Microstructural evolution characterization of Fe–Nb–B ternary systems processed by ball milling

Fe–Nb–B alloys prepared by ball milling can undergo a complex microstructural evolution during milling. In order to overcome the limitations imposed by traditional X-ray bulk analysis, a comprehensive multi-technique approach was devised to systematically characterize samples with the required resolution. A combination of in situ FIB (focused ion beam) lift-out and high-resolution ATEM (analytical transmission electron microscopy) has allowed the characterization of the phase evolution during milling. In particular, boron inclusions, not detected by X-ray diffraction, have been found to remain undissolved in the Fe matrix.     

Imaging of electron potential landscapes on Au(111)

The Hohenberg-Kohn theorem states that the ground state electron density completely determines the external potential acting on an electron system. Inspired by this fundamental theorem, we developed a novel approach to map directly the electron potential in surface systems: linear response theory applied to the total electron density as measured with scanning tunneling microscopy determines the external potential. Potential imaging is demonstrated for the s-p derived surface state on Au(111), where the "herringbone" reconstruction induces a periodic potential modulation, the details of which are revealed by our technique.     

In situ lift-out: steps to improve yield and a comparison with other FIB TEM sample preparation techniques

Steps to improve the success yield of the in situ lift-out technique are presented. These include tapping the plinth of the system and monitoring the grounding current to check the lift-out needle is fixed to the material being removed. In addition, the relative success yields and the time to prepare a TEM lamella for the three main FIB methods are discussed and compared.     

光学移频超分辨成像技术进展

光学显微镜具有无损、样品友好、速度快等优点,一直是人类探索微观世界的主要手段。但是,由于受到衍射极限限制,长期以来,光学成像系统的分辨率最高仅能达到可见光半波长量级,逐渐成为科学技术发展的桎梏。对于荧光标记样品,可以利用荧光超分辨光学显微成像技术打破光学衍射极限,填补电子显微镜(约为1 nm)和普通可见光学显微镜(200~250 nm)之间的空缺。然而,对于大多数样品特别是非荧光标记样品而言,利用现有技术进行超分辨成像依旧存在相当难度。近年来,科研人员从合成孔径成像原理出发,提出了光学移频超分辨成像方法,开辟了光学超分辨成像的新思路。光学移频超分辨成像不拘泥于荧光非线性效应的限制,兼具非荧光标记样品以及荧光标记样品的超分辨成像能力,而且因为其成像速度快、样品普适性高和光毒性低等优点,在材料学、生物学和医学等领域展现了很好的应用前景。本文从原理和方法上详细综述了移频超分辨光学显微成像技术,并对未来发展方向进行了评述和展望。     

Surface imaging using diffracted electrons

Refection electron microscopy has been applied for studying single crystal surfaces of noble metals in a conventional electron microscope using diffracted electrons for the imaging. This is equivalent to dark field imaging in transmission electron microscopy. The image contains far more contrast due to deviation from the ideal surface structure than any other surface imaging technique. A resolution of the order of 20 Å has been achieved for directions perpendicular to the incident beam. In directions parallel to the incident beam the resolution is much worse and the magnification is reduced with a factor 15 to 100 because the surface is viewed at the angle of the diffracted beam used for imaging (foreshortening). Nevertheless the achieved resolution is one to two orders of magnitude better than for other surface imaging methods. Since a correspondence can be established between image and diffraction pattern by means of selected area diffraction, the present technique should be very useful in structure analysis of surfaces.     

Organization of chromatin in the interphase mammalian cell

The use of imaging techniques has become an essential tool in cell biology. In particular, advances in fluorescence microscopy and conventional transmission electron microscopy have had a major impact on our understanding of chromatin structure and function. In this review we attempt to chart the conceptual evolution of models describing the organization and function of chromatin in higher eukaryotic cells, in parallel with the advances in light and electron microscopy over the past 50 years. In the last decade alone, the application of energy filtered transmission electron microscopy (EFTEM), also referred to as electron spectroscopic imaging (ESI), has provided many new insights into the organization of chromatin in the interphase nucleus. Based on ESI imaging of chromatin in situ, we propose a 'lattice' model for the organization of chromatin in interphase cells. In this model, the chromatin fibers of 10 and 30nm diameter observed by ESI, produce a meshwork that accommodates an extensive and distributed interchromosomal (IC) space devoid of chromatin. The functional implications of this model for nuclear activity are discussed.     

High contrast hollow-cone dark field transmission electron microscopy for nanocrystalline grain size quantification

In this paper, we describe hollow-cone dark field (HCDF) transmission electron microscopy (TEM) imaging, with a slightly convergent beam, as an improved technique that is suitable to form high contrast micrographs for nanocrystalline grain size quantification. We also examine the various factors that influence the HCDF TEM image quality, including the conditions of microscopy (alignment, focus and objective aperture size), the properties of the materials imaged (e.g., atomic number, strain, defects), and the characteristics of the TEM sample itself (e.g., thickness, ion milling artifacts). Sample preparation was found to be critical and an initial thinning by wet etching of the substrate (for thin film samples) or tripod polishing (for bulk samples), followed by low-angle ion milling was found to be the preferred approach for preparing high-quality electron transparent samples for HCDF imaging.     

In situ electron beam irradiated rapid growth of bismuth nanoparticles in bismuth-based glass dielectrics at room temperature

In this study, in situ control growth of bismuth nanoparticles (Bi⁰ NPs) was demonstrated in bismuth-based glass dielectrics under an electron beam (EB) irradiation at room temperature. The effects of EB irradiation were investigated in situ using transmission electron microscopy (TEM), selected-area electron diffraction and high-resolution transmission electron microscopy. The EB irradiation for 2–8 min enhanced the construction of bismuth nanoparticles with a rhombohedral structure and diameter of 4–9 nm. The average particle size was found to increase with the irradiation time. Bismuth metal has a melting point of 271 °C and this low melting temperature makes easy the progress of energy induced structural changes during in situ TEM observations. This is a very useful technique in nano-patterning for integrated optics and other applications.     

多光子皮肤成像技术及其应用

多光子成像技术是一种层析能力好、信噪比高的新型光学成像技术。在皮肤光学三维检测中,多光子技术已经应用于无创在体成像,且已得到产业化开发。本文将首先介绍多光子皮肤检测系统的若干核心技术,即双光子自发荧光技术、二次谐波成像技术、荧光寿命成像技术、相干反斯托克斯-拉曼成像技术等,然后简要介绍多光子成像系统在皮肤疾病成像检测上的应用,最后分析该系统的优势和未来可能的发展趋势。     

Mapping the intracellular distribution of carbon nanotubes after targeted delivery to carcinoma cells using confocal Raman imaging as a label-free technique

The uptake of carbon nanotubes (CNTs) by mammalian cells and their distribution within cells is being widely studied in recent years due to their increasing use for biomedical purposes. The two main imaging techniques used are confocal fluorescence microscopy and transmission electron microscopy (TEM). The former, however, requires labeling of the CNTs with fluorescent dyes, while the latter is a work-intensive technique that is unsuitable for in situ bio-imaging. Raman spectroscopy, on the other hand, presents a direct, straightforward and label-free alternative. Confocal Raman microscopy can be used to image the CNTs inside cells, exploiting the strong Raman signal connected to different vibrational modes of the nanotubes. In addition, cellular components, such as the endoplasmic reticulum and the nucleus, can be mapped. We first validate our method by showing that only when using the CNTs' G band for intracellular mapping accurate results can be obtained, as mapping of the radial breathing mode (RBM) only shows a small fraction of CNTs. We then take a closer look at the exact localization of the nanotubes inside cells after folate receptor-mediated endocytosis and show that, after 8-10 h incubation, the majority of CNTs are localized around the nucleus. In summary, Raman imaging has enormous potential for imaging CNTs inside cells, which is yet to be fully realized.     

Focused ion beam milling and ultramicrotomy of mineralised ivory dentine for analytical transmission electron microscopy

The use of focused ion beam (FIB) milling for preparation of sections of mineralised ivory dentine for transmission electron microscopy (TEM) is investigated. Ivory dentine is essentially composed of fibrillar type-I collagen and apatite crystals. The aim of this project is to gain a clearer understanding of the relationship between the organic and inorganic components of ivory dentine using analytical TEM, in order to utilise these analytical techniques in the context of common skeletal diseases such as osteoporosis and arthritis. TEM sections were prepared in both single and dual beam FIB instruments, using two standard lift-out techniques, in situ and ex situ. The FIB sections were systematically compared with sections prepared by ultramicrotomy, the traditional preparation route in biological systems, in terms of structural and chemical differences. A clear advantage of FIB milling over ultramicrotomy is that dehydration, embedding and section flotation can be eliminated, so that partial mineral loss due to dissolution is avoided. The characteristic banding of collagen fibrils was clearly seen in FIB milled sections without the need for any chemical staining, as is commonly employed in ultramicrotomy. The FIB milling technique was able to produce high-quality TEM sections of ivory dentine, which are suitable for further investigation using electron energy-loss spectroscopy (EELS) and energy-filtering TEM (EFTEM) to probe the collagen/apatite interface.     

Spectrally encoded confocal microscopy

An endoscope-compatible, submicrometer-resolution scanning confocal microscopy imaging system is presented. This approach, spectrally encoded confocal microscopy (SECM), uses a quasi-monochromatic light source and a transmission diffraction grating to detect the reflectivity simultaneously at multiple points along a transverse line within the sample. Since this method does not require fast spatial scanning within the probe, the equipment can be miniaturized and incorporated into a catheter or endoscope. Confocal images of an electron microscope grid were acquired with SECM to demonstrate the feasibility of this technique.     

Optical and structural characterization of a self-aligned single electron transitor structure by cathodoluminescence microscopy

We report about optical and structural investigations of a self-aligned single electron transistor (SET) structure using cathodoluminescence-(CL) and transmission electron microscopy (TEM). The SET structures were fabricated by MBE growth of GaAs/AlAs on different prepatterned GaAs (1 0 0) substrates. This technique for the in situ formation of nanoscopic semiconductor heterostructures is presently a widely used and promising approach for the fabrication of low-dimensional systems like quantum wires and quantum dots (QD). The active region of the SET structure consists of a GaAs/AlGaAs-QD formed by thickness modulation of a single quantum well (SQW) during the MBE growth. The position and the size of the QD is defined by the design of the substrate pattern. The thickness modulation of the GaAs-SQW is evidenced by TEM investigations. The lateral confinement potential given by the thickness modulation of GaAs-SQW is directly imaged by CL microscopy.     

Computer-assisted quantification of the multi-scale structure of films made of nanofibrillated cellulose

Films made of nanofibrillated cellulose (NFC) are most interesting for use in packaging applications. However, in order to understand the film-forming capabilities of NFC and their properties, new advanced methods for characterizing the different scales of the structures are necessary. In this study, we perform a comprehensive characterisation of NFC-films, based on desktop scanner analysis, scanning electron microscopy in backscatter electron imaging mode (SEM-BEI), laser profilometry (LP) and field-emission scanning electron microscopy in secondary electron imaging mode (FE-SEM-SEI). Objective quantification is performed for assessing the (i) film thicknesses, (ii) fibril diameters and (iii) fibril orientations, based on computer-assisted electron microscopy. The most frequent fibril diameter is 20–30 nm in diameter. A method for acquiring FE-SEM images of NFC surfaces without a conductive metallic layer is introduced. Having appropriate characterisation tools, the structural and mechanical properties of the films upon moisture were quantified.     

Approaches for ultrafast imaging of transient materials processes in the transmission electron microscope

The growing field of ultrafast materials science, aimed at exploring short-lived transient processes in materials on the microsecond to femtosecond timescales, has spawned the development of time-resolved, in situ techniques in electron microscopy capable of capturing these events. This article gives a brief overview of two principal approaches that have emerged in the past decade: the stroboscopic ultrafast electron microscope and the nanosecond-time-resolved single-shot instrument. The high time resolution is garnered through the use of advanced pulsed laser systems and a pump-probe experimental platforms using laser-driven photoemission processes to generate time-correlated electron probe pulses synchronized with laser-driven events in the specimen. Each technique has its advantages and limitations and thus is complementary in terms of the materials systems and processes that they can investigate. The stroboscopic approach can achieve atomic resolution and sub-picosecond time resolution for capturing transient events, though it is limited to highly repeatable (>10(6) cycles) materials processes, e.g., optically driven electronic phase transitions that must reset to the material's ground state within the repetition rate of the femtosecond laser. The single-shot approach can explore irreversible events in materials, but the spatial resolution is limited by electron source brightness and electron-electron interactions at nanosecond temporal resolutions and higher. The first part of the article will explain basic operating principles of the stroboscopic approach and briefly review recent applications of this technique. As the authors have pursued the development of the single-shot approach, the latter part of the review discusses its instrumentation design in detail and presents examples of materials science studies and the near-term instrumentation developments of this technique.     

Preparing surfaces for the analysis of magnetic structures

The effect of plasma cleansing on the surface state of samples studied by electron microscopy is experimentally investigated. This technique of sample preparation allowed diagnostics of the magnetic states of objects by scanning electron microscopy with the analysis of spin polarization of secondary electrons.     

Electron microscopy and in situ testing of mechanical deformation of carbon nanotubes

In this paper, the electron diffraction technique to determine the helicity and atomic structure of carbon nanotubes is reviewed, as well as different mechanical test methods, tensile test, bending test, compression test and vibration test of carbon nanotubes by in situ electron microscopy are summarized while the relationship between mechanical properties and structures revealed by experiments is addressed. Except for these, the electric current and electron beam irradiation effect and some other novel electron microscopy experiments are also incorporated.     

Influence of surface topography on the secondary electron yield of clean copper samples

Secondary electron yield (SEY) due to electron impact depends strongly on surface topography. The SEY of copper samples after Ar-ion bombardment is measured in situ in a multifunctional ultrahigh vacuum system. Increasing the ion energy or duration of ion bombardment can even enlarge the SEY, though it is relatively low under moderate bombardment intensity. The results obtained with scanning electron microscopy and atomic force microscopy images demonstrate that many valley structures of original sample surfaces can be smoothed due to ion bombardment, but more hill structures are generated with stronger bombardment intensity. With increasing the surface roughness in the observed range, the maximum SEY decreases from 1.2 to 1.07 at a surface characterized by valleys, while it again increases to 1.33 at a surface spread with hills. This phenomenon indicates that hill and valley structures are respectively effective in increasing and decreasing the SEY. These obtained results thus provide a comprehensive insight into the surface topography influence on the secondary electron emission characteristics in scanning electron microscopy.     

Fabrication of high quality plan-view TEM specimens using the focused ion beam

We describe a technique using a focused ion beam instrument to fabricate high quality plan-view specimens for transmission electron microscopy studies. The technique is simple, site-specific and is capable of fabricating multiple large, >100 μm² electron transparent windows within epitaxially grown thin films. A film of La_0.67Sr_0.33MnO₃ is used to demonstrate the technique and its structural and functional properties are surveyed by high resolution imaging, electron spectroscopy, atomic force microscopy and Lorentz electron microscopy. The window is demonstrated to have good thickness uniformity and a low defect density that does not impair the film's Curie temperature. The technique will enable the study of in-plane structural and functional properties of a variety of epitaxial thin film systems.     

Calibration of a scanning Joule expansion microscope (SJEM)

Scanning thermal microscopy (SThM) is a scanning probe technique based on atomic force microscopy (AFM) enabling high-resolution topographical imaging together with visualization of the temperature distribution in the studied sample. For the thermal mapping, rather expensive, micro-fabricated cantilevers with integrated thermocouples have to be used. The spatial resolution is typically limited to 100 nm. Scanning Joule expansion microscopy (SJEM) uses an alternative approach to detect the temperature of the sample with a regular silicon cantilever and lock-in detection. By monitoring the thermal expansion of the sample (due to Joule heating), the local temperature can be monitored. The resolution of SJEM is comparable to that of contact AFM, which is an order of magnitude better than for SThM. Our research involves implementing a SJEM for the study of heating phenomena in mesoscopic structures prepared by electron beam lithography and lift-off techniques. In particular, we calibrated our SJEM in order to make quantitative temperature maps of the studied samples.