Chapter 8 - Nonequilibrium synthesis and processing approaches to tailor heterogeneity in 2D materials

Abstract

Defects in two-dimensional materials are largely incurred during nucleation and growth where stochastic variations in chemical potential, temperature, flux of different species push the synthesis environment out of equilibrium. Cooperative effects, such as strain accumulation due to coalescence with other crystalline domains during growth, can also induce both localized and long-range heterogeneity. In this chapter we focus on such effects of nonequilibrium synthesis variations on heterogeneity in the growth of two-dimensional (2D) transition metal dichalcogenide (TMD) materials, where such effects are manifested as changes in optoelectronic properties. We describe a synergistic approach to reveal the synthetic origins of heterogeneity in 2D TMD materials that involves a combination of: (1) temporally- and spatially-resolved in situ diagnostics of growth environment, using primarily optical spectroscopic and electron microscopy techniques, (2) a correlation between spectroscopic maps of optoelectronic properties and atomistic characterization of heterogeneity, using primarily Z-contrast scanning transmission electron microscopy, and (3) correlated theory of electronic and vibrational properties, leading to computational modeling simulations of synthesis dynamics. We then consider novel methods to control heterogeneity using novel nonequilibrium laser-based techniques. Recent progress is reviewed in the use of pulsed laser deposition (PLD) to explore the roles of different ‘building blocks’, from atoms and ions to clusters and amorphous nanoparticles, in the growth of 2D TMD crystals. Through in-situ optical spectroscopy diagnostics, the advantages of PLD for controlling the flux, kinetic energy, and nature of the ‘building blocks’ for 2D TMD materials are illustrated with associated in situ laser crystallization within the TEM to explore non-classical crystallization pathways for 2D materials synthesis. Finally, recent progress in demonstrating the role of kinetic energy on defect generation in atomically thin 2D TMD crystals is reviewed for the formation of Janus TMD monolayers.