Mass Spectrometric Analysis and Ion Soft Landing of Atomically Precise Nanoclusters

Habib Gholipour-Ranjbar, Purdue University

Abstract

Mass spectrometry (MS) plays an important role in nanomaterials research by facilitating the discovery of superatomic clusters and fullerenes, enabling the identification of atomically substituted clusters, and contributing to understanding mechanisms of cluster formation. In this dissertation, we used different mass spectrometry methods as well as ion soft-landing to address some of the ongoing challenges in cluster science. The first challenge is to extend the atom-by-atom substitution method, which is a promising strategy for designing new cluster-based materials, to a wider range of molecular clusters. Due to challenges associated with the synthesis, purification, and crystallization, this approach has been achieved only for a handful of gold and silver clusters. We extended this approach to enable the substitution of the first-row transitional metals into the core of Co6S8(PEt3)6 cluster, a well-defined metal chalcogenide superatomic cluster and a popular building block for designing novel 2D materials. This cluster is widely used in energy and electronic applications and is an excellent model system for computational studies of cluster-based materials. High-resolution MS analysis identified the formation of Co5MS8(PEt3)6+ (M=Mn, Fe, Ni) clusters, indicating that only Mn, Fe, and Ni atoms can be incorporated into the Co6S8core using our synthetic method. A combination of mass spectrometric analysis and theoretical studies reaved that each heteroatom has different impact on the relative stability, core-ligand interaction, as well as optical, magnetic, and electrochemical properties of the cluster. Another challenge in the cluster science addressed in this work is the controlled activation of fully ligated clusters by ligand removal. Conventional activation methods such as thermolysis or chemical treatment do not provide sufficient control of the number of the removed ligands and often suffer from sintering of NCs as a result of excessive ligand removal and degradation of the destabilized NC cores. Using a specially-designed ion soft-landing instrument, we achieved a controlled removal of one or two phosphine ligands form the synthesized cobalt sulfide clusters using collision-induced dissociation (CID). The resulting fragments were mass selected and soft-landed onto surfaces. We found out that the reactivity of the fragment ions on surfaces may be controlled by altering the composition of the cluster core and ligand binding energy to the cluster. Although some of the fragments formed by removing one ligand including Co5FeS8(PEt3)5+ and Co6S8L(PPh3)5+ remain unreactive on surfaces, other fragments including Co6S8(PEt3)5+, Co5NiS8(PEt3)5+, and Co6S8(PEt3-xPhx) (x=1-2) undergo selective dimerization. We observe that the reactivity of fragment ions produced by removing one surface ligand is controlled by the relative stability of the corresponding precursor ions towards fragmentation. In particular, fragment ions generated from more stable precursors undergo a selective dimerization reaction.

Degree

Ph.D.

Advisors

Laskin, Purdue University.

Subject Area

Materials science|Analytical chemistry|Chemistry|Energy|Nanotechnology

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