Oxidation behavior and atomic structural transition of size-selected coalescence-resistant tantalum nanoclusters.

Abstract

Herein a series of size-selected TaN (N=147, 309, 561, 923, 1415, 2057, 6525, 10000, 20000) clusters are generated using a gas-phase condensation cluster beam source equipped with a lateral time-of-flight (TOF) mass-selector. Aberration-corrected scanning transmission electron microscopy (AC-STEM) imaging reveals good thermal stability of TaN clusters in this study. The oxidation-induced amorphization is observed from AC-STEM imaging and further demonstrated through X-ray photoelectron spectroscopy (XPS) and energy-dispersive spectroscopy (EDS). The oxidized Ta predominantly exists in the +5 oxidation state and the maximum spontaneous oxidation depth of Ta cluster is observed to be 5 nm under prolonged atmosphere exposure. Furthermore, the size-dependent sintering and crystallization process of oxidized TaN clusters is observed with in-situ heating technique, and eventually, ordered structures are restored. As temperature reaches 1300 °C, a fraction of oxidized Ta309 clusters exhibit decahedral and icosahedral structures, but the five-fold symmetry structures are absent in larger clusters, instead, these clusters exhibit ordered structures resembling those of the crystalline Ta2O5 films. 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 condition.&#xD.