Showing 1–49 of 49 results for author: Lupini, A

A versatile generalized digital twin for Electron Microscopy

Authors: Thomas W. Pfeifer, Andrew R. Lupini, Eric R. Hoglund

Abstract: Development of specialized imaging and spectroscopy states in transmission electron microscopy is needed for novel applications, such as flexible momentum-resolved high energy-resolution spectroscopy. This task is complicated by the need to align and configure many different lenses, and by ambiguity as to the locations of various imaging and diffraction planes, which are often not well documented.… ▽ More Development of specialized imaging and spectroscopy states in transmission electron microscopy is needed for novel applications, such as flexible momentum-resolved high energy-resolution spectroscopy. This task is complicated by the need to align and configure many different lenses, and by ambiguity as to the locations of various imaging and diffraction planes, which are often not well documented. Here we develop a versatile digital twin for simulating the electron beam trajectory throughout an electron microscope. Our code package contains simple tools for modeling the microscope column, and we present multiple procedures for dialing in precise lens locations and calibrating lens models. This enables use of the model as a predictive tool, allowing the user to quickly and easily determine the correct lens values for setting up new condenser or projector modes. Automatic procedures to change lens settings and measure the resulting changes can be used to close the loop. With the accelerating development of machine learning and artificial intelligence tools, we also believe a physically-informed model of the microscope can serve as a valuable tool for automated microscopy and AI/ML integration. △ Less

Submitted 31 July, 2026; originally announced July 2026.

Inference-Sufficient Representations for High-Throughput Measurement: Lessons from Lossless Compression Benchmarks in 4D-STEM

Authors: Ondrej Dyck, Andrew R. Lupini, Albina Borisevich, Miaofang Chi, Rama K. Vasudevan, Stephen Jesse

Abstract: Four-dimensional scanning transmission electron microscopy (4D-STEM) generates multi-gigabyte datasets, creating a growing mismatch between acquisition rates and practical storage, transfer, and interactive visualization capabilities. We systematically benchmark 13 lossless compression implementations across 5 representative datasets (8~MiB to 8~GiB, 49.5--92.8\% sparsity), with 10 independent run… ▽ More Four-dimensional scanning transmission electron microscopy (4D-STEM) generates multi-gigabyte datasets, creating a growing mismatch between acquisition rates and practical storage, transfer, and interactive visualization capabilities. We systematically benchmark 13 lossless compression implementations across 5 representative datasets (8~MiB to 8~GiB, 49.5--92.8\% sparsity), with 10 independent runs per method. HDF5 provides built-in gzip compression, of which gzip-9 typically achieves the highest compression ratio but is slow. We therefore evaluate widely available alternatives (via hdf5plugin), including the Blosc family. As a representative comparison, blosc\_zstd achieves compression comparable to gzip-9 (mean 13.5$\times$ vs 12.3$\times$) while compressing 19--69$\times$ faster and reading 1.9--2.6$\times$ faster across datasets. Compression ratios are deterministic, and timing measurements are highly reproducible (CV $<$2\%). Compression performance follows a power law with sparsity ($R^2 = 0.99$), ranging from 5$\times$ for moderately sparse data to 35$\times$ for highly sparse data. We identify six top-performing implementations optimized for different use cases and demonstrate that 4D-STEM data can be routinely compressed by $>$10$\times$. While these results provide practical guidance for lossless compression selection, the broader conclusion is that lossless compression preserves measurements but does not by itself guarantee sustainable high-throughput workflows. As detector rates rise, data handling will increasingly require inference-driven representations -- i.e., deciding what must be preserved to support a scientific inference, rather than defaulting to storing fully dense raw measurements. △ Less

Submitted 26 March, 2026; originally announced April 2026.

Atomically Precise Electron Beam Sculpting of Bilayer h-BN: The Role of Crystallographic Orientation and Milling Strategy

Authors: Ondrej Dyck, Andrew R. Lupini, Ivan Vlassiouk, Matthew Brahlek, Rob Moore, Stephen Jesse

Abstract: Achieving atomic precision in top-down manufacturing remains a fundamental challenge nanofabrication technology. Here, the focused electron beam of a scanning transmission electron microscope is used to demonstrate atomically precise sculpting of hexagonal boron nitride (h-BN) bilayers, achieving nanoribbons as narrow as 6 Å with atomically smooth edges. The key to this precision lies in understan… ▽ More Achieving atomic precision in top-down manufacturing remains a fundamental challenge nanofabrication technology. Here, the focused electron beam of a scanning transmission electron microscope is used to demonstrate atomically precise sculpting of hexagonal boron nitride (h-BN) bilayers, achieving nanoribbons as narrow as 6 Å with atomically smooth edges. The key to this precision lies in understanding how the underlying atomic structure, particularly in twisted bilayer systems, influences the milling process. High-angle annular dark-field imaging combined with multislice simulations reveals distinct intensity signatures that allow identification of different stacking arrangements within moiré patterns. Mathematical analysis of moiré lattices provides a predictive framework for determining optimal cutting directions, with cuts along armchair directions yielding superior edge quality compared to zigzag orientations. Surprisingly, a sequential milling approach, where a small electron beam subscan area is translated during the process, produces significantly better results than parallel milling of the entire target region. To understand these differences we implemented a stochastic milling model that reveals that sequential milling minimizes unwanted exposure to surrounding material through beam tail effects. These findings establish a framework for achieving atomic precision in electron beam sculpting of two-dimensional materials and provide fundamental insights applicable to the broader challenge of top-down nanofabrication. △ Less

Submitted 18 February, 2026; originally announced February 2026.

Topology-Driven Vibrations in a Chiral Polar Vortex Lattice

Authors: Eric R. Hoglund, Harrison A. Walker, Peter Meisenheimer, Thomas W. Pfeifer, Niels De Vries, Dipanjan Chaudhuri, Ting-Ran Liu, Amber M. Nelson-Quillin, Sandhya Susarla, De-Liang Bao, Patrick E. Hopkins, Andrew R. Lupini, Peter Abbamonte, Yu-Tsun Shao Ramamoorthy Ramesh, Sokrates T. Pantelides, Jordan A. Hachtel

Abstract: The ordering of magnetic or electric dipoles leading to real-space topological structures is at the forefront of materials research as their quantum mechanical nature often lends itself to emergent properties. Atomic lattice vibrations (phonons) are often a key contributor to the formation of long-range dipole textures based on ferroelectrics and impact the properties of the emergent phases. Her… ▽ More The ordering of magnetic or electric dipoles leading to real-space topological structures is at the forefront of materials research as their quantum mechanical nature often lends itself to emergent properties. Atomic lattice vibrations (phonons) are often a key contributor to the formation of long-range dipole textures based on ferroelectrics and impact the properties of the emergent phases. Here, using monochromated, momentum-resolved electron energy-loss spectroscopy (qEELS) with nanometer spatial resolution and meV-spectral-precision, we demonstrate that polar vortex lattices in PbTiO$_3$ spatially modulate the material's vibrational spectrum in patterns that directly reflect the overlying symmetry of the topological patterns. Moreover, by combining experiments with molecular dynamics simulations using machine learned potentials we reveal how these structures modify phonon modes across the vibrational spectrum. Beyond simple intensity modulation, we find that the chirality of the vortex topology imparts its unique symmetry onto phonons, producing a distinctive asymmetrical spectral shift across the vortex unit cell. Finally, the high spatial resolution of the technique enables topological defects to be probed directly, demonstrating a return to trivial PbTiO$_3$ modes at vortex dislocation cores. These findings establish a fundamental relationship between ferroelectric-ordering-induced topologies and phonon behavior, opening new avenues for engineering thermal transport, electron-phonon coupling, and other phonon-mediated properties in next-generation nanoscale devices. △ Less

Submitted 16 September, 2025; v1 submitted 12 September, 2025; originally announced September 2025.

Comments: 16 pages main text, 4 figures, 6 pages of methods, 3 pages of references, 1 extended figure, 7 supporting figures

Sub-nm2 ferroelectric domains via charged 180 degree walls in ZrO2

Authors: Nashrah Afroze, Hamoon Fahrvandi, Guodong Ren, Pawan Kumar, Christopher Nelson, Sarah Lombardo, Mengkun Tian, Ping-Che Lee, Jiayi Chen, Manifa Noor, Kisung Chae, Sanghyun Kang, Prasanna Venkat Ravindran, Matthew Bergschneider, Gwan Yeong Jung, Pravan Omprakash, Gardy K. Ligonde, Nujhat Tasneem, Dina Triyoso, Steven Consiglio, Kanda Tapily, Robert Clark, Gert Leusink, Jayakanth Ravichandran, Shimeng Yu , et al. (9 additional authors not shown)

Abstract: Flat phonon bands in fluorite ferroelectrics (HfO2 or ZrO2) shrink polar domains laterally to an irreducible half-unit-cell width (0.27 nm) within which the vertical arrangement of dipoles is expected to remain uniform. We report on the direct observation of nonuniform and nearly discrete vertical arrangements of dipoles in ZrO2 thin films consisting of closely spaced head-to-head (HH) and tail-to… ▽ More Flat phonon bands in fluorite ferroelectrics (HfO2 or ZrO2) shrink polar domains laterally to an irreducible half-unit-cell width (0.27 nm) within which the vertical arrangement of dipoles is expected to remain uniform. We report on the direct observation of nonuniform and nearly discrete vertical arrangements of dipoles in ZrO2 thin films consisting of closely spaced head-to-head (HH) and tail-to-tail (TT) charged 180 degree walls, each exhibiting a distinct bulk-like structure. These charged domain walls (CDWs) further compress the irreducibly narrow, laterally stacked domains vertically to a thickness of 1-2.75 nm, yielding in-plane domains with sub-nm2 footprints-among the smallest ever reported for any ferroelectric material. The HH and TT walls form due to their flat longitudinal optical (LO) polar bands and are electrostatically stabilized by bound-charge compensation via interstitial oxygen atoms, which act as natural structural defects at the HH walls. Moreover, these walls are predicted to be conducting and to exhibit ultralow propagation barriers, with HH walls (1.6 meV) being far more mobile than TT walls (22.3 meV), indicating strong potential for low-voltage, domain-wall-based nanoelectronics. △ Less

Submitted 5 February, 2026; v1 submitted 24 July, 2025; originally announced July 2025.

Comments: 4 main figures, 16 supplementary figures

Mic-hackathon 2024: Hackathon on Machine Learning for Electron and Scanning Probe Microscopy

Authors: Utkarsh Pratiush, Austin Houston, Kamyar Barakati, Aditya Raghavan, Dasol Yoon, Harikrishnan KP, Zhaslan Baraissov, Desheng Ma, Samuel S. Welborn, Mikolaj Jakowski, Shawn-Patrick Barhorst, Alexander J. Pattison, Panayotis Manganaris, Sita Sirisha Madugula, Sai Venkata Gayathri Ayyagari, Vishal Kennedy, Ralph Bulanadi, Michelle Wang, Kieran J. Pang, Ian Addison-Smith, Willy Menacho, Horacio V. Guzman, Alexander Kiefer, Nicholas Furth, Nikola L. Kolev , et al. (48 additional authors not shown)

Abstract: Microscopy is a primary source of information on materials structure and functionality at nanometer and atomic scales. The data generated is often well-structured, enriched with metadata and sample histories, though not always consistent in detail or format. The adoption of Data Management Plans (DMPs) by major funding agencies promotes preservation and access. However, deriving insights remains d… ▽ More Microscopy is a primary source of information on materials structure and functionality at nanometer and atomic scales. The data generated is often well-structured, enriched with metadata and sample histories, though not always consistent in detail or format. The adoption of Data Management Plans (DMPs) by major funding agencies promotes preservation and access. However, deriving insights remains difficult due to the lack of standardized code ecosystems, benchmarks, and integration strategies. As a result, data usage is inefficient and analysis time is extensive. In addition to post-acquisition analysis, new APIs from major microscope manufacturers enable real-time, ML-based analytics for automated decision-making and ML-agent-controlled microscope operation. Yet, a gap remains between the ML and microscopy communities, limiting the impact of these methods on physics, materials discovery, and optimization. Hackathons help bridge this divide by fostering collaboration between ML researchers and microscopy experts. They encourage the development of novel solutions that apply ML to microscopy, while preparing a future workforce for instrumentation, materials science, and applied ML. This hackathon produced benchmark datasets and digital twins of microscopes to support community growth and standardized workflows. All related code is available at GitHub: https://github.com/KalininGroup/Mic-hackathon-2024-codes-publication/tree/1.0.0.1 △ Less

Submitted 27 June, 2025; v1 submitted 9 June, 2025; originally announced June 2025.

Journal ref: Mach. Learn.: Sci. Technol. 6 (2025) 040701

Towards Atomic-Scale Control over Structural Modulations in Quasi-1D Chalcogenides for Colossal Optical Anisotropy

Authors: Guodong Ren, Shantanu Singh, Gwan Yeong Jung, Wooseon Choi, Huandong Chen, Boyang Zhao, Kevin Ye, Andrew R. Lupini, Miaofang Chi, Jordan A. Hachtel, Young-Min Kim, Jayakanth Ravichandran, Rohan Mishra

Abstract: Optically anisotropic materials are sought after for tailoring the polarization of light. Recently, colossal optical anisotropy was reported in a quasi-one-dimensional chalcogenide, Sr1.125TiS3. Compared to SrTiS3, the excess Sr in Sr1.125TiS3 leads to periodic structural modulations and introduces additional electrons that undergo charge ordering on select Ti atoms to form a highly polarizable cl… ▽ More Optically anisotropic materials are sought after for tailoring the polarization of light. Recently, colossal optical anisotropy was reported in a quasi-one-dimensional chalcogenide, Sr1.125TiS3. Compared to SrTiS3, the excess Sr in Sr1.125TiS3 leads to periodic structural modulations and introduces additional electrons that undergo charge ordering on select Ti atoms to form a highly polarizable cloud oriented along the c-axis, hence, resulting in the colossolal optical anisotropy. Here, further enhancement of the colossal optical anisotropy to 2.5 in Sr1.143TiS3 is reported through control over the periodicity of the atomic-scale modulations. The role of structural modulations in tuning the optical properties in a series of SrxTiS3 compounds has been investigated using DFT calculations. The structural modulations arise from various stacking sequences of face-sharing TiS6 octahedra and twist-distorted trigonal prisms, and are found to be thermodynamically stable for x larger than 1 but smaller than 1.5. As x increases, an indirect-to-direct band gap transition is predicted for x equal to and larger than 1.143 along with an increased occupancy of Ti-dz2 states. Together, these two factors result in a theoretically predicted maximum birefriengence of 2.5 for Sr1.143TiS3. Single crystals of Sr1.143TiS3 were grown using a molten-salt flux method. Atomic-scale observations using scanning transmission electron microscopy confirm the feasibility of synthesizing SrxTiS3 with varied modulation periodicities. Overall, these findings demonstrate compositonal tunability of optical properties in SrxTiS3 compounds, and potentially in other hexagonal perovskites having structural modulations. △ Less

Submitted 14 May, 2025; originally announced May 2025.

Defect Engineering in Large-Scale CVD-Grown Hexagonal Boron Nitride: Formation, Spectroscopy, and Spin Relaxation Dynamics

Authors: Ivan V. Vlassiouk, Yueh-Chun Wu, Alexander Puretzky, Liangbo Liang, John Lasseter, Bogdan Dryzhakov, Ian Gallagher, Sujoy Ghosh, Nickolay Lavrik, Ondrej Dyck, Andrew R. Lupini, Marti Checa, Liam Collins, Huan Zhao, Farzana Likhi, Kai Xiao, Ilia Ivanov, David Glasgow, Alexander Tselev, Benjamin Lawrie, Sergei Smirnov, Steven Randolph

Abstract: Recently, numerous techniques have been reported for generating optically active defects in exfoliated hexagonal boron nitride (hBN), which hold transformative potential for quantum photonic devices. However, achieving on-demand generation of desirable defect types in scalable hBN films remains a significant challenge. Here, we demonstrate that formation of negative boron vacancy defects, VB-, in… ▽ More Recently, numerous techniques have been reported for generating optically active defects in exfoliated hexagonal boron nitride (hBN), which hold transformative potential for quantum photonic devices. However, achieving on-demand generation of desirable defect types in scalable hBN films remains a significant challenge. Here, we demonstrate that formation of negative boron vacancy defects, VB-, in suspended, large-area CVD-grown hBN is strongly dependent on the type of bombarding particles (ions, neutrons, and electrons) and irradiation conditions. In contrast to suspended hBN, defect formation in substrate-supported hBN is more complex due to the uncontrollable generation of secondary particles from the substrate, and the outcome strongly depends on the thickness of the hBN. We identify different defect types by correlating spectroscopic and optically detected magnetic resonance features, distinguishing boron vacancies (formed by light ions and neutrons) from other optically active defects emitting at 650 nm assigned to anti-site nitrogen vacancy (NBVN) and reveal the presence of additional dark paramagnetic defects that influence spin-lattice relaxation time (T1) and zero-field splitting parameters, all of which strongly depend on the defect density. These results underscore the potential for precisely engineered defect formation in large-scale CVD-grown hBN, paving the way for the scalable fabrication of quantum photonic devices. △ Less

Submitted 28 March, 2025; v1 submitted 24 March, 2025; originally announced March 2025.

Strain-driven stabilization of a room-temperature chiral multiferroic with coupled ferroaxial and ferroelectric order

Authors: Guodong Ren, Gwan Yeong Jung, Huandong Chen, Chong Wang, Boyang Zhao, Rama K. Vasudevan, Jordan A. Hachtel, Andrew R. Lupini, Miaofang Chi, Di Xiao, Jayakanth Ravichandran, Rohan Mishra

Abstract: Noncollinear ferroic materials are sought after as testbeds to explore the intimate connections between topology and symmetry, which result in electronic, optical and magnetic functionalities not observed in collinear ferroic materials. For example, ferroaxial materials have ordered rotational structural distortions that break mirror symmetry and induce chirality. When ferroaxial order is coupled… ▽ More Noncollinear ferroic materials are sought after as testbeds to explore the intimate connections between topology and symmetry, which result in electronic, optical and magnetic functionalities not observed in collinear ferroic materials. For example, ferroaxial materials have ordered rotational structural distortions that break mirror symmetry and induce chirality. When ferroaxial order is coupled with ferroelectricity arising from a broken inversion symmetry, it offers the prospect of electric-field-control of the ferroaxial distortions and opens up new tunable functionalities. However, chiral multiferroics, especially ones stable at room temperature, are rare. We report the discovery of a strain-stabilized, room-temperature chiral multiferroic phase in single crystals of BaTiS$_3$, a quasi-one-dimensional (1D) hexagonal chalcogenide. Using first-principles calculations, we predict the stabilization of this multiferroic phase having $P6_3$ space group for biaxial tensile strains exceeding 1.5% applied on the basal ab-plane of the room temperature $P6_3cm$ phase of BaTiS$_3$. The chiral multiferroic phase is characterized by rotational distortions of select TiS$_6$ octahedra around the long $c$-axis and polar displacement of Ti atoms along the $c$-axis. We used an innovative approach using focused ion beam milling to make appropriately strained samples of BaTiS$_3$. The ferroaxial and ferroelectric distortions, and their domains in $P6_3$-BaTiS$_3$ were directly resolved using atomic resolution scanning transmission electron microscopy. Landau-based phenomenological modeling predicts a strong coupling between the ferroelectric and the ferroaxial order making $P6_3$-BaTiS$_3$ an attractive test bed for achieving electric-field control of chirality-related phenomena such as circular photo-galvanic current and the Rashba effect. △ Less

Submitted 2 December, 2024; v1 submitted 7 August, 2024; originally announced August 2024.

Comments: 20 pages, 4 figures

Antiferroelectric Hafnia Down to the 2D Limit

Authors: Xin Li, Guodong Ren, Haidong Lu, Kartik Samanta, Amit Kumar Shah, Kai Huang, Pravan Omprakash, Yu Yun, Pratyush Buragohain, Huibo Cao, Yan Wu, Jordan A. Hachtel, Andrew R. Lupini, Miaofang Chi, Juan Carlos Idrobo, Evgeny Y. Tsymbal, Alexei Gruverman, Rohan Mishra, Xiaoshan Xu

Abstract: Antiferroelectricity is a material property characterized by alternating electric dipoles spontaneously ordered in antiparallel directions. Antiferroelectrics are promising for energy storage, solid-state cooling, and memory technologies; however, these materials are scarce, and their scalability remains largely unexplored. In this work, we demonstrate that single-crystalline hafnia, a lead-free C… ▽ More Antiferroelectricity is a material property characterized by alternating electric dipoles spontaneously ordered in antiparallel directions. Antiferroelectrics are promising for energy storage, solid-state cooling, and memory technologies; however, these materials are scarce, and their scalability remains largely unexplored. In this work, we demonstrate that single-crystalline hafnia, a lead-free CMOS-compatible material, exhibits antiferroelectricity under compressive-strain conditions. We observe antiparallel sublattice polarization and stable double-hysteresis in single-crystalline (111)-oriented epitaxial La-doped hafnia films grown on yttrium-stabilized zirconia and show that the antipolar orthorhombic phase of hafnia adheres to the Kittel model of antiferroelectricity. Notably, compressive strain strengthens the antiferroelectric order in thinner La-doped hafnia films, achieving an unprecedented 850 C ordering temperature in the two-dimensional limit, highlighting hafnia's potential for advanced antiferroelectric devices. △ Less

Submitted 2 August, 2025; v1 submitted 3 August, 2024; originally announced August 2024.

A Versatile Side Entry Laser System for Scanning Transmission Electron Microscopy

Authors: Ondrej Dyck, Olugbenga Olunloyo, Kai Xiao, Benjamin Wolf, Thomas M. Moore, Andrew R. Lupini, Stephen Jesse

Abstract: We present the design and implementation of a side entry laser system designed for an ultra-high vacuum scanning transmission electron microscope. This system uses a versatile probe design enclosed in a vacuum envelope such that parts can be easily aligned, modified, or exchanged without disturbing the vacuum. The system uses a mirror mounted on the sample holder such that the sample can be illumi… ▽ More We present the design and implementation of a side entry laser system designed for an ultra-high vacuum scanning transmission electron microscope. This system uses a versatile probe design enclosed in a vacuum envelope such that parts can be easily aligned, modified, or exchanged without disturbing the vacuum. The system uses a mirror mounted on the sample holder such that the sample can be illuminated without being tilted. Notably the mirror can be removed and replaced with an ablation target and a higher power laser used to ablate material directly onto the sample. We argue that new capabilities hold the potential to transform the electron microscope from an analysis tool towards a more flexible synthesis system, where atomic scale fabrication and atom-by-atom experiments can be performed. △ Less

Submitted 16 October, 2024; v1 submitted 12 July, 2024; originally announced July 2024.

Your Clean Graphene is Still Not Clean

Authors: Ondrej Dyck, Aisha Okmi, Kai Xiao, Sidong Lei, Andrew R. Lupini, Stephen Jesse

Abstract: Efforts aimed at scaling fabrication processes to the level of single atoms, dubbed atom-by-atom fabrication or atomic fabrication, invariably encounter the obstacle of atomic scale cleanliness. When considering atomic fabrication, cleanliness of the base material and purity of the source reservoir from which atomic structures will be built are invariable constraints imposed by laws of physics and… ▽ More Efforts aimed at scaling fabrication processes to the level of single atoms, dubbed atom-by-atom fabrication or atomic fabrication, invariably encounter the obstacle of atomic scale cleanliness. When considering atomic fabrication, cleanliness of the base material and purity of the source reservoir from which atomic structures will be built are invariable constraints imposed by laws of physics and chemistry. As obvious as such statements may be, and regardless of the inevitable consequences for successful atomic fabrication, there is a poignant lack of understanding of the "dirt" (contamination/impurities). Here, we examine hydrocarbon contamination on graphene. Graphene has formed the base substrate for many e-beam-based atomic fabrication studies and many strategies for cleaning graphene have been presented in the literature. One popular method is heating to high temperatures (>500 °C). It is usually inferred that volatile hydrocarbons evaporate into the microscope vacuum system leaving behind pristine graphene. Here, we show through direct image intensity analysis that what appears to be clean graphene can be coated with a thin layer of dynamically diffusing hydrocarbons. This result holds significant implications for approaches to e-beam based atomic fabrication, updates the conceptual model of e-beam induced hydrocarbon deposition, and may extend to hot surfaces generally. △ Less

Submitted 16 October, 2024; v1 submitted 2 July, 2024; originally announced July 2024.

Giant Modulation of Refractive Index from Picoscale Atomic Displacements

Authors: Boyang Zhao, Guodong Ren, Hongyan Mei, Vincent C. Wu, Shantanu Singh, Gwan-Yeong Jung, Huandong Chen, Raynald Giovine, Shanyuan Niu, Arashdeep S. Thind, Jad Salman, Nick S. Settineri, Bryan C. Chakoumakos, Michael E. Manley, Raphael P. Hermann, Andrew R. Lupini, Miaofang Chi, Jordan A. Hachtel, Arkadiy Simonov, Simon J. Teat, Raphaële J. Clément, Mikhail A. Kats, J. Ravichandran, Rohan Mishra

Abstract: Structural disorder has been shown to enhance and modulate magnetic, electrical, dipolar, electrochemical, and mechanical properties of materials. However, the possibility of obtaining novel optical and optoelectronic properties from structural disorder remains an open question. Here, we show unambiguous evidence of disorder in the form of anisotropic, picoscale atomic displacements modulating the… ▽ More Structural disorder has been shown to enhance and modulate magnetic, electrical, dipolar, electrochemical, and mechanical properties of materials. However, the possibility of obtaining novel optical and optoelectronic properties from structural disorder remains an open question. Here, we show unambiguous evidence of disorder in the form of anisotropic, picoscale atomic displacements modulating the refractive index tensor and resulting in the giant optical anisotropy observed in BaTiS$_3$, a quasi-one-dimensional hexagonal chalcogenide. Single crystal X-ray diffraction studies reveal the presence of antipolar displacements of Ti atoms within adjacent TiS$_6$ chains along the c-axis, and three-fold degenerate Ti displacements in the a-b plane. $^{47/49}$Ti solid-state NMR provides additional evidence for those Ti displacements in the form of a three-horned NMR lineshape resulting from a low symmetry local environment around Ti atoms. We used scanning transmission electron microscopy to directly observe the globally disordered Ti a-b plane displacements and find them to be ordered locally over a few unit cells. First-principles calculations show that the Ti a-b plane displacements selectively reduce the refractive index along the ab-plane, while having minimal impact on the refractive index along the chain direction, thus resulting in a giant enhancement in the optical anisotropy. By showing a strong connection between structural disorder with picoscale displacements and the optical response in BaTiS$_3$, this study opens a pathway for designing optical materials with high refractive index and functionalities such as large optical anisotropy and nonlinearity. △ Less

Submitted 19 March, 2024; v1 submitted 6 October, 2023; originally announced October 2023.

Comments: 24 pages, 3 figures

Controlling hydrocarbon transport and electron beam induced deposition on single layer graphene: toward atomic scale synthesis in the scanning transmission electron microscope

Authors: Ondrej Dyck, Andrew R. Lupini, Philip D. Rack, Jason Fowlkes, Stephen Jesse

Abstract: Focused electron beam induced deposition (FEBID) is a direct write technique for depositing materials on a support substrate akin to 3D printing with an electron beam (e-beam). Opportunities exist for merging this existing technique with aberration-corrected scanning transmission electron microscopy to achieve molecular- or atomic-level spatial precision. Several demonstrations have been performed… ▽ More Focused electron beam induced deposition (FEBID) is a direct write technique for depositing materials on a support substrate akin to 3D printing with an electron beam (e-beam). Opportunities exist for merging this existing technique with aberration-corrected scanning transmission electron microscopy to achieve molecular- or atomic-level spatial precision. Several demonstrations have been performed using graphene as the support substrate. A common challenge that arises during this process is e-beam-induced hydrocarbon deposition, suggesting greater control over the sample environment is needed. Various strategies exist for cleaning graphene in situ. One of the most effective methods is to rapidly heat to high temperatures, e.g., 600 C or higher. While this can produce large areas of what appears to be atomically clean graphene, mobile hydrocarbons can still be present on the surfaces. Here, we show that these hydrocarbons are primarily limited to surface migration and demonstrate an effective method for interrupting the flow using e-beam deposition to form corralled hydrocarbon regions. This strategy is effective for maintaining atomically clean graphene at high temperatures where hydrocarbon mobility can lead to substantial accumulation of unwanted e-beam deposition. △ Less

Submitted 11 July, 2023; originally announced July 2023.

Doping transition-metal atoms in graphene for atomic-scale tailoring of electronic, magnetic, and quantum topological properties

Authors: Ondrej Dyck, Lizhi Zhang, Mina Yoon, Jacob L. Swett, Dale Hensley, Cheng Zhang, Philip D. Rack, Jason D. Fowlkes, Andrew R. Lupini, Stephen Jesse

Abstract: Atomic-scale fabrication is an outstanding challenge and overarching goal for the nanoscience community. The practical implementation of moving and fixing atoms to a structure is non-trivial considering that one must spatially address the positioning of single atoms, provide a stabilizing scaffold to hold structures in place, and understand the details of their chemical bonding. Free-standing grap… ▽ More Atomic-scale fabrication is an outstanding challenge and overarching goal for the nanoscience community. The practical implementation of moving and fixing atoms to a structure is non-trivial considering that one must spatially address the positioning of single atoms, provide a stabilizing scaffold to hold structures in place, and understand the details of their chemical bonding. Free-standing graphene offers a simplified platform for the development of atomic-scale fabrication and the focused electron beam in a scanning transmission electron microscope can be used to locally induce defects and sculpt the graphene. In this scenario, the graphene forms the stabilizing scaffold and the experimental question is whether a range of dopant atoms can be attached and incorporated into the lattice using a single technique and, from a theoretical perspective, we would like to know which dopants will create technologically interesting properties. Here, we demonstrate that the electron beam can be used to selectively and precisely insert a variety of transition metal atoms into graphene with highly localized control over the doping locations. We use first-principles density functional theory calculations with direct observation of the created structures to reveal the energetics of incorporating metal atoms into graphene and their magnetic, electronic, and quantum topological properties. △ Less

Submitted 11 July, 2023; originally announced July 2023.

Doping of Cr in Graphene Using Electron Beam Manipulation for Functional Defect Engineering

Authors: Ondrej Dyck, Mina Yoon, Lizhi Zhang, Andrew R. Lupini, Jacob L. Swett, Stephen Jesse

Abstract: Chromium atoms in graphene have been proposed to exhibit magnetic moments and spin-selective conducting states depending on the local bonding geometry within the graphene structure, which could lead to interesting applications in spintronics. Despite this interest, there are few direct experimental reports of Cr dopants in graphene even though it is theorized to be stable. Here, we demonstrate the… ▽ More Chromium atoms in graphene have been proposed to exhibit magnetic moments and spin-selective conducting states depending on the local bonding geometry within the graphene structure, which could lead to interesting applications in spintronics. Despite this interest, there are few direct experimental reports of Cr dopants in graphene even though it is theorized to be stable. Here, we demonstrate the introduction of single Cr dopant atoms into the graphene lattice and onto graphene edges through the controlled use of a focused electron beam in a scanning transmission electron microscope. We show local control of doping locations and when coupled with targeted in situ milling during scanning of the e-beam, these strategies demonstrate an important component of the fabrication of tailored nanostructured devices in the electron microscope. The approach is validated with first principles calculations to understand synthesis pathways during fabrication and reveal the energetics and local properties of Cr atoms embedded in graphene; e.g., Cr doping can convert graphene into a magnetic and semiconducting material, which suggests Cr-doped graphene can be used as a building block for potential electronic devices and a means to construct them. △ Less

Submitted 11 July, 2023; originally announced July 2023.

The role of temperature on defect diffusion and nanoscale patterning in graphene

Authors: Ondrej Dyck, Sinchul Yeom, Sarah Dillender, Andrew R. Lupini, Mina Yoon, Stephen Jesse

Abstract: Graphene is of great scientific interest due to a variety of unique properties such as ballistic transport, spin selectivity, the quantum hall effect, and other quantum properties. Nanopatterning and atomic scale modifications of graphene are expected to enable further control over its intrinsic properties, providing ways to tune the electronic properties through geometric and strain effects, intr… ▽ More Graphene is of great scientific interest due to a variety of unique properties such as ballistic transport, spin selectivity, the quantum hall effect, and other quantum properties. Nanopatterning and atomic scale modifications of graphene are expected to enable further control over its intrinsic properties, providing ways to tune the electronic properties through geometric and strain effects, introduce edge states and other local or extended topological defects, and sculpt circuit paths. The focused beam of a scanning transmission electron microscope (STEM) can be used to remove atoms, enabling milling, doping, and deposition. Utilization of a STEM as an atomic scale fabrication platform is increasing; however, a detailed understanding of beam-induced processes and the subsequent cascade of aftereffects is lacking. Here, we examine the electron beam effects on atomically clean graphene at a variety of temperatures ranging from 400 to 1000 C. We find that temperature plays a significant role in the milling rate and moderates competing processes of carbon adatom coalescence, graphene healing, and the diffusion (and recombination) of defects. The results of this work can be applied to a wider range of 2D materials and introduce better understanding of defect evolution in graphite and other bulk layered materials. △ Less

Submitted 11 July, 2023; originally announced July 2023.

A platform for in situ synthesis in a STEM

Authors: Ondrej Dyck, Andrew R. Lupini, Stephen Jesse

Abstract: The engineering of quantum materials requires the development of tools able to address various synthesis and characterization challenges. These include the establishment and refinement of growth methods, material manipulation, and defect engineering. Material modification at the atomic level will be a key enabling factor for the engineering of quantum materials where desired phenomena are critical… ▽ More The engineering of quantum materials requires the development of tools able to address various synthesis and characterization challenges. These include the establishment and refinement of growth methods, material manipulation, and defect engineering. Material modification at the atomic level will be a key enabling factor for the engineering of quantum materials where desired phenomena are critically determined by local atomic structures. Successful use of scanning transmission electron microscopes (STEMs) for atomic scale material manipulation has opened the door for a transformed view of what can be accomplished using electron-beam-based strategies. However, serious obstacles exist on the pathway from possibility to practical reality. One such obstacle is the in situ delivery of atomized material in the STEM to the region of interest for further fabrication processes. Here, we present progress on this front with a view toward performing synthesis (deposition and growth) processes in a scanning transmission electron microscope. An in situ thermal deposition platform is presented, tested, and deposition and growth processes are demonstrated. In particular, we show that isolated Sn atoms can be evaporated from a filament and caught on the nearby sample, demonstrating atomized material delivery. This platform, and future variations, are envisioned to facilitate real-time atomic resolution imaging of growth processes and open new pathways toward atomic fabrication. △ Less

Submitted 27 February, 2023; originally announced February 2023.

Automatic and quantitative measurement of spectrometer aberrations

Authors: Yueming Guo, Andrew R. Lupini

Abstract: The performance of electron energy-loss spectrometers can often be limited by their electron-optical aberrations. Due to recent developments in high energy-resolution and momentum-resolved electron energy loss spectroscopy (EELS), there is renewed interest in optimizing the performance of such spectrometers. For example, the "ω-q" mode of momentum-resolved EELS, which uses a small convergence angl… ▽ More The performance of electron energy-loss spectrometers can often be limited by their electron-optical aberrations. Due to recent developments in high energy-resolution and momentum-resolved electron energy loss spectroscopy (EELS), there is renewed interest in optimizing the performance of such spectrometers. For example, the "ω-q" mode of momentum-resolved EELS, which uses a small convergence angle and requires aligning diffraction spots with a slot aperture, presents a challenge for realigning the spectrometer after adjusting the projection lenses. Automated and robust alignment can greatly benefit such a process. The first step towards this goal is automatic and quantitative measurement of spectrometer aberrations. Here we demonstrate the measurement of geometric aberrations and distortions in EELS within a monochromated scanning transmission electron microscope(STEM). To better understand the results, we present a wave mechanical simulation of the experiment. Using the measured aberration and distortion coefficients as inputs to the simulation, we find a good match between the simulation and experiment, verifying the approach used in the simulation, allowing us to assess the accuracy of the measurements. Understanding the errors and inaccuracies in the procedure can guide further progress in aberration measurement and correction for new spectrometer developments. △ Less

Submitted 24 February, 2023; originally announced February 2023.

Comments: Submitted to Microscopy and Microanalysis

Revealing intrinsic vortex-core states in Fe-based superconductors through machine-learning-driven discovery

Authors: Yueming Guo, Hu Miao, Qiang Zou, Mingming Fu, Athena S. Sefat, Andrew R. Lupini, Sergei V. Kalinin, Zheng Gai

Abstract: Electronic states within superconducting vortices hold crucial information about paring mechanisms and topology. While scanning tunneling microscopy/spectroscopy(STM/S) can image the vortices, it is difficult to isolate the intrinsic electronic states from extrinsic effects like subsurface defects and disorders. We combine STM/S with unsupervised machine learning to develop a method for screening… ▽ More Electronic states within superconducting vortices hold crucial information about paring mechanisms and topology. While scanning tunneling microscopy/spectroscopy(STM/S) can image the vortices, it is difficult to isolate the intrinsic electronic states from extrinsic effects like subsurface defects and disorders. We combine STM/S with unsupervised machine learning to develop a method for screening out the vortices pinned by embedded disorder in Fe-based superconductors. The approach provides an unbiased way to reveal intrinsic vortex-core states and may address puzzles on Majorana zero modes. △ Less

Submitted 18 February, 2023; originally announced February 2023.

The Synthescope: A Vision for Combining Synthesis with Atomic Fabrication

Authors: Ondrej Dyck, Andrew R. Lupini, Stephen Jesse

Abstract: The scanning transmission electron microscope, a workhorse instrument in materials characterization, is being transformed into an atomic-scale material manipulation platform. With an eye on the trajectory of recent developments and the obstacles toward progress in this field, we provide a vision for a path toward an expanded set of capabilities and applications. We reconceptualize the microscope a… ▽ More The scanning transmission electron microscope, a workhorse instrument in materials characterization, is being transformed into an atomic-scale material manipulation platform. With an eye on the trajectory of recent developments and the obstacles toward progress in this field, we provide a vision for a path toward an expanded set of capabilities and applications. We reconceptualize the microscope as an instrument for fabrication and synthesis with the capability to image and characterize atomic-scale structural formation as it occurs. Further development and refinement of this approach may have substantial impact on research in microelectronics, quantum information science, and catalysis where precise control over atomic scale structure and chemistry of a few "active sites" can have a dramatic impact on larger scale functionality and where developing a better understanding of atomic scale processes can help point the way to larger scale synthesis approaches. △ Less

Submitted 30 May, 2023; v1 submitted 16 February, 2023; originally announced February 2023.

Direct-Writing Atom-by-Atom

Authors: Ondrej Dyck, Andrew R. Lupini, Stephen Jesse

Abstract: Direct-write processes enable the alteration or deposition of materials in a continuous, directable, sequential fashion. In this work we demonstrate an electron beam direct-write process in an aberration-corrected scanning transmission electron microscope. This process has several fundamental differences from conventional electron beam induced deposition techniques, where the electron beam dissoci… ▽ More Direct-write processes enable the alteration or deposition of materials in a continuous, directable, sequential fashion. In this work we demonstrate an electron beam direct-write process in an aberration-corrected scanning transmission electron microscope. This process has several fundamental differences from conventional electron beam induced deposition techniques, where the electron beam dissociates precursor gases into chemically reactive products that bond to a substrate. Here, we use elemental tin (Sn) as a precursor and employ a different mechanism to facilitate deposition. The atomic-sized electron beam is used to generate chemically reactive point defects at desired locations in a graphene substrate. Temperature control of the sample is used to enable the precursor atoms to migrate across the surface and bond to the defect sites thereby enabling atom-by-atom direct-writing. △ Less

Submitted 20 March, 2023; v1 submitted 6 January, 2023; originally announced January 2023.

Top-down fabrication of atomic patterns in twisted bilayer graphene

Authors: Ondrej Dyck, Sinchul Yeom, Andrew R. Lupini, Jacob L. Swett, Dale Hensley, Mina Yoon, Stephen Jesse

Abstract: Atomic-scale engineering typically involves bottom-up approaches, leveraging parameters such as temperature, partial pressures, and chemical affinity to promote spontaneous arrangement of atoms. These parameters are applied globally, resulting in atomic scale features scattered probabilistically throughout the material. In a top-down approach, different regions of the material are exposed to diffe… ▽ More Atomic-scale engineering typically involves bottom-up approaches, leveraging parameters such as temperature, partial pressures, and chemical affinity to promote spontaneous arrangement of atoms. These parameters are applied globally, resulting in atomic scale features scattered probabilistically throughout the material. In a top-down approach, different regions of the material are exposed to different parameters resulting in structural changes varying on the scale of the resolution. In this work, we combine the application of global and local parameters in an aberration corrected scanning transmission electron microscope (STEM) to demonstrate atomic scale precision patterning of atoms in twisted bilayer graphene. The focused electron beam is used to define attachment points for foreign atoms through the controlled ejection of carbon atoms from the graphene lattice. The sample environment is staged with nearby source materials, such that the sample temperature can induce migration of the source atoms across the sample surface. Under these conditions, the electron-beam (top-down) enables carbon atoms in the graphene to be replaced spontaneously by diffusing adatoms (bottom-up). Using image-based feedback-control, arbitrary patterns of atoms and atom clusters are attached to the twisted bilayer graphene with limited human interaction. The role of substrate temperature on adatom and vacancy diffusion is explored by first-principles simulations. △ Less

Submitted 4 January, 2023; originally announced January 2023.

Direct Imaging of Electron Orbitals with a Scanning Transmission Electron Microscope

Authors: Ondrej Dyck, Jawaher Almutlaq, Jacob L. Swett, Andrew R. Lupini, Dirk Englund, Stephen Jesse

Abstract: Recent studies of secondary electron (SE) emission in scanning transmission electron microscopes suggest that material's properties such as electrical conductivity, connectivity, and work function can be probed with atomic scale resolution using a technique known as secondary electron e-beam-induced current (SEEBIC). Here, we apply the SEEBIC imaging technique to a stacked 2D heterostructure devic… ▽ More Recent studies of secondary electron (SE) emission in scanning transmission electron microscopes suggest that material's properties such as electrical conductivity, connectivity, and work function can be probed with atomic scale resolution using a technique known as secondary electron e-beam-induced current (SEEBIC). Here, we apply the SEEBIC imaging technique to a stacked 2D heterostructure device to reveal the spatially resolved electron orbital ionization cross section of an encapsulated WSe2 layer. We find that the double Se lattice site shows higher emission than the W site, which is at odds with first-principles modelling of ionization of an isolated WSe2 cluster. These results illustrate that atomic level SEEBIC contrast within a single material is possible and that an enhanced understanding of atomic scale SE emission is required to account for the observed contrast. In turn, this suggests that subtle information about interlayer bonding and the effect on electron orbitals can be directly revealed with this technique. △ Less

Submitted 1 December, 2022; originally announced December 2022.

A roadmap for edge computing enabled automated multidimensional transmission electron microscopy

Authors: Debangshu Mukherjee, Kevin M. Roccapriore, Anees Al-Najjar, Ayana Ghosh, Jacob D. Hinkle, Andrew R. Lupini, Rama K. Vasudevan, Sergei V. Kalinin, Olga S. Ovchinnikova, Maxim A. Ziatdinov, Nageswara S. Rao

Abstract: The advent of modern, high-speed electron detectors has made the collection of multidimensional hyperspectral transmission electron microscopy datasets, such as 4D-STEM, a routine. However, many microscopists find such experiments daunting since such datasets' analysis, collection, long-term storage, and networking remain challenging. Some common issues are the large and unwieldy size of the said… ▽ More The advent of modern, high-speed electron detectors has made the collection of multidimensional hyperspectral transmission electron microscopy datasets, such as 4D-STEM, a routine. However, many microscopists find such experiments daunting since such datasets' analysis, collection, long-term storage, and networking remain challenging. Some common issues are the large and unwieldy size of the said datasets, often running into several gigabytes, non-standardized data analysis routines, and a lack of clarity about the computing and network resources needed to utilize the electron microscope fully. However, the existing computing and networking bottlenecks introduce significant penalties in each step of these experiments, and thus, real-time analysis-driven automated experimentation for multidimensional TEM is exceptionally challenging. One solution is integrating microscopy with edge computing, where moderately powerful computational hardware performs the preliminary analysis before handing off the heavier computation to HPC systems. In this perspective, we trace the roots of computation in modern electron microscopy, demonstrate deep learning experiments running on an edge system, and discuss the networking requirements for tying together microscopes, edge computers, and HPC systems. △ Less

Submitted 5 October, 2022; originally announced October 2022.

Comments: Perspective on automated microscopy. 3 figures

Journal ref: Microscopy Today, 30(6), 10-19. (2022)

Electron-beam Introduction of Heteroatomic Pt-Si Structures in Graphene

Authors: Ondrej Dyck, Cheng Zhang, Philip D. Rack, Jason D. Fowlkes, Bobby Sumpter, Andrew R. Lupini, Sergei V. Kalinin, Stephen Jesse

Abstract: Electron-beam (e-beam) manipulation of single dopant atoms in an aberration-corrected scanning transmission electron microscope is emerging as a method for directed atomic motion and atom-by-atom assembly. Until now, the dopant species have been limited to atoms closely matched to carbon in terms of ionic radius and capable of strong covalent bonding with carbon atoms in the graphene lattice. In s… ▽ More Electron-beam (e-beam) manipulation of single dopant atoms in an aberration-corrected scanning transmission electron microscope is emerging as a method for directed atomic motion and atom-by-atom assembly. Until now, the dopant species have been limited to atoms closely matched to carbon in terms of ionic radius and capable of strong covalent bonding with carbon atoms in the graphene lattice. In situ dopant insertion into a graphene lattice has thus far been demonstrated only for Si, which is ubiquitously present as a contaminant in this material. Here, we achieve in situ manipulation of Pt atoms and their insertion into the graphene host matrix using the e-beam deposited Pt on graphene as a host system. We further demonstrate a mechanism for stabilization of the Pt atom, enabled through the formation of Si-stabilized Pt heteroatomic clusters attached to the graphene surface. This study provides evidence toward the universality of the e-beam assembly approach, opening a pathway for exploring cluster chemistry through direct assembly. △ Less

Submitted 17 March, 2022; originally announced March 2022.

Comments: 23 pages, 6 figures

Journal ref: Carbon 161, 750-757 (2020)

Mapping Conductance and Switching Behavior of Graphene Devices In Situ

Authors: Ondrej Dyck, Jacob L. Swett, Charalambos Evangeli, Andrew R. Lupini, Jan A. Mol, Stephen Jesse

Abstract: Graphene has been proposed for use in various nanodevice designs, many of which harness emergent quantum properties for device functionality. However, visualization, measurement, and manipulation become non-trivial at nanometer and atomic scales, representing a significant challenge for device fabrication, characterization, and optimization at length scales where quantum effects emerge. Here, we p… ▽ More Graphene has been proposed for use in various nanodevice designs, many of which harness emergent quantum properties for device functionality. However, visualization, measurement, and manipulation become non-trivial at nanometer and atomic scales, representing a significant challenge for device fabrication, characterization, and optimization at length scales where quantum effects emerge. Here, we present proof of principle results at the crossroads between 2D nanoelectronic devices, e-beam-induced modulation, and imaging with secondary electron e-beam induced currents (SEEBIC). We introduce a device platform compatible with scanning transmission electron microscopy investigations. We then show how the SEEBIC imaging technique can be used to visualize conductance and connectivity in single layer graphene nanodevices, even while supported on a thicker substrate (conditions under which conventional imaging fails). Finally, we show that the SEEBIC imaging technique can detect subtle differences in charge transport through time in non-ohmic graphene nanoconstrictions indicating the potential to reveal dynamic electronic processes. △ Less

Submitted 17 March, 2022; originally announced March 2022.

Comments: 25 pages, 15 figures

Journal ref: Small Methods 2021, 2101245

Contrast mechanisms in secondary electron e-beam induced current (SEEBIC) imaging

Authors: Ondrej Dyck, Jacob L. Swett, Charalambos Evangeli, Andrew R. Lupini, Jan Mol, Stephen Jesse

Abstract: Over the last few years, a new mode for imaging in the scanning transmission electron microscope (STEM) has gained attention as it permits the direct visualization of sample conductivity and electrical connectivity. When the electron beam (e-beam) is focused on the sample in the STEM, secondary electrons (SEs) are generated. If the sample is conductive and electrically connected to an amplifier, t… ▽ More Over the last few years, a new mode for imaging in the scanning transmission electron microscope (STEM) has gained attention as it permits the direct visualization of sample conductivity and electrical connectivity. When the electron beam (e-beam) is focused on the sample in the STEM, secondary electrons (SEs) are generated. If the sample is conductive and electrically connected to an amplifier, the SE current can be measured as a function of the e-beam position. This scenario is similar to the better-known scanning electron microscopy (SEM)-based technique, electron beam induced current (EBIC) imaging except the signal in STEM is generated by the emission of SEs, hence the name SEEBIC, and in this case the current flows in the opposite direction. Here, we provide a brief review of recent work in this area, examine the various contrast generation mechanisms associated with SEEBIC, and illustrate its use for the characterization of graphene nanoribbon devices. △ Less

Submitted 15 March, 2022; originally announced March 2022.

Comments: 49 pages, 13 figures

Discovering Invariant Spatial Features in Electron Energy Loss Spectroscopy Images on the Mesoscopic and Atomic Levels

Authors: Kevin M. Roccapriore, Maxim Ziatdinov, Andrew R. Lupini, Abhay P. Singh, Usha Philipose, Sergei V. Kalinin

Abstract: Over the last two decades, Electron Energy Loss Spectroscopy (EELS) imaging with a scanning transmission electron microscope (STEM) has emerged as a technique of choice for visualizing complex chemical, electronic, plasmonic, and phononic phenomena in complex materials and structures. The availability of the EELS data necessitates the development of methods to analyze multidimensional datasets wit… ▽ More Over the last two decades, Electron Energy Loss Spectroscopy (EELS) imaging with a scanning transmission electron microscope (STEM) has emerged as a technique of choice for visualizing complex chemical, electronic, plasmonic, and phononic phenomena in complex materials and structures. The availability of the EELS data necessitates the development of methods to analyze multidimensional datasets with complex spatial and energy structures. Traditionally, the analysis of these data sets has been based on analysis of individual spectra, one at a time, whereas the spatial structure and correlations between individual spatial pixels containing the relevant information of the physics of underpinning processes have generally been ignored and analyzed only via the visualization as 2D maps. Here we develop a machine learning-based approach and workflows for the analysis of spatial structures in 3D EELS data sets using a combination of dimensionality reduction and multichannel rotationally-invariant variational autoencoders. This approach is illustrated for the analysis of both the plasmonic phenomena in a system of nanowires and in the core excitations in functional oxides using low loss and core loss EELS, respectively. The code developed in this manuscript is open sourced and freely available and provided as a Jupyter notebook for the interested reader here. △ Less

Submitted 1 February, 2022; originally announced February 2022.

Comments: Associated Jupyter Notebook: https://github.com/kevinroccapriore/Multichannel-rVAE

Spatial symmetry constraint of charge-ordered kagome superconductor CsV$_3$Sb$_5$

Authors: Haoxiang Li, Yu-Xiao Jiang, J. X. Yin, Sangmoon Yoon, Andrew R. Lupini, Y. Pai, C. Nelson, A. Said, Y. M. Yang, Q. W. Yin, C. S. Gong, Z. J. Tu, H. C. Lei, Binghai Yan, Ziqiang Wang, M. Z. Hasan, H. N. Lee, H. Miao

Abstract: Elucidating the symmetry of intertwined orders in exotic superconductors is at the quantum frontier. Recent surface sensitive studies of the topological kagome superconductor CsV$_3$Sb$_5$ discovered a cascade 4a$_0$ superlattice below the charge density wave (CDW) ordering temperature, which can be related to the pair density modulations in the superconducting state. If the 4a$_0$ phase is a bulk… ▽ More Elucidating the symmetry of intertwined orders in exotic superconductors is at the quantum frontier. Recent surface sensitive studies of the topological kagome superconductor CsV$_3$Sb$_5$ discovered a cascade 4a$_0$ superlattice below the charge density wave (CDW) ordering temperature, which can be related to the pair density modulations in the superconducting state. If the 4a$_0$ phase is a bulk and intrinsic property of the kagome lattice, this would form a striking analogy to the stripe order and pair density wave discovered in the cuprate high-temperature superconductors, and the cascade ordering found in twisted bilayer graphene. High-resolution X-ray diffraction has recently been established as an ultra-sensitive probe for bulk translational symmetry-breaking orders, even for short-range orders at the diffusive limit. Here, combining high-resolution X-ray diffraction, scanning tunneling microscopy and scanning transmission electron microscopy, we demonstrate that the 4a$_0$ superstructure emerges uniquely on the surface and hence exclude the 4a$_0$ phase as the origin of any bulk transport or spectroscopic anomaly. Crucially, we show that our detected 2$\times$2$\times$2 CDW order breaks the bulk rotational symmetry to C2, which can be the driver for the bulk nematic orders and nematic surface superlattices including the 4a$_0$ phase. Our high-resolution data impose decisive spatial symmetry constraints on emergent electronic orders in the kagome superconductor CsV$_3$Sb$_5$. △ Less

Submitted 23 September, 2021; v1 submitted 7 September, 2021; originally announced September 2021.

Journal ref: Nature Communications 13:6348 (2022)

Correlated Oxide Dirac Semimetal in the Extreme Quantum Limit

Authors: Jong Mok Ok, Narayan Mohanta, Jie Zhang, Sangmoon Yoon, Satoshi Okamoto, Eun Sang Choi, Hua Zhou, Megan Briggeman, Patrick Irvin, Andrew R. Lupini, Yun-Yi Pai, Elizabeth Skoropata, Changhee Sohn, Haoxiang Li, Hu Miao, Benjamin Lawrie, Woo Seok Choi, Gyula Eres, Jeremy Levy, Ho Nyung Lee

Abstract: Quantum materials (QMs) with strong correlation and non-trivial topology are indispensable to next-generation information and computing technologies. Exploitation of topological band structure is an ideal starting point to realize correlated topological QMs. Herein, we report that strain-induced symmetry modification in correlated oxide SrNbO3 thin films creates an emerging topological band struct… ▽ More Quantum materials (QMs) with strong correlation and non-trivial topology are indispensable to next-generation information and computing technologies. Exploitation of topological band structure is an ideal starting point to realize correlated topological QMs. Herein, we report that strain-induced symmetry modification in correlated oxide SrNbO3 thin films creates an emerging topological band structure. Dirac electrons in strained SrNbO3 films reveal ultra-high mobility (100,000 cm2/Vs), exceptionally small effective mass (0.04me), and non-zero Berry phase. More importantly, strained SrNbO3 films reach the extreme quantum limit, exhibiting a sign of fractional occupation of Landau levels and giant mass enhancement. Our results suggest that symmetry-modified SrNbO3 is a rare example of a correlated topological QM, in which strong correlation of Dirac electrons leads to the realization of fractional occupation of Landau levels. △ Less

Submitted 18 August, 2021; originally announced August 2021.

Comments: accepted in Sci. Adv

Journal ref: Sci. Adv. 7, eabf9631 (2021)

Atomic structure of initial nucleation layer in hexagonal perovskite BaRuO$_3$ thin films

Authors: Sangmoon Yoon, Jong Mok Ok, Sang A Lee, Jegon Lee, Andrew R. Lupini, Woo Seok Choi, Ho Nyung Lee

Abstract: Hexagonal perovskites are an attractive group of materials due to their various polymorph phases and rich structure-property relationships. BaRuO3 (BRO) is a prototypical hexagonal perovskite, in which the electromagnetic properties are significantly modified depending on its atomic structure. Whereas thin-film epitaxy would vastly expand the application of hexagonal perovskites by epitaxially sta… ▽ More Hexagonal perovskites are an attractive group of materials due to their various polymorph phases and rich structure-property relationships. BaRuO3 (BRO) is a prototypical hexagonal perovskite, in which the electromagnetic properties are significantly modified depending on its atomic structure. Whereas thin-film epitaxy would vastly expand the application of hexagonal perovskites by epitaxially stabilizing various metastable polymorphs, the atomic structure of epitaxial hexagonal perovskites, especially at the initial growth stage, has rarely been investigated. In this study, we show that an intriguing nucleation behavior takes place during the initial stabilization of a hexagonal perovskite 9R BaRuO3 (BRO) thin film on a (111) SrTiO3 (STO) substrate. We use high-resolution high-angle annular dark field scanning transmission electron microscopy in combination with geometrical phase analysis to understand the local strain relaxation behavior. We find that nano-scale strained layers, composed of different RuO6 octahedral stacking, are initially formed at the interface, followed by a relaxed single crystal9R BRO thin film. Within the interface layer, hexagonal BROs are nucleated on the STO (111) substrate by both corner- and face-sharing. More interestingly, we find that the boundaries between the differently-stacked nucleation layers, i.e. heterostructural boundaries facilitates strain relaxation, in addition to the formation of conventional misfit dislocations evolving from homostructural boundaries. Our observations reveal an important underlying mechanism to understand the thin-film epitaxy and strain accommodation in hexagonal perovskites. △ Less

Submitted 15 July, 2021; originally announced July 2021.

Comments: 18 pages, 3 figures

Journal ref: Advanced Materials Interfaces 8 (2021) 2100023

Mechanism of electron-beam manipulation of single dopant atoms in silicon

Authors: Alexander Markevich, Bethany M Hudak, Jacob Madsen, Jiaming Song, Paul C Snijders, Andrew R Lupini, Toma Susi

Abstract: The precise positioning of dopant atoms within bulk crystal lattices could enable novel applications in areas including solid-state sensing and quantum computation. Established scanning probe techniques are capable tools for the manipulation of surface atoms, but at a disadvantage due to their need to bring a physical tip into contact with the sample. This has prompted interest in electron-beam te… ▽ More The precise positioning of dopant atoms within bulk crystal lattices could enable novel applications in areas including solid-state sensing and quantum computation. Established scanning probe techniques are capable tools for the manipulation of surface atoms, but at a disadvantage due to their need to bring a physical tip into contact with the sample. This has prompted interest in electron-beam techniques, followed by the first proof-of-principle experiment of bismuth dopant manipulation in crystalline silicon. Here, we use first principles modeling to discover a novel indirect exchange mechanism that allows electron impacts to non-destructively move dopants with atomic precision within the silicon lattice. However, this mechanism only works for the two heaviest group V donors with split-vacancy configurations, Bi and Sb. We verify our model by directly imaging these configurations for Bi, and by demonstrating that the promising nuclear spin qubit Sb can be manipulated using a focused electron beam. △ Less

Submitted 25 June, 2021; originally announced June 2021.

Comments: 23 pages, 4 figures, 1 table

Twin-domain formation in epitaxial triangular lattice delafossites

Authors: Jong Mok Ok, Sangmoon Yoon, Andrew R. Lupini, Panchapakesan Ganesh, Amanda Huon, Matthew F. Chisholm, Ho Nyung Lee

Abstract: Twin domains are often found as structural defects in symmetry mismatched epitaxial thin films. The delafossite ABO2, which has a rhombohedral structure, is a good example that often forms twin domains. Although bulk metallic delafossites are known to be the most conducting oxides, the high conductivity is yet to be realized in thin film forms. Suppressed conductivity found in thin films is mainly… ▽ More Twin domains are often found as structural defects in symmetry mismatched epitaxial thin films. The delafossite ABO2, which has a rhombohedral structure, is a good example that often forms twin domains. Although bulk metallic delafossites are known to be the most conducting oxides, the high conductivity is yet to be realized in thin film forms. Suppressed conductivity found in thin films is mainly caused by the formation of twin domains, and their boundaries can be a source of scattering centers for charge carriers. To overcome this challenge, the underlying mechanism for their formation must be understood, so that such defects can be controlled and eliminated. Here, we report the origin of structural twins formed in a CuCrO2 delafossite thin film on a substrate with hexagonal or triangular symmetries. A robust heteroepitaxial relationship is found for the delafossite film with the substrate, and the surface termination turns out to be critical to determine and control the domain structure of epitaxial delafossites. Based on such discoveries, we also demonstrate a twin-free epitaxial thin films grown on high-miscut substrates. This finding provides an important synthesis strategy for growing single domain delafossite thin films and can be applied to other delafossites for epitaxial synthesis of high-quality thin films. △ Less

Submitted 4 May, 2021; originally announced May 2021.

Sculpting the plasmonic responses of nanoparticles by directed electron beam irradiation

Authors: Kevin M. Roccapriore, Shin-Hum Cho, Andrew R. Lupini, Delia J. Milliron, Sergei V. Kalinin

Abstract: Spatial confinement of matter in functional nanostructures has propelled these systems to the forefront of nanoscience, both as a playground for exotic physics and quantum phenomena and in multiple applications including plasmonics, optoelectronics, and sensing. In parallel, the emergence of monochromated electron energy loss spectroscopy (EELS) has enabled exploration of local nanoplasmonic funct… ▽ More Spatial confinement of matter in functional nanostructures has propelled these systems to the forefront of nanoscience, both as a playground for exotic physics and quantum phenomena and in multiple applications including plasmonics, optoelectronics, and sensing. In parallel, the emergence of monochromated electron energy loss spectroscopy (EELS) has enabled exploration of local nanoplasmonic functionalities within single nanoparticles and the collective response of nanoparticle assemblies, providing deep insight into the associated mechanisms. However, modern synthesis processes for plasmonic nanostructures are often limited in the types of accessible geometry and materials, and even then, limited to spatial precisions on the order of tens of nm, precluding the direct exploration of critical aspects of the structure-property relationships. Here, we use the atomic-sized probe of the scanning transmission electron microscope (STEM) to perform precise sculpting and design of nanoparticle configurations. Furthermore, using low-loss (EELS), we provide dynamic analyses of evolution of the plasmonic response during the sculpting process. We show that within self-assembled systems of nanoparticles, individual nanoparticles can be selectively removed, reshaped, or arbitrarily patterned with nanometer-level resolution, effectively modifying the plasmonic response in both space and energy domains. This process significantly increases the scope for design possibilities and presents opportunities for arbitrary structure development, which are ultimately key for nanophotonic design. Nanosculpting introduces yet another capability to the electron microscope. △ Less

Submitted 5 April, 2021; originally announced April 2021.

Van der Waals Epitaxy on Freestanding Monolayer Graphene Membrane by MBE

Authors: Jason Lapano, Ondrej Dyck, Andrew Lupini, Wonhee Ko, Haoxiang Li, Hu Miao, Ho Nyung Lee, An-Ping Li, Matthew Brahlek, Stephen Jesse, Robert G. Moore

Abstract: Research on two-dimensional materials has expanded over the past two decades to become a central theme in condensed matter research today. Significant advances have been made in the synthesis and subsequent reassembly of these materials using mechanical methods into a vast array of hybrid structures with novel properties and ever-increasing potential applications. The key hurdles in realizing this… ▽ More Research on two-dimensional materials has expanded over the past two decades to become a central theme in condensed matter research today. Significant advances have been made in the synthesis and subsequent reassembly of these materials using mechanical methods into a vast array of hybrid structures with novel properties and ever-increasing potential applications. The key hurdles in realizing this potential are the challenges in controlling the atomic structure of these layered hybrid materials and the difficulties in harnessing their unique functionality with existing semiconductor nanofabrication techniques. Here we report on high-quality van der Waals epitaxial growth and characterization of a layered topological insulator on freestanding monolayer graphene transferred to different mechanical supports. This templated synthesis approach enables direct interrogation of interfacial atomic structure of these as-grown hybrid structures and opens a route towards creating device structures with more traditional semiconductor nanofabrication techniques. △ Less

Submitted 29 March, 2021; originally announced March 2021.

Comments: 13 pages, 4 figures

Automated and Autonomous Experiment in Electron and Scanning Probe Microscopy

Authors: Sergei V. Kalinin, Maxim A. Ziatdinov, Jacob Hinkle, Stephen Jesse, Ayana Ghosh, Kyle P. Kelley, Andrew R. Lupini, Bobby G. Sumpter, Rama K. Vasudevan

Abstract: Machine learning and artificial intelligence (ML/AI) are rapidly becoming an indispensable part of physics research, with domain applications ranging from theory and materials prediction to high-throughput data analysis. In parallel, the recent successes in applying ML/AI methods for autonomous systems from robotics through self-driving cars to organic and inorganic synthesis are generating enthus… ▽ More Machine learning and artificial intelligence (ML/AI) are rapidly becoming an indispensable part of physics research, with domain applications ranging from theory and materials prediction to high-throughput data analysis. In parallel, the recent successes in applying ML/AI methods for autonomous systems from robotics through self-driving cars to organic and inorganic synthesis are generating enthusiasm for the potential of these techniques to enable automated and autonomous experiment (AE) in imaging. Here, we aim to analyze the major pathways towards AE in imaging methods with sequential image formation mechanisms, focusing on scanning probe microscopy (SPM) and (scanning) transmission electron microscopy ((S)TEM). We argue that automated experiments should necessarily be discussed in a broader context of the general domain knowledge that both informs the experiment and is increased as the result of the experiment. As such, this analysis should explore the human and ML/AI roles prior to and during the experiment, and consider the latencies, biases, and knowledge priors of the decision-making process. Similarly, such discussion should include the limitations of the existing imaging systems, including intrinsic latencies, non-idealities and drifts comprising both correctable and stochastic components. We further pose that the role of the AE in microscopy is not the exclusion of human operators (as is the case for autonomous driving), but rather automation of routine operations such as microscope tuning, etc., prior to the experiment, and conversion of low latency decision making processes on the time scale spanning from image acquisition to human-level high-order experiment planning. △ Less

Submitted 22 March, 2021; originally announced March 2021.

Probing atomic-scale symmetry breaking by rotationally invariant machine learning of multidimensional electron scattering

Authors: Mark P. Oxley, Maxim Ziatdinov, Ondrej Dyck, Andrew R. Lupini, Rama Vasudevan, Sergei V. Kalinin

Abstract: The 4D scanning transmission electron microscopy (STEM) method has enabled mapping of the structure and functionality of solids on the atomic scale, yielding information-rich data sets containing information on the interatomic electric and magnetic fields, structural and electronic order parameters, and other symmetry breaking distortions. A critical bottleneck on the pathway toward harnessing 4D-… ▽ More The 4D scanning transmission electron microscopy (STEM) method has enabled mapping of the structure and functionality of solids on the atomic scale, yielding information-rich data sets containing information on the interatomic electric and magnetic fields, structural and electronic order parameters, and other symmetry breaking distortions. A critical bottleneck on the pathway toward harnessing 4D-STEM for materials exploration is the dearth of analytical tools that can reduce complex 4D-STEM data sets to physically relevant descriptors. Classical machine learning (ML) methods such as principal component analysis and other linear unmixing techniques are limited by the presence of multiple point-group symmetric variants, where diffractograms from each rotationally equivalent position will form its own component. This limitation even holds for more complex ML methods, such as convolutional neural networks. Here, we propose and implement an approach for the systematic exploration of symmetry breaking phenomena from 4D-STEM data sets using rotationally invariant variational autoencoders (rrVAE), which is designed to disentangle the general rotation of the object from other latent representations. The implementation of purely rotational rrVAE is discussed as are applications to simulated data for graphene and zincblende structures that illustrate the effect of site symmetry breaking. Finally, the rrVAE analysis of 4D-STEM data of vacancies in graphene is illustrated and compared to the classical center-of-mass (COM) analysis. This approach is universal for probing of symmetry breaking phenomena in complex systems and can be implemented for a broad range of diffraction methods exploring the 2D diffraction space of the system, including X-ray ptychography, electron backscatter diffraction (EBSD), and more complex methods. △ Less

Submitted 22 September, 2020; originally announced September 2020.

Gaussian process analysis of Electron Energy Loss Spectroscopy (EELS) data: parallel reconstruction and kernel control

Authors: Sergei V. Kalinin, Andrew R. Lupini, Rama K. Vasudevan, Maxim Ziatdinov

Abstract: Advances in hyperspectral imaging modes including electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) bring forth the challenges of exploratory and subsequently physics-based analysis of multidimensional data sets. The (by now common) multivariate unsupervised linear unmixing methods and their nonlinear analogs generally explore similarities in the energy d… ▽ More Advances in hyperspectral imaging modes including electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) bring forth the challenges of exploratory and subsequently physics-based analysis of multidimensional data sets. The (by now common) multivariate unsupervised linear unmixing methods and their nonlinear analogs generally explore similarities in the energy dimension but ignore correlations in the spatial domain. At the same time, Gaussian process (GP) methods that explicitly incorporate spatial correlations in the form of kernel functions tend to be extremely computationally intensive, while the use of inducing point-based sparse methods often leads to reconstruction artefacts. Here, we suggest and implement a parallel GP method operating on the full spatial domain and reduced representations in the energy domain. In this parallel GP, the information between the components is shared via a common spatial kernel structure while allowing for variability in the relative noise magnitude or image morphology. We explore the role of common spatial structures and kernel constraints on the quality of the reconstruction and suggest an approach for estimating these factors from the experimental data. Application of this method to an example EELS dataset demonstrates that spatial information contained in higher-order components can be reconstructed and spatially localized. This approach can be further applied to other hyperspectral and multimodal imaging modes. The notebooks developed in this manuscript are freely available as part of a GPim package (https://github.com/ziatdinovmax/GPim). △ Less

Submitted 21 May, 2020; originally announced May 2020.

Deep learning of interface structures from the 4D STEM data: cation intermixing vs. roughening

Authors: Mark P. Oxley, Junqi Yin, Nikolay Borodinov, Suhas Somnath, Maxim Ziatdinov, Andrew R. Lupini, Stephen Jesse, Rama K. Vasudevan, Sergei V. Kalinin

Abstract: Interface structures in complex oxides remain one of the active areas of condensed matter physics research, largely enabled by recent advances in scanning transmission electron microscopy (STEM). Yet the nature of the STEM contrast in which the structure is projected along the given direction precludes separation of possible structural models. Here, we utilize deep convolutional neural networks (D… ▽ More Interface structures in complex oxides remain one of the active areas of condensed matter physics research, largely enabled by recent advances in scanning transmission electron microscopy (STEM). Yet the nature of the STEM contrast in which the structure is projected along the given direction precludes separation of possible structural models. Here, we utilize deep convolutional neural networks (DCNN) trained on simulated 4D scanning transmission electron microscopy (STEM) datasets to predict structural descriptors of interfaces. We focus on the widely studied interface between LaAlO3 and SrTiO3, using dynamical diffraction theory and leveraging high performance computing to simulate thousands of possible 4D STEM datasets to train the DCNN to learn properties of the underlying structures on which the simulations are based. We validate the DCNN on simulated data and show that it is possible (with >95% accuracy) to identify a physically rough from a chemically diffuse interface and achieve 85% accuracy in determination of buried step positions within the interface. The method shown here is general and can be applied for any inverse imaging problem where forward models are present. △ Less

Submitted 20 February, 2020; originally announced February 2020.

Comments: 18 pages, 4 figures

Manifold Learning of Four-dimensional Scanning Transmission Electron Microscopy

Authors: Xin Li, Ondrej E. Dyck, Mark P. Oxley, Andrew R. Lupini, Leland McInnes, John Healy, Stephen Jesse, Sergei V. Kalinin

Abstract: Four-dimensional scanning transmission electron microscopy (4D-STEM) of local atomic diffraction patterns is emerging as a powerful technique for probing intricate details of atomic structure and atomic electric fields. However, efficient processing and interpretation of large volumes of data remain challenging, especially for two-dimensional or light materials because the diffraction signal recor… ▽ More Four-dimensional scanning transmission electron microscopy (4D-STEM) of local atomic diffraction patterns is emerging as a powerful technique for probing intricate details of atomic structure and atomic electric fields. However, efficient processing and interpretation of large volumes of data remain challenging, especially for two-dimensional or light materials because the diffraction signal recorded on the pixelated arrays is weak. Here we employ data-driven manifold leaning approaches for straightforward visualization and exploration analysis of the 4D-STEM datasets, distilling real-space neighboring effects on atomically resolved deflection patterns from single-layer graphene, with single dopant atoms, as recorded on a pixelated detector. These extracted patterns relate to both individual atom sites and sublattice structures, effectively discriminating single dopant anomalies via multi-mode views. We believe manifold learning analysis will accelerate physics discoveries coupled between data-rich imaging mechanisms and materials such as ferroelectric, topological spin and van der Waals heterostructures. △ Less

Submitted 13 January, 2019; v1 submitted 18 October, 2018; originally announced November 2018.

Journal ref: npj Computational Materials volume 5, Article number: 5 (2019)

Deep analytics of atomically-resolved images: manifest and latent features

Authors: Maxim Ziatdinov, Ondrej Dyck, Artem Maksov, Bethany M. Hudak, Andrew R. Lupini, Jiaming Song, Paul C. Snijders, Rama K. Vasudevan, Stephen Jesse, Sergei V. Kalinin

Abstract: Recent advances in scanning transmission electron and scanning tunneling microscopies allow researchers to measure materials structural and electronic properties, such as atomic displacements and charge density modulations, at an Angstrom scale in real space. At the same time, the ability to quickly acquire large, high-resolution datasets has created a challenge for rapid physics-based analysis of… ▽ More Recent advances in scanning transmission electron and scanning tunneling microscopies allow researchers to measure materials structural and electronic properties, such as atomic displacements and charge density modulations, at an Angstrom scale in real space. At the same time, the ability to quickly acquire large, high-resolution datasets has created a challenge for rapid physics-based analysis of images that typically contain several hundreds to several thousand atomic units. Here we demonstrate a universal deep-learning based framework for locating and characterizing atomic species in the lattice, which can be applied to different types of atomically resolved measurements on different materials. Specifically, by inspecting and categorizing features in the output layer of a convolutional neural network, we are able to detect structural and electronic 'anomalies' associated with the presence of point defects in a tungsten disulfide monolayer, non-uniformity of the charge density distribution around specific lattice sites on the surface of strongly correlated oxides, and transition between different structural states of buckybowl molecules. We further extended our method towards tracking, from one image frame to another, minute distortions in the geometric shape of individual Si dumbbells in a 3-dimensional Si sample, which are associated with a motion of lattice defects and impurities. Due the applicability of our framework to both scanning tunneling microscopy and scanning transmission electron microscopy measurements, it can provide a fast and straightforward way towards creating a unified database of defect-property relationships from experimental data for each material. △ Less

Submitted 16 January, 2018; originally announced January 2018.

Direct atomic fabrication and dopant positioning in Si using electron beams with active real time image-based feedback

Authors: Stephen Jesse, Bethany M. Hudak, Eva Zarkadoula, Jiaming Song, Artem Maksov, Miguel Fuentes-Cabrera, Panchapakesan Ganesh, Ivan Kravchenko, Paul C. Snijders, Andrew R. Lupini, Albina Borisevich, Sergei V. Kalinin

Abstract: Semiconductor fabrication is a mainstay of modern civilization, enabling the myriad applications and technologies that underpin everyday life. However, while sub-10 nanometer devices are already entering the mainstream, the end of the Moore's Law roadmap still lacks tools capable of bulk semiconductor fabrication on sub-nanometer and atomic levels, with probe-based manipulation being explored as t… ▽ More Semiconductor fabrication is a mainstay of modern civilization, enabling the myriad applications and technologies that underpin everyday life. However, while sub-10 nanometer devices are already entering the mainstream, the end of the Moore's Law roadmap still lacks tools capable of bulk semiconductor fabrication on sub-nanometer and atomic levels, with probe-based manipulation being explored as the only known pathway. Here we demonstrate that the atomic-sized focused beam of a scanning transmission electron microscope can be used to manipulate semiconductors such as Si on the atomic level, inducing growth of crystalline Si from the amorphous phase, reentrant amorphization, milling, and dopant-front motion. These phenomena are visualized in real time with atomic resolution. We further implement active feedback control based on real-time image analytics to control the e-beam motion, enabling shape control and providing a pathway for atom-by-atom correction of fabricated structures in the near future. These observations open a new epoch for atom-by-atom manufacturing in bulk, the long-held dream of nanotechnology. △ Less

Submitted 15 November, 2017; originally announced November 2017.

Persistent photoconductivity in 2-dimensional electron gases at different oxide interfaces

Authors: Emiliano Di Gennaro, Umberto Scotti di Uccio, Carmela Aruta, Claudia Cantoni, Alessandro Gadaleta, Andrew R. Lupini, Davide Maccariello, Daniele Marré, Ilaria Pallecchi, Domenico Paparo, Paolo Perna, Muhammad Riaz, Fabio Miletto Granozio

Abstract: We report on the transport characterization in dark and under light irradiation of three different interfaces: LaAlO3/SrTiO3, LaGaO3/SrTiO3, and the novel NdGaO3/SrTiO3 heterostructure. All of them share a perovskite structure, an insulating nature of the single building blocks, a polar/non- polar character and a critical thickness of four unit cells for the onset of conductivity. The interface st… ▽ More We report on the transport characterization in dark and under light irradiation of three different interfaces: LaAlO3/SrTiO3, LaGaO3/SrTiO3, and the novel NdGaO3/SrTiO3 heterostructure. All of them share a perovskite structure, an insulating nature of the single building blocks, a polar/non- polar character and a critical thickness of four unit cells for the onset of conductivity. The interface structure and charge confinement in NdGaO3/SrTiO3 are probed by atomic-scale- resolved electron energy loss spectroscopy showing that, similarly to LaAlO3/SrTiO3, extra electronic charge confined in a sheet of about 1.5 nm in thickness is present at the NdGaO3/SrTiO3 interface. Electric transport measurements performed in dark and under radiation show remarkable similarities and provide evidence that the persistent perturbation induced by light is an intrinsic peculiar property of the three investigated oxide-based polar/non-polar interfaces. Our work sets a framework for understanding the previous contrasting results found in literature about photoconductivity in LaAlO3/SrTiO3 and highlights the connection between the origin of persistent photoconductivity and the origin of conductivity itself. An improved understanding of the photo- induced metastable electron-hole pairs might allow to shed a direct light on the complex physics of this system and on the recently proposed perspectives of oxide interfaces for solar energy conversion. △ Less

Submitted 22 February, 2014; v1 submitted 12 September, 2013; originally announced September 2013.

Comments: 11 pages, 7 figures

Journal ref: Advanced Optical Materials, Volume 1, Issue 11, pages 834-843, November 2013

Reversible and Persistent Photoconductivity at the NdGaO3/SrTiO3 Conducting Interface

Authors: Umberto Scotti di Uccio, Carmela Aruta, Claudia Cantoni, Emiliano Di Gennaro, Alessandro Gadaleta, Andrew R. Lupini, Davide Maccariello, Daniele Marré, Ilaria Pallecchi, Domenico Paparo, Paolo Perna, Muhammad Riaz, Fabio Miletto Granozio

Abstract: The interface between the band gap insulators LaAlO3 and SrTiO3 is known to host a highly mobile two-dimensional electron gas. Here we report on the fabrication and characterization of the NdGaO3/SrTiO3 interface, that shares with LaAlO3/SrTiO3 an all-perovskite structure, the insulating nature of the single building block and the polar-non polar character. Our work demonstrates that in NdGaO3/SrT… ▽ More The interface between the band gap insulators LaAlO3 and SrTiO3 is known to host a highly mobile two-dimensional electron gas. Here we report on the fabrication and characterization of the NdGaO3/SrTiO3 interface, that shares with LaAlO3/SrTiO3 an all-perovskite structure, the insulating nature of the single building block and the polar-non polar character. Our work demonstrates that in NdGaO3/SrTiO3 a metallic layer of mobile electrons is formed, with properties comparable to LaAlO3/SrTiO3. The localization of the injected electrons at the Ti sites, within a few unit cells from the interface, was proved by Atomic-scale-resolved EELS analyses. The electric transport and photoconduction of samples were also investigated. We found that irradiation by photons below the SrTiO3 gap does not increase the carrier density, but slightly enhances low temperature mobility. A giant persistent photoconductivity effect was instead observed, even under irradiation by low energy photons, in highly resistive samples fabricated at non-optimal conditions. We discuss the results in the light of different mechanisms proposed for the two-dimensional electron gas formation. Both the ordinary and the persistent photoconductivity in these systems are addressed and analyzed. △ Less

Submitted 22 June, 2012; originally announced June 2012.

Comments: 24 pages, 7 figures, submitted to ACS Nano

Electron transfer and ionic displacements at the origin of the 2D electron gas at the LAO/STO interface: Direct measurements with atomic-column spatial resolution

Authors: C. Cantoni, J. Gazquez, F. Miletto Granozio, M. P. Oxley, M. Varela, A. R. Lupini, S. J. Pennycook, C. Aruta, U. Scotti di Uccio, P. Perna, D. Maccariello

Abstract: The discovery that the interface between two band gap insulators LaAlO3 and SrTiO3 is highly conducting has raised an enormous interest in the field of oxide electronics. The LAlO3/SrTiO3 interface can be tuned using an electric field and switched from a superconducting to an insulating state. Conducting paths in an insulating background can be written applying a voltage with the tip of an atomic… ▽ More The discovery that the interface between two band gap insulators LaAlO3 and SrTiO3 is highly conducting has raised an enormous interest in the field of oxide electronics. The LAlO3/SrTiO3 interface can be tuned using an electric field and switched from a superconducting to an insulating state. Conducting paths in an insulating background can be written applying a voltage with the tip of an atomic force microscope, creating great promise for the development of a new generation of nanoscale electronic devices. However, the mechanism for interface conductivity in LaAlO3/SrTiO3 has remained elusive. The theoretical explanation based on an intrinsic charge transfer (electronic reconstruction) has been strongly challenged by alternative descriptions based on point defects. In this work, thanks to modern aberration-corrected electron probes with atomic-scale spatial resolution, interfacial charge and atomic displacements originating the electric field within the system can be simultaneously measured, yielding unprecedented experimental evidence in favor of an intrinsic electronic reconstruction. △ Less

Submitted 20 June, 2012; originally announced June 2012.

Comments: Accepted for publication in Advanced Materials. Published online on 06/20/2012

Journal ref: Advanced Materials 24, 3952 (2012)

Direct Observation of an Interface Dipole between Two Metallic Oxides Caused by Localized Oxygen Vacancies

Authors: A. Y. Borisevich, A. R. Lupini, J. He, E. A. Eliseev, A. N. Morozovska, G. S. Svechnikov, P. Yu, Y. H. Chu, R. Ramesh, S. T. Pantelides, S. V. Kalinin, S. J. Pennycook

Abstract: Oxygen vacancies are increasingly recognized to play a role in phenomena observed at transition-metal oxide interfaces. Here we report a study of SrRuO3/La0.7Sr0.3MnO3 (SRO/LSMO) interfaces using a combination of quantitative aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and density-functional calculations. Cation displacements are observed at t… ▽ More Oxygen vacancies are increasingly recognized to play a role in phenomena observed at transition-metal oxide interfaces. Here we report a study of SrRuO3/La0.7Sr0.3MnO3 (SRO/LSMO) interfaces using a combination of quantitative aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and density-functional calculations. Cation displacements are observed at the interface, indicative of a dipole-like electric field even though both materials are nominally metallic. The observed displacements are reproduced by theory if O vacancies are present in the near-interface LSMO layers. The results suggest that atomic-scale structural mapping can serve as a quantitative indicator of the presence of O vacancies at interfaces. △ Less

Submitted 31 October, 2011; originally announced November 2011.

Journal ref: Phys. Rev. B 86, 140102(R) (2012)

Direct measurement of charge transfer phenomena at ferromagnetic/superconducting oxide interfaces

Authors: M. Varela, A. R. Lupini, V. Pena, Z. Sefrioui, I. Arslan, N. D. Browning, J. Santamaria, S. J. Pennycook

Abstract: YBa2Cu3O7-x/La0.67Ca0.33MnO3 ferromagnetic/superconducting interfaces are analyzed by scanning transmission electron microscopy and electron energy loss spectroscopy with monolayer resolution. We demonstrate that extensive charge transfer occurs between the manganite and the superconductor, in a manner similar to modulation-doped semiconductors, which explains the reduced critical temperatures o… ▽ More YBa2Cu3O7-x/La0.67Ca0.33MnO3 ferromagnetic/superconducting interfaces are analyzed by scanning transmission electron microscopy and electron energy loss spectroscopy with monolayer resolution. We demonstrate that extensive charge transfer occurs between the manganite and the superconductor, in a manner similar to modulation-doped semiconductors, which explains the reduced critical temperatures of heterostructures. This behavior is not seen with insulating PrBa2Cu3O7 layers. Furthermore, we confirm directly that holes in the YBa2Cu3O7-x are located on the CuO2 planes. △ Less

Submitted 15 May, 2006; v1 submitted 23 August, 2005; originally announced August 2005.

Comments: 16 pages, 4 figures

Spectroscopic imaging of single atoms within a bulk solid

Authors: M. Varela, S. D. Findlay, A. R. Lupini, H. M. Christen, A. Y. Borisevich, N. Dellby, O. L. Krivanek, P. D. Nellist, M. P. Oxley, L. J. Allen, S. J. Pennycook, .

Abstract: The ability to localize, identify and measure the electronic environment of individual atoms will provide fundamental insights into many issues in materials science, physics and nanotechnology. We demonstrate, using an aberration-corrected scanning transmission microscope, the spectroscopic imaging of single La atoms inside CaTiO3. Dynamical simulations confirm that the spectroscopic information… ▽ More The ability to localize, identify and measure the electronic environment of individual atoms will provide fundamental insights into many issues in materials science, physics and nanotechnology. We demonstrate, using an aberration-corrected scanning transmission microscope, the spectroscopic imaging of single La atoms inside CaTiO3. Dynamical simulations confirm that the spectroscopic information is spatially confined around the scattering atom. Furthermore we show how the depth of the atom within the crystal may be estimated. △ Less

Submitted 28 January, 2004; v1 submitted 9 January, 2004; originally announced January 2004.

Comments: 4 pages and 3 figures. Accepted in Phys.Rev.Lett