Femtochemistry:  Atomic-Scale Dynamics of the Chemical Bond†

2000 ◽  
Vol 104 (24) ◽  
pp. 5660-5694 ◽  
Author(s):  
Ahmed H. Zewail
Keyword(s):  
Chemical Bond ◽  
Atomic Scale

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

1978 ◽  
Vol 36 (1) ◽  
pp. 284-285 ◽  
Author(s):  
H. Hashimoto ◽  
Y. Sugimoto ◽  
Y. Takai ◽  
H. Endoh
Keyword(s):  
Electron Microscope ◽  
Rotation Angle ◽  
Crystal Surface ◽  
Electron Gun ◽  
Atomic Scale ◽  
Present Observation ◽  
Twist Boundary ◽  
Optical Magnification ◽  
Zinc Crystal ◽  
And Migration

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.

Resolution in the scanning tunneling microscope

1991 ◽  
Vol 49 ◽  
pp. 488-489
Author(s):  
R. J. Wilson ◽  
D. D. Chambliss ◽  
S. Chiang ◽  
V. M. Hallmark
Keyword(s):  
Scanning Tunneling Microscope ◽  
Fundamental Principle ◽  
Atomic Scale ◽  
Lateral Resolution ◽  
Scanning Tunneling ◽  
Electronic Noise ◽  
Vertical Resolution ◽  
Tunneling Microscopy ◽  
Spatial Profile ◽  
Theoretical Analyses

Scanning tunneling microscopy (STM) has been used for many atomic scale observations of metal and semiconductor surfaces. The fundamental principle of the microscope involves the tunneling of evanescent electrons through a 10Å gap between a sharp tip and a reasonably conductive sample at energies in the eV range. Lateral and vertical resolution are used to define the minimum detectable width and height of observed features. Theoretical analyses first discussed lateral resolution in idealized cases, and recent work includes more general considerations. In all cases it is concluded that lateral resolution in STM depends upon the spatial profile of electronic states of both the sample and tip at energies near the Fermi level. Vertical resolution is typically limited by mechanical and electronic noise.

What catalysis needs from the electron microscopist

1988 ◽  
Vol 46 ◽  
pp. 694-695
Author(s):  
Alexis T. Bell
Keyword(s):  
Active Sites ◽  
Heterogeneous Catalysts ◽  
Catalyst Deactivation ◽  
Surface Composition ◽  
Internal Surface ◽  
Active Phase ◽  
Atomic Scale ◽  
Metal Catalyst ◽  
Internal Surface Area ◽  
Porous Support

Heterogeneous catalysts, used in industry for the production of fuels and chemicals, are microporous solids characterized by a high internal surface area. The catalyticly active sites may occur at the surface of the bulk solid or of small crystallites deposited on a porous support. An example of the former case would be a zeolite, and of the latter, a supported metal catalyst. Since the activity and selectivity of a catalyst are known to be a function of surface composition and structure, it is highly desirable to characterize catalyst surfaces with atomic scale resolution. Where the active phase is dispersed on a support, it is also important to know the dispersion of the deposited phase, as well as its structural and compositional uniformity, the latter characteristics being particularly important in the case of multicomponent catalysts. Knowledge of the pore size and shape is also important, since these can influence the transport of reactants and products through a catalyst and the dynamics of catalyst deactivation.

Electron holography of catalysts

1995 ◽  
Vol 53 ◽  
pp. 416-417
Author(s):  
A. K. Datye ◽  
D. S. Kalakkad ◽  
L. F. Allard ◽  
E. Völkl
Keyword(s):  
Heterogeneous Catalysts ◽  
High Surface ◽  
High Surface Area ◽  
Atomic Scale ◽  
Nano Particles ◽  
Phase Image ◽  
Electron Holography ◽  
Specimen Thickness ◽  
Phase Information ◽  
Particle Structure

The active phase in heterogeneous catalysts consists of nanometer-sized metal or oxide particles dispersed within the tortuous pore structure of a high surface area matrix. Such catalysts are extensively used for controlling emissions from automobile exhausts or in industrial processes such as the refining of crude oil to produce gasoline. The morphology of these nano-particles is of great interest to catalytic chemists since it affects the activity and selectivity for a class of reactions known as structure-sensitive reactions. In this paper, we describe some of the challenges in the study of heterogeneous catalysts, and provide examples of how electron holography can help in extracting details of particle structure and morphology on an atomic scale.Conventional high-resolution TEM imaging methods permit the image intensity to be recorded, but the phase information in the complex image wave is lost. However, it is the phase information which is sensitive at the atomic scale to changes in specimen thickness and composition, and thus analysis of the phase image can yield important information on morphological details at the nanometer level.

Limits for high-resolution electron microscopy of inorganic materials?

1987 ◽  
Vol 45 ◽  
pp. 4-5
Author(s):  
David J. Smith
Keyword(s):  
Electron Microscopy ◽  
Spherical Aberration ◽  
High Resolution Electron Microscopy ◽  
Inorganic Materials ◽  
Atomic Scale ◽  
Resolution Limit ◽  
Contrast Transfer Function ◽  
Existing Problems ◽  
Resolution Electron ◽  
Electron Microscopes

The era of atomic-resolution electron microscopy has finally arrived. In virtually all inorganic materials, including oxides, metals, semiconductors and ceramics, it is possible to image individual atomic columns in low-index zone-axis projections. A whole host of important materials’ problems involving defects and departures from nonstoichiometry on the atomic scale are waiting to be tackled by the new generation of intermediate voltage (300-400keV) electron microscopes. In this review, some existing problems and limitations associated with imaging inorganic materials are briefly discussed. The more immediate problems encountered with organic and biological materials are considered elsewhere.Microscope resolution. It is less than a decade since the state-of-the-art, commercially available TEM was a 200kV instrument with a spherical aberration coefficient of 1.2mm, and an interpretable resolution limit (ie. first zero crossover of the contrast transfer function) of 2.5A.

Off-axis electron holography applied to the study of interfaces

1992 ◽  
Vol 50 (1) ◽  
pp. 244-245
Author(s):  
J.K. Weiss ◽  
M. Gajdardziska-Josifovska ◽  
M. R. McCartney ◽  
David J. Smith
Keyword(s):  
Electric Fields ◽  
Resolution Enhancement ◽  
Interfacial Structure ◽  
Atomic Scale ◽  
Bulk Property ◽  
Electron Holography ◽  
Specimen Thickness ◽  
Composition And Structure ◽  
Chemical Segregation ◽  
Diffraction Effects

Interfacial structure is a controlling parameter in the behavior of many materials. Electron microscopy methods are widely used for characterizing such features as interface abruptness and chemical segregation at interfaces. The problem for high resolution microscopy is to establish optimum imaging conditions for extracting this information. We have found that off-axis electron holography can provide useful information for the study of interfaces that is not easily obtained by other techniques.Electron holography permits the recovery of both the amplitude and the phase of the image wave. Recent studies have applied the information obtained from electron holograms to characterizing magnetic and electric fields in materials and also to atomic-scale resolution enhancement. The phase of an electron wave passing through a specimen is shifted by an amount which is proportional to the product of the specimen thickness and the projected electrostatic potential (ignoring magnetic fields and diffraction effects). If atomic-scale variations are ignored, the potential in the specimen is described by the mean inner potential, a bulk property sensitive to both composition and structure. For the study of interfaces, the specimen thickness is assumed to be approximately constant across the interface, so that the phase of the image wave will give a picture of mean inner potential across the interface.

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