Utilizing cation exchange method to produce core/shell PbS/CdS quantum dots with stable infrared emission

PbS quantum dots (QDs) have enormous potential for applications ranging from tunable infrared lasers to solar cells due to their efficient emission over a large spectral range in the infrared. Especially, multiple exciton generation has been observed in PbS QDs, which makes PbS QDs have great potential for high-efficiency solar cells. However, these applications have been limited by instability in emission quantum yield and peak position on exposure to ambient conditions. An effective strategy to improve PbS QDs' stability is overgrowth with a shell of a more stable semiconductor, such as CdS, resulting in core/shell PbS/CdS QDs. The PbS/CdS QDs were fabricated in a two-step method. In the first step, PbS QDs with a 4.8 nm diameter were prepared by using organic metal. Second, PbS/CdS QDs with 3.8 nm PbS core and 0.5 nm CdS shell were fabricated by exposing PbS QDs in Cd²⁺ solution for 24 h at 65°C. In this article, cation exchange method was adopted to moderate reaction temperature in a low temperature, so that Ostwald ripening in high temperature was avoided. The results of transmission electron microscopy (TEM) and high resolution TEM showed that PbS QDs were sphere and in a cubic cystal without obvious lattice defects. After PbS QDs being cation exchanged, PbS/CdS QDs' size and crystalline form almost kept the same. The results of X-ray diffraction also showed both the crystalline form of PbS and PbS/CdS QDs were cubic. That PbS/CdS QDs had strong absorption and bright photo fluorescence in near infrared region was proved in UV/Vis/NIR and PL spectrum. The stability was proved by comparing fresh QDs' and 3 months later QDs' PL spectrum, in which little blue shift and decrease in PL intensity was found in PbS/CdS QDs. Besides, the results showed that the degree of cation exchange was limited on the surface of the QDs. For example, 0.5 nm CdS shell was formed on the surface of PbS QDs after being exchanged for 24 h in 65°C. Thin as CdS shell was, it could effectively passivate PbS QDs' surface defects and significantly improve the photooxidation stability. By employing cation exchange method, highly crystallized core/shell PbS/CdS QDs with superior and stable infrared emission were fabricated at a low temperature.