Formation of prenucleation clusters and transformation to ZnSe quantum dots and magic-size clusters

Prenucleation clusters (PNCs) play an important role in the synthesis of colloidal semiconductor quantum dots (QDs) and magic-size clusters (MSCs) of binary II-VI metal chalcogenide (ME). Here, we explore the formation and transformation of ZnSe PNCs, using a reaction of zinc oleate (Zn(OA)₂), tri-n-octylphosphine selenide (SeTOP), and diphenylphosphine (HPPh₂, DPP). The PNC forms above 120 °C, and relatively high feed concentrations and Zn-to-Se molar ratios favor the formation. When a prenucleation-stage sample (160 °C/30 min) is dispersed in a mixture of cyclohexane (CH) and octylamine (OTA) at 25 °C, MSC-299 (displaying optical absorption peaking at 299 nm) develops from the PNC isomerization. The PNC is the precursor compound (PC-299) of MSC-299 and is relatively transparent. When the reaction is heated at higher temperatures (such as 220 °C for 15 min), the PNC fragments to monomers (Mos) and the nucleation and growth (N/G) of QDs occur (via the one-by-one addition of Mos). In a mixture of CH and CH₃OH at 25 °C, MSC-299 transforms to MSC-320 and MSC-340. The present study provides a deeper understanding of the formation and transformation of ZnSe PNCs.