Real-space Resolution Research Articles

Multidimensional images and aberrations in STEM

Abstract Recent advances in scanning transmission electron microscopy (STEM) have led to increased development of multi-dimensional STEM imaging modalities and novel image reconstruction methods. This interest arises because the main electron lens in a modern transmission electron microscope usually has a diffraction-space information limit that is significantly better than the real-space resolution of the same lens. This state-of-affairs is sometimes shared by other scattering methods in modern physics and contributes to a broader excitement surrounding multidimensional techniques that scan a probe while recording diffraction-space images, such as ptychography and scanning nano-beam diffraction. However, the contrasting resolution in the two spaces raises the question as to what is limiting their effective performance. Here, we examine this paradox by considering the effects of aberrations in both image and diffraction planes, and likewise separate the contributions of pre- and post-sample aberrations. This consideration provides insight into aberration-measurement techniques and might also indicate improvements for super-resolution techniques.

Reconstruction of martensite variant microstructures in grains of deformed NiTi shape memory alloy by TEM

Two transmission electron microscopy (TEM) based methods were applied to reconstruct martensite variant microstructures in entire grains of plastically deformed nanocrystalline NiTi wires. First method consists in manual TEM analysis of composite electron diffraction patterns and dark field images of microstructures taken using carefully selected Bragg spots. Second method involves automated orientation/phase mapping of martensite variant microstructures in TEM (ASTAR). Application of these two methods helped us to discover new mechanism of the plastic deformation of monoclinic B19’ martensite in NiTi shape memory alloys called kwinking as it combines dislocation slip based kinking and deformation twinning in martensite.In this work, we describe how the martensite variant microstructures in nanograins of plastically deformed NiTi shape memory wire are reconstructed using the TEM based methods and discuss their advantages and disadvantages. The application of the first manual method requires tilting the TEM lamella so that all martensite variants in the analysed grain are oriented in a common low index zone. Such common crystal lattice direction exists in each grain of plastically deformed martensitic NiTi wire because the microstructures were created by the kwinking deformation mechanism. The requirement to orient the selected grain into a common unique direction in monoclinic lattice, on the other hand, is not unique to kwinking and represents a severe limitation of the SAED-DF method due to the constraints on tilting the TEM lamella. Application of the automated crystal orientation mapping ASTAR method enables detailed orientation analysis of nanoscale martensite variant microstructures in generally oriented grains of deformed NiTi wires as well as pointwise ex-situ analysis of interfaces with excellent resolution in real and orientation space.

Single-molecule electron spin resonance by means of atomic force microscopy

Understanding and controlling decoherence in open quantum systems is of fundamental interest in science, whereas achieving long coherence times is critical for quantum information processing1. Although great progress was made for individual systems, and electron spin resonance (ESR) of single spins with nanoscale resolution has been demonstrated2–4, the understanding of decoherence in many complex solid-state quantum systems requires ultimately controlling the environment down to atomic scales, as potentially enabled by scanning probe microscopy with its atomic and molecular characterization and manipulation capabilities. Consequently, the recent implementation of ESR in scanning tunnelling microscopy5–8 represents a milestone towards this goal and was quickly followed by the demonstration of coherent oscillations9,10 and access to nuclear spins11 with real-space atomic resolution. Atomic manipulation even fuelled the ambition to realize the first artificial atomic-scale quantum devices12. However, the current-based sensing inherent to this method limits coherence times12,13. Here we demonstrate pump–probe ESR atomic force microscopy (AFM) detection of electron spin transitions between non-equilibrium triplet states of individual pentacene molecules. Spectra of these transitions exhibit sub-nanoelectronvolt spectral resolution, allowing local discrimination of molecules that only differ in their isotopic configuration. Furthermore, the electron spins can be coherently manipulated over tens of microseconds. We anticipate that single-molecule ESR-AFM can be combined with atomic manipulation and characterization and thereby paves the way to learn about the atomistic origins of decoherence in atomically well-defined quantum elements and for fundamental quantum-sensing experiments.

Capturing the non-equilibrium state in light–matter–free-electron interactions through ultrafast transmission electron microscopy

Ultrafast transmission electron microscope (UTEM) with the multimodality of time-resolved diffraction, imaging, and spectroscopy provides a unique platform to reveal the fundamental features associated with the interaction between free electrons and matter. In this review, we summarize the principles, instrumentation, and recent developments of the UTEM and its applications in capturing dynamic processes and non-equilibrium transient states. The combination of the transmission electron microscope with a femtosecond laser via the pump–probe method guarantees the high spatiotemporal resolution, allowing the investigation of the transient process in real, reciprocal and energy spaces. Ultrafast structural dynamics can be studied by diffraction and imaging methods, revealing the coherent acoustic phonon generation and photo-induced phase transition process. In the energy dimension, time-resolved electron energy-loss spectroscopy enables the examination of the intrinsic electronic dynamics of materials, while the photon-induced near-field electron microscopy extends the application of the UTEM to the imaging of optical near fields with high real-space resolution. It is noted that light–free-electron interactions have the ability to shape electron wave packets in both longitudinal and transverse directions, showing the potential application in the generation of attosecond electron pulses and vortex electron beams.

Nanoscale X-ray Imaging of Composition and Ferroelastic Domains in Heterostructured Perovskite Nanowires: Implications for Optoelectronic Devices.

Metal halide perovskites (MHPs) have garnered significant interest as promising candidates for nanoscale optoelectronic applications due to their excellent optical properties. Axially heterostructured CsPbBr3-CsPb(Br(1-x)Clx)3 nanowires can be produced by localized anion exchange of pregrown CsPbBr3 nanowires. However, characterizing such heterostructures with sufficient strain and real space resolution is challenging. Here, we use nanofocused scanning X-ray diffraction (XRD) and X-ray fluorescence (XRF) with a 60 nm beam to investigate a heterostructured MHP nanowire as well as a reference CsPbBr3 nanowire. The nano-XRD approach gives spatially resolved maps of composition, lattice spacing, and lattice tilt. Both the reference and exchanged nanowire show signs of diverse types of ferroelastic domains, as revealed by the tilt maps. The chlorinated segment shows an average Cl composition of x = 66 and x = 70% as measured by XRD and XRF, respectively. The XRD measurements give a much more consistent result than the XRF ones. These findings are consistent with photoluminescence measurements, showing x = 73%. The nominally unexchanged segment also has a small concentration of Cl, as observed with all three methods, which we attribute to diffusion after processing. These results highlight the need to prevent such unwanted processes in order to fabricate optoelectronic devices based on MHP heterostructures.

Real-time imaging of sulfhydryl single-stranded DNA aggregation

The structure and functionality of biomacromolecules are often regulated by chemical bonds, however, the regulation process and underlying mechanisms have not been well understood. Here, by using in situ liquid-phase transmission electron microscopy (LP-TEM), we explored the function of disulfide bonds during the self-assembly and structural evolution of sulfhydryl single-stranded DNA (SH-ssDNA). Sulfhydryl groups could induce self-assembly of SH-ssDNA into circular DNA containing disulfide bonds (SS-cirDNA). In addition, the disulfide bond interaction triggered the aggregation of two SS-cirDNA macromolecules along with significant structural changes. This visualization strategy provided structure information at nanometer resolution in real time and space, which could benefit future biomacromolecules research.

An inclined detector geometry for improved X-ray total scattering measurements.

X-ray total scattering measurements are implemented using a digital flat-panel area detector in an inclined geometry and compared with the traditional geometry. The traditional geometry is defined here by the incident X-ray beam impinging on and normal to the center-most pixel of a detector. The inclined geometry is defined here by a detector at a pitch angle α, set to 15° in this case, bisected by the vertical scattering plane. The detector is positioned such that the incident X-ray beam strikes the pixels along the bottom edge and 90° scattered X-rays impinge on the pixels along the top edge. The geometric attributes of the inclined geometry translate into multiple benefits, such as an extension of the measurable scattering range to 90°, a 47% increase in the accessible magnitudes of the reciprocal-space vector Q and a leveling of the dynamic range in the measured total scattering pattern. As a result, a sixfold improvement in signal-to-noise ratios is observed at higher scattering angles, enabling up to a 36-fold reduction in acquisition time. Additionally, the extent of applied modification functions is reduced, decreasing the magnitude of termination ripples and improving the real-space resolution of the pair distribution function G(r). Taken all together, these factors indicate that the inclined geometry produces higher quality data than the traditional geometry, usable for simultaneous Rietveld refinement and total scattering studies.

Probing charge density in materials with atomic resolution in real space

Probing charge density in materials with atomic resolution in real space

Pressure cells for in situ neutron total scattering: time and real-space resolution during deuterium absorption.

In situ gas-loading sample holders for two-dimensionally arranged detectors in time-of-flight neutron total scattering experiments have been developed to investigate atomic arrangements during deuterium absorption using time and real-space resolution. A single-crystal sapphire container was developed that allows conditions of 473 K and 10 MPa hydrogen gas pressure. High-resolution transient measurements detected deuterium absorption by palladium that proceeded within a few seconds. A double-layered container with thick- and thin-walled vanadium allowed conditions of 423 K and 10 MPa hydrogen gas pressure. The deuterium occupation sites of a lanthanum-nickel-aluminium alloy are discussed in detail on the basis of real-space high-resolution data obtained from in situ neutron scattering measurements and reverse Monte Carlo structural modeling.

Entangled polarizations in ferroelectrics: A focused review of polar topologies

Ferroelectric crystals feature asymmetric or polar structures that are switchable under an external electric field, holding promise for information storage. Nanoscale ferroelectrics might exhibit various exotic domain configurations and polar topologies, such as full flux-closure, vortex, skyrmion, and meron. These topological domains were theoretically switchable and may give rise to an unusually high density of memory bits. They would also undergo unusual phase transitions and form hidden, collective polar topological states under external stimulations. Similar domains and spin topologies are well known in ferromagnetic materials, and their topological properties and dynamics are under intensive investigation. However, in ferroelectric materials, the coupling of polarizations to spontaneous strains would be so pronounced that the formations of polar topologies were believed to be impossible. How to stabilize the polar topologies in ferroelectrics, especially in nanoscale ferroelectrics, was known as a big challenge.In this overview, we summarize the recent progress in polar topologies in ferroelectric oxides. We start from a review the discovery of polar topologies, including flux-closure quadrant, vortex, bubble, skyrmion, meron lattice, polar waves, and center-type domains. We also focus on the effects of mechanical and electrical boundary conditions and sample size on the formation of topological structures. In the meanwhile, we emphasize the use of aberration-corrected transmission electron microscope which enables to visualize the ion displacement at a sub-Ångström resolution in real space. And at the end, we envision several aspects to be considered in the future, such as imaging three dimensional (3D) atomic morphology of the topological polar structures, exploring novel polar topologies in other possible systems, and addressing the coupling of polar topologies with flexoelectricity by a combination of quantitative transmission electron microscopy and relevant theoretical approaches.

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.

Structure of binary antimony phosphate glasses by diffraction methods

Antimony phosphate glasses (Sb2O3)x(P2O5)1-x with x = 0.05 and 0.80 were measured by neutron and X-ray diffraction with high real-space resolution. The P−O and O−O distance peaks of the PO4 tetrahedra were found with the expected parameters. For the glass of x = 0.80 the Sb−O and O−O coordination numbers and distances suggest highly distorted SbO3 and SbO4 environments according to strong lone-pair effects of the Sb3+. The fraction of SbO4 has the right portion that all oxygens can form Sb−O−Sb or Sb−O−P bridges. Significant numbers of secondary Sb−O bonds (> 0.25 nm) are extracted. The first diffraction peak at 13 nm−1 is related to distances between stacked layers of corner-connected oxygen triangles as in the orthorhombic Sb2O3. For the glass of x = 0.05 the medium-range order reminds that of vitreous P2O5. Its SbO4 units are formed with mean Sb−O bonds of 0.210 nm.

Real-space anisotropy of the superconducting gap in the charge-density wave material 2H-NbSe2

We present a scanning tunneling microscopy (STM) and ab-initio study of the anisotropic superconductivity of 2H-NbSe2 in the charge-density-wave (CDW) phase. Differential-conductance spectra show a clear double-peak structure, which is well reproduced by density functional theory simulations enabling full k- and real-space resolution of the superconducting gap. The hollow-centered (HC) and chalcogen-centered (CC) CDW patterns observed in the experiment are mapped onto separate van der Waals layers with different electronic properties. We identify the CC layer as the high-gap region responsible for the main STM peak. Remarkably, this region belongs to the same Fermi surface sheet that is broken by the CDW gap opening. Simulations reveal a highly anisotropic distribution of the superconducting gap within single Fermi sheets, setting aside the proposed scenario of a two-gap superconductivity. Our results point to a spatially localized competition between superconductivity and CDW involving the HC regions of the crystal.

Realizing Kagome Band Structure in Two-Dimensional Kagome Surface States of RV_{6}Sn_{6} (R=Gd, Ho).

We report angle resolved photoemission experiments on a newly discovered family of kagome metals RV_{6}Sn_{6} (R=Gd, Ho). Intrinsic bulk states and surface states of the vanadium kagome layer are differentiated from those of other atomic sublattices by the real-space resolution of the measurements with a small beam spot. Characteristic Dirac cone, saddle point, and flat bands of the kagome lattice are observed. Our results establish the two-dimensional (2D) kagome surface states as a new platform to investigate the intrinsic kagome physics.

Rmc-discord: reverse Monte Carlo refinement of diffuse scattering and correlated disorder from single crystals.

A user-friendly program has been developed to analyze diffuse scattering from single crystals with the reverse Monte Carlo method. The approach allows for refinement of correlated disorder from atomistic supercells with magnetic or structural (occupational and/or displacive) disorder. The program is written in Python and optimized for performance and efficiency. Refinements of two user cases obtained with legacy neutron-scattering data demonstrate the effectiveness of the approach and the developed program. It is shown with bixbyite, a naturally occurring magnetic mineral, that the calculated three-dimensional spin-pair correlations are resolved with finer real-space resolution compared with the pair distribution function calculated directly from the reciprocal-space pattern. With the triangular lattice Ba3Co2O6(CO3)0.7, refinements of occupational and displacive disorder are combined to extract the one-dimensional intra-chain correlations of carbonate molecules that move toward neighboring vacant sites to accommodate strain induced by electrostatic interactions. The program is packaged with a graphical user interface and extensible to serve the needs of single-crystal diffractometer instruments that collect diffuse-scattering data.

Neutron diffraction investigation of copper tellurite glasses with high real-space resolution

High real-space resolution neutron diffraction measurements up to 34 Å−1 were performed on a series of xCuO–(100 − x)TeO2 (x = 30, 40 and 50 mol%) glasses that were synthesized by the melt-quenching technique. The Fourier transformation of neutron diffraction structure factors was used to generate the pair distribution functions, with the first peak at 1.90 Å due to the overlapping Te–O and Cu–O atomic pairs. Reverse Monte Carlo (RMC) simulations were performed on the structure factors and the six partial atomic pair distributions of Cu–Cu, Cu–Te, Cu–O, Te–Te, Te–O and O–O were calculated. The Te–O and Cu–O distributions are very similar and asymmetrical, which revealed that there is a significant short-range disorder in the tellurite network due to the existence of a wide range of Te—O and Cu—O bond lengths. A high-Q (magnitude of momentum transfer function) neutron diffraction study revealed that the average Te–O coordination number decreases steadily from 3.45 to 3.18 with an increase in CuO concentration from 30 to 50 mol% in the glass network. Similar coordination number modifications were earlier found by the RMC analysis of neutron diffraction data sets of copper tellurite glasses that were performed up to lower Q maximum values of 9.5 Å−1. The comparison of high-Q and low-Q neutron diffraction studies reveals that RMC is a powerful and possibly the only technique that is available to elucidate the glass short-range and medium-range structural properties when diffraction data are available up to low Q values of, say, 9.5 Å−1, and when cation–oxygen bond lengths are strongly overlapping and cannot be resolved by Fourier transformation. In situ high-temperature (473 K) neutron diffraction studies of 50CuO–50TeO2 glass revealed that significant distortion of the tellurite network occurs with heating.

Probes to measure kinetic and magnetic phenomena in plasmas.

Diagnostic tools are of fundamental importance in experimental research. In plasma physics, probes are usually used to obtain the plasma parameters, such as density, temperature, electromagnetic fields, and waves. This Review focuses on low-temperature plasma diagnostics where in situ probes can be used. Examples of in situ and remote diagnostics will be shown, proven by many experimental verifications. This Review starts with Langmuir probes and then continues with other diagnostics such as waves, beams, and particle collectors, which can provide high accuracy. A basic energy analyzer has been advanced to measure distribution functions with three-dimensional velocity resolution, three directions in real space and time resolution. The measurement of the seven-dimensional distribution function is the basis for understanding kinetic phenomena in plasma physics. Non-Maxwellian distributions have been measured in magnetic reconnection experiments, scattering of beams, wakes of ion beams, etc. The next advance deals with the diagnostics of electromagnetic effects. It requires magnetic probes that simultaneously resolve three field components, measured in three spatial directions and with time resolution. Such multi-variable data unambiguously yield field topologies and related derivatives. Examples will be shown for low frequency whistler modes, which are force-free vortices, flux ropes, and helical phase rotations. Thus, with advanced probes, large data acquisition and fast processing further advance in the fields of kinetic plasma physics and electromagnetic phenomena can be expected. The transition from probes to antennas will also be stimulated. Basic research with new tools will also lead to new applications.

2D real space visualization of d values in polycrystalline bulk materials of different hardness

Conventional X-ray diffraction measurements provide some average structural information, mainly on the crystal structure of the whole area of the given specimen, which might not be very uniform and may include different crystal structures, such as co-existing crystal phases and/or lattice distortion. The way in which the lattice plane changes due to strain also might depend on the position in the sample, and the average information might have some limits. Therefore, it is important to analyse the sample with good lateral spatial resolution in real space. Although various techniques for diffraction topography have been developed for single crystals, it has not always been easy to image polycrystalline materials. Since the late 1990s, imaging technology for fluorescent X-rays and X-ray absorption fine structure has been developed via a method that does not scan either a sample or an X-ray beam. X-ray diffraction imaging can be performed when this technique is applied to a synchrotron radiation beamline with a variable wavelength. The present paper reports the application of X-ray diffraction imaging to bulk steel materials with varying hardness. In this study, the distribution of lattice distortion of hardness test blocks with different hardness was examined. Via this 2D visualization method, the grains of the crystals with low hardness are large enough to be observed by X-ray diffraction contrast in real space. The change of the d value in the vicinity of the Vickers mark has also been quantitatively evaluated.

Cross-examining Polyurethane Nanodomain Formation and Internal Structure.

Structural and morphological interplay between hard and soft phases determine the bulk properties of thermoplastic polyurethanes. Commonly employed techniques rely on different physical or chemical phenomena for characterizing the organization of domains, but detailed structural information can be difficult to derive. Here, total scattering pair distribution function (PDF) analysis is used to determine atomic-scale insights into the connectivity and molecular ordering and compared to the domain size and morphological characteristics measured by AFM, TEM, SAXS, WAXS, and solid-state NMR 1H–1H spin-diffusion. In particular, density distribution functions are highlighted as a means to bridging the gap from the domain morphology to intradomain structural ordering. High real-space resolution PDFs are shown to provide a sensitive fingerprint for indexing aromatic, aliphatic, and polymerization-induced bonding characteristics, as well as the hard phase structure, and indicate that hard phases coexist in both ordered and disordered states.

Observation of flat bands in twisted bilayer graphene

Transport experiments in twisted bilayer graphene revealed multiple superconducting domes separated by correlated insulating states. These properties are generally associated with strongly correlated states in a flat mini-band of the hexagonal moir\'e superlattice as it was predicted by band structure calculations. Evidence for such a flat band comes from local tunneling spectroscopy and electronic compressibility measurements, reporting two or more sharp peaks in the density of states that may be associated with closely spaced van Hove singularities. Direct momentum resolved measurements proved difficult though. Here, we combine different imaging techniques and angle resolved photoemission with simultaneous real and momentum space resolution (nano-ARPES) to directly map the band dispersion in twisted bilayer graphene devices near charge neutrality. Our experiments reveal large areas with homogeneous twist angle that support a flat band with spectral weight that is highly localized in momentum space. The flat band is separated from the dispersive Dirac bands which show multiple moir\'e hybridization gaps. These data establish the salient features of the twisted bilayer graphene band structure.