2026/07/15 by Tiago E. C. Magalhães, Tânia M. Ribeiro, Oili Ylivaara +2
Physics and Astronomy · #physics.optics #cond-mat.mes-hall
We report the femtosecond time-resolved measurements of confined acoustic phonons in free-standing nanocrystalline silicon membranes and compare them directly with the crystalline silicon counterpart. While the latter exhibit well-resolved higher-order modes, a strong suppression of these modes is observed in nanocrystalline samples with grain size distribution controlled by thermal annealing. The suppression is strongly frequency dependent and becomes more pronounced as the phonon wavelength approaches the characteristic grain size. By separating intrinsic and extrinsic contributions to the phonon lifetime, we identify an additional frequency-dependent decay channel associated with grain boundaries, with scattering rates following a power-law dependence close to f2, where f is the frequency. The measured sound velocity is consistent with previous reports for nanocrystalline silicon and indicates an effective elastic response arising from multiple crystallographic orientations. These results establish coherent phonons as a sensitive probe of microstructure-dependent scattering in nanocrystalline materials and indicate that grain boundaries act as an effective spectral filter for high-frequency acoustic phonons.