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.     

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.     

Reductive electron transfer in phenothiazine-modified DNA is dependent on the base sequence

A new DNA assay has been designed, prepared and applied for the chemical investigation of reductive electron transfer through the DNA. It consists of 5-(10-methyl-phenothiazin-3-yl)-2'-deoxyuridine (Ptz-dU, 1) as the photoexcitable electron injector and 5-bromo-2'-deoxyuridine (Br-dU) as the electron trap. The Ptz-dU-modified oligonucleotides were synthesised by means of a Suzuki-Miyaura cross-coupling protocol and subsequent automated phosphoramidite chemistry. Br-dU represents a kinetic electron trap, since it undergoes a chemical modification after its one-electron reduction that can be analysed by piperidine-induced strand cleavage. The quantification of the strand cleavage yields from irradiation experiments reveals important information about the electron-transfer efficiency. The performed DNA studies focused on the base sequence dependence of the electron-transfer efficiency with respect to the proposal that C*- and T*- act as intermediate electron carriers during electron hopping. From our observations it became evident that excess-electron transfer is highly sequence dependent and occurs more efficiently over T-A base pairs than over C-G base pairs.     

Three‐Dimensional Characterization of Noble‐Metal Nanoparticles and their Assemblies by Electron Tomography

New developments in the field of nanomaterials drive the need for quantitative characterization techniques that yield information down to the atomic scale. In this Review, we focus on the three‐dimensional investigations of metal nanoparticles and their assemblies by electron tomography. This technique has become a versatile tool to understand the connection between the properties and structure or composition of nanomaterials. The different steps of an electron tomography experiment are discussed and we show how quantitative three‐dimensional information can be obtained even at the atomic scale.     

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.     

Monte Carlo model of the temperature rise at a GaAs surface under an electron beam

Scanning electron microscopy (SEM) has frequently been used to study semiconductor materials. It offers the possibility of obtaining reliable qualitative and quantitative information on relevant local material parameters. The temperature rise due to electron‐beam bombardment can influence some semiconductor parameters, which then will influence the SEM information. In this work we propose a model calculation based on the Monte Carlo (MC) method to calculate the temperature rise due to electron‐beam heating. The results show that the temperature rise increases with increasing numbers of electrons (electron‐beam current), and the inverse behavior is observed with respect to the electron energy (electron‐beam voltage). The decrease in temperature rise with depth is also obtained. Copyright © 2006 John Wiley & Sons, Ltd.     

DNA内电子传递特性的研究现状*

DNA是生物体中储存和传递遗传信息的重要物质。双链DNA分子中碱基对的紧密堆积为电子传递提供了有利条件,DNA内的电子转移与许多生物学功能密切相关,可能诱发遗传信息的错读和引起DNA损伤,导致细胞的突变和癌变。本文介绍了DNA电子传递的多种可能机理,就DNA电子传递的各种理论模型进行了讨论,详细介绍了实验体系的设计和研究方法,分析了各种影响电子传递的因素,对近10多年来DNA电子传递的研究工作进行了综述。     

Photosensitizing Electron Transfer Processes of Fullerenes,Carbon Nanotubes,and Carbon Nanohorns

In this account, studies on the photosensitizing electron transfer of nanocarbons, such as fullerenes, single‐walled carbon nanotubes (SWCNTs), and carbon nanohorns (CNH), performed in our laboratory for about 15 years in the early 21st century have been briefly reviewed. These novel nanocarbons act as excellent electron acceptors, when they are linked to light‐absorbing electron donors, such as porphyrins or phthalocyanines. For such molecule–nanocarbon hybrids, the direct confirmation of fast, transient, electron‐transfer phenomena must be performed with time‐resolved spectroscopic methods, such as transient absorption spectral measurements, in addition to fluorescence time‐profile measurements in the wide‐wavelength regions. Careful use of these methods affords useful information to understand photoinduced electron‐transfer mechanisms. In addition, kinetic data obtained by these methods can assist in the construction of light‐active devices, such as photovoltaic cells and solar H₂‐generation systems.     

Revealing the Bonding Pattern from the Molecular Electron Density Using Single Exponential Decay Detector: An Orbital‐Free Alternative to the Electron Localization Function

We introduce a new tool (single exponential decay detector: SEDD) to extract information about bonding and localization in atoms, molecules, or molecular assemblies. The practical evaluation of SEDD does not require any explicit information about the orbitals. The only quantity needed is the electron density (calculated or experimental) and its derivatives up to the second order.     

Laser experiments in a scanning electron microscope

In order to increase the information content of electron microscopic investigations a scanning electron microscope was combined with a pulsed Nd-glass laser in both the free running and the Q-switch mode. The equipment allows an in situ observation of melting, sintering and evaporation processes as well as of crack generation and growth at target surfaces. This article describes evaporation experiments for laser PVD of HTSC layers, Langmuir probe measurements in the corresponding plasma and crack processes by thermal shock loading in the SEM.     

Interactions of Amino Acids with Oxidized Guanine in the Gas Phase Associated with the Protection of Damaged DNA

Density functional theory calculations were employed to study the stabilization process of the guanine radical cation through amino acid interactions as well as to understand the protection mechanisms. On the basis of our calculations, several protection mechanisms are proposed in this work subject to the type of the amino acid. Our results indicate that a series of three‐electron bonds can be formed between the amino acids and the guanine radical cation which may serve as relay stations supporting hole transport. In the three‐electron‐bonded, π–π‐stacked, and H‐bonded modes, amino acids can protect guanine from oxidation or radiation damage by sharing the hole, while amino acids with reducing properties can repair the guanine radical cation through proton‐coupled electron transfer or electron transfer. Another important finding is that positively charged amino acids (ArgH⁺, LysH⁺, and HisH⁺) can inhibit ionization of guanine through raising its ionization potential. In this situation, a negative dissociation energy for hydrogen bonds in the hole‐trapped and positively charged amino acid–Guanine dimer is observed, which explains the low hole‐trapping efficiency. We hope that this work provides valuable information on how to protect DNA from oxidation‐ or radiation‐induced damages in biological systems.     

Electron energy loss spectrometry mapping

Among electron beam microanalytical techniques, electron energy loss spectrometry (EELS) offers unique advantages in terms of information content, sensitivity, limits of detection. This paper describes new methods and tools for acquiring families of spectra over many pixels on the specimen, i.e. spectrumimages, and for processing them. Applications in different fields of research, both in materials science and in life sciences, demonstrate the potential impact of the technique for characterizing nano-sized structures.     

从表面电化学实验参数理解活性基团的微观结构

从分子水平上理解电子传递中电活性基团的结构变化能够直接促进我们对电化学、分子/纳米电子学、生命过程中电子传递的认识。表面电化学是经典的宏观实验方法,从宏观的实验数据中也可以了解分子结构和电活性基团所处的微环境的信息。对此,本文从表面电化学的转移系数、表观转移电子数和重组能三个方面进行了介绍。表面电化学的转移系数的变化规律可以用来指示分子间的相互作用,表观转移电子数能够给出生物大分子基团间的连接信息,重组能可以反应出电活性中心溶剂化环境的变化。本文旨在充分发挥宏观实验方法在分子电化学方面的研究价值。     

Electron and positron pair emission by low energy positron impact on surfaces

The emission of electron pairs from surfaces has the power to reveal details about the electron–electron interaction in condensed matter. This process, stimulated by a primary electron or photon beam, has been studied both in experiment and theory over the last two decades. An additional pathway, namely positron–electron pair emission, holds the promise to provide additional information. It is based on the notion that the Pauli exclusion principle does not need to be considered for this process.We have commissioned a laboratory based positron source and performed a systematic study on a variety of solid surfaces. In a symmetric emission geometry we can explore the fact that positron and electron are distinguishable particles. Following fundamental symmetry arguments we have to expect that the available energy is shared unequally among positron and electron. Experimentally we observe such a behavior for all materials studied. We find an universal feature for all materials in the sense that on average the positron carries a larger fraction of the available energy. This is qualitatively accounted for by a simplified scattering model. Numerical results, which we obtained by a microscopic theory of positron–electron emission from surfaces, reveal however that there are also cases in which the electron carries more energy. Whether the positron or the electron is more energetic depends on details of the bound electron state and of the emission geometry. The coincidence intensity is strongly material dependent and there exists an almost monotonic relation between the singles and coincidence intensity. These results resemble the findings obtained in electron and photon stimulated electron pair emission. An additional reaction channel is the emission of an electron pair upon positron impact. We will discuss the energy distributions and the material dependence of the coincidence signal which shows similar features as those for positron–electron pairs.     

透射电镜像转角的测定及应用

论述了完全标定透射电镜像转角所需拍摄的二次曝光电子显微像的数量,推导了由已测的像转角间接计算其它像转角的一般公式,以Philips Cm200电镜为实例,说明了测定像转角的基本步骤,并以实例说明怎样利用像转角,把衍射花样的晶体学信息传递给电子显微像。     

Three‐Dimensional Structure of Olefinic Thermoplastic Elastomer Blends Using Electron Tomography

Summary: The present communication reports the first use of electron tomography in reconstructing the three‐dimensional morphology in thermoplastic elastomer blends. The blends investigated were dynamically vulcanized blends of ethylene‐propylene‐diene (EPDM) rubber/poly(propylene)/oil and polystyrene‐block‐(ethylene‐co‐butylene)‐block‐polystyrene (SEBS)/poly(propylene)/oil. An easy identification of blend morphology could be carried out at blend compositions, where conventional transmission electron microscopic imaging gives misleading information. This technique gives a higher resolution than any other microscopic technique, and is applicable to blends with dispersed as well as co‐continuous morphologies.

Example of a tomographic model of partially co‐continuous SEBS phases in a SEBS/PP/oil thermoplastic blend. Only the contours of the SEBS phase are shown.     

A measure of electron localizability

A functional of the same‐spin electron pair density is proposed as a measure of electron localizability. This functional yields the average number of same‐spin electron pairs in a region Ω enclosing a fixed charge. The functional equals zero if the fixed charge in Ω originates from one electron only, with all other same‐spin electrons outside the region Ω. Then, the correlation of the electronic motion in Ω and thus the localizability of an electron is high. If the motion of the same‐spin electrons becomes less correlated, more electrons participate in the fixed charge contained in Ω, the average number of same‐spin electron pairs (the functional) increases. In the Hartree–Fock approximation the Taylor expansion of the proposed localizability functional can be related to the electron localization function of Becke and Edgecombe without using an arbitrary reference to the uniform electron gas. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Theory on the molecular characteristic contour (II)--Molecular intrinsic characteristic contours of several typical organic molecules

The molecular intrinsic characteristic contour (MICC) is defined based on the classical turning point of electron movement in a molecule. Three typical organic molecules, I.e. Methane, methanol and formic acid, were employed as examples for detailed introduction of our method. Investigations on the cross-sections of MICC provide important information about atomic size changing in the process of forming molecules. The electron density distributions on the MICCs of these molecules were calculated and shown for the first time. Results showed that the electron density distribution on the MICC correlates closely with molecular chemical properties, and it provides a new insight into molecular boundary.     

Theory on the molecular characteristic contour (II)——Molecular intrinsic characteristic contours of several typical organic molecules

Shape and size of a molecule are the most fun-damental concepts in modern chemistry, and its appli-cations, especially about molecular surface area and molecular volume, are numerous in many fields[1—10]. Most properties of a molecule, including the proc…