2011 REU Poster Session Abstracts

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• Synthesis of ZnO nanowires with Liquid Crystal for Photovoltaic cell
• Self-Assembled Monolayers on Topological Insulator Bismuth Selenide
• Sputter deposition of copper oxide on virus-structured current collectors for the photoelectrolysis of water
• Atomic Layer Deposition Films as Oxidant Barriers for Silver Artifacts
• Simulations and Magnetic Force Microscopy Images of Vortices and Cross-Tie Walls on a Magnetic Thin Film Structure
• Imaging Charge Puddles in Graphene
• Nanofibers for Trapping Atoms
• Improvements of a Cavity QED Experiment

Synthesis of ZnO nanowires with Liquid Crystal for Photovoltaic cell

Xenia Barbosa^1,*, Lourdes Salamanca-Riba^1,2, Luz Martinez-Miranda^1,2

Materials Research Science and Engineering Center¹, Department of Materials Science and Engineering², University of Maryland, College Park, Maryland 20742-4111, USA.

We have methodically investigated the growth behavior and the surface morphology of ZnO nanowires (NW’s) for the fabrication of photovoltaic cells. The photovoltaic cells consist of a hybrid system with organic-inorganic compounds. Hybrid photovoltaic cells have recently shown improvements in the efficiency. They also have the advantage of low cost fabrication which makes them desirable for the fabrication of solar cells. We use liquid crystal octylcyanobiphenyl (8CB) as the organic compound and ZnO NW’s as the inorganic compound. Well-aligned single-crystalline zinc oxide (ZnO) nanowires were successfully fabricated on silicon (001) substrates. As the catalytic material we used a thin film of Au with a thickness of 2 nm. The silicon substrates were inserted into a tube furnace and heated to a temperature of 850°C. The growth of ZnO nanowires is controlled by the conventional vapor–liquid–solid (VLS) mechanism. Transmission electron microscopy (TEM), Scanning electron microscopy (SEM) observations and x-ray diffraction (XRD) analysis showed that the ZnO nanowires are singlecrystalline with a hexagonal wurzite structure. The Optical properties of ZnO NW’s by themselves and with the liquid crystal were studied using reflection and photoluminescence. Conductivity properties were also studied by acquisition of I-V curves using a two-point probe. The I-V curves show very low current indicating that there is poor contact with the nanowires and the Si substrate. In order to improve the electrical contact between the nanowires and the Si a Platinum (Pt) layer could be deposited on the Si substrate before the Au film.

*Currently an Undergraduate at University of Puerto Rico-Humacao

Atomic Layer Deposition Films as Oxidant Barriers for Silver Artifacts

Matthew Lewis^1,*, Amy Marquardt², Ray Phaneuf^1,2

Materials Research Science and Engineering Center¹, Department of Materials Science and Engineering², University of Maryland, College Park, Maryland 20742-4111, USA.

The tarnishing of silver artifacts is an immense problem for art conservators trying to preserve museum collections. The rate of tarnishing can be significantly decreased by depositing TiO2 thin films onto the surface of both fine and sterling silver substrates using atomic layer deposition. Variations in substrate preparation methods, and film thickness were investigated to determine the optimum combination for tarnish prevention while meeting the rigorous standards of conservators. X-ray photoelectric spectroscopy and atomic force microscopy were used to characterize sample substrate preparation methods and reflectance spectroscopy was used to characterize the optical properties of the thin films. The substrate preparation method providing the cleanest and smoothest surface involved polishing with polishing paper and solvent cleaning with trichloroethylene (TCE), acetone, isopropanol, and ethylenediaminetetraacetic acid (EDTA). This method gave a root mean square (RMS) surface roughness of 5.35 nm±2.35 nm for sterling silver substrates and 7.76 nm±1.83 nm for fine silver substrates, and a sulfur content of 0.24 At%±0.09 At%. TiO2 films slowed the tarnishing rate of the silver substrates, however they did alter their appearance.

*Currently an Undergraduate at Millersville University

Simulations and Magnetic Force Microscopy Images of Vortices and Cross-Tie Walls on a Magnetic Thin Film Structure

Timothy Middleton^1,*, Ting Xie², Jin Seock², Romel Gomez^1,2

Materials Research Science and Engineering Center¹, Department of Electrical and Computer Engineering², University of Maryland, College Park, Maryland 20742-4111, USA.

Understanding vortices and anti-vortices on a magnetic structure has many applications this is why understanding vortices is so important. Vortices on a magnetic structure have certain microwave properties that are helpful. The interaction with magnetic structures is encountered every day. The simulation predicts the configuration that will minimize the total magnetic energy. The most important energies are Zeeman energy, exchange, anisotropy, and demagnetization or magnetostatic energy. Zeeman is the energy that we associated when a compass is oriented in the north south direction. The exchange energy is related to the interaction between to neighboring moments. The anisotropy energy has to do with the preferred directions in the crystal that the moments would like to be aligned with. Finally the demagnetization energy or magnetostatic energy tends to realign the magnetic moments due to its own field. All of these energies can be grouped into an effective magnetic field, H effective, which interacts with the magnetic moment.

Object Oriented Micro-Magnetic Framework (OOMMF) attempts to simulate the orientation of magnetic moments inside a given magnetic object. As inputs, one can put the shape of the object, the dimensions, the type of material, as well as a guess of what the initial magnetic distribution might look like. Magnetic Force Microscopy (MFM) scans a magnetic thin film structure and shows the structure at the micro scale.

The task is to understand how the thickness of the material affects the structure. In these simulations, we show the effect of increasing numbers of vortices as a function of the thickness. Vortices are these whirlpool shaped structures. They are interesting because they come paired with “anti-vortex” structures. The results show that at a certain thickness vortices increase and there after dissipates gradually.

*Currently an Undergraduate at Bowie State University

Imaging Charge Puddles in Graphene

Frazier Mork^1,*, Kristen Burson^1,2, Michael Fuhrer^1,2

Materials Research Science and Engineering Center¹, Department of Physics², University of Maryland, College Park, Maryland 20742-4111, USA.

Graphene, a gapless semiconductor, has a nearly zero charge carrier density at the Dirac point in its band structure. However, graphene exhibits a nonzero minimum conductivity at those points, despite the lack of charge carriers. Previous studies have found puddles of charge in graphene, or regions that are p- or n- doped, so that while there may be no charge carriers on average, there are still regions that conduct. However, previous images of these puddles were either heavily resolution limited or incapable of imaging a large portion of graphene. Scanning gate microscopy, a technique using an AFM tip to apply a local top-gate to the sample while measuring the current going through the graphene, has the potential to resolve these puddles at an intermediate scale. This will make it possible to combine the data from previous studies and fully characterize these unique charge patterns. However, scanning gate measurements are difficult to perform because of the possibility for electrical damage to the fragile wires used. These problems have been eliminated, and an optimal set-up for scanning gate measurements will be presented.

*Currently an Undergraduate at Millersville University

Martin et al., Nature Physics 4, 144 - 148 (2008)
Zhang et al., Nature Physics 5, 722 - 726 (2009)
Castro Neto et al., Rev. Mod. Phys. 81, 109–162 (2009)

Nanofibers for trapping atoms

Dalia Ornelas*, Austin Wood, Jonathan Hoffman, Jeff Grover, Kristen Voigt, Zaeill Kim, Luis Orozco

Joint Quantum Institute, Department of Physics, University of Maryland, College Park, Maryland 20742-4111, USA.

Experiments to study the coupling between atoms and superconducting quantum interference devices require trapping neutral 87Rb atoms on the evanescent light fields, detuned from the D₂ line, on the outside of a nanofiber, the construction of this kind of hybrid systems is an valuable for fields such as quantum information and computing. We present a procedure to make tapered optical nanofibers with radii of the order of 500 nm, using a localized oxygen-hydrogen flame to heat the fiber while pulling both ends by two computer-controlled stepper motors. We measure the fiber radius by means of diffraction with a 532 nm laser; this allows us to have a nondestructive, in house measurement. The Fast Fourier Transform to the image of the diffraction pattern is related to the radius size. The destructive measurement using a confocal microscope shows a 482 nm radius fiber waist with an exponential taper to the 100 microns of the original fiber.

*Currently an Undergraduate at University of Guanajuato, Guanajuato, Mexico

Improvements of a Cavity QED Experiment

Ana Valdés Curiel*, Luis Orozco

Joint Quantum Institute, Department of Physics, University of Maryland, College Park, Maryland 20742-4111, USA.

Cavity Quantum Electrodynamics studies the interaction between atoms and a finite number of electromagnetic modes of light. A high finesse optical cavity, stable laser frequency and certainty in the detection of the polarization of the photon are required conditions when performing cavity QED experiment. We present a series of improvements in the performance of a current cavity QED experiment. We were able to achieve 0.1 °C temperature stability of an electro optical modulator by implementing a PID controller that will allow to lock the laser frequency to the D₂ line of ⁸⁵Rb. We designed and constructed a new high finesse Fabry-Perot optical cavity that will replace the one that is currently used. Finally, we designed an apparatus that will measure birefringence in the windows of the new vacuum chamber. With this improvements, a better signal to noise ratio is expected when detecting quantum signals from the cavity.

*Currently an Undergraduate at National Autonomous University of Mexico, Mexico City, Mexico