Nanoscale infrared imaging and spectroscopy of few-layer hexagonal boron nitride

M. Takamura, K. Watanabe, T. Taniguchi and Y. Taniyasu

Journal of Applied Physics 132, 174301 (2022)
Nanoscale evaluation of the number of layers and boundaries in two-dimensional (2D) materials is crucial for understanding relationships between structure and property. Here, using scattering-type scanning near-field optical microscopy, we systematically studied on a nanoscale the infrared spectra and imaging of hexagonal boron nitride (h-BN), an ideal 2D insulating material. We revealed that the main factor determining the infrared amplitude changes at an optical frequency of about 1370 cm−1, corresponding to the in-plane phonon mode of h-BN. At lower frequencies, the amplitude is mainly determined by the local dielectric function of a sample and depends on the number of h-BN layers. At higher frequencies, it is affected by the phonon polariton waves of h-BN, and thus edges and grain boundaries of h-BN can be visualized due to the reflection of the waves at the boundary. The infrared spectra show a shoulder peak at higher frequencies, derived from the resonance with the phonon polaritons, in addition to a peak due to the in-plane phonon mode.