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Resonance-Enhanced Excitation of Interlayer Vibrations in Atomically Thin Black Phosphorus

Publication Type
Journal Name
Nano Letters
Publication Date
Page Numbers
4809 to 4815

The strength of interlayer coupling critically affects the physical properties of two-dimensional (2D) materials. In particular, black phosphorus (BP) – an anisotropic elemental 2D semiconductor – has been theoretically predicted to have strong interlayer interactions due to significant charge redistribution, giving rise to the well-known layer dependence of the electronic structure in few-layer BP. However, the strength of interlayer coupling in BP has not been experimentally validated. In principle, rigid-layer vibrations reflect directly the interlayer coupling strength in 2D van der Waals solids, but measurement of these characteristic frequencies is made difficult by sample instability and small Raman scattering cross-sections in atomically-thin elemental semiconductors. Here we overcome these challenges in BP by performing resonance-enhanced low-frequency Raman scattering under an argon protective environment. Interlayer breathing modes for atomically-thin BP were unobservable under conventional (non-resonant) excitation but became strongly enhanced when the excitation energy matched the subband electronic transitions of few-layer BP, down to bilayer thicknesses. Notably, the measured out-of-plane interlayer force constant was found to be 7.3 N/atom in BP, which is 2.8 times larger than graphene. These measurements directly confirm strong interlayer coupling in BP, and lay the foundation for future exploration of BP twisted structures and heterostructures.