In Situ Nucleation and Growth Mechanisms of Chiral Tellurium on 2D Materials

The nucleation and growth mechanisms of chiral crystals have been a topic of open discussion since their initial investigation by Pasteur in 1848. Chiral materials exhibit great potential for integration into emerging nanoscale devices including polarization detectors, chiral catalysts, non-linear photonics and drug testing. However, integration of chiral nanomaterials in devices relies on the ability to selectively synthesize chiral nanostructures with high crystallinity and enantiomeric selectivity. Understanding fundamental nucleation and growth mechanisms has the potential to unlock new regimes of reproducible solid-state chiral nanostructure synthesis, compatible with large-scale manufacturing.<br/> <br/>Here, we utilize a unique <i>in situ</i> environmental transmission electron microscope (ETEM) equipped with molecular beam epitaxy (MBE) deposition sources to observe and analyze the epitaxial growth mechanism of chiral Te nanoribbons on 2D materials. Growth on freestanding membranes enables atomic resolution observation. Previous studies of growth on van der Waals (vdW) materials have been conducted primarily on three-fold symmetric materials such as transition metal dichalcogenides (TMDs), where the epitaxial orientation and structure of nanoislands are strongly dependent on the three-fold symmetric surface structure, chemical identity, and defect density.Compared to three-fold symmetric TMDs, growth on more asymmetric 2D material surfaces results in facetted rods or ribbons with a single growth orientation. <i>In situ </i>observation of nucleation and growth allows us to determine step flow mechanisms and distinguish between nucleation-driven and growth-driven models of chiral nanostructures. We elucidate the effect of substrate symmetry, temperature, and flux rate on the resulting interface, and determine kinetic limiting factors such as diffusion distance, morphology, and activation barriers. Our results aid us in determining routes towards enhancing the enantiomeric excess, and expanding the materials toolbox in which chiral vdW epitaxy occurs.