Probing optical and acoustic phonons in heated nano-Si/epoxy composites

Understanding thermal response of optical and acoustic phonons is crucial for designing functional polymer nanocomposites. We study silicon nanoparticle–epoxy composites with filler contents of 0.02–2 wt. % using combined Raman and Brillouin spectroscopy under local laser-induced and global stage-controlled heating. Raman spectra reveal localized terahertz silicon–silicon optical phonon softening and spectral broadening in silicon fillers under local heating, indicating nanoscale hot spots and interfacial scattering. Brillouin data track propagating gigahertz longitudinal acoustic phonons in the effective polymer–filler medium, showing temperature- and concentration-dependent thermoelastic response and acoustic damping. Comparing the two heating methods reveals silicon loading thresholds for isolated thermal absorbers, thermal crosstalk, acoustic attenuation and elastic homogenization. Local heating induces greater phonon softening and damping than global heating, with this disparity amplified at higher loadings by thermal gradients and interfacial dissipation. Raman thermometry combined with finite element opto-thermal modeling shows that silicon nanoparticle loading improves thermal conductive performance compared with previously reported silicon carbide nanowire–epoxy composites at 2 wt. %. Our results establish silicon nanoparticle–epoxy composites as interface-engineered phonon-damping materials with tunable thermal conductivity and demonstrate multimodal spectroscopy's power to resolve thermo-phononic processes relevant to vibration damping and thermal management.