Hybrid SiO2-coated nanocrystal-based heterostructures: Assembly, morphology transition, and photoluminescence at room temperature

Coating CdTe nanocrystals (NCs) with a hybrid SiO2 shell with CdS-like clusters drastically increased their photoluminescence (PL) efficiency and substantially red-shifted their PL peak. The coating was done either by a two-step synthesis including a sol-gel process and a subsequent reflux with a short reaction time or by a sol–gel procedure with a long reaction time at room temperature. When the solution of the coated NCs containing thiolglycolic acid (TGA), Cd2+, and SiO2 monomers was left at room temperature, the coated NCs self-assembled into heterostructures. The self-assembly behavior of the NCs is ascribed to an electrostatic force that led to the attachment of the NCs to a composite of TGA, Cd2+, and SiO2 monomers. The PL of the resulting assemblies depended strongly on the hybrid nanostructure, i.e., CdS-like clusters formed near the CdTe core. Experimental conditions such as the concentration of the hybrid NCs in solution and the growth time were adjusted to control the assembly morphology. When the NC concentration was sufficiently high, assemblies with a sheaf-like morphology were created at room temperature. Systematic reduction of the NC concentration resulted in the creation, in turn, of belt-like fibers, solid fibers, and long nanowires. When an aqueous solution with a low concentration of hybrid NCs was kept at room temperature for six months, the assemblies exhibited tubal and helical morphologies. This morphological evolution is ascribed to the concentration dependence of the growth kinetics.