Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session F36: DMP Past Chair Symposium on Visualizing EpitaxyInvited
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Sponsoring Units: DMP Chair: Rachel Goldman, University of Michigan; James Rondinelli, Northwestern University Room: Room 236 |
Tuesday, March 7, 2023 8:00AM - 8:36AM |
F36.00001: Uncovering atomic arrangements in epitaxial systems using synergistic computational and experimental approaches Invited Speaker: Wissam A Saidi In nanostructured materials, every atom counts; thus, determining the atomic arrangements is critical for their precise design and fabrication for targeted applications in energy and electronics. However, it is often challenging to unambiguously identify the atomic structure, particularly for heterogenous systems, using either theoretical modeling or experimental imaging alone. In our recent studies, we developed and applied a synergistic computational and experimental approach to precisely map the atomic structure and infer growth mechanisms at the nanoscale. In this talk, I will demonstrate our approach in two different case studies. In the first one, we applied a multi-tier density functional theory (DFT), classical force fields, and genetic algorithms in conjunction with in situ characterization to determine atom-by-atom the structure of supported metal clusters on a two-dimensional substrate. In the second application, we examined the Cu2O nano-island epitaxial growth mechanism using correlated in situ environmental transmission electron microscopy, statistically-validated quantitative analysis, and DFT calculations. We showed that the oxide islands grow layer-by-layer along Cu2O(110) planes, regardless of substrate orientation, contradicting classical models that predict multi-layer growth parallel to substrate surfaces. Growth kinetics show cubic relationships with time, indicating individual oxide monolayers follow Frank-van der Merwe growth, whereas oxide islands follow Stranski-Krastanov growth. Our results demonstrate that epitaxial systems can be “visualized” at the atomic scale using concerted approaches, which is critical for advanced manufacturing at the nanoscale. |
Tuesday, March 7, 2023 8:36AM - 9:12AM |
F36.00002: Dynamic Atomic-Scale Fundamental Mechanisms of the Initial Stages of Cu Oxidation Revealed by Environmental TEM Invited Speaker: Judith C Yang Surface oxidation is an important process for corrosion, which costs a few percent of the U.S. Gross Domestic Product (GDP) each year. Much is known about oxygen interaction with metal surfaces and the macroscopic growth of thermodynamically stable oxides. At present, however, the transient stages of oxidation - from nucleation of the metal oxide to formation of the thermodynamically stable oxide - represent a scientifically challenging and technologically important terra incognito. These issues can only be understood through a detailed study of the relevant microscopic processes at the nanoscale in situ. We have previously demonstrated via in situ transmission electron microscopy (TEM) that the formation of epitaxial Cu2O islands during the transient oxidation of Cu(100), (110), and (111) films bear a striking resemblance to heteroepitaxy, where the initial stages of growth are dominated by oxygen surface diffusion and strain impacts the evolution of the oxide morphologies. To deepen our understanding of the atomic-scale dynamic processes of Cu2O island formation on Cu during oxidation in situ, we are presently using correlated in situ high-resolution environmental TEM (ETEM) and atomistic theoretical simulations. As an example of this approach, preferential monolayer-by-monolayer growth along Cu2O(110) planes was noted instead of along Cu2O(100) planes. Correlated Density Functional Theory (DFT) simulations on the surface and diffusion energies during Cu2O growth on various Cu2O surface orientations and terminations were carried out. Our DFT results show that monolayer formation of Cu2O along Cu2O(110) was both thermodynamically and kinetically preferred over that of Cu2O(100) during Cu2O growth, which explains the observed phenomenon. These results shed new light on the epitaxial oxide growth mechanism and provide a deeper understanding of the dynamic processes involved in initial oxidation, which will ultimately help to precisely predict, design, and control nanostructured oxide growth for either corrosion-protection or creating nano-oxides for their functional properties. |
Tuesday, March 7, 2023 9:12AM - 9:48AM |
F36.00003: Imaging and controlling epitaxial nucleation and growth using ultra high vacuum transmission electron microscopy Invited Speaker: Frances M Ross Transmission electron microscopy offers a unique spatially and temporally resolved view of epitaxial nucleation and growth. Continuous imaging of a sample during deposition can provide quantitative measurements of the evolution of strain fields, dislocations, interface structure and surface configurations. Microscope designs that maintain ultra high vacuum at the sample region enable atomically clean surfaces to be prepared on which reactive materials can be deposited. I will describe the application of these techniques to two types of epitaxial process. I will first discuss chemical vapor deposition of Ge or SiGe on Si, where in situ movies show nucleation events, island shape evolution and the formation and motion of dislocations. I will compare the results with observations made during van der Waals epitaxy of metals on 2D materials such as graphene and MoS2. Evaporation of metals onto UHV-cleaned substrates enables the nucleation sites, equilibrium crystal shapes, interfacial moirés and degree of epitaxy to be quantified. Using patterning and sequential and catalytic deposition, it is possible to form metal and semiconductor nanostructures with complex shapes and compositions and achieve some degree of control over nucleation. Future developments in instrumentation are broadening the scope of in situ electron microscopy to explore the underlying mechanisms of a wider range of epitaxial processes and use the results to create functional epitaxial structures. |
Tuesday, March 7, 2023 9:48AM - 10:24AM |
F36.00004: Real-time TEM observations of III-V nanowire growth by molecular beam epitaxy Invited Speaker: Federico Panciera Here, we present experimental observations of the growth of III-V nanowires using a transmission electron microscope (TEM) equipped with molecular-beam-epitaxy (MBE) sources. Nanowires are grown directly inside the microscope, and their growth is monitored in situ and in real-time with high spatial and temporal resolution. When GaAs nanowires are grown using the vapor-liquid-solid (VLS) method, the stable zincblende (ZB) phase coexists with metastable wurtzite (WZ) structure, resulting in nanowires having a mixed-phase structure. Remarkably, the valence and conduction bands of the two phases are misaligned so that small sections of one phase within the other effectively confine charge carriers. These heterostructures are defined as crystal-phase quantum dots (CPQDs) and have been proposed for several potential applications in photonics and quantum computing. The technological application of CPQDs has been severely limited by the poor understanding of the crystal-phase selection mechanism and the difficulty of controlling their formation. Only recently, thanks to in situ TEM, could we shed new light on the phase selection mechanism and demonstrate how to achieve structural control. In fact, in situ TEM provides unparalleled imaging resolution and allows one to capture in real-time the growth dynamics and the effects of changing growth parameters. By using in situ TEM we demonstrate that the phase selection can be controlled by simply tuning the contact angle. These findings are explained within a dedicated model based on surface energetics and provide a clear route for the crystal phase control in III-V nanowires. |
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