Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y17: Thin Film Growth, Processing, and Surface Properties |
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Sponsoring Units: DCMP DMP Chair: Daniel Dougherty, North Carolina State Univ Room: LACC 306A |
Friday, March 9, 2018 11:15AM - 11:27AM |
Y17.00001: Fabrication of Thin Films from Aligned Single-Walled Carbon Nanotubes Desmond Jeff, Jeffrey Fagan, Adewale Adepoju, Quinton Williams, Thomas Searles In this study, an investigation of a recently discovered vacuum filtration and wet transfer method for the spontaneous alignment of single-walled carbon nanotubes in thin films was performed. These films may be employed to investigate their non-linear optical properties and improve the performance of LiFePO4 batteries. The method for fabricating these films is highly efficient, being able to produce and transfer thin films to a substrate within an hour. Using a linear fit of the thickness data, the films were found to be within 15 nm of expectations. Nanotube alignment with a nematic order parameter S = 0.030856 was observed in a film fabricated with 20.3 μg of unsorted carbon nanotubes. This indicated a much smaller alignment within the film, given the limitations of the setup, than reported in previous works. Raman analysis showed the expected RBM, G, and 2D peaks with position percent error of 0.97 %, 0.38 %, and 1.07 %, respectively. Using the Raman RBM peak of the partially aligned film, an average film diameter of 1.515 nm was obtained which is consistent with expected values for single-walled carbon nanotubes fabricated by the arc-discharge technique. |
Friday, March 9, 2018 11:27AM - 11:39AM |
Y17.00002: Investigations of the optical and structural properties of amorphous Ta2O5 prepared with ion assist during deposition Le Yang, Gabriele Vajente, Alena Ananyeva, Emmett Randel, Eric Gustafson, Carmen Menoni The performance of next generation gravitational wave detector is fundamentally limited by the thermal noise of the mirror coatings in the interferometer. Amorphous tantalum pentoxide (Ta2O5) is the component in the multilayer coating with the highest mechanical loss. This work investigates the optical and structural properties of tantalum pentoxide aimed at identifying conditions that lead to ultra-high thermodynamic stability. Amorphous tantalum pentoxide single layers were deposited by dual ion beam sputtering. The films were bombarded during growth by an assist Ar ion beam at different conditions. All samples were found to have good optical quality with an absorption loss at 1 µm as low as 7.2 ppm. The tantalum bonding states investigated through x-ray photoelectron spectroscopy (XPS) showed a stoichiometric composition in sample prepared with 100 eV Ar ion bombardment. The use of the Ar assist beam during film growth is shown to favorably affect the films’ mechanical properties. |
Friday, March 9, 2018 11:39AM - 11:51AM |
Y17.00003: Structural and Electrical Properties of Pulsed Laser Deposited Yttrium Doped Zirconium Oxide Thin Films Stabilization William Cockerell, Matthew Melfi, Joshua Steier, Rory Vander Valk, Stephen Kelty, Mehmet Sahiner Solid oxide fuel cells, or SOFC’s, are electrochemical conversion devices that produce electricity and potentially a strong candidate to produce clean energy for the future. We are attempting to lower the operating temperature of SOFC’s to minimize startup times and mechanical/chemical issues, yet preserve electrical efficiency. This can be achieved by lowering the solid oxide electrolyte resistivity in between the anode and cathode of the SOFC. This electrolyte compound of Yttrium Zirconium Oxide (Y2ZrO5) or known as Yttria Stabilized Zirconia (YSZ), is what causes the electrochemical conversion for the cell, but is also responsible for the operating temperature of the cells from 800–1000 οC. Our research goal is to determine the relationship between the surface structures. This includes roughness, periodicity, and molecular structure. The resistivity changes due to these characteristics are observed. After production of samples by pulse laser deposition, structural characterization is performed using ellipsometry, scanning electron microscopy equipped with an energy dispersive x-ray spectroscopy, and x-ray diffraction. Electrical properties are analyzed by using four-point probe conductivity measurements. Structural versus electrical properties will be discussed. |
Friday, March 9, 2018 11:51AM - 12:03PM |
Y17.00004: The surface properties of elemental crystalline solids Richard Tran, Zihan Xu, Balachandran Radhakrishnan, Donald Winston, Wenhao Sun, Joseph Montoya, Xiangguo Li, Kristin Persson, Shyue Ong The anisotropic surface energies and work functions of a crystal are crucial in understanding phenomena such as crystal morphology and the design of technologies such as Schottky barriers. To better understand and implement these properties in screening for optimal materials, a comprehensive and standardized database is needed. In this talk, we will discuss the development of the world’s largest open, rigorously-validated database of surface energies, Wulff shapes and work functions for elemental crystalline solids using high-throughput density functional theory (DFT) calculations. We will describe the methodology for constructing the database, extensive validation with previous experiments and computed data, and the analysis of structure-property relationships. This database spans more than 140 polymorphs and 70 elements, up to a maximum Miller index of two and three for non-cubic and cubic crystals, respectively with both metals and non-metals along with well-known surface reconstructions. The database is continuously updated with the addition of more crystal structures, Miller indices and surface reconstruction schemes. |
Friday, March 9, 2018 12:03PM - 12:15PM |
Y17.00005: Ultrathin layers of β-tellurium grown on highly oriented pyrolytic graphite by molecular-beam epitaxy Yawei Dai, Jinglei Chen, Yaqiang Ma, Xianqi Dai, Wingkin Ho, MAOHAI XIE Two-dimensional (2D) materials have been the subject of intensive research in recent years. New 2D materials are continuously put forward by theory and realized by experiments. A recent theory has predicted some layered structures of tellurium (Te), the monolayers of which represent new two-dimensional (2D) systems with attractive electronic and optoelectronic properties. In this work, we employ molecular-beam epitaxy to fabricate ultrathin Te films on highly oriented pyrolytic graphite (HOPG). Using scanning tunneling microscopy, we find that the grown Te ultrathin layers have rectangular surface cells with the cell size being consistent with the predicted β-tellurene. For thicker films, the cell size is found more consistent with that of the [10-10] surface of the bulk Te crystal. Scanning tunneling spectroscopy measurements suggest the β-tellurium films are semiconductors with the energy bandgaps decreasing with increasing film thickness and the gap narrowing occurs predominantly at the valance-band maximum (VBM). The latter can be explained by the strong coupling of states at the VBM but a weak coupling at conduction band minimum as revealed by density functional theory (DFT) calculations. |
Friday, March 9, 2018 12:15PM - 12:27PM |
Y17.00006: A Versatile UHV Molecular Beam Epitaxy for High-purity Thin Film Growth Somesh Ganguli, Guannan Chen, Waclaw Swiech, Vidya Madhavan Molecular beam epitaxy (MBE) has long been at the forefront of high quality thin film growth. Here we describe the design and construction of a universal MBE system with the capability of thin film growth on various sample holders that are currently used in experimental set ups such as angle-resolved photoemission spectroscopy (ARPES), electron energy loss spectroscopy (EELS) and scanning tunneling microscopy (STM). The MBE has seven effusion cells (High-T, low-T and standard) and electron beam evaporators for evaporating a host of different materials (Ti, Se, Bi, Sb, Mo, Nb etc). This universal MBE system uses a sample holder-holder to accommodate different sample holders. After deposition, the sample holder-holder can be unloaded via a vacuum shuttle for in-situ transfer to the respective systems. We will describe the growth and characterization of dichalcogenides on a variety of substrates using this MBE system. |
Friday, March 9, 2018 12:27PM - 12:39PM |
Y17.00007: Nitrogen Doping of MoSe2 by Plasma Treatment YIPU XIA, Jin-Peng Xu, Jun-Qiu Zhang, BIN LI, Wingkin Ho, MAOHAI XIE Ultrathin two-dimensional layered transition metal dichalcogenides (TMDs) have instigated great research interests due to their unique electrical, optical and spin-electronic properties. Similar to traditional semiconductors, doping plays a critical role in controlling TMDs’ properties and potential applications. In this work, we use molecular beam epitaxy (MBE) to fabricate monolayer Molybdenum diselenide (MoSe2) on graphene substrate. Post-growth doping is achieved by in-situ nitrogen plasma treatment of the surface. Using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS), we confirm the p-type doping of MoSe2 where selenium atoms are partially substituted by nitrogen atoms. Doping concentration can be controlled through the change of substrate temperature and different nitrogen plasma exposure time. The results demonstrate effective tuning of carrier density in epitaxial MoSe2 that is necessary for device applications. |
Friday, March 9, 2018 12:39PM - 12:51PM |
Y17.00008: SrTiO3-xNx thin films synthesized using nitrogen plasma assisted hybrid molecular beam epitaxy Forrest Brown, Arnab Sen Gupta, Yoonsang Park, Venkatraman Gopalan, Roman Engel-Herbert For perovskite oxynitrides, it has been proven challenging to maintain cation stoichiometry while incorporating nitrogen at the anion site in conventional thin film deposition approaches, such as pulsed laser deposition and sputtering. Post-synthesis nitridation routes for bulk perovskites using high temperature NH3 anneals have been applied with limited success. Nonequilibrium thin film synthesis techniques offer an alternative strategy allowing large amounts of nitrogen into the film lattice during growth. We report the synthesis of strontium titanium oxynitride (SrTiO3-xNx) thin films using nitrogen plasma assisted hybrid molecular beam epitaxy. The self-regulated growth regime offered by hybrid molecular beam epitaxy can still be accessed in the presence of a nitrogen plasma. Nitrogen incorporation rates larger than 10% have been obtained, accompanied by substantial changes of the film lattice parameter. Temperature dependent impedance spectroscopy, second harmonic generation, and spectroscopic ellipsometry will be discussed to elucidate the effect of nitrogen on the ferroelectric, dielectric, and optical properties in SrTiO3-xNx films. |
Friday, March 9, 2018 12:51PM - 1:03PM |
Y17.00009: Epitaxial growth of uniaxial antiferromagnetic FeF2 thin films Junxue Li, Mark Lohmann, Weimin Zhou, Peng Wei, Jing Shi Antiferromagnetic (AFM) materials are potential candidates for future spintronic applications owing to their ultrafast spin dynamics. AFM spintronics devices require high-quality thin films with controlled spin orientations. Here, we report the epitaxial growth of the (110)-oriented FeF2 thin films, an AFM material, on MgF2(110) substrate using molecular beam epitaxy. The structure of the FeF2(110) thin film is characterized by both ex-situ X-ray diffraction (XRD) and in-situ reflection high energy electron diffraction (RHEED) studies. The uniaxial AFM spin configuration of FeF2(110) films is further confirmed by the exchange bias effect in both FeF2/Co and FeF2/Ni bilayers. A clear exchange bias is observed when an external magnetic field (HFC) is applied along the [001] direction while absent when the HFC is along the [1-10] direction. The blocking temperature of a 50 nm FeF2(110) film is ~80K, which is close to the Néel temperature of bulk FeF2 (78K). Our epitaxial FeF2(110) thin films will open up a promising direction for exploring AFM spintronic devices. |
Friday, March 9, 2018 1:03PM - 1:15PM |
Y17.00010: LabVIEW Implementation of Single-Laser, Alternating-Target Pulsed-Laser Deposition Keller Andrews, Anthony Kaye Alternating target pulsed laser deposition is a thin-film growth technique used for fabricating materials that require the use of more than one solid target. While it is possible to adapt a standard pulsed-laser deposition (PLD) system to accommodate the alternating-target PLD technique, very little literature exists to assist in this effort. We have developed a LabVIEW implementation of this technique that synchronizes and controls the necessary system components to enable alternating-target PLD using up to xx targets. To test this implementation, we grew thin films of tungsten-doped vanadium dioxide (WxV1–xO2), a material with a temperature-dependent metal-insulator transition. Since the temperature at which the films transition to the metallic state is linearly dependent upon the tungsten doping fraction, we were able to use the results of films with doping fractions in the range from x = 0 (i.e., pure VO2) to x = 0.02 (i.e., 2.0at%) in steps of x = 0.0025 (i.e., 0.25at%) to verify the implementation of this method in our PLD system. In this presentation, we will show both the LabVIEW implementation and the results of our verification experiment. |
Friday, March 9, 2018 1:15PM - 1:27PM |
Y17.00011: Study of Magnetoelectric Coupling at the Ferroelectric Vinylidene Fluoride/Ferroelectric Cobalt Interface. Aashish Subedi, Sajib Saha, Keith Foreman, Nowsherwan Sultan, Shireen Adenwalla The implementation of organic ferroelectric materials in devices could lead to significantly increased device functionality, as they effectively combine the numerous advantages of organic-based electronics with the useful bi-stable polarization characteristic of ferroelectricity. Magnetoelectric coupling, for example, between poly(vinylidene fluoride), or PVDF, and ferromagnetic cobalt thin films has been well established . However, open questions remain, concerning the role and structure of the Co/PVDF interface. The VDFn oligomer (n=15), with high remanent polarization, better crystallinity, and vacuum deposition capability, has an edge over its polymer counterpart. We have shown that the Co/VDF interface is pristine , but only displays weak magnetoelectric coupling. Inserting a high-K dielectric MgO thin film between the ferroelectric VDF and ferromagnetic Co layers may help strengthen the coupling. We also expound on the effects that annealing has on the Co/MgO interface: it results in the migration of oxygen from Co to MgO, changing the magnetic anisotropy from out-of-plane to in-plane. We present data on the magnetoelectric coupling in these metal/organic heterostructures with and without the MgO interlayer |
Friday, March 9, 2018 1:27PM - 1:39PM |
Y17.00012: Magnetic properties of intercalated rare-earth metals under graphene Nathaniel Anderson, Myron Hupalo, David Keavney, John Freeland, Michael Tringides, David Vaknin X-ray magnetic circular dichroism (XMCD) measurements of graphene intercalation with rare-earth metals (REs; Eu, Dy, and Gd) on silicon carbide reveal similar surface magnetic properties in these single layer REs. The intercalation of the REs (Eu, Dy, and Gd) is achieved by depositing the metal on graphene that is grown on SiC and by annealing at high temperatures to allow diffusion through the graphene. STM scans reveal that each of the metals has a distinct nucleation pattern after intercalation, whereby Eu has distinct nano-clusters, Dy has a amorphous clusters, and Gd forms long streaks. Furthermore, the field dependence and temperature dependence of the RE magnetic moments extracted from the XMCD shows paramagnetic behavior, while having a slightly lower moment than predicted by the Brillouin function. We will also provide evidence that the graphene protects these intercalated films by comparing the samples to common oxides and showing the XMCD time dependence after months of exposure to air. |
Friday, March 9, 2018 1:39PM - 1:51PM |
Y17.00013: In-situ ARPES Study of STO/BTO Surface Terminations Slavko Rebec, Tao Jia, Hafiz Sohail, Robert Moore, Zhi-Xun Shen SrTiO3 (STO) has a host of interesting properties, particularly when layered with other complex oxide materials. One such property is a high mobility two-dimensional electron gas (2DEG), which can be generated at the surface of STO when exposed to VUV radiation. Here we study how the 2DEG formation is affected by STO layering and termination. Here we present recent in-situ angle-resolved photo emission studies of MBE grown STO and BTO samples. |
Friday, March 9, 2018 1:51PM - 2:03PM |
Y17.00014: Dielectric properties of interfaces Huihuo Zheng, Marco Govoni, Giulia Galli The description of dielectric properties of interfaces is a key ingredient for models of heterogeneous materials. We investigated several interfaces, including Si/Si3N4, Si/H2O, Si/vacuum, using first principle electronic structure methods. We present a definition of the dielectric response of a semiconducting interface obtained by using a spectral decomposition of the dielectric matrix, and by localizing its eigenvectors in appropriately chosen regions of space. The interfacial dielectric response is then used to define an electronic thickness of the interface and spatial dependent band offsets. |
Friday, March 9, 2018 2:03PM - 2:15PM |
Y17.00015: Tin Starved Pseudo-Phase in the Growth of Superconducting Thin-Film Nb3Sn: a DFT and Monte Carlo Study Nathan Sitaraman, Tomas Arias, Daniel Hall, Ryan Porter While the art of making Nb3Sn wires was perfected long ago, vapor diffusion grown Nb3Sn layers have so far fallen well short of their theoretical potential, leaving applications like high-Tc SRF cavities tantalizingly out of reach. Recently, high-resolution microscopy has revealed sharply defined regions of tin depleted Nb0.83Sn0.17 stoichiometry just beneath the surface of these layers. Because the superconducting properties of Nb3Sn are known to be sensitive to even slight changes in stoichiometry, these regions could be limiting the performance of the layers. |
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