INTRAZEOLITE TOPOTACTIC MOCVD - 3-DIMENSIONAL STRUCTURE-CONTROLLED SYNTHESIS OF II-VI SEMICONDUCTOR NANOCLUSTERS

A versatile synthetic method is described that blends conventional semiconductor chemistry with 3-D structure-controlled topotactic growth of semiconductor nanoclusters. The approach is utilized to assemble and organize II-VI semiconductor nanoclusters from MOCVD-type reagents within the diamond lattice of 13-angstrom spherical cavities found in zeolite Y. Specifically, these experiments involve the room-temperature reaction of volatile (CH3)2M MOCVD reagents (where M = Zn, Cd) with Bronsted acid sites in HnNa56-nY. This yields materials containing ZOMCH3 (where ZO represents the zeolite oxide framework) MOCVD moieties anchored at alpha-cage (supercage) sites of zeolite Y. Exposure of (CH3M)48Na8Y, containing six ZOMCH3 MOCVD precursors per alpha-cage, to H2X (where X = S, Se) induces a subsequent transformation to materials which analyze close to the ideal unit cell formula (M6X4)8H16Na8Y. These are shown by a combination of IR, EXAFS, and Rietveld PXRD structure analyses to contain a diamond lattice of alpha-cage encapsulated M6X4(4+) nanoclusters. From the results of these analyses, an ideal structural model based on M4X4 cubane-type clusters, anchored through two of their chalcogenide vertices to the oxide framework of the zeolite by site II and III alpha-cage M2+ cations, namely, M2(M4X4)4+, is favored over an alternative M6X44+ adamantane geometry. Six (CH3)2Cd species per alpha-cage can also be loaded into partially exchanged HnNa56-nY having n = 0, 8, 16, 24, 32, and 40. These react with, for example, H2Se to yield nanoclusters of the type Cd6Se6 for n = 0, 8, Cd6Se52+ for n = 16, 24, 32 and Cd6Se44+ for n = 40. The latter is most similar to the Cd6Se44+ nanoclusters formed in the n = 48 zeolite host. The optical reflectance spectra of the M6X44+ nanoclusters display a blue shift of their absorption edges with respect to the bulk II-VI semiconductors, characteristic of quantum confinement. The order of the absorption edges of the II-VI nanoclusters, namely, Zn6S44+ > Zn6Se44+ > Cd6S44+ > Cd6Se44+ follows that of the bandgap energies of the parent II-VI semiconductors. The luminescence emission and excitation profiles of the Cd6S44+ nanocluster have been studied over the 300-19 K temperature range. A 625-nm emission is proposed to originate from localized surface states associated with improperly terminated dangling bonds. The temperature dependence of the observed luminescence quenching of the 625-nm emission is consistent with a multiphonon radiationless relaxation process. This analysis yields an average mediating phonon frequency of 378 cm-1 and a Frank-Condon displacement parameter of 42.1 and supports an intermediate electron-phonon coupling case, all in accord with the model of a zeolite-anchored Cd2(Cd4S4)4+ nanocluster. A comparison is drawn between the process of depositing epitaxial layers of a II-VI semiconductor from MOCVD precursors on a 2-D planar substrate, with that of the intrazeolite topotactic stepwise assembly of a II-VI semiconductor nanocluster lattice on the 3-D curved internal surface of a zeolite.