Credit: © 2010 AAAS

The assembly of nanoparticles into complex architectures results in interesting morphologies and properties — helices, in particular, have continued to intrigue chemists and materials scientists. Now, an international team led by Sharon Glotzer and Nicholas Kotov at the University of Michigan has used visible light irradiation to control the self-assembly of cadmium telluride (CdTe) nanoparticles into helical ribbons1.

Semiconductor CdTe nanoparticles were dispersed in an aqueous solution, with a lower concentration of stabilizer (thioglycolic acid) than that normally used — the concentration influences the geometry, dipole and reactivity of the nanoparticles. After precipitation and re-dispersion in solution, the nanoparticles self-assembled over 72 hours into left- and right-handed helical ribbons, aggregated into bundles of the same chirality. A closer look at the ageing process revealed that it was accompanied by the oxidation of tellurium — from Te2− to Te0 — and the replacement of some tellurium by sulfur. The cadmium sulfide, predominant in the resulting CdS/CdTe nanoparticles, is most likely to be located at the surface, preventing re-crystallization into thin CdTe nanowires.

Comparing the ageing processes under visible light irradiation and in the dark revealed that in both cases the assemblies evolve through a variety of unusual shapes to form bundled straight nanoribbons. In the dark, however, the ribbons remain straight, but under continuing irradiation with visible light photo-oxidation of CdS occurs, which increases the charge–dipole forces and causes the formation of helices. Stronger twisting was obtained under more intense irradiation. Calculations show that the formation of particular architectures — chains, ribbons or sheets — depends on the interplay between face–face attraction and electrostatic repulsion forces.