The formation pathway of colloidal semiconductor ZnSe magic-size clusters (MSCs) in a reaction that display an optical absorption doublet remains poorly understood. The reaction of Zn(OAc)₂/OLA (made from zinc acetate and oleylamine) and tri-n-octylphosphine selenide (SeTOP) in OLA in the presence of diphenylphosphine (HPPh₂) is studied, in which dMSC-345 displays a doublet peaking at 328/345 nm. We suggest that the development is from the clusters that form in the initial prenucleation stage of the reaction. The clusters are the precursor compound (PC-299) of MSC-299 (displaying an absorption singlet peaking at 299 nm). PC-299 transforms to PC-345 at a later stage. The presence of alcohol (such as methanol or ethylene glycol) promotes another pathway, which is the PC-299 to PC-320 transformation. PC-320 transforms to dMSC-320 (with a doublet at 305/320 nm), followed by dMSC-345 via PC-345. The present study provides additional evidence that clusters (PC-299) form and transform (such as to dMSC-345 via PC-345) in the prenucleation stage of ZnSe quantum dots (QDs).

Little is known about how to precisely promote the selective production of either colloidal semiconductor metal chalcogenide (ME), magic-size clusters (MSCs), or quantum dots (QDs). Recently, a two-pathway model has been proposed to comprehend their evolution; here, we reveal for the first time that the size of precursors plays a decisive role in the selected evolution pathway of MSCs and QDs. With the reaction of cadmium myristate (Cd(MA)₂) and tri-n-octylphosphine selenide (SeTOP) in 1-octadecene (ODE) as a model system, the size of Cd precursors was manipulated by the steric hindrance of carboxylic acid (RCOOH) additive. Without RCOOH, the reaction produced both CdSe MSCs and QDs (from 100 to 240 °C). With RCOOH, the reaction produced MSCs or QDs when R was small (such as CH₃−) or large (such as C₆H₅−), respectively. According to the two-pathway model, the selective evolution is attributed to the promotion and suppression of the self-assembly of Cd and Se precursors, respectively. We propose that the addition of carboxylic acid may occur ligand exchange with Cd(MA)₂, causing the different sizes of Cd precursor. The results suggest that the size of Cd precursors regulates the self-assemble behavior of the precursors, which dictates the directed evolution of either MSCs or QDs. The present findings bring insights into the two-pathway model, as the size of M and E precursors determine the evolution pathways of MSCs or QDs, the understanding of which is of great fundamental significance toward mechanism-enabled design and predictive synthesis of functional nanomaterials.