Keywords:Homogeneous Nucleation, Cosmic Dust
A gaseous molecule can have a chemical formula that is inconsistent with its ability to act as a building block for crystals; for example, hot AlO and AlO2 vapors cannot solidify directly to crystals. The contributions of such gases to homogeneous nucleation processes are unknown. Using in situ IR spectroscopy on nucleating nanoparticles, we show that a liquid-like behavior of nucleating nanoparticles in the Al–O system induces immiscible phase segregation. In an oxygen-deficient atmosphere, aluminum oxide nanoparticles formed with a unique shape composed of an Al metal head and an anisotropic Al2O3 crystalline tail. The anisotropic nanoparticles are larger than Al2O3 nanoparticles formed in a sufficient oxygen condition. Fewer nuclei may be available in the supersaturated gas, indicating that homogeneous nucleation is initiated by oxygen-bearing species. The nuclei grow by incorporation of Al-bearing species, changing the composition of the particle as a whole toward an Al-rich material. In situ IR measurements revealed that the degree of anisotropy continues to increase, suggesting that oxygen-bearing species also formed by oxidation on the surface of the molten Al-rich head. The scenario is supported by quantum-chemical calculations indicating that oxygen-deficient (AlO)n clusters as few as 16 molecules induce aggregation of Al atoms. Because Al metal has a significantly lower melting temperature than Al2O3, the immiscibility of the nucleating nanoparticles leads to vapor–liquid–solid growth. Astronomical implication will be given in the presentation.