Controlling nanotube dimensions: correlation between composition, diameter, and internal energy of single-walled...

Control over the diameter of nanotubes is of significance in manipulating their properties, which depend on their dimensions in addition to their structure and composition. This aspect has remained a challenge in both carbon and inorganic nanotubes, since there is no obvious aspect of the formation mechanism that allows facile control over nanotube curvature. Here we develop and analyze a quantitative correlation between the composition, diameter, and internal energy of a class of single-walled mixed oxide aluminosilicogermanate (AlSiGeOH) nanotubes. A series of synthetic AlSiGeOH nanotubes with varying Si/Ge ratio are characterized by X-ray photoelectron spectroscopy, vibrational spectroscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction to relate their compositions and diameters. We then study these nanotubes computationally by first parametrizing and validating a suitable interatomic potential model, and then using this potential model to investigate the internal energy of the nanotube as a function of diameter and composition via molecular dynamics simulations. There are minima in the internal energy as a function of diameter which progressively shift to larger nanotube diameters with increasing Ge content. An approximate analytical theory of nanotube diameter control, which contains a small number of physically significant fitted parameters, well describes the computational data by relating the composition and geometry to the strain energy of bending into a nanotube. The predicted composition-dependent shift in the energetically favored diameter follows the experimental trends. We suggest related methods of controlling nanotube energetics and their role in engineering nanotubes of controlled dimensions by liquid-phase chemistry.