Session D40: SPS Undergraduate Research and Outreach II

Oxide Reliability of SiC MOSFETs

SiC is one of the materials that presents the most promise for harsh environment electronics. Its ability to operate under high temperature and high power, as well as under radiation, made it the material of choice for this study. SiC MOSFETs constitute an important step towards the development of the next generation of resistant electronics. The eventual industrial manufacturing of this type of field effect transistor depends on the effectiveness to improve its performance. Currently, a sudden current degradation, and an unsatisfactory low mobility are observed during the operation of these devices. In this work, we studied both of these drawbacks as a function of temperature. The devices used were SiC nMOSFETs with a SiO$_{2}$ oxide. Two types of measurements (ultra fast and conventional) were performed during this experience in order to observe 8 decades of current degradation. From our experience, it was observed that as the temperature was lowered the threshold voltage (V$_{TH})$ increased, while the mobility and the drain current (I$_{D})$ decreased.   

X-ray degradation studies of Nafion in a PEM fuel cell

The overall goal of this research is to test for degradation of the Polymer Electrolyte Membrane (PEM) fuel cells due to exposure to ionizing radiation. We have successfully developed a Membrane Electrode Assembly (MEA) that can be fully disassembled down to the bare Proton Exchange Membrane (PEM) and reassembled repeatedly. This is crucial for testing the degradation effects on the individual components of the MEA. It was also important to establish baseline repeatability of the polarization curves of the MEAs. Therefore, we systematically varied different parameters to test their effect as well as to establish consistent experimental procedures. Hydration of the fuel cell has been found to be crucial for repeatable results. These polarization curves showed voltages that ranged from 0.4V to 1.0V and current densities up to 11mA/cm2. The Nafion can then be exposed to an x-ray source and the respective polarization data can be studied. A working fuel cell has also been built that fits into the microwave cavity of an electron paramagnetic resonance spectrometer. This allows for study of in situ behavior of free radicals formed in a normal operational fuel cell as well as fuel cells with x-ray exposed membranes.   

Polymer Nanocomposite Gyroids

Self-assembled polymer phases are increasingly being used in the development of nanocomposite materials. The polymer matrix provides a template for nanoparticles added to the system, transferring the structure of the polymer to the nanoparticles. We perform Molecular Dynamics simulations of these polymer nanocomposite materials and characterize their phase diagrams. Two striking results are found. First, a specific interaction of the polymer and the nanoparticles is required for a successful templating. Second, the presence of nanoparticles can change the pure polymer phase entirely. For instance, a small nanoparticle concentration turns a polymer system from a hexagonal phase into a gyroid phase, both for the polymers and the nanoparticles. In fact, the gyroid is the most prevalent phase over a wide range of interaction strengths and polymer composition.   

Sol-gel synthesis and characterization of terbium doped tin-oxide

Rare earth doped tin oxide nanocrystals emit visible light when excited in the ultra-violet. Using a sol-gel process, we embedded Tb$^{3+}$ doped SnO$_2$ nanocrystals in silica glass and characterized the samples using x-ray diffraction, photoexcitation and emission spectroscopy, and transmission electron microscopy. We synthesized four sets of samples, SnO$_2$-99SiO$_2$, 3SnO$_2$-97SiO$_2$, 5SnO$_2$-95SiO$_2$, 7SnO$_2$-93SiO$_2$ with constant weight ratios of Tb$^{3+}$ to measure the effects of varying the molar concentrations of Tin-Oxide on the photoluminescence properties of the nanocrystals.   

Generation of 279nm Light for Single Photon Ionization of Laser Cooled Rubidium

The ionization of rubidium for the formation of ultracold plasma is often done by a two photon process; a 479nm photon ionizes the rubidium from an excited state pumped by the 780nm trapping beams. We detail the generation and characterization of this 297nm light from a Nd:YAG pumped dye laser and a tracking doubling crystal. Supported by Rowan University College of Liberal Arts and Sciences, Research Corporation grant CC6180 and NSF grant PHY-0613659.   

Bio-Photonic Detection of Various Cellular Cultures

Since it is non-invasive, there has been increased research in the field of bio-optics. Many biological systems display an unusual phenomenon, delayed luminescence, produced by what is known as bio-photons. We present an apparatus and procedure for the detection of these ultra-weak photonic emissions using a single photon detection device. The results of bread yeast, saccramyces, and algae will be presented and compared to other reports in the literature   

Epitaxial Thin Film Growth of CMR Manganites on Silicon: The Effect of Thermal Stress

Our research addresses some of the challenges associated with growing epitaxial thin films of the CMR manganite material, Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ (NSMO) on Silicon for application as a bolometric x-ray sensor. Due to the chemical incompatibility between NSMO and Silicon, the formation of amorphous SiO$_{2}$ and crystal lattice mismatch issues, `buffer layers' and `template layers' of other suitable materials need to be interposed between NSMO and the Silicon substrate. Even with such schemes in place, there exists a mismatch between the thermal expansion coefficients of Silicon ($\alpha _{Si}$=2.618$\times $10$^{-6}$K$^{-1}$ at 300K) and NSMO ($\alpha _{NSMO}\sim $3$\times $10$^{-5}$K$^{-1})$. This large mismatch induces thermal stresses that deteriorate the film properties. Our research investigates how the thermal stress evolves as a function of the thickness of the multi-layers, and how the process parameters such as the film growth kinetics and thermal kinetics can be optimized to minimize the stress. We are using the Pulsed Laser Deposition technique for thin film growth and characterizing the properties of the sensor layer using X-ray diffraction, electrical resistance measurements, optical microscopy and atomic force microscopy. Acknowledgement: We acknowledge support for this research from Lawrence Livermore National Laboratory.   

Synthesis and Characterization of CrAlC Thin Films

We have synthesized and characterized Cr2AlC thin films grown on substrates Al2O3, MgO and seed layers of VC, and TiC at room temperature up to 850$^{\circ}$C. Texture films were successfully grown above 550$^{\circ}$C while Raman spectroscopy shows vibrations down to 500$^{\circ}$C. Films below 500$^{\circ}$C down to room temperature show texturing upon annealing at 750$^{\circ}$C. The films were prepared using RF magnetron sputtering from elemental targets. Electrical transport shows metallic behavior of the films down to 10 K. EDS was used to verify chemistry from which the MA ratios were found that a slight deviation still allowed formation of the MAX phase. X-ray diffraction shows that when the chemistry is off it results in secondary phases of Cr26C6 and Cr2Al. Atomic Force Microscopy (AFM) shows smoother films at lower temperatures and rough at higher temperatures with a surface roughness $>$ 20 nm. Friction test results will be presented.   

Spatially resolved quasiparticle tunneling spectra in the vortex state of optimally hole-doped YBa$_{2}$Cu$_{3}$O$_{x}$ (Y-123)

We report cryogenic scanning tunneling spectroscopic (STS) studies of superconducting single crystalline Y-123 (T$_{c}$ = 93 K) as a function of magnetic field. We study and model the influence of competing orders (COs), which coexist with superconductivity (SC), on the quasiparticle (QP) excitation spectra. The spatial dependence of the QP tunneling spectra is probed via STS to quantify the presence and spatial extent of SC and CO. Zero-field spatial maps of the QP spectra (100$\times $100 nm$^{2})$ in Y-123 exhibit long-range spatial homogeneity of SC ($\Delta _{SC }$= 23$\pm $1 meV) associated with the spectral coherence peaks and the presence of CO (V$_{CO }$= 33$\pm $2 meV) that gives rise to the spectral satellite features at $\Delta _{eff}$ = [($\Delta _{SC})^{2}$+(V$_{CO})^{2}$]$^{1/2}$. Conductance maps of the Y-123 in finite fields demonstrate spatially varying spectra consistent with the periodicity $a_{0}$ of the vortex lattice, with pseudogap (PG) like features at $\sim $V$_{CO}$ inside the vortex core and SC gap features remaining at $\sim \Delta _{SC}$ outside the vortex core. Moreover, conductance histograms of the vortex state reveal that the ratio of the areas associated with $\Delta _{SC}$ and V$_{CO}$ is comparable to ($a_{0}$/\textit{$\xi $}$_{ab})^{2}$, (\textit{$\xi $}$_{ab}$: in-plane SC coherence length). These results therefore suggest the important role of COs in the cuprate QP excitations. This work is supported by NSF Grant DMR-0405088.   

High Frequency Electrical Properties of Carbon Nanotubes

We report on measurements of the high frequency electrical properties of single-walled carbon nanotubes. These measurements are accomplished by incorporating a single nanotube into a microwave stripline and using optical pulses from a femtosecond laser to create short electrical pulses on the stripline. By varying the time delay between the pulses, it is possible to determine the frequency dependence of the response of the nanotube.   

Current State of Research of Alternate Fuel Sources for Passenger Vehicles

The purpose of this project is to report on the current state of research in the field of alternate fuel sources for passenger vehicles. Because the number of alternate fuel options is very large, this study focuses on selected bio-fuels and briefly describes a couple of the most popular non-bio and non-renewable alternatives. The fuel and energy sources studied are compared using well-to-wheel and well-to-tank net energy balances. Data also includes relative production capabilities by volume in terms of current fossil fuels. Qualitative data includes production methods and transportability.   

Universal Properties of Population Dynamics with Fluctuating Resources

Starting from the well-known field theory for directed percolation, we describe an evolving population, near extinction, in an environment with its own nontrivial spatio-temporal dynamics. Here, we consider the special case where the environment follows a simple relaxational (Model A) dynamics. Two new operators emerge, with upper critical dimension of four, which couple the two theories in a nontrivial way. While the Wilson-Fisher fixed point remains completely unaffected, a mismatch of time scales destabilizes the usual DP fixed point, suggesting a crossover to a first order transition from the active (surviving) to the inactive (extinct) state.   

First-principles density-functional theory investigation of FOX-7

Due to the expense and difficulty of experimental investigation of the chemical and physical properties of energetic materials (EMs), computational methods provide a unique opportunity for accurate determination of the chemical and physical properties of EM molecular crystals based on underlying atomic structure. In this presentation, we discuss the results of first-principles density functional theory (DFT) calculations of hydrostatic and uniaxial compression of the important energetic material, FOX-7. The calculated equilibrium properties, such as lattice parameters, elastic constants, and the bulk modulus will be reported and compared with experiment, as well as the isothermal equation of state. Due to the anisotropic nature of energetic molecular crystals, physical properties such as cohesive energy, band gap, and stress-strain relationships are reported as functions of each uniaxial compression studied. In addition, the shear stress behavior upon uniaxial compression will be discussed, as well as its possible relation to anisotropic shock-sensitivity in FOX-7.   

Variational Wavefunction Monte Carlo method applied to electrons in a two dimensional square lattice with zero doping

We present the theoretical results from the Variational Wavefunction Monte Carlo method applied to electrons in cuprates, of a two dimensional square lattice with zero doping. Since the true Hamiltonian of the cuprates is not definitively known, much study has gone into identifying the best possible Hamiltonian. To do this, we vary the terms in the Extended Heisenberg Hamiltonian - the neighbor spin coupling term J, the spin next-neighbor term J' and the spin ring exchange term Jring, where each variation represents a different electronic interaction. We then use the variational approach to find the best groundstate wavefunction for each model Hamiltonian. Once we find the best groundstate wavefunction for each Hamiltonian, we can deduce the magnetization predicted by that model. Hence, by comparing our results for the magnetization to known experimental results, we can identify the most suitable model.   

Quantum Criticality and Neutron Scattering Solutions for a Spin-1/2 Ladder Model

Exact solutions for a two dimensional, $S = 1/2$ quantum spin ladder model are obtained through mapping the Hamiltonian and correlation functions onto those of a one dimensional Ising chain model [1]. These solutions include a three dimensional ground state phase diagram, establishing states of ladder rung singlets, triplets, and alternating singlets and triplets in terms of interaction parameters and applied magnetic field. Evidence of quantum criticality is uncovered for select regions of the phase diagram through explorations into ladder site correlations and correlation lengths. Neutron scattering solutions for scattering intensities provide insight into the energy spectra associated with various rung spin configurations. \\$[1]$ J. H. Barry and M. W. Meisel, \textit{Phys. Rev. B} \textbf{58}, 3129 (1998).