  Natural materials usually consist of isotopic mixtures, for which different
isotopic ratios can lead to distinct material properties such as thermal
conductivity and nucleation process. However, the knowledge of isotopic
interface remains largely unexplored mainly due to the challenges in isotopic
identification and property measurement at an atomic scale. Here, by using
monochromated electron energy-loss spectroscopy in a scanning transmission
electron microscope, we reveal momentum-transfer-dependent lattice vibration
behavior at an artificial h-10BN/h-11BN heterostructure with sub-unit-cell
resolution. We find the vibrational energy changes across the isotopic
interface gradually, featuring a wide transition regime, which suggests strong
delocalization of the out-of-plane optical phonons at the interface. In
addition, we identify phonons near the Brillouin zone center have a transition
regime ~3.34 nm (10 atomic layers), whereas phonons at the Brillouin zone
boundary transition in ~1.66 nm (5 atomic layers). We propose that the
isotope-induced charge effect at the interface accounts for the distinct
delocalization behavior. Moreover, intra-atomic layer variation of vibration
energy is also sensitive to the momentum transfer, thus at the interface it
depends on both of momentum transfer and mass change. The revealed lattice
vibration behavior at an isotopic interface provides new insights to understand
the isotopic effects on properties in natural materials.
