Van der Waals materials exhibit intriguing structural, electronic, and photonic properties. Electron energy loss spectroscopy within scanning transmission electron microscopy allows for nanoscale mapping of such properties. However, its detection is typically limited to energy losses in the eV range-too large for probing low-energy excitations such as phonons or mid-infrared plasmons. Here, we adapt a conventional instrument to probe energy loss down to 100 meV, and map phononic states in hexagonal boron nitride, a representative van der Waals material. The boron nitride spectra depend on the flake thickness and on the distance of the electron beam to the flake edges. To explain these observations, we developed a classical response theory that describes the interaction of fast electrons with (anisotropic) van der Waals slabs, revealing that the electron energy loss is dominated by excitation of hyperbolic phonon polaritons, and not of bulk phonons as often reported. Thus, our work is of fundamental importance for interpreting future low-energy loss spectra of van der Waals materials.Here the authors adapt a STEM-EELS system to probe energy loss down to 100 meV, and apply it to map phononic states in hexagonal boron nitride, revealing that the electron loss is dominated by hyperbolic phonon polaritons.
Conflict of interest statement
The authors declare no competing financial interests.
Nanoscale Guiding of Infrared Light With Hyperbolic Volume and Surface Polaritons in Van Der Waals Material RibbonsI Dolado et al. Adv Mater e1906530. PMID 31977111.Van der Waals (vdW) materials host a variety of polaritons, which make them an emerging material platform for manipulating light at the nanoscale. Due to the layered stru …
Optical Nanoimaging of Hyperbolic Surface Polaritons at the Edges of Van Der Waals MaterialsP Li et al. Nano Lett 17 (1), 228-235. PMID 27966994.Hyperbolic polaritons in van der Waals (vdW) materials recently attract a lot of attention, owing to their strong electromagnetic field confinement, ultraslow group veloc …
Efficiency of Launching Highly Confined Polaritons by Infrared Light Incident on a Hyperbolic MaterialS Dai et al. Nano Lett 17 (9), 5285-5290. PMID 28805397.We investigated phonon-polaritons in hexagonal boron nitride-a naturally hyperbolic van der Waals material-by means of the scattering-type scanning near-field optical mic …
Polaritons in Layered Two-Dimensional MaterialsT Low et al. Nat Mater 16 (2), 182-194. PMID 27893724. - ReviewIn recent years, enhanced light-matter interactions through a plethora of dipole-type polaritonic excitations have been observed in two-dimensional (2D) layered materials …
Magnetism in Two-Dimensional Van Der Waals MaterialsKS Burch et al. Nature 563 (7729), 47-52. PMID 30382199. - ReviewThe discovery of materials has often introduced new physical paradigms and enabled the development of novel devices. Two-dimensional magnetism, which is associated with s …
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