Unit for Nanocharacterization Equipment and Techniques TEM

High Resolution Transmission Scanning Electron Microscope Tecnai F20 G2

TEM - Overview
TEM - Basics and Tutorials
TEM - Specifications

Basics and Tutorials

The transmission electron microscope uses a high energy electron beam transmitted through a very thin sample to image and analyze the microstructure of materials with atomic scale resolution. The electrons are focused with electromagnetic lenses and the image is observed on a fluorescent screen, or recorded on film or digital camera. The electrons are accelerated at several hundred kV, giving wavelengths much smaller than that of light and X-rays: 200 kV electrons have a wavelength of 0.025 Angstrom. However, whereas the resolution of the optical microscope is limited by the wavelength of light, that of the electron microscope is limited by aberrations inherent in electromagnetic lenses, to about 1-2 Angstrom.
Because even for very thin samples one is looking through many atoms, one does not usually see individual atoms. Rather the high resolution imaging mode of the microscope images the crystal lattice of a material as interference pattern between the transmitted and diffracted beams. This allows one to observe planar and line defects, grain boundaries, interfaces, etc. with atomic scale resolution. The brightfield/darkfield imaging modes of the microscope, which operate at intermediate magnification, combined with electron diffraction, are also invaluable for giving information about the morphology, crystal phases, and defects in a material.
The TEM is also capable of forming a focused electron probe, as small as 2 Angstrom, which can be positioned on very fine features in the sample for microdiffraction information or analysis of x-rays for compositional information with energy dispersive X-ray spectroscopy (EDS) detector. The spatial resolution for this compositional analysis in TEM is very high, on the order of the probe size, because the sample is so thin (1000 - 2000 Angstrom).
Transmission of fast electrons through the thin sample causes their inelastic scattering, i.e. energy losses of primary electrons due to the excitations of samples' atoms. Typically, for 200 KeV primary electrons the energy losses are within 1KeV range. The distribution of transmitted electrons as a function of energy loss is called an energy loss spectrum. Such spectrum carries invaluable information about chemical composition of the samples as well as information about bonding, coordination and charge transfer on atomic level. Combination of electron energy loss spectroscopy and imaging ability of GATAN Imaging Filter (GIF) extends the analytical power of TEM up to nano-scale and allows chemical analysis of grain boundaries, thin interface layers and many other nano-size objects.
Restrictions on Samples:
Sample preparation for TEM generally requires more time and experience than for most other characterization techniques. A TEM specimen must be approximately 1000Angstrom or less in thickness in the area of interest. The entire specimen must fit into a 3 mm diameter cup and be less than about 100 microns in thickness. A thin, disc shaped sample with a hole in the middle, the edges of the hole being thin enough for TEM viewing, is typical. The initial disk is usually formed by cutting and grinding from bulk or thin film/substrate material, and the final thinning done by ion milling. Other specimen preparation possibilities include direct deposition onto a TEM-thin substrate (Si3N4, carbon); direct dispersion of powders on such a substrate; grinding and polishing using special devices like tripod; chemical etching and electro-polishing; and lithographic patterning of walls and pillars for cross-section viewing. A focused ion beam (FIB) may be used to make cross-sections; however this capability is not currently available at HUJ.
For basic education and principle understanding of Transmission Electron Microscopy, we kindly ask you to use following web resources:

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Electron Diffraction in TEM

One of the main interaction events occurring when fast electrons of primary beam pass through the thin sample in TEM is diffraction. This process provides a basis for studying the structure of crystals and of identifying materials. Metals tend to give very strong electron diffraction patterns, whereas biological specimens generally diffract quite weakly. The basic advantage of electron diffraction as compared with diffraction of X-rays is an opportunity to study very small specimen areas.
A crystalline specimen will diffract the electron beam strongly through well-defined directions (given in angles, Theta) dependent on electron wavelength and crystal lattice spacing according to Bragg's law,

where n is integer, is electron wavelength, d is crystal lattice spacing between atomic planes, is angle of incidence and also of reflection.
This relation gives the conditions for constructive interference of the scattered electron waves. There is reinforcement of reflections from successive parallel planes when the angles of incidence and reflection satisfy Bragg's law. The dimensions and spacing of spots are reciprocally related to the lattice dimensions in the specimen. For example, with 100 kV electrons, (= 0.0037 nm) and a d-spacing for a typical biological crystal = 10 nm, sin = 0.000185 and the Bragg angle = 0.0106░. (A typical d-spacing for a metallic crystal such as nickel is 0.203 nm, so for 200 kV electrons (= 0.0025 nm) sin = 0.00616 and = 0.353░).
The electrons, which passed through the specimen, contain various information about it structure and composition.
The image formed by the transmitted electrons at the back focal plane of the objective lens contains crystallographic information and is called Diffraction Pattern, since this is the focused image of the diffraction maxima formed according to Bragg's low from the initial coherent parallel electron beam illuminating the limited area in the specimen, which contains diffracting media (groups of atom planes).
Depending on the nature of the specimen, a diffraction pattern usually consists of a series of rings (for specimens consisting of many randomly oriented microcrystals) or a discrete lattice of sharp spots (for specimens with a single, crystalline domain). Each sharp spot in the diffraction pattern is an image of the electron source since the imaging system is set to bring the image of the electron source to the viewing screen.
The pattern formed at the image plane of the objective lens contains information about mass-thickness distribution through the illuminated area of the sample. Here the contrast originates from the fact that thicker regions of the sample and regions containing heavier elements cause more scattering events in the initial probing beam. Thus, this image is essentially the picture of losses which initial electron beam suffered when passed through the specimen.
Appearance of diffraction pattern in the back focal plane of the objective lens does not depend on the illumination condition, but the crystallographic information it carries differs for patterns obtained at parallel or convergent beam illumination conditions.

Typically, two types of diffraction patterns could be obtained at parallel, or close to parallel, illumination: selected area diffraction pattern (SAD) and microdiffraction (nanoprobe diffraction). The diffracted pattern obtained upon the convergent beam illumination condition is called convergent beam diffraction pattern (CBED).
SAD is obtained in the TEM mode of the microscope, when specimen is illuminated by parallel electron beam and special selected area aperture is used to limit the area from which diffraction pattern is acquired. Because of some optical limitations, even for the best modern microscopes, it is impossible to use this technique to study the regions smaller then 0.1-0.5 Ám.
Microdiffraction technique was developed to overcome this limitation. In this method the smallest possible probe is formed by the illumination system and optics of the column keeps the beam close to parallel. Thus, modern microscopes are suitable to acquire the diffraction information from the areas of tens nanometers size.
Due to the convergent beam illumination condition, the diffraction information could be acquired from regions as small as nanometer and even sub-nanometer size. Therefore, this technique provides the highest spatial resolution for diffraction analysis. In addition, the CBED patterns contain more crystallographic information then SAD and microdiffraction. It comprises specimen thickness, unit cell and precise lattice parameters, crystal system and true 3D crystal symmetry (point group and space group) and enantiomorphism, if present.

For basic education and principle understanding of diffraction analysis in TEM, please, use following web resources:


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Energy Dispersive X-Ray Spectroscopy (EDS)

In our lab we use two type of EDS detectors: EDAX LN2 cooled Si(Li) detector and Oxford LN2 free X-MAX Silicon drift detector (SDD).
HR TEM Tecnai F20 G2 and ESEM Quanta 200 are equipped with EDAX Energy Dispersive X-Ray Spectrometers with SAPPHIRE Si(Li) detectors. TEM Tecnai F20 G2 is equipped with special EDAX retractable detector (r-TEM) designed for the high-energy environment of the TEM. Its spectral resolution is better than 135 eV. Spectral resolution of ESEM Quanta EDS with UT window is better than 132 eV, and its operating software Genesis allows EDS quantification at LV conditions.

HR SEM Sirion and UHR SEM Magellan are both equipped with the brand new large area (20 mm2) Silicon Drift Detectors X-Max 20 SDD with Inca Energy 450 software package (Oxford Instruments, UK). Spectral resolution of these detectors is better than 129 eV, they are mounted with SATW Light Element Window for detection of elements from boron at the count rates up to 100Kps.

An electron microscope uses a focused electron beam to interact with the atoms in a sample. One of the phenomena occurring in this interaction is generation of characteristic X-rays. When an element is bombarded with a particle beam, in this case, an electron beam, the specimen will release some of the absorbed energy as X-rays. Much of the time, the energy is the result of changes in the speed of an electron, which is random; however, when this interaction removes an electron from a specimen's atom, frequently a vacancy in the inner electron shell appears. In order to return the atom to its normal state, an electron from an outer atomic shell "drops" into the vacancy in the inner shell. This drop results in the loss of a specific amount of energy, namely, the difference in energy between the vacant shell and the shell contributing the electron. This energy is given up in the form of electromagnetic radiation X-rays. Since energy levels in all elements are different, element-specific, or characteristic, X-rays are generated.

Energy Dispersive X-ray Microanalysis uses detection equipment to measure the energy values of the characteristic X-rays generated within the electron microscope. Using semiconductor material (typically, Si/Li single crystal) to detect the X-rays and a multi-channel analyzer, an X-ray microanalysis system converts an X-ray energy into an electronic count. The accumulation of these energy counts creates a spectrum representing the chemical analysis of the sample. Therefore, while the electron microscope produces an image of the sample's topography, energy dispersive X-ray microanalysis tells the microscopist what elements are present in the sample.

For basic education and principle understanding of Microanalysis, please, use following web resources:


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Electron Energy Loss Spectroscopy (EELS)

Electron Energy Loss Spectroscopy (EELS) is the analysis of the energy distribution of electrons that have interacted inelastically with the specimen. There inelastic collisions carries an information about the electron structure of the specimen atoms, which in turn revels details of the nature of these atoms, their bonding and nearest neighbor distributions, and their dielectric response. In order to examine the spectrum of electron energies, a magnetic prism spectrometer is used as an additional electromagnetic lens in TEM. The magnetic prism is highly sensitive device with resolving power of approximately 1 eV, which is sufficient to distinguish all the elements in the periodic table. Magnetic field of spectrometer deflects the trajectories of transmitted electrons through 900, therefore the electrons with different energy losses exit at different points and spectrum is formed in the dispersion plane. The spectrum consists of a distribution of electron accounts versus energy loss. This process is exactly analogous to the dispersion of white light by glass prism. EELS spectrum has specific features of absorption spectrum, because, unlike EDS and other microanalytical techniques, EELS concerns with detecting the primary event, namely the loss of electron energy due to an inelastic interaction and not a secondary event connected with the return of the atom to ground state.

Gatan Imaging Filter (GIF) produces energy filtered electron images and diffraction patterns, as well as electron energy loss spectra (EELS). One of the major applications of the imaging filter is in chemical microanalysis, where the spectra provide information about the different chemical elements present, and the images show how the elements are distributed in the sample. Another important application of the GIF is improving the contrast of images and diffraction patterns by removing the inelastic scattering contribution.

For basic education and principle understanding of the Electron Energy Loss Spectroscopy, please, refer to the following web resources:


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