Edge vibrations of boron nitride

05 Jan 2016 NUS scientists measured phonon modes during hexagonal boron nitride growth using a surface sensitive technique, and obtained insights into whether islands have merged seamlessly into a continuous film devoid of grain boundaries.

Prof LOH Kian Ping and his PhD student NAI Chang Tai from the Department of Chemistry in NUS have studied the phonons of hexagonal boron nitride (h-BN), a single-atom thick 2D material, using High-resolution Electron Energy Loss Spectroscopy (HREELS) during its bottom-up growth on metal surfaces. The phonons were found to occur at lower energies during the initial stages of growth on Ru(0001), which was attributed to the incompletely bonded edges of the island domains. This phonon property could also be used to indicate the presence of defects or grain boundaries in h-BN films. Comparison across substrates also showed a correlation between the phonon energy and the film-substrate interaction.

h-BN has remarkable thermal conductivity, while exhibiting a wide electronic bandgap, making it ideal when used as an ultrathin dielectric thermal dissipation material. Phonons and their scattering are critical to the thermal conductivity of a material, and edge-localized phonons have been shown to impact thermal transport in graphene, which is structurally similar to h-BN. Nai and Loh’s work on the factors that affect phonon energies of h-BN enhances fundamental understanding of the thermal conductivity of h-BN and will prove useful in applications exploiting this property of h-BN.

Modification of h-BN with other elements or molecules, such as incorporating carbon into the lattice, is one way to tailor its properties for various applications. However, such modifications alter the structure of h-BN, and the effect on the structural properties can be analyzed by studying the phonons of h-BN. Further studies on the changes in the phonons due to modifications in the structure or interaction with other 2D materials, can provide more information that will prove useful in shaping the material properties for the desired application.

Illustration of the out-of-plane phonon of a triangular h-BN island within the bulk of the island (red arrows) and at the edges (green arrows) [Image credit: Nai Chang Tai]

Reference

Nai CT, Lu J, Zhang K, Loh KP. “Studying Edge Defects of Hexagonal Boron Nitride Using High-Resolution Electron Energy Loss Spectroscopy.” Journal of Physical Chemistry Letters. 6 (2015) 4189.