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

Abstract

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.

Room‐Temperature Synthesis of 2D Janus Crystals and their Heterostructures

“…The observed Raman spectra also differ significantly from 2D alloy WSSe [25] and MoSSe [26,27] (see Figure S3, Supporting Information). Moreover, our calculated vibrational dispersion, together with the published predictions for WSSe and MoSSe [7,8,14,28] match reasonably well with our experimental datasets with a deviation of around 1.1-2.8%, as shown in Figures S4 and S5, Supporting Information, and Table 1. These observations again validate that the as-synthesized layers have Janus structure instead of random alloying.…”

“…[12] While the creation of thermodynamically stable 2D alloys is easier, [13] Janus layers, unlike their alloy counterparts, must be produced at unit cell level with atomic precision. Because of these problems-to the best of our knowledge-there are only three approaches [2,7,14] recently reported in the literature. The first method replaced the top selenium layer in MoSe 2 in the presence of sulfur vapor (S 2 ) at a high temperature (800 °C) to produce Janus MoSSe.…”

“…The third method relied on low energy implantation of Se atoms in WS 2 to form Janus WSSe at 300 °C. [14] These methods require processing well above room temperature. While this is welcome in the traditional semiconductor industry, it is highly undesirable for the production of 2D Janus layers.…”

Non‐Equilibrium Synthesis of Highly Active Nanostructured, Oxygen‐Incorporated Amorphous Molybdenum Sulfide HER Electrocatalyst

“…[ 72 ] The diffusive expansion of the collisionally‐thermalized material near the substrate is responsible for the deposition of nearly pure architectures of ultrasmall nanoparticles that condensed in the thermalized plasma, as shown in a previous work (with the sole difference of the slow‐moving 4th component of the plume to be described next). [ 61 ] Finally, a significant “tail” of clusters emitted thermally from the laser‐irradiated spot is observable in all the images (4th component in Figure 1) and, as shown for the PLD synthesis of a Janus‐phase TMD, [ 72 ] can be observed via collisional dissociation with the background gas into excited states of smaller clusters. This low‐energy (0.12 eV per Mo‐atom) component of material arrives as small cluster aggregates and represents the final fraction of material to deposit.…”

Heterogeneities at multiple length scales in 2D layered materials: From localized defects and dopants to mesoscopic heterostructures

“…To reduce the defect level, it is important to control the kinetic energy of the incoming chalcogen atoms. In our work, we developed a hyperthermal implantation method to implant Se species with a kinetic energy less than 10 eV/atom into a WS2 ML using pulse laser plasmas [104]. By controlling the background Ar pressure, the kinetic energy can be tuned to allow selective selenization of the top layer of the WS2 ML to form a high-quality WSSe Janus structure or to fully convert to a WSe2 ML even at 300 o C. To be more specific, the process uses the natural hyperthermal velocities of atoms and clusters in pulsed laser ablation plasmas, as shown in Figs.…”