Cu₂Se and Cu Nanocrystals as Local Sources of Copper in Thermally Activated In Situ Cation Exchange

Abstract

Among the different synthesis approaches to colloidal nanocrystals, a recently developed toolkit is represented by cation exchange reactions, where the use of template nanocrystals gives access to materials that would be hardly attainable via direct synthesis. Besides, postsynthetic treatments, such as thermally activated solid-state reactions, represent a further flourishing route to promote finely controlled cation exchange. Here, we report that, upon in situ heating in a transmission electron microscope, Cu2Se or Cu nanocrystals deposited on an amorphous solid substrate undergo partial loss of Cu atoms, which are then engaged in local cation exchange reactions with Cu “acceptor” phases represented by rod- and wire-shaped CdSe nanocrystals. This thermal treatment slowly transforms the initial CdSe nanocrystals into Cu2–xSe nanocrystals, through the complete sublimation of Cd and the partial sublimation of Se atoms. Both Cu “donor” and “acceptor” particles were not always in direct contact with each other; hence, the gradual transfer of Cu species from Cu2Se or metallic Cu to CdSe nanocrystals was mediated by the substrate and depended on the distance between the donor and acceptor nanostructures. Differently from what happens in the comparably faster cation exchange reactions performed in liquid solution, this study shows that slow cation exchange reactions can be performed at the solid state and helps to shed light on the intermediate steps involved in such reactions.

Electrically driven cation exchange for in situ fabrication of individual nanostructures

“…Cd sublimation of heterogeneous NCs was previously observed in colloidal CdSe, CdSe/Cu 3 P/CdSe NCs, NCs and CdSe/PbSe NCs68910. The sublimation temperatures are as low as 450 K (ref.…”

The New Youth of the In Situ Transmission Electron Microscopy

“…The study of Casu et al [61] provides a further example, focusing on modifications occurring during CE reactions in solid state at the nanoscale during in situ TEM annealing. In fact, while the out-diffusion and motion of chemical species have previously been accounted for as a generic possibility during in situ exchange reactions happening at the nanoscale, it was never actively investigated or used as a tool for in situ ionic exchange reactions.…”

“…Green and red color coding indicates the presence of Cu and Cd in the EFTEM maps, respectively. Adapted with permission from[61].…”

Developments of cation-exchange by in situ electron microscopy

“…In the fourth chapter the concept of in situ CE reaction at the solid state is definitely extended to a more complex phenomenological configuration: two NPs populations, one capable of expelling a chemical species under heating and the other one capable of adsorbing that expelled species, are deposited close-by on the same heating substrate and undergo a CE reaction under heating [31].…”