Probing Local Phonon Polariton Signals at Edges of Folded Boron Nitride Sheets

Hyperbolic phonon polaritons (HPhPs) are an emerging physical concept related to several intriguing phenomena such as hyperlensing, negative refraction, and subdiffractional resonator [1]. Low-loss and tunable HPhPs have been detected in several single-phase natural materials including hexagonal boron nitride (h-BN) and α-MoO3 using both scanning near-field optical microscopy [2] and monochromoated electron energy-loss spectroscopy (EELS) [3,4]. However, the study of 1D propagating HPhPs, promising for integrated optical circuits and mid-infrared optical communication, is still absent in h-BN nanostructures. Owing to the ultrahigh energy resolution, monochromated EELS can detect multiple excitations with high spatial resolution in the low energy-loss range from 10 meV to a few eV, including lattice vibrations, phonon polaritons, plasmons, and excitons. Here, we combine the high spatial and energy resolutions of the low voltage (60 kV) aberration-corrected STEM-EELS approach to discover the curvature-induced phonon polaritons in folded h-BN sheets with a few atomic layers [5]. Fig. 1a shows a schematic model of the folded 1L h-BN, which is formed due to the van der Waals attraction between two overlapping regions and can be frequently found in the freestanding sample transferred on a TEM grid. The typical vibrational spectrum in Fig. 1b contains a prominent peak at about 170 meV, which is assigned to HPhP modes according to previous studies [3,4]. Fig. 1c compares four high-quality representative spectra from different regions. The peak center of the HPhP signal in the vacuum (aloof) spectrum is 169.26 ± 0.03 meV, while that of the HPhP signal in the unfolded monolayer is 170.06 ± 0.03 meV. The reduced peak energy in the aloof spectrum can be interpreted by the excitation of the hyperbolic surface phonon polaritons (HSPhPs). The folded monolayer region exhibits a HPhP peak at 170.16 ± 0.03 meV with a tiny blue-shift of 0.1 meV compared to the unfolded monolayer region. Interestingly, at the crease of the folding region, the HPhP peak further blue shifts to 171.27 ± 0.04 meV with an increment of 1.1 meV. We then investigated the location dependence of HPhPs in another folded 2L h-BN (Fig. 2). The atomic structure at the folding edge and corresponding twist angle between the top and bottom flakes were characterized by ADF-STEM imaging and electron diffraction (Fig. 2a). The observed sample is found to be a folded bilayer with a twist angle of 52°, leading to a Moiré pattern. Figs. 2b-c present a line-scan vibrational spectra across the sample edge. The HPhPs undergo an energy blue-shift of 1.1 meV and an intensity enhancement at the folding edge, while the HSPhP signal is detected with a lower energy in the vacuum (Fig. 2d). Such edge-localized variation takes place within 8 nm of the edge, which cannot be resolved by optical methods. The crease-localized energy blue-shifts are also observed in other samples with different layer numbers (1-3L) and twisting angles. Most of the measured energy shifts span from 0.2 to 1.4 meV. To delve into the nature of the observed signals, we performed angle-resolved vibrational EELS experiments at the Brillouin zone center and observed a similar energy blue-shift also occurs at folding edges. More experimental evidence precludes the influence of the beam−sample geometry, illumination condition, and even the strong anisotropy of h-BN on the variation of HPhPs. The possible explanation is narrowed down to the curvature at the folding edges. We used BN nanotube (BNNT) structures to mimic the edge of folded sheets for calculating phonon structures and dielectric functions, which can vary the HPhP peak center [4]. First-principles calculations reveal that the transverse optical (TO) modes in (4,4) and (8,0) BNNTs are blue-shifted by 3.1 and 6.7 meV, respectively, compared to the bilayer structure, giving rise to an energy blue-shift of HPhP modes of 3.1 and 6.7 meV, respectively. In summary, monochromated STEM-EELS reveals an unexpected energy blue-shift of the phonon polaritons near the edges of folded atomically thin h-BN films. We further demonstrated that the crease-localized phonon polariton modes are attainable via both bottom-up and top-down methods, especially by electron beam etching. The generation and manipulation of this new-type crease-localized 1D phonon polariton mode in 2D h-BN are essential for many applications such as integrated optical circuits [6]. Vibrational spectra and confined HPhP signals in the folded 1L h-BN. (a) Schematics of moving electron probe at different locations (vacuum, edge, folded and unfolded regions) of a folded 1L h-BN. The pink and blue balls denote B and N atoms, respectively. (b) Representative vibrational spectrum of an unfolded 1L sample containing the zero-loss peak (ZLP) and HPhP peak. (c) Averaged vibrational spectra of vacuum (black), unfolded 1L (green), folded 1L (blue), and edge (red) regions. The solid curves are Gaussian fitting curves, while vertical dashed lines indicate the peak centers. Spatially resolved vibrational spectra at the edge of folded 2L h-BN. (a) Atomic-resolution ADF-STEM image. Inset: fast Fourier transform pattern. (b) Low-magnification ADF-STEM image. (c) Line scan of vibrational spectra along the white arrow in (b). The peak centers of the phonon polariton signal are indicated as open dots. (d) Line profile of ADF-STEM intensity, peak center of the phonon polariton mode, and signal intensity at 171 meV in (c). The light gray, red, and green regions represent the vacuum, edge, and folded regions, respectively.