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
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
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:
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
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
where n is integer,
is electron wavelength, d is crystal lattice spacing between atomic
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 =
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
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,
For basic education and principle understanding of diffraction
analysis in TEM, please, use following web resources:
Energy Dispersive X-Ray Spectroscopy
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:
Electron Energy Loss Spectroscopy
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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