Showing papers on "Resolution (electron density) published in 1998"

Electron microscopy image enhanced

Abstract: One of the biggest obstacles in improving the resolution of the electron microscope has always been the blurring of the image caused by lens aberrations. Here we report a solution to this problem for a medium-voltage electron microscope which gives a stunning enhancement of image quality.

High-Resolution X-Ray Scattering from Thin Films and Multilayers

Abstract: Part 1 Experimental realization: basic elements of an equipment resolution elements diffractometers and reflectometers. Part 2 The theory of X-ray diffraction and its realization by the experiment: kinematical X-ray scattering from ideal crystals kinematical X-ray diffraction from deformed thin layers kinematical X-ray diffraction from randomly disturbed layers dynamical X-ray diffraction in perfect layers dynamical X-ray diffraction in slightly deformed layers optical reflection of X-rays from ideal layers optical reflection of X-rays from layers with rough interfaces dynamical X-ray diffraction in strongly asymmetric cases grazing incidence diffraction (GID). Appendices: elements of the formal theory of scattering structure factors, dispersion corrections and extinction length.

Resolution and contrast in Kelvin probe force microscopy

Abstract: The combination of atomic force microscopy and Kelvin probe technology is a powerful tool to obtain high-resolution maps of the surface potential distribution on conducting and nonconducting samples. However, resolution and contrast transfer of this method have not been fully understood, so far. To obtain a better quantitative understanding, we introduce a model which correlates the measured potential with the actual surface potential distribution, and we compare numerical simulations of the three-dimensional tip–specimen model with experimental data from test structures. The observed potential is a locally weighted average over all potentials present on the sample surface. The model allows us to calculate these weighting factors and, furthermore, leads to the conclusion that good resolution in potential maps is obtained by long and slender but slightly blunt tips on cantilevers of minimal width and surface area.

Full-field optical coherence microscopy.

Abstract: We present a new microscopy system for imaging in turbid media that is based on the spatial coherence gate principle and generates in parallel a complete two-dimensional head-on image without scanning. This system has been implemented in a commercial microscope and preserves the lateral resolution of the optics used. With a spatially incoherent source, speckle-free images with diffraction-limited resolution are recorded at successive depths with shot-noise-limited detection. The setup comprises a photoelastic modulator for path difference modulation and a two-dimensional CCD array and uses a multiplexed lock-in detection scheme.

Quantitative microwave near-field microscopy of dielectric properties

Abstract: A theoretical model analysis for a recently developed scanning evanescent microwave microscope has been performed. The result enables a quantitative microscopy of local complex dielectric constant profiles for dielectric materials. Various experiments were performed and found to be in good agreement with the theoretical results. The estimation of intrinsic resolution of the microscope suggests that nanometer spatial resolution is achievable. System analysis gives a limiting sensitivity of about δe/e∼1×10−5.

SPELEEM: Combining LEEM and Spectroscopic Imaging

Abstract: At present the only surface electron microscope which allows true characteristic XPEEM (photoemission electron microscopy using synchrotron radiation) and structural characterization is the spectroscopic LEEM developed at the Technical University Clausthal in the early nineties. This instrument has in the past been used mainly for LEEM studies of various surface and thin film phenomena, because it had very limited access to synchrotron radiation. Now the microscope is connected quasipermanently to the undulator beamline 6.2 at the storage ring ELETTRA, operating successfully since the end of 1996 under the name SPELEEM (Spectroscopic PhotoEmission and Low Energy Electron Microscope). The high brightness of the ELETTRA light source, together with an optimized instrument, results in a spatial resolution better than 25 nm and an energy resolution better than 0.5 eV in the XPEEM mode. The instrument can be used alternately for XPEEM, LEEM, LEED (low energy electron diffraction), MEM (mirror electron microscop...

Contrast, resolution, pixelation, dynamic range and signal‐to‐noise ratio: fundamental limits to resolution in fluorescence light microscopy

Abstract: In a perfect optical system numerical aperture and wavelength determine resolution. In a real optical system, however, the number of photons collected from a specimen determines the contrast and this limits the resolution. Contrast is affected by the number of picture elements per unit area, the number of photons and the aberrations present in every optical system. The concept of contrast vs. distance functions is used to compare the resolution achievable in confocal and wide-field fluorescence microscopes and the effect of a further reduction of the observable volume. In conclusio: (a) real optical systems will never be able to achieve the theoretical resolution, (b) wide-field fluorescence microscopy will often provide a better resolution than confocal fluorescence microscopy, (c) decreasing the observed volume does not necessarily increase the resolution and (d) using multiple fluorophores can improve the accuracy with which distances are measured. Some numbers for typical situations are provided.

Chapter 3 Structural Transitions in the Group IV, III-V, and II-VI Semiconductors under Pressure

Abstract: Publisher Summary This chapter discusses structural transitions in the group IV, III–V, and II–VI semiconductors under pressure. The most preferred technique of structural transitions is energy-dispersive diffraction (EDX) in which the full white beam of the synchrotron falls on the sample. The diffraction pattern is recorded as a function of X-ray energy at a fixed scattering angle. The resolution of EDX powder patterns is limited to that of the solid-state detector. The patterns are unavoidably contaminated with fluorescence lines from the sample. The tight collimation of the diffracted beam required to define the scattering angle often leads to poor-powder averaging with the effect that peak intensities are unreliable—for example, the ability to detect and characterize sample microstructure effects, which are known to cause serious problems in high-pressure work. Preferred orientation (PO) can alter peak intensities dramatically leading to a strong reflection that is apparently absent from a powder pattern.

Resolution Enhancement and Band Assignments for the First Overtone of OH Stretching Modes of Butanols by Two-Dimensional Near-Infrared Correlation Spectroscopy. 2. Thermal Dynamics of Hydrogen Bonding in n- and tert-Butyl Alcohol in the Pure Liquid States

Abstract: The temperature-dependent near-infrared (NIR) spectral variations of n- and tert-butyl alcohols in the pure liquid state have been studied by generalized two-dimensional (2D) correlation analysis. ...

The relationship between the electrochemistry and the crystallography of microcrystals. The case of TCNQ (7,7,8,8-tetracyanoquinodimethane)†‡

Abstract: Microcrystals of TCNQ, in the size range 100–2000 nm, may be attached to the surfaces of graphite, glassy carbon, gold, platinum and RAMTM electrodes by a process of dry abrasion. When the resulting surfaces are placed in aqueous solutions of Group I cations, such as Na+, K+, Rb+ and Cs+, and the electrode potential is cycled, reversible phase transformations take place between the TCNQ and its corresponding cation salts. The electrochemical responses of these reversible phase transformations show that, in all cases, nucleation–growth kinetics are rate-determining. Ancillary techniques, such as optical microscopy, scanning electron microscopy and X-ray diffractometry, elucidate the corresponding crystal structure changes. In combination, these techniques provide deep insights into the relationship between the electrochemistry and crystallography of microcrystals.Optical microscopy reveals a colour change from yellow to blue–green upon electrochemical reduction of TCNQ microcrystals, and also reveals that small crystals react faster than large crystals. Unfortunately, analysis of morphological changes in situ in real time is prevented by the limited resolution of optical techniques (500 nm). However, scanning electron microscopy is able to provide ex situ ‘snapshots’ of the microcrystal morphologies before and after the phase transformations with a resolution of 1 nm, and these can be used to reconstitute the reaction pathway. Finally, X-ray diffractometry allows the spatial coordinates of the TCNQ molecules to be determined both before and after the phase transformations with accuracies of ± 0.001 nm. Such data reveal, for the first time, the changes that occur in molecular orientation during electrochemically induced solid–solid phase transformations in pi-stacked organic conductors.

On the Extendibility of X‐ray Crystallography to Noncrystals

Abstract: This paper discusses the concept that crystallinity is not an essential requirement for applying the techniques of X-ray crystal structure analysis Assuming this to be true, the removal of crystallinity as a prerequisite for the techniques would allow the imaging of structures well beyond the present range of sizes accessible to X-ray crystallography An example of an imageable structure could be a single small biological cell, containing perhaps 1013–1014 Da The proposed concept differs from the usual diffraction method of studying noncrystalline structure, ie small-angle scattering, in carrying out the diffraction experiment and subsequent processing as if the structure being studied were in fact the asymmetric unit of a crystal: ie orienting the structure in all directions in the X-ray beam needed to explore its Fourier transform (F transform), and phasing and inverting the transform to obtain the electron-density image of the structure The one actual difference from the crystal case is that the F transform is faint and also continuous, rather than displaying discrete intense Bragg spots As a result, to get a readable pattern, the structure must be exposed to high levels of radiation This last fact creates the principal limitation of the technique With single air-dried biological cells at room temperature as diffracting specimens and soft X-rays in the wavelength range 18–32 A, patterns to date have not been observed beyond resolutions of 140–300 A before radiation damage has become evident At this resolution, the technique nevertheless would lie on the same curve of resolution vs specimen size as do the existing major imaging techniques of X-ray crystallography, electron microscopy and light microscopy, falling directly between the latter two Thus, X-ray diffractive imaging is not destroyed by the withdrawal of crystallinity but instead is shifted to a new size range of structures, which have hitherto been somewhat inaccessible to imaging A method for phasing the diffraction pattern, based on the ability to sample the pattern more finely than in the case of the crystalline specimen, is giving good results in preliminary testing The principal need at present is for better instrumentation for collecting the diffraction data, including the additional motions needed for collecting data in three dimensions

Second-harmonic imaging of ferroelectric domains in linbo3 with micron resolution in lateral and axial directions

Abstract: We demonstrate that scanning second-harmonic microscopy with a mode-locked laser can be used as a nondestructive technique to image ferroelectric domain structures with micron resolution in both lateral and axial directions. This method is expected to have significant impact particularly on the further development of nonlinear optical bulk and waveguide devices with periodically poled ferroelectric crystals. PACS: 07.60.Pb; 42.30; 77.80.Dj Ferroelectrics such as LiNbO3, KNbO3, or BaTiO3 play an important role in nonlinear optics and electro-optics because of their large second-order nonlinearities. A ferroelectric crystal may be composed of domains with different polar orientations.LiNbO3, for example, whose crystal symmetry is 3m, can show two different domain orientations. They differ in the signs of all non-vanishing components of the nonlinear optical tensor χ(2). Hence, for nonlinear optical experiments and applications, it is crucial to control the domain structure of the crystals. Devices for holographic data storage and optical parallel processing [1, 2] or for nonlinear frequency conversion with birefringent phase-matching usually require crystals with a single domain, whereas in recent years a lot of interest has been attracted by crystals fabricated with a periodic domain structure, mainly for quasi-phase-matched frequency conversion [3, 4]. The technique of quasi-phasematching (QPM) opens a number of very attractive possibilities in nonlinear optics: virtually any nonlinear interaction of waves within the transparency region of the crystal can be noncritically phase-matched at room temperature, and in addition QPM devices can be significantly less sensitive to the photorefractive effect [5], which is particularly important for the generation of visible light. The crucial prerequisite ∗ To whom correspondence should be addressed ∗∗ Present address:Institute of Quantum Electronics, Swiss Federal Institute of Technology, ETH-Hönggerberg, HPT E 17, CH-8093 Zürich, Switzerland, Phone: +41-1 /633-6825, Fax: +41-1 /633-1059, E-mail: paschott@iqe.phys.ethz.ch for using QPM is the ability to fabricate crystals with periodic domain structures of good quality, typically with periods in the range from3μm to 30μm. Various techniques have been used for this purpose, most successfully the technique of electric-field poling [6–8]. In any case, the further development of such methods requires techniques for the characterization of the obtained domain structures. First we briefly review the currently available techniques and later describe our new technique which is non-destructive, reproducible, quick and versatile, and provides images of domain structures with micron resolution in both lateral and axial directions. 1 Alternative techniques for the observation of ferroelectric domains Polarization microscopy is well suitable for the observation of the anisotropic grains in polycrystalline matter. The linear optical technique, however, does not allow us to distinguish between the antiparallel domains in ferroelectrics. Domain boundaries can cause visible structures due to stress birefringence or due to the electro-optic effect originating from charges on the boundaries. The unambiguous identification of domain structures, however, seems not to be possible with this method. Selective etching can be used to transform the domain structure on the surface of a crystal into a topographic structure [9, 10] that can be observed with a usual microscope. This method, however, is destructive, provides information about the domain structure on a polished surface but not from the interior of the specimen, and it works only on certain crystal faces (for example, on the Y and Z face, but not on the X face of aLiNbO3 crystal). Another method applied to crystal surfaces is the deposition of electrically charged powder particles [11]. This technique is non-destructive but suffers from a poor accuracy of the obtained images. Other nondestructive techniques such as atomic force microscopy [12– 14] and scanning secondary-electron microscopy [15, 16] have recently been applied to ferroelectric crystal surfaces. However, they also do not provide information on the interior domain structure. X-ray topography studies can reveal strain,

Modified phased translation functions and their application to molecular-fragment location.

Abstract: Direct methods at high resolution have depended on the resolution of atomic like features in the map. At data resolutions more typical for protein structures (2-3 A) individual atoms may not be resolved, so larger features must be identified. At one extreme the whole molecule may be located using the diffraction magnitudes alone by the molecular-replacement method. At the other extreme it is possible to locate individual residues in a well phased map. In this paper an intermediate problem is addressed: the location of multi-residue fragments on the basis of weak phase information. An agreement function based on the mean-squared difference between model and map over a masked region is shown to be more effective than a simple overlap integral, and may be efficiently calculated by Fourier methods. The techniques are compared using poorly phased electron-density maps at approximately 3 A for the proteins RNAse and O6-methylguanine-DNA-methyltransferase.

Resolution for temporal logics of knowledge

Abstract: A resolution based proof system for a temporal logic of knowledge is p re ented and shown to be correct. Such logics are useful for proving properties of distribut ed and multi-agent systems. Examples are given to illustrate the proof system. An extension of th e basic system to the multimodal case is given and illustrated using the ‘muddy children problem’.

Choosing Objective Lenses: The Importance of Numerical Aperture and Magnification in Digital Optical Microscopy

Abstract: Microscopic images are characterized by a number of microscope-specific parameters--numerical aperture (NA), magnification (M), and resolution (R)--and by parameters that also depend on the specimen--for example, contrast, signal-to-noise ratio, dynamic range, and integration time. In this article, issues associated with the microscope-specific parameters NA, M, and R are discussed with respect to both widefield and laser scanning confocal microscopies. Although most of the discussion points apply to optical microscopy in general, the main application considered is fluorescence microscopy.

Information depth and spatial resolution in BSE microtomography in SEM

Abstract: Various methods of visualizing subsurface structures using the scanning electron microscope (SEM) in the backscattered electrons (BSE) mode are analyzed. The problems of image contrast and “in-depth” resolution of layered microstructures are discussed. The potentials of BSE microtomography, i.e. layer-by-layer formation of images of undersurface inhomogeneities are demonstrated by experiment.

High resolution imaging microellipsometry of soft surfaces at 3 μm lateral and 5 Å normal resolution

Abstract: We report on the design of an imaging microellipsometer enabling the generation of maps of the two ellipsometric angles Δ and Ψ. Areas of 60×200 μm2 are imaged at a rate of 1–2 images per minute. By working at angles (45°) much smaller than the Brewster angle (≈73° for Si/SiO2/air) a lateral resolution of 3 μm and a height resolution of 5 A is achieved. The performance is demonstrated by thickness measurement of a laterally structured polymer film and a transient thickness measurement of dewetting fluid film of n-hexadecane on a Si/SiO2 wafer.

The Quantum World Unveiled by Electron Waves

Abstract: By electron microscopy, the intensity of an electron beam transmitted through an object is observed. While by electron holography, the whole information, i. e. both intensity and phase of the electron beam can be observed. Therefore, higher resolution images are obtained by compensating for the lens aberrations, and hitherto-unobservable objects can be also observed using the phase information of an electron beam.