Collision-induced dissociation of II-VI semiconductor nanocrystal precursors, Cd²⁺ and Zn²⁺ complexes with trioctylphosphine oxide, sulfide, and selenide

The metal (M = Cd²⁺ and Zn²⁺) complexes with trioctylphosphine chalcogenide (TOPE, E = O, S, and Se) are prepared by electrospray ionization, and their relative stabilities and intramolecular reactions are studied by collision-induced dissociation (CID) with Xe under single collision conditions. These metal-TOPE complexes are considered as molecular precursors for the colloidal synthesis of II-VI compound semiconductor nanocrystals employing TOPO as a metal-coordinating solvent and TOPS or TOPSe as a chalcogen precursor. Of the various [M + nTOPE]²⁺ (n = 2-7) ions generated by ESI, the n = 2-4 complexes are characterized by CID as a function of collision energy. The collision energy at 50% dissociation (E_50%) is determined from the cracking curve and the relative stabilities of the complexes are established. Between the two metal ions, the zinc-TOPE complexes are more stable than the cadmium-TOPE complexes when n = 2-3, whereas their stabilities are reversed when n = 4. Of the TOPE, TOPO binds most strongly to the metal ion, while TOPSe does most weakly. Upon CID, loss of TOPE occurs exclusively from the tetra-TOPE complexes, while extensive fragmentation of TOPE takes place from the di-TOPE complexes, showing the signature of the metal chacogenide formation. The nucleation of nanocrystals appears to begin with cracking of [M + 2TOPE]^2- (E = S and Se).