A study of self-assembled calix[4]hydroquinone nanostructures

Abstract

In nanotechnology, it is of great significance to construct nanoscale structures with predicted sizes, shapes, and chemical compositions. It determines unusual physical, chemical and mechanical properties showing superb capabilities in a vast area of applications. Molecular self-assembly is the spontaneous organization of molecules through non-covalent weak interactions, which is a promising strategy for nanofabrication. However, although self-assembling phenomena are common in nature, it is not easy to design and actualize well-defined structures as nature does.Calix[4]hydroquinone, which is a bowl-shaped organic molecule, has the constituents creating non-covalent bonds such as hydrogen bonds and ？？？ stacking interactions. The various CHQ nanostructures self-assembled by bottom-up integration of CHQ molecules have very distinctive structures like tube, sphere, polygonal plate/film and lens. Especially, the self-assembled nanoscale lens has an ideal ？plano-spherical convex？ lens-shaped structure which has not been fabricated in nanoscale. The CHQ nanolens shows the new nano-optical phenomena in the case that the size of lens is comparable to wavelength of light. Based on the optical experiments and the accurate electromagnetic simulations we confirm that the CHQ nanolens has a remarkably short near-field focal length in contrast to geometrical optics. It leads to high resolution that is able to resolve sub-wavelength fine features beyond the diffraction limit that any lens-based system has not overcome. The optical information for fine features, which exist only in near-field region, can be transmitted to far-field through the near-field focusing and magnification of the CHQ nanolens. The unique optical properties of CHQ nanolens are expected to be applied to bio-imaging, near-field lithography, optical memory storage, spectral signal enhancing, and optical nano-sensing.The CHQ nanostructures with diverse shapes are formed through morphology conversions from nanotube/hexagonal crystal structures into nanospheres and nanolenses depending on nucleation and growth conditions. We have tried to understand their nucleation, growth, and conversion mechanism, by nucleation theory combined with ab-initio calculation. In the self-assembling process, the crystal structures are made prior to nanospheres because the isotropic formation of the spherical shaped CHQ morphology requires a higher kinetic barrier than that of nanotube crystals. Kinetics favors the formation of CHQ nanotubes, but thermodynamics ultimately drives CHQ nanotubes to transform into CHQ nanospheres. In this conversion process, CHQ nanospheres nucleate and grow anisotropically on the surface of crystals to reduce the surface energy barrier. That is, the growth pathways of the anisotropic formation of CHQ nanospheres via CHQ nanotubes are more energetically favorable than those of the isotropic formation. The CHQ nanolenses are formed within a small gap between film-like structures and crystals with fast nucleation and growth, driven by hyper density of CHQ molecules and short diffusion distance towards nucleus. In this process, film-like structures play a role as a substrate for nucleation and growth

finally are separated as CHQ nanolens. These thermodynamic and kinetic studies of morphology conversions provide not only the comprehensive understanding for the formation mechanism of nanostructures but also an insight into the strategy to synthesize organic nanomaterials towards desired shape, size, and properties.