high resolation of electron microscope

8/4/2019 HIGH RESOLATION OF ELECTRON MICROSCOPE

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TOPIC: HIGH RESOLUTIONELECTRON MICROSCOPY

COURSE: ELECTRON MICROSCOPY (MM-535)

CONDUCTED BY: Dr. ASHRAF ALI MEO

PRESENTATION PREPARED BY: SYED UMAIRAZHER (MM-03)

DATE: 02-11-2009

NED UNIVERSITY OF ENGINEERING & TECHNOLOGY


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HIGH RESOLUTION ELECTRON MICROSCOPY

HIGH RESOLUTION ELECTRON MICROSCOPY is an imaging modeof the electron microscope that allows the imaging of thecrystallographic structure of a sample at an atomic scale.

High resolutions is not high magnifications; its primarily about

being able to see more, see smaller, etc. At present, the highest resolution realised is 0.8 withmicroscopes.

At these small scales, individual atoms and crystalline defectscan be imaged. HREM technique allows the direct observation ofcrystal structure and therefore has an advantage over other

methods in that there is no displacement between the locationof a defect and the contrast variation caused in the image.



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WORKING PRINCIPLE

Image formation: Contrast arises from interference of theelectron wave with itself.

Phase contrast imaging: When the sample is thin enough andthe amplitude variations do not contribute to the images.

We generally consider images formed from one beam, either adiffracted or undiffracted beam, and these are examples ofamplitude contrast.

If more than one beam is selected in the objective aperture,interference between these beams leads to image contrastarising from phase differences between the beams.



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WORKING PRINCIPLE



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WORKING PRINCIPLE

For two beams at the Bragg condition, the interference patterngenerated has an intensity:

Ig = A2 + B2 2AB sin 2|g.r|

Were,

g = k-k,A, B are the amplitudes of the two waves.

This results in an image with a sinusoidal oscillation of intensitynormal to g with periodicity 1/|g|

In the back focal plane: In the image plane:

Fringe spacing isproportional to 1/|g|



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WORKING PRINCIPLE

If an array of diffracted spots were included in the aperturecentered on the direct (unscattered) beam, then the image willbe a cross-grating of sine fringes arising from interferencebetween all of the beams included in the aperture.

These lattice fringes are not necessarily direct images of thestructure because the phase relationship between the beams isdistorted by the aberrations of the lenses. Chromatic aberrations. Spherical aberrations. Astigmatism.



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WORKING PRINCIPLE:Lens aberrations


Spherical aberration:

The magnetic field of the electron lens is notuniform causing the off axis electrons to befocused more strongly, thus creating avariable focal length with distance from theoptic axis.


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WORKING PRINCIPLE:Lens aberrations

Chromatic aberration:Focal length depends on wavelength ofelectrons. All electron sources demonstratea degree of temporal coherence, with arange of electron energies emitted.

Astigmatism:Focal length of the lens varies azimuthally.At the disc of least confusion the beam iscircular, away from this point the beam iselliptical.



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DRAWBACK:

One of the difficulties with HREM is that image formation relies onphase contrast.

In phase-contrast imaging, contrast is not spontaneously interpretableas the image is influenced by strong aberrations of the imaging lensesin the microscope.

Resolution of the HREM is limited by spherical and chromaticaberration, but a new generation of aberration correctors has beenable to overcome spherical aberration.

For exemple, software correction of spherical aberration has allowedthe production of images with sufficient resolution to show carbonatoms in diamond separated by only 0.89 and atoms in silicon at

0.78 at magnifications of 50 million times. Improved resolution has also allowed the imaging of lighter atoms thatscatter electrons less efficiently lithium atoms have been imaged inlithium battery materials.



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Why HREM: High resolution electron microscopy can resolve object details smaller

than 1 nm (10-9 m). It can be used to image the interior structure of the specimen

(comparing to atomic resolution scanning tunneling microscopy, onlyat the surface).

Comparing to atomic resolution provided by X-ray diffraction(averaging information), HREM can provide information on the localstructure.

Direct imaging of atom arrangements, in particular the structuraldefects, interface, dislocations.

The ability to determine the positions of atoms within materials has

made the HREM an essential tool for nanotechnology research anddevelopment in many fields, including heterogeneous catalysis and thedevelopment of semiconductor devices for electronics and photonics.



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High-Resolution Electron Microscopy: Carbon nanotube

Discovery of the carbon nanotube

S. Iijima, Nature 354, 56 (1991).



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High-Resolution Electron Microscopy: Au Nanoparticles

Shape Determination of Au Nanoparticles



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High-Resolution Electron Microscopy: Interface



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INTERPRETATION OF HREM IMAGES

The use of HREM images and/or electron diffraction patterns forstructure determination is called electron crystallography.

Two electron crystallography techniques based on HREM images haverecently emerged:1- The reconstruction of the exit wave by acquiring HREM images at

different defocus values named through focus exit wave reconstruction(TF-EWR). The resulting structure is refined with the diffractionintensities recorded on a CCD camera, with a software which includesthe excitation error and the dynamical effects, called multi-slice leastsquares (MSLS).2- The processing of HREM images of thin samples by crystallographic

image processing (CRISP). This permits to extract the phase and theamplitudes of the HREM image and correct them by considering thechanges introduced by the transfer function, the astigmatism (andpossibly the tilt).



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REQUIREMENTS FOR HIGH RESOLUTION WORK

The microscope site must be acceptable. Mechanical vibration, straymagnetic fields, and room temperature must all be within acceptablelimits.

In order to record an image at a specified focus defect it will benecessary to measure the change in focus between focus control steps

(clicks) using different methods. The resolution obtained in a transmission image depends amongst

other things on the specimen position in the objective lens. Theoptimum specimen height must be found by trial and error. Inreducing the specimen height and so increasing the objective lenscurrent needed for focus, the lens focal length and spherical aberrationconstant are reduced, leading to improved resolution.



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REQUIREMENTS FOR HIGH RESOLUTION WORK

A vacuum of 0.5 107 torr or better is needed (measured in the rearpumping line). The simplest way to trace vacuum leaks is to use apartial pressure gauge.

A reliable, constant temperature and pressure supply of clean coolingwater must be assured.

The microscope high voltage must be sufficiently stable to allow high-resolution images to be obtained. The room containing the microscope must be easily darkened

completely.



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REFERENCES:

Spence, J. C. H. (2003). High-resolution electron microscopy, NewYork

Agar, A. W., Alderson, R. H., and Chescoe, D. (1974). Principles andpractice of electron microscope operation. In Practical Methods inElectron Microscopy (ed. A. M. Glauert). North-Holland, Amsterdam.

Alderson, R. H. (1974). The design of the electron microscopelaboratory. In Practical Methods in Electron Microscopy (ed. A. M.Glauert). North-Holland, Amsterdam.

Bennett, A. H., Jupnik, H., Osterberg, H., and Richards, O. W. (1951).Phase Microscopy. Principles and Applications. Wiley, New York.

Cosslett. V. E. (1958). Quantitative aspects of electron staining. J. R.

Microsc. Soc. 78, 18. Downing, K. H. and Siegel, B. M. (1973). Phase shift determination in

single-sideband holography. Optik38, 21.



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Lipson, S. G. and Lipson, H. (1969). Optical Physics. CambridgeUniversity Press, London.

Spence, J. C. H. (1974). Complex image determination in the electronmicroscope. Opt. Acta 21, 835.

Unwin, N. (1971). Phase contrast and interference microscopy with the

electron microscope. Phil. Trans. R. Soc., Lond. B261, 95. Goodman, J. W. (1968). Introduction to Fourier Optics. McGraw-Hill,

New York.

Rambu, A. P, Curecheriu. L. P & Mihalache, G. High Resolution ElectronMicroscopy of Soft Condensed Matter Systems. University AlexandruIoan Cuza, Romania

REFERENCES:



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high resolation of electron microscope

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