Hexagonal Boron Nitride on Liquid and Single‐Crystal Copper: Operando X‐Ray and Atomistic Insights into Growth and Interfacial Structure

ABSTRACT Two‐dimensional (2D) hexagonal boron nitride (hBN) is a key dielectric for van der Waals nanoelectronics, however, its controlled synthesis by chemical vapor deposition remains challenging and poorly understood. In this context, the growth of hBN on liquid metal catalysts is promising, as the atomically flat liquid surfaces are assumed to promote high‐quality 2D growth, an expectation largely informed by graphene. Here, we implement an involved operando methodology to monitor and quantify hBN growth on molten copper (Liq‐Cu) and re‐solidified single‐crystal copper (SC‐Cu) under near‐ambient‐pressure conditions, enabling real‐time identification of growth stages, morphology, and interfacial structure. Contrary to expectation, Liq‐Cu promotes multilayer and three‐dimensional domain formation, whereas SC‐Cu predominantly yields monolayer‐limited growth. This substrate‐phase dependence correlates with a larger adsorption height of hBN on Liq‐Cu than on SC‐Cu, as determined by X‐ray reflectivity and supported by machine‐learning‐accelerated molecular dynamics simulations. Direct comparison with graphene on Cu further reveals a distinct directional bonding character at the hBN/Cu interface, which rationalizes the observed adsorption‐height trends. More generally, these trends across 2D materials and substrates identify the resulting interfacial stabilization, together with macroscopic factors such as precursor solubility, as a complementary design parameter governing mono‐ vs. multilayer growth in 2D material synthesis.