Accurate measurements of mean inner potential of crystal wedges using electron holography

1992 ◽  
Vol 50 (1) ◽  
pp. 134-135
Author(s):  
M. Gajdardziska-Josifovska ◽  
M. R. McCartney ◽  
J. K. Weiss
Keyword(s):  
Phase Change ◽  
Electron Holography ◽  
Specimen Thickness ◽  
Electron Wave ◽  
Flat Surfaces ◽  
Numerical Reconstruction ◽  
Digital Holograms ◽  
The Mean ◽  
Energy Dependent ◽  
Magnetic Electron

The phase of an electron wave which has interacted with a material is measured in electron holography experiments with respect to a coherent reference wave which has travelled through vacuum. In non-magnetic electron-transparent materials, and under kinematical diffracting conditions, the phase change (Δφ) of the transmitted electron wave depends only on the thickness (t) and the mean inner potential (Ui) of the material: Δφ = c |Ui| t; c being an energy-dependent constant. This phase change measured from electron holograms has been used previously to determine the mean inner potential of amorphous and polycrystalline films of known thicknesses. Refraction effects in RHEED patterns have also been used to determine the mean inner potential of several crystals with flat surfaces. The reported accuracies in these studies have ranged from 2.5% to 9.5%, although uncertainties in specimen thickness and the unknown effects of surface contamination and/or reconstruction are very likely sources of systematic errors. This paper shows that numerical reconstruction of digital holograms, combined with use of cleaved crystal wedges, enables measurement of the mean inner potential of crystals with enhanced accuracy.

Electron Refraction of Amorphous Nanospheres

1997 ◽  
Vol 3 (S2) ◽  
pp. 1055-1056
Author(s):  
Y.C. Wang ◽  
T.M. Chou ◽  
M. Libera
Keyword(s):  
Refractive Index ◽  
Electron Scattering ◽  
Unit Volume ◽  
High Energy ◽  
Covalent Bonding ◽  
Electron Holography ◽  
High Energy Electron ◽  
Electron Wave ◽  
Transmission Electron ◽  
The Mean

The phase shift imparted to an incident high-energy electron wave in a TEM is related to the specimen’s electron-refractive properties. These, in turn, are related to the electrostatic potential and, by Fourier transform (1), to the electron scattering factors fei(s) for the various atom species i in the specimen and scattering vectors s. The average refractive index is determined by the mean electrostatic (inner) potential, Φo, and can be modelled as Φo = (C/Ω) Σfei(s0) [equation 1] where C = 47.878 (V-Å2) and the summation runs over all of the atoms in the unit volume Ω (2). Calculated fei(s) data are available from the literature (e.g. 3). These calculations have only been done for neutral atoms and some fully ionized cations and anions. They do not account for electron redistribution due to covalent bonding to which Φo is quite sensitive (4).This research is making Φo measurements using transmission electron holography. Holograms were collected using a 200keV Philips CM20 FEG TEM equipped with a non-rotatable biprism (5) and a Gatan 794 Multiscan camera.

Development of electron holography system in field emission electron microscrope

1995 ◽  
Vol 53 ◽  
pp. 594-595
Author(s):  
M. Takeguchi ◽  
T. Tomita ◽  
T. Honda ◽  
M. Kersker
Keyword(s):  
Field Emission ◽  
Power Supply ◽  
Electric Field Distribution ◽  
Practical Method ◽  
Electron Holography ◽  
Specimen Thickness ◽  
Electron Wave ◽  
Beam Splitting ◽  
Physical Information ◽  
Application Data

Electron holography is a powerful and practical method to know the phase of electron wave quantitatively and enables us to obtain various kinds of physical information such as magnetic and electric field distribution inside/outside of the specimen, thickness of the crystal etc. Since this method requires an FE-TEM equipped with an electron biprism and some electron hologram reconstruction methodology, the general EM users is typically not yet familiar with electron holography practice. The recent introduction of commercial type FE-TEM, however, brings some users the opportunities of practical electron holography applications. We have recently developed an electron holography system for our FE-TEMs, the JEM-2010F and the JEM-3000F. The components, specifications and some application data are presented below:Our electron holography system consists of electron biprism, power supply and hologram reconstruction processor(PC + software). The electron biprism is rotatable and the beam-splitting electrode(a Pt wire of 0.6 μm in diameter) can be cleaned by direct heating. The electron biprism is of the cartridge type for easy maintenance, thus removing the necessity of wire exchange.

Measurement of Polystyrene Mean Inner Potential by Transmission Electron Holography of Latex Spheres

1998 ◽  
Vol 4 (2) ◽  
pp. 146-157 ◽  
Author(s):  
Y.C. Wang ◽  
T.M. Chou ◽  
M. Libera ◽  
E. Voelkl ◽  
B.G. Frost
Keyword(s):  
Phase Image ◽  
Electron Holography ◽  
Specimen Thickness ◽  
Polystyrene Spheres ◽  
Reconstruction Procedure ◽  
A Value ◽  
Transmission Electron ◽  
Phase Images ◽  
The Mean ◽  
Beam Damage

This study describes the use of transmission electron holography to determine the mean inner potential of polystyrene. Spherical nanoparticles of amorphous polystyrene are studied so that the effect of specimen thickness on the phase shift of an incident electron wave can be separated from the intrinsic refractive properties of the specimen. A recursive four-parameter χ-squared minimization routine is developed to determine the sphere center, radius, and mean inner potential (Φ0) at each pixel in the phase image. Because of the large number of pixels involved, the statistics associated with determining a single Φ0 value characteristic of a given sphere are quite good. Simulated holograms show that the holographic reconstruction procedure and the χ-squared analysis method are robust. Averaging the Φ0 data derived from ten phase images from ten different polystyrene spheres gives a value of Φ0PS = 8.5 V (σ) = 0.7 V). Specimen charging and electron-beam damage, if present, affect the measurement at a level below the current precision of the experiment.

Mean inner potential measurements of polymer latexes by transmission electron holography

1996 ◽  
Vol 54 ◽  
pp. 168-169
Author(s):  
Young-Chung Wang ◽  
Matthew Libera
Keyword(s):  
Refractive Index ◽  
Polymeric Materials ◽  
High Energy ◽  
Electron Holography ◽  
Specimen Thickness ◽  
Phase Imaging ◽  
Rest Energy ◽  
Polymer Microstructure ◽  
Transmission Electron ◽  
Energy Dependent

Because polymeric materials consist primarily of light elements, weak contrast is often observed when imaging polymer microstructure in a transmission electron microscope. Preferential staining of microstructural features by heavy elements such as osmium, ruthenium, or uranium is commonly used to induce amplitude contrast. Because of its ability to recover the entire exit-face electron wavefunction, transmission electron holography raises the possibility of using phase contrast to measure polymer microstructure without the need for heavy-element stains. Under kinematic scattering conditions, the phase shift, ΔΦ, imposed on an incident high-energy electron wave is given by the product of the electron-optical refractive index, neo, and the specimen thickness, t: Δφ=(2π/λ)(neo−1)t. The refractive index is related to the specimen’s mean coulombic (inner) potential Φ0: neo−1 = (e |Φ0|/E) [(E0+E)/(2E0+E)] = CEΦ0 where e is the electron charge, E is the kinetic energy of the incident electrons, E0 is the rest energy, and CE is an energy-dependent constant. Quantitative measurements of Φ0 and neo can be made using holographic phase imaging to determine from specimens of known thickness.

Electron holography in materials science

1991 ◽  
Vol 49 ◽  
pp. 670-671
Author(s):  
Hannes Lichte ◽  
Edgar Voelkl
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Materials Science ◽  
Fourier Spectrum ◽  
Object Wave ◽  
Electron Holography ◽  
Reconstruction Procedure ◽  
Numerical Reconstruction ◽  
Transmission Electron ◽  
Unique Interpretation

The object wave o(x,y) = a(x,y)exp(iφ(x,y)) at the exit face of the specimen is described by two real functions, i.e. amplitude a(x,y) and phase φ(x,y). In stead of o(x,y), however, in conventional transmission electron microscopy one records only the real intensity I(x,y) of the image wave b(x,y) loosing the image phase. In addition, referred to the object wave, b(x,y) is heavily distorted by the aberrations of the microscope giving rise to loss of resolution. Dealing with strong objects, a unique interpretation of the micrograph in terms of amplitude and phase of the object is not possible. According to Gabor, holography helps in that it records the image wave completely by both amplitude and phase. Subsequently, by means of a numerical reconstruction procedure, b(x,y) is deconvoluted from aberrations to retrieve o(x,y). Likewise, the Fourier spectrum of the object wave is at hand. Without the restrictions sketched above, the investigation of the object can be performed by different reconstruction procedures on one hologram. The holograms were taken by means of a Philips EM420-FEG with an electron biprism at 100 kV.

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.

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.

On the Faceting of Polar Ceramic Surfaces: Microscopy and Holography Studies of MGO(111) Surfaces

1996 ◽  
Vol 54 ◽  
pp. 366-367
Author(s):  
M. Gajdardziska-Josifovska ◽  
B. G. Frost ◽  
E. Völkl ◽  
L. F. Allard
Keyword(s):  
Electron Microscopy ◽  
High Vacuum ◽  
Ultra High Vacuum ◽  
Electron Holography ◽  
Flat Surfaces ◽  
Transmission Electron ◽  
Excess Surface ◽  
Polar Surfaces ◽  
Salt Structure ◽  
Crystallographic Planes

Polar surfaces are those crystallographic faces of ionically bonded solids which, when bulk terminated, have excess surface charge and a non-zero dipole moment perpendicular to the surface. In the case of crystals with a rock salt structure, {111} faces are the exemplary polar surfaces. It is commonly believed that such polar surfaces facet into neutral crystallographic planes to minimize their surface energy. This assumption is based on the seminal work of Henrich which has shown faceting of the MgO(111) surface into {100} planes giving rise to three sided pyramids that have been observed by scanning electron microscopy. These surfaces had been prepared by mechanical polishing and phosphoric acid etching, followed by Ar+ sputtering and 1400 K annealing in ultra-high vacuum (UHV). More recent reflection electron microscopy studies of MgO(111) surfaces, annealed in the presence of oxygen at higher temperatures, have revealed relatively flat surfaces stabilized by an oxygen rich reconstruction. In this work we employ a combination of optical microscopy, transmission electron microscopy, and electron holography to further study the issue of surface faceting.

On the influence of specimen thickness in TEM images of super-conducting vortices

1996 ◽  
Vol 54 ◽  
pp. 724-725
Author(s):  
J. Bonevich ◽  
D. Capacci ◽  
G. Pozzi ◽  
K. Harada ◽  
H. Kasai ◽  
...  
Keyword(s):  
Flux Line ◽  
Analytical Form ◽  
Realistic Model ◽  
Electron Holography ◽  
Specimen Thickness ◽  
Flux Tubes ◽  
Analytical Expression ◽  
Flux Lines ◽  
Normal Core ◽  
Two Parameters

The successful observation of superconducting flux lines (fluxons) in thin specimens both in conventional and high Tc superconductors by means of Lorentz and electron holography methods has presented several problems concerning the interpretation of the experimental results. The first approach has been to model the fluxon as a bundle of flux tubes perpendicular to the specimen surface (for which the electron optical phase shift has been found in analytical form) with a magnetic flux distribution given by the London model, which corresponds to a flux line having an infinitely small normal core. In addition to being described by an analytical expression, this model has the advantage that a single parameter, the London penetration depth, completely characterizes the superconducting fluxon. The obtained results have shown that the most relevant features of the experimental data are well interpreted by this model. However, Clem has proposed another more realistic model for the fluxon core that removes the unphysical limitation of the infinitely small normal core and has the advantage of being described by an analytical expression depending on two parameters (the coherence length and the London depth).

Is there an electron wind?

1990 ◽  
Vol 48 (4) ◽  
pp. 798-799
Author(s):  
L. D. Marks ◽  
J. P. Zhang
Keyword(s):  
Electron Microscopy ◽  
High Resolution Electron Microscopy ◽  
Zone Axis ◽  
Electron Holography ◽  
Electron Wave ◽  
Small Particles ◽  
Resolution Electron ◽  
Pass Through ◽  
Electron Wind ◽  
Inelastic Scatter

A not uncommon question in electron microscopy is what happens to the momentum transferred by the electron beam to a crystal. If the beam passes through a crystal and is preferentially diffracted in one direction, is the momentum ’lost’ by the beam transferred to the crystal? Newton’s third law implies that this must be the case. Some experimental observations also indicate that this is the case; for instance, with small particles if the particles are supported on the top surface of a film they often do not line up on the zone axis, but if they are on the bottom they do. However, if momentum is transferred to the crystal, then surely we are dealing with inelastic scattering, not elastic scattering and is not the scattering probability different? In addition, normally we consider inelastic scatter as incoherent, and therefore the part of the electron wave that is inelastically scattered will not coherently interfere with the part of the wave that is scattered; but, electron holography and high resolution electron microscopy work so the wave passing through a specimen must be coherent with the wave that does not pass through the specimen.

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