The auxetic effect in two-dimensional (2D) materials can not only enhance the mechanical properties of materials but also bring additional tunability of their physical properties. Here, we employ density-functional-theory calculations...

The epitaxial growth of wafer-scale single-crystalline two-dimensional materials requires precise control over the crystallographic orientation and morphology of clusters formed during the initial stages of nucleation. However, there is limited knowledge about the critical nucleus and its growth mechanism for h-BN on high-symmetry surfaces of transition metals. In this study, we provide atomic insights into h-BN nucleation on Ir(111) using scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM), corraborated by density functional theory (DFT) calculations. The atomic-resolved structural characterization reveals that the smallest h-BN cluster, exhibiting a non-1:1 stoichiometric ratio of boron and nitrogen atoms, maintains a triangular shape with zigzag-type edges. Through AFM force spectroscopy, individual B and N atoms within the cluster, as well as the N-termination edge, are clearly identified. The achievement of elemental sensitivity at the atomic scale is attributed to the chemical reactivity of the metallic tip. The local registry of h-BN nuclei that align with the prevailing crystalline orientation is consistently identified as NtopBhcp. Despite DFT calculations indicating the energetically favorable NtopBfcc configuration for larger h-BN clusters, the alignment of the nuclei with respect to the underlying substrate remains unchanged throughout the growth of the clusters. This orientation pinning behavior, which impeding the rotational alignment of nuclei following the formation of initial clusters during the early stages of nucleation, presents a valuable avenue for exploring the large-scale growth of binary and ternary materials.