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.     

Spectroscopic imaging techniques for characterization of polymer morphologies: a combination of infrared and electron microscopy

The morphological characterization of polymer blends consisting of polyamide and poly(tetrafluoroethylene) using FT-IR spectroscopy and electron microscopy is described. To enhance the lateral resolution - one of the main limits in infrared spectroscopy - a combination with scanning electron microscopy and analytical electron microscopic methods of a transmission electron microscope was made. The possibilities of electron energy loss spectroscopy and energy filtered transmission electron microscopy (EFTEM) in the area of polymer characterization are outlined.     

Quantitative determination of site occupancy of multi-rare-earth elements doped into Ca2SnO4 phosphor by electron channeling microanalysis

X-ray fluorescence analysis based on electron channeling effects in transmission electron microscopy (TEM) was performed on Ca₂SnO₄ phosphor materials doped with Eu³⁺/Y³⁺ at various concentrations, which showed red photoluminescence associated with the ⁵D₀-⁷F₂ electric dipole transition of Eu³⁺ ions. The method provided direct information on which host element site dopant elements occupy, the results of which were compared with those of X-ray diffraction (XRD)-Rietveld analysis. The obtained results indicated that while it is not favorable for a part of Eu³⁺ to occupy the smaller Sn⁴⁺ site, this is still energetically better than creating Ca vacancies or any other of the possible charge balance mechanisms. The local lattice distortions associated with dopant impurities with different ionic radii were also examined by TEM-electron energy-loss spectroscopy (TEM-EELS). The change in PL intensity as a function of dopant concentration is discussed based on the experimental results, although the general concept of concentration quenching applies.     

Application of EELS to the microanalysis of materials

Electron energy loss spectroscopy (EELS) is used in analytical electron microscopy (AEM) because it can provide results on the chemical composition and structure of a small volume of material. The practical application of EELS was demonstrated by the investigation of a refractory hard metal of the type WC-TiC-Co and by the investigation of a BaTiO₃ ceramic material. To demonstrate the present status of quantitative analysis by EELS, the spectra of Be₂SiO₄, TiB₂ and BaTiO₃ were quantified and the results indicate that quantitative analysis is feasible for major concentrations of light elements and also of heavier elements even in the presence of severe edge overlap.     

The Development of iDPC-STEM and Its Application in Electron Beam Sensitive Materials

The main aspects of material research: material synthesis, material structure, and material properties, are interrelated. Acquiring atomic structure information of electron beam sensitive materials by electron microscope, such as porous zeolites, organic-inorganic hybrid perovskites, metal-organic frameworks, is an important and challenging task. The difficulties in characterization of the structures will inevitably limit the optimization of their synthesis methods and further improve their performance. The emergence of integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM), a STEM characterization technique capable of obtaining images with high signal-to-noise ratio under lower doses, has made great breakthroughs in the atomic structure characterization of these materials. This article reviews the developments and applications of iDPC-STEM in electron beam sensitive materials, and provides an outlook on its capabilities and development.     

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.     

PbTiO3/SrTiO3(010)异质界面上的周期性失配位错及电子富集

It is important to determine the effects of misfit dislocations and other defects on the domain structure, ferroelectricity, conductivity, and other physical properties of ferroelectric thin films to understand their ferroelectric and piezoelectric behaviors. Much attention has been given to ferroelectric PbTiO₃/SrTiO₃ or PbZr_0.2Ti_0.8O₃/SrTiO₃ heterointerfaces, at which improper ferroelectricity, a spin-polarized two-dimensional electron gas, and other physical phenomena have been found. However, those heterointerfaces were all (001) planes, and there has been no experimental studies on the growth of (010) PbTiO₃/SrTiO₃ heterointerface due to the 6.4% misfit between two materials. In this study, we selected an atomically flat (010) PbTiO₃/SrTiO₃ heterointerface grown using a two-step hydrothermal method as the research subject, and this is the first experimental report on that interface. Interfacial dislocations can play a significant role in causing dramatic changes in the Curie temperature and polarization distribution near the dislocation cores, especially when the size of a ferroelectric thin film is scaled down to the nanoscale. The results of previous studies on the effects of interfacial dislocations on the physical properties of ferroelectric thin films have been contradictory. Thus, this issue needs to be explored more deeply in the future. This study used aberration corrected scanning transmission electron microscopy (STEM) to study the atomic structure of a (010) PbTiO₃/SrTiO₃ heterointerface and found periodic misfit dislocations with a Burgers vector of a[001]. The extra planes at the dislocation cores could relieve the misfit strain between the two materials in the [001] direction and thus allowed the growth of such an atomically sharp heterointerface. Moreover, monochromated electron energy-loss spectroscopy with an atomic scale spatial resolution and high energy resolution was used to explore the charge distribution near the periodic misfit dislocation cores. The fine structure of the Ti L edge was quantitatively analyzed by linearly fitting the experimental spectra recorded at various locations near and at the misfit dislocation cores with the Ti³⁺ and Ti⁴⁺ reference spectra. Therefore, the accurate valence change of Ti could be determined, which corresponded to the charge distribution. The probable existence of an aggregation of electrons was found near the a[001] dislocation cores, and the density of the electrons calculated from the valence change was 0.26 electrons per unit cell. Based on an analysis of the fine structure of the oxygen K edge, it could be argued that the electrons aggregating at the dislocation cores came from the oxygen vacancies in the interior regions of the PbTiO₃. This aggregation of electrons will probably increase the electron conductivity along the dislocation line. The physics of two-dimensional charge distributions at oxide interfaces have been intensively studied, however, little attention had been given to the one-dimensional charge distribution. Therefore, the results of this study can stimulate research interest in exploring the influence of the interfacial dislocations on the physics of ferroelectric heterointerfaces.     

Visualizing the Stoichiometry of Industrial‐Style Co‐Mo‐S Catalysts with Single‐Atom Sensitivity

The functional properties of transition metal dichalcogenides (TMDs) may be promoted by the inclusion of other elements. Here, we studied the local stoichiometry of single cobalt promoter atoms in an industrial‐style MoS₂‐based hydrotreating catalyst. Aberration‐corrected scanning transmission electron microscopy and electron energy loss spectroscopy show that the Co atoms occupy sites at the (?100) S edge terminations of the graphite‐supported MoS₂ nanocrystals in the catalyst. Specifically, each Co atom has four neighboring S atoms that are arranged in a reconstructed geometry, which reflects an equilibrium state. The structure agrees with complementary studies of catalysts that were prepared under vastly different conditions and on other supports. In contrast, a small amount of residual Fe in the graphite is found to compete for the S edge sites, so that promotion by Co is strongly sensitive to the purity of the raw materials. The present single‐atom‐sensitive analytical method therefore offers a guide for advancing preparative methods for promoted TMD nanomaterials.     

New Tools for Investigating Electromagnetic Hot Spots in Single‐Molecule Surface‐Enhanced Raman Scattering

Surface‐enhanced Raman scattering (SERS) is quickly growing as an analytical technique, because it offers both molecular specificity and excellent sensitivity. For select substrates, SERS can even be observed from single molecules, which is the ultimate limit of detection. This review describes recent developments in the field of single‐molecule SERS (SM‐SERS) with a focus on new tools for characterizing SM‐SERS‐active substrates and how they interact with single molecules on their surface. In particular, techniques that combine optical spectroscopy and microscopy with electron microscopy are described, including correlated optical and transmission electron microscopy, correlated super‐resolution imaging and scanning electron microscopy, and correlated optical microscopy and electron energy loss spectroscopy.     

Analytical electron microscopy of materials

Analytical electron microscopy enables combined crystallographic and chemical information with a high spatial resolution to be gained from microregions of electron-transparent specimens. This is reached by the combined application of imaging, diffraction and spectroscopic methods, using either a dedicated scanning transmission electron microscope or a conventional high-resolution electron microscope (having a strong objective lens) equipped with suitable X-ray or electron spectrometers. Of the diffraction methods especially the technique of convergent beam diffraction is used, yielding valuable information on crystal structures, lattice parameter changes, symmetry variations and crystal perfection, respectively. For chemical analysis, either energy-dispersive X-ray spectroscopy (EDX) is used or electron energy loss spectroscopy (EELS). Finally, high-resolution electron microscopy in the lateral resolution range of some 0.1 nm allows the reliable geometrical inspection of extreme microregions.     

Imaging the Atomic Position of Light Cations in a Porous Network and the Europium(III) Ion Exchange Capability by Aberration‐Corrected Electron Microscopy

In the present work, ETS‐10 microporous titanosilicate has been synthesized and its structure characterized by means of powder XRD and aberration corrected scanning transmission electron microscopy (C_s‐corrected STEM). For the first time, sodium ions have been imaged sitting inside the 7‐membered rings. The ion‐exchange capability has been tested by the inclusion of rare earth metals (Eu, Tb and Gd) to produce a luminescent material which has been studied by atomic‐resolution C_s‐corrected STEM. The data produced has allowed unambiguous imaging of light atoms in a microporous framework as well as determining the cationic metal positions for the first time, providing evidence of the importance of advanced electron microscopy methods for the study of the local environment of metals within zeolitic supports providing unique information of both systems (guest and support) at the same time.     

Advanced TEM Characterization for Single-atom Catalysts: from Ex-situ Towards In-situ

Clean energy innovation has triggered the development of single-atom catalysts(SACs) due to their excellent catalytic activity, high tunability and low cost. The success of SACs for many catalytic reactions has opened a new field, where the fundamentals of catalytic property-structure relationship at atomic level await exploration, and thus raises challenges for structural characterization. Among the characterization techniques for SACs, aberration-corrected transmission electron microscopy(TEM) has become an essential tool for direct visualization of single atoms. In this review, we briefly summarize recent studies on SACs using advanced TEM. We first introduce TEM methods, which are particularly important for SACs characterization, and then discuss the applications of advanced TEM for SAC characterization, where not only atomic dispersion of single atoms can be studied, but also the distribution of elements and the valence state with local coordination can be resolved. We further extend our review towards in-situ TEM, which has increasing importance for the fundamental understanding of catalytic mechanism. Perspectives of TEM for SACs are finally discussed.     

Atomic Force Microscopy,Transmission Electron Microscopy,and Photon Correlation Spectroscopy: Three Techniques for Rapid Characterization of Optimized Self-Nanoemulsiying Drug Delivery System of Glibenclamide,Carvedilol, and Lovastatin

This article looks at atomic force microscopy as an important aid to characterize the self-nanoemulsifying formulation of glibenclamide, lovastatin, and carvedilol in conjunction with other sophisticated technique, viz., transmission electron microscopy and photon correlation spectroscopy. Sizes obtained by processing the atomic force microscopy (AFM) image are comparable with those obtained from transmission electron microscope. Although in the present study, the mean particle size obtained from photon correlation spectroscopy does not correlate to the findings of atomic force microscopy and transmission electron microscopy, but the poly-disperse index values correlate well with the findings of AFM and transmission electron microscopy where uniform particle size was observed in aqueous dispersion of self-nanoemulsifying formulation of glibenclamide, lovastatin, and carvedilol.     

High-resolution analytical transmission electron microscopy of semiconductor quantum structures

Analytical transmission electron microscopy was applied to characterize the size, shape, real structure, and, in particular, the composition of different semiconductor quantum structures. Its potential applicability is demonstrated for heterostructures of III-V semiconducting materials and II-VI ones, viz. (In,Ga)As quantum wires on InP and (In,Ga)As quantum dots on GaAs both grown by metal organic chemical vapor deposition, and CdSe quantum dots on ZnSe grown by molecular beam epitaxy. The investigations carried out show that the element distribution even of some atomic layers can be detected by energy-dispersive X-ray spectroscopy, however, exhibiting a smeared profile. Contrary to that, sub-nanometre resolution has been achieved by using energy-filtered transmission electron microscopy to image quantum dot structures.     

Application of analytical transmission electron microscopy to the characterization of interlayers in fibre-reinforced composites with ceramic and glass matrices

The microchemistry of interfaces and corresponding interlayers in different fibre-reinforced ceramic and glass composite systems has been investigated by using a dedicated scanning transmission electron microscope demonstrating the potential applicabilities of such an instrument to this large field of materials science. Energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy were used to determine the materials composition on a nanometre scale. Besides analyses performed in the spot mode of the electron probe the distributions of the elements present in the interface region were measured as line profiles across the relevant interface structure by X-ray spectroscopy with a lateral resolution of about 5 nm, even for the detection of a light element as carbon. Moreover, in the composite systems under investigation the two-dimensional element distribution was also attained by energy-filtered imaging. In addition, first results of energy loss near edge structure analyses are presented indicating variations of the chemical bonding of silicon at the interface in a Nicalon fibre/Duran glass composite.     

Carbon‐Nanotube‐Based Hybrid Materials: Nanopeapods

This review article focuses on the structures and properties of novel hybrid nanocarbon materials, which are created by incorporating atoms and molecules into the hollow spaces of carbon nanotubes (CNTs); thus they are called nanopeapods. After dealing with synthesis procedures, we discuss the structures and electronic properties of the hybrid materials based on high‐resolution transmission electron microscopy (HRTEM), electron energy‐loss spectroscopy (EELS), X‐ray and electron diffraction, scanning tunneling microscopy (STM), and field‐effect transistor transport measurements. Utilization of the low‐dimensional nanosized spaces of CNTs to produce novel low‐dimensional nanocluster, nanowire, and nanotube materials is also discussed.     

电子束敏感材料的原子尺度结构研究

分子筛和金属有机骨架(MOF)材料以其独特的孔道和骨架结构在催化、 储能、 干燥及净化和吸附分离等领域有着广泛应用, 对其结构的原子尺度表征对于深入理解其构效关系具有重要意义. 但其大孔道结构和有机骨架使得它们对电子束辐照极为敏感, 在常规透射电镜成像模式下结构会很快被破坏变为非晶, 从而无法获得孔道和骨架的原子排列信息. 最近发展起来的基于积分差分相位衬度扫描透射电子显微(iDPC-STEM)技术在电子敏感材料和轻元素组分成像方面展现出明显优势, 使得对多孔骨架材料及烃池物种的表征成为了可能. 本文综述了本课题组近期利用该技术对分子筛和MOF材料原子尺度结构方面的研究. 将iDPC-STEM技术应用到ZSM-5分子筛催化剂中, 实现了对该分子筛的原子级骨架结构的成像分析. 在MOF体系中, 利用该技术观察到MIL-101骨架内部有机连接体与金属节点的配位方式, 在此基础上解析了MIL-101结构中有机配体的连接和金属节点的苯环结构, 并观察了MOF的原子级表面、 界面和缺陷等局域结构特征. 最后对iDPC-STEM技术在原子尺度成像方面的应用潜力进行了总结与展望.     

Synthesis and characterization of heat-stabilized albumin magnetic microspheres

Human serum albumin magnetic microspheres containing 30% iron oxide particles were synthesized by a heat-stabilization process. The average diameter, the size distribution and the morphology were characterized by scanning electron microscopy, atomic force microscopy and transmission electron microscopy. The distribution of the iron oxide nanoparticles within the microspheres was confirmed by the contrast obtained in the morphology by backscattered electron imaging in scanning electron microscopy. Energy-dispersive X-ray spectroscopy showed the presence of iron in the microspheres. The cabbage like surface structure in some of the microspheres obtained in scanning electron microscopy can be better understood by atomic force microscopy. This peculiar surface structure in the microsphere may be due to the cross-linking in the protein molecule by heat. The amount of iron oxide in the microsphere was analyzed by atomic absorption spectroscopy. The magnetic properties of the particles were measured in a superconducting quantum interference device magnetometer. Received: 12 September 2000 Accepted: 5 February 2001