bluuebunny/crossref_metadata_2025_split huggingface dataset:
Towards Composition Determination in Compound Semiconductors by High-Angle Annular Dark-Field Imaging
Authors: Martin J L , Chapman J N , Nicholson W A P , Stanley C R , Kean A H 
Published: June 2020

An analytical technique with high spatial resolution is required to investigate the composition of thin layers in compound semiconductor multilayer specimens. The techniques currently in use in TEM can broadly be divided into those in which element-specific signals are collected (e.g. EDX) and those in which compositional information is inferred from the variation in the position or magnitude of some contrast feature in the image (e.g. the displacement of thickness fringes in a wedge-shaped sample ). In this paper we report on the determination of the atomic fraction of Al, denoted x, in the Alx Ga1-x As/GaAssystem by analysis of high-angle annular dark field (HADF) images. The principal advantage of using an imaging technique is that the required information is contained in the variation of a signal proportional to an elastic scattering or a thermal-diffuse scattering cross-section rather than one related to an inner shell excitation. As the former are generally a few orders of magnitude larger than the latter it is possible to obtain an image from a wide area of specimen in the time that is required to collect an adequate EDX spectrum from a single position. Thus accumulation of the required information is much more rapid. Instances where it is desirable to perform a quantitative assay over a wide area include specimens in which there are graded interfaces where the atomic fraction varies continuously (e.g. to obtain specific light-guiding properties) or specimens which are deposited at elevated temperatures where Ga desorption can lead to significant variations from the expected composition. In both circumstances it is useful to make measurements at several locations in planes perpendicular to the growth direction. This is possible using techniques where cross-sectional specimens are employed but not where the presence of a wedge is essential.
Nanoseconds Double-Frame and Streak Transmission Electron Microscopy
Authors: Bostanjoglo Oleg , Kornitzky Jochen 
Published: June 2020

Material processing and synthesis is increasingly done by lasers. In order to apply this modern tool effectively, the laser-induced physical processes must be well known. As transmission electron microscopy is a powerful method to study the structure of the treated material, it seemed worthwhile to extend this technique for fast phase transitions, as are triggered by laser radiation. High-speed TEM may be realized either by pulsing the detector /l/ or the illuminating electron beam. The latter technique is more convenient and is described here.Fig. 1 shows a high-speed TEM designed for taking either double frame images (exposure/ repetition times ≿ 10 ns/≿ 50 ns) or streak images of transitions induced by a laser in the thin film specimen. It consists of a modified commercial TEM, an attached Q-switched (FWHM 50 ns), frequency-doubled (532 nm) Nd:YAG laser for treating the specimen, and electronics for electron beam pulsing and image storage. The TEM is equipped with focusing/deflecting optics for the laser radiation, an electron beam pulser generating either the exposure times for double frame pictures or the streak, and an image shifter. The image detector is a proximity focusing double stage MicroChannel Plate (MCP)/scintillator assembly. A CCD camera transfers the image to a PC-backed digitizing and frame grabbing card. The components are synchronized by a specially designed logic unit /2/.
New Feasible Concepts of Aberration Correction for Realizing High-Resolution Electron Microscopes
Authors: Rose H. 
Published: June 2020

The imaging performance of the light optical lens systems has reached such a degree of perfection that nowadays numerical apertures of about 1 can be utilized. Compared to this state of development the objective lenses of electron microscopes are rather poor allowing at most usable apertures somewhat smaller than 10-2 . This severe shortcoming is due to the unavoidable axial chromatic and spherical aberration of rotationally symmetric electron lenses employed so far in all electron microscopes.The resolution of such electron microscopes can only be improved by increasing the accelerating voltage which shortens the electron wave length. Unfortunately, this procedure is rather ineffective because the achievable gain in resolution is only proportional to λ1/4 for a fixed magnetic field strength determined by the magnetic saturation of the pole pieces. Moreover, increasing the acceleration voltage results in deleterious knock-on processes and in extreme difficulties to stabilize the high voltage. Last not least the cost increase exponentially with voltage.
Electron Holography Improving Transmission Electron Microscopy
Authors: Lichte Hannes 
Published: June 2020

Generally, the electron object wave o(r) is modulated both in amplitude and phase. In the image plane of an ideal imaging system we would expect to find an image wave b(r) that is modulated in exactly the same way, i. e. b(r) =o(r). If, however, there are aberrations, the image wave instead reads as b(r) =o(r) * FT(WTF) i. e. the convolution of the object wave with the Fourier transform of the wave transfer function WTF . Taking into account chromatic aberration, illumination divergence and the wave aberration of the objective lens, one finds WTF(R) = Echrom(R)Ediv(R).exp(iX(R)) . The envelope functions Echrom(R) and Ediv(R) damp the image wave, whereas the effect of the wave aberration X(R) is to disorder amplitude and phase according to real and imaginary part of exp(iX(R)) , as is schematically sketched in fig. 1.Since in ordinary electron microscopy only the amplitude of the image wave can be recorded by the intensity of the image, the wave aberration has to be chosen such that the object component of interest (phase or amplitude) is directed into the image amplitude. Using an aberration free objective lens, for X=0 one sees the object amplitude, for X= π/2 (“Zernike phase contrast”) the object phase. For a real objective lens, however, the wave aberration is given by X(R) = 2π (.25 Csλ3R4 + 0.5ΔzλR2), Cs meaning the coefficient of spherical aberration and Δz defocusing. Consequently, the transfer functions sin X(R) and cos(X(R)) strongly depend on R such that amplitude and phase of the image wave represent only fragments of the object which, fortunately, supplement each other. However, recording only the amplitude gives rise to the fundamental problems, restricting resolution and interpretability of ordinary electron images:
Holographic Interference Electron Microscopy by Numerical Reconstruction Method
Authors: Ohshita Akinori , Minamide Hiroshi , Saito Yahachi , Tomita Hiroshi 
Published: June 2020

Holographic interference electron microscopy has recently been put to practical use with a field-emission electron microscope. In this microscopy, the optical image reconstruction method are usually used, but many practical difficulties such as optical adjustment are always accompanied. On the other hand, the numerical image reconstruction method has many advantages. In this paper, therefore, holographic interference electron microscopy are tried by using a numerically reconstructed image.In the first step, an off-axis Fresnel electron hologram (Fig.1) was formed in a 100 kV thermionic-emission electron microscope equipped with an electron biprism. MgO smoke particles were used as a specimen. A reconstructed image is given by the convolution of amplitude transmittance of an electron hologram with propagation function from a hologram plane to an image reconstruction plane. In the second step for the numerical reconstruction, the hologram was enlarged and printed on a photographic paper.
CRYO-Electron Microscopy of Reovirus Intermediate Subviral Particles
Authors: Wang Guoji , Dryden Kelly A. , Coombsf Kevin M. , Furlongt Deirdre B. , Timothy Baker S. 
Published: June 2020

Reovirus, the type member of the Reoviridae family, contains a segmented dsRNA genome, a multi-shell protein capsid structure, and transcriptase activity. The virus has been extensively characterized biochemically and genetically, although details of the structural organization of the RNA and protein components are not completely established. Reovirus contains eight structural proteins, three in the outer capsid (σ1, σ3, μ1C) and five associated with the core capsid (σ2, λl, λ2, λ3, μ2) . A number of morphogenetically related viral products including mature virions, cores and intermediate subviral particles (ISVP) exist naturally in the host cell. ISVPs are similar to virions, but lack σ3, and the cell recognition protein, σl extends radially outward from the viral surface by ∽40 nm. We have initialized a study of the structure of unstained, frozen-hydrated ISVPs using cryo-electron microscopy and three-dimensional image reconstruction techniques.ISVP samples were generated by digesting purified reovirus with α-chymotrypsin for 20 minutes.Aliquots of 4μl were applied to freshly-prepared, perforated carbon films, blotted with filter paper and rapidly plunged into liquid ethane. The sample was maintained at∽-170°C and images were recorded in a Philips EM420 microscope at ∽1 μm underfocus and at a nominal magnification of 30,000 × with an electron dose <1000e-/nm2.
Three-Dimensional Structure of Cloned T3 Connector Protein at 1.6nm Resolution
Authors: Carrascosa José L. , Valpuesta José M. , Fujisawa Hisao 
Published: June 2020

The head to tail connector of bacteriophages plays a fundamental role in the assembly of viral heads and DNA packaging. In spite of the absence of sequence homology, the structure of connectors from different viruses (T4, Ø29, T3, P22, etc) share common morphological features, that are most clearly revealed in their three-dimensional structure. We have studied the three-dimensional reconstruction of the connector protein from phage T3 (gp 8) from tilted view of two dimensional crystals obtained from this protein after cloning and purification.DNA sequences including gene 8 from phage T3 were cloned, into Bam Hl-Eco Rl sites down stream of lambda promotor PL, in the expression vector pNT45 under the control of cI857. E R204 (pNT89) cells were incubated at 42°C for 2h, harvested and resuspended in 20 mM Tris HC1 (pH 7.4), 7mM 2 mercaptoethanol, ImM EDTA. The cells were lysed by freezing and thawing in the presence of lysozyme (lmg/ml) and ligthly sonicated. The low speed supernatant was precipitated by ammonium sulfate (60% saturated) and dissolved in the original buffer to be subjected to gel nitration through Sepharose 6B, followed by phosphocellulose colum (Pll) and DEAE cellulose colum (DE52). Purified gp8 appeared at 0.3M NaCl and formed crystals when its concentration increased above 1.5 mg/ml.
Study of Tip-Surface Interactions in Scanning Tunneling Microscopy (STM) by Reflection Electron Microscopy (REM)
Authors: Lo W. , Spence J.C.H. , Kuwabara M. 
Published: June 2020

Work on the integration of STM with REM has demonstrated the usefulness of this combination. The STM has been designed to replace the side entry holder of a commercial Philips 400T TEM. It allows simultaneous REM imaging of the tip/sample region of the STM (see fig. 1). The REM technique offers nigh sensitivity to strain (<10−4) through diffraction contrast and high resolution (<lnm) along the unforeshortened direction. It is an ideal technique to use for studying tip/surface interactions in STM.The elastic strain associated with tunnelling was first imaged on cleaved, highly doped (S doped, 5 × 1018cm-3) InP(110). The tip and surface damage observed provided strong evidence that the strain was caused by tip/surface contact, most likely through an insulating adsorbate layer. This is consistent with the picture that tunnelling in air, liquid or ordinary vacuum (such as in a TEM) occurs through a layer of contamination. The tip, under servo control, must compress the insulating contamination layer in order to get close enough to the sample to tunnel. The contaminant thereby transmits the stress to the sample. Elastic strain while tunnelling from graphite has been detected by others, but never directly imaged before. Recent results using the STM/REM combination has yielded the first direct evidence of strain while tunnelling from graphite. Figure 2 shows a graphite surface elastically strained by the STM tip while tunnelling (It=3nA, Vtip=−20mV). Video images of other graphite surfaces show a reversible strain feature following the tip as it is scanned. The elastic strain field is sometimes seen to extend hundreds of nanometers from the tip. Also commonly observed while tunnelling from graphite is an increase in the RHEED intensity of the scanned region (see fig.3). Debris is seen on the tip and along the left edges of the brightened scan region of figure 4, suggesting that tip abrasion of the surface has occurred. High resolution TEM images of other tips show what appear to be attached graphite flakes. The removal of contamination, possibly along with the top few layers of graphite, seems a likely explanation for the observed increase in RHEED reflectivity. These results are not inconsistent with the “sliding planes” model of tunnelling on graphite“. Here, it was proposed that the force due to the tunnelling probe acts over a large area, causing shear of the graphite planes when the tip is scanned. The tunneling current is then modulated as the planes of graphite slide in and out of registry. The possiblity of true vacuum tunnelling from the cleaned graphite surface has not been ruled out. STM work function measurements are needed to test this.
Characterization of Surface Resonance Conditions for Surface Imaging
Authors: Yao Nan , Cowley John M. 
Published: June 2020

In order to increase intensity and contrast in the image of a surface, the surface resonance conditions have been widely used to enhance the Bragg reflection for image formation in REM (Reflection Electron Microscopy). However, detailed studies of how the resonance conditions relate to the imaging contrast have not been reported. This paper will concentrate on the general properties of the different resonance conditions, as well as the resulting image contrast.Figure 1 shows a series of RHEED (Reflection High Energy Electron Diffraction) patterns and REM images from the same region of a Pt(l11) surface with the incident electron beam in a direction close to the [112] zone axis at 200 KeV, with a glancing incident angle of about 24 mrad which corresponds to the (555) Bragg reflection condition inside the crystal. For the purpose of convenience in discussion, the four different diffraction conditions shown in figures l(al)-(dl) have been named as D1-D4. With Dl, the specular reflected spot falls in an intersection of a parallel Kikuchi line with a parabola; with D2, the specular reflected spot coincides with an intersection of the Kikuchi lines running parallel to and inclined to the crystal surface; with D3, the specular reflected spot crosses only the parallel Kikuchi line; and with D4, the specular reflected spot intersects only with a parabola. It was found that the diffraction conditions Dl and D2 can not be considered as identical, although the specular reflected spots for both cases are commonly regarded as (555) Bragg reflection in the RHEED pattern. Detailed inspection indicates that for Dl, both the Bragg reflection and the electron surface channelling wave are excited, and for D2, the excitement of simultaneous Bragg reflection occurs closely associated with the properties of three-dimensional dynamical diffraction for a bulk crystal.
Surface Structure Study by High Resolution Electron Holography: Contrast Simulation
Authors: Tanji Takayoshi , Yada Keiji 
Published: June 2020

The potential advantage of an electron holography in researching the atomic structure at crystal surfaces with high spatial resolution has been reveald by using the computer simulation for the 7×7 super structure rearranged on a Si(111) clean surface. In order to realize the holography of the surface, problems still remain unsolved in addition to instrumentation of a high resolutional TEM equipped with a highly coherent electron source and a high vacuum specimen chamber. The most essential one among such problems is the determination of imaging conditions necessary for restoring the image blured by the aberration of an objective lens. Especially, the amount of out-of-focus is difficult to know directly from the hologram recorded as the interference pattern of the specimen transmitted wave with the reference wave passing in a vacuum area.Restricting to the case of a weak phase object, Franke et al. suggested the possibility of restoring the image without the knowledge of focussing condition. Electron scattering at a surface is very weak and may be considered approximately kinematical. Therefore the applicability of the Franke's method to surface structures of Si(lll) thin films was examined. As the atomic positions of the 7×7 structure, DAS model was adopted, and the electron diffraction was treated dynamically with the multi-slice method. Reconstructing a wave front from a hologram, we obtain the wave function in the reciprocal space as,
The Possibilities for Analytical Methods in Photoemission and Low-Energy Electron Microscopy
Authors: Bauer Ernst 
Published: June 2020

One of the major shortcomings of conventional PEEM and of LEEM is the lack of chemical information about the surface. Although the imaging of the LEED pattern in the back focal plane of the objective lens of a LEEM instrument allows chemical characterization via the crystalline structure derived from the LEED pattern, this method fails in the absence of a characteristic LEED pattern. Direct information about the atomic composition of the surface is then needed which can be best obtained from inner shell electrons either directly by x-ray-induced photoemission (XPEEM) or by x-ray- or electron-induced Auger electron emission (AEEM). These modes of excitation and imaging can be combined with conventional PEEM and LEEM in one instrument which is presently being developed. Thus a complete structural and chemical characterization becomes possible in one instrument, with parallel detection and high resolution.In contrast to LEEM, in which up to more than 50% of the incident intensity is available for image formation, the intensity of the emitted electrons is much lower in XPEEM and AEEM and the signal is much lower than the background in AEEM. Therefore, intensity I and resolution d have to be optimized simultaneously which is best done by maximizing Q = I/d2 with respect to maximum emission angle α and relative energy distribution ε = ΔVo/V accepted by the instrument. For a well-designed magnetic lens section of the cathode lens its aberrations are determined by the accelerating field F in front of the specimen. For a homogeneous accelerating field F and a cosine emission distribution one obtains for the optimum α and ε values αo,εo a radius of the minimum disc of confusion of
Low-Energy Electron Microscope of Novel Design
Authors: Liebl Helmut , Senftinger Berthold 
Published: June 2020

The pioneering work of Bauer and Telieps stimulated us to construct a low-energy electron microscope intended mainly for imaging solid surfaces with reflected and diffracted electrons in the 10 to 100 eV range, as well as with photoelectrons. Several novel design features were incorporated.Figure 1 shows the layout of the instrument. The primary-beam column consists essentially of the electron gun, the primary-beam lens, the energy-focusing deflector and the magnet. al lenses are electrostatic. The electron gun comprises the cathode and two acceleration electrodes, the first of which is shaped semi-spherical about the cathode tip and is connected to a variable voltage supply, and the second is the tubular vacuum housing. This constitutes an immersion lens with variable focal length. The cathode is operated in the diode mode. Its virtual diameter is assumed to be dv = dtip √2.7 kT/eVa because in thermionic emission the energy distribution drops to half the peak value at 2.7 kT. The final acceleration voltage is Va = 15 KV. The variable immersion lens together with the primary-beam lens provides a magnification range of about 1 to about 70 without a crossover from the cathode to the backfocal plane of the emission lens. For example, a pointed W cathode with a tip diameter of 1μm imaged with 15× magnification yields a lμm diameter image.
Scanning vs Direct Imaging Emission Microscopy
Authors: Veneklasen Lee H. 
Published: June 2020

With the addition of an imaging energy analyzer to a low energy emission microscope (LEKM) (1), it becomes possible to use inelastic scattering (secondaries, Auger and x-ray emissions) to form high resolution images. One can then obtain complimentary diffraction, structural, topological (2) and chemical information in the same instrumental environment. The favorable noise/bandwidth associated with the parallel imaging of many pixels may allow dynamic surface processes to be observed. The goal of this paper is to compare hypothetical scanning (SEM/SAM) with direct imaging (SPEC-LEEM) instruments in terms of resolution and signal/noise ratio vs integration time (visibility). By comparison, one may extrapolate experience with the more common scanning instrument (3), in order to predict the advantages and disadvantages of the newer analytic imaging technology.The complete analysis will be published in “Ultramicroscopy”.Briefly, the signal to noise ratio within an image element is expressed in terms of incident flux density, net collection efficiency, integration time and quantum yields for spectral lines upon a diffuse background. While ultimate resolution is determined by lens aberrations, practical resolution is defined by a minimum tolerable signal/noise criteria. Aperture angles, energy window, lens field strength, and electron gun configuration are considered (within technical limits) to be free variables that can be optimized for a given resolution requirement, emission spectrum, and surface standoff field strength. The optimization techniques are developed in a form applicable to both instruments, and comparative predictions are made for a specific numerical example.
Comparison of Beam Damage of Hydrated Biological Specimens in High-Pressure Scanning Electron Microscopy and Low-Temperature Scanning Electron Microscopy
Authors: Farley A.N. , Beckett A. , Shah J.S. 
Published: June 2020

High Pressure Scanning Electron Microscopy (HPSEM) is a technique in which biological materials are viewed close to their natural state in a saturated water vapour environment and at ambient temperatures. The absence of chemical or cryogenic fixation in this technique can offer considerable advantages in the reduction of preparative artifacts in labile specimens but it is also necessary to consider the effects of the electron beam interaction with the specimen. A comparative study has been undertaken with low temperature scanning electron microscopy (LTSEM) to evaluate the relative damage rates in the labile specimen Narcissus sp. . The results are compared with those expected from theoretical models of the beam-specimen interaction developed by other workers. Full details of the theory and practice of HPSEM have been presented elsewhere.Fresh, untreated perianth segments of Narcissus sp. were excised and examined in HPSEM at a saturated water vapour pressure of 1716 Pa, with a beam voltage of 10 kV and beam current of approximately 100 pA. The beam dose rate was therefore approximately 15 MJ.Kg−1s−1 over the scanned area. Fully frozen hydrated specimens were prepared for LTSEM and examined in an uncoated condition at a beam voltage of 3kV to reduce charging artifacts. No metal films were applied to the LTSEM specimens since this would artificially increase the thermal conductivity of the surface.