Bulletin of the American Physical Society
2013 Annual Meeting of the California-Nevada Section of the APS
Volume 58, Number 14
Friday–Saturday, November 1–2, 2013; Rohnert Park, California
Session H2: Condensed Matter II |
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Chair: Kendall Mallory, Point Loma Nazarene University Room: Darwin 29 |
Saturday, November 2, 2013 2:00PM - 2:12PM |
H2.00001: Band folding of the image potential states at the phthalocyanine/Ag interface Benjamin Caplins, David Suich, Alex Shearer, Eric Muller, Charles Harris Two-photon photoemission is used to probe image potential states at the metal-molecule interface. We compare model systems of monolayer metal-free phthalocyanine and cobalt phthalocyanine adsorbed on Ag(100) and Ag(111). Both phthalocyanines show very similar image state binding energies and dispersions. Strikingly, the spectra clearly show the opening of a band gap in the first image potential state, regardless of the presence of the electron dense metal center. Experiments conducted on both Ag(100) and Ag(111) determined the role of the surface symmetry and projected bulk band structure. Our results are contrasted to the PTCDA/Ag(100) interface. Finally, we compare to experiments a model that takes into account both the band structure of the substrate and the phthalocyanine induced corrugation of the potential energy landscape. [Preview Abstract] |
Saturday, November 2, 2013 2:12PM - 2:24PM |
H2.00002: Electronic Excited States at the NaCl/Ag(100) Interface David Suich, Benjamin Caplins, Alex Shearer, Charles Harris The alkali halide/metal interface represents a model insulator/metal system and has attracted much attention recently for their ability to decouple molecular properties from bulk metal. There are, however, few time resolved studies of these systems. We used two photon photoemission to study the dynamics of the electronic states at the NaCl/Ag(100) interface. We find a series of image potential states, where the lowest lying (n=1) member of the series is hybridized with the conduction band of NaCl. Electrons in this hybrid state undergo significant trapping, as shown by a change in their energy and spatial extent on the ultrafast time scale. Our studies correlate coverage and temperature with the dynamics and energetic trapping of electrons in this state. Momentum resolved measurements reveal that electrons in this state are initially delocalized, but become localized within a few hundred femtoseconds either due to small polaron formation or trapping at a defect. Qualitatively similar behavior has been observed for several other alkali halides on both the Ag(100) and Ag(111) surfaces, proving the generality of the phenomenon. [Preview Abstract] |
Saturday, November 2, 2013 2:24PM - 2:36PM |
H2.00003: Tuning magnetic anisotropy in (001) oriented L10 (Fe$_{1-x}$Cu$_{x})_{55}$Pt$_{45}$ films Dustin A. Gilbert, Kai Liu, Liang-Wei Wang, Chih-Huang Lai, Timothy Klemmer, Jan-Ulrich Thiele The development of high anisotropy magnetic materials that are compatible with industrial processing is critical in advancing magnetic recording, permanent magnet, and spintronic technologies. Specifically, high anisotropy materials are necessary to ensure long-term thermal stability in magnetic nanoelements, such as ultra-high density recording media and magnetic memory. A material of particular interest is $L$1$_{0}$ ordered FePt because of its large magneto-crystalline anisotropy (K$_{U})$, saturation magnetization (M$_{S})$, and chemical stability. A key limiting factor has been the high annealing temperature necessary to transform the as-deposited disordered face centered cubic (fcc) $A$1 phase into the ordered tetragonal$ L$1$_{0}$ phase. We have achieved (001) oriented $L$1$_{0}$ (Fe$_{1-x}$Cu$_{x})_{55}$Pt$_{45}$ thin films, with K$_{U}$ up to 3.6 $\times$ 10$^{7}$ erg/cm$^{3}$, using atomic-scale multilayer sputtering and post annealing at 400 $^{\circ}$C for 10 seconds, which is a much lower temperature annealing for a much shorter time compared to earlier studies. By fixing the Pt concentration, structure and magnetic properties are systematically tuned by the Cu addition. Increasing Cu content results in an increase in the tetragonal distortion of the $L$1$_{0}$ phase, significant changes to the film microstructure, and lowering of the M$_{S}$ and K$_{U}$. The relatively convenient synthesis conditions, along with the tunable magnetic properties, make such materials highly desirable for future magnetic recording technologies. [Preview Abstract] |
Saturday, November 2, 2013 2:36PM - 2:48PM |
H2.00004: Activation Energies of Copper Phthalocyanine Thin Films Using Impedance Spectroscopy Kyle Robinson, Thomas Gredig Impedance spectroscopy is used to study copper phthalocyanine thin films in order to disentangle the effective activation energies of the crystalline bulk and the grain boundaries. The spectroscopy data is fit with an equivalent circuit model to determine the activation energy for samples with different grain morphologies. The copper phthalocyanine thin films are deposited via thermal evaporation on platinum interdigitated electrodes on glass substrates at temperatures from 300~K to 530~K with constant thickness of 22~nm. The AC measurements, implementing a precision LCR meter are taken from 20~Hz~-~2~MHz, and at measurement temperatures from 25~-~90~$^{\circ}$C. The activation energy for the grain boundary peaks at 1.29~$ \pm $~0.12~eV near the structural phase-transition temperature for CuPc thin films, whereas the crystalline bulk component has a constant activation energy of 0.34~$\pm$~0.08~eV for all samples with different grain sizes. \\[4pt] This research has been funded by the National Science Foundation (NSF) grant DMR-0847552. [Preview Abstract] |
Saturday, November 2, 2013 2:48PM - 3:00PM |
H2.00005: Simulations of the Time Dependent Crystallization of Amorphous Thin Films George Wang, Andreas Bill We recently developed an analytic theory that determines the non-equilibrium grain size distribution during the crystallization of an amorphous solid. One of the key new elements introduced in the theory is a time-dependent effective growth rate. We present numerical simulations of the crystallization process and the growth rate for the isotropic nucleation and growth model of thin films. The simulation yields the functional form of the grain size distribution and the effective growth rate and their dependence on time and microscopic parameters of the system. [Preview Abstract] |
Saturday, November 2, 2013 3:00PM - 3:12PM |
H2.00006: Fabrication and Magneto-optical Characterization of Magnetic Nanostructures on 2-D Self Assembled Colloidal Crystals Xiaoyu Zou, Hung Ngo, Josh Mendez, Jiyeong Gu Patterned ferromagnetic thin films show different magnetic behaviors than flat ones. Corrugated permalloy (Py) thin films were fabricated using self assembly of colloidal polystyrene nanospheres on Si wafers followed by sputter deposition of Py, which introduces periodic curvature. To study the effect of discontinuities, Py was also deposited on nanospheres treated with RF substrate biasing to yield a discontinuous thin film. Magneto-optical Kerr effect (MOKE) magnetometry was used to measure the magnetic switching behavior. The total MOKE signal was separated into longitudinal and quadratic MOKE components to measure the surface magnetization, which are inaccessible to traditional magnetometry. The coercivity of the samples increased as the thin films increased in discontinuity. Flat Py thin films have coercivity at around 5 Oe, while the corrugated thin film has coercivity around 15 Oe. However, the substrate biased nanosphere thin film showed a much greater coercivity around 80 Oe. Finally, the symmetry properties of the hysteresis loop of both the corrugated Py thin film and the substrate biased thin film were significantly different. While the flat sample has almost no QMOKE signal, the patterned samples showed moderate QMOKE signals with peaks near the coercivity. [Preview Abstract] |
Saturday, November 2, 2013 3:12PM - 3:24PM |
H2.00007: Reconstructing the surface-height autocorrelation function of a randomly rough dielectric surface using incoherently scattered light Sangeeta Chakrabarti, Alexei Maradudin We present an analytic approach for obtaining the normalized surface height autocorrelation function of a one-dimensional randomly rough dielectric surface from experimental scattering data. It is based on the contribution to the mean differential reflection coefficient, obtained in the Kirchhoff approximation, from the light scattered incoherently. The incident light is s polarized, and its plane of incidence is perpendicular to the generators of the surface. Good agreement with numerically generated experimental data was obtained. We present several examples illustrating our approach for different power spectra and surface parameters. This approach can be used to determine the rms height, transverse correlation length and the dielectric constant of a one-dimensional randomly rough surface by inverting the incoherent contribution to the mean differential reflection coefficient. [Preview Abstract] |
Saturday, November 2, 2013 3:24PM - 3:36PM |
H2.00008: Ensemble Behavior of Magnetic Nanoparticles Dispersed in a Liquid Crystalline Matrix Jacky Wan, J. Jussi Amaral, Sayantani Ghosh, Makkiko Quint, Andrea Rodarte We are investigating the ensemble behavior of magnetic nanoparticles (MNPs) when dispersed in an electro-optically active liquid crystalline (LC) matrix. Using high-resolution scanning magneto-optical Kerr effect (MOKE), we characterize the spatial distribution of 10 nm Fe3O4 MNPs in a room temperature nematic LC, 5CB. Our results show that the presence of MNPs in the LC material affects the host itself, by changing the threshold magnetic field required for molecular re-orientation. In addition, as the LC material is cooled from the isotropic to the nematic phase, it also allows clustering of the MNPs suspended in it. We are following up on this part by using quantum dots (QDs) in conjunction with MNPs in LC and using the emission from QDs as spatial labels of MNP cluster positions. Our studies indicate the possibilities of modulation of both LC and MNP properties in the composite samples. [Preview Abstract] |
Saturday, November 2, 2013 3:36PM - 3:48PM |
H2.00009: Development and Calibration of a Ruby Fluorescence Pressure Measurement System Blaine Bush, Peter Klavins, Matthew Lawson, Kent Shirer, Nicholas Curro We present an integrated system for continuous pressure measurements inside of a clamp-type pressure cell using ruby fluorescence. The conventional method is based on the superconducting transition temperature of tin, is slow, requires thermal cycling of the sample, and is magnetic field dependent. The ruby fluorescence method is based on the pressure-dependent frequency shift of the fluorescence lines of a small ruby chip. This method allows for rapid in situ pressure measurements, which eliminates the need to thermally cycle the pressure cell and allows for measurement of the pressure at any given temperature. Preliminary calibration data is also presented. [Preview Abstract] |
Saturday, November 2, 2013 3:48PM - 4:00PM |
H2.00010: Effect of glycerol doping of PEDOT:PSS on solar cell characteristics Emma Lewis, Bhaskar Mantha, Richard Barber We have performed resistivity measurements of poly[3,4-ethylenedioxythiophene]: poly[styrenesulfonate] (PEDOT:PSS) films with varying concentrations of glycerol. Resistivity is seen to decrease exponentially from roughly 3x10$^{-2} \Omega $-m for pure PEDOT:PSS to 3x10$^{-4} \Omega $-m for 35 mg/cm$^{3}$ glycerol in PEDOT:PSS. Beyond this concentration adding glycerol does not significantly change resistivity. Bulk heterojunction P3HT/PCBM solar cells using these variously doped PEDOT:PSS layers as electrodes were studied to characterize the effects on efficiency and lifetime. Although our data display significant scatter, lowering the resistance of the PEDOT:PSS layers results in lower device resistance and higher efficiency as expected. We also note that the lifetime of the devices tends to be reduced as the glycerol content of PEDOT:PSS is increased. Many devices show an initial increase in efficiency followed by a roughly exponential decay. This effect is explained based on concomitant changes in the zero bias conductance of the samples under dark conditions. [Preview Abstract] |
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