Date of Award:

12-2011

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Physics

Committee Chair(s)

D. Mark Riffe

Committee

D. Mark Riffe

Committee

Eric Held

Committee

JR Dennison

Abstract

The explosion in electronic devices over the last half century is a result of the successful development and application of theories that explain the physical properties of solids. For example, the theory of lattice vibrations developed in the first half of the 20th century has had a huge impact on our ability to understand and design devices. The idea that atoms vibrate together in grouped vibrational modes, called phonons, has enabled scientists to quantify the impact that atomic motion has on mechanical, thermal, electrical, and optical properties. This has aided the creation of all sorts of useful technology ranging from electronics to optical devices.

As technology becomes miniaturized, the size of device components becomes so small that the theories that have had so much success over the past half century must be modified to include the impact of small size. For example, the theory of bulk atomic vibrations ignores the impact of a solid’s surface. This is okay if the smallest dimension of a device component is a few microns (millionths of a meter) because only a tiny percentage of the atoms are on the surface. However, as device components approach a few nanometers (billionth of a meter), a significant percentage of the atoms are on or near the surface. For example, in gold nanorods being used in novel cancer research for radiating tumors, as many as one quarter of the atoms are on the surface. For this type of structure, ignoring the impact a surface has on physical properties is not okay. The surroundings of a surface atom are drastically different than an atom on the interior of the material. Most notably, surface atoms have one half as many surrounding atoms to interact with. This results in dramatic changes in the vibrational motion of these atoms, and can lead to effects like atoms rising in temperature quicker than typical for a bulk atom.

This work investigates how the vibrations of atoms on or near a surface differ from atoms deep in the bulk. This is done by using a model called the Embedded Atom Method (EAM) that predicts the forces that atoms exert on each other when arranged in different structures. The first part of this work evaluates the ability of the EAM model to accurately predict bulk properties that are well known, demonstrating the predictive power of the model. The second part of this work uses the model to describe how atomic density and geometric arrangement of atoms near the surface impact the atomic vibrations. I find that being near a surface results in atoms vibrating at frequencies and amplitudes at which bulk atoms cannot vibrate. Finally, I explain how the surface vibrations change the thermal properties of atoms near a surface compared to atoms on the interior of the crystal.

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Comments

This work made publicly available electronically on April 24, 2012.

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