Research Experiences for Undergraduates
Summer 2013

Vanderbilt University
Physics & Astronomy

Research Projects: Condensed Matter, Atomic, Molecular, Optical, and Nano Physics

Nanostructured Thin Film Fabrication
(Prof. James Dickerson)
A recently developed technique for nanostructured thin film fabrication, electrophoretic deposition of nanocrystals, involves the locomotion of charged, dipolar, and polarizable nanoparticles, suspended in solution, due to an ambient electric field. Traditionally, high conductance electrodes, gold, platinum, indium tin oxide, have been used to engender the electric field, which yields the nanocrystal accretion onto the electrodes. Such methods have produced robust nanocrystalline heterostructures, which could be implemented in a variety of possible applications, including thin film optical devices, biologically sensitive detectors, and others. However, there has been scant research on the electrokinetic deposition of nanocrystals onto poorly conducting or semiconducting electrodes, such as doped silicon, tungsten, and gallium arsenide. The project for the REU student is to investigate the electrophoretic deposition of nanocrystals onto selected, non-metallic substrates. This project will include learning how to synthesize the nanocrystals for the electrophoretic deposition, modeling the necessary applied voltages and substrates to produce strong enough fields to yield electrophoretic deposition, performing electrophoretic deposition experiments using our partially automated system, and examining the nanocrystals and nanocrystal thin films, before and after deposition, via optical, electro-optical, and electrical characterization.

Nanocrystal/Quantum Dot Analysis
(Prof. Leonard Feldman)
Feldman's group focuses on nanoscale science. Current activities include interface studies associated with organic/inorganic interfaces, nanocrystal fabrication and passivation, and the interface science 7 associated with wide band-gap semiconductors. Equipment for fabrication, deposition and analysis is on-campus. REU students will use Rutherford backscattering for the compositional and structural analysis of nanostructured thin films. The goal is to provide accurate elemental analysis of the composition of II-VI semiconductor nanocrystals, both cores and core-shell structures. Ideal and well-controlled compositions are required for applications of these nanostructures to fields as diverse as biological marking, opto-electronic devices and nanofluidic structures. Students will learn the Rutherford scattering concept and employ it in a practical application. Included in the process will be the design of simple experiments, the opportunity to do actual "hands-on" data accumulation in our ion beam laboratory and the detailed analysis of the data. In addition to the ion scattering activity the student will learn about the fundamental aspects of the nature of materials and the scientific collaboration process, which is an intimate part of modern materials science. Finally the student will have an opportunity to learn the needs and applications of these new materials in exploratory device structures. Through such discussions the student will see the interplay between materials science, applied physics and the technologies that rely on these underpinning sciences.

Nonlinear optics in metal/metal-oxide nanostructures
(Prof. Richard Haglund)
An REU student can work in Haglund's group on an investigation of ultrafast optical switching in nanostructures that incorporate metal nanoparticles and vanadium dioxide. Depending on the nonlinear optical effect to be studied (e.g., second-harmonic generation, nonlinear refraction), model nanostructured materials are prepared using focused ion-beam machining or electron-beam or colloid-mask lithography; the metal and vanadium dioxide deposition is done by either evaporation or pulsed laser deposition. The semiconductor-to-metal phase transition in vanadium dioxide is then initiated by heating or a fast laser pulse; the temporal and spectral responses are studied by ultrafast pump-probe spectroscopy, confocal dark-field microscopy, scanning near-field optical microscopy (SNOM), and single-nanoparticle Raman spectroscopy. REU students will have the opportunity to learn about clean-room technology; lithographic fabrication of metal/metal-oxide nanostructures; thin-film deposition; materials characterization by atomic-force and scanning-electron microscopies; and optical spectroscopy and microscopy using lasers emitting at wavelengths from the visible to the mid-infrared and at pulse durations as short as 20 fs.

Free-Electron Lasers and Electron Beams
(Prof. Charles Brau)
Electron sources of unprecedented brightness (possibly up to the quantum limit) are being developed in our laboratory for free-electron lasers, electron microscopes, and other applications. In collaboration with the microelectronics group in the Department of Electrical Engineering, we are developing and testing arrays of diamond tips as electron sources for high-power free-electron lasers. In addition, we have initiated an experiment to investigate the quantum degeneracy of electron beams emitted from the tip of a carbon nanotube. A degenerate electron beam would have properties unlike those of any ordinary electron beam, just as the degenerate electron gas in a metal is different from a plasma. In our lab, REU students will have the opportunity to gain experience with lasers, high-vacuum systems, and high-speed diagnostics of optical and electron beams. Possible REU projects include experiments to investigate doping a nanotube to increase its emission, or analysis and design of an electron microscope on a chip.

Computational Materials Physics
(Prof. Sokrates Pantelides, Prof. Kalman Varga)
Members of Pantelides' group carry out first-principles calculations of electronic and structural properties of various materials. An assortment of computer codes are available that REU students can easily learn how to run. As an entry point, a student could reproduce some well-known results such as energy bands for crystalline Si, determination of the lattice constant of Si, determination of the structure of a nanocluster (small molecule), and so on. The student gets to appreciate the quantum mechanics that underlie the calculations. Once comfortable with the codes, the student will be given a real problem that has a good likelihood for either completion or at least significant progress within the available time. A problem relating to the physics of nanoclusters is likely. There are many options and the determination will be made at the time the student is accepted to participate. As an example, a previous undergraduate student tackled the problem of how an aspirin molecule interacts with a local site at a protein, to probe the physics of how aspirin works in the human body. Another student participated in research on how a single La atom binds on the surface of alumina, a problem that relates to the catalytic properties of alumina. The group is also working on several problems in nanocatalysis with gold nanoclusters on different substrates.

Ultra-Fast Laser Studies of Surfaces and Interfaces
(Prof. Norman Tolk)
The Tolk group studies ultra-fast tunable laser induced electronic and vibrational excitation at surfaces and interfaces. REU students will have the opportunity to be actively engaged in one or more of the following research thrusts: (A) Non-thermal resonant photodesorption of hydrogen from silicon and diamond crystal surfaces, using the Vanderbilt Free-Electron Laser. This novel and unanticipated effect was reported in the May 2006 issue of the magazine Science. This research effort is not only fundamental but also has very exciting possible applications including low-temperature growth of silicon and diamond crystals, hydrogen storage and room temperature refining. (B) Spin Dynamics of Ultra-Fast Laser Photoinduced Magnetization in Expitaxial GaMnAs. We have initiated a study of the dynamics of photoinduced magnetization in ferromagnetic Ga1-xMnxAs (x=0.05) by time-resolved polar Kerr rotation over a wide range of temperatures. Measured spin relaxation times were found to vary from tens to hundreds of picoseconds. The GaMnAs magnetic semiconductor system has received considerable attention in recent years because it is anticipated that it will play a major role in developing future spin-based devices. (C) Near-bandgap wavelength-dependent studies of long-lived traveling coherent longitudinal acoustic phonon oscillations in GaSb/GaAs systems. The oscillations arise from a photo-generated coherent longitudinal acoustic phonon wave, which travels from the top surface of GaSb across the interface into the GaAs substrate, thus providing information on the optical properties of the material as a function of time/depth. Wavelength-dependent studies of the oscillations near the bandgap of GaAs indicate strong correlations to the optical properties of GaAs.

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