In situ heating characterization of structural evolution and size-dependent melting point depression in gold nanoclusters: a comprehensive thermodynamic investigation

The investigation of nanocluster behaviors at elevated temperatures is important because it encompasses temperature-dependent structural evolution and size-dependent melting points. Size-selected Au_2057±52, Au_923±24, Au_1846±48, and Au_2769±72 clusters were generated using a gas-phase condensation cluster beam source equipped with a lateral time-of-flight mass selector. Comprehensive in situ heating characterization was conducted, revealing the structural evolution and size-dependent melting point depression of Au_N clusters at atomic resolution via aberration-corrected scanning transmission electron microscopy (AC-STEM). Using quantitative (Q)STEM simulations, a comprehensive statistical analysis was conducted to investigate the structural characteristics of the Au clusters. These clusters tended to be kinetically trapped in metastable structures during nucleation, which subsequently served as "growth templates" for the formation of many metastable Au clusters. In situ heating experiments performed on Au_2057±52 revealed a structural evolution trend from icosahedron (Ih) to decahedron (Dh) and finally to face-centered cubic (FCC) structures, with noticeable competition being observed between the Dh and FCC structures. AC-STEM imaging revealed that the melting of the Au clusters began with the formation of molten liquid shells on the surface. The liquid shells thickened at higher temperatures, and the solid core suddenly melted when its diameter decreased to a critical size. Furthermore, the melting points of the Au clusters were linearly dependent on the reciprocal diameter. Compared with the theoretical models, it was found that the liquid nucleation and growth model is in good agreement with the experimental results, indicating its suitability for describing the surface core melting processes of Au clusters at the studied scales.