Size effects in metallic clusters using field emission and field ion microscopy
Abstract
The investigation of nanometer-sized clusters has become an intriguing area of research spanning many scientific fields. Difficulties in studying these ultra-small materials arise due to their small dimensions. Many experimental techniques are incapable of such spatial resolution. In these thesis we rely on the high magnification and resolution of the field emission and field ion microscopes to study the electronic and structural properties of individual nanometer-sized supported metallic clusters. The clusters are grown in a well controlled aerosol reactor (MECS) capable of producing particles with a controlled mean size. Utilizing this reactor, deposition techniques have been developed which allow a single cluster to be placed routinely on the end of a field emitter and then studied. The experimental apparatus designed to accomplish this will be described. The sensitivity of the field emission microscope to slight changes in cluster shape allows a determination of the melting temperature of a single supported cluster as a function of size over a wide size range ($\sim$1-20 nm). The results show a size dependent reduction in the melting temperature from the bulk value for both silver and gold cluster supported on a tungsten substrate. The results are in agreement with a classical thermodynamic model up to a diameter of $\sim$2 nm. Below this value a clear deviation from the model is observed. In this size range a saturation in the melting point ($\sim$540 K) is observed for gold clusters. The ability of the field emission microscope to measure electronic properties of individual clusters is clearly demonstrated by measuring the work function of gold clusters as a function of size. These preliminary results indicate an increase in work function as the diameter of the cluster is reduced. A simple model is discussed that attributes this increase to single cluster charging effects. Field ion microscopy permits the individual atoms comprising the surface of a cluster to be viewed. Under ideal conditions a single cluster may be imaged. Typically, cluster agglomerates are imaged due to the field induced diffusion of the clusters over the tip surface that takes place in a field range of $\sim$1.3$-$2.2V/A. This aspect of the work will also be discussed.
Degree
Ph.D.
Advisors
Reifenberger, Purdue University.
Subject Area
Condensation
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