Through-focus Image Series Research Articles

An improved iterative wave function reconstruction algorithm in high-resolution transmission electron microscopy

Exit wavefunction reconstruction is a powerful image processing technique to enhance the resolution and the signal-to-noise ratio for atomic-resolution imaging in both aberration uncorrected and corrected transmission electron microscopes. The present study aims to improve the performance of the iterative wavefunction reconstruction algorithm in comparison not only with its conventional form but also with the popular commercial Trueimage software for exit wavefunction reconstruction. It is shown that by implementing a wave propagation procedure for refining its image alignment, the iterative wavefunction reconstruction algorithm can be greatly improved in accurately retrieving the wavefunctions while keeping its original advantages, which allow the reconstruction be performed with less images and a larger defocus step in the data set of through-focus image series. In addition, calculations of this algorithm can be accelerated drastically by the graphic processing unit (GPU) hardware programming using the popular computer unified device architecture language, whose computing speed can be 25–38 times as fast as a central processing unit (CPU) program.

Digital differential confocal microscopy based on spatial shift transformation.

Differential confocal microscopy is a particularly powerful surface profilometry technique in industrial metrology due to its high axial sensitivity and insensitivity to noise. However, the practical implementation of the technique requires the accurate positioning of point detectors in three-dimensions. We describe a simple alternative based on spatial transformation of a through-focus series of images obtained from a homemade beam scanning confocal microscope. This digital differential confocal microscopy approach is described and compared with the traditional Differential confocal microscopy approach. The ease of use of the digital differential confocal microscopy system is illustrated by performing measurements on a 3D standard specimen.

Study of titanate nanotubes by X-ray and electron diffraction and electron microscopy

The structure of titanate nanotubes (Ti-NTs) was studied by a combination of powder X-ray diffraction (PXRD), electron diffraction and high resolution transmission electron microscopy (HRTEM). Ti-NTs are prepared by hydrothermal treatment of TiO2 powder. The structure is identified by powder X-ray diffraction as the one based on the structure of H2Ti2O5·H2O phase. The same structure is obtained by projected potential from HRTEM through-focus image series. The structure is verified by simulated PXRD pattern with the aid of the Debye formula. The validity of the model is tested by computing Fourier transformation of a single nanotube which is proportional to measured electron diffraction intensities. A good agreement of this calculation with measured precession electron diffraction data is achieved.

Contrast mechanisms in scanning transmission x-ray microscopy

We present a derivation of the main contrast mechanisms accessible in scanning transmission x-ray microscopy with a pixel-array detector. We consider the effect of the probe defocus and show that it can produce strong differential absorption contrast. The effect of noise is derived and used to compare the relative merits of absorption and differential contrast imaging in various experimental conditions. We illustrate the main results with an experiment that combines a through-focus series with the near-edge signal of a cerium oxide sample. The measurements are seen to follow closely the derived contrast expressions, including the defocus-dependent differential absorption contrast. The analysis includes additional considerations about the application of principal component analysis on a through-focus image series.

High-resolution wave field reconstruction using a through-focus series of images taken under limited electron dose

The three-dimensional Fourier filtering method and Schiske's Wiener filtering method are compared with the aim of high-resolution wave field reconstruction of an unstained deoxyribonucleic acid (DNA) molecular fiber using a through-focus series of images taken under a limited electron dose. There were some definite differences between the two reconstructed images, although the two kinds of processing are essentially equivalent except for the dimension and the filter used for processing. Through theoretical analyses together with computer simulations, the differences were proved to be primarily due to specimen drift during the experiment. Although the observed structure of the DNA molecular fiber was heavily damaged by electron beam irradiation, reconstructed images by the three-dimensional Fourier filtering method provided higher resolution information on the molecular structure even when relatively large specimen drift was included in the through-focus series. In contrast, in Schiske's Wiener filtering method, the detailed information of the structure was lost because of the drift, although the reconstructed image showed a higher signal-to-noise ratio. The three dimensional Fourier filtering method seems to be more applicable for observing radiation-sensitive materials under an extremely low electron dose, because specimen drift cannot be completely avoided.

Astigmatic intensity equation for electron microscopy based phase retrieval

Phase retrieval, in principle, can be performed in a transmission electron microscope (TEM) using arbitrary aberrations of electron waves; provided that the aberrations are well-characterised and known. For example, the transport of intensity equation (TIE) can be used to infer the phase from a through-focus series of images. In this work an “astigmatic intensity equation” (AIE) is considered, which relates phase gradients to intensity variations caused by TEM objective lens focus and astigmatism variations. Within the paraxial approximation, it is shown that an exact solution of the AIE for the phase can be obtained using efficient Fourier transform methods. Experimental requirements for using the AIE are the measurement of a through-focus derivative and another intensity derivative, which is taken with respect to objective lens astigmatism variation. Two quasi-experimental investigations are conducted to test the validity of the solution.

HRTEM and EELS Studies of Py/Al and Co/Al Multilayers

ABSTRACTFerromagnetic/non-magnetic (F/N) metallic multilayers in the Current-Perpendicular-to-Plane (CPP) geometry show giant Magnetoresistance (MR) and are promising candidates for potential use in high density storage devices. F/Al interfaces were recently shown to have large interface specific resistances that enhance the CPP-resistance. However, the CPP resistances showed instability over time at room temperature and also upon annealing to 453K. To help understand both the large interface specific resistances and their instabilities, we have undertaken cross-sectional High Resolution Transmission Electron Microscopy (HRTEM) and Electron Energy Loss Spectroscopy (EELS) studies of both as-sputtered and annealed Py/Al and Co/Al multilayers. We find well-layered, but rough structures with local F/Al interfaces being tilted up to ˜15°from the plane perpendicular to the growth direction. HRTEM images appear to show diffuse interfaces, but a through-focus series of images suggests considerable grain overlap in the electron beam direction, thereby complicating interpretation. This combination of HRTEM imaging and EELS analysis suggests that any interfacial mixing is limited in scale, and shows no evidence of intermetallic compound formation. No obvious differences are seen between assputtered and annealed samples.

Development of computer-assisted minimal-dose system with beam blanker for TEM

A computer-assisted minimal-dose system has been developed for the high-resolution observation of radiation-sensitive samples using a transmission electron microscope (TEM). This system consists of a CCD camera, a beam blanker and a control computer (PC) that also controls the TEM. A sample is illuminated by an electron beam only when the CCD camera takes images; otherwise, the beam blanker cuts off the electron beam. Emulated images, which are calculated from the images taken and the variable parameters of the TEM, such as magnification and sample stage position, are displayed on the control PC display. After a few times of repetition of exposures and emulations, the sample is positioned to final observation area. Subsequently to select the observation area, the system automatically adjusts the focus position from two different illuminating angle images and the TEM conditions are appropriate for taking a final image. The total electron dose before the final image is taken can be markedly decreased because the sample is irradiated by an electron beam only when images are taken. For the final image, a three-dimensional Fourier filtering method (3DFFM), which corrects the spherical aberration for the through-focus image series and enables the selection of the optimum focus condition later, is also included in our system.

Separation of linear and non-linear imaging components in high-resolution transmission electron microscope images

Linear and non-linear image components in high-resolution transmission electron microscope images were successfully separated by applying a bandpass filter to the three-dimensional Fourier spectrum of its through-focus series of images. In the observed lattice image of a wedgeshaped Si [110] crystal, we determined the magnitude of the contribution of the non-linear imaging components to the total image intensity distribution. The contribution was proved to become sometimes larger than that of the linear imaging component, even at a thickness of 13 nm.

A Direct Phasing Electron Image of Oxygen Atoms in a Ga-In-Sn-O Ceramic

In solving complex inorganic structures, often the greatest challenge is to obtain good initial atom positions for Rietveld refinement using x-ray diffraction intensities. In this work, a technique is presented which combines direct phasing with HRTEM and exploits dynamical diffraction effects to obtain an image of oxygen atoms. The technique was used in the structure determination of a monoclinic (Ga0.7In0.3)2SnO5 phase to provide accurate first-guess positions for oxygen atoms for subesquent Rietveld refinement.The solution of the cation positions of the structure relied on HRTEM images. The image in Fig. 1, taken along the crystal’s short b-axis, has a defocus of approximately -50 nm based on the locations of minima in a weak amorphous component in the power spectra of the through-focus image series. Because this is near Scherzer defocus, one expects dark areas of the image to correspond a large projected potential (making the weak phase object approximation). This was used to get successful initial cation positions for a Rietveld refinement of powder x-ray diffraction data. The final refined structure is shown in Fig. 2, including the oxygen positions.

Enhanced 3-D reconstruction from confocal scanning microscope images 1: Deterministic and maximum likelihood reconstructions

Significant improvement in the longitudinal (optical axis) resolution of a microscope has previously been obtained either by posterior digital processing of optical sections collected with a conventional microscope, or by collecting sections with a confocal scanning microscope. In this paper we report the feasibility of obtaining longitudinal resolution comparable to the lateral diffraction limit by posterior processing of confocal sections. A confocal through-focus image series was simulated numerically and restored by constrained iterative deconvolution and by maximum likelihood. Several typical imaging situations were simulated. The results support the possibility of achieving equal longitudinal and lateral resolution.

Dynamical diffraction calculations for RHEED and REM

The multislice formulation of the many-beam dynamical diffraction theory has been applied to the Bragg case of electron diffraction for the extended surface of a perfect crystal and also for a crystal surface with a surface step. The wavefunctions within and outside the crystal have been calculated and used to derive the standing-wave pattern in the top atomic layers of the crystal, the intensities of the reflection high-energy electron diffraction (RHEED) pattern and the contrast of the reflection electron microscopy (REM) image. Calculations made for the diffraction of 19, 40 and 80 keV electrons from (111) surfaces of Pt and Au demonstrate the channeling of electrons under the conditions of surface resonance, the perturbation of the standing-wave field in the crystal by a one-atom-high surface step and the REM contrast for a through-focus series of images of a surface step. The method is applicable to models including surface relaxations and reconstructions and any kind of local defect of the surface or of the bulk crystal.

Imaging of polytypes and stacking faults in tetrahedral compounds

The resolution attained with a JEOL 200CX electron microscope in top-entry configuration has allowed the direct elucidation of the structure of tetrahedrally bonded compounds. Computer simulation methods were used to calculate through-focus series of images at a range of thicknesses and the conditions for which structural features are interpretable on an atomic scale have been ascertained. In particular, various polytypes are differentiated and stacking faults are intuitively interpreted.