General hypothesis for nanowire synthesis. II: Universality

A universal model for basic science of nanowire, nanotube, and nanodot syntheses by the solid-phase, liquid-phase, and vapor-phase mechanisms must be established. To our knowledge, the syntheses of these nanomaterials by the solid-liquid-solid (SLS) and fluid-liquid-solid (FLS) mechanisms have not been understood well. Extensive investigations of the basic features of SLS and FLS mechanisms for nanowire synthesis in the framework of general hypothesis put forth in the preceding paper have been carried out. These have explained why nanowires grown by the SLS mechanism are almost always amorphous. These have explained also why nanowires produced by the catalyst-mediated FLS mechanism have diameters almost always smaller than the diameters of catalyst seed. SLS growth is believed to be a high-temperature process. The actual temperature for this process has been examined. Concept of component seeds (CSDs), component droplets (CODs) from CSDs, and droplets from CODs, has been exploited for the investigations. Evidential (experimental and theoretical) demonstration of the hypothesis for the SLS and FLS growths of nanowires has been carried out. Possible relationship between the activation energy and the precursor decomposition on the droplet surface at the lowest possible temperature has been examined. Evidences of the role of dipole moment in the catalyst-mediated and catalyst-free FLS growth of nanowires, and of the role of droplets in the multiple nucleation of nanowires have been articulated. Evidences have also been presented to highlight the importance of the concentration gradient of the nanowire species and the motive force resulting from this concentration gradient. Quantum dots are miniaturized nanowires. Shortcomings of the conventional methods for large-scale quantum dot synthesis have been discussed. The usefulness of the self-catalytic mechanism for large-scale synthesis of the quantum dots has been elaborated. The importance of thermodynamic imbalance of seeds in nonmaterial growths has been established. Droplet-free scenario for nanowire growth has been envisioned. Ground rules for nanowire heterostructures have been proposed. Experimental results support the findings and demonstrations. The investigations appear to reveal a unified synthetic route of all nanomaterials (e.g., nanowires, nanotubes, and nanodots). They suggest that growths of these nanomaterials may be explained just from one single platform.