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Applications of superatom theory in metal cluster chemistry

Date

2016

Authors

Tofanelli, Marcus A., author
Ackerson, Christopher J., advisor
Prieto, Amy L., committee member
Shores, Mathew, committee member
Farmer, Delphine, committee member
Roberts, Jacob, committee member

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Abstract

One of the largest modern scientific debates is understanding the size dependent properties of a metal. While much effort has been performed on understanding metal particles from the top down to much less work has been accomplished from the bottom up. This has lead to a great deal of interest in metal clusters. Metal clusters containing 20 to 200 metal atoms are similar yet strikingly different to both to normal coordination chemistry and continuous bulk systems, therefore neither a classical understanding for bulk or molecular systems appears to be appropriate. Superatom theory has emerged as a useful concept for describing the properties of a metal cluster in this size range. In this model a new set of ‘superatomic’ orbitals arises from the valence electrons of all the metals in a cluster. From superatom theory the properties of a metal cluster, such as stability, ionization energy, reactivity, and magnetism, should depend on valence of the superatomic orbitals, similar to a normal atom. However superatom theory has largely been used to describe the high stabilities of metal clusters with completed electronic configurations. Thus many features of superatom theory have remained largely untested and the extent that the superatom model truly applies has remained in question for many years. Over the past decade increases in synthetic and analytical techniques have allowed for the isolation of a series of stable monodisperse gold thiolate monolayer protected clusters (MPCs) containing from 10 to 500 gold atoms. The wide range in sizes and high stability of gold thiolate clusters provides an instrumental system for understanding superatom theory and the transition from molecular-like cluster to bulk-like system. In the first part of this thesis the effects of the superatomic valence is investigated under superatomic assumptions. Au25(SR)18 (where SR= any thiolate) can be synthesized in 3 different oxidation states without any major distortions to the geometry of the cluster, thus it is possible to test 3 different superatomic configurations for a single cluster. These studies show that the superatom model correctly predicts changes observed in the stability, absorption spectrum, crystal structures, and magnetic susceptibility for each charge state of Au25(SR)18. In addition, the superatom model is shown to also apply to the isoelectronic PdAu24(SR)18 superatomic cluster. This work is discussed in Chapters 2, 3, and 4. The second part of this thesis focuses on the transition from superatomic metal clusters to metal nanoparticles. Au144(SR)60 is studied in order to understand this transition. Although the plasmon is not immediately apparent through linear absorption spectroscopy, a plasmonic feature is observed in transient absorption spectroscopy. This observation in combination with the absence of a HOMO-LUMO gap suggests that Au144(SR)60 can be treated with bulk assumptions. However Au144(SR)60 shows quantized behavior and powder x-ray diffraction reveals that symmetry of the metal core does not represent what is observed in the bulk. Au144(SR)60 appears to show both superatomic and bulk behavior making it an instrumental tool for understanding the transition from superatomic to bulk behavior. This work is discussed in Chapters 2, 5, and 6.

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2016 Fall.
Includes bibliographical references.

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