RESUMO
Herein a series of size-selected TaN(N = 147, 309, 561, 923, 1415, 2057, 6525, 10 000, 20 000) clusters are generated using a gas-phase condensation cluster beam source equipped with a lateral time-of-flight mass-selector. Aberration-corrected scanning transmission electron microscopy (AC-STEM) imaging reveals good thermal stability of TaNclusters in this study. The oxidation-induced amorphization is observed from AC-STEM imaging and further demonstrated through x-ray photoelectron spectroscopy and energy-dispersive spectroscopy. The oxidized Ta predominantly exists in the +5 oxidation state and the maximum spontaneous oxidation depth of the Ta cluster is observed to be 5 nm under prolonged atmosphere exposure. Furthermore, the size-dependent sintering and crystallization processes of oxidized TaNclusters are observed with anin situheating technique, and eventually, ordered structures are restored. As the temperature reaches 1300 °C, a fraction of oxidized Ta309clusters exhibit decahedral and icosahedral structures. However, the five-fold symmetry structures are absent in larger clusters, instead, these clusters exhibit ordered structures resembling those of the crystalline Ta2O5films. Notably, the sintering and crystallization process occurs at temperatures significantly lower than the melting point of Ta and Ta2O5, and the ordered structures resulting from annealing remain well-preserved after six months of exposure to ambient conditions.
RESUMO
The investigation of nanocluster behaviors at elevated temperatures is important because it encompasses temperature-dependent structural evolution and size-dependent melting points. Size-selected Au2057±52, Au923±24, Au1846±48, and Au2769±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 AuN 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 Au2057±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.
RESUMO
The emerging technique of nano-welding (NW) via precisely regulating the fusion of nanoclusters (NCs) in nanotechnology has attracted significant attention for its innovative approach. Employing the gas-phase condensation cluster source with a lateral time-of-flight (TOF) mass-selector, size-selected gold (Au), and tantalum (Ta) NCs were prepared. This study explores the coalescence behavior of size-selected Au and Ta NCs under electron beam irradiation, aiming to investigate the related mechanism governing the welding process. Intrinsically driven by the reduction of excess surface energy, electron beam induces atomic thermal migration, fostering sintering neck growth at cluster interfaces. During this process, atomic diffusion and recrystallization enable NCs to alter shape while retaining stable facet planes. Aberration-corrected scanning transmission electron microscopy (AC-STEM) showcases the formation of single or polycrystalline sintered clusters, during which some lattice distortions can be eliminated. Interestingly, oxidized Ta clusters experience knock-on damage caused by elastic scattering of electron beams, partially deoxidizing them. Additionally, electron-phonon inelastic scattering transforms oxidized Ta clusters from amorphous to crystalline structures. Moreover, the quantum size effect and surface effect of NCs facilitate the surpassing of miscibility limits during Au-Ta heterogeneous welding processes. This investigation bridges the gap between fundamental research on cluster materials and their practical applications.