts for practical applications of 2D materials.
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