Emerging Electron Microscopy Techniques for Probing Functional Interfaces in Energy Materials

Interfaces play a fundamental role in many areas of chemistry. However, their localized nature requires characterization techniques with high spatial resolution in order to fully understand their structure and properties. State‐of‐the‐art atomic resolution or in situ scanning transmission electron microscopy and electron energy‐loss spectroscopy are indispensable tools for characterizing the local structure and chemistry of materials with single‐atom resolution, but they are not able to measure many properties that dictate function, such as vibrational modes or charge transfer, and are limited to room‐temperature samples containing no liquids. Here, we outline emerging electron microscopy techniques that are allowing these limitations to be overcome and highlight several recent studies that were enabled by these techniques. We then provide a vision for how these techniques can be paired with each other and with in situ methods to deliver new insights into the static and dynamic behavior of functional interfaces.     

Diradical Organic One‐Dimensional Polymers Synthesized on a Metallic Surface

We report on the synthesis and characterization of atomically precise one‐dimensional diradical peripentacene polymers on a Au(111) surface. By means of high‐resolution scanning probe microscopy complemented by theoretical simulations, we provide evidence of their magnetic properties, which arise from the presence of two unpaired spins at their termini. Additionally, we probe a transition of their magnetic properties related to the length of the polymer. Peripentacene dimers exhibit an antiferromagnetic (S=0) singlet ground state. They are characterized by singlet–triplet spin‐flip inelastic excitations with an effective exchange coupling (J_eff) of 2.5 meV, whereas trimers and longer peripentacene polymers reveal a paramagnetic nature and feature Kondo fingerprints at each terminus due to the unpaired spin. Our work provides access to the precise fabrication of polymers featuring diradical character which are potentially useful in carbon‐based optoelectronics and spintronics.     

Fluorine in Shark Teeth: Its Direct Atomic‐Resolution Imaging and Strengthening Function

Atomic‐resolution imaging of beam‐sensitive biominerals is extremely challenging, owing to their fairly complex structures and the damage caused by electron irradiation. Herein, we overcome these difficulties by performing aberration‐corrected electron microscopy with low‐dose imaging techniques, and report the successful direct atomic‐resolution imaging of every individual atomic column in the complex fluorapatite structure of shark tooth enameloid, which can be of paramount importance for teeth in general. We demonstrate that every individual atomic column in shark tooth enameloid can be spatially resolved, and has a complex fluorapatite structure. Furthermore, ab initio calculations show that fluorine atoms can be covalently bound to the surrounding calcium atoms, which improves understanding of their caries‐reducing effects in shark teeth.     

Molecular Spintronics Based on Single‐Molecule Magnets Composed of Multiple‐Decker Phthalocyaninato Terbium(III) Complex

Unlike electronics, which is based on the freedom of the charge of an electron whose memory is volatile, spintronics is based on the freedom of the charge, spin, and orbital of an electron whose memory is non‐volatile. Although in most GMR, TMR, and CMR systems, bulk or classical magnets that are composed of transition metals are used, this Focus Review considers the growing use of single‐molecule magnets (SMMs) that are composed of multinuclear metal complexes and nanosized magnets, which exhibit slow magnetic‐relaxation processes and quantum tunneling. Molecular spintronics, which combines spintronics and molecular electronics, is an emerging field of research. Using molecules is advantageous because their electronic and magnetic properties can be manipulated under specific conditions. Herein, recent developments in [LnPc]‐based multiple‐decker SMMs on surfaces for molecular spintronic devices are presented. First, we discuss the strategies for preparing single‐molecular‐memory devices by using SMMs. Next, we focus on the switching of the Kondo signal of [LnPc]‐based multiple‐decker SMMs that are adsorbed onto surfaces, their characterization by using STM and STS, and the relationship between the molecular structure, the electronic structure, and the Kondo resonance of [TbPc₂]. Finally, the field‐effect‐transistor (FET) properties of surface‐adsorbed [LnPc₂] and [Ln₂Pc₃] cast films are reported, which is the first step towards controlling SMMs through their spins for applications in single‐molecular memory and spintronics devices.     

Structural and Chemical Analysis of Materials with High Spatial Resolution

 An understanding of the correlation between microstructures and properties of materials require the characterization of the material on many different length scales. Often the properties depend primarily on the atomistics of defects, such as dislocations and interfaces. The different techniques of transmission electron microscopy allow the characterization of the structure and of the chemical composition of materials with high spatial resolution to the atomic level: high resolution transmission electron microscopy allows the determination of the position of the columns of atoms (ions) with high accuracy. The accuracy which can be achieved in these measurements depends not only on the instrumentation but also on the quality of the transmitted specimen and on the scattering power of the atoms (ions) present in the analyzed column. The chemical composition can be revealed from investigations by analytical microscopy which includes energy dispersive X-ray spectroscopy, mainly quantitatively applied for heavy elements, and electron energy-loss spectroscopy. Furthermore, the energy-loss near-edge structure of EELS data results in information on the local band structure of unoccupied states of the excited atoms and, therefore, on bonding. A quantitative evaluation of convergent beam electron diffraction results in information on the electron charge density distribution of the bulk (defect-free) material. The different techniques are described and applied to different problems in materials science. It will be shown that nearly atomic resolution can be achieved in high resolution electron microscopy and in analytical electron microscopy. Recent developments in electron microscopy instrumentation will result in atomic resolution in the foreseeable future.     

Electronic Stabilization Effect of a Spin‐Delocalized Neutral Radical: Synthesis of an 8‐Cyano‐6‐oxophenalenoxyl Derivative and Quantitative Evaluation of the Electronic Spin Structure in terms of Resonance Structures

A new 2,5‐di‐tert‐butyl‐6‐oxophenalenoxyl (6OPO) derivative with a cyano group at the 8‐position, where a large spin density resides, has been synthesized. This neutral radical exhibits high stability in the solid state in air despite the low steric protection on the 8‐position; the stability is comparable to that of a corresponding 8‐tert‐butylated 6OPO derivative. EPR/¹H‐ENDOR/TRIPLE (electron paramagnetic resonance/¹H‐electron‐nuclear double resonance/TRIPLE) spectroscopy and cyclic voltammetry showed an extended spin delocalization on the cyano group and a significant increase in electron‐accepting ability relative to that of the 8‐tert‐butylated 6OPO derivative. DFT calculations indicated the extension of a singly occupied molecular orbital (SOMO) onto the cyano group and the lower‐lying SOMO and LUMO in comparison with those of the 8‐tert‐butylated 6OPO derivative, which was consistent with experimental results. Furthermore, the extended nature of π conjugation onto the cyano group was quantitatively evaluated by calculating the contributing weights of resonance structures in terms of a molecular orbital (MO)‐based valence‐bond (VB) method. Herein, the synthesis and physical properties of the 8‐cyano‐6OPO derivative are described, emphasizing that the high stability arises from the electronic effect of the cyano group. Also, the usefulness of the quantitative resonance structure analysis is shown.     

Unpaired electrons at the second‐order reduced density matrix level: Covalent bonding,and coulomb and fermi correlations in closed shell systems

The usual one‐electron populations in atomic orbitals of closed shell systems are split into unpaired and paired at the (spin‐dependent) second‐order reduced density matrix level. The unpaired electron in an orbital is defined as the “simultaneous occurrence of an electron and an electron hole of opposite spins in the same spatial orbital,” which for simplicity is called “electropon.” The electropon population in a given orbital reveals whether and to what degree the Coulomb correlations, and hence, the chemical bonding between this orbital and the remaining orbitals of the system are globally favorable or unfavorable. The interaction of two electropons in two target orbitals reveals the quality (favorable or unfavorable) and the strength of the covalent bonding between these orbitals; this establish a bridge between the notion of “unpaired electrons” and the traditional covalent structure of valence‐bond (VB) theory. Favorable/unfavorable bonding between two orbitals is characterized by the positive/negative (Coulomb) correlation of two electropons of opposite spins, or alternatively, by the negative/positive (Fermi) correlation of two parallel spin electropons. A spin‐free index is defined, and the relationship between the electropon viewpoint for chemical bonding and the well‐known two‐electron Coulomb and Fermi correlations is established. Benchmark calculations are achieved for ethylene, hexatriene, benzene, pyrrole, methylamine, and ammonia molecules on the basis of physically meaningful natural orbitals. The results, obtained in the framework of both orthogonal and nonorthogonal population analysis methods, provide the same conceptual pictures, which are in very good agreement with elementary chemical knowledge and VB theory. © 2013 Wiley Periodicals, Inc.     

Electron Diffraction with the Transmission Electron Microscope as a Phase-Determining Diffractometer—From Spatial Frequency Filtering to the Three-Dimensional Structure Analysis of Ribosomes

Chemists recognize X-ray crystal structure analysis and electron microscopy as powerful methods of analysis. In the last 20 years the basic ideas of X-ray diffraction analysis have been extended to the field of electron microscopy, whereby an image-forming apparatus is converted into an electron diffractometer, and through which an old dream of crystallographers can be realized—the measurement of the phase shift of scattered waves, a prerequisite for the direct calculation of structures. Its most important area of application, like that of the X-ray diffractometer, is in three-dimensional structure analysis—in all fields of science. However, beyond crystallography, aperiodic structures (comparable to crystals with a single unit cell) can also be analyzed three-dimensionally. In this progress report, the development of the first idea (spatial frequency filtering) to the analysis of ribosomal particles is outlined. Attention will be focused primarily on quantitative methods for the measurement of scattered rays, which are also usable beyond the conventional limit of resolution, down to atomic resolution. In the course of this work in 1968, the principle of the three-dimensional analysis of native biological crystal structures using the electron microscope, as worked with today in many laboratories, was developed. In Munich, however, further research focused on the three-dimensional analysis of aperiodic and individual (especially biological) objects. The analysis of 50S-subunits of the procaryotic ribosome of E. coli showed surprisingly good reproducibility of the results (although only within the same orientation), allowing the deduction of almost ideal average model structures from a limited number of particles.     

Direct Imaging of Octahedral Distortion in a Complex Molybdenum Vanadium Mixed Oxide

Complex Mo,V‐based mixed oxides that crystallize in the orthorhombic M1‐type structure are promising candidates for the selective oxidation of small alkanes. The oxygen sublattice of such a complex oxide has been studied by annular bright field scanning transmission electron microscopy. The recorded micrographs directly display the local distortion in the metal oxygen octahedra. From the degree of distortion we are able to draw conclusions on the distribution of oxidation states in the cation columns at different sites. The results are supported by X‐ray diffraction and electron paramagnetic resonance measurements that provide integral details about the crystal structure and spin coupling, respectively.     

Pulsed Electron-Nuclear Double Resonance in the Fourier Regime

Nuclear magnetic resonance (NMR) spectroscopy provides atomic-level molecular structural information. However, in molecules containing unpaired electron spins, NMR signals are difficult to measure directly. In such cases, data is obtained using the electron-nuclear double resonance (ENDOR) method, where nuclei are detected through their interaction with nearby unpaired electron spins. Unfortunately, electron spins spread the ENDOR signals, which challenges current acquisition techniques, often resulting in low spectral resolution that provides limited structural details. Here, we show that by using miniature microwave resonators to detect a small number of electron spins, integrated with miniature NMR coils, one can excite and detect a wide bandwidth of ENDOR data in a single pulse. This facilitates the measurement of ENDOR spectra with narrow lines spread over a large frequency range at much better spectral resolution than conventional approaches, which helps reveal details of the paramagnetic molecules’ chemical structure that were not accessible before.     

Intracrystalline Transport Resistances in Nanoporous Zeolite X

By applying pulsed‐field gradient nuclear magnetic resonance (PFG NMR) in comparison to quasi‐elastic neutron scattering (QENS), we sense by measurement of the diffusion of n‐octane on different length scales, transport resistances in faujasite type X (which is isostructural with type Y and differs by the lower Si/Al ratio only) with mutual distances of less than 300 nm. Direct observation of the real structure of zeolite X by transmission electron microscopy identifies them as stacking faults of mirror‐twin type on (111)‐type planes of the cubic framework. Thus, direct experimental proof is given that, in general, nanoporous host systems such as zeolite crystals cannot be considered as a mere arrangement of cavities. It is rather the presence of structural defects that dominates their properties as soon as transport phenomena with practical relevance, including chemical conversion by heterogeneous catalysis and chemical separation by molecular sieving and selective adsorption, become relevant.     

Diradical Organic One-Dimensional Polymers Synthesized on a Metallic Surface

We report on the synthesis and characterization of atomically precise one-dimensional diradical peripentacene polymers on a Au(111) surface. By means of high-resolution scanning probe microscopy complemented by theoretical simulations, we provide evidence of their magnetic properties, which arise from the presence of two unpaired spins at their termini. Additionally, we probe a transition of their magnetic properties related to the length of the polymer. Peripentacene dimers exhibit an antiferromagnetic (S=0) singlet ground state. They are characterized by singlet–triplet spin-flip inelastic excitations with an effective exchange coupling (J_eff) of 2.5 meV, whereas trimers and longer peripentacene polymers reveal a paramagnetic nature and feature Kondo fingerprints at each terminus due to the unpaired spin. Our work provides access to the precise fabrication of polymers featuring diradical character which are potentially useful in carbon-based optoelectronics and spintronics.     

Electronic structure of [(CpCr)[(CO)3M]]mu-Cot (M = Cr, Fe): a theoretical study

The electronic structure of two cyclooctatetraene-bridged dinuclear first-row transition metal complexes of the type [(CpM)[(CO)3M']]mu-Cot (M = Cr; M' = Fe (1), Cr (2)) was investigated by complete active space self-consistent field (CASSCF) calculations. In this context the differences in the binding capabilities of the complex fragments CpM and (CO)3M are discussed on the basis of extended Huckel molecular orbital (MO) calculations. The geometries used for the CASSCF calculations for complex 1 were obtained from the crystal structure. For 2 a model structure was established by geometry optimization using density functional methods. The CASSCF results agree well with the experimental findings and provide insight into the binding situation of the two compounds. Complex 1 can be regarded as being composed of a chromocene-like subunit CpCr(eta5-C5H5) and the fragment (CO)3Fe(eta3-C3H3). A direct metal-metal bond is found, involving one initially singly occupied orbital of each fragment, leading to a doublet ground state for 1 with the remaining unpaired electron localized at the chromium center. For 2 no such direct metal-metal bond can be recognized. A very weak direct metal-metal interaction is induced by electron donation from the Cot2- ligand into a formally unoccupied metal-metal binding orbital combination. In the quartet ground state all three unpaired electrons are localized at the chromium center of the formally doubly positive charged CpCr unit, on which complex fragment [(CO)3Cr(eta5-Cot)]2- acts like a cyclopentadienyl ligand. The coordination sphere of the chromium center of the CpCr unit resembles that of a metallocene metal center and its metal 3d occupation scheme corresponds to that of vanadocene.     

Protein‐Specific,Multicolor and 3D STED Imaging in Cells with DNA‐Labeled Antibodies

Photobleaching is a major challenge in fluorescence microscopy, in particular if high excitation light intensities are used. Signal‐to‐noise and spatial resolution may be compromised, which limits the amount of information that can be extracted from an image. Photobleaching can be bypassed by using exchangeable labels, which transiently bind to and dissociate from a target, thereby replenishing the destroyed labels with intact ones from a reservoir. Here, we demonstrate confocal and STED microscopy with short, fluorophore‐labeled oligonucleotides that transiently bind to complementary oligonucleotides attached to protein‐specific antibodies. The constant exchange of fluorophore labels in DNA‐based STED imaging bypasses photobleaching that occurs with covalent labels. We show that this concept is suitable for targeted, two‐color STED imaging of whole cells.     

基于光学成像技术的单颗粒电化学研究

光学显微镜技术具有高时空分辨率、高通量、高灵敏度、非接触等优点,十分适合于微观、异相界面的电子转移过程研究,因而在单颗粒电化学分析中展现出良好的应用前景.结合本课题组的研究工作,本文主要介绍了单分子荧光显微镜、表面等离激元共振显微镜、暗场显微镜及电化学发光四种光学显微技术在单纳米粒子电化学研究方面取得的最新研究进展,最...     

High-Resolution Transmission Electron Microscopy Re-examined as a Tool in Solid State Chemistry

Transmission electron microscopy (TEM) can be used with crystalline solids to obtain direct images of small structural groups comprising a few coordination polyhedra with resolution nearly down to atomic scale (“lattice imaging”). More exact knowledge of the conditions required for direct imaging, as well as improvements in the instruments themselves, have now made it possible to examine very small defect regions (microdomains), faults in the stacking sequence of structural groups or atom layers (planar or Wadsley defects), and isolated defects in narrowly delimited areas that may actually be below the dimensions of the unit cell. The structural principle of the very smallest ordered regions can even be determined when X-ray structure analysis proves unable to do this. “Block structures” are particularly suitable as models for the testing and further development of the high-resolution method; the detection of three-dimensional, two-dimensional, and one-dimensional defects has been studied on such structures.     

Femtosecond Time-Resolved Spectroscopic Photoemission Electron Microscopy for Probing Ultrafast Carrier Dynamics in Heterojunctions

The fast developing semiconductor industry is pushing to shrink and speed up transistors. This trend requires us to understand carrier dynamics in semiconductor heterojunctions with both high spatial and temporal resolutions. Recently, we have successfully set up a timeresolved photoemission electron microscopy (TR-PEEM), which integrates the spectroscopic technique to measure electron densities at specific energy levels in space. This instrument provides us an unprecedented access to the evolution of electrons in terms of spatial location, time resolution, and energy, representing a new type of 4D spectro-microscopy. Here in this work, we present measurements of semiconductor performance with a time resolution of 184 fs, electron kinetic energy resolution of 150 meV, and spatial resolution of about 150 nm or better. We obtained time-resolved micro-area photoelectron spectra and energy-resolved TR-PEEM images on the Pb island on Si(111). These experimental results suggest that this instrument has the potential to be a powerful tool for investigating the carrier dynamics in various heterojunctions, which will deepen our understanding of semiconductor properties in the submicron/nanometer spatial scales and ultrafast time scales.     

Haruspex: A Neural Network for the Automatic Identification of Oligonucleotides and Protein Secondary Structure in Cryo‐Electron Microscopy Maps

In recent years, three‐dimensional density maps reconstructed from single particle images obtained by electron cryo‐microscopy (cryo‐EM) have reached unprecedented resolution. However, map interpretation can be challenging, in particular if the constituting structures require de‐novo model building or are very mobile. Herein, we demonstrate the potential of convolutional neural networks for the annotation of cryo‐EM maps: our network Haruspex has been trained on a carefully curated set of 293 experimentally derived reconstruction maps to automatically annotate RNA/DNA as well as protein secondary structure elements. It can be straightforwardly applied to newly reconstructed maps in order to support domain placement or as a starting point for main‐chain placement. Due to its high recall and precision rates of 95.1 % and 80.3 %, respectively, on an independent test set of 122 maps, it can also be used for validation during model building. The trained network will be available as part of the CCP‐EM suite.     

High-yield Synthesis of Branched Gold Nanocrystals by a Sodium Diphenylamine-4-Sulfonate Reduction Process in Polyethylene Glycol Aqueous Solution

For developing new excellent electrocatalysts toward methanol and oxygen, branched Au nanocrystals have been prepared in high yield by the reaction of HAuCl₄ and sodium diphenylamine‐4‐sulfonate in the presence of PEG (polyethylene glycol) at room temperature. When the amount of PEG was in the range of 1–3 mL, branched Au nanocrystals were all obtained, and the amounts of sodium diphenylamine‐4‐sulfonate and HAuCl₄ had no obvious effect on the morphology of gold nanocrystals. PEG molecules play an important role in the formation of branched gold nanocrystals. The nanocrystals were characterized by transmission electron microscopy (TEM), selected area electron diffraction, high‐resolution transmission electron microscopy (HRTEM) and UV‐Vis absorption spectrum. HRTEM research suggests that the tips of gold nanocrystals grow selectively in the [111] directions. The UV‐Vis absorption spectrum displays two‐separated surface plasmon resonance peaks.