Sub-angstrom Resolution Research Articles

In-situ sub-angstrom characterization of laser-lubricant interaction in a thermo-tribological system

Laser-lubricant interaction has been a critical reliability issue in a thermo-tribological system named heat-assisted magnetic recording, one of the next generation hard disk drive solutions to increasing data storage. The lubricant response under laser irradiation and the subsequent lubricant recovery are crucial to the system’s reliability and longevity, however, they cannot be diagnosed locally and timely so far. Here, we propose a thermal scheme to in-situ characterize the mechanical laser-lubricant interaction. The nanometer-thick lubricant has a thermal barrier effect on the near-field thermal transport in the system, according to which the lubricant thickness can be determined. As demonstrations, this paper reports the first quantitative in-situ measurements of the laser-induced lubricant depletion and the subsequent reflow dynamics. The proposed scheme shows a sub-angstrom resolution (~0.2 Å) and a fast response time within seconds, rendering in-situ real-time lubricant diagnosis feasible in the practical hard disk drive products.

Picometer-Level In Situ Manipulation of Ferroelectric Polarization in Van der Waals layered InSe.

Ferroelectric 2D van der Waals (vdW) layered materials are attracting increasing attention due to their potential applications in next-generation nanoelectronics and in-memory computing with polarization-dependent functionalities. Despite the critical role of polarization in governing ferroelectricity behaviors, its origin and relation with local structures in 2D vdW layered materials have not been fully elucidated so far. Here, intralayer sliding of approximately six degrees within each quadruple-layer of the prototype 2D vdW ferroelectrics InSe is directly observed and manipulated using sub-angstrom resolution imaging and in situ biasing in an aberration-corrected scanning transmission electron microscope. The in situ electric manipulation further indicates that the reversal of intralayer sliding can be achieved by altering the electric field direction. Density functional theory calculations reveal that the reversible picometer-level intralayer sliding is responsible for switchable out-of-plane polarization. The observation and manipulation of intralayer sliding demonstrate the structural origin of ferroelectricity in InSe and establish a dynamic structural variation model for future investigations on more 2D ferroelectric materials.

Atomic-level quantitative analysis of electronic functional materials by aberration-corrected STEM

Abstract The stable sub-angstrom resolution of the aberration-corrected scanning transmission electron microscope (AC-STEM) makes it an advanced and practical characterization technique for all materials. Owing to the prosperous advancement in computational technology, specialized software and programs have emerged as potent facilitators across the entirety of electron microscopy characterization process. Utilizing advanced image processing algorithms promotes the rectification of image distortions, concurrently elevating the overall image quality to superior standards. Extracting high-resolution, pixel-level discrete information and converting it into atomic-scale, followed by performing statistical calculations on the physical matters of interest through quantitative analysis, represent an effective strategy to maximize the value of electron microscope images. The efficacious utilization of quantitative analysis of electron microscope images has become a progressively prominent consideration for materials scientists and electron microscopy researchers. This article offers a concise overview of the pivotal procedures in quantitative analysis and summarizes the computational methodologies involved from three perspectives: contrast, lattice and strain, as well as atomic displacements and polarization. It further elaborates on practical applications of these methods in electronic functional materials, notably in piezoelectrics/ferroelectrics and thermoelectrics. It emphasizes the indispensable role of quantitative analysis in fundamental theoretical research, elucidating the structure–property correlations in high-performance systems, and guiding synthesis strategies.

A quantum sensor for atomic-scale electric and magnetic fields

The detection of faint magnetic fields from single-electron and nuclear spins at the atomic scale is a long-standing challenge in physics. While current mobile quantum sensors achieve single-electron spin sensitivity, atomic spatial resolution remains elusive for existing techniques. Here we fabricate a single-molecule quantum sensor at the apex of the metallic tip of a scanning tunnelling microscope by attaching Fe atoms and a PTCDA (3,4,9,10-perylenetetracarboxylic-dianhydride) molecule to the tip apex. We address the molecular spin by electron spin resonance and achieve ~100 neV resolution in energy. In a proof-of-principle experiment, we measure the magnetic and electric dipole fields emanating from a single Fe atom and an Ag dimer on an Ag(111) surface with sub-angstrom spatial resolution. Our method enables atomic-scale quantum sensing experiments of electric and magnetic fields on conducting surfaces and may find applications in the sensing of spin-labelled biomolecules and of spin textures in quantum materials.

Electron Ptychography for Deep Sub-angstrom Resolution Without an Aberration Corrector

Electron Ptychography for Deep Sub-angstrom Resolution Without an Aberration Corrector

Real-time tracking of three-dimensional atomic dynamics of Pt trimer on TiO2 (110).

Single and multi-atoms supported on oxide substrates ultimately increase the efficiency of noble metal atom use, and moreover, catalytic activity and selectivity are also improved substantially. However, single and multi-atoms are unstable under catalytic conditions, and these metal atoms spontaneously aggregate and grow into nanoparticles. Catalytic performance is strongly related to local atomic configurations, and hence, it is essential to determine the three-dimensional (3D) atomic structures of multi-atoms on the substrate and their structural dynamics. Here, we show the real-time tracking of the 3D structural evolution of a Pt trimer on TiO2 (110) substrate at a high temperature, using high-spatiotemporal-resolution scanning transmission electron microscopy, where sub-angstrom spatial resolution is maintained, while the temporal resolution reaches 40 milliseconds. With the aid of prior structural knowledge of a Pt trimer for 3D reconstruction, the present method could open the way to characterize in situ atomic-scale structural dynamics, especially meta-stable structural transition.

Understanding the Electron Beam Resilience of Two-Dimensional Conjugated Metal-Organic Frameworks.

Knowledge of the atomic structure of layer-stacked two-dimensional conjugated metal-organic frameworks (2D c-MOFs) is an essential prerequisite for establishing their structure-property correlation. For this, atomic resolution imaging is often the method of choice. In this paper, we gain a better understanding of the main properties contributing to the electron beam resilience and the achievable resolution in the high-resolution TEM images of 2D c-MOFs, which include chemical composition, density, and conductivity of the c-MOF structures. As a result, sub-angstrom resolution of 0.95 Å has been achieved for the most stable 2D c-MOF of the considered structures, Cu3(BHT) (BHT = benzenehexathiol), at an accelerating voltage of 80 kV in a spherical and chromatic aberration-corrected TEM. Complex damage mechanisms induced in Cu3(BHT) by the elastic interactions with the e-beam have been explained using detailed ab initio molecular dynamics calculations. Experimental and calculated knock-on damage thresholds are in good agreement.

Achieving sub-0.5-angstrom-resolution ptychography in an uncorrected electron microscope.

Subangstrom resolution has long been limited to aberration-corrected electron microscopy, where it is a powerful tool for understanding the atomic structure and properties of matter. Here, we demonstrate electron ptychography in an uncorrected scanning transmission electron microscope (STEM) with deep subangstrom spatial resolution down to 0.44 angstroms, exceeding the conventional resolution of aberration-corrected tools and rivaling their highest ptychographic resolutions​. Our approach, which we demonstrate on twisted two-dimensional materials in a widely available commercial microscope, far surpasses prior ptychographic resolutions (1 to 5 angstroms) of uncorrected STEMs. We further show how geometric aberrations can create optimized, structured beams for dose-efficient electron ptychography. Our results demonstrate that expensive aberration correctors are no longer required for deep subangstrom resolution.

Dislocation descriptors of low and high angle grain boundaries with convolutional neural networks

Multiscale modeling of plasticity in polycrystalline metals is a long-standing challenge in part because of the lack of an accepted grain boundary descriptor, which has hindered the bridging of scales between atomistic simulations and meso-scale discrete dislocation dynamics (DDD) models. While grain boundary dislocations (GBDs) for low angle grain boundaries can be ascertained by Burgers circuit analyses, the dislocation structures of high angle grain boundaries have remained elusive because of overlapping dislocation core fields. Here, we use convolutional neural networks (CNNs) to establish the locations of GBDs responsible for the misorientations of <001> symmetrical-tilt Cu grain boundaries, from the local atomistic stress fields modeled with molecular dynamics (MD) simulations. We achieve accurate CNN predictions of GBDs with sub-angstrom resolution across an extensive set of low- to high-tilt-angle grain boundaries through a training dataset, generated from superposing continuum-representations of the MD stress fields of single dislocations that consider the contributions of both the Volterra and dislocation core fields. The approach paves the way for dislocation representations of the atomistic grain boundary structures modeled by MD simulations or density functional theory (DFT) calculations in mesoscale DDD models to elucidate multiscale plasticity effects.

Photo-induced structural dynamics of o-nitrophenol by ultrafast electron diffraction.

The photo-induced dynamics of o-nitrophenol, particularly its photolysis, has garnered significant scientific interest as a potential source of nitrous acid in the atmosphere. Although the photolysis products and preceding photo-induced electronic structure dynamics have been investigated extensively, the nuclear dynamics accompanying the non-radiative relaxation of o-nitrophenol on the ultrafast timescale, which include an intramolecular proton transfer step, have not been experimentally resolved. Herein, we present a direct observation of the ultrafast nuclear motions mediating photo-relaxation using ultrafast electron diffraction. This work spatiotemporally resolves the loss of planarity which enables access to a conical intersection between the first excited state and the ground state after the proton transfer step, on the femtosecond timescale and with sub-Angstrom resolution. Our observations, supported by ab initio multiple spawning simulations, provide new insights into the proton transfer mediated relaxation mechanism in o-nitrophenol.

Local Structure in Sodium Antiperovskite Ionic Conductors

Antiperovskites, which can serve as solid state electrolytes, have recently received great attention due to their high ionic conductivity[1]. Compared to typical solid-state ionic conductors, Na3OCl antiperovskite exhibits a unique ionic conductivity behavior that deviates from a linear Arrhenius behavior, in which the ionic conductivity increases rapidly with increasing temperature[2,3]. Many different structural origins affecting the ion conduction mechanism in antiperovskite have been proposed using calculation, such as vacancy/dumbbell migration[4]. The lack of direct experimental local structural investigations makes it challenging to fully understand the structural origins for high ionic conductivity, even resulting in limitations in the development of material designs for good ionic conductors. Complicating their investigation, these materials are also extremely sensitive to air, preventing many structural studies.Here, local structural investigations of Na3OCl are performed using aberration corrected scanning transmission electron microscopy (STEM) to elucidate the atomistic mechanisms underlying its high ionic conduction. To avoid degradation in air, we use an inert gas transfer system that has several advantages. First, transfer in an inert gas environment without air/moisture exposure enables the sample to retain the on-stoichiometry. Second, both room temperature and cryo experiments are possible, which allows pristine structure investigation at application relevant temperatures. Beyond the environment, Na3OCl is also very electron beam sensitive. We will show that low-dose imaging methods, such as parallel beam electron diffraction and electron ptychography, can provide new insights and allow for direct imaging of the local structure. For example, diffuse electron scattering (scattering between the Bragg reflections) is used to investigate subtle differences resulting from local structure, atomic defects, and phonon dispersion relations through combination with electron scattering simulations and Molecular Dynamics (MD) using machine learned potentials. We will then demonstrate how electron ptychography can achieve deep sub-angstrom spatial resolution with low dose conditions and provide direct insights into the migration of Na-ions. Overall, the inert gas transfer system and low-dose diffraction and STEM ptychography enable direct imaging of local structures even for reactive and electron beam sensitive materials, such as antiperovskite electrolytes. The understanding of local structure related to the high ionic conductivity will be crucial for designing high-performance solid electrolytes.

Degrees of freedom of atoms in a rigid molecule for local temperature calculation in molecular dynamics simulation

ABSTRACT In molecular dynamics simulation, it is quite common to calculate a precise temperature profile with an atomic or sub-angstrom resolution. While this calculation typically requires the degrees of freedom (DOF) of individual atoms, those in a molecule subject to geometric constraints is undefined in general. Conventionally, the approximation of subtracting from the atom DOF by 1/2 per one distance constraint has been used, but this approximation is valid for spatially homogeneous systems only. In the present study, on the basis of statistical mechanics, we derived more general expressions for the DOF of atoms in fully or partially rigid molecules. The expressions ensure that in an equilibrium state, all constrained atoms have the same temperature as the equilibrium temperature of the whole system and are also applicable to inhomogeneous systems.

Nucleation dependence of atomic layer deposition on diamond surface termination

Surface termination and interfacial interactions are critical for advanced solid-state quantum applications. In this paper, we demonstrate that atomic layer deposition (ALD) can both provide valuable insight on the chemical environment of the surface, having sufficient sensitivity to distinguish between the common diamond (001) surface termination types and passivate these interfaces as desired. We selected diamond substrates exhibiting both smooth and anomalously rough surfaces to probe the effect of morphology on ALD nucleation. We use high resolution in situ spectroscopic ellipsometry to monitor the surface reaction with sub-angstrom resolution, to evaluate the nucleation of an ALD Al2O3 process as a function of different ex and in situ treatments to the diamond surface. In situ water dosing and high vacuum annealing provided the most favorable environment for nucleation of dimethylaluminum isopropoxide and water ALD. Hydrogen termination passivated both smooth and rough surfaces while triacid cleaning passivated the smooth surface only, with striking effectiveness.

The Long and Winding Road: On-Demand DNA Synthesis in High Demand

The Long and Winding Road: On-Demand DNA Synthesis in High Demand

Ptychographic measurements of varying size and shape along zeolite channels.

Sub-angstrom resolution imaging of porous materials like zeolites is important to reveal their structure-property relationships involved in ion exchange, molecule adsorption and separation, and catalysis. Using multislice electron ptychography, we successfully measured the atomic structure of zeolite at sub-angstrom lateral resolution for 100-nanometer-thick samples. Both lateral and depth deformations of the straight channels are mapped, showing the three-dimensional structural inhomogeneity and flexibility. Since most zeolites in industrial applications are usually tens to hundreds of nanometers thick, the sub-angstrom resolution imaging and accurate measurements of depth-dependent local structures with electron ptychography at low-dose condition will find wide applications in porous materials close to their industrially relevant conditions.

Deep-Learning Electron Diffractive Imaging

We report the development of deep-learning coherent electron diffractive imaging at subangstrom resolution using convolutional neural networks (CNNs) trained with only simulated data. We experimentally demonstrate this method by applying the trained CNNs to recover the phase images from electron diffraction patterns of twisted hexagonal boron nitride, monolayer graphene, and a gold nanoparticle with comparable quality to those reconstructed by a conventional ptychographic algorithm. Fourier ring correlation between the CNN and ptychographic images indicates the achievement of a resolution in the range of 0.70 and 0.55Å. We further develop CNNs to recover the probe function from the experimental data. The ability to replace iterative algorithms with CNNs and perform real-time atomic imaging from coherent diffraction patterns is expected to find applications in the physical and biological sciences.

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Imaging Atomic Structure, Strain, and Disorder By Atomic Electron Tomography

Knowledge of the atomic structure of materials is critical to understanding their functionality in many fields such as biology, microelectronics, condensed matter, and nanotechnology. Traditionally, x-ray crystallography has been the main method used to solve structures of all types. New techniques such as micro electron diffraction and single particle cryo electron microscopy are now utilizing electrons to study crystals that cannot be crystallized at the scales necessary for x-ray experiments. However, these techniques require ensembles of identical unit cells (like molecules or proteins) to solve the structure from a series of images or diffraction patterns. The need to average over many unit cells means that important material properties such as defects, dopants, and disorder are not captured. Investigations of unique arrangements of atoms requires a direct imaging method that does not rely on structural averaging or the assumption of crystallinity.Transmission electron microscopy (TEM) provides the ability to directly image atomic structure, and the development of powerful, stable aberration correctors in the last decade now routinely provide sub-Angstrom imaging resolution. Scanning TEM (STEM) has also become an effective method for measuring the atomic structure of inorganic materials based on so-called Z-contrast where the image intensities are proportional to the atomic number of the material being imaged. However, by their nature TEMs are limited to producing only two-dimensional projections of a structure. Electron tomography is a technique that can reconstruct the three-dimensional structure of unique nanoscale objects from a series of two-dimensional images acquired at different viewing angles. Tomography was traditionally limited to nanoscale resolution due to experimental difficulties and a lack of high-quality reconstruction algorithms.We have developed atomic electron tomography (AET) by utilizing the sub-Angstrom real-space resolution of STEM with advanced tomographic reconstruction algorithms to solve the structure of materials at the single atom level without averaging or the assumption of crystallinity. This talk will present recent progress and capabilities in the field of AET for reconstructing order and disorder in materials with 20 picometer precision. Our success in resolving chemical order/disorder in metallic FePt nanoparticles was extended to include the time domain by capturing nucleation and growth or ordered phases in the same nanoparticle. Also, the ability to reconstruct atomic structure without the need for averaging or crystallinity led to the achievement of directly imaging three-dimensional atomic arrangements in amorphous solids. We determined the chemical and structural disorder of an eight-element metallic glass to quantitatively characterize short- and medium-range order, and we determined the disordered atomic packing of monatomic amorphous materials.The field of electron microscopy is rapidly changing with the advent of improved detectors, aberration correctors, and monochromators. We are exploring the use of these new hardware to implement new imaging algorithms such as ptychography to allow AET to explore all new classes of materials especially lightly scattering materials like oxygen. We have also established the Materials Databank to share experimentally determined 3D atomic structures with other scientists. As more structures and classes of materials are solved, we expect AET to become an important technique in the field of atomic structural characterization.

Automatic parameter selection for electron ptychography via Bayesian optimization

Electron ptychography provides new opportunities to resolve atomic structures with deep sub-angstrom spatial resolution and to study electron-beam sensitive materials with high dose efficiency. In practice, obtaining accurate ptychography images requires simultaneously optimizing multiple parameters that are often selected based on trial-and-error, resulting in low-throughput experiments and preventing wider adoption. Here, we develop an automatic parameter selection framework to circumvent this problem using Bayesian optimization with Gaussian processes. With minimal prior knowledge, the workflow efficiently produces ptychographic reconstructions that are superior to those processed by experienced experts. The method also facilitates better experimental designs by exploring optimized experimental parameters from simulated data.

Deep sub-angstrom resolution imaging by electron ptychography with misorientation correction.

Superresolution imaging of solids is essential to explore local symmetry breaking and derived material properties. Electron ptychography is one of the most promising schemes to realize superresolution imaging beyond aberration correction. However, to reach both deep sub-angstrom resolution imaging and accurate measurement of atomic structures, it is still required for the electron beam to be nearly parallel to the zone axis of crystals. Here, we report an efficient and robust method to correct the specimen misorientation in electron ptychography, giving deep sub-angstrom resolution for specimens with large misorientations. The method largely reduces the experimental difficulties of electron ptychography and paves the way for widespread applications of ptychographic deep sub-angstrom resolution imaging.