Bulletin of the American Physical Society
85th Annual Meeting of the APS Southeastern Section
Volume 63, Number 19
Thursday–Saturday, November 8–10, 2018; Holiday Inn at World’s Fair Park, Knoxville, Tennessee
Session B01: Condensed Matter I |
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Chair: Albert Gapud, Southern Alabama Room: Holiday Inn Knoxville Downtown Summit |
Thursday, November 8, 2018 11:00AM - 11:12AM |
B01.00001: Martensitic transformation in a single crystal of V3Si, as measured by NMR, magnetic susceptibility, and magnetoresistivity Albert A Gapud The Martensitic transformation (MT), a temperature-driven, displacive structural transformation in the crystal lattice, is observed in a single crystal of V3Si using measurements of magnetoresistivity and magnetic susceptibility, and NMR spectroscopy -- all on the same sample. This was to uncover further details of the MT, insofar as changes in physical structure, as well as in the scattering of charge carriers. The latter issue is explored in the context of the well-known Köhler’s Rule, which postulates that the field (H) dependence F of the resistivity (from zero-field ρo) is a power law: F(H/ρo) ~ (H/ρo)b, from which a deviation may occur because of a MT. We found that the MT may not occur uniformly throughout the crystal, possibly propagating as a wave that may even be pinned or unpinned by defects. |
Thursday, November 8, 2018 11:12AM - 11:24AM |
B01.00002: Electron-phonon coupling in binary organic semiconducting crystals Rohan Isaac, Laurie E McNeil Semiconducting materials made from small organic molecules show great promise for applications in energy conversion, display technologies, and flexible electronics among many others. However, the electronic mobilities of these materials are much lower than traditional inorganic materials, which limits their application, and despite extensive research in this field, the fundamental processes that govern the generation and motion of charge in these materials is not completely understood. It is known that vibrational motions (phonons) in the material have a strong impact on the electronic, optical and transport properties. We explore a range of binary semiconducting single crystals made from two different small organic molecules, a donor and an acceptor, arranged in mixed stacks. We have completed resonance Raman measurements to quantify the electron-phonon coupling constants in these materials, which will give us information on the types of vibrational modes have a strong impact on charge transfer, and help in selecting and designing materials more suitable characteristics. |
Thursday, November 8, 2018 11:24AM - 11:36AM |
B01.00003: Computational Search for Novel Color Centers in Diamond for Quantum Technologies Caroline M Collins, Richard H Mattish, Ted Zhan, Nicholas W Gothard As quantum technologies continue to attain viability, increased focus is needed to identify novel materials with the requisite characteristics. Quantum technologies can be divided into two types: 1) quantum information processing, which requires stable sources of single photons, and 2) quantum sensing, which relies on electro-photonic interactions at the atomic scale. To identify new materials that satisfy these criteria, a survey of optical defects in diamond has been undertaken, focusing primarily on transition metal dopants. Defects are introduced into both nano- and bulk diamond, and the defect properties are calculated with respect to the relevant electro-optical transition energies using ground and excited state density functional theory. Comparison is made between the nano- and bulk formulations and potential application to quantum information and quantum sensing technologies is considered. |
Thursday, November 8, 2018 11:36AM - 11:48AM |
B01.00004: Charge transfer of C-related centers in C-doped GaN Subash Paudel, U. R. Sunay, W. R. Willoughby, M. E. Zvanut Incorporation of carbon impurities in the concentration range of 1017-1019 creates semi-insulating GaN layers as required for electronic power devices. We used electron paramagnetic resonance (EPR) spectroscopy to study the point defects in 2x1017 -1019 C-doped GaN substrates grown by hydride vapor phase epitaxy. EPR was performed at 3.5K. An isotropic signal with g=1.987+/- 0.001 was observed in all samples. EPR intensity of the signal increased monotonically with carbon concentration indicating the defect is carbon-related. Under the photo-EPR measurements, the intensity of the signal increased with photon energy greater than 2.75+/- 0.15 eV and the photo-induced signal began to decrease at 0.95+/- 0.05 eV. Another signal, a well- known shallow donor with gpar=1.951+/-0.001 and gperp=1.950+/-0.001, also appeared along with the g~1.987 signal in the most lightly doped samples under illumination. The appearance of the donor confirms that the photo-excitation is caused by excitation of an electron from the defect to the conduction band. This implies that the defect level for C-related centers is about 1 eV above the valence band edge, consistent with temperature-dependent Hall measurements.
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Thursday, November 8, 2018 11:48AM - 12:00PM |
B01.00005: Effect of Visible Light on the Electrical Performance of difTES ADT Thin-Film Transistors Zafrullah Jagoo, Oana Jurchescu, Laurie E McNeil Photocurrent microscopy can be used to probe the underlying mechanisms of charge generation and charge transport in organic thin-film transistors. We report on the effects of monochromatic illumination on the electrical performance of bottom-gated bottom-contact solution-processed difTES ADT transistors. The wavelength, 532 nm, was selected since photons of that wavelength are readily absorbed. When incident light was focused at the center of the channel, the magnitude of the drain current was found to be higher than in the dark in both the linear and saturation regimes, signifying the dissociation of excitons into holes (and electrons) which migrated to the drain electrode due to the drain-source bias. In addition to the enhancement of the drain current under illumination, we also observe an increase in the hole mobility. Injected carriers from the electrodes along with photo-induced carriers that are generated midway in the channel fill localized in-gap (deep traps) states of the semiconductor, associated with impurities and defects in the channel. Mobile carriers that are near the mobility edge are able to traverse the active layer easier resulting in a larger mobility. We also noted a significant shift in both the threshold voltage and subthreshold swing under illumination. |
Thursday, November 8, 2018 12:00PM - 12:12PM |
B01.00006: Plasmonic Properties of Silver Nanoparticle Clusters Prepared by AFM Manipulation Longyu Hu, Ramakrishna Podila, Jeffrey N Anker, Apparao Rao The plasmonic resonance of nanostructures is extremely sensitive to the size and shape of the constituting nanoparticles, geometric configuration, inter-particle separation, and the dielectric property of the medium. While there has been much theoretical advancement in the plasmonic properties of ensembles of noble metal nanoparticles, practical realization of such ensembles has been challenging. Here, we present a versatile method for fabricating plasmonic nano-assemblies using atomic force microscope (AFM) manipulation, which allows precise fabrication of well-defined ensembles of single crystalline nanoparticles, and to some extent the control of inter-particle separations as well. Polyhedral single crystalline AgNPs were prepared via the hydrogen reduction method with a uniform diameter of ~110 nm. Combined with dark-field microscopy and spectroscopy, our method also enables the collection of the scattering spectrum for each ensemble with excellent signal-to-noise ratio. We manipulated chemically synthesized high purity AgNPs into linear chains, rings, dimers, trimers and other various geometries. The scattering spectra of these ensembles will be presented within the framework of plasmonic hybridization. |
Thursday, November 8, 2018 12:12PM - 12:24PM |
B01.00007: Organic Semiconductor and Plasmonic Gold Nanoparticle Based Transistor Brandon T Yost, Zafrullah Jagoo, Laurie E McNeil Organic semiconductor-based devices such as field effect transistors (OFET) show promise in electronics and photodetection due to low-temperature and solution processability as well as the ability to be fabricated on flexible substrates such as polymer films. However, charge carrier mobilities are low relative to inorganics, ranging from 0.001-1 cm2/Vs compared to 100-500 cm2/Vs in silicon devices. The incorporation of metal nanoparticles into the active layer of an OFET device offers enhanced optical properties such as an increased absorption cross-section which we predict leads to enhanced sensitivity in a photodetector due to the excitation of the surface plasmon resonance of the gold nanoparticle. In the absence of illumination, we predict the gold nanoparticles will act as traps leading to suppression of dark current. 6,13-Bis(triisopropylsilylethynyl) (TIPS)-pentacene OFETs are fabricated on a heavily-doped silicon wafer with an SiO2 gate dielectric layer and gold electrodes. The TIPS-pentacene active layer is drop cast from solution to create a bottom-gate bottom-contact geometry. Results will be presented to elucidate the effect of gold nanoparticles in the active layer on charge carrier mobility and on/off ratio. |
Thursday, November 8, 2018 12:24PM - 12:36PM |
B01.00008: Silver nanodiscs for enhanced fluorescence emission Longyu Hu, Bipin Sharma, Achyut Raghavendra, Apparao M Rao, Ramakrishna Podila Here, we used a novel method to deform silver nanoparticles (Ag NPs) into nanodiscs through the application of transverse mechanical force using a home-built Teflon puncher. The application of 50 N force for 50 times resulted in the formation of Ag nanodiscs (88 nm high, 113 nm wide) from polyhedral Ag NPs (~105 nm diameter). We observed a clear red-shift in the plasmonic resonance for Ag nanodiscs. We show that the fluorescence emission from Rhodamine B (RhB) is enhanced by >30 fold (with respect to RhB on bare glass) in the presence of Ag nanodiscs due to a simultaneous increase in the excitation intensity and photon mode density. The observed increase in Ag nanodiscs is explained in terms of its red-shifted scattering closer to RhB emission, which results in an increased local electric field and higher photonic mode density for Ag nanodiscs. |
Thursday, November 8, 2018 12:36PM - 12:48PM |
B01.00009: Modeling Thermal Lensing in Soy Sauce Heath Martin II, Sanichiro Yoshida, David Feldbaum, Kayla Wilson Interesting thermal lensing effects in ordinary soy sauce have been demonstrated in previous experiments in the form of rings of the concentric circular intensity profiles. Here we produce more quantitative results for thermal lens produced by this substance. More specifically we are concerned with the absorbance of the medium and with defining the lensing needed to produce the observed image. This process utilizes a vertical setup, two photodiodes, two pick-off glasses, a ccd camera, a handheld green laser, and a thermistor. The absorbance is measured by comparing the radiant flux of the light source before and after interacting with the medium. The lens produced is measured by using imagery from the camera to try to determine the intensity of the light rings. We recreate the absorbance and final image in a mathematical model. |
Thursday, November 8, 2018 12:48PM - 1:00PM |
B01.00010: Temperature dependence of transverse magnetic focusing in a high-mobility heterostructure Adbhut Gupta, Jean J. Heremans, Saeed Fallahi, Michael J. Manfra The temperature dependence of Transverse Magnetic Focusing (TMF) is investigated in a two-dimensional electron system in a high-mobility GaAs/AlGaAs heterostructure at low temperatures T (electron mean free path 81 $\mu$m at 4.2 K). Two TMF configurations are studied $—$ a regular in-line configuration with distance between the injector and collector of L = 7 $\mu$m, and a bent configuration with L = 5 $\mu$m horizontal and 2 $\mu$m vertical. In both configurations, the TMF amplitude (non-local resistance R$_f$) shows a monotonic decrease as T is increased from 4 K to 20 K. This decay is analyzed by fitting R$_f$ according to different scattering mechanisms. Limitation of R$_f$ by acoustic phonon scattering is ruled out because 1/T does not provide a good fit to the experimental data. In fact, we observe R$_f$ $\sim$ 1/T$^2$, indicating the dominance of inelastic electron-electron interactions in limiting R$_f$. The role of electron-electron interactions hints at the existence of a hydrodynamic transport regime in a particular temperature range. The hydrodynamic regime has recently received increasing attention. The finding suggests that TMF, usually considered a purely ballistic phenomenon, may present hydrodynamic aspects as well. |
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