Quasi 2D Colloidal CdSe Platelets with Thicknesses Controlled at the Atomic Level

Well documented procedures to grow zero-dimensional systems, dots, and one-dimensional systems, wires and tubes, as colloidal particles in solution have been reported. In contrast, there are no methods of preparation that yield optically active two-dimensional soluble particles. Yet, ultrathin films (quantum wells) of II−VI and III−V semiconductors epitaxially grown on substrates by molecular beam epitaxy for example have proven extremely useful for both fundamental studies and a wealth of applications in optoelectronics. We show that II−VI cadmium selenide platelets, with thicknesses tuned at the atomic level, can be synthesized in solution. We describe the method for the preparation of these new colloidal nanocrystals and characterize them structurally and optically. We identified three platelets populations with emission maximums at 462, 513, and 550 nm with corresponding thicknesses estimated at 1.9, 2.2, and 2.5 nm, respectively. Despite the fact that the platelet aspect ratio within a population can range from 4 to several hundreds, the emission spectra full width half-maximum of each population is <10 nm at room temperature with quantum yields that can reach 30%. The platelets we have synthesized are an extension of the quantum wells epitaxially grown on substrates, with the advantages that they can be easily synthesized in solution at low cost and used as building blocks for more advanced structures and have a uniform thickness that can be tuned within one CdSe monolayer and finite lateral dimensions ranging from 10 nm to a few 100 nm.