Stripping and Transforming Alloyed Semiconductor Quantum Dots Via Atomic Interdiffusion

We report the transformation of near-infrared CdSeTe/ZnS quantum dots (QDs) that are exposed to water. When the colloidal QDs with 840 nm emission wavelength and 75 nm spectral line width are self assembled on water surface and transferred to an oxide-coated silicon wafer using a Langmuir-Blodgett (LB) procedure, two prominent relatively sharp photoluminescence (PL) bands are observed at 1530 and similar to 660 nm peak wavelengths with line width of similar to 23 and similar to 39 nm, respectively. On the other hand, the PL spectrum of the QDs as they are assembled on the water surface is essentially the same as that of the solution phase. Structural analysis of the LB films shows that the QDs are stripped off the stabilizing excess surfactant molecules by the preferential interaction at the water air interface. After the film is transferred, the QDs are interfaced with each other and with the substrate directly, while covered with the stack of surfactant molecules from the top. Based on analysis of the chemical composition using X-ray photoelectron spectroscopy of the LB film, the transformation of the CdSeTe/ZnS nanocrystals is attributed to a diffusion of Te atoms from the core to the shell that can initiate inward diffusion of S atoms. This atomic interdiffusion minimizes lattice mismatch as the larger Te atoms are replaced by the smaller S atoms and can lead to formation of either CdSe/CdS or CdSeS nanocrystals that emit at 630 and 660 nm wavelengths, respectively.