Bulletin of the American Physical Society
Spring 2011 Meeting Ohio-Region Section of the APS
Volume 56, Number 3
Friday–Saturday, April 15–16, 2011; University Heights, Ohio
Session D3: Condensed Matter Physics and Materials Science |
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Chair: Walter Lambrecht, Case Western Reserve University Room: Dolan Science Center E130 |
Saturday, April 16, 2011 8:10AM - 8:22AM |
D3.00001: Measurements of Large Dielectric Constants in Phthalocyanine Tetramers Khalil Hamam, C.A. Burns, G. Mezei, M. Al-Amer Understanding the dielectric constant of organic materials is important for applications including organic transistors and photovoltaics. We have measured the dielectric constant and dissipation factor of oligomer metal-phthalocyanine (MePcs) pellets. Zn and Cu based tetramers (MeC$_{30}$H$_{10}$N$_{8}$O$_{8})_{4}$ are water soluble materials with high dielectric constant. We investigated these materials in the frequency range 20--10$^{6}$ Hz and at temperatures up to 110\r{ } C. Both the dielectric constant and dissipation factor were found to increase strongly with temperature and to decrease with frequency. [Preview Abstract] |
Saturday, April 16, 2011 8:22AM - 8:34AM |
D3.00002: Charge Transfer Ferromagnetism Christopher Porter, Plamen Stamenov, Vatsal Dwivedi, J.M.D. Coey A model is developed to describe high-temperature ferromagnetism in defect-ridden materials such as thin films of oxides or nanoparticles. The proposed model consists of a defect-based impurity band and a charge reservoir that can be identified with multivalent dopant cations. Even when the Stoner criterion is not satisfied for the impurity band alone, transfer of charge from the reservoir to the impurity bands may raise the density of states at the Fermi level sufficiently for spin splitting to occur. Whether or not the free energy is lowered by spin splitting depends on the Stoner parameter$ I$, the defect bandwidth $W$, and the energy cost of electron transfer from the reservoir. The free energy minimization has been carried out both in the canonical and grand canonical ensemble. The resulting phase diagrams show half-metallic regions in which spin wave suppression may yield extremely high Cure temperatures of order W. The magnetic phase transitions appear to be first order to within our computational accuracy. [Preview Abstract] |
Saturday, April 16, 2011 8:34AM - 8:46AM |
D3.00003: Magnetic-Field Induced Diffraction Patterns from Ferrofluids Corneliu Rablau, Prem Vaishnava, Gavin Lawes, Ratna Naik Ferrofluids are stable colloidal suspensions of superparamagnetic nanoparticles in a carrier liquid. We report studies of magneto-optic properties of two ferrofluid systems consisting of tetramethyl-ammonium-hydroxide (TMAH)-coated and of dextran-coated Fe$_{3}$O$_{4}$ nanoparticles of nominal sizes of 6 nm and 12 nm respectively suspended in water. Both samples showed superparamagnetic behavior. The static and time-dependent DC-magnetic-field-induced light scattering patterns produced by two orthogonal He-Ne laser beams passing through the ferrofluid samples revealed significant different optical signatures for the two surfactants. Notably, in contrast to the linear diffraction pattern produced by TMAH-coated nanoparticles, a circular diffraction pattern is reported -- for the first time -- in the dextran-coated ferrofluid. [Preview Abstract] |
Saturday, April 16, 2011 8:46AM - 8:58AM |
D3.00004: Epitaxial Zinc Oxide Semiconductor Film deposited on Gallium Nitride Substrate Michael McMaster, Tom Oder Zinc oxide (ZnO) is a wide bandgap semiconductor which is very promising for making efficient electronic and optical devices. The goal of this research was to produce high quality ZnO film on gallium nitride (GaN) substrate by optimizing the substrate temperature. The GaN substrates were chemically cleaned and mounted on a ceramic heater and loaded into a vacuum deposition chamber that was pumped down to a base pressure of 3 x 10$^{-7}$ Torr. The film deposition was preceded by a 30 minute thermal desorption carried in vacuum at 500 $^{\circ}$C. The ZnO thin film was then sputter-deposited using an O$_{2}$/Ar gas mixture onto GaN substrates heated at temperatures varying from 20 $^{\circ}$C to 500 $^{\circ}$C. Post-deposition annealing was done in a rapid thermal processor at 900 $^{\circ}$C for 5 min in an ultrapure N$_{2}$ ambient to improve the crystal quality of the films. The films were then optically characterized using photoluminescence (PL) measurement with a UV laser excitation. Our measurements reveal that ZnO films deposited on GaN substrate held at 200 $^{\circ}$C gave the best film with the highest luminous intensity, with a peak energy of 3.28 eV and a full width half maximum of 87.4 nm. Results from low temperature (10 K) PL measurements and from x-ray diffraction will also be presented. [Preview Abstract] |
Saturday, April 16, 2011 8:58AM - 9:10AM |
D3.00005: Optimizing Deposition gas and Annealing Conditions for ZnO Films Andrew Smith, Tom Oder Zinc Oxide (ZnO) is a transparent II-VI semiconductor with a direct band gap and has potential applications for making efficient optoelectronic devices such as laser diodes and light emitting diodes as well as in solar panels, taking advantage of its transparency. ZnO films have been deposited onto sapphire using radio frequency sputtering in different gases including nitrogen and different mixtures of argon and oxygen. The films were then annealed for different durations at 900 $^{\circ}$C and characterized using photoluminescence spectroscopy measurements with a HeCd laser to examine the crystal quality of the samples. The conditions that give the best quality film were for films deposited in 100{\%} argon at 500 $^{\circ}$C. The optimum annealing condition was 3 min and five minutes. Photoluminescence analysis yielded peaks at 377 nm for each sample measured at room temperature 368 nm at 10 K. It was also found that pre-heating the sapphire substrate in pure oxygen prior to deposition greatly improves the quality of the ZnO film. [Preview Abstract] |
Saturday, April 16, 2011 9:10AM - 9:22AM |
D3.00006: ZnO/sapphire by RF Magnetron sputtering deposition Tom Oder, Nagaraju Velpukonda Zinc Oxide (ZnO) thin films with thickness around 200 nm were deposited on sapphire substrates by RF magnetron sputtering deposition method from an ultra pure ZnO solid target. The substrate temperature was varied from room temperature to 800~$^{\circ}$C and the resulting films were annealed in a rapid thermal processor in N$_{2}$ gas. Analyses carried using photoluminescence (PL) spectroscopy and X-ray diffraction measurements indicate that films deposited at 500~$^{\circ}$C using Ar:O$_{2}$ (1:1) had the best crystalline qualities. This film had a luminescence peak at 3.35 eV with a full-width-half maximum (FWHM) value of 24 meV when measured at 10 K. The X-ray 2$\theta$-scan had a peak at 41.7$^{\circ}$ with a FWHM value of 0.096$^{\circ}$. The results from our attempts to dope the films to p-type will be presented. [Preview Abstract] |
Saturday, April 16, 2011 9:22AM - 9:34AM |
D3.00007: Two Dimensional Transport Induced Linear Magnetoresistance in Topological Insulator Bi$_{2}$Se$_{3}$ Nanoribbons Richard L.J. Qiu, Dong Liang, Hao Tang, Xuan P.A. Gao Bulk Bi$_{2}$Se$_{3}$ has been proposed and confirmed as a type of three dimensional (3D) topological insulators (TI's) with a single Dirac cone for the surface state. Although the existence of topological surface state in Bi$_{2}$Se$_{3}$ has been established by surface sensitive techniques (ARPES, STM), the transport properties of two dimensional (2D) surface state in 3D TI's has been plagued by the dominating conductivity from bulk carriers. Here, we report the study of a novel linear magneto-resistance (MR) under perpendicular magnetic fields in Bi$_{2}$Se$_{3}$ nanoribbons, and show that this linear MR is purely due to 2D transport by angular dependence experiments. The 2D magneto-transport induced linear MR in Bi$_{2}$Se$_{3}$ nanoribbons is in agreement with the recently discovered linear MR from topological surface state in bulk Bi$_{2}$Te$_{3}$, and the MR of other gapless semiconductors and graphene. We further show that the linear MR of Bi$_{2}$Se$_{3}$ nanoribbons persists up to room temperature, underscoring the potential of exploiting TI's for room temperature magnetoelectronic applications. Reference: arXiv:1003.6099, arXiv:1101.2152 [Preview Abstract] |
Saturday, April 16, 2011 9:34AM - 9:46AM |
D3.00008: High Efficiency Ultra-thin CdS/CdTe Solar Cells Naba Paudel, Kristopher Wieland, Alvin Compaan Polycrystalline thin-film CdTe is currently the dominant thin-film technology in world-wide PV manufacturing. Typically a 2-8 $\mu $m thick CdTe layer is used for large scale CdS/CdTe based solar cells and modules. With finite Te resources, it is appropriate to limit the utilization of Te by reducing the thickness of the CdTe layer in these devices. But thinning the CdTe in solar cells and modules often decreases the conversion efficiency due to incomplete photon absorption and increased probability of shunting. In this study, ultra-thin CdTe devices were prepared by magnetron sputtering which appears to be well suited to control growth rate, grain size, and film stress. 0.25 -- 2.1 $\mu $m CdTe was sputtered on Pilkington TEC15 SnO2:F-coated soda-lime glass substrates with a high resistivity transparent interfacial layer after 60-80 nm of sputtered CdS. With optimum cell post-deposition processing, we obtained cells with efficiencies of 8{\%}, 10.3{\%}, 12.0{\%} for CdTe thicknesses, respectively, of 0.25, 0.50, 0.75 $\mu $m. We believe that these represent the highest efficiencies yet obtained for CdS/CdTe cells with these submicron absorber-layer thicknesses. [Preview Abstract] |
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