Real-Time Diagnostics of 2D Crystal Transformations by Pulsed Laser Deposition: Controlled Synthesis of Janus WSSe Monolayers and Alloys

Abstract

Energetic processing methods such as hyperthermal implantation hold special promise to achieve the precision synthesis of metastable two-dimensional (2D) materials such as Janus monolayers; however, they require precise control. Here, we report a feedback approach to reveal and control the transformation pathways in materials synthesis by pulsed laser deposition (PLD) and apply it to investigate the transformation kinetics of monolayer WS 2 crystals into Janus WSSe and WSe 2 by implantation of Se clusters with different maximum kinetic energies (<42 eV/Se-atom) generated by laser ablation of a Se target. Real-time Raman spectroscopy and photoluminescence are used to assess the structure, composition, and optoelectronic quality of the monolayer crystal as it is implanted with well-controlled fluxes of selenium for different kinetic energies that are regulated with in situ ICCD imaging, ion probe, and spectroscopy diagnostics. First-principles calculations, XPS, and atomic-resolution HAADF STEM imaging are used to understand the intermediate alloy compositions and their vibrational modes to identify transformation pathways. The real-time kinetics measurements reveal highly selective top-layer conversion as WS 2 transforms through WS 2(1−x) Se 2x alloys to WSe 2 and provide the means to adjust processing conditions to achieve fractional and complete Janus WSSe monolayers as metastable transition states. The general approach demonstrates a real-time feedback method to achieve Janus layers or other metastable alloys of the desired composition, and a general means to adjust the structure and quality of materials grown by PLD, addressing priority research directions for precision synthesis with real-time adaptive control.

Nonlinear Dispersion Relation and Out‐of‐Plane Second Harmonic Generation in MoSSe and WSSe Janus Monolayers

“…[28] Since their theoretical prediction in 2013 [15] and first synthesis in 2017 [29,30] a multitude of morphotaxy-based techniques [31] have emerged breaching bottlenecks toward the synthesis of high-quality MoSSe and WSSe Janus monolayers. [32][33][34][35][36][37][38] Further studies have already provided insight into their vibrational properties, [39,40] interlayer coupling strength, [27,41,42] exciton complexes, [43] second harmonic generation, [44] and dipole emission profile. [45] Atomically thin crystals provide two distinct advantages compared to conventional bulk materials used in nonlinear optics.…”

Top‐Down Fabrication of Atomic Patterns in Twisted Bilayer Graphene

“…[ 55 ] It seems plausible that, if another atomic species that is known to chemically bond at the desired lattice location were to be supplied concurrently with such damage, atomic patterning and direct atom‐by‐atom writing may also be possible on these materials. One could conceive of precisely converting, for example, WS 2 to the Janus WSSe structure [ 56 ] with atomic precision. The challenge with these other 2D materials will likely not be the creation of damage sites but the prevention of unintentional damage that forms attachment sites for the source atoms in locations that are not desired.…”

A Researcher’s Perspective on Unconventional Lab-to-Fab for 2D Semiconductor Devices

“…Real-time photoluminescence and Raman spectroscopy are also important tools in determining the structure and global properties of the materials. This has been demonstrated in the investigation of the transformation kinetics of monolayer WS 2 crystals to Janus WSSe and WSe 2 by Se cluster implantation using pulsed layer deposition (PLD) . During the implantation process with controlled fluxes of Se for different kinetic energies, in situ Raman and PL help in assessing the structure, composition, and optoelectronic properties of the monolayers.…”

“…This has been demonstrated in the investigation of the transformation kinetics of monolayer WS 2 crystals to Janus WSSe and WSe 2 by Se cluster implantation using pulsed layer deposition (PLD). 122 During the implantation process with controlled fluxes of Se for different kinetic energies, in situ Raman and PL help in assessing the structure, composition, and optoelectronic properties of the monolayers. The implantation of Se in this case is also monitored using in situ ICCD imaging, ion probe, and spectroscopy diagnostics.…”