Elastic properties and cluster-substrate interactions of preformed nanometer-size gold clusters measured using atomic force microscopy
Abstract
An atomic force microscope has been constructed at Purdue capable of contact and noncontact mode measurements in air, moderate vacuum, and inert gas environments. The resolution of this instrument was sufficient to observe atomic features on highly oriented pyrolytic graphite, mica, and highly oriented boron nitride substrates when operating in the contact mode. Extensive studies of preformed nanometer-size gold clusters were made using the AFM. The longstanding problem of reliably imaging individual clusters on substrates was solved by operating the AFM in the noncontact mode. It was shown that because of the weak binding of the clusters to substrates, contact-mode scanning probe operation tended to dislodge clusters from their as deposited positions. Operating the AFM in the noncontact mode produced reliable images of clusters on various surfaces. With the ability to image clusters, the surface induced deformation of gold clusters on $\alpha$-Al$\sb2$O$\sb3$, HOPG, and MoS$\sb2$ was measured. Predictions using continuum theory showed that the deformation could be described by a plastic deformation due to surface forces. The AFM was also used as an indentation device to study the elastic properties of individual gold clusters as a function of cluster size. The Young's modulus for clusters ranging from 3.1 nm to 18.1 nm was studied in air and vacuum on $\alpha$-Al$\sb2$O$\sb3$, HOPG, and mica substates. The elastic modulus was found to be fairly constant for large cluster heights, with a slight decrease in elastic modulus for the smaller clusters studied. The elastic modulus was also determined for unannealed gold clusters and found to be significantly lower than for annealed clusters. Finally, the AFM was used to study the adhesive properties between micron-size spheres and various substrates. Force studies between a bare silicon AFM tip and an HOPG substrate were also performed.
Degree
Ph.D.
Advisors
Reifenberger, Purdue University.
Subject Area
Condensation
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