Digital Repository at the University of Maryland (DRUM)  >
Theses and Dissertations from UMD  >
UMD Theses and Dissertations 

Please use this identifier to cite or link to this item: http://hdl.handle.net/1903/13104

Title: On Mapping Electron Clouds with Force Microscopy
Authors: Wright, Charles Alan
Advisors: Solares, Santiago D.
Department/Program: Mechanical Engineering
Type: Dissertation
Sponsors: Digital Repository at the University of Maryland
University of Maryland (College Park, Md.)
Subjects: Atomic physics
Mechanical engineering
Quantum physics
Keywords: atomic force microscopy
frequency modulation
higher harmonics
scanning tunneling microscopy
subatomic
ultra high vacuum
Issue Date: 2012
Abstract: At its core, this is a story about electrons. Electrons drive the interactions of matter at the nanoscale, so an understanding of electron behavior offers significant insight into the behavior of nanoscale materials. Atomic force microscopy (AFM) has demonstrated great success as a tool for probing matter at the nanoscale, and recent reports suggest that it may even be capable of mapping electron clouds on atomic surfaces. The most recent of these claims came in 2004, when Hembacher <italic>et al</italic>. [<italic>Science</italic> <bold>305</bold>] observed subatomic features while imaging a graphite surface with a tungsten tip using higher-harmonics frequency modulation AFM (FM-AFM). The authors' interpretation of these features as the footprint of the electron density at the tungsten tip's apex atom has been met with much skepticism. But despite the potential significance of the results, a detailed theoretical study has not been performed. In this work, a computational method based in density functional theory (DFT) is developed in order to simulate the imaging process and draw fundamental conclusions regarding the feasibility of subatomic imaging with higher harmonics FM-AFM. The application of this method to the tungsten/graphite system reveals that the bonding lobes of increased charge density are in fact present at the tungsten tip's apex atom and that the corresponding higher harmonics images can exhibit subatomic features similar to those observed experimentally. We further show that the filtering process used to experimentally measure the harmonics does not introduce imaging artifacts but that harmonics averaging is not an appropriate method for enhancing contrast. We then suggest an alternate approach: the individual mapping of the first two harmonics, which are expected to dominate the contrast under the experimental conditions studied. Finally, we demonstrate the important role played by the surface atom used to probe the AFM tip. We find that a small, non-reactive atom is necessary for resolving subatomic features. Most importantly, we show that the observed features are <italic>not a direct reflection</italic> of the electron density at the AFM tip's front atom. Instead, they represent a measure of the bonding stiffness between the tip's front atom and the atoms in the layer above.
URI: http://hdl.handle.net/1903/13104
Appears in Collections:Mechanical Engineering Theses and Dissertations
UMD Theses and Dissertations

Files in This Item:

File Description SizeFormatNo. of Downloads
Wright_umd_0117E_13605.pdf16.37 MBAdobe PDF130View/Open

All items in DRUM are protected by copyright, with all rights reserved.

 

DRUM is brought to you by the University of Maryland Libraries
University of Maryland, College Park, MD 20742-7011 (301)314-1328.
Please send us your comments