Mapping the reaction zones for CdTe magic-sized clusters and their emission properties

Abstract

CdTe magic-sized clusters (MSCs) are promising building blocks for semiconductor devices because of their single size, consistent properties, and reproducible synthesis. However, the synthetic conditions for CdTe MSCs vary significantly in different reports, which hinders the general understanding of their formation mechanisms. Here, we employed Cd(oleate)₂, trioctylphosphine telluride (TOPTe), and oleylamine, which are commonly used for larger quantum dot (QD) synthesis, as standard reaction precursors, and systematically investigated the effects of solvent, phosphine amount, oleylamine amount, Cd : Te ratio, and temperature on the evolution of MSCs with time. These conditions compose the “reaction coordinates” to map out the “reaction zones” for CdTe MSCs and QDs. We found that CdTe MSCs with the first excitonic transition (E₁) at 449 nm can be synthesized in high purity with excess TOPTe using toluene as the solvent at 100 °C. Whereas higher temperature, excess of Cd(oleate)₂, or more viscous solvent led to the aggregation of 449 nm MSC into larger magic-sized species with E₁ at 469 nm as well as QDs with E₁ > 500 nm. Increasing phosphine concentration simply enhanced the rate and yield of CdTe MSCs, while a critical amount of oleylamine was required to “turn on” the MSC formation. Interestingly, the pure 449 nm MSCs were non-emissive, but colorful emissions were observed for the reaction mixtures containing both MSCs and QDs. The emissions could be attributed to a small amount of QDs formed during the reaction. The mapping of reaction zones is a crucial step towards the rational synthesis of CdTe MSCs and further understanding of their formation mechanism.