Wave front reconstruction of Fresnel off-axis holograms with compensation of aberrations by means of phase-shifting digital holography

Off-axis holograms recorded with a CCD camera are numerically reconstructed in amplitude by calculating through the Fresnel–Kirchhoff integral. A phase-shifting Mach–Zehnder interferometer is used for recording four-quadrature phase-shifted off-axis holograms. The basic principle of this technique and its experimental verification are described. We show that the application of this algorithm allows for the suppression of the zero order of diffraction and of the twin image and that the contrast of the reconstructed images can be further enhanced by digital compensation of the aberrations introduced by the holographic recording system     

A new theta modulation multiplexing scheme for computer-generated holograms

In this paper, a new theta modulation multiplexing scheme for producing multiplexed computer-generated holograms (CGHs) is proposed. Under this scheme, normal Lohmann-type CGHs are individually modulated by gratings with different orientations and then superimposed to form a single hologram. The proposed method results in a higher storage capacity without the increase of hologram space. Theoretical analysis and experimental results are presented.     

High-resolution transmission electron microscopy investigation of the nanostructure of undoped and Pt-doped nanocrystalline pulsed laser deposited SnO2 thin films

Pulsed laser deposition (PLD) was used to grow nanocrystalline SnO₂ thin films onto alumina substrates. The reactive PLD process was carried out at different substrate deposition temperatures (T_d) between 20 and 600 °C under an oxygen background pressure of 150 mtorr. The same PLD technique was used to produce SnO₂ films in situ-doped with Pt (at the level of ∼2 at. %) through the concomitant ablation of both SnO₂ target and Pt strips. Conventional and high-resolution transmission electron microscopy (HRTEM) observations have revealed that the microstructure of the PLD SnO₂ films is highly sensitive to their deposition temperature. Indeed, its changes from a porous granular structure with extremely fine equiaxed grains (∼4 nm diameter), at T_d=20 °C to a very compact and textured columnar structure characterized by SnO₂ columns (∼25 nm diameter) composed of grains of ∼12 nm of diameter, at T_d=600 °C. In addition, the PLD SnO₂ films were found to exhibit the highest nanoporosity at T_d=300 °C which also coincides with the granular-to-columnar microstructural transition. On the other hand, the microstructure of the Pt-doped SnO₂ films, deposited at 300 °C, was found to contain a high density of defects, such as twin boundaries and edge dislocations. By combining HRTEM and EDS microanalysis, we were able to show that the Pt-dopant self-organizes into spherical nanoparticles (1-2 nm diameter) randomly distributed at the SnO₂ grain boundaries. Finally, doping the films with such platinum nanoclusters is found to affect the SnO₂ nanostructure by particularly reducing the SnO₂ mean grain size (from ∼10 nm when undoped to ∼6 nm for the doped films).     

Observing gas-catalyst dynamics at atomic resolution and single-atom sensitivity

Transmission electron microscopy (TEM) has become an indispensable technique for studying heterogeneous catalysts. In particular, advancements of aberration-corrected electron optics and data acquisition schemes have made TEM capable of delivering images of catalysts with sub-Ångström resolution and single-atom sensitivity. Parallel developments of differentially pumped electron microscopes and of gas cells enable in situ observations of catalysts during the exposure to reactive gas environments at pressures of up to atmospheric levels and temperatures of up to several hundred centigrade. Here, we outline how to take advantage of the emerging state-of-the-art instrumentation and methodologies to study surface structures and dynamics to improve the understanding of structure-sensitive catalytic functionality. The concept of using low electron dose-rates in TEM in conjunction with in-line holography and aberration-correction at low voltage (80 kV) is introduced to allow maintaining atomic resolution and sensitivity during in situ observations of catalysts. Benefits are illustrated by exit wave reconstructions of TEM images of a nanocrystalline Co₃O₄ catalyst material acquired in situ during their exposure to either a reducing or oxidizing gas environment.     

High-temperature pretreatment of Ni nanoparticles enhances the growth of high-density carbon fiber bundles during microwave plasma chemical vapor deposition

This paper presents an experimental study on the effect of the pretreatment procedure of Ni nanoparticles (NPs) on the growth of multiwalled carbon fiber (CNF) bundles by means of microwave plasma chemical vapor deposition (MPCVD). We used atomic force microscopy to investigate a series of pretreated Ni films. The structures and compositions of the CNFs on the via were investigated using scanning electron microscopy, high-resolution transmission electron microscopy, and Raman spectroscopy.The geometric shape of the Ni NPs was identified in terms of their roughness, which decreased upon increasing the pretreatment temperature, resulting subsequently in the synthesis of high-density CNFs. The diameter and shape of the Ni NPs were the dominant factors affecting the size and density of the CNFs bundles. We obtained CNFs that fully filled the via effectively; they might serve as potential interconnects in future nanodevices.     

Atomic resolution imaging of oxygen atoms close to heavy atoms by HRTEM and ED,using the superconductor SmFeAsO0.85F0.15 as an example

Imaging of light atoms has always been a challenge in high-resolution electron microscopy. Image resolution is mainly limited by lens aberrations, especially the spherical aberration of the objective lens. Image deconvolution could correct for the image distortion by lens aberrations and restore the structure projection, the resolution of which is limited by the information limit of the microscope. Electron diffraction unrestricted by lens aberrations could overcome this resolution limit. Here we show a combination of electron diffraction and image deconvolution to reveal simultaneously the atomic columns of O and considerably heavier Sm at a very close distance (1.17 Å) in iron-based superconductor SmFeAsO_0.85F_0.15 using a conventional 200 kV electron microscope. The approach used here, starting from an image and an electron diffraction pattern, has an advantage for those radiation-sensitive samples. Besides, it can be applied to simultaneously imaging light and heavy atoms, even though they have a big difference in atomic number and a much smaller atomic distance than the microscope resolution.     

纳米结构中磁斯格明子的原位电子全息研究

斯格明子(skyrmion)磁序结构与晶体微观结构的关联是新型功能磁材料和器件研发的重要问题.本文利用微纳加工技术制备了形状、尺寸均可控的磁纳米结构,通过电子全息术观察定量地分析了斯格明子磁序结构,确定了材料晶格缺陷和空间受限效应对斯格明子磁结构形成和稳定机制的影响,系统地分析了斯格明子基元的磁功能与材料微结构的关联.文中主要探讨了两个问题:1)斯格明子在磁纳米结构中的空间受限效应.重点研究斯格明子磁序随外磁场和温度变化的演变规律,探索其演变过程的拓扑属性和稳定性;2)晶格缺陷对斯格明子磁结构的影响,重点考察晶界原子结构手性反转对斯格明子磁序的影响.这些研究结果可为研发以磁斯格明子为基元的磁信息存储器及自旋电子学器件提供重要实验基础.     

Contamination mitigation strategies for scanning transmission electron microscopy

Modern scanning transmission electron microscopy (STEM) enables imaging and microanalysis at very high magnification. In the case of aberration-corrected STEM, atomic resolution is readily achieved. However, the electron fluxes used may be up to three orders of magnitude greater than those typically employed in conventional STEM. Since specimen contamination often increases with electron flux, specimen cleanliness is a critical factor in obtaining meaningful data when carrying out high magnification STEM. A range of different specimen cleaning methods have been applied to a variety of specimen types. The contamination rate has been measured quantitatively to assess the effectiveness of cleaning. The methods studied include: baking, cooling, plasma cleaning, beam showering and UV/ozone exposure. Of the methods tested, beam showering is rapid, experimentally convenient and very effective on a wide range of specimens. Oxidative plasma cleaning is also very effective and can be applied to specimens on carbon support films, albeit with some care. For electron beam-sensitive materials, cooling may be the method of choice. In most cases, preliminary removal of the bulk of the contamination by methods such as baking or plasma cleaning, followed by beam showering, where necessary, can result in a contamination-free specimen suitable for extended atomic scale imaging and analysis.     

Off-axis electron holography of hetero-interfaces

Off-axis electron holography is one of the new emerging methods for high spatial resolution characterization of interfaces in materials. It enables recording and retrieval of both amplitude and phase of an electron wavefunction scattered by a specimen. Phase changes introduced by magnetic and electrostatic fields have been studied in the first applications of electron holography to domain walls in ferromagnetic and ferroelectric materials and to p-n junctions. Planar defects in single crystals, such as stacking faults, have been observed with strong phase contrast due to dynamical diffraction.Applications to heterogeneous interfaces have only started. High phase contrast due to mean inner potential differences is found for interfaces between high and low atomic number materials. Dynamical contributions to the phase of the transmitted beam are important for epitaxial interfaces in strongly diffracting orientations. Numerical hologram reconstructions yield quantitative amplitude and phase images of an interface which are energy filtered and are in perfect registry. Both are function of specimen thickness. The thickness dependence can be eliminated by division of the phase image with a logarithm of the amplitude image. This ratio maps the product of the mean inner potential and the mean free path for inelastic scattering across a hetero-interface in weekly diffracting orientations.Resolution enhancement through aberration correction has not been demonstrated for interfaces as yet. Holography of interfaces in plan view is unexplored.     

Direct observations of La ordering and domain structures in La0.61Li0.17TiO3 by high resolution electron microscopy

La_0.61Li_0.17TiO₃ microstructures have been studied by high resolution electron microscopy. A local lattice distortion occurs in the vicinity of the domain boundary region due to the twin with an angle of 89°. The average domain size of La_0.61Li_0.17TiO₃ is greater than 20 nm. The domain size and structures of La_0.61Li_0.17TiO₃ differ greatly from those of La-poor compounds, such as La_0.55Li_0.35TiO₃. At a nanoscopic level, microdomains of 20–100 nm in size construct a two-dimensional structure in La-rich compounds, while microdomains of 5–10 nm in size construct a three-dimensional structure in La-poor compounds. In addition, the Li-ion conduction mechanisms for La-rich and La-poor compounds are two- and three-dimensional, respectively.     

Graphene oxide monolayers as supporting films for high resolution transmission electron microscopy

A novel kind of TEM support composed of graphene oxide monolayers was fabricated by a facile solution-cast method. Compared to the conventional carbon supporting films, such graphene oxide film (GO-film) performed better stability under high-energy electron beams and better dispersion effect for water-soluble nanoparticles. Quantitative improvement in TEM imaging resolution for nanomaterials on such GO-films compared to the commercial ultrathin carbon films was demonstrated.     

2D strain measurement in sub-10 nm SiGe layer with dark-field electron holography

In this paper, we carried out the two-dimensional (2D) strain measurement in sub-10 nm SiGe layer; images were obtained by dark-field electron holography (DFEH). This technique is based on transmission electron microscopy (TEM), in which dark-field holograms were obtained from a (400) diffraction spot. The measured results were compared to the X-ray diffraction (XRD) results in terms of the strain value and the depth of strain distribution in a very thin SiGe layer. Subsequently, we were able to successfully analyze the 2D strain maps along the [100] growth direction of the nanoscale SiGe region. The strain was measured and found to be in the range of 1.8–2.4%. The strain precision was estimated at 2.5 × 10⁻³. As a result, the DFEH technique is truly useful for measuring 2D strain maps in very thin SiGe layers with nanometer resolution and high precision.     

石墨烯的透射电子显微学研究

石墨烯（Graphene）是近几年迅速发展起来的研究热点材料之一.利用透射电子显微镜（TEM）研究Graphene的结构特征和原子动态过程,是 Graphene研究的重要进展.文章评述了利用透射电子衍射方法对Graphene的层数、堆垛方式、取向和表面形貌等结构特征进行的研究工作,介绍了利用高分辨透射电子显微术在Graphene的表面缺陷、边缘结构及吸附原子等研究领域取得的最新结果.     

数字滤波法再现电子全息图

在分析电子全息图特性的基础上，提出频谱滤波法对电子全息图进行数字再现，以解决其再像之间相互重叠的问题，此方法首先将数字化的电子全息图进行傅里叶变换得到全息图的空间频谱，然后对此频谱进行滤波处理并作反傅里叶变换得到一幅新的全息图，最后再用一般的数字再现法对此新的全息图进行再现，用这种方法，可以很方便地得到不受干扰的再现实像。     

Imaging polycrystalline and smoke MgO surfaces with atomic force microscopy: a case study of high resolution image on a polycrystalline oxide

Atomic force microscopy in contact, non-contact and in high resolution modes have been used to image MgO powder samples, obtained at different degree of sintering, starting from Mg(OH)₂ decomposition or obtained in form of smoke. From high resolution AFM images of MgO smoke, the lattice periodicity on regular surfaces has been revealed for the first time, under ambient conditions. The high surface perfection of the microcrystals has been further confirmed by HRTEM analysis. To obtain more information on the local structure of the single faces, in terms of type and distribution of the surface active sites, the adsorption of a simple probe molecule (CO) on such surfaces has been investigated by means of FTIR spectroscopy.     

Structure and orbital ordering of ultrathin LaVO3 probed by atomic resolution electron microscopy and Raman spectroscopy

Orbital ordering has been less investigated in epitaxial thin films, due to the difficulty to evidence directly the occurrence of this phenomenon in thin film samples. Atomic resolution electron microscopy enabled us to observe the structural details of the ultrathin LaVO₃ films. The transition to orbital ordering of epitaxial layers as thin as ≈4 nm was probed by temperature‐dependent Raman scattering spectroscopy of multilayer samples. From the occurrence and temperature dependence of the 700 cm^–1 Raman active mode it can be inferred that the structural phase transition associated with orbital ordering takes place in ultrathin LaVO₃ films at about 130 K.     

采用高分辨电镜分析Cu掺杂聚-4-甲基-1-戊烯泡沫材料采用高分辨电镜分析Cu掺杂聚-4-甲基-1-戊烯泡沫材料

利用物理掺杂的方法,制备了铜掺杂聚-4-甲基-1-戊烯(PMP)低密度泡沫材料,采用高分辨扫描电子显微镜和透射电子显微镜,分析了泡沫材料的微观结构、成分及微区成分分布。结果表明：与PMP泡沫相比,铜掺杂PMP泡沫的孔洞直径和网络骨架尺寸变大;铜颗粒镶嵌在PMP泡沫的有机骨架上且有团聚现象;泡沫材料中除碳、铜外,还残留有杂质元素氧;铜颗粒被PMP泡沫包裹,与碳网络骨架之间接触紧密。     

直拉硅单晶中原生氧沉淀的透射电镜研究

利用透射电镜对掺氮(NCZ) 和普通 (CZ) 直拉硅单晶中的原生氧沉淀进行研究. 研究表明，在NCZ样品中，有高密度的粒径为5nm的氧沉淀生成，而在CZ样品中，没有观察到这种氧沉淀. 初步认为，这种细小的氧沉淀是以650℃低温下形成的N-O复合体为核心在随后的冷却过程中形成. 关键词： 直拉硅 透射电镜 氧沉淀