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Lecture 1
Introduction to TEM Techniques
What is TEM?
Transmission electron microscopy (TEM) is a microscopy technique in which a
beam of electrons is transmitted through an ultra-thin specimen, interacting
with the specimen as it passes through. An image is formed from the interaction
of the electrons transmitted through the specimen; the image is magnified
and focused onto an imaging device, such as a fluorescent screen, on a layer
of photographic film, or to be detected by a sensor such as a CCD camera.
Why Electron?
The resolution limit: It is the smallest distance Y separating two distinct objects.
Below, their image cannot be distinguished anymore. The smaller the wave
length is, the smaller Y is and the higher the number of details visible in the
image.
The idea is then to use radiation with a wave length as small as possible for
example an electron wave.
Resolution for an accelerated electron wave:
Abbe's equation. Resolution in a perfect optical system can be described
mathematically by Abbe's equation. In this equation:
d = _0.612 * l_
n sin a
where: d = resolution, l = wavelength of imaging radiation, n = index of refraction
of medium between point source and lens, relative to free space,
a = half the angle of the cone of light from specimen plane accepted by the
objective (half aperture angle in radians) and n sin a is often expressed as NA
(numerical aperture).
This is the diffraction-limited resolution of an optical system. If all aberrations
and distortions are eliminated from the optical system, this will be the limit to
resolution. If aberrations and distortions are present, they will determine the
practical limit to resolution.
De Broglie equation: By combining some of the principles of classical physics
with the quantum theory, de Broglie proposed that moving particles have wavelike properties and that their wavelength can be calculated, based on their mass
and energy levels. The general form of the de Broglie equation is as follows:
l = __h__
m*v
where: l = wavelength , h = Planck's constant (6.6 X 10-27), m = mass of the
particle (9.1 X 10-28) and v = velocity of the particle.
When an electron passes through a potential difference (accelerating voltage
field) V, its kinetic energy with be equal to the energy of the field, i.e. eV (energy
in electron volts) = V (the accelerating voltage). As you may recall, e = mc2. By
restating this for velocities below the speed of light and particles with true mass,
the energy of an electron may be stated as follows:
eV = 1/2 mv2
where: eV = energy in electron volts (e = 4.8 X 10-10), m = mass of the particle
and v = velocity of the particle
By using some assumptions about the velocity of the particle and its mass, it is
possible to express either wavelength (l) or velocity (v) in terms of the
accelerating voltage (V). By further substituting the values of h and m above, the
equation for l reduces to the following:
l = _1.23 nm_
V1/2
One caveat is that as the velocity of the electron approaches the speed of light,
Einstein's special equations of relativity need to be used for greater accuracy as
the mass and momentum of electrons increases with velocity.
Equation for resolution in TEM: This value for l can then be substituted into
Abbe's equation. Since angle a is usually very small, for example 10-2 radians (a
likely figure for TEM), the value of a approaches that of sin a, so we replace it.
Since n (refractive index) is essentially 1, we eliminate it, and we multiply 0.612
by 12.3 to obtain 0.753. Therefore, the equation reduces to the following:
d = __0.753__
a V1/2
where: d = resolution in nm, a = half aperture angle and V = accelerating velocity
Now, solving for 100,000 volts, the result is 0.24 nm or 2.4 Å. This improves with
higher accelerating voltage and gets worse with lower voltages. (Using
Einsteinian calculations, the resolution is: 0.22 nm or 2.2 Å.) Each lens and
aperture has its own set of aberrations and distortions. If aberrations and
distortions are present, they will determine the practical limit to resolution.
Thus at 100 eV we have .2 nm as the resolution which is the range of interatomic
distances thus enabling the user to examine very fine details.
What can be done with a TEM?
Experimental methodologies which employs (electron-optical) instrumentation
to spatially characterize matter and those which will be covered in the workshop
is given below:
Imaging:
Bright Field (BF) Microscopy:
Bright field imaging is the simplest form of microscopy where white light is either
passed through, or reflected off, a specimen. Illumination is not altered by
devices that alter the properties of light (such as polarizers or filters). The
contrast in the sample is caused by absorbance of some of the transmitted light
in dense areas of the sample. Bright-field microscopy’s simplicity makes it a
popular technique. The typical appearance of a bright-field microscopy image is
a dark sample (because of absorbance of the light by the sample) on a bright
background.
In biological applications, bright field observation is widely used for stained or
naturally pigmented or highly contrasted specimens mounted on a glass
microscope slide. The specimen is illuminated from below and observed from
above. The specimen appears bright, but darker than the bright background.
This technique is widely used in pathology to view fixed tissue sections or cell
films / smears. Bright field imaging is not very useful for unstained living cells or
unstained tissue sections as, in most cases, the light passes through transparent
or translucent samples with little or no definition of structure.
Light is reflected from opaque samples and this is exploited in industrial
environments where bright field imaging is used for wafer inspection and liquid
crystal board inspection.
Nowadays colored (usually blue) or polarizing filter on the light source are used
to highlight features not visible under white light. The use of filters is especially
useful with mineral samples.
Dark Field (DF) Microscopy:
Dark field microscopy (dark ground microscopy) describes microscopy methods,
in both light and electron microscopy, which excludes the unscattered beam
from the image whereas the scattered beam produces the image. As a result,
the field around the specimen (i.e., where there is no specimen to scatter the
beam) is generally dark.
Dark field optics are a low cost alternative to phase contrast optics. The contrast
and resolution obtained with inexpensive dark field equipment may be superior
to what you have with student grade phase contrast equipment. It is surprising
that few manufacturers and vendors promote the use of dark field optics.
Dark field illumination is most readily set up at low magnifications (up to 100x),
although it can be used with any dry objective lens. Any time you wish to view
everything in a liquid sample, debris and all, dark field is best. Even tiny dust
particles are obvious. Dark field is especially useful for finding cells in
suspension. Dark field makes it easy to obtain the correct focal plane at low
magnification for small, low contrast specimens.
High angle annular Dark Field (HAADF):
HAADF images are formed by collecting high-angle scattered electrons with an
annular dark-field detector in dedicated scanning transmission electron
microscopy (STEM) instruments.
The main difference between the traditional dark field imaging and HAADF is
that in case of dark field imaging, the objective aperture is placed in the
diffraction plane so as to only collect electrons scattered through this aperture,
thus avoiding the main beam. Whereas for HAADF the optics distinguishing
between bright field and dark field modes is positioned further downstream,
after the converged beam has interacted with the specimen. Consequently, the
contrast specimen.
An annular dark field image formed only by very high angle, incoherently
scattered electrons — as opposed to Bragg scattered electrons — is highly
sensitive to variations in the atomic number of atoms in the sample (Z-contrast
images). This technique is also known as high-angle annular dark-field imaging
(HAADF).
Scanning transmission Electron Microscopy (STEM):
Scanning transmission electron microscopy (STEM) combines the principles
of transmission electron microscopy and scanning electron microscopy and can
be performed on either type of instrument. Like TEM, STEM requires very thin
samples and looks primarily at beam electrons transmitted by the sample. The
main difference between STEM and TEM is that the former focuses electron
beams into a narrow spot which is scanned over the entire sample in a raster
pattern. One of its principal advantages over TEM is in enabling the use of other
of signals that cannot be spatially correlated in TEM, including secondary
electrons, scattered beam electrons, characteristic X-rays, and electron energy
loss, whereas its primary advantage over conventional SEM imaging is the
improvement in spatial resolution.
High-resolution transmission electron microscopy (HRTEM):
It is an imaging mode of the transmission electron microscope (TEM) that allows
for direct imaging of the atomic structure of the sample. HRTEM can provide
structural information at better than 0.2 nm spatial resolution. As a result it is
suitable for the study on atomic scale of the materials like semiconductors,
metals, nanoparticles, etc. At these small scales, a 2 dimensional projection of
individual atoms of a crystal and its defects can be resolved but it makes sense
only in some low index direction, so that atoms are exactly on top of each other.
For 3 dimensional analysis, several views can be combined from different angles
into a 3D map, this technique is called Electron Crystallography. One of the
limitations with HRTEM is that image formation relies on phase contrast, but
contrast is not necessarily interpretable because the image is influenced by the
aberrations of the imaging lenses in the microscope.
X-ray Diffraction:
It is a technique through which we can determine crystal structure and defects
present in a material, due to the crystalline atoms diffracting the incident x-rays
into many specific directions. By measuring the angles and intensities of these
diffracted beams, a 3D image of the electron density within the crystal can be
produced. This enables us to determine the mean positions of the atoms in the
crystals, chemical bonds and disorders.
Spectroscopy:
Spectroscopy is the study of the interaction of electromagnetic radiation in all
its forms with matter. By performing this dissection and analysis of an object's
light, the physical properties of that object (such as temperature, mass,
luminosity and composition) can be inferred.
X-ray Energy Dispersive Spectroscopy:
It is an analytical technique used for the elemental analysis or chemical
characterization of a sample. It relies on an interaction of some source of
X-ray excitation and a sample. The EDS technique detects x-rays emitted from
the sample during bombardment by an electron beam to characterize the
elemental composition of the analysed volume. Features or phases as small as 1
µm or less can be analysed.
When the sample is bombarded by the electron beam, electrons are ejected
from the atoms comprising the sample's surface. The resulting electron
vacancies are filled by electrons from a higher state, and an x-ray is emitted to
balance the energy difference between the two electrons' states. The x-ray
energy is characteristic of the element from which it was emitted.
The EDS x-ray detector measures the relative abundance of emitted x-rays
versus their energy. The detector is typically a lithium-drifted silicon, solid-state
device. When an incident x-ray strikes the detector, it creates a charge pulse
that is proportional to the energy of the x-ray. The charge pulse is converted to
a voltage pulse (which remains proportional to the x-ray energy) by a chargesensitive preamplifier. The signal is then sent to a multichannel analyser where
the pulses are sorted by voltage. The energy, as determined from the voltage
measurement, for each incident x-ray is sent to a computer for display and
further data evaluation. The spectrum of x-ray energy versus counts is evaluated
to determine the elemental composition of the sampled volume.
X-ray Wavelength Dispersive Spectrometry:
It measures and counts X-rays by their wavelength (a correlate of energy). A
wavelength spectrometer uses a crystal or grating with known spacing to diffract
characteristic X-rays. This technique is complementary to energy-dispersive
spectroscopy (EDS) but WDS spectrometers have significantly higher spectral
resolution and enhanced quantitative potential.
Electron Energy Loss Spectroscopy:
It measures the changes in the energy distribution of an electron beam
transmitted through a thin specimen. Each type of interaction between the
electron beam and the specimen produces a characteristic change in the energy
and angular distribution of scattered electrons. The energy loss process is the
primary interaction event. All other sources of analytical information (i.e. X-rays,
Auger electrons, etc.) are secondary products of the initial inelastic event. Thus,
EELS has the highest potential yield of information/inelastic event Energy Field
TEM.
The amount of energy loss can be measured via an electron spectrometer and
interpreted in terms of what caused the energy loss. Inelastic interactions
includes phonon excitations, inter and intra band transitions, plasmon
excitations, inner shell ionizations and Cherenkov radiation. The inner shell
ionizations are particularly useful for detecting the elemental components of a
material. With some care, and looking at a wide range of energy losses, one can
determine the types of atoms, and the numbers of atoms of each type, being
struck by the beam. The scattering angle (that is, the amount that the electron's
path is deflected) can also be measured, giving information about the dispersion
relation of whatever material excitation caused the inelastic scattering.
Energy Filtered Transmission Electron Microscopy (EFTEM):
It is a technique used in Transmission electron microscopy, in which only
electrons of particular kinetic energies are used to form the image or diffraction
pattern. The technique can be used to aid chemical analysis of the sample in
conjunction with complementary techniques such as electron crystallography.
When a very thin sample is illuminated with a beam of high-energy electrons,
then a majority of the electrons pass unhindered through the sample but some
interact with the sample, scattering elastically as well as inelastically
(phonon scattering, plasmon scattering or inner shell ionisation). Inelastic
scattering results in both a loss of energy and a change in momentum, which in
the case of inner shell ionisation is characteristic of the element in the sample.
If the electron beam emerging from the sample is passed through a magnetic
prism, then the flight path of the electrons will vary depending on their energy.
This technique is used to form spectra in Electron energy loss
spectroscopy (EELS), but it is also possible to place an adjustable slit to allow
only electrons with a certain range of energies through, and reform an image
using these electrons on a detector. This adjusted slit if allows only the passage
of electrons which did not lose their energy for the formation of image, then the
result we get is an enhanced contrast image.
Improved elemental maps can be obtained by taking a series of images, allowing
quantitative analysis and improved accuracy of mapping where more than one
element is involved. By taking a series of images, it is also possible to extract the
EELS profile from particular features.