Phonon transition across an isotopic interface

TL;DR

This study uses advanced electron energy-loss spectroscopy to investigate how lattice vibrations transition across an isotopic interface in h-10BN/h-11BN, revealing gradual energy changes and phonon delocalization at atomic scales.

Contribution

It provides the first atomic-scale observation of phonon behavior across an isotopic interface, highlighting the influence of momentum transfer and isotope-induced charge effects.

Findings

Phonons at the zone center transition over ~3.34 nm

Zone boundary phonons transition over ~1.66 nm

Isotope-induced charge effects influence vibrational delocalization

Abstract

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.