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 X41: Thin Film Growth and ProcessingLive
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Sponsoring Units: DCMP Chair: Jacques Amar, University of Toledo |
Friday, March 19, 2021 8:00AM - 8:12AM Live |
X41.00001: Synthesis and Characterization of Nitrogen-substituted Nickel Oxide Simon Godin, Ilya Elfimov, Fengmiao Li, Bruce Davidson, Ronny Sutarto, George Albert Sawatzky, Ke Zou Nickel monoxide (NiO) displays emergent properties when substituted by other atoms. We present here the synthesis of NiO thin films with low substitution concentrations of oxygen (O) by nitrogen (N), using NO gas-assisted molecular beam epitaxy method. This technique allows us to have a fine control over the N-doping concentration and maintain a high crystallinity in the films. We measure the changes in the electronic structures of these films introduced by N doping, using x-ray absorption and x-ray photoelectron spectroscopies. We identify a strong interaction between the N and Ni atoms. This leads to drastic changes in the electronic states, magnetic ordering, and conductivity of the films. |
Friday, March 19, 2021 8:12AM - 8:24AM Live |
X41.00002: Effects of short-range attachment barriers on island nucleation in irreversible growth Sameer S Hamadna, Indiras Khatri, Ehsan Sabbar, Jacques Amar A variety of effects can lead to short-range attachment barriers in submonolayer growth. While it has been predicted that the exponent χ describing the dependence of the island density on deposition rate F crosses over from the diffusion-limited (DL) value i/(i+2) (where i is the critical island size) in the absence of a barrier to the attachment-limited (AL) value 2i/(i+3) for a strong attachment barrier, this prediction has not been confirmed. Furthermore the dependence of the effective value of χ on barrier strength and ratio R = D/F (where D is the monomer hopping rate) has not been studied. Here we present the results of kinetic Monte Carlo (KMC) simulations for the case of deposition on a 2D substrate and irreversible growth corresponding to i = 1. In particular, we have studied the case of a barrier to island nucleation and attachment as well as that of a barrier to island attachment only. In both cases the effective value of χ increases with increasing R to a peak value χpk(Rpk) larger than 1/3 before decreasing very slowly towards the DL value. In addition, both χpk and Rpk increase with increasing barrier strength. A self-consistent rate-equation approach is also presented and good agreement is found with our KMC simulations for all values of barrier strength and D/F. |
Friday, March 19, 2021 8:24AM - 8:36AM Live |
X41.00003: Fast Diffusive Behavior of Pb on Ge(111) at Low to Room Temperatures During Island Formation Andrew Rakyoung Kim, Eli Baum, Shirley Chiang, Michael C Tringides, Andre Childs, Duy Le, Talat Rahman Lead deposited on Ge(111) at low temperatures (<260K) was found to show unusual collective diffusion behavior upon heating towards room temperature. As the sample was heated, Pb was seen diffusing from high coverage regions into a region of low Pb coverage, corresponding to islands forming from the wetting layer. Experiments studied the number density and island size as a function of coverage and temperature, as well as interaction with the electron beam, which was found to form regions with less Pb coverage. Eventually the regions of low coverage filled with enough Pb to form a uniform layer. Similar behavior was seen with Pb on Si(111) at low temperatures, with island formation whose heights were determined by quantum size effects,[1] as well as unusually fast diffusion speed.[2] In order to explain the diffusing behavior of Pb, we also present a first-principles study of the structure of Pb overlayers on Ge(111) using DFT calculations, which were inspired by experimental observations of Pb/Ge(111) phases.[3] |
Friday, March 19, 2021 8:36AM - 8:48AM Live |
X41.00004: On the crystal growth and magnetism of Fe on MgO and MgAl2O4 Anna Ravensburg, Gunnar Karl Pálsson, Björgvin Hjörvarsson, Vassilios Kapaklis We report on the epitaxial growth of thin Fe layers on single-crystalline MgAl2O4 substrates. Compared to Fe thin films grown on single-crystalline MgO substrates, exhibiting strain induced by the 4% lattice mismatch, sputter-deposited Fe films on single-crystalline MgAl2O4 substrates have a lattice mismatch of around 0%. Their structural and magnetic properties are systematically analyzed for different deposition parameters and Fe layer thicknesses and compared to identically grown films on MgO substrates. The results of this study show that Fe thin films deposited on MgAl2O4 substrates exhibit improved crystal quality resulting in the observation of clear Laue oscillations around the Fe(002) peak as well as an approximately 40 times smaller FWHM of the rocking curve compared to Fe grown on MgO. Fits of the Laue oscillations suggest that over 91% of these 50 and 100 Å thick Fe layers scatter coherently. Investigations of the magnetic properties show smaller saturation fields for Fe deposited on MgAl2O4 resulting from the low stress in the Fe lattice. Such Fe films may potentially find application in devices which depend strongly on the overall crystal structure and/or interface quality. |
Friday, March 19, 2021 8:48AM - 9:00AM Live |
X41.00005: Design and Engineering of Gibbs Free Energy for Heterostructure Nano-Bonding™ of Piezo-Electric Crystals to Si-based materials at T < 500 K Mohammed Sahal, Abbie Elison, Shefali Prakash, Srivatsan Swaminathan, Riley Rane, Brian Baker, Thilina Balasooriya, Wesley Peng, Aashi R Gurijala, Nikhil Suresh, Lauren Puglisi, Robert J Culbertson, Nicole Herbots Forming heterostructures between piezo-electrics such as LiTaO3 and LiNbO3 with Si-based materials is limited by the mismatch of their lattice constants and thermal expansion. |
Friday, March 19, 2021 9:00AM - 9:12AM Live |
X41.00006: An efficient numerical method for constructing heterojunctions between two crystal lattices having the same in-plane lattice parameter ratio Weon-Gyu Lee, Jung-Hoon Lee Important physical properties of two-dimensional (2D) electronic devices are mainly determined by interface geometries in heterojunctions. Here we propose a new numerical method for efficiently constructing heterojunctions between two different crystal lattices having the same in-plane lattice parameter ratio such as a hexagonal graphene-hexagonal MoS2 heterostructure. The bases of two 2D lattices 1, 2 ((αmx, αmy), m=1, 2, α=a, b) can be regarded as the 2D complex vector l=(za zb) where (zα=αmx+iαmy) in the complex plane. Then, the transformation matrix between two lattices and the complex lattice bases satisfy the eigenvalue problem. We find that a ratio of two lattice matrices c=|a1x/a2x| should be the same as the ratio of norms (h2+Dk2) of complex quadratic integers p=h+k√(-D) (h, k are integer, D>0) when two lattices form a commensurate heterostructure. Consequently, using the ratio table, the smallest commensurate heterojunction can be efficiently constructed by finding a quadratic integer without scanning all possible supercells. This is significantly more time-efficient (~O(log n) where n is the maximum value of the transformation matrix elements) than the currently widely used algorithm for construing heterojunctions (~O(n4)). |
Friday, March 19, 2021 9:12AM - 9:24AM Live |
X41.00007: The competition of phase transitions and growth in thin films Anna Werkovits, Johannes Cartus, Lukas Hörmann, Andreas Jeindl, Oliver T. Hofmann Growth processes of organic thin films are strongly influenced by the interplay of thermodynamics and kinetics. Following Ostwald’s rule of stages, metastable structures often form first during the deposition process. Transitions to more stable structures may be prevented by fast aggregation and growth of the thermodynamically less stable seed, leading to a kinetically trapped polymorph. The selective growth of specific polymorphs, therefore, requires balancing growth and phase transitions via appropriately chosen deposition conditions. This, in turn, is only possible if the corresponding transition rates are known. |
Friday, March 19, 2021 9:24AM - 9:36AM Live |
X41.00008: Electrochemical Deposition of Mn+3 rich δ-MnO2 on Epitaxial Graphene-Silicon Carbide Substrates for Selective Gas Sensing Michael Pedowitz, Soaram Kim, Balaadithya Uppalapati, Digangana Khan, Ferhat Bayram, Goutam Koley, Kevin Daniels Mixed valence manganese oxides (MnOx) have attracted significant research interest due to the easily reversible redox reactions between manganese oxidation states (Mn+2, Mn+3, Mn+4)1. Of particular interest among these compounds is the layered(δ) phase of MnO2 due to its tunable interlayer spacing and high Mn+3 content2. These Mn+3 sites are desirable as they are highly reactive and improve the catalytic activity of MnO2 structures3. However, the speed of these redox reactions is sharply limited by the low electronic conductivity of MnO2. To counteract this, we electrodeposited δ-MnO2 on quasi-freestanding bilayer epitaxial graphene (EG)-silicon carbide substrates. The resulting junction between the EG and the δ-MnO2 layers leads to improved charge transfer between the materials, improving the reactive speed of δ-MnO2. We confirmed this by constructing a four-contact gas sensor from the resulting heterostructure, which demonstrated a high selectivity towards nitrogen-based gases and an enhanced response/recovery time. We also confirmed the Mn+3 rich structure through Raman spectroscopy and X-ray photoelectron spectroscopy. |
Friday, March 19, 2021 9:36AM - 9:48AM Live |
X41.00009: Kinetic Processes and surfactant design of Group I
elements on CZTS (1\bar1\bar2\bar) surface Kejie Bao, Haolin Liu, Kinfai Tse, Junyi Zhu Cu2ZnSnS4 (CZTS) is a promising thin-film solar-cell material consisted of earth abundant and nontoxic elements. Yet, there exists a fundamental bottle neck that hinders the performance of the device due to complexed intrinsic defects properties and detrimental secondary phases. Recently, it was proven experimentally that Na and K in co-evaporation growth of CZTS can enlarge the grain size and suppress ZnS secondary phase near surface, but the reasons are not well understood. We used first principle calculations to investigate the kinetic processes on CZTS (1\bar1\bar2\bar) surface involving Group I elements, including Na, K, and Cs, to demonstrate their surfactant effects. Both the structure of the reconstructed surfaces involving Group I elements and various diffusion paths of a Zn ad-atom in these reconstructed surfaces were explored. The advantages and concerns of the surfactant effects of Na, K, Cs, were systematically compared and discussed. Also, a disordered metastable configuration with a diffusion barrier of about 400meV was found. In addition, our studies provide a systematical design principle for surfactant effects during the growth. |
Friday, March 19, 2021 9:48AM - 10:00AM Live |
X41.00010: Evolution of metastable structures on bimetallic surfaces from microscopy and machine-learning molecular dynamics Jin Soo Lim, Jonathan Vandermause, Matthijs A. van Spronsen, Albert Musaelian, Yu Xie, Lixin Sun, Christopher O'Connor, Tobias Egle, Nicola Molinari, Jacob Florian, Kaining Duanmu, Robert J. Madix, Philippe Sautet, Cynthia Friend, Boris Kozinsky Restructuring of interfaces plays a crucial role in materials science and heterogeneous catalysis. In particular, bimetallic surfaces often adopt very different composition and morphology compared to the bulk. For the first time, we reveal a detailed long-timescale restructuring of Pd deposited on Ag by combining microscopy, spectroscopy, and novel simulation methods. We develop and perform accelerated machine-learning molecular dynamics, followed by automated mechanistic characterization, to discover surface restructuring events in an unbiased fashion, including Pd-Ag place exchange and Ag pop-out. We find that encapsulation of Pd islands by Ag always precedes layer-by-layer dissolution of Pd into Ag, resulting in significant migration of Ag out of the surface and leaving behind extensive vacancy pits within microseconds. Because Ag-encapsulated Pd remains much more accessible to reactants than bulk-dissolved Pd, these metastable structures are of vital catalytic importance. Systematic investigation of other bimetallic systems (e.g. Pd/Au, Pt/Ag, Co/Pt) and adsorbate-induced restructuring (e.g. CO, H2) is underway. |
Friday, March 19, 2021 10:00AM - 10:12AM Live |
X41.00011: Atomic description of the atomic layer deposition of diethylzinc and water on carboxyl functionalized carbon nanotubes: a first principle study. José Israel Paez Ornelas, Hector Noé Fernández Escamilla, Hugo Alejandro Borbón-Nunez, Hugo Tiznado, Noboru Takeuchi, Jonathan Guerrero Sanchez In this work, we describe -at the atomic level- the full reaction mechanism involved in the formation of a hybrid inorganic ZnO/CNT structure. The chemically inert CNTs are activated with a carboxylic acid. Diethylzinc (DEZ) and water are used as gas-phase precursors to form ZnO. Our findings show that DEZ is physically adsorbed on the CNT; after a ligand-exchange mechanism involving overcome an energy barrier of 0.06 eV, ethyl zinc and ethane are formed. The energy barrier is minimal compared with previous values of 0.6 eV for OH functionalized surfaces and is associated with the CO group that mediates DEZ's proximity. |
Friday, March 19, 2021 10:12AM - 10:24AM Live |
X41.00012: Tuning the phase structure of multicomponent PVD glasses by controlling surface mobility Marie Fiori, Kushal Bagchi, Sebastian Schneider, Zhenxuan Chen, Lian Yu, Michael Toney, Mark Ediger Physical vapor deposition (PVD) leverages the mobility at the free surface of a glass to create glassy films with exceptional properties beyond those of a liquid-cooled glass. As PVD continues, subsequent material locks the surface molecules into the bulk glass. Deposition conditions that affect the mobility at the surface control the structure of the PVD glass. We expect surface mobility in multicomponent PVD glasses to affect their structure and morphology. Here, we deposit two organic semiconductors, Alq3 and DSA-Ph, into multicomponent films. Using GIWAXS and GISAXS, we find that we can control the distribution of each material within the PVD glass by controlling the deposition conditions. A multicomponent glass of a set composition can be prepared as either a well-mixed glass or with phase separated regions rich with either Alq3 or DSA-Ph. When Alq3 and DSA-Ph are deposited in alternating small amounts, we can create either well-defined layers of each material or 3D nanodomains within the glass. We hypothesize that 3D Alq3/DSA-Ph nanodomains are created if a small amount of one material is not able to wet a film before the second material is deposited. This kinetically traps the structure of the organic islands formed on the substrate in the first monolayers of deposition. |
Friday, March 19, 2021 10:24AM - 10:36AM Live |
X41.00013: Superhydrophobicity induced by CO2 plasma treatment of Magnesium Sinchul Yeom, Gyoung Gug Jang, Mina Yoon Poor corrosion resistance of light weight magnesium (Mg) is one of the major bottlenecks limiting broad application of Mg alloy for lightweight vehicle frame, aircraft component and bio-engineering implants application. Here we report a new protective coating approach by an atmospheric, room temperature CO2 plasma (CO2-AP) technique. The CO2-AP treatment demonstrates the formation of Mg carbonate, superhydrophobic anticorrosion layer on bare Mg surface resulting in more than 10-fold reduction in the corrosion rate, compared to the untreated Mg specimens. Surface characterizations such as XPS, SEM, TEM-EDX indicate the CO2-AP forms nanostructured MgO, MgCO3, and carbon layers on the Mg surface. Because MgCO3 and MgO are known as hydrophilic, it is likely that the hydrophobic carbon layer leads to superhydrophobicity. We test our hypothesis by performing molecular dynamic (MD) simulations based on force-field potentials that are well established in the field. In particular, we investigate the fundamental mechanisms of interaction between water nanodroplets and different substrates at a finite temperature. They provide insight into the understanding of superhydrophobicity at the atomistic level. Our results should be instrumental in the development of new superhydrophobic surfaces. |
Friday, March 19, 2021 10:36AM - 10:48AM Live |
X41.00014: Mechanism of Stacking Fault Formation in Metal(100) Heteroepitaxial Growth Indiras Khatri, Ehsan Sabbar, Yunsic Shim, Jacques Amar Strain in metal(100) heteroepitaxial thin-film growth can lead to a variety of effects such as the formation of stacking faults (SFs). To gain a better understanding we have carried out temperature-accelerated dynamics simulations of the submonolayer growth of Cu islands on a biaxially strained Cu substrate at 200 K. In the case of 4% compressive strain we find that SF formation occurs with a morphology very similar to that found experimentally in Cu/Ni(100) growth. We also find that islands play a key role by lowering the barrier for vacancy formation. In addition, once two substrate vacancies are formed and diffuse to form the appropriate configuration this leads to the formation of a SF in both the substrate and island. While the activation barrier for SF formation is very high, due to the presence of a large number of low-frequency vibrational modes, the saddle-point entropy is large while the corresponding Vineyard prefactor is more than 14 orders of magnitude larger than typical values. In contrast, due to the large entropy of the SF state the reverse prefactor is much smaller, while the free energy is significantly lower. |
Friday, March 19, 2021 10:48AM - 11:00AM Live |
X41.00015: Epitaxial growth and surface characterization of TiO2 anatase Shrinkhala Sharma, Zheng Ren, Hong Li, He Zhao, Ilija Zeljkovic Titanium dioxide TiO2 is semi-metallic in nature and has several optical and photocatalytic applications. We study the step-by-step formation of anatase phase of TiO2 grown on SrTiO3 (001) using plasma-assisted molecular beam epitaxy (MBE) growth, and explore the effects of growth temperature, substrate choice and lattice mismatch on the growth pattern. Initially, the nucleation of 3D islands in the TiO2 anatase phase is observed, identified by the appearance of transmission spots in the reflection high energy electron diffraction (RHEED) image. Previous studies suggested that these 3D structures may appear due to the migration and self-organization of Sr ions into the epitaxially strained TiO2 layer. Upon further TiO2 deposition, the RHEED pattern evolves into a series of streaks, signaling a smoother stepped surface, with a (4x1) surface reconstruction. Lastly, we study the electronic properties using low-temperature scanning tunneling microscopy/spectroscopy and ex-situ magnetization measurements. Our experiments shed light on the nanoscale structural and electronic information of the films, complementary to the electronic characterization by bulk probes. |
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