Multiscale analysis of Benjamin Franklin's innovations in American paper money.

Benjamin Franklin was a preeminent proponent of the new colonial and Continental paper monetary system in 18th-century America. He established a network of printers, designing and printing money notes at the same time. Franklin recognized the necessity of paper money in breaking American dependence on the British trading system, and he helped print Continental money to finance the American War of Independence. We use a unique combination of nondistractive, microdestructive, and advanced atomic-level imaging methods, including Raman, Infrared, electron energy loss spectroscopy, X-ray diffraction, X-ray fluorescence, and aberration-corrected scanning transmission electron microscopy, to analyze pre-Federal American paper money from the Rare Books and Special Collections of the Hesburgh Library at the University of Notre Dame. We investigate and compare the chemical compositions of the paper fibers, the inks, and fillers made of special crystals in the bills printed by Franklin's printing network, other colonial printers, and counterfeit money. Our results reveal previously unknown ways that Franklin developed to safeguard printed money notes against counterfeiting. Franklin used natural graphite pigments to print money and developed durable "money paper" with colored fibers and translucent muscovite fillers, along with his own unique designs of "nature-printed" patterns and paper watermarks. These features and inventions made pre-Federal American paper currency an archetype for developing paper money for centuries to come. Our multiscale analysis also provides essential information for the preservation of historical paper money.

An introduction to scanning transmission electron microscopy for the study of protozoans.

Since its inception in the 1930s, transmission electron microscopy (TEM) has been a powerful method to explore the cellular structure of parasites. TEM usually requires samples of <100 nm thick and with protozoans being larger than 1 µm, their study requires resin embedding and ultrathin sectioning. During the past decade, several new methods have been developed to improve, facilitate, and speed up the structural characterisation of biological samples, offering new imaging modalities for the study of protozoans. In particular, scanning transmission electron microscopy (STEM) can be used to observe sample sections as thick as 1 µm thus becoming an alternative to conventional TEM. STEM can also be performed under cryogenic conditions in combination with cryo-electron tomography providing access to the study of thicker samples in their native hydrated states in 3D. This method, called cryo-scanning transmission electron tomography (cryo-STET), was first developed in 2014. This review presents the basic concepts and benefits of STEM methods and provides examples to illustrate the potential for new insights into the structure and ultrastructure of protozoans.

Controllable Chiral Light Generation and Vortex Field Investigation Using Plasmonic Holes Revealed by Cathodoluminescence.

Control of the angular momentum of light is a key technology for next-generation nano-optical devices and optical communications, including quantum communication and encoding. We propose an approach to controllably generate circularly polarized light from a circular hole in a metal film using an electron beam by coherently exciting transition radiation and light scattering from the hole through surface plasmon polaritons. The circularly polarized light generation is confirmed by fully polarimetric four-dimensional cathodoluminescence mapping, where angle-resolved spectra are simultaneously obtained. The obtained intensity and Stokes maps show clear interference fringes as well as almost fully circularly polarized light generation with controllable parities by the electron beam position. By applying this approach to a three-hole system, a vortex field with a phase singularity is visualized in the middle of three holes.

Atomically Unraveling Highly Crystalline Self-Intercalated Tantalum Sulfide with Correlated Stacking Registry-Dependent Magnetism.

In self-intercalated two-dimensional (ic-2D) materials, understanding the local chemical environment and the topology of the filling site remains elusive, and the subsequent correlation with the macroscopically manifested physical properties has rarely been investigated. Herein, highly crystalline gram-scale ic-2D Ta1.33S2 crystals were successfully grown by the high-pressure high-temperature method. Employing combined atomic-resolution scanning transmission electron microscopy annular dark field imaging and density functional theory calculations, we systematically unveiled the atomic structures of an atlas of stacking registries in a well-defined √3(a) × âˆš3(a) Ta1.33S2 superlattice. Ferromagnetic order was observed in the AC' stacking registry, and it evolves into an antiferromagnetic state in AA/AB/AB' stacking registries; the AA' stacking registry shows ferrimagnetic ordering. Therefore, we present a novel approach for fabricating large-scale highly crystalline ic-2D crystals and shed light on a powerful means of modulating the magnetic order of ic-2D systems via stacking engineering, i.e., stackingtronics.

Engineering Oxide Epitaxy beyond Substrate Constraint.

Orientation engineering is a crucial aspect of thin film growth, and it is rather challenging to engineer film epitaxy beyond the substrate constraint. Guided by density functional theory calculations, we use SrRuO3 (SRO) as a buffer layer and successfully deposit [111]-oriented CoFe2O4 (CFO) on [001]-, [110]-, and [111]-oriented SrTiO3 (STO) substrates. This enables subsequent growth of [111]-oriented functional oxides, such as PbTiO3 (PTO), overcoming the constraint of the substrate. This strategy is quite general and applicable to lanthanum aluminate and yttria-stabilized zirconia substrates as well. X-ray Φ scans and atomic resolution aberration-corrected scanning transmission electron microscopy (AC-STEM) reveal detailed epitaxial relations in each of the cases, with four variants of [111]-CFO found on [001]-STO and two variants found on [110]-STO, formed to mitigate the large lattice misfit strain between the film and substrate. Our strategy thus provides a general pathway for orientation engineering of oxide epitaxy beyond substrate constraint.

Engineering Charge Density Waves by Stackingtronics in Tantalum Disulfide.

In the realm of condensed matter physics and materials science, charge density waves (CDWs) have emerged as a captivating way to modulate correlated electronic phases and electron oscillations in quantum materials. However, collectively and efficiently tuning CDW order is a formidable challenge. Herein, we introduced a novel way to modulate the CDW order in 1T-TaS2 via stacking engineering. By introducing shear strain during the electrochemical exfoliation, the thermodynamically stable AA-stacked TaS2 consecutively transform into metastable ABC stacking, resulting in unique 3a × 1a CDW order. By decoupling atom coordinates, we atomically deciphered the 3D subtle structural variations in trilayer samples. As suggested by density functional theory (DFT) calculations, the origin of CDWs is presumably due to collective excitations and charge modulation. Therefore, our works shed light on a new avenue to collectively modulate the CDW order via stackingtronics and unveiled novel mechanisms for triggering CDW formation via charge modulation.

Subnanometer Scale Mapping of Hydrogen Doping in Vanadium Dioxide.

Hydrogen donor doping of correlated electron systems such as vanadium dioxide (VO2) profoundly modifies the ground state properties. The electrical behavior of HxVO2 is strongly dependent on the hydrogen concentration; hence, atomic scale control of the doping process is necessary. It is however a nontrivial problem to quantitatively probe the hydrogen distribution in a solid matrix. As hydrogen transfers its sole electron to the material, the ionization mechanism is suppressed. In this study, a methodology mapping the doping distribution at subnanometer length scale is demonstrated across a HxVO2 thin film focusing on the oxygen-hydrogen bonds using electron energy loss spectroscopy (EELS) coupled with first-principles EELS calculations. The hydrogen distribution was revealed to be nonuniform along the growth direction and between different VO2 grains, calling for intricate hydrogenation mechanisms. Our study points to a powerful approach to quantitatively map dopant distribution in quantum materials relevant to energy and information sciences.

4D-STEM Mapping of Nanocrystal Reaction Dynamics and Heterogeneity in a Graphene Liquid Cell.

Chemical reaction kinetics at the nanoscale are intertwined with heterogeneity in structure and composition. However, mapping such heterogeneity in a liquid environment is extremely challenging. Here we integrate graphene liquid cell (GLC) transmission electron microscopy and four-dimensional scanning transmission electron microscopy to image the etching dynamics of gold nanorods in the reaction media. Critical to our experiment is the small liquid thickness in a GLC that allows the collection of high-quality electron diffraction patterns at low dose conditions. Machine learning-based data-mining of the diffraction patterns maps the three-dimensional nanocrystal orientation, groups spatial domains of various species in the GLC, and identifies newly generated nanocrystallites during reaction, offering a comprehensive understanding on the reaction mechanism inside a nanoenvironment. This work opens opportunities in probing the interplay of structural properties such as phase and strain with solution-phase reaction dynamics, which is important for applications in catalysis, energy storage, and self-assembly.

Offcut Substrate-Induced Defect Trapping at Step Edges.

We report step edge-induced localized defects suppressing subsequent antiphase boundary formation in the bulk structure of a trilayer oxide heterostructure. The heterostructure encompasses a layer of La0.66Sr0.34MnO3 sandwiched between a superconducting La1.84Sr0.16CuO4 bottom layer and an insulating La2CuO4 top layer. The combination of a minor a-axis mismatch (0.11 Å) and a pronounced c-axis mismatch (2.73 Å) at the step edges leads to the emergence of localized defects exclusively forming at the step edge. Employing atomically resolved electron energy-loss spectroscopy maps, we discern the electronic state of those structures in the second La0.66Sr0.34MnO3 unit cell near the step edge. In particular, a reduction in the pre-edge region of the O-K edge indicates the formation of oxygen vacancies induced by the strained step edge. This study underscores our capability to control defects at the nanoscale.

Texture Transfer in Dielectric Layers via Nanocrystalline Networks: Insights from in Situ 4D-STEM.

Transition metal oxide dielectric layers have emerged as promising candidates for various relevant applications, such as supercapacitors or memory applications. However, the performance and reliability of these devices can critically depend on their microstructure, which can be strongly influenced by thermal processing and substrate-induced strain. To gain a more in-depth understanding of the microstructural changes, we conducted in situ transmission electron microscopy (TEM) studies of amorphous HfO2 dielectric layers grown on highly textured (111) substrates. Our results indicate that the minimum required phase transition temperature is 180 °C and that the developed crystallinity is affected by texture transfer. Using in situ TEM and 4D-STEM can provide valuable insights into the fundamental mechanisms underlying the microstructural evolution of dielectric layers and could pave the way for the development of more reliable and efficient devices for future applications.

Ultraclean Suspended Graphene by Radiolysis of Adsorbed Water.

Access to intrinsic properties of a 2D material is challenging due to the absence of a bulk that would dominate over surface contamination, and this lack of bulk also precludes effective conventional cleaning methods that are almost always sacrificial. Suspended graphene and carbon contaminants represent the most salient challenge. This work has achieved ultraclean graphene, attested by electron energy loss (EEL) spectra unprecedentedly exhibiting fine-structure features expected from bonding and band structure. In the cleaning process in a transmission electron microscope, radicals generated by radiolysis of intentionally adsorbed water remove organic contaminants, which would otherwise be feedstock of the notorious electron irradiation induced carbon deposition. This method can be readily adapted to other experimental settings and other materials to enable previously inhibited undertakings that rely on the intrinsic properties or ultimate thinness of 2D materials. Importantly, the method is surprisingly simple and robust, easily implementable with common lab equipment.

In Situ Single Particle Reconstruction Reveals 3D Evolution of PtNi Nanocatalysts During Heating.

Tailoring nanoparticles' composition and morphology is of particular interest for improving their performance for catalysis. A challenge of this approach is that the nanoparticles' optimized initial structure often changes during use. Visualizing the three dimensional (3D) structural transformation in situ is therefore critical, but often prohibitively difficult experimentally. Although electron tomography provides opportunities for 3D imaging, restrictions in the tilt range of in situ holders together with electron dose considerations limit the possibilities for in situ electron tomography studies. Here, an in situ 3D imaging methodology is presented using single particle reconstruction (SPR) that allows 3D reconstruction of nanoparticles with controlled electron dose and without tilting the microscope stage. This in situ SPR methodology is employed to investigate the restructuring and elemental redistribution within a population of PtNi nanoparticles at elevated temperatures. The atomic structure of PtNi is further examined and a heat-induced transition is found from a disordered to an ordered phase. Changes in structure and elemental distribution are linked to a loss of catalytic activity in the oxygen reduction reaction. The in situ SPR methodology employed here can be extended to a wide range of in situ studies employing not only heating, but gaseous, aqueous, or electrochemical environments to reveal in-operando nanoparticle evolution in 3D.

Crystallographic Pathways to Tailoring Metal-Insulator Transition through Oxygen Transport in VO2.

The metal-insulator (MI) transition of vanadium dioxide (VO2) is effectively modulated by oxygen vacancies, which decrease the transition temperature and insulating resistance. Oxygen vacancies in thin films can be driven by oxygen transport using electrochemical potential. This study delves into the role of crystallographic channels in VO2 in facilitating oxygen transport and the subsequent tuning of electrical properties. A model system is designed with two types of VO2 thin films: (100)- and (001)-oriented, where channels align parallel and perpendicular to the surface, respectively. Growing an oxygen-deficient TiO2 layer on these VO2 films prompted oxygen transport from VO2 to TiO2. Notably, in (001)-VO2 film, where oxygen ions move along the open channels, the oxygen migration deepens the depleted region beyond that in (100)-VO2, leading to more pronounced changes in metal-insulator transition behaviors. The findings emphasize the importance of understanding the intrinsic crystal structure, such as channel pathways, in controlling ionic defects and customizing electrical properties for applications.

Novel Polytype of III-VI Metal Chalcogenides Nano Crystals Realized in Epitaxially Grown InTe.

III-VI metal chalcogenides have garnered considerable research attention as a novel group of layered van der Waals materials because of their exceptional physical properties and potential technological applications. Here, the epitaxial growth and stacking sequences of InTe is reported, an essential and intriguing material from III-VI metal chalcogenides. Aberration-corrected scanning transmission electron microscopy (STEM) is utilized to directly reveal the interlayer stacking modes and atomic structure, leading to a discussion of a new polytype. Furthermore, correlations between the stacking sequences and interlayer distances are substantiated by atomic-resolution STEM analysis, which offers evidence for strong interlayer coupling of the new polytype. It is proposed that layer-by-layer deposition is responsible for the formation of the unconventional stacking order, which is supported by ab initio density functional theory calculations. The results thus establish molecular beam epitaxy as a viable approach for synthesizing novel polytypes. The experimental validation of the InTe polytype here expands the family of materials in the III-VI metal chalcogenides while suggesting the possibility of new stacking sequences for known materials in this system.

In Situ TEM/STEM Investigation of Crystallization in Y3Al5O12:Ce at High Temperatures Inside a Transmission Electron Microscope.

Y3Al5O12:Ce (YAG:Ce) phosphors are extensively used in the field of white light-emitting diodes (LEDs) due to their efficient luminescent properties. To optimize the performance of YAG:Ce phosphors, a comprehensive understanding of their synthesis and structural evolution is essential. This paper presents a direct in situ transmission electron microscopy (TEM) /scanning TEM (STEM) investigation on the transformation process of a precursor comprising nanocrystalline CeO2 dispersed in an amorphous Y-Al oxide matrix into crystalline YAG:Ce particles. The study reveals that nanocrystalline CeO2 particles dissolve completely in the Y-Al oxide matrix at a temperature above 900 °C, while YAlO3 (YAP)-type crystalline particles with Al2O3 phase in grain boundaries are observed above 1000 °C. Finally, YAG:Ce-type crystalline particles are formed above 1180 °C. Atomic-resolution energy-dispersive X-ray spectroscopy (EDS) elemental mapping demonstrates that the doped cerium (Ce) atoms occupy the same atomic sites as yttrium (Y). Photoluminescence measurements validate the efficient luminescent properties of the obtained YAG:Ce phosphor.

DFT-Assisted Investigation of the Electric Field and Charge Density Distribution of Pristine and Defective 2D WSe2 by Differential Phase Contrast Imaging.

Most properties of solid materials are defined by their internal electric field and charge density distributions which so far are difficult to measure with high spatial resolution. Especially for 2D materials, the atomic electric fields influence the optoelectronic properties. In this study, the atomic-scale electric field and charge density distribution of WSe2 bi- and trilayers are revealed using an emerging microscopy technique, differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM). For pristine material, a higher positive charge density located at the selenium atomic columns compared to the tungsten atomic columns is obtained and tentatively explained by a coherent scattering effect. Furthermore, the change in the electric field distribution induced by a missing selenium atomic column is investigated. A characteristic electric field distribution in the vicinity of the defect with locally reduced magnitudes compared to the pristine lattice is observed. This effect is accompanied by a considerable inward relaxation of the surrounding lattice, which according to first principles DFT calculation is fully compatible with a missing column of Se atoms. This shows that DPC imaging, as an electric field sensitive technique, provides additional and remarkable information to the otherwise only structural analysis obtained with conventional STEM imaging.

Important aspects of investigating optical excitations in semiconductors using a scanning transmission electron microscope.

Since semiconductor structures are becoming smaller and smaller, the examination methods must also take this development into account. Optical methods have long reached their limits here, but small dimensions are also a challenge for electron beam techniques, especially when it comes to determining optical properties. In this paper, electron microscopic methods of investigating optical properties are discussed. Special attention is given to the physical limits and how to deal with them. We will cover electron energy loss spectrometry as well as cathodoluminescence spectrometry. We pay special attention to inelastic delocalisation, radiation damage, the Cerenkov effect, interference effects of optical excitations and higher diffraction orders on a grating analyser for the cathodoluminescence signal.

X-ray linear dichroic ptychography.

Biominerals such as seashells, coral skeletons, bone, and tooth enamel are optically anisotropic crystalline materials with unique nanoscale and microscale organization that translates into exceptional macroscopic mechanical properties, providing inspiration for engineering new and superior biomimetic structures. Using Seriatopora aculeata coral skeleton as a model, here, we experimentally demonstrate X-ray linear dichroic ptychography and map the c-axis orientations of the aragonite (CaCO3) crystals. Linear dichroic phase imaging at the oxygen K-edge energy shows strong polarization-dependent contrast and reveals the presence of both narrow (<35°) and wide (>35°) c-axis angular spread in the coral samples. These X-ray ptychography results are corroborated by four-dimensional (4D) scanning transmission electron microscopy (STEM) on the same samples. Evidence of co-oriented, but disconnected, corallite subdomains indicates jagged crystal boundaries consistent with formation by amorphous nanoparticle attachment. We expect that the combination of X-ray linear dichroic ptychography and 4D STEM could be an important multimodal tool to study nano-crystallites, interfaces, nucleation, and mineral growth of optically anisotropic materials at multiple length scales.

Atomic-scale insights into quantum-order parameters in bismuth-doped iron garnet.

Bismuth and rare earth elements have been identified as effective substituent elements in the iron garnet structure, allowing an enhancement in magneto-optical response by several orders of magnitude in the visible and near-infrared region. Various mechanisms have been proposed to account for such enhancement, but testing of these ideas is hampered by a lack of suitable experimental data, where information is required not only regarding the lattice sites where substituent atoms are located but also how these atoms affect various order parameters. Here, we show for a Bi-substituted lutetium iron garnet how a suite of advanced electron microscopy techniques, combined with theoretical calculations, can be used to determine the interactions between a range of quantum-order parameters, including lattice, charge, spin, orbital, and crystal field splitting energy. In particular, we determine how the Bi distribution results in lattice distortions that are coupled with changes in electronic structure at certain lattice sites. These results reveal that these lattice distortions result in a decrease in the crystal-field splitting energies at Fe sites and in a lifted orbital degeneracy at octahedral sites, while the antiferromagnetic spin order remains preserved, thereby contributing to enhanced magneto-optical response in bismuth-substituted iron garnet. The combination of subangstrom imaging techniques and atomic-scale spectroscopy opens up possibilities for revealing insights into hidden coupling effects between multiple quantum-order parameters, thereby further guiding research and development for a wide range of complex functional materials.

Deep-Learning Aided Atomic-Scale Phase Segmentation toward Diagnosing Complex Oxide Cathodes for Lithium-Ion Batteries.

Phase transformation─a universal phenomenon in materials─plays a key role in determining their properties. Resolving complex phase domains in materials is critical to fostering a new fundamental understanding that facilitates new material development. So far, although conventional classification strategies such as order-parameter methods have been developed to distinguish remarkably disparate phases, highly accurate and efficient phase segmentation for material systems composed of multiphases remains unavailable. Here, by coupling hard-attention-enhanced U-Net network and geometry simulation with atomic-resolution transmission electron microscopy, we successfully developed a deep-learning tool enabling automated atom-by-atom phase segmentation of intertwined phase domains in technologically important cathode materials for lithium-ion batteries. The new strategy outperforms traditional methods and quantitatively elucidates the correlation between the multiple phases formed during battery operation. Our work demonstrates how deep learning can be employed to foster an in-depth understanding of phase transformation-related key issues in complex materials.