Aberration Free Focus Research Articles

Pseudo-aberration free focus condition for atomic resolution electron microscope images

Spherical Aberration Free Focus (AFF) conditions for taking atomic resolution electron microscope images of single crystals and pseudo-Aberration Free Focus (ψ-AFF) conditions for taking atomic images of more than two adjoining single crystals are discussed by using diagrams of C s (spherical aberration coefficient) and Δf (defocus). Calculated image contrast in those conditions is shown for three individual crystals of MgO, MgAl 2O 4, Al 2O 3 and their adjoining crystal blocks. It is shown that at least two kinds of Bragg reflected waves can be in the AFF condition in each case, and thus the size of the aperture for limiting the waves becomes important in some cases, C s can be changed by changing the position of the specimen along the objective lens axis but highly coherent illuminating waves are needed for large C sand δf .

Pseudo aberration free focus for atomic resolution imaging of two adjoining crystals

The atomic structure images of two crystals on both sides of the interface made by conventional TEM are not so easy to be recorded at the same focus without the effect of spherical aberration of imaging lenses even though the spacings of the crystal lattices of both crystals are within Scherzer focus limit, because the specimen is generally larger than 1 Onm thickness and cannot be treated as weak phase objects and Bragg reflected waves from tow crystals generally cannot be in phase at the image plane. In TEM, the shift of the phases of several Bragg reflected waves from a crystal due to the spherical aberration (coefficient Cs) can be compensated by the defocus (Δf), which was named as aberration free focus (AFF). The current images of crystals on both sides of an interface are formed by the waves reflected from one or two kinds of lattice planes with low indexes.

Detection of small displacement of atoms in crystals by atom resolution electron microscopy.

Four kinds of works on the detection of displacement of atoms in crystals are shown. The irregular small displacement of atoms has been detected, with an accuracy of about 0.1 A around dislocations and stacking faults in Au crystals as shown by their electron microscope images. The displacement of the atomic images is recorded by aberration-free focus (AFF). Even when the periodic displacement of atoms in SiC and TaS2 crystals is around 0.1% of the lattice constant, this displacement has been revealed as the weak-contrast anomaly in the images. Using in situ observations by a TV system attached to an electron microscope, the rapid movements of atoms that have taken place within 1/30 sec have been recorded. Using the technique of successive subtraction of the images by TV system and image sigma, only the images of moving atoms in Au crystal have been recorded each 1/10 sec.

Electron microscope observations of lattice imperfections and their movement on the atomic scale

The effect of partial coherence of illuminating electron waves on the image contrast is discussed. It is shown that aberration-free focus (AFF) imaging in axial and symmetrical illumination, which is generally believed to be the best imaging condition for coherent illumination, produces an inherent contrast which does not correspond to the atomic structure of the specimen. However, tilted-illumination AFF imaging produces the correct image. Atomic structure images of gold crystals containing edge, screw and mixed dislocations have been photographed, with tilted-illumination AFF imaging and by adjusting the specimen thickness to the optimum value, and the atomic structure of the core of these dislocations is discussed. The movement of these dislocations has been recorded on an atomic scale using a TV system, and the movement of atom columns accompanying the movement of dislocations is analyzed.

Visualization of small displacement of atoms in crystals by the processing of atom resolution images

Detection of small displacement of atoms was carried out using diffuse and irregular diffraction spots and Aberration Free Focus (AFF) imaging.Fig. 1 (a) shows the position of atoms of a Ti-Mo alloy where white and black discs represent the position of atoms in β-phase and ω-phase respectively. Namely, the ω-phase is formed by the displacement of atoms in B and C positions.Fig. 1 (b) is an electron diffraction pattern of an as-quenched alloy that contains 16 diffuse diffraction spots due to the displacement of atoms. The corresponding electron micrograph photographed with AFF condition is shown in (c). The images of displaced atoms are rather difficult to see. (d) is the processed image formed by using only diffuse spots in the Fourier Transform (FT) of (c). The bright spots correspond to the shaded displaced atom regions shown in (a) and not to the atom positions which are not displaced [1].Charge Density Waves (CDW) occur as periodic modulations of conduction electrons and produce the periodic lattice distortions.

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

SUMMARYFine 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.

The selective HREM imaging of sublattices of atoms in complicated structures

It is found experimentally that by a suitable choice of defocus it is possible to reveal preferentially sublattices of atom columns in complicated structures as bright dots. Examples of observations on Au 4Mn, Au 77Mg 23 and Ba 2NaNb 5O 15 are presented and analysed. It is demonstrated that these observations can be understood in terms of an extension of the aberration-free focus method. It is shown in particular that in the cases considered the most intense beams, which carry the essential information concerning the geometry of the sublattice, have diffraction vectors situated in circular regions of which the radius is directly related to the average distance between atom columns in the considered sublattice. By a suitable choice of defocus, one can create “windows” in the image transfer function through which these essential beams are transmitted “in phase” through the microscope. Theoretical considerations support this interpretation and allow graphs to be made which establish a relationship between the defocus required for a zero relative phase shift between essential beams and the length of the g-vector of these beams. It is shown that the experimental images can be explained satisfactorily on the basis of these graphs.

Interpretation of atomic-resolution electron microscope images of Guiner-Preston zones in aluminium-copper alloys

Abstract Contrast calculations by the dynamical theory of electron diffraction have been performed using two kinds of models of Guiner-Preston (G.P.) (I) and (II) zones in aluminium-copper alloys, and results have been compared with high-resolution electron microscope images in the zones. Image contrast changes of the calculated images are demonstrated. The contrast is dependent both on crystal thickness and objective lens defocus, and the dependence on the latter is in good agreement with observations. The observed images of G.P. (I) zones are well understood by the copper atom monolayer model of Yoshida et al. (1976). A contrast anomaly in the observed images of G.P. (II) zones is attributed to the effect of defocus for a modified model containing shear displacements. It is proposed that the optimum imaging condition for a crystal containing lattice distortion due to G.P. zones is the tilted beam aberration free focus condition.

Electron-microscope images of the crystal lattice of gold containing planar defects

Using the aberration-free focus (AFF) imaging condition in tilted illumination and by adjusting the thickness of gold crystals to optimum values, the electron-microscope images of gold atoms in perfect crystals are formed at the correct positions of atoms with a resolution higher than the theoretical resolution limit in normal operation. This method is applied to observe the images of atoms in gold crystals containing planar defects such as twin boundaries and stacking faults. The observed contrast of the images is compared with theoretical calculations based on both the Howie-Whelan theory of electron diffraction in many-beam form and an electron image-formation theory. The agreement between observations and theoretical calculations is fairly good.

Structure and migration of planar defects in crystals observed in atomic scale

As was demonstrated by the present authors that atomic structure of simple crystal can be photographed by the conventional 100 kV electron microscope adjusted at “aberration free focus (AFF)” condition. In order to operate the microscope at AFF condition effectively, highly stabilized electron beams with small energy spread and small beam divergence are necessary. In the present observation, a 120 kV electron microscope with LaB6 electron gun was used. The most of the images were taken with the direct electron optical magnification of 1.3 million times and then magnified photographically.1. Twist boundary of ZnSFig. 1 is the image of wurtzite single crystal with twist boundary grown on the surface of zinc crystal by the reaction of sulphur vapour of 1540 Torr at 500°C. Crystal surface is parallel to (00.1) plane and electron beam is incident along the axis normal to the crystal surface. In the twist boundary there is a dislocation net work between two perfect crystals with a certain rotation angle.

High resolution lattice images of aluminum-copper alloy containing Guinier-Preston zones

Imaging of atomic structure of sinple crystals such as A1 and Au by the conventional high resolution electron microscope operated at 100 kV is still not very easy, because the lattice spacing of the crystals are smaller than about 2. 3 A, which equivalent to the resolving power of the microscopes. However, if the crystal lattice image is taken at the “Aberration Free Focus” condition(l), the fine structure of the crystal lattice can be photographed. Using this technique, atomic structure arround G.P. zone is studied in the present paper. Since local orientation in the specimen film disturbes the Bragg condition, we did not succeed to observe the atomic structure but could see the lattice fringes, which are formed by the systematic reflection from the crystal position. The fringe position obtained such condition corresponds to the position of atomic rows.Disc specimens were spark-cut from the sheet of an Al-3.97%Cu alloy.