Mechanical size effects of novel core-shell structured liquid gallium nanoparticles

Liquid metal nanoparticles (LM NPs) find extensive applications in flexible electronics, nanomedicine, and various other fields owing to their deformability and distinctive solid-liquid core-shell configuration. Nevertheless, the dynamic structural transformations and size-dependent mechanical characteristics of gallium-based core-shell nanoparticles are not well comprehended due to the constraints of high-precision nanoparticle mechanical characterization techniques. This limitation significantly hampers their utilization and advancement. This study systematically investigates the synthesis approaches of thiolated and nonthiolated gallium NPs and comprehensively examines the evolution of their structures and mechanical properties in relation to time and particle dimensions, leading to several significant findings. Initially, scanning electron microscopy revealed the formation of a 3 nm thick gallium oxide (Ga2O3) shell layer on both thiolated and nonthiolated gallium nanoparticles during synthesis. The gallium oxide shells on these two types of nanoparticles did not form simultaneously; instead, they underwent further oxidation and thickening over time, with the thickness stabilizing at day 10 for thiolated NPs and day 15 for nonthiolated NPs. Subsequently, the impact of gallium NPs’ size on mechanical properties, such as elastic modulus and critical fracture force, was investigated using atomic force microscopy and finite element simulation. It was observed that the elastic modulus of gallium NPs increased exponentially as the particle size decreased. This change in modulus was not only solely influenced by Young’s modulus of the gallium oxide shells as predicted by the classical Reissner’s theory but also by the increase in the bulk modulus of the gallium core due to its size effects. A significant discovery was made regarding the impact of internal pressure from the liquid gallium core on preventing the inward buckling of the gallium oxide shell during compression. This discovery notably increased the deformability of gallium nanoparticles, allowing them to withstand strains from 10% to 18% without fracturing. This unique behavior has not been documented in prior studies on the mechanical properties of liquid metal nanoparticles, indicating promising opportunities for utilizing and designing core-shell structured LM NPs.