Transmission electron microscope (TEM)

Transmission Electron Microscope (TEM)
Presented By :Presented By :
Mr. Sanjeet Kumar Paswan
Research Scholar
Department of Nanoscience & Technology
Central University of Jharkhand , Ranchi-835205

 Introduction
 Theoretical Background
 Transmission Electron Microscope (TEM)
 Electron Gun & Condensor Aperture
 Condensor Lens, Objective Lens, Fluorescent Screen and vacuum System
 Resolution and Abbe’s Equation
Difference between Light Microscopy, TEM and SEM
 Electron Matter Interactions
 Different types of TEM Technique
 Principle of TEM
OUTLINE
 Principle of TEM
 TEM Operation
 TEM Specimen Holder
 Limitation of TEM
 Advantages & Disadvantages
 Application of TEM
 High Resolution TEM
 Working Principle of HRTEM
 Selected Area Electron Diffraction Pattern
 References

Introduction
 Microscopes are useful to investigate morphology, size, structure and even
composition of solids depending upon the type of microscope and microscope
are used to see objects that cannot be seen by naked eyes, the range can be
between mm to nm.
 Some of the powerful microscopes are able to resolve structures up to
atomic resolution.
 Optical microscope
 Confocal microscope
 Scanning Probe Microscope (SPM) Scanning Probe Microscope (SPM)
 Atomic Force Microscope (AFM)
 Scanning Near-Field Optical Microscope (SNOM)
 Electron Microscope
1. Transmission Electron Microscope (TEM)
1. High Resolution TEM
2. SAED

Theoretical Background
Q. Why we need Electron microscope
 Light microscopes are limited by the physics of light to 500X or 1000X
magnification and a resolution of 0.2μm
 In the early 1030’s there was a scientific desire to see the fine details of the
interior structures of organic cells (nucleus, mitochondria ..etc).
 This required 10,000X plus magnification which was just not possible using
light Microscopes.light Microscopes.
 Light Microscope, resolving power is 0.25 μm, maximum magnification is
about 250 μm/0.25 μm = 1000X. Any magnification above this value represents
empty magnification.
 But in TEM at 60,000 volts has a resolving power of about 0.0025nm.
Maximum useful magnification of about 100 million times.
 In Light Microscope : Optical glass lens, Small depth of field , Lower
magnification, Do not require vacuum and Low price.
 In Electron Microscope : Magnetic lens, Large depth of field, Higher
magnification and better resolution, Operates in High vacuum and Price tag.

 TEM is a microscopy technique where beam
of electrons is transmitted through a ultra thin
specimen. An image is formed from the
interaction of electrons transmitted through the
specimen; the image is magnified and focused
onto an imaging device, such as a fluorescent
screen, on a layers of photographic film or to be
detected by a sensor such as a CCD Camera.
 A TEM can appear in several different forms,
such as HRTEM, STEM, and EFTEM.such as HRTEM, STEM, and EFTEM.
 Significant impact on fields such as:
materials science, biological science, medical
science, geology, environmental science, among
others.
 Can be used for investigating the morphology
and structure in physical and biological science.
 Also enables the investigation of crystal
structures, orientations and chemical
compositions of phases and nano-structured
materials
 Highly energetic beam of electrons used in
TEM

What can be observed by TEM:
• Thin films and foils;
• meso- micro- and nanoparticles;
• biological specimens;
TEM Hitachi HT 7700
Basic requirements for TEM
specimens:
• specimen thickness max 0.1 um;
• Stability under the electron beam
and vacuum influence.

Electron Gun & Condensor Aperture
 Electron beam is generated in the electron gun. The
function of electron gun is to provide an intense beam of
high energy electrons. There are two basic types of electron
gun are used.
 Thermionic Gun : Based on two types of filaments (1)
Tungsten (w) and (2) Lanthanum Hexaboride (LaB6)
 Field Emission Gun : Employs either a thermally assisted
cold field emitter or Schottky emitter.
 The Condensor Aperture controls the fraction of the
beam which is allowed to hit the specimen. It therefore helps
to control the intensity of illumination.
 Apertures in each lens limit the amount of electrons
striking the specimen (protecting it from excessive
irradiation) and limit the number of x-rays generated from
electrons hitting parts of the microscope column.

Condensor Lens, Objective Lens, Fluorescent
Screen and vacuum SystemCondensor Lens
 Illuminates the specimen.
 Relatively weak lens.
 Longer focal length than Objective lens.
 May bring electron beam into focus directly upon specimen, above the specimen or
below the specimen.
Objective Lens
 Strong lens.
 It has highly concentrated magnetic field and short focal length.
 Total magnification in the TEM is a combination of the magnification from the Total magnification in the TEM is a combination of the magnification from the
objective lens times the magnification of the projector lens. Each of which is capable of
approximately 100X.
Mob Mint Mproj = Total Mag
Fluorescent Screen
 In TEM, screen coated with a material in the visible range Ex: ZnS is installed beneath
the projector lens in the path of the electron beam.
 Screen emits visible light when bombarded with electrons.
Vacuum System
 Electron can not travel more than a few Å without colliding with gas molecules.
 Distance b/w photographic plate & electron gun is approximately 1m.
 Two types of vacuum pump are used: (1) Rotary (2) Diffusion pump

Resolution and Abbe’s Equation
The limit of Resolution is defined as the minimum distances by which two structures
can be separated and still appear as two distinct objects. Ernst Abbe proved that the
limit of resolution depends on the wavelength of the illumination source. At certain
wavelength, when resolution exceeds the limit, the magnified image blurs. Because of
diffraction and interference, a point of light cannot be focused as a perfect dot.
Instead, the image will have the appearance of a larger diameter than the source,
consisting of a disk composed of concentric circles with diminishing intensity. This is
known as an Airy disk.
Abbe’s equation
Fig. Illustration of resolution in (a) Airy disk and (b) wave front.
Abbe’s equation
d = resolution
λ = wavelength of imaging
radiation
n = index of refraction
α = half aperture angle in
radians

Difference between LM, TEM and SEM

Electron Matter Interactions
Scattering Characterization of
TEM :
 Elastic Scattering is usually
coherent if the specimen is thin and
crystalline.
 Inelastic Scattering electrons are
usually almost incoherent.
 Elastic Scattering usually occurs
at relatively low angles (1-10) that isat relatively low angles (1-10) that is
in the forward direction.
 At high angles (>10) elastic
scattering becomes more
incoherent.
 As the specimen gets thin, less
electrons are formed scattered and
more and back scattered until
primarily incoherent back
scattering.

Different types of TEM Technique
Diffraction
 Selected-area diffraction (SAD)
 Convergent beam electron diffraction (CBED)
Imaging
 Bright/Dark field image
 High-resolution TEM (HRTEM)
 Scanning TEM (STEM Scanning TEM (STEM
Spectroscopy
 Energy-dispersive X-ray (EDX) spectroscopy
 Electron Energy-loss Spectroscopy (EELS)
Other techniques
 3D Tomography
 Cryo-TEM

Diffractions
 Imagining of tiny structures in a thin
specimen and diffraction pattern of
the same structures, one of the main
advantage of TEM.
 The basis of all image formation in
the TEM.
 Diffraction pattern (DP) formed in
back focal plane of objective lens. The
location of back focal plane
determined by strength of objective
Fig. There are several kinds of diffraction pattern
determined by strength of objective
lens.
 Intermediate lens must focus at this
point.
DP can be used :
 Crystallographic
 Orientation of crystal
 Analysis of interfaces
 Twinning and certain
crystal defects.

Convergent beam electron diffraction (CBED)
 Very useful for nanocrystalline materials

Bright Field & Dark Field
 Bright Field image formed from direct beam and Dark Field image formed from
the diffracted beam

Principle of TEM
 In TEM, a highly collimated beam of electrons incident on a sample of thickness < 100
μm and diameter 3mm.
 Different characteristic signals are obtained after interaction of electron beam with the
sample.
 TEM images are formed using transmitted electrons (instead of the visible light) which
can produce magnification details up to 1,000,000 X with resolution better than 10Å.
 A series of powerful magnetic lenses above and below the sample position are
responsible for delivering the signal to the detector fluorescent screen, a film plate and
camera.
 X-ray produced by the interaction between the accelerated electrons with the sample X-ray produced by the interaction between the accelerated electrons with the sample
allows determining the elemental composition of the sample with high spatial resolution.
 The signal transmission is a magnification of the spatial information in the signal by as
little as 50 times to as much as a factor of 106.
 The remarkable magnification range is facilitated by the small wavelength of the
incidents electrons and it is associated with TEM.
 The ray paths of both unscattered and scattered electrons beneath the sample.
The higher the operating voltage of a TEM instrument, the greater its lateral spatial
resolution.
 The theoretical instrument point to point resolution is proportional to λ3/4.

TEM Operation
 TEM offers two method of specimen observation (a) to form diffraction
patterns by using selected area apertures and focusing the intermediate lens on
the diffraction pattern formed in the back focal plane of the objective lens. (b) to
form images by bright field , dark field , or lattice image phase contrast modes-
 Diffraction mode of operation : In diffraction mode an electron diffraction
pattern is obtained on the fluorescent screen.
 Imaging mode of operation : Imaging mode produce an image of the
illuminated the sample area. There are three types of imaging :illuminated the sample area. There are three types of imaging :
1). Mass thickness contrast : Incoherent scattering from the sample and Z –
contrast imagining
2). Diffraction contrast : Either the direct beam or one of the diffracted beams
is selected to form image.
3). Phase contrast : Direct and diffracted beams undergo phase shifts in the
material.

TEM Specimen Holder
Grid hole
Specimen Holder
 In TEM the orientation of the thin foil
maybe varied using the sample
manipulation capabilities of tilting
specimen holder.
 Holder come with the range of tilt
capabilities including single axis tilt, double
axis tilt and tilt rotate stages with
upto ±60 capabilities.
 The higher resolution of the instrument
Supporting grid for
TEM specimens
 The higher resolution of the instrument
limited the tilting capabilities (to as low as
±10).
 For the studies of single crystal or
epitaxial thin film. Its important to had
access to as much tilts to capabilities as
possible.
 Other types of holder Cooling, Heating,
Streaming and Low emission (Be) holder.

Limitation of TEM
Sampling (0.3mm3) :
 Very small sample size. But fortunately we are dealing with nanostructures.
Interpreting transmission images:
 TEM presents 2D images of 3D specimens.
Electron beam damage and safety:
 TEM is a potential dangerous instrument that generates radiation level that
is enough to kill human being.
Specimen preparation:Specimen preparation:
 Your specimens have to be thin, very thin below 100nm (has to be electron
transparent). If you are going to get any information.
 Limited depth of resolution.
 Electron Scattering originated from the 3D sample but projected on 2D
detection.
 Sample preparation is very tedious (Hard).
 Estimated analysis time varies for 3 to 30 hrs per specimen (not include
sample preparation).
 Highly skilled operator required for operation.

Advantages & Disadvantages
Advantages :
 Highly lateral and spatial resolution ( normally larger than 0.2 nm point to point).
 Aberration corrected TEM gives 0.07nm resolution.
 TEMs offer the most powerful magnification, potentially over one million times or
more.
 TEMs provide information on element and compound structure.
 They are easy to operate with proper training.
TEMs are able to yield information of surface features, shape, size and structure.
 Gives image, diffraction and compositional information from the single sample.
 Highly energetic beam of electrons is used in TEM which interacts with sample and Highly energetic beam of electrons is used in TEM which interacts with sample and
produces signals of different characteristics. These signals are captured by different
detectors.
 In Situ : Studies like deformation, heating , cooling and irradiation etc is possible.
Disadvantages:
 To prepare an electron transparent sample fro the bulk is difficult ( due to the
conducting or electron density) and sample thickness.
 TEMs are large and very expensive.
 Laborious sample preparation.
 Operation and analysis requires special training.
 TEMs require special housing and maintenance.
 Images are black and white.

Application of TEM
 The imagining of any types of features i.e microstructural features at 100 to
10,00,000X in conventional TEM and HRTEM. But in Aberration corrected TEM is
5-10 million X.
 Qualitative and Quantitative elemental analysis of Microstructural/
Nanostructural features are possible by using special detector i.e HAADF and
EELS.
 Lattice imaging of crystal with the interplanar spacing > 0.1 nm is possible.
 Crystallographic information such as crystal structure orientation. Relationship
between different phases are possible by analyzing SAD pattern.between different phases are possible by analyzing SAD pattern.
 TEMs provide topographical, morphological, compositional and crystalline
information.
 The images allow researchers to view samples on a molecular level, making it
possible to analyze structure and texture.
 This provide information which is useful in the study of crystals and metals, but
also has industrial applications.
 Crystal defect characterization
 Ultra small area elemental analysis by EDX and EELS.
 Failure analysis of integrated circuits.

High Resolution TEM
 The image is formed by the interference of the diffracted beam
with the direct beam (phase contrast image) .
 The interpretation of HRTEM images has to be confirmed by
image simulation, like JEMS.
 Typically requires very thin TEM specimens free of preparation
artefacts. <50 nm (the optimum is 5 ~20 nm).
HRTEM
HRTEM image :
Dark – Metal
atoms columns.
HAADF-STEM : Bright Spots – Metal Atoms.

Working Principle of HRTEM
 The basic principle involved in the image formation in
both the microscopes (TEM and HRTE<) is similar.
 However, HRTEM provides high resolution images at
atomic scale level.
 Most precisely, HRTEM is a type of TEM.
 The HRTEM uses both the transmitted and scattered
beams to create an interference image. It is a phase
contrast image & can be as small as the unit cell of
crystal.
 All electrons emerging from the specimen are All electrons emerging from the specimen are
combined at a point in the image plane.
 Consider a very thin slice of crystal that has been tilted so that a low index direction is
exactly perpendicular to the electron beam. All lattice planes about parallel to the electron
beam will be close enough to the Bragg position and will diffract the primary beam.
 The diffraction pattern is the Fourier Transform of the periodic potential for the electrons
in 2D. In the objective lens , all diffracted beams and the primary beam are brought together
again: their interferences provide a back transformation and leads to an enlarged picture of
the periodic potential.
 This picture is magnified by the following electron optical system and finally seen on the
screen at magnifications of typically 106.

Selected Area Electron Diffraction Pattern
 SAED Pattern which will guide us about
the indices based on which we can calculate
the d-spacing. Additionally the pattern also
informs us about the crystallinity of the
samples. The brighter the spots the
crystalline is the particles.
 SAED pattern:
 (i) if your sample is amprohous (diffuse
rings).
 (ii) Crystalline (bright spots)
 (iii) Polynanocrystalline (small spots (iii) Polynanocrystalline (small spots
making up a rings)
How to Calculate SAED Pattern ?
1. Measure the diameter (2R) of each ring using some software such as Image tool.
Ex: if 2R = 5.37 [1/nm]
2. Obtain the value of radius R (w.r.t central spot); R= 2.68 [1/nm], the unit 1/nm means the
distance is in reciprocal lattice.
3. Obtain the interplanar distance (d) in real space as 1/R; so d= 1/R = 1/2.68 =0.372 nm.
4. Now you found the d value for 1 ring. Do the same for procedure to obtain d-values for
other rings.
5. Ex: TiO2, I know (by comparison with d-value of different phases in literature) that a d-
value of 0.372 correspond to (101) plane.

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