Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A67: Undergraduate Research IUndergraduate
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Sponsoring Units: APS/SPS Chair: Brad Conrad, American Institute of Physics Room: BCEC 050 |
Monday, March 4, 2019 8:00AM - 8:12AM |
A67.00001: In-Operando Characterization and Investigation of Individual Defects in GaAs Solar Cells Sunny Y Zhang, Qiong Chen, Chang-Kui Hu, Timothy Hurley Gfroerer, Mark W Wanlass, Yong Zhang The ultimate constraint for a solar cell to reach its theoretical efficiency limit is the quality of the absorber material. It is common knowledge that the existence of structural defects degrades device performance. However, in a real device, it is unclear (1) how individual defects affect performance via different cell parameters, (2) how the impact depends on the operation conditions, and (3) how the impact varies from one defect to another. This work answers these questions through investigating individual dislocation defects in GaAs solar cells under different illumination conditions. An array of correlative and spatially-resolved techniques, including electroluminescence, photoluminescence, Raman, and current-voltage (I-V) characteristics, is used to identify and characterize the defects. By comparing the I-V characteristics with the laser beam focused at the defect and defect-free site, the adverse impact of the defects is quantitatively measured through changes in the key device parameters, including short-circuit current, open-circuit voltage, fill factor, energy-conversion efficiency, and shunt resistance. This study provides insights for both a fundamental understanding of defect physics and practical knowledge of defects in the single defect level. |
Monday, March 4, 2019 8:12AM - 8:24AM |
A67.00002: Charge-transfer-state energy reduction at various donor/acceptor interfaces Sara Anjum Organic solar cells have several properties that make them desirable over inorganic solar cells such as cost-effectiveness, flexibility, and lightweight. While organic solar cells tend to exhibit energy losses due to the need to break up charge-transfer (CT) states, a phenomenon that does not exist in its inorganic counterparts, highly-crystalline rubrene is a donor that has been shown to allow for delocalized CT states. This project aimed to examine the effects of the acceptor choice in the delocalization of charge transfer states by examining the energies of these states in the acceptors Cl6-subPc-Cl and Cl12-subPc-Cl. We found that the CT state energy dropped by 0.25 eV when going from amorphous to crystalline rubrene with Cl6-subPc-Cl as the acceptor. The CT state energy dropped by 0.38 eV when going from amorphous to crystalline rubrene with Cl12-subPc-Cl, which compares to the energy drop of 0.38 eV observed with C60 as the acceptor. This indicates that the choice of acceptor does have an impact on the CT-state energies. |
Monday, March 4, 2019 8:24AM - 8:36AM |
A67.00003: Determining the Interface Conductivity of Nb-SrTiO3 (Nb-STO) with Various Electrodes Gillian Hagen, Daehee Lee, Ruiyun Huang, Sossina Haile Solid oxide fuel cells have the potential to function within efficient systems of energy infrastructure. Nb-doped SrTiO3 (Nb-STO) (001) is an n-type semiconductor which has the potential to provide electrical contact for application in solid oxide fuel cells. We study the interface conductivity of Nb-STO with various metal and oxide contacts as a function of temperature (200°C-550°C) and oxygen partial pressure (10-5 - 0.2 atm). We consider the interface of Nb-STO with metal contacts of Cr and oxide contacts of vertically aligned STO (100) and Sm-doped ceria (SDC) (100) through the fabrication of symmetric electrode devices. This presentation examines the interface electrical characteristics of Nb-STO with the two electrode materials studied. Overall, we demonstrate that Cr metal can provide contact to Nb-STO without resulting in a Schottky barrier, while vertically aligned STO (100) and Sm-doped ceria (100) nanocolumns cannot provide contact at elevated temperatures. The results of this study have implications on the use of single crystal Nb-STO as a conductive substrate for electrolyte materials in solid oxide fuel cells. |
Monday, March 4, 2019 8:36AM - 8:48AM |
A67.00004: Ultrafast Carrier Dynamics of Exfoliated Transition Metal Dichalcogenides with Optical-Pump Terahertz-Probe Microscopy Kenneth Lin, Satoshi Kusaba, Takashi Arikawa, François Blanchard, Koichiro Tanaka Atomically thin two-dimensional transition metal dichalcogenides (TMD) exhibit extraordinary properties similar to graphene but features an intrinsic bandgap, opening vast potential applications in photonics and optoelectronics as semiconductors. The characterization of these materials is essential for developing such technologies and the emerging technique of terahertz (THz) microscopy enables contactless probing that directly reveals the carrier dynamics of TMDs. We employ a unique, optical-pump THz-probe microscope to measure the electric field response of mechanically exfoliated bulk TMD MoS2. We measure the decay lifetime of bulk MoS2 electron carriers with a temporal resolution of approximately 500 femtoseconds in the 1 THz range. We observe the relaxation of free carriers to be uneven on a single sample and the decay constant was found to vary with position from 36.9 ps to 82.5 ps. Using atomic force microscopy, we demonstrate a correlation between the thickness of a given position on the MoS2 sample with the decay rate, where a thinner region or sample edge position corresponded to a faster decay. We attribute this nonuniform carrier relaxation rate to the edge state and surface defect effects. |
Monday, March 4, 2019 8:48AM - 9:00AM |
A67.00005: Development of Low Cost Room Temperature STM for the Preliminary Analysis of 2D Materials and Twisted Graphene Bilayers Param Patel, Michael Altvater, Guohong Li, Eva Andrei Scanning tunneling microscopy (STM) is a valuable tool for studying atomic positions, electronic energy properties of materials, and distributions of electrons at the surface of a material. Using 3D printing, we developed a cost-effective room temperature STM for the analysis of 2D materials. After noise reduction and calibration, we can examine Moiré patterns in CVD-grown twisted bilayer graphene to efficiently identify the relative angle between the bilayers. This work demonstrates how modern tunneling microscopes can be used as a cost-effective preliminary diagnostic tool for studying atomic positions and electronic superlattices at material surfaces. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A67.00006: Thermal and Structural Properties of Alkali Modified TeO2 Glasses Martha Jesuit, Michael Joseph Packard The structural and thermal properties of lithium, sodium, and potassium tellurite glasses, written as JM2O-TeO2 where M is an alkali metal, were studied. Raman spectra were measured on all glasses along with the glass transition onset (Tg), crystallization (Tx), and melting (Tm) temperatures. All thermal measurements were measured on a Perkin-Elmer DSC-7. The thermal measurements were compared to the coordination of the tellurium. Coordination data was found by deconvoluting Raman spectra using Igor Pro to find the percent of Q4 and Q3 units in the glass. |
Monday, March 4, 2019 9:12AM - 9:24AM |
A67.00007: Effect of varying the gate voltage scan rate in a MoS2/ferroelectric polymer field effect transistor Luis Rijos, Nicholas Pinto, Mengqiang Zhao, William Parkin, Alan T Johnson A ferroelectric field effect transistor (FE-FET) using chemical vapor deposition (CVD) grown monolayer MoS2 as the semiconductor was fabricated and tested at room temperature. Ferroelectric poly(vinylidene fluoride-trifluoroethylene)-PVDF-TrFE was used as the gate insulator, and the effects of varying the gate voltage scan rate from 200 mV/s to 4 mV/s on device performance were investigated. Prior to the device switching on, a negative trans-conductance was observed for all scan rates. It was followed by a rapid increase in the channel current to the on state, corresponding to the polarized down configuration of the FE. This effect was independent of the drain-source voltage. Our results revealed a narrowing in the memory window width, an increase in the mobility (μ) from 0.02 – 10 cm2/V-s, and a decrease in the sub-threshold voltage swing (SS) as the scan rate was lowered. These parameters appeared to stabilize at slower scan rates suggesting an asymptotic limit to their values. A model based on nucleation and unrestricted domain growth was used to explain these results. By lowering the gate voltage scan rate, the performance of polymer based FE-FET’s can therefore be improved. |
Monday, March 4, 2019 9:24AM - 9:36AM |
A67.00008: Fabrication of planar tunnel junctions on bulk Pb1-xSnxSe Rakin Baten, Derrick VanGennep, Tiffany Paul, James Hamlin The topological crystalline insulator Pb1-xSnxSe can undergo a trivial to non-trivial transition as a function of temperature or pressure. Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS) are both useful measurements for studying the nature of these topological transitions. However, ARPES and STS are incompatible with high pressure conditions. Planar tunneling spectroscopy (PTS) can provide similar information as STS but can be deployed at high pressure. I will discuss our efforts to fabricate Pb-alumina-Pb1-xSnxSe planar tunnel junctions and will present the results obtained thus far. |
Monday, March 4, 2019 9:36AM - 9:48AM |
A67.00009: Optimizing Conditions For Growth of Transition Metal Dichalcogenides Using Chemical Vapor Deposition Zoe Phillips, Humberto Rodriguez Gutierrez Two-dimensional materials are receiving growing attention due to their physical properties that often differ from their bulk counterparts. 2D niobium diselenide (NbSe2), a metallic compound, and 2D molybdenum disulfide (MoS2), a semiconductor, both have great potential for integration into novel 2D structures and devices. So far, few layers of NbSe2 has been synthesized using non-chemical vapor deposition (CVD), methods to study their physical properties. MoS2 has been synthesized using CVD but not continuous monolayers using water-assisted CVD approach. The purpose of this work is to determine optimal conditions to grow niobium diselenide continuous layers and molybdenum diselenide continuous layers by CVD. With this aim, we varied the amount of precursors, the growth time, the growth temperature, and the carrier gas. Powder niobium (IV) chloride and selenium were used as solid precursors for evaporation for niobium diselenide growth. Powder molybdenum disulfide and powder sulfur were used as solid precursors for molybdenum disulfide growth. For both, SiO2 substrates were pleased downstream at different positions in the furnace. The samples were characterized by using Raman spectroscopy, SEM, and AFM. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A67.00010: Spectroscopic ellipsometry characterization of Zn1-xCdxO thin films Omar Aguilar, Suelen De Castro, Marcio P. F. Godoy, Mariama Rebello Sousa Dias Despite previous investigations on the fabrication and optical characterization of Zn1−xCdxO thin films spanning the whole composition range, the permittivity has yet to be determined. In this work, we deposited Zn1−xCdxO alloyed thin films via the spray pyrolysis method. We optically characterized the samples through spectroscopy ellipsometry and transmittance measurements in the [300 nm – 3200 nm] wavelength range. We show that the optical band gap of ZnO is in the ultraviolet region of the electromagnetic spectrum, and while increasing Cd concentration, we notice a nonlinear red-shift. Also, we find that most of our films were conductive with a sheet resistance as low as 19.8 Ω for a composition of Zn0.25Cd0.75O. Moreover, the thin films have an average transmittance of 0.8 over a wide range of the electromagnet spectrum. Understanding the optoelectronic properties of Zn1−xCdxO thin films can pave the way to a more efficient Transparent Conducting Oxide (TCO). |
Monday, March 4, 2019 10:00AM - 10:12AM |
A67.00011: Understanding the Effects of Controlled Strain on Low-Dimensional Material Properties Michael O'Connor, Manoj Singh, Michael C Boyer A material is typically considered low-dimensional if electron movement within the material is limited, for example, to within a particular ionic plane or along an axis. When strain is introduced to an ideal material, lattice deformations may cause substantial changes in structural and electronic properties. Studying the effects strain have on a material can lead to basic insights into the physics governing the material as well as an understanding of how a material’s properties can be manipulated for use in applications. Strain can be introduced to a low-dimensional material through processes such as sample cleaving, elemental doping, chemical pressure, and external mechanical application. Here we detail our efforts to implement a system for controlled external straining of a sample. We present resistivity measurements showing the effects strain has on the bulk phase transition temperature of a charge density wave compound. We will discuss how this system can be used in further studies which characterize strain on the nanoscale. |
Monday, March 4, 2019 10:12AM - 10:24AM |
A67.00012: Structural and transport analysis of (BixSb1-x)2Te3 topological insulator thin films Ahmad Matar Abed, Timothy Pillsbury, Run Xiao, Anthony Richardella, Nitin Samarth Topological insulators are in the focus of research due to their unique characteristics. Their unusual metallic surface states allow the spin of electrons moving through them to be controlled and used for spintronics and even potentially quantum computing. Here we present a structural analysis of bismuth antimony telluride (BixSb1-x)2Te3 thin films grown using molecular beam epitaxy. The films were analyzed using the in-situ characterization technique reflection high-energy electron diffraction (RHEED) as they were grown and afterward using ex-situ x-ray diffraction (XRD). The work focused on having a better analysis for understanding the crystal quality of (BixSb1-x)2Te3 films. With RHEED and XRD we obtained information about different aspects of the crystal structure and how they were correlated. Further, we made devices of these films and measured their transport properties allowing us to correlate their structural and electronic properties. |
Monday, March 4, 2019 10:24AM - 10:36AM |
A67.00013: Cassie-Baxter Transition: Gibbs Energy Analysis and CFD Simulations Using Newly Developed, Validated Algorithms Chae Rohrs, Ping He, Arash Azimi A liquid droplet on a textured substrate equalizes into either the Cassie-Baxter, or Wenzel state. In addition, metastable states between these conditions are reported in the literature. The Cassie-to-Wenzel transition has been understood as the intersection of the Cassie-Baxter and Wenzel equations, which are functions of the Young’s angle; however, a major issue exists: the texture shape and dimensions are not considered. What’s more, the transition point of the Young’s model has not been experimentally verified. Because changing texture dimensions will also cause the droplet to transit from one state to another, we plan to build a new transition model through a Gibbs energy analysis of a liquid-gas-solid system. The Young’s angle is held constant and the wetting phenomena are computed as a function of texture dimensions. Each dimension set will have an equilibrium state identified by its penetration depth and apparent contact angle. Lastly, the energy barrier describing the obstacle to a droplet moving from a meta-stable state toward the stable state can simultaneously be computed. Our newly developed, experimentally validated CFD method will be used to confirm our Gibbs energy analysis, and to explore the dynamic behaviors of a droplet during the Cassie-to-Wenzel transition. |
Monday, March 4, 2019 10:36AM - 10:48AM |
A67.00014: Magnetic vortex disks for magneto-mechanotransduction Georgia Nelson, Meredith Xu, Xiao Wang, Andy Clark, David Keavney, Ralu Divan, Dafei Jin, Xuemei Cheng A magnetic vortex, the ground state existing in micron- and submicron-sized ferromagnetic disks, is a three-dimensional spin structure that consists of a circulating in-plane magnetization and an out-of-plane vortex core. Magnetic vortex disks are promising mechanotransduction toolkits due to their capabilities in spatiotemporal manipulation of mechanical forces via the magnetic field. Here we report fabrication and magnetic imaging of patterned magnetic vortex disks. An array of Fe20Ni80 (Py) disks 3 um in diameter and 40 nm in thickness were fabricated using the photolithography, sputtering deposition, and lift off process. Element-specific photoemission electron microscopy imaging at the Ni L3 edge, performed at Beamline 4-ID-C of the Advanced Photon Source of Argonne National Laboratory, confirmed that the magnetic configuration of these Py disks are indeed magnetic vortices as expected. In the future, these fabricated magnetic vortex disks will be used as force or torque transducers in biomedical systems. |
Monday, March 4, 2019 10:48AM - 11:00AM |
A67.00015: Thermoelectric Effects in a Double Mushroom Phase Change Memory Cell Noah Del Coro, Jake Scoggin, Ali Gokirmak Phase change memory (PCM) is a non-volatile memory that uses crystalline (set) and amorphous (reset) states to hold information. We use a finite-element PCM model1,2 to demonstrate thermoelectric effects in a cell with a novel “double mushroom” geometry. We analyze the impact of thermoelectric power (Peltier and Thomson heating) on device performance. We also compare the double and single mushroom cell write (reset and set) requirements and find better performance in the double mushroom due to enhanced thermal isolation. |
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