Nanotubes are important ‘building block' materials for nanotechnology, but a synthesis process for short (sub-100-nm) nanotubes with structural order and monodisperse diameter has remained elusive. To achieve this goal, it is critical to possess a definitive mechanistic framework for control over nanotube dimensions and structure at very small length scales.

While no general strategy has thus far been proposed, we are interested in a unique model system that offers mechanistic insights into the assembly of nanotubular objects that could lead to a more general synthesis process. We have employed solution-phase and solid-state characterization tools to elucidate the mechanism governing the formation of short (20 nm), ordered, monodisperse (3.3 nm diameter), aluminum-germanium-hydroxide(‘aluminogermanate') nanotubes in aqueous solution. We show - via mechanistic synthesis experiments, dynamic light scattering, UV-Vis/Raman/IR spectroscopy, microscopy, and diffraction techniques - that the central phenomena underlying this mechanism are: (1) the generation (via pH control) of a precursor solution containing aluminate and germanate precursors chemically bonded to each other, (2) the formation of amorphous nanoscale (~ 6 nm) condensates via temperature control, and (3) the self-assembly of short nanotubes from the amorphous nanoscale condensates.

Simultaneously, atomistic simulations reveal that the formation of ordered monodisperse nanotubes is strongly related to the existence of unique energy minima in the nanotube structure as a function of diameter. This provides an additional handle for directing the assembly of metal oxide nanomaterials towards energetically favorable structures. Our mechanism provides a model for controlled low-temperature assembly of small, monodisperse, ordered nanotube objects.