Substrate controlled thermal transport governs coalescence, disordering, and liquid-like transitions in gold...

Gold nanoparticles (AuNPs) exhibit dynamic structural evolution under external stimuli, yet how substrate-mediated heat dissipation governs inter-particle interactions and phase behaviour during electron beam irradiation remains not fully understood. Here, we combine in situ transmission electron microscopy with COMSOL Multiphysics simulations to elucidate the roles of substrate thickness and surface chemistry in regulating the nanoscale sintering dynamics of a pair of AuNPs on silicon nitride (SiN) substrates. We show that electron beam-induced heating drives a competition between diffusion-mediated coalescence, structural disordering, and transition from a crystalline state to a highly disordered state exhibiting characteristics consistent with a melting-like transformation, dictated by substrate-controlled thermal transport. On thin (10 nm) SiN substrates, efficient heat dissipation stabilizes the crystalline phases of nanoparticles, promoting gradual neck formation, lattice reconstruction and the emergence of multi-twinned, rod-like structures with a delayed melting-like transformation. In contrast, thicker SiN (40-50 nm) substrates enhance local heat accumulation, accelerating coalescence while simultaneously inducing structural disorder and phase transitions. Plasma treatment further modulates interfacial thermal resistance, thereby governing local thermal transport and affecting nanoparticle transformation. COMSOL Multiphysics simulations reveal that increasing SiN thickness suppresses plasmonic near-field enhancement while amplifying beam-induced thermal deposition. On the other hand, thinner substrates favour localized energy dissipation and structural stability. Overall, these results suggest that substrate thermal properties and heat transport at particle-substrate interfaces are key parameters governing electron beam irradiation-driven nanoparticle dynamics.