Showing papers on "Contrast transfer function published in 2009"
TL;DR: A spherical aberration-corrected electron microscope has been developed recently, which is equipped with a 300-kV cold field emission gun and an objective lens of a small chromatic aberration coefficient, and the observed image was compared with a simulated image obtained by dynamical calculation.
Abstract: A spherical aberration-corrected electron microscope has been developed recently, which is equipped with a 300-kV cold field emission gun and an objective lens of a small chromatic aberration coefficient. A dumbbell image of 47 pm spacing, corresponding to a pair of atomic columns of germanium aligned along the [114] direction, is resolved in high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) with a 0.4-eV energy spread of the electron beam. The observed image was compared with a simulated image obtained by dynamical calculation.
149 citations
TL;DR: It is shown that a major reason for this well-known contrast discrepancy lies in the neglect of the detector modulation-transfer function in image simulations.
Abstract: A fully quantitative approach to high-resolution transmission electron microscopy requires a satisfactory match between image simulations and experiments. While almost perfect agreement between simulations and experiments is routinely achieved on a relative contrast level, a huge mutual discrepancy in the absolute image contrast by a factor of 3 has been frequently reported. It is shown that a major reason for this well-known contrast discrepancy, which is often called Stobbs-factor problem, lies in the neglect of the detector modulation-transfer function in image simulations.
117 citations
TL;DR: Aberration-corrected transmission electron microscopy allows us to image the structure of matter at genuine atomic resolution and a prominent role for the imaging of crystalline samples is played by the negative spherical aberration imaging technique.
Abstract: Aberration-corrected transmission electron microscopy allows us to image the structure of matter at genuine atomic resolution. A prominent role for the imaging of crystalline samples is played by the negative spherical aberration imaging (NCSI) technique. The physical background of this technique is reviewed. The especially high contrast observed under these conditions owes its origin to an enhancing combination of amplitude contrast due to electron diffraction channelling and phase contrast. A number of examples of the application of NCSI are reviewed in order to illustrate the applicability and the state-of-the-art of this technique.
100 citations
TL;DR: Depth-sectioning experiments using a 33-mrad 100 keV C(5)-corrected aberration-corrected STEM demonstrate the elongation effect and the missing-cone problem in real and reciprocal space and the performance limits of different S/TEM-based imaging modes are compared.
Abstract: The short depth of focus of aberration-corrected scanning transmission electron microscopes (STEMs) could potentially enable 3D reconstruction of nanomaterials through acquisition of a through-focal series. However, the contrast transfer function of annular dark-field (ADF)-STEM depth sectioning has a missing-cone problem similar to that of tilt-series tomography. The elongation as a function of the probe-forming angle is found to be (square root of 3/2) x 1/alphamax. For existing aberration-corrected STEMs operated at optimal imaging conditions, the elongation factor for depth sectioning is larger than 30. This large elongation factor results in highly distorted shapes of 3D objects and unexpected artifacts due to the loss of information. Depth-sectioning experiments using a 33-mrad 100 keV C(5)-corrected aberration-corrected STEM demonstrate the elongation effect and the missing-cone problem in real and reciprocal space. The performance limits of different S/TEM-based imaging modes are compared. There is a missing cone of information for bright-field S/TEM, ADF-STEM, hollow-cone ADF-STEM and coherent scanning confocal electron microscopy (SCEM). Only incoherent SCEM fills the missing cone.
87 citations
TL;DR: This approach gives an improvement in the structure obtained by sub-tomogram averaging from cryo-electron tomograms, and it is shown that errors in defocus determination are well tolerated when correcting a series of tomograms collected at a range of defocus values.
84 citations
TL;DR: In this article, the authors compare the experimental image features, such as contrast reversals, intensities, and image contrast, with image simulations that account for elastic scattering and the effect of phonon scattering.
Abstract: Quantitative, atomic resolution bright-field scanning transmission electron microscopy experiments are reported. The image intensities are placed on an absolute scale relative to the incident beam intensity. Features in the experimental images, such as contrast reversals, intensities, and the image contrast, are compared with image simulations that account for elastic scattering and the effect of phonon scattering. Simulations are carried out using both the multislice absorptive and frozen phonon simulation methods. For a ${\text{SrTiO}}_{3}$ sample with thicknesses between 4 and 25 nm, both models agree within the experimental uncertainty. We demonstrate excellent agreement between the simulated and the experimentally observed image contrast. The implications for the contrast mismatch commonly reported for high-resolution transmission electron microscopy using plane-wave illumination are discussed.
61 citations
TL;DR: The method is based on inverse ray tracing and can generate correction phase patterns whose peak-to-valley values are minimized and is useful in cases where a large phase modulation is needed, such as when employing a high-NA lens or focusing a beam deep inside a sample.
Abstract: We propose a simple method to correct a large amount of spherical aberration caused by a refractive index mismatch The method is based on inverse ray tracing and can generate correction phase patterns whose peak-to-valley values are minimized We also demonstrated spherical aberration correction in a transparent acrylic block using a liquid-crystal-on-silicon spatial light modulator (LCOS-SLM) A distorted focal volume without correction was substantially improved with correction This method is useful in cases where a large phase modulation is needed, such as when employing a high-NA lens or focusing a beam deep inside a sample
54 citations
TL;DR: An achromatic electron–optical aplanat, which is free of chromatic aberration, spherical aberration and total off-axial coma is proposed, to achieve atomic resolution at voltages below approximately 100 kV and a large number of equally resolved image points.
Abstract: The attainable specimen resolution is determined by the instrumental resolution limit d(i) and by radiation damage. Solid objects such as metals are primarily damaged by atom displacement resulting from knock-on collisions of the incident electrons with the atomic nuclei. The instrumental resolution improves appreciably by means of aberration correction. To achieve atomic resolution at voltages below approximately 100 kV and a large number of equally resolved image points, we propose an achromatic electron-optical aplanat, which is free of chromatic aberration, spherical aberration and total off-axial coma. Its anisotropic component is eliminated either by a dual objective lens consisting of two separate windings with opposite directions of their currents or by skew octopoles employed in the TEAM corrector. We obtain optimum imaging conditions by operating the aberration-corrected electron microscope at voltages below the knock-on threshold for atom displacement and by shifting the phase of the non-scattered wave by pi/2 or that of the scattered wave by -pi/2. In this negative contrast mode, the phase contrast and the scattering contrast add up with the same sign. The realization of a low-voltage aberration-corrected phase transmission electron microscope for the visualization of radiation-sensitive objects is the aim of the proposed SALVE (Sub-A Low-Voltage Electron microscope) project. This microscope will employ a coma-free objective lens, an obstruction-free phase plate and a novel corrector compensating for the spherical and chromatic aberrations.
53 citations
TL;DR: A method is demonstrated that extends the ultimate resolution of the latest generation of aberration corrected transmission electron microscopes by 41% relative to that achievable using conventional axial imaging.
Abstract: Transmission electron microscopy is an extremely powerful technique for direct characterization of local structure at the atomic scale. However, the resolution of this technique is fundamentally limited by the partial coherence of the electron beam. In this Letter we demonstrate a method that extends the ultimate resolution of the latest generation of aberration corrected transmission electron microscopes by 41% relative to that achievable using conventional axial imaging. Experimental results verify that a real space resolution of 78 pm has been achieved at 200 kV.
48 citations
Patent•
21 Jul 2009TL;DR: In this article, a full-field x-ray imaging system for quantitatively reconstructing the phase shift through a specimen comprises a source that generates X-ray radiation, a condenser xray lens for projecting the xray radiation onto the specimen, an objective x-Ray lens for imaging the x-rays transmitted through the specimen and a phaseshifting device to shift the phase of portions of the x ray radiation by a determined amount.
Abstract: The principle of reciprocity states that full-field and scanning microscopes can produce equivalent images by interchanging the roles of condenser and detector. Thus, the contrast transfer function inversion previously used for images from scanning systems can be applied to Zernike phase contrast images. In more detail, a full-field x-ray imaging system for quantitatively reconstructing the phase shift through a specimen comprises a source that generates x-ray radiation, a condenser x-ray lens for projecting the x-ray radiation onto the specimen, an objective x-ray lens for imaging the x-ray radiation transmitted through the specimen, a phase-shifting device to shift the phase of portions of x-ray radiation by a determined amount, and an x-ray detector that detects the x-ray radiation transmitted through the specimen to generate a detected image. An image processor then determines a Fourier filtering function and reconstructs the quantitative phase shift through the specimen by application of the Fourier filtering function to the detected image. As a result, artifacts due to absorption contrast can be removed from the detecting image. This corrected image can then be used in generating three dimensional (3D) images using computed tomography.
48 citations
TL;DR: A computational algorithm based on constrained non‐linear optimization is developed to estimate the essential parameters in the CTF and envelope function model simultaneously and automatically.
Abstract: The three-dimensional reconstruction of macromolecules from two-dimensional single-particle electron images requires determination and correction of the contrast transfer function (CTF) and envelope function. A computational algorithm based on constrained non-linear optimization is developed to estimate the essential parameters in the CTF and envelope function model simultaneously and automatically. The application of this estimation method is demonstrated with focal series images of amorphous carbon film as well as images of ice-embedded icosahedral virus particles suspended across holes.
TL;DR: In this paper, the formation and direct observation of self-interstitials in surface proximity of an elemental semiconductor by exploiting subthreshold effects in a new generation of aberration-corrected transmission electron microscopes was reported.
Abstract: We report the formation and direct observation of self-interstitials in surface proximity of an elemental semiconductor by exploiting subthreshold effects in a new generation of aberration-corrected transmission electron microscopes We find that the germanium interstitial atoms reside close to hexagonal, tetragonal, and $S$-interstitial sites Using phase-contrast microscopy, we demonstrate that the three-dimensional position of interstitial atoms can be determined from contrast analysis, with subnanometer precision along the electron-beam direction Comparison with a first-principles study suggests a strong influence of the surface proximity or a positively charged interstitial More generally, our investigation demonstrates that imaging of single atom can now be utilized to directly visualize single-defect formation and migration These high-resolution electron microscopy studies are applicable to a wide range of materials since the reported noise level of the images even allows the detection of single-light atoms
TL;DR: The state of the art for phase plates and what is innovated with them in biological electron microscopy are reviewed and the extension of thin-film phase plates to the material-less type using electrostatic field or magnetic field is addressed.
Abstract: After slow progress in the efforts to develop phase plates for electron microscopes, functional phase plate using thin carbon film has been reported recently. It permits collecting high-contrast images of close-to-life biological structures with cryo-fixation and without staining. This report reviews the state of the art for phase plates and what is innovated with them in biological electron microscopy. The extension of thin-film phase plates to the material-less type using electrostatic field or magnetic field is also addressed.
TL;DR: It is demonstrated that semi-quantitative structural analysis on the nanometer scale can be achieved by comparing experimental C(s)- corrected HAADF STEM images with their corresponding simulated images when the effects of the C(c) coefficient and spatial incoherence are included.
TL;DR: The signals of atomically fine structures are better defined for quantitative evaluation as, using the hardware corrector, the point-spread function shrinks considerably, the current density increases and the signal-to-noise ratio improves correspondingly.
Abstract: Electron holography allows the reconstruction of the complete electron wave, and hence offers the possibility of correcting aberrations. In fact, this was shown by means of the uncorrected CM30 Special Tubingen transmission electron microscope (TEM), revealing, after numerical aberration correction, a resolution of approximately 0.1 nm, both in amplitude and phase. However, it turned out that the results suffer from a comparably poor signal-to-noise ratio. The reason is that the limited coherent electron current, given by gun brightness, has to illuminate a width of at least four times the point-spread function given by the aberrations. As, using the hardware corrector, the point-spread function shrinks considerably, the current density increases and the signal-to-noise ratio improves correspondingly. Furthermore, the phase shift at the atomic dimensions found in the image plane also increases because the collection efficiency of the optics increases with resolution. In total, the signals of atomically fine structures are better defined for quantitative evaluation. In fact, the results achieved by electron holography in a Tecnai F20 Cs-corr TEM confirm this.
TL;DR: In this article, the atomic structure of FePd nanoparticles has been studied by spherical aberration (Cs) corrected high-resolution transmission electron microscopy, and the periodic arrangement of atoms arising from chemical order is clearly seen as bright contrast due to the small negative value of corrected Cs.
Abstract: The atomic structure of FePd nanoparticles has been studied by spherical aberration (Cs) corrected high-resolution transmission electron microscopy. The periodic arrangement of atoms arising from chemical order is clearly seen as bright contrast due to the small negative value of corrected Cs. The amount of optimal defocus (Scherzer defocus) is markedly reduced by the small Cs value. The interface between crystalline particles and the amorphous matrix can also be observed, free of imaging artifacts, at a small defocus value. The reconstructed phase image directly shows the projected potential distribution within the specimen and reveals the elemental differences due to chemical order. The clear-cut long-range order is lost when particle size is smaller than about 5 nm, at which locally ordered mixed-phase particles begin to dominate.
TL;DR: In this paper, a wave optical treatment of surface step contrast in a low-energy electron microscopy (LEEM) imaging system is presented, where the aberrations of an idealised LEEM imaging system are directly incorporated into a transfer function (TF) and image simulations of surface steps are evaluated in one and two dimensions.
Abstract: A wave optical treatment of surface step contrast in a low-energy electron microscopy (LEEM) is presented. The aberrations of an idealised LEEM imaging system are directly incorporated into a transfer function (TF) and image simulations of surface steps are evaluated in one and two dimensions. Under the special circumstances of a weak phase object, the simplified form of the contrast transfer function (CTF) is used to discuss LEEM image contrast and optimum defocus conditions.
TL;DR: In this paper, a projection phase contrast microscopy experiment using hard x-ray pink beam undulator radiation focused by an adaptive mirror system to 100-200 nm spot size is reported.
Abstract: We report a projection phase contrast microscopy experiment using hard x-ray pink beam undulator radiation focused by an adaptive mirror system to 100--200 nm spot size. This source is used to illuminate a lithographic test pattern with a well-controlled range of spatial frequencies. The oscillatory nature of the contrast transfer function with source-to-sample distance in this holographic imaging scheme is quantified and the validity of the weak phase object approximation is confirmed for the experimental conditions.
TL;DR: Electron holography has been shown to allow a posteriori aberration correction, and an aberration corrector in the transmission electron microscope does not seem to be needed to achieve atomic lateral resolution, which is necessary to reach a signal resolution sufficient for detecting single light atoms and very small interatomic fields.
Abstract: Electron holography has been shown to allow a posteriori aberration correction. Therefore, an aberration corrector in the transmission electron microscope does not seem to be needed with electron holography to achieve atomic lateral resolution. However, to reach a signal resolution sufficient for detecting single light atoms and very small interatomic fields, the aberration corrector has turned out to be very helpful. The basic reason is the optimized use of the limited number of “coherent” electrons that are provided by the electron source, as described by the brightness. Finally, quantitative interpretation of atomic structures benefits from the holographic facilities of fine-tuning of the aberration coefficients a posteriori and from evaluating both amplitude and phase.
TL;DR: Contrast details in beta-Si3N4 are clarified by means of extended image calculations and a simple procedure has been shown for pure phase imaging, which is restricted to linear imaging conditions.
TL;DR: Which are the most sensitive CTF parameters as well as the mostsensitive background parameters and a methodology to reveal the internal structure of the CTF model and to estimate the accuracy of each model parameter are provided.
Abstract: Background
The transmission electron microscope is used to acquire structural information of macromolecular complexes. However, as any other imaging device, it introduces optical aberrations that must be corrected if high-resolution structural information is to be obtained. The set of all aberrations are usually modeled in Fourier space by the so-called Contrast Transfer Function (CTF). Before correcting for the CTF, we must first estimate it from the electron micrographs. This is usually done by estimating a number of parameters specifying a theoretical model of the CTF. This estimation is performed by minimizing some error measure between the theoretical Power Spectrum Density (PSD) and the experimentally observed PSD. The high noise present in the micrographs, the possible local minima of the error function for estimating the CTF parameters, and the cross-talking between CTF parameters may cause errors in the estimated CTF parameters.
TL;DR: In this paper, the state of the chemical ordering in a decahedral FePt nanoparticle was studied using aberration corrected high resolution transmission electron microscopy, and it was shown that the image intensity oscillation from one atomic layer to another can be interpreted as L10-like chemical ordering.
Abstract: The state of the chemical ordering in a decahedral FePt nanoparticle was studied using aberration corrected high resolution transmission electron microscopy. With the reduced image delocalization effect as a result of spherical aberration correction, it is possible to directly correlate the image intensity with the local state of chemical ordering through the help of a multislice image simulation. We have found direct evidence for the image intensity oscillation from one atomic layer to another. It is interpreted as L10-like chemical ordering, i.e., the alternate occupation of Fe and Pt atoms in the (002) planes. The result suggests that chemical ordering survives even in decahedral nanoparticles down to 3 nm size despite the possible surface effects.
TL;DR: An experimental investigation of the optical transfer functions for an X-ray microscope operated in defocus phase-contrast mode and a theoretical model of partially coherent image formation are compared and are found to be in excellent agreement.
Patent•
02 Mar 2009TL;DR: In this article, an aberration corrector that guarantees freedom in designing a coma-free plane transfer portion even when the mechanical configuration is already decided, and has a flexible adjustment margin regarding the corrector exterior is proposed.
Abstract: To provide an aberration corrector (2) that guarantees freedom in designing a coma-free plane transfer portion even when the mechanical configuration of the aberration corrector is already decided, and has a flexible adjustment margin regarding the corrector exterior. The aberration corrector causes an electron beam trajectory emanating from a specimen plane (3,15) (physical surface of objective lens) to be incident in parallel with a multipole lens (HEX1 18), and causes an electron beam trajectory emanating from an objective-lens coma-free plane (16) or a minimum plane of a fifth-order aberration (objective lens center) to form an image on a center plane of a multipole lens of the 4f system. Thus, antisymmetric transfer is performed between two multipole lenses (HEX1 18, HEX2 19) to correct a spherical aberration in the 4f system, and a coma-free plane or a minimum plane of a fifth-order aberration is transferred to suppress occurrence of coma aberrations or fifth-order aberrations.
01 Jan 2009
TL;DR: In this article, the transmission electron microscope (TEM) and related techniques, such as HRTEM, scanning transmission electron microscopy (STEM), and principles and applications in the field of electrochemical power sources are discussed.
Abstract: This article briefly describes transmission electron microscope (TEM) and related techniques, such as high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), and principles and applications in the field of electrochemical power sources. Instrumentation and operation are discussed together on the basis of the electron scattering phenomena that generate contrast both in the real (imaging mode) and reciprocal space (diffraction mode). Applications of TEM on the study of materials in electrochemical devices are discussed, demonstrating the capability of this technique in the characterization of electrochemically active materials down to the atomic scale.
TL;DR: In this paper, a new approach to reduce spherical and chromatic aberration in electron microscopy allows for low-energy imaging of single-layer boron nitride, a novel 2D nanostructure that is analogous to graphene.
Abstract: A new approach to reduce spherical and chromatic aberration in electron microscopy allows for low-energy imaging of single-layer boron nitride, a novel 2D nanostructure that is analogous to graphene.
01 Dec 2009
TL;DR: In this article, the structural information that can be extracted from a HRTEM image of an approximately 5 to 10 nm thick nanocrystal is the projected reciprocal lattice geometry, the plane symmetry group, and a few structure factor amplitudes and phase angles.
Abstract: It is well known that nanocrystals cannot be fingerprinted structurally from laboratory based powder X-ray diffraction patterns [1]. Three novel strategies for the structurally identification of nanocrystals in a Transmission Electron Microscope (TEM) are, therefore, presented. Either a single High-Resolution Transmission Electron Microscopy (HRTEM) image or a single Precession Electron Diffractogram (PED) can be employed. The structural identification information is in both cases collected from an individual nanocrystal. PED from fine-grained crystal powders may also be utilized. Automation of the former two strategies shall lead to statistically significant results on ensembles of nanocrystals and is currently in progress. The structural information that can be extracted from a HRTEM image of an approximately 5 to 10 nm thick nanocrystal is the projected reciprocal lattice geometry, the plane symmetry group, and a few structure factor amplitudes and phase angles. While the structure factor amplitudes suffer from dynamical diffraction effects and are in addition modified by the (not precisely known) contrast-transfer function of the objective lens, the structure factor phase angles are remarkably stable against dynamical diffraction effects and slight crystal misorientations [1,2].
Journal Article•
TL;DR: In this article, a new approach to reduce spherical and chromatic aberration in electron microscopy allows for low-energy imaging of single-layer boron nitride, a novel 2D nanostructure that is analogous to graphene.
Abstract: A new approach to reduce spherical and chromatic aberration in electron microscopy allows for low-energy imaging of single-layer boron nitride, a novel 2D nanostructure that is analogous to graphene.
Patent•
04 Mar 2009TL;DR: In this paper, a scanning transmission electron microscope (STEM) and method of aberration correction are offered which can correct defocus and astigmatism during imaging and which can provide atomic resolution.
Abstract: Scanning transmission electron microscope (STEM) and method of aberration correction are offered which can correct defocus and astigmatism during imaging and which can provide atomic resolution. The STEM (2) has plural electron optical means. Furthermore, the STEM has autocorrelation function calculation means, aberration coefficient calculation means, and feedback control means. At least two images are obtained by varying a value at which one of the electron optical means is set. The autocorrelation function calculation means calculates autocorrelation functions of the at least two images. The aberration coefficientcalculation meansfitsaberrationfunctions to iso-intensity lines of the autocorrelation functions and calculates aberration coefficients based on the obtained aberration functions. The feedback control means provides feedback control of the electron optical means (23,25) based on the aberration coefficients.
TL;DR: A lens system is proposed that not only provides spherical correction of the objective lens by charges that are induced on a thin foil, but also provides Zernike phase contrast by means of an appropriate phase shift of the scattered electrons within the foil.
Abstract: A lens system is proposed that not only provides spherical correction of the objective lens by charges that are induced on a thin foil, in the way proposed in a paper by Otto Scherzer [Optik56(2), 133–147, 1980], but also provides Zernike phase contrast by means of an appropriate phase shift of the scattered electrons within the foil. This system has the potential to provide strong phase contrast from very low spatial frequencies to frequencies above 1/(100 pm).