Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session Y41: Surface Electronic Structure, Dynamics, and ReactivityLive
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Sponsoring Units: DCMP Chair: Masa Ishigami, University of Central Florida |
Friday, March 19, 2021 11:30AM - 11:42AM Live |
Y41.00001: Work Function Studies of Carbon Nanospikes by XPS and UPS Arthur Baddorf, Adam J. Rondinone, Dale K. Hensley Carbon nanospikes show excellent energy and selectivity for electrochemical conversion of CO2 to ethanol and N2 to NH3. Reactivity is attributed to an enhanced electric field from the spike topography. We have identified the composition of nanospike surfaces with x-ray photoelectron spectroscopy (XPS) and their absolute work function using ultraviolet photoemission spectroscopy (UPS). Annealing as-grown samples above 275 °C produces a clean surface which has a 4.13 eV work function, a half volt lower than that of flat graphite. Contamination of the spiked surface by exposure to air, oxidation at elevated temperature, or immersion in water or hydrocarbons, increases the work function. Blunting the spikes, by exposure to an oxygen plasma, argon sputtering, or annealing to 800 °C results in a work function close to that of flat graphite. An unusual double onset in the UPS secondary electron intensity is observed on as-grown nanospike samples and is reproduced by absorbing hydrocarbons on clean nanospikes. This double onset appears to be unique to carbon substrates and may originate in inelastic scattering of photoelectrons. |
Friday, March 19, 2021 11:42AM - 11:54AM Live |
Y41.00002: Investigation of Low-Energy H and He Implanted 4H-SiC using X-Ray and Neutron Reflectivity Mitchel B Vaninger, Alessandro R Mazza, Thomas Z Ward, Helmut Kaiser, Thomas W Heitmann, Katherine G Schaefer, Gavin Mcl King, Edward H Conrad, Paul F Miceli The atomic number density profiles extracted from grazing angle x-ray and neutron reflectivity are relatively unexplored for investigating ion implantation. Here they are used to explore the effects of low energy implantation of light ions, H and He, into 4H-SiC with energies in the range of 360 to 2000 eV and fluences up to 2x10^17 ions/cm^2. For both ions, combined x-ray and neutron reflectivity measurements reveal a top layer of SiC having a reduced density of 60-65% while the thickness of this layer increases with fluence. Atomic force microscopy imaging (AFM) shows topographic defects which appear to be etch pits. Our results indicate that these implanted light ions do not form gas bubbles below the surface of SiC and we discuss how these combined AFM and reflectivity measurements provide insight on the interaction of the light ions with SiC. |
Friday, March 19, 2021 11:54AM - 12:06PM Live |
Y41.00003: Atomic Resolution from Friction Force Microscopy Using the Jarzynski Equality Yasmin Watanabe, Renata Simão, Rodrigo Capaz Atomic resolution in Atomic Force Microscopy (AFM) has been achieved in a number of studies regarding surface properties in atomic scale. Most of these works are performed using the frequency-modulation analysis of the normal force. In the present work, we explore the possibility of achieving atomic resolution using Friction Force Microscopy (FFM). We simulate the scanning process using the Tomlinson model and Langevin dynamics. Tests are performed on the graphene surface, using different scanning speeds, temperatures and contact forces. By varying these parameters, it is possible to observe two distinct motion regimes of the tip of the microscope on the surface of sample: The “stick-and-slip” movement and the thermolubricity regime. We use the Jarzynski equality to map the potential energy of graphene in both cases. In the “stick-and-slip” case, it is only possible to use Jarzynski’s equation in a portion of the distance between two energy minima, but the whole potential curve can be extrapolated, with an energy barrier in agreement with the reference values. For the thermolubricity mode, we were able to obtain the surface potential for the entire scanning distance, which indicates it may be possible to apply these results in order to obtain FFM images with better resolution. |
Friday, March 19, 2021 12:06PM - 12:18PM Live |
Y41.00004: Challenges for integrating first principles theory and X-ray reflectivity measurements to predict interfacial structure Kendra Letchworth-Weaver, Katherine Harmon, Nicholas Cheung, Maria Chan, Giulia Galli, Dillon D Fong, Paul Fenter Determining atomic-scale structure, composition, and electronic properties of interfaces is crucial for technological applications but remains a challenge in liquid environments and under growth conditions. Defect sites on solvated catalyst surfaces change electron energy alignment between the surface and reacting molecules, impacting catalyst performance. Precise control of surface termination during epitaxial growth of novel materials is affected by processes such as dynamic layer rearrangement. |
Friday, March 19, 2021 12:18PM - 12:30PM Live |
Y41.00005: Deep Learning Enhanced STM: Recognizing the Herringbone Surface Reconstruction and Atomic Lattice of Au(111) Darian Smalley, Jesse Thompson, John Thomas, Alexander Weber-Bargioni, Masa Ishigami Performing high quality scanning tunneling microscopy (STM) requires a clean, sharp, and stable STM probe tip. Many research groups use Au(111) to prepare and pre-characterize the STM tip for experiments. The preparations vary from imaging Au(111) until the images improve, applying voltage pulses while tunneling, to diving the tip into Au(111). Good tips are identified by imaging and performing tunneling spectroscopy of Au(111). All preparation techniques and the final inspection are time consuming and tedious, limiting the productivity of the microscope operator. Here we present a deep learning approach using a convolutional neural network (CNN) to recognize certain features in STM images of Au(111) to automatically identify tips which are suitable for high resolution imaging. These features include the herringbone surface reconstruction and atomic lattice of Au(111) samples. The dataset used to train our model comprised of STM images of Au(111) collected over several years, under a variety of imaging conditions. Even though the dataset was non-curated, our CNN was able to achieve 96.7% accuracy in identifying high quality tips. We will show that our technique potentially has broader utility than efforts based on more highly curated datasets. |
Friday, March 19, 2021 12:30PM - 12:42PM Live |
Y41.00006: Spectroscopic Characterization and Molecular Dynamics Simulation of Tin Dioxide Hawazin Alghamdi, Sugata Chowdhury, Prabhakar Misra Tin dioxide (SnO2) is a metal oxide with highly symmetric structure at the point that has been studied theoretically and experimentally; however, its band structure is not yet completely understood. The present spectroscopic investigation focuses on rutile SnO2 in powder form. Raman spectroscopy with laser excitation at 780 nm has been used to characterize the different vibrational modes of SnO2. Thermal effects associated with the vibrational features in the Raman spectra have been studied in the range 303 – 443 K. We have demonstrated a red-shift in the Raman spectra as the temperature increases for both A1g (634 cm-1) and B2g (775 cm-1) modes, while the Eg (475 cm-1) mode exhibited no change. FT-IR spectra have been obtained in order to study the IR-active vibrational modes for tin dioxide: Sn-O stretching vibration (467.4 cm-1) and Sn-O-Sn asymmetric vibration (569.4 cm-1). XRD spectra have been recorded to confirm the rutile structure (a=b=4.737Å, c=3.186Å) of tin dioxide. SEM images have provided information regarding surface morphology. Molecular Dynamics simulations have been performed in order to study the various vibrational modes using the LAMMPS software. |
Friday, March 19, 2021 12:42PM - 12:54PM Live |
Y41.00007: The adsorption of oxygen on bimetallic Pd3M2 clusters (M= Ag, Au, Co, Cu, Mn, Ni, Pt and Ru) with and without alumina support by Density Functional Theory Nusaiba Zaman, Kah Chun Lau, Abdelkader Kara We use density functional theory to systematically investigate the adsorption of oxygen on the bimetallic Pd3M2 clusters (M = Ag, Au, Co, Cu, Mn, Ni, Pt, and Ru) with and without supported alumina. Small bimetallic clusters with high surface area to volume ratio often offers, higher stability, greater selectivity, and sometimes superior activity than the pure metal counterparts. We explore different adsorption sites for molecular oxygen, which can be oriented in a vertical or horizontal direction with respect to the cluster, as well as atomic oxygen on these bimetallic Pd3M2 clusters. We also investigate how the presence of an alumina support affect the molecular and atomic oxygen adsorption energy on these bimetallic clusters. Moreover, we will present the effect of oxygen adsorption on the electronic properties of these Pd3M2 clusters with and without alumina support. Bader charge analysis is performed to probe how the charges are transferred between the atomic and molecule oxygen with these bimetallic clusters and its effect is compared with the presence and absence of the alumina substrate. |
Friday, March 19, 2021 12:54PM - 1:06PM Live |
Y41.00008: Titanium monoxide (TiO): an exceptional member of 3d transition-metal monoxides Fengmiao Li, Yuting Zou, Myung-Geun Han, Kateryna Foyevtsova, Hyungki Shin, Sangjae Lee, Chong Liu, Kidae Shin, Stephen D Albright, Ronny Sutarto, Feizhou He, Bruce Davidson, Frederick Walker, Yimei Zhu, Zhigang Cheng, Ilya Elfimov, George Albert Sawatzky, Ke Zou The 3d transition-metal monoxides (3d-TMOs) with rock-salt structure, including TiO, VO, (CrO), MnO, FeO, CoO, NiO, and (CuO), have been studied for decades for their strongly correlated electrons. Besides the non-stable CrO and CuO, titanium monoxide (TiO), an important member of 3d-TMOs, has not been studied in the stoichiometric single-crystal form. It has been challenging to prepare stoichiometric TiO due to the highly reactive Ti2+. We report the successful growth of single-crystalline TiO(001) film using molecular beam epitaxy. This enables a first-time study of stoichiometric TiO in the thin film form, showing TiO is metal but in proximity to Mott insulating state. Remarkably, we also observe a transition to the superconducting phase at a low temperature. Our findings leave single-crystalline TiO film the only metal and superconductor among 3d-TMOs. |
Friday, March 19, 2021 1:06PM - 1:18PM Live |
Y41.00009: Thermodynamic and optoelectronic properties of ultra-thin-film crystalline and amorphous selenium from a first-principles investigation Sajib Kumar Barman, Muhammad Huda, Jonathan Asaadi, Elena Gramellini, David Nygren Selenium is a vital earth-abundant and non-toxic material that has been used for many practical applications for decades. Both experimental and theoretical investigations are going on the different selenium phases across the scientific communities for understanding its thermodynamic, electronic, and optical properties. However, stabilizing the amorphous phase of it and the role of various defects in crystalline (c-Se) and amorphous (a-Se) phases still warrant rigorous studies. In this case, the first-principles based calculations can provide a robust understanding of how crystalline to amorphous phase transition occurs and pinpoints a practical route to stabilize it in the amorphous phase for its most promising application as a photo-sensitive material for efficient photodetection technologies. We present our density functional theory (DFT) based investigations for both c-Se and a-Se, focusing on the thermodynamic and optoelectronic properties in this work. Our ab initio molecular dynamics studies reproduce the experimentally observed structural properties of a-Se. DFT studies on (100) surface of trigonal selenium show promising optical properties compared to the pure bulk phase's optical properties. |
Friday, March 19, 2021 1:18PM - 1:30PM Live |
Y41.00010: Tip-enhanced Photoluminescence of Freestanding Lateral Heterostructures Abdallah Albagami, Sharad Ambardar, Dmitri Voronine Two-dimensional (2D) materials have promising applications that are based on the tunability of their optical and electronic properties. We investigated the photoluminescence (PL) signals of freestanding lateral WSe2-MoSe2 heterostructures transferred from a Si/SiO2 substrate onto a TEM copper grid under the influence of strain exerted by a scanning probe microscope (SPM) gold-coated plasmonic tip. We studied the nanobubbles formed in heterostructures by nanoindentation as a function of the tip-sample distance. We investigated the near-field tip-enhanced PL (TEPL) signals from both MoSe2 and WSe2 nanobubbles and their junction heterostructures. The observed tunability and nanoscale control open new possibilities to control the performance of optoelectronic nanodevices. |
Friday, March 19, 2021 1:30PM - 1:42PM Live |
Y41.00011: The quantum physical reality of polar-nonpolar oxide heterostructures Summayya Kouser, Sokrates T Pantelides Conducting interfaces between polar and nonpolar insulating oxides, e.g., LaAlO3/SrTiO3 [1], have generated interest for both fundamental physics and oxide-electronics applications. Current understanding is based on an amalgamation of a classical electrostatic model (polar catastrophe model) that was originally derived for semi-infinite solids and quantum density-functional-theory (DFT) results on ultrathin films. Here we report comprehensive DFT calculations that unveil a very different purely quantum physical reality. We first reassess the PCM’s foundations and utility. We show that, for ultrathin polar films, the interfacial dipole does not control the electrostatic potential in the polar film -- the surface and interface play equal roles, and the absence or presence of centrosymmetry in the physical LAO film result in different, purely quantum mechanisms for the generation of a conducting interface, neither involving physical-charge transfer. Our conclusion regarding the origin of the conducting 2DEG is supported by SHG data [2]. Predictions are made that can be tested and can guide technology development. |
Friday, March 19, 2021 1:42PM - 1:54PM Live |
Y41.00012: Ammonia formation on hexagonal Molybdenum Nitride by Langmuir−Hinshelwood Mechanism: a DFT study Muhammad Sajid, William Kaden, Abdelkader Kara By 2030, Ammonia production, worldwide is estimated to increase to 280 million tons. Ammonia is industrially synthesized by Iron-based catalysts through energy intensive Haber-Bosch Reaction. More efficient Ru-based catalysts are also theorized but they are very costly compared to Iron ones. In recent years, scientific community has shown great interest in Molybdenum Nitride as a promising catalyst for ammonia production. Using Density Functional Theory (DFT), we studied the reaction steps involved in the production of Ammonia in Langmuir− Hinshelwood Mechanism. Adsorption characteristics of N2, H2, NH, NH2 and NH3 are explored on hexagonal MoN surface. Using Transition State Theory, we identified possible routes for dissociation of H2 and N2 molecules and hydrogenation of NHx-species (0<x≤2). Role of hydrogen coverage on the surface in minimizing transition barriers was also studied. Our calculations predict that presence of H atoms on the surface decreases the large barriers present for hydrogenation of N species. Our work will contribute to the search of the most suitable catalyst for a very important chemical reaction. |
Friday, March 19, 2021 1:54PM - 2:06PM Live |
Y41.00013: Chemical-state specific atomic mapping of MoOx catalyst on α-TiO2(110) using X-ray standing wave excited XPS Anusheela Das, Yanna Chen, Devika Choudhury, Leighton Jones, Denis Keane, Anil U Mane, Jeffrey W Elam, George C Schatz, Michael J Bedzyk X-ray standing wave (XSW) excited photoelectron emission was used to study the atomic-scale structure of a one monolayer molybdenum oxide catalyst supported on a rutile TiO2(110) single crystal substrate. The experiments were performed at the Advanced Photon Source DND-CAT 5IDC beamline. The modulations in the Mo 2p and O 1s XPS signals were monitored while scanning through 3 different substrate Bragg peaks. This was done for different stages of a redox reaction allowing us to track the surface sites for chemically distinct Mo and O atoms, important for understanding their catalytic behaviour. Mo6+ and Mo4+ 2p XPS peaks were found to be separated by 1 eV and have slightly different XSW hkl Fourier amplitudes and phases. Summation of these Fourier components generates chemical-state sensitive 3D atomic maps of Mo and O. These results are compared with DFT calculations to give us a more complete picture of this catalytically relevant interface. |
Friday, March 19, 2021 2:06PM - 2:18PM Live |
Y41.00014: Confined Polymer Between Rough Silica Surfaces: An Atomistic Simulation Study Petra Bacova, Wei Li, Pritam Kumar Jana, Craig Burkhart, Patrycja Polinska, Manolis Doxastakis, Marcus Mueller, Vagelis Harmandaris We use atomistic molecular dynamics simulations to get an insight on the behavior of polybutadiene and polyisoprene films, confined between realistic amorphous silica surfaces. Characteristic polymer conformations and relaxation times are analyzed as a function of polymer-surface separation, chain length and the thickness of the polymer film. Particular attention is paid to the estimation of the width of the polymer/silica interphase from static as well as dynamical properties. In contrast to the polymer behavior on ideal perfectly planar surfaces, the perpendicular as well as the lateral motion of the polymer chains in the vicinity of the rough surfaces is altered. |
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