Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Q60: Electron Microscopy Imaging and Spectroscopy of 2D MaterialsFocus Recordings Available
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Sponsoring Units: DMP Chair: Pinshane Huang, University of Illinois at Urbana-Champaign Room: Hyatt Regency Hotel -DuSable C |
Wednesday, March 16, 2022 3:00PM - 3:36PM |
Q60.00001: Picometer-scale characterization of structure, strain and defects in 2D materials using 4D-STEM Invited Speaker: David A Muller Electron microscopy is a widespread and often essential tool for structural and chemical analysis at the atomic level. Image resolution is dominated by the energy (or wavelength) of the electron beam and the quality of the lens. Two-dimensional materials are imaged with low beam energies to avoid damaging the samples, limiting spatial resolution to ~1 Å. A new generation of direct electron detectors have the speed, sensitivity and dynamic range to record the complete momentum distribution of transmitted electrons at every beam position, allowing us to build up complete 4-dimensional phase space maps (4D-STEM). Using ptychographic phase retrieval algorithms to process this data, we have been able to increase the spatial resolution well beyond the traditional lens limitations reaching a 39 Å resolution for MoS2, at the same dose and imaging conditions where conventional imaging modes reach only 0.98 Å [1]. The ultimate limit to spatial resolution in an electron microscope is set by the thermal vibrations of the atoms themselves, which are on the order of 10-20 pm [2]. Using multislice electron ptychography, we are now able to see the details of thermal vibrations of individual atom columns. |
Wednesday, March 16, 2022 3:36PM - 3:48PM |
Q60.00002: Visualizing lattice relaxation and strain fields in twisted bilayer graphene Madeline Van Winkle, Daniel K Bediako, Nathanael P Kazmierczak Moiré superlattices, formed by stacking two-dimensional van der Waals layers with a slight lattice mismatch, have electronic band structures that are highly sensitive to structural changes, such as interlayer twist angle. For example, twisted bilayer graphene (TBG) exhibits unconventional superconductivity and ferromagnetism at a 'magic' interlayer twist angle of 1.1°, associated with formation of flat electronic bands. At the same time, lattice deformations, strain, and disorder can also dramatically influence the behavior observed in these systems. Visualizing the structure and strain fields of moiré materials is therefore paramount to understanding and controlling their emergent electronic behavior. In this talk, I will present work on the development of a technique termed Bragg interferometry, based on four-dimensional scanning transmission electron microscopy (4D-STEM), for directly and quantitatively mapping interlayer atomic displacements, structural relaxation, and strain fields in TBG. The results uncover two regimes of lattice relaxation in TBG based on twist angle, in contrast to previous models depicting one continuous process, as well as intrinsic nanoscale twist angle and strain disorder and unique striped strain phases arising from extrinsic uniaxial heterostrain. This work sheds light on structural changes underpinning the twist angle dependent electronic properties of TBG and provides a framework for visualizing lattice relaxation, disorder, and strain in other moiré materials. |
Wednesday, March 16, 2022 3:48PM - 4:00PM |
Q60.00003: Simultaneous atomic resolution imaging and electrical characterization of 2D quantum devices Joachim Dahl Thomsen, Julian Klein, Frances Ross, Prineha Narang Direct correlation between (opto-/magneto-)electronic properties and structure is critical for the understanding of nanoscale and quantum devices. Recent developments in (scanning) transmission electron microscopy ((S/TEM) allow acquisition of atomic resolution images at cryogenic temperatures while measuring electrical properties. The S/TEM therefore acts as a cryostat with added functionalities of imaging, 4D STEM and electron energy loss spectroscopy to investigate excitations. We present our progress towards in-situ electrical characterization of 2D quantum devices. Electronic properties of 2D materials are commonly measured using graphene/BN/X/BN heterostructures, where graphene is the gate electrode, BN the dielectric, and in our case X=WSe2, MoS2, Cr2Ge2Te6. We use atomic mass contrast to obtain atomic resolution images of the X layers and show that that atomic rearrangements at edges and defects depend on the beam current and acceleration voltage, and that BN reduces radiation damage effects. Finally, we measure the structure of defects in helium ion-irradiated MoS2 and WSe2 devices and perform optical spectroscopy ex-situ. |
Wednesday, March 16, 2022 4:00PM - 4:12PM |
Q60.00004: Identifying Single-Atom Defects of 2D Materials on the Million-Atom Scale via Deep Learning Chia-Hao Lee, Abid A Khan, Di Luo, Chuqiao Shi, Yue Zhang, M. Abir Hossain, Arend M van der Zande, Bryan K Clark, Pinshane Y Huang Aberration-corrected scanning transmission electron microscopy (STEM) is an important tool to study how atomic defects such as vacancies and substitutions impact the structure and properties of 2D materials1-2. Yet, high-precision characterization of defects in 2D materials remains challenging because they are irradiation sensitive, making it difficult to achieve high resolution and signal-to-noise ratio (SNR) measurements without modifying the intrinsic structure. Here, we combine automated acquisition, class averaging, and machine learning to acquire and analyze large datasets of STEM images of 2D materials. We analyzed an atomic resolution, ~million atom dataset to determine the precise atomic structures and spatial distributions of 7 different types of point defects in a 2D transition metal dichalcogenide (TMDC). By summing thousands of images from nominally identical defects, we improve the measurement precision to ~0.3 pm, sufficient to detect pm-scale strain field oscillations from a single atom defects up to ~1 nm away. This technique also allows us to observe how defect clusters interact with each other through coupled strain fields. |
Wednesday, March 16, 2022 4:12PM - 4:24PM |
Q60.00005: Using Cycle-GANS to Generate Realistic STEM Images for Defect Identification Abid A Khan, Chia-Hao Lee, Pinshane Y Huang, Bryan K Clark Identifying atomic defects in aberration-corrected scanning transmission electron microscopy (STEM) data is critical to understanding the structure and properties of 2D materials. Recent advances in machine learning techniques now allow very fast defect identification in STEM imaging. The training sets for these machine learning models are constructed with simulation codes that replicate realistic data via the manual addition of Gaussian noise, probe jittering, image shear, and background contamination. This procedure is not only time consuming, but the manual tuning of noise may not cover all the realistic factors of the experimental data. We present an alternative approach to generating realistic STEM images by employing a cycle-GAN to automatically add realistic noise to simulated data. In a cycle-GAN, the noise present in the experiment STEM images are "transferred" over to the simulated images. We train our defect-identification model using these generated images and evaluate the model on real STEM images to locate atomic defects within them. The application of Cycle-GAN removes the need for human intervention in the machine-learning workflow allowing for higher throughput results as well providing other machine learning models with more realistic data for any type of supervised learning. |
Wednesday, March 16, 2022 4:24PM - 4:36PM Withdrawn |
Q60.00006: Structural transformations creating atomistic spin textures on-demand in the van der Waals layered magnet CrSBr Thang Pham, Julian Klein, Joachim Thomsen, Jan Luxa, Zdenek Sofer, Frances Ross The observation of long-range magnetic order in few-to-single layers of two-dimensional van der Waals (vdW) magnets has sparked new interest in studying low-dimensional magnetism. However, the air-sensitivity of many 2-D magnets, such as CrI3, hinders nanoscale investigations and potential applications. Recently, an air-stable 2-D magnet, CrSBr, has been rediscovered. It is a semiconductor with a bandgap of approximately 1.6 eV and a high Néel temperature of 145 K. It shows strong light-matter interaction, making it an ideal candidate to study magneto-transport and magneto-optical excitations at the atomically thin limit. The air-stable nature of CrSBr enables exciting opportunities for atomistic measurements and manipulation. |
Wednesday, March 16, 2022 4:36PM - 4:48PM |
Q60.00007: Direct Imaging of Step-induced Phonon Softening with 4D Ultrafast Electron Microscopy Yichao Zhang, David J Flannigan The abrupt lattice termination at surface steps leads to modulation of local electronic, vibrational, and structural properties via edge atom relaxation. [1-3] However, relevant spatiotemporal scales are such that resolving ultrafast structural dynamics at individual steps is challenging. Here, we report direct imaging of the step-induced softening of photoexcited phonons in MoS2 using ultrafast electron microscopy (UEM). [4] The sensitivity of UEM to diffraction contrast dynamics allows us to resolve few-percent reductions in phonon frequency spanning tens of nanometers laterally away the step, displaying an upward exponential behavior. This arises from a combination of anisotropic bonding inherent to layered materials and incoherent photoinduced atomic displacements at the discontinuity. The behavior is in quantitative agreement with a finite element transient deformation model. The results provide new insights into the structure-function relationships of defect-sensitive materials on the atomic scale. |
Wednesday, March 16, 2022 4:48PM - 5:00PM |
Q60.00008: Anisotropic Photoinduced Lattice Dynamics of Black Phosphorus Revealed by Reflection Ultrafast Electron Diffraction Mazhar Chebl, Xing He, Ding-Shyue Yang Black phosphorus (BP) is emerging as a fascinating two-dimensional (2D) material given its anisotropic structure and optical, electrical, and mechanical properties, which have been readily observed through steady-state measurements of, e.g., absorption, carrier and thermal transports. Such anisotropic behavior is also prominent on ultrafast time scales following photoexcitation, which is distinct among 2D materials. Detailed knowledge of photoinduced responses in BP is important to the understanding of the material’s relaxation pathways in various degrees of freedom. However, to date, most reports of ultrafast dynamics have been focused on the in-plane components, whereas the out-of-plane counterpart remains unclear. In this presentation, we shed light on the photoinduced BP dynamics in the out-of-plane direction using ultrafast electron diffraction in reflection geometry as a direct structure-probing technique, whose results are cross-examined with those by optical transient reflectivity. It is found that following an optical excitation by 515-nm photons, the large excess energy of photoinjected carriers lead to an interlayer lattice compression at ultrashort times, which appears to be somewhat independent of the laser fluence used. We further show that the electron-phonon coupling takes place within 1 ps, followed by a slower relaxation process on the order of few tens of picoseconds to reach a thermalized phonon bath. A schematic of the different temporal regimes and the corresponding physical processes, as well as the production of coherent acoustic phonons moving into the bulk, will also be discussed. |
Wednesday, March 16, 2022 5:00PM - 5:12PM |
Q60.00009: Atomic-resolution imaging and characterization of ferroelectric domain walls in two-dimensional In2Se3 Edmund Han, Shahriar M Nahid, Yue Zhang, Tawfiqur Rakib, Gillian Nolan, Andre Schleife, Elif Ertekin, SungWoo Nam, Arend M van der Zande, Pinshane Y Huang Two-dimensional (2D) ferroelectric materials have garnered interest for their applications in nanoscale electronics and memory storage devices [1]. α-In2Se3 was recently discovered to exhibit strong out-of-plane ferroelectricity [2], ideal for these next-generation devices. In our work, we study the atomic structure and electronic properties of domain walls in α-In2Se3 using scanning transmission electron microscopy (STEM), piezoelectric force microscopy (PFM), and density functional theory (DFT). We identify two main types of ferroelectric domain boundaries in α-In2Se3: transverse and lateral domain walls, which respectively are perpendicular and parallel to the basal plane. By creating buckled structures, we find that we can induce polarization switching via mechanical deformation, forming transverse domain walls at highly localized bends. Using 4D-STEM, we measure the charge density and width of the domain walls, and we show that lateral domain walls are strongly charged. Finally, we also show that the lateral domain wall can readily travel between the In2Se3 layers to tune the overall polarization of a multilayer flake. |
Wednesday, March 16, 2022 5:12PM - 5:24PM |
Q60.00010: Cryogenic Transmission Electron Microscopy Analysis and Characterization of 2D Metal-Organic Hybrid MXenes Francisco J Lagunas Vargas, Chenkun Zhou, Dmitri V Talapin, Robert F Klie MXenes are a large and fast-growing family of two-dimensional functional materials. MXenes (where M is transition metal, X is either carbon or nitrogen and T is a surface group) owe their large design space, and consequently their broad variety of properties, to a wide range of chemical modifications possible in M and T groups. In this contribution, we probe a new region in MXene design space by presenting a study of MXenes with organic surface groups. To conduct our analysis at electron beam dosages required for high-resolution imaging and spectroscopy we will borrow elements of CryoTEM, namely cooling our samples in-situ to liquid nitrogen temperatures. The primary instrument we will utilize is an aberration-corrected cold field emission JEOL ARM200CF operating at 200kV primary electron energy. We find that unlike MXene polymer composites which form loosely connected wavey sheets, organic terminated MXenes are highly ordered with detectable structure within organic regions. |
Wednesday, March 16, 2022 5:24PM - 5:36PM |
Q60.00011: Atomically resolved phonons localized at defects in monolayer graphene Deliang Bao, Mingquan Xu, Aowen Li, Stephen J Pennycook, Sokrates T Pantelides, Wu Zhou Phonons at defects in materials have long attracted interest, not only because defects are inevitable in materials preparation, but also because defects can be used to engineer desirable properties. Advances in scanning transmission electron microscopy have recently enabled electron-energy-loss spectroscopy (EELS) with energy resolution similar to that of optical spectroscopies, with the extra advantage of atomic-scale spatial resolution. Here we report atomic-resolution EELS of phonons localized at various impurities in graphene, mapping the energy-loss intensity at the surrounding atomic shells, and analyze them using density-functional-theory (DFT) calculations of phonon densities of states (DOS) projected on individual atoms. The agreement between DOS and EELS enables the identification of the observed phonons and provides insights in the role of the defect atomic structures and local symmetries in controlling the localization of the various phonon modes. This research lays the groundwork for experimental measurements and theoretical understanding of phonons associated with defects that can play a role in thermal properties of materials such as thermal expansion, thermal conductivity etc. |
Wednesday, March 16, 2022 5:36PM - 5:48PM |
Q60.00012: Spin−orbit induced magnetic modulation at the V5Se8 / NbSe2 van der Waals heterostructures Hideki Matsuoka, Masaki Nakano, Stewart E Barnes, Jun'ichi Ieda, Sadamichi Maekawa, Mohammad S Bahramy, Bruno S Kenichi, Yukiharu Takeda, Hiroki Wadati, Yue Wang, Satoshi Yoshida, Kyoko Ishizaka, Yoshihiro Iwasa A van der Waals (vdW) heterostructure provides an indispensable material platform in modern condensed-matter researches. There, weak interlayer bonding nature ensures formation of an atomically abrupt heterointerface beyond fundamental constraint imposed by lattice matching condition, while strong electronic coupling enables creation of an emergent electronic ground state that is missing in individual materials. The most studies on the vdW heterostructures have been made by the top-down approach, exfoliation, pick-up, and dry-transfer. On the other hand, the bottom-up approach by MBE has remained almost totally undeveloped, probably due to difficulties in fabrication of high-enough quality samples. However, MBE should enable fabrication of a few layer samples even for hardly-cleavable materials as well as thermally-metastable compounds, and therefore, the MBE-based approach should be in principle extremely powerful and important for broadening the scope of vdW heterostructures. |
Wednesday, March 16, 2022 5:48PM - 6:00PM |
Q60.00013: Investigation of van der Waals antiferromagnet MnPS3 using X-ray Magnetic Linear Dichroism (XMLD). Tianye Wang, Qian Li, Mengmeng Yang, Christoph Klewe, Padraic Shafer, Alpha T N'Diaye, Chanyong Hwang, Zi Q. Qiu As compared to ferromagnetic van der Waals (vdW) materials, there has not been many direct measurement on the antiferromagnetic vdW materials. In this talk, we present our experimental result on the bulk van der Waals antiferromagnet MnPS3 using element-resolved X-ray Magnetic Linear Dichroism (XMLD). By taking the X-ray absorption spectra (XAS) at the Mn L-edge absorption energies at various temperatures and with two orthogonal linear polarizations of the x-rays, we show that the luminescence yield (LY) indicates a neat shape of XMLD while total electron yield (TEY) indicates charging effect due to its semiconductor energy gap. There exists a non-zero XMLD effect due to the Mn antiferromagnetic order. Temperature-dependent XMLD indicates that the Neel temperature of MnPS3 is around ~78K, consistent with previous reports. XAS shows that all Mn elements in bulk MnPS3 are at Mn2+ state. |
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