In-situ observation of shape and atomic structure of Xe nanocrystals embedded in aluminium

An imaging technique to determine in situ the shape and atomic structure of nanosized Xe crystals embedded in Al is described using high-resolution transmission electron microscopy (HRTEM). The Xe nanocrystals, with sizes less than 5 nm were prepared by the implantation of 30 keV Xe⁺ into Al at room temperature. The fcc Xe nanocrystals are mesotactic with the Al lattice and have a lattice parameter ≈ 50% larger than that of Al. HRTEM images of the Xe were not clear in [110] zone axis illumination because of the small number of Xe atoms relative to Al atoms in any atom column. An off-axial imaging technique that consists of tilting the specimen several degrees from a zone axis and defocusing to suppress the Al lattice fringes is employed for the 110 projection of the Xe/Al system and the structure of the Xe nanocrystals is successfully imaged. The Xe images clearly represent projections of cuboctahedra with faces parallel to eight Al {111} planes truncated by six {100} planes. The results of multislice image simulations using a three-dimensional atomic model agreed well with the results obtained by the off-axial imaging technique. The usefulness of the technique is demonstrated with observations of crystal defects introduced into the Xe under intense 1000 keV electron irradiation.     

Atomic imaging in aberration-corrected high-resolution transmission electron microscopy

In recent years, the successful implementation of a spherical-aberration corrector in a Philips CM 200 FEG ST microscope achieved by Haider et al. has attracted a great deal of attention. However, thus far extensive applications of this novel high-resolution transmission electron microscope (HRTEM) to materials research have been hampered by the problems concerning optimum imaging conditions and image interpretation. In this paper, we present our points of view concerning atomic imaging in an aberration-corrected HRTEM. Since atomic resolution images can also be obtained with other techniques such as through-focus exit-wave function reconstruction (TF-EWR), we have to emphasis that the strength of the aberration-corrected HRTEM particularly lies on its ability to resolve the atomic structure in real time. However, for this purpose it is mandatory that the image contrast be related in a one-to-one function with the projected structure of the object. We analyzed the atomic imaging conditions in much detail and we come to the following conclusion: this novel facility is no doubt a powerful and advanced HRTEM instrument in achieving atomic images with its highest resolution (information limit). We furthermore demonstrate that the combination of the new microscope and TF-EWR will yield optimal results.     

Resolving power of lens-less low energy electron point source microscopy

Simulations show the resolving power of lens-less low energy electron point source microscopy to be approximately 15 Å  for electron energies of between 14 and 105 eV. This resolution can be improved to 10 Å  by employing a composite hologram method. However, these values fall short of predictions of at least 3 Å  resolution for electron energies in this range because the limited divergence angle of the electron beam had not previously been taken into account. Also shown is that electron coherence from field emitting tips that terminate in a single atom will not limit the resolving power as much as the beam divergence angle. The penetration depth of electrons with energies of between 50 and 100 eV, into biological molecules, was estimated from the inelastic mean free path length to be 25 Å . This places an upper limit on the size of biological molecules for which internal structural information can be obtained using low energy electrons.     

Fine structure of the images of atoms in 011-gold crystals in various Bragg reflecting conditions

Fine structures with petal and horseshoe shape contrast have been observed in the high resolution electron microscope images of atoms in a 011 gold crystal in deviated Bragg reflecting conditions. The images with such fine structures have been recorded by using only the beams of 000, four 111 and two 200 reflections with axial illumination. The contrast of the images has been discussed by the many beam dynamical theory of electron diffraction and the image formation theory considering the spatial and temporal coherence of the electron waves. It has been noted that the temporal coherence has much more effect on the change of image contrast than the spatial coherence. The imaging conditions have been analysed in terms of Fourier images and the aberration free focus conditions.     

Mid-infrared scanning near-field optical microscope resolves 30 nm

We explore the performance of a scanning near-field infrared microscope, which works by scattering tightly focused CO₂ laser radiation (λ = 10  μ m) from the apex of a metallized atomic force microscope tip. The infrared images of test samples prove a spatial resolution of 30 nm and are free of topographical and inertial artefacts, thus they should be of great interest for practical applications. We also observe that the infrared contrast vanishes when the input beam polarization is orthogonal to the tip axis, in accordance with theoretical expectations for a mechanism of longitudinal field interaction.     

Non-kinkwise field evaporation and kink relaxation on stepped W(112) surface

Low-temperature field evaporation of the [111] steps on a W(211) surface was investigated by field ion microscopy (FIM). The atoms at the (112)[1;1;1] kink site are anomalously stable against field evaporation. This effect results in non-kinkwise field evaporation near the kink of this type. The non-kinkwise field evaporating steps [211]<111> usually produce the reconstructed atomic chain with double-space arranged adatoms. The experimental results reveal atomic relaxation effects at steps and kinks. The normal to surface differential relaxation of the kink-site atoms was estimated by the geometrical method of indirect magnification and by simulation of the FIM image.     

On the interpretation of the forbidden spots observed in the electron diffraction patterns of flat Au triangular nanoparticles

In many cases nanostructures present forbidden spots in their electron diffraction patterns when they are observed by transmission electron microscopy (TEM). To interpret their TEM and high resolution transmission electron microscopy (HRTEM) images properly, an understanding of the origin of these spots is necessary. In this work we comment on the origin of the forbidden spots observed in the [111] and [112] electron diffraction patterns of flat gold triangular nanoparticles. The forbidden spots were successfully indexed as corresponding to the first laue Zone (FOLZ) and the HRTEM images presented a contrast produced by the interference of the zero-order Laue zone (ZOLZ) and FOLZ spots. We discuss the use of the forbidden spots in the study of the structure of nanoparticles and show that they are related to the shape and incompleteness of layers in the very thin particles.     

Reduction of charging in protein electron cryomicroscopy

Charging causes a loss of resolution in electron cryomicroscopy with biological specimens prepared without a continuous carbon support film. Thin conductive films were deposited onto catalase crystals prepared across holes using ion-beam sputtering and thermal evaporation and evaluated for the effectiveness of charge reduction. Deposits applied by ion-beam sputtering reduced charging but concurrently resulted in structural damage. Coatings applied by thermal evaporation also reduced charging, and preserved the specimen structure beyond 5 Å resolution as judged from electron diffraction patterns and images of glucose-embedded catalase crystals tilted to 45° in the microscope. This study demonstrates for the first time the feasibility of obtaining high-resolution data from unstained, unsupported protein crystals with a conductive surface coating.     

Imaging of individual atoms on an Al(111) surface by scanning tunnelling microscopy

Due to the delocalized character of metal valence electrons the atomic corrugation of metal surfaces observed in Scanning Tunnelling Microscopy (STM) is found to be much smaller than in the case of semiconductor surfaces. In fact, there is only a single study in the literature which reports the resolution of the individual atoms on a metal surface (Hallmark et al., 1987). The present paper demonstrates the resolution of individual atoms on a close packed surface of a nearly free electron metal, Al(111), and presents systematic experiments on the physical origin of this phenomenon. A more detailed discussion will be given elsewhere (Wintterlin et al., 1988).     

Increased resolution in neutral atom microscopy

The neutral atom microscope uses a beam of thermal noncharged atoms or molecules to probe an atomic surface with very low interaction energies (<70 meV). Continued optimization of the ‘pinhole’ neutral atom microscope has improved resolution to 0.35 μm. Recent images are presented demonstrating resolution and the contrast mechanisms identified so far. The future potential for sub‐100 nm resolution is discussed.     

Seeing atoms with aberration-corrected sub-Angström electron microscopy

High-resolution electron microscopy is able to provide atomic-level characterization of many materials in low-index orientations. To achieve the same level of characterization in more complex orientations requires that instrumental resolution be improved to values corresponding to the sub-Ångström separations of atom positions projected into these orientations. Sub-Ångström resolution in the high-resolution transmission electron microscope has been achieved in the last few years by software aberration correction, electron holography, and hardware aberration correction; the so-called “one-Ångström barrier” has been left behind. Aberration correction of the objective lens currently allows atomic-resolution imaging at the sub-0.8 Å level and is advancing towards resolutions in the deep sub-Ångström range (near 0.5 Å). At current resolution levels, images with sub-Rayleigh resolution require calibration in order to pinpoint atom positions correctly. As resolution levels approach the “sizes” of atoms, the atoms themselves will produce a limit to resolution, no matter how much the instrumental resolution is improved. By arranging imaging conditions suitably, each atom peak in the image can be narrower, so atoms are imaged smaller and may be resolved at finer separations.     

Scanning and transmission electron microscopy for evaluation of order/disorder in bone structure

A comparative characterization of the structure of normal and abnormal (osteoporotic) human lumbar and thoracic vertebrae samples was carried out to reveal the type of possible disorder. Samples from the bone fragments extracted during the surgery due to vertebra fractures were examined by scanning electron microscopy (SEM), conventional and high resolution transmission electron microscopy (TEM and HRTEM), and X-ray energy dispersive spectroscopy (EDS). Contrary to what might be expected in accordance with possible processes of dissolution, formation and remineralization of hard tissues, no changes in phase composition of mineral part, crystal sizes (length, width, and thickness), and arrangement of crystals on collagen fibers were detected in abnormal bones compared to the normal ones. The following sizes were determined by HRTEM for all bone samples: 相似文献   

Optical far-field microscopy of single molecules with 3.4 nm lateral resolution

Optical far‐field imaging of single molecules in a frozen solution at 1.2 K with a lateral resolution of 3.4 nm is reported. The mechanical stability of the fluorescence microscope, especially of the low‐temperature insert, allows for the localization of fluorescing molecules with a reproducibility of better than 5 nm within observation times up to 10 min. For observation times of 9 h the reproducibility of the lateral position is limited to about 20 nm due to mechanical drift. Lateral position and orientation of 314 single molecules, present within the confocal detection volume of ~10 µm³, are obtained. The possibility to correct for mechanical drift by monitoring the position of a spatial reference in the sample is demonstrated.     

High resolution electron microscope observations of microstructures in A15 type Nb3X superconductors

Microstructures of superconducting Nb₃X(A15) compounds are studied by means of high resolution electron microscopy. Structure images are obtained with the 400kV high resolution electron microscope for both the annealed and ion-irradiated crystals. The images obtained from the annealed crystals show bright contrast patterns similar to the projections of Nb and X atoms comprising the A15 structure. Defects observed in the annealed crystal irradiated with 40 kV Nb⁺ ions show characteristic features in contrast. The dependence of image contrast on the element X and the atomic structures of the defects are discussed by comparison with the calculated images.     

Imaging and analysis of nanowires

We used vapor-liquid-solid (VLS) methods to synthesize discrete single-element semiconductor nanowires and multicomposition nanowire heterostructures, and then characterized their structure and composition using high-resolution electron microscopy (HRTEM) and analytical electron microscopy techniques. Imaging nanowires requires the modification of the established HRTEM imaging procedures for bulk material to take into consideration the effects of finite nanowire width and thickness. We show that high-resolution atomic structure images of nanowires less than 6 nm in thickness have lattice "streaking" due to the finite crystal lattice in two dimensions of the nanowire structure. Diffraction pattern analysis of nanowires must also consider the effects of a finite structure producing a large reciprocal space function, and we demonstrate that the classically forbidden 1/3 [422] reflections are present in the [111] zone axis orientation of silicon nanowires due to the finite thickness and lattice plane edge effects that allow incomplete diffracted beam cancellation. If the operating conditions are not carefully considered, we found that HRTEM image delocalization becomes apparent when employing a field emission transmission electron microscope (TEM) to image nanowires and such effects have been shown to produce images of the silicon lattice structure outside of the nanowire itself. We show that pseudo low-dose imaging methods are effective in reducing nanowire structure degradation caused by electron beam irradiation. We also show that scanning TEM (STEM) with energy dispersive X-ray microanalysis (EDS) is critical in the examination of multicomponent nanowire heterostructures.     

X-ray microscopy and imaging of Escherichia coli, LPS and DNA

Ultrastructural examination by transmission and scanning electron microscopy involves a series of specialized preparation steps which may introduce artefacts in the micrographs. X-ray microscopy can take instant images of speci-mens but is mostly restricted to a few synchrotron X-ray sources. We have utilized a bench-top nanosecond laser-plasma to produce a single-shot source of nanosecond X-rays tuned for maximum contrast with carbon-rich material. To examine the ultrastructure by absorption profiles, we utilized a laser-produced plasma generated by a single-shot laser (1.06 μm wavelength, 5 × 10¹² W cm⁻² intensity) focused on to a silicon target as an X-ray source for high-resolution X-ray microscopy. This approach eliminates the specimen preparation steps. Whole hydrated cells of Escherichia coli and purified preparations of lipopolysaccharide (LPS) and chromosomal DNA (cDNA) were streaked onto poly(methyl methacrylate) (PMMA)-coated grids (resist). This resist was exposed to X-rays under vacuum at a distance of 2.5 cm from the target disc. The silicon plasma produced by a 10-ns burst of laser energy (at 20 J) radiates strong emission lines in the region of 300 eV. The X-rays penetrate the sample and their absorption profile is transferred on to the resist where PMMA acts as a negative to generate an image. By atomic force microscopy imaging of this photoresist we have visualized layers around cells of E. coli , darker areas inside the cell probably corresponding to cDNA, and preliminary images of LPS and DNA molecules. This technique has resolution at the 100 Å level, produces images similar to the space-filling models of macromolecules and may be of great value in the study of the ultrastructure of hydrated live biological specimens.     

Comparing electron tomography and HRTEM slicing methods as tools to measure the thickness of nanoparticles

Nanoparticles’ morphology is a key parameter in the understanding of their thermodynamical, optical, magnetic and catalytic properties. In general, nanoparticles, observed in transmission electron microscopy (TEM), are viewed in projection so that the determination of their thickness (along the projection direction) with respect to their projected lateral size is highly questionable. To date, the widely used methods to measure nanoparticles thickness in a transmission electron microscope are to use cross-section images or focal series in high-resolution transmission electron microscopy imaging (HRTEM “slicing”). In this paper, we compare the focal series method with the electron tomography method to show that both techniques yield similar particle thickness in a range of size from 1 to 5 nm, but the electron tomography method provides better statistics since more particles can be analyzed at one time. For this purpose, we have compared, on the same samples, the nanoparticles thickness measurements obtained from focal series with the ones determined from cross-section profiles of tomograms (tomogram slicing) perpendicular to the plane of the substrate supporting the nanoparticles. The methodology is finally applied to the comparison of CoPt nanoparticles annealed ex situ at two different temperatures to illustrate the accuracy of the techniques in detecting small particle thickness changes.     

Imaging of hydrogen-induced Si(111) surface with the scanning tunnelling microscope

We have directly observed the hydrogen-induced changes of the Si(111)7times7 surface using a scanning tunnelling microscope (STM). The 7times7 reconstructed atomic structure was formed on a clean surface of Si(111). But when the clean surface was dosed with typically 1–2 L [1 L (Langmuir) = 1·33 times 10^?4 Pa. sec] hydrogen, the 7 times 7 image was gradually smeared out and then a 1 times 1 unreconstructed pattern appeared. After dosing with 5–10 L hydrogen, the STM image exhibited a new long-periodic structure together with the 1times1 structure underneath. These experimental results may be ascribed to the chemisorption of hydrogen atoms on clean Si surfaces.     

Nanobeam electron diffraction and high resolution imaging analysis of InN films grown on sapphire

A JEOL JEM-3000F field emission, analytical, high-resolution transmission electron microscope (HRTEM) was used to study InN films grown on sapphire substrates. It was found that, while the InN films maintained the hexagonal (wurtzite) structure, InN nanodomains with a cubic (zincblende) structure were also formed in the films. Nanobeam electron diffraction techniques were applied for identification of the cubic phase. The identification of the cubic InN was confirmed by HRTEM structural imaging. The cubic InN nanodomains are 3-10 nm in diameter, and are orientated in two different orientations with their [110](cubic) and [110](cubic) axes parallel to each other and their (111)(cubic) planes parallel to the (0001)(hex) plane of the hexagonal InN.     

Polarization contrast in reflection near-field optical microscopy with uncoated fibre tips

Using cross-hatched, patterned semiconductor surfaces and round 20-nm-thick gold pads on semiconductor wafers, we investigate the imaging characteristics of a reflection near-field optical microscope with an uncoated fibre tip for different polarization configurations and light wavelengths. It is shown that cross-polarized detection allows one to effectively suppress far-field components in the detected signal and to realize imaging of optical contrast on the sub-wavelength scale. The sensitivity window of our microscope, i.e. the scale on which near-field optical images represent mainly optical contrast, is found to be ≈100 nm for light wavelengths in the visible region. We demonstrate imaging of near-field components of a dipole field and purely dielectric contrast (related to well-width fluctuations in a semiconductor quantum well) with a spatial resolution of ≈100 nm. The results obtained show that such a near-field technique can be used for polarization-sensitive imaging with reasonably high spatial resolution and suggest a number of applications for this technique.