Real-Time Adaptive Control of 2D Crystal Synthesis and Transformations via in situ Diagnostics: Janus Monolayers

Controlling precision synthesis via real-time adaptive control is a priority research direction in synthesis science and is especially important to enable the scaled synthesis of atomically-thin 2D materials. Here, we describe a general feedback approach to reveal and control the transformation pathways in materials synthesis by pulsed laser deposition (PLD). Here, we focus on the transformation kinetics of monolayer WS₂ crystals into Janus WSSe and WSe₂ by hyperthermal implantation of laser-vaporized Se clusters (< 42 eV/Se-atom). In situ ICCD imaging, ion probe, and spectroscopy diagnostics characterize the PLD plasma and are used to precisely control the maximum kinetic energies of the Se species arriving at the substrate. At the same time, in situ Raman spectroscopy, PL, and optical reflectivity are used to assess the structure, composition, thickness, and optoelectronic quality of the monolayer crystal as it evolves. The experimental apparatus and data acquisition are automated and designed to enable rapid AI/ML exploration of metastable intermediate phases, and provide a pathway for the autonomous optimization of optoelectronic properties. First principles calculations, XPS, and atomic-resolution HAADF STEM are used to identify the fractional Janus alloy compositions and vibrational modes, revealing a layer-by-layer transformation of the monolayer crystal and a method to achieve predesigned metastable states. Bottom-up PLD synthesis of 2D crystals will also be described, along with in situ laser/HRTEM/EELS approaches to understand the assembly of amorphous nanoscale PLD precursors.