Retaining the square: Pb-to-Cd ion exchange in square PbSe superlattices

Abstract

A novel class of two-dimensional semiconducting materials can be prepared by oriented attachment of colloidal nanocrystals (NCs). In the process of oriented attachment, a monolayer of NCs are epitax- ially connected into ordered superstructures called "superlattices". Novel band structures that depend on the geometry and the chemical composition of the lattice are predicted for these superlattices. A rich electronic structure is predicted for the honeycomb zincblende CdSe superlattice in particular. How- ever, the oriented attachment of NCs into superlattices has only been demonstrated for PbSe NCs and cation exchange is required to obtain superlattices of other chemical compositions. The Pb-to-Cd cation exchange reaction of PbSe superlattices has been reported in previous work. However, the CdSe crystal structure that is obtained after cation exchange and the exact mechanism of the exchange reaction re- main unclear. Studying cation exchanged honeycomb superlattices is hindered by the buckled geometry of the silicene-type lattice. In the present work, cation exchange is investigated in square superlattices, as the NCs in superlattices with a square geometry only occupy one bonding plane. Nevertheless, for the square superlattices retention of large domain sizes after transformation into CdSe has not yet been reported. Here we show that nearly full retention of the square superlattice structure is achieved by reducing the cation exchange reaction temperature down to 100 °C. Moreover, we show that annealing of the superlattice before cation exchange negatively impacts retention of the long-range order of the superlattice. The cation exchanged square superlattices obtained by the optimized exchange methods can be used to further study the obtained CdSe crystal structure and the exchange reaction mechanism in superlattices.