Bottom-up Synthesis of Metastable 2D Hexagonal Copper(I) Iodide on Monolayer and within Bilayer Graphene
Copper(I) iodide (CuI) is a wide-bandgap semiconductor crystallizing in the 3D $\gamma$-phase under ambient conditions; its layered van der Waals bulk phase $\beta$-CuI is stable only between 643 and 673 K. The two-dimensional (2D) h-CuI form has been obtained via liquid-phase exfoliation of mechanochemically prepared precursors and via encapsulation between graphene sheets, whereas bottom-up growth of 2D h-CuI on open surfaces has not yet been demonstrated. Here, we report a vapor-phase synthesis of h-CuI directly on low-defect, large-area monolayer and within bilayer reduced oxo-graphene (r-oxo-G) at low temperatures. Using a copper TEM grid as the solid-state precursor for copper, HI-vapor exposure at 40 $\deg$C initiates nucleation, while annealing at 180 $\deg$C promotes the growth of extended h-CuI domains. Aberration-corrected HRTEM resolves the atomic structure, local twist angles, and lattice anisotropy of the CuI/r-oxo-G nanohybrid, while STEM-EDX yields a Cu:I ratio consistent with 1:1. First-principles calculations show that van der Waals adhesion to graphene stabilizes the supported hexagonal layer. Under the presented low-temperature precursor conditions, pathways for nucleation of the $\gamma$-phase are not available, allowing the hexagonal phase to form selectively at the graphene interface. Ab initio molecular dynamics simulations show that the heterostructure retains its hexagonal lattice order at 600 K, including on an open monolayer graphene support. The lateral extent of the growth is limited mainly by remaining interfacial adsorbates. These results establish a route to metastable 2D h-CuI on a chemically inert graphene template, which may be useful for wide-bandgap electronic and optoelectronic devices.