Date of Award

Summer 8-15-2017

Author's School

Graduate School of Arts and Sciences

Author's Department

Chemistry

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

This dissertation presents the isolation, characterization and reactivity of two magic size clusters, (ZnSe)34 and (CdTe)34, as well as the L-, Z- and X-type ligand exchange on the CdSe QBs or CdS QP surface.

The dissertation first reports the spectroscopically observed magic-size nanoclusters (ZnSe)34 and (CdTe)34 are isolable as amine derivatives. The nanoclusters [(ZnSe)34(n-octylamine)29±6(di-n-octylamine)5±4] and [(CdTe)34(n-octylamine)4±3(di-n-pentylamine)13±3] are fully characterized by UV-visible spectroscopy, IR spectroscopy, elemental analysis, and mass spectrometry. Amine derivatives of both (ZnSe)34 and (CdTe)34 are observed to convert to the corresponding (ZnSe)13 and (CdTe)13 derivatives, indicating that the former are kinetic products and the latter thermodynamic products, under the conditions employed. This conversion process is significantly inhibited in the presence of secondary amines. The isolation of the two new nanocluster derivatives adds to a total of nine of twelve possible isolated derivatives in the (II-VI)13 and (II-VI)34 families (II = Zn, Cd; VI = S, Se, Te), allowing comparisons of their properties. The members of these two families exhibit extensive spectroscopic homologies. In both the (II-VI)13 and (II-VI)34 families, linear relationships are established between the lowest-energy nanocluster electronic transition and the band gap of the corresponding bulk semiconductor phase.

Then, the research interest expanded from the magic-size clusters to the flat colloidal nanocrystals. The dissertation demonstrates that reaction of n-octylamine-passivated {CdSe[n-octylamine]0.53} QBs with anhydrous metal carboxylates M(oleate)2 (M = Cd, Zn) results in a rapid exchange of the L-type amine passivation to Z-type M(oleate)2 passivation. The cadmium-carboxylate derivative is determined to have the composition {CdSe[Cd(oleate)2]0.19±0.02}. The morphologies and crystal structures of the quantum belts are largely unaffected by the exchange processes. Addition of n-octylamine or oleylamine to the M(oleate)2-passivated quantum belts removes M(oleate)2, and restores the L-type amine passivation. Analogously, reversible surface exchanges are also demonstrated for CdS QPs. The absorption and emission spectra of the QBs and QPs are reversibly shifted to lower energy by M(oleate)2 passivation vs. amine passivation. The largest shift of 140 meV is observed for the Cd(oleate)2-passivated CdSe quantum belts. We establish that changes in strain states, which can be calculated from high-angle XRD patterns, and confinement dimensions contribute roughly equally to the spectral shifts in the Cd(oleate)2-passivated nanocrystals. Notably, addition of Cd(oleate)2, which electronically couples to the nanocrystal lattices, increases the effective thickness of the belts and platelets by approximately a half of a momolayer, thus increasing the confinement dimension. However, these shifts are attributed entirely to changes in the strain states in the Zn(oleate)2-passivated nanocrystals.

Last, the dissertation describes facile interchange of neutral-donor amine (L-type) and anionic (X-type) ligation on CdSe QBs. Reaction of n-octylamine-passivated QBs with protic acids HX (X = halide, nitrate, or carboxylate) results in displacement, protonation and liberation of the amines. The newly formed n-octylammonium ions will balance the surface charge of X– anions in the form of surface-bound ion pairs. Addition of n-octylamine to the bound-ion-pair X-type ligation quantum belts restores the L-type amine passivation. These ligand exchanges are readily monitored spectroscopically. The shifts in the lowest-energy feature ranged from 49 meV to 112 meV, depending on the different X– anions. We attribute the red spectral shifts in X-type ligation to negative surface charges and associated dipoles, which slightly increase the energy of the valence-band edge, and decrease the energy of the conduction-band edge.

Language

English (en)

Chair and Committee

William Buhro

Committee Members

Richard Loomis, John Bleeke, Bryce Sadtler, Elijah Thimsen,

Comments

Permanent URL: https://doi.org/10.7936/K7571BFJ

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