|Spontaneous Superlattice Formation in Nanorods through Partial Cation Exchange |
|Richard Robinson, |
10:00 AM CCR 201
Lattice mismatch strains are widely known to control nanoscale pattern formation in heteroepitaxy, but such effects have not been exploited in colloidal nanocrystal growth. In this talk I will discuss our recent results which demonstrate a strain-mediated colloidal route to synthesizing CdS-Ag2S nanorod superlattices through partial cation exchange .
Cation exchange provides a facile method for systematically varying the chemical composition within a colloidal nanocrystal. We have previously shown that cation exchange can be used to fully (and reversibly) convert CdSe, CdS, and CdTe nanocrystals to the corresponding silver chalcogenide nanocrystal by a complete replacement reaction of the Cd2+ cations for Ag+ cations . The resultant material is the silver-anion analog of the starting material (i.e., Ag2Se, Ag2S, and Ag2Te). The reaction is rapid enough that complete exchange can be performed on nonequilibrium shaped nanocrystals -- such as rods, tetrapods, and hollow spheres -- without changing the shape of the crystal. Here I investigate the step-wise evolution of heterostructures as the degree of cation exchange is gradually increased. A striped pattern on each nanorod is created spontaneously at a critical Ag+ concentration. Further examination of the pattern shows that it is periodic and equally spaced on each nanorod and thus a superlattice. We believe that three factors contribute to the superlattice self-organization: a positive value for the interfacial formation energy between the materials, the fast diffusion of the cations in the solids, and an epitaxial strain from the mismatched lattices. To study these forces we use ab initio calculations to determine material properties and interfacial energies, and a valence force field model (VFF) to determine the strain energies. The nanorod superlattices exhibit high stability against ripening and phase mixing. These materials are tunable near-infrared emitters with potential applications as nanometer-scale optoelectronic devices. This work was supported by the U.S. Department of Energy under the contract number DE-AC02-05CH11231.
 R.D. Robinson, B. Sadtler, D.O. Demchenko, C.K. Erdonmez, L.-W. Wang, A.P. Alivisatos,
Science 317, 355 (2007)
 D.H. Son, S.M. Hughes, Y. Yin, and A.P. Alivisatos, Science 306, 1009 (2004)