The transition between four- and six-coordinate structures in CdSe nanocrystals displays simple transition kinetics as compared with the extended solid, and we determined activation volumes from the pressure dependence of the relaxation times. Our measurements indicate that the transformation takes place by a nucleation mechanism and place strong constraints on the type of microscopic motions that lead to the transformation. The type of analysis presented here is difficult for extended solids, which transform by complicated kinetics and involve ill-defined domain volumes. Solids patterned on the nanoscale may prove to be powerful models for the general study of structural transitions in small systems, as well as in extended solids.

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) TITLE RUNNING HEAD. Monodisperse Iron Oxide Nanodisks by Delayed Nucleation 2 ABSTRACT A comprehensive study of iron oxide nanocrystal growth through non-hydrolitic, surfactantmediated thermal reaction of iron pentacarbonyl and an oxidizer has been conducted, which includes size control, anisotropic shape evolution, and crystallographic phase transition of monodisperse iron oxide colloidal nanocrystals. The reaction was monitored by in situ UV-Vis spectroscopy taking advantage of the color change accompanying the iron oxide colloid formation allowing measurement of the induction time for nucleation. Features of the synthesis such as the size control and reproducibility are related to the occurrence of the observed delayed nucleation process. As a separate source of iron and oxygen is adopted, phase control could also be achieved by sequential injections of oxidizer.3

The study of nanoscale materials with well-controlled size and shape can be used to learn more about critical length scales for numerous physical and chemical phenomena in solids and extended systems. , Small nanocrystals (below 5-nm diameter) have been shown to exhibit fully reversible single-domain structural phase transformations with large volume changes over multiple cycles. The same transformations in extended solids are accompanied by irreversible domain formation. − Here we investigate the crossover between these regimes by studying a pressure-induced structural transformation in 4-nm-diameter nanorods varying in aspect ratio from 1 to 10. We find that above a critical length the nanorods fracture at the moment of the structural transformation. This work demonstrates the use of simple, well-defined nanoscale systems to examine fundamental structural phenomena found in extended solids.