Symmetry-Breaking for Formation of Rectangular CdSe Two-Dimensional Nanocrystals in Zinc-Blende Structure.

Formation of CdSe nanocrystals with two-dimensional quantum confinement (CdSe 2D nanocrystals) was studied with preformed CdSe nanocrystals in the size range between 1.7 and 2.2 nm as seeds. Specifically, the 2D CdSe nanocrystals were encased with six {100} facets of the zinc-blende (face-center-cubic) structure, that is, 1.5 nm in thickness with quite large atomically flat {100} basal planes (∼8 nm width and X ≈ 45 nm length). Symmetry breaking between the thickness and lateral directions occurred in the early stage by rapid formation of single-dot intermediates with flat yet polar {100} basal planes and the desired thickness from the seeds through intraparticle ripening. Two single-dot intermediates fused together through their reactive side facets-mostly the nonpolar {110} ones-to form 2D embryos with the same thickness. Such oriented attachment continued selectively onto the reactive side facets of the 2D embryos. Simultaneously, intraparticle ripening occurred slowly on the side facets of the 2D nanocrystals, which converted unstable side facets gradually to four stable {100} ones. When ∼3 stable {100} side facets were developed, oriented attachment would continue on the remaining active one, which would result in the second symmetry breaking between two lateral directions. Cadmium acetate assisted both formation of single-dot intermediates and oriented attachment. Cadmium alkanoates with a long hydrocarbon chain selectively stabilized polar {100} facets on the nanocrystals including single-dot intermediates and shuttled insoluble acetate to the reactive surface of the nanocrystals.