2189341 Material Characterisation - WordPress.com · 2020. 8. 30. · Materials characterization using nondestructive evaluation (NDE) methods. Woodhead Publishing, 2016. Consists

Transmission Electron Microscopy

(TEM)

Lecturer: Charusluk Viphavakit, PhD

ISE, Chulalongkorn University

Email: [email protected]: https://charuslukv.wordpress.com

2189341 Material Characterisation

Transmission Electron Microscopy (TEM)

2189341 Material Characterisation 2https://charuslukv.wordpress.com

❑ A technique of choice for analysis of specimen internal microstructure, evaluation of nanostructures such as particles, fibers, and thin films, and imaging of atoms.

Transmission light microscope Transmission Electron Microscopy

Specimen requirements Polished surface and thin section (<100μm)

Thin section (<200 nm)

Best resolution ∼200 nm ∼0.2 nm

Magnification range 2–2000x 500–1,000,000x

Source of illumination

Lens

Operation environment

Image formation for viewing

Transmission Electron Microscopy (TEM)


Instrumentation


❑ Optical arrangement

X-ray detector

Magnetic prism

Anode

Electron source

Condenser aperture

STEM scan coils

BF, ADF, HAADF detector

Sample

CCD camera

Inkson, B. J. "Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for materials characterization." Materials characterization using nondestructive evaluation (NDE) methods. Woodhead Publishing, 2016.

➢ Consists of the electron gun, electromagnetic lenses to focus the electrons before and after the specimen, fluorescent screen and a transmitted electron detection system in high vacuum chamber.

Objective aperture

Selected area aperture

EELS detectorCCD camera or

photographic plates

Viewing screen

Instrumentation


➢ Electron gun

▪ The electron gun in TEM typically accelerates electrons through 80-300 kV accelerating voltage (1-30 kV for SEM) to give them sufficient energy to pass through up to 1 µm of material.

Acceleration voltage (kV) Electron wavelength (nm) TEM resolution (nm)

40 0.00601 0.56

60 0.00487 0.46

80 0.00418 0.39

100 0.00370 0.35

200 0.00251 0.24

500 0.00142 0.13

Instrumentation


➢ Control of the e- beam

▪ TEM microscopes have many more electromagnetic lenses than SEM.

Objective lens

Condenser lenses

Sample

Projector lens

▪ Condenser lenses before the specimen focus the electrons into a beam of controlled diameter and convergence.

▪ Objective lens focuses the transmitted electrons to form the diffraction pattern and first image.

▪ Projector lenses then magnify the image/diffraction pattern onto the detection system.

Instrumentation


➢ Aperture

▪ A piece of metal with a small hole in the center.

▪ To limit light scattering and/or to select the non-diffracted or diffracted beams.

▪ The hole diameter of the objective aperture in the TEM is only in the order of tens of micrometers.

Condenser aperture

Sample

Objective aperture


Instrumentation


➢ Specimen and stage

▪ TEM specimen holders designed for specimens of maximum 3 mm diameter and maximum 200 µm thickness in the electron beam direction.

▪ All TEM specimens must have regions of electron transparent material through which the electrons can be transmitted (electronically transparent).

Instrumentation



Copper TEM grid

▪ Smaller specimens can be mounted on a 3-mm mesh disc.

▪ The meshes, usually made from copper, prevent the specimens from falling into the TEM vacuum column.

▪ A copper mesh can be coated with a thin film of amorphous carbon in order to hold specimen pieces even smaller than the mesh size.

3 mmRim mark

Centre mark

Carbon

Grid

Instrumentation



▪ Specialized grids include:

✓ Bar grids✓ Mixed bar grids✓ Folding grids (Oyster grids)✓ Slot grids✓ Hexagonal grids✓ Finder grids✓ Support films

Instrumentation



▪ The specimen holder is a rod-like device that not only holds the specimen but also is able to tilt it for better viewing inside the TEM column.

▪ Types of specimen holders used include single-tilt and double-tilt.

✓ The single-tilt holder allows a specimen to be titled along one axis in the disc plane.

✓ The double-tilt holder allows the specimen be tilted independently along two axes in the disc plane. It is necessary for analytical studies of crystalline structures.

Instrumentation


➢ Detection of electrons

▪ The image can be monitored live on a phosphor screen, or a wide-angle camera.

▪ All modern TEMs are fitted with digital electron detection systems, the most common being charged coupled devices (CCD), which convert incoming electrons into an electronic pulse per pixel.

Magnetic prism

BF, ADF, HAADF detector

CCD camera

EELS detectorCCD camera or

photographic plates

Viewing screen

Instrumentation


Specimen preparation


❑ TEM specimen preparation is a varied and often complex job.

➢ Materials made of heavy elements (high Z-number) may need to be 100 nm or thinner to transmit electrons.

➢ True nondestructive characterization by TEM is therefore only possible for specimens such as nanoparticles, nanofibers, and light materials such as molecules and nanocarbons, which are electron transparent in their natural state.

➢ Specimen cleanliness and specimen handling protocols are even more important for TEM than SEM, especially if the goal is sub-nm spatial resolution.



❑ Pre-Thinning: The process of reducing the specimen thickness to about 0.1mm before final thinning to 100 nm thickness.

Leng, Yang. Materials characterization: introduction to microscopic and spectroscopic methods. John Wiley & Sons, 2009.

❑ Final Thinning

➢ Electrolytic thinning for metal specimens that are good electric conductors.

➢ Ion milling for metal and ceramic specimens.

➢ Ultramicrotome cutting for polymeric and biological specimens.



Ion mill to make thin (< 0.1mm) area or hole

Ar+

Final thinning

SpecimenGuard ringLock ring

Central post



Ion milling

3 mm

300 µm

100 µm

20 µm

ArFinal thinning



▪ Electrolytic thinning: Methods for reducing specimen thickness to the scale of 100 nm.


➢ Create a dimpled area on pre-thinned specimens because it is almost impossible to reduce the thickness of specimens uniformly to the level of electron transparency.

Anode

Sample holder for disk specimen

Light guideJet (cathode) stream

➢ A specimen is placed in an electrochemical cell with the specimen as anode. A suitable electrolyte (usually an acidic solution) is used to electrochemically reduce specimen thickness.

Electrochemical jet-polishing

➢ Only the edge of a tiny hole contains thin sections of electron transparency

3–15 minutes



▪ Ion Milling: Methods for reducing specimen thickness to the scale of 100 nm.


➢ Ion milling uses a beam of energetic ions to bombard specimen surfaces in order to reduce the thickness by knocking atoms out of a specimen.

➢ Before ion milling, the specimen is often ground with a dimple grinding device in order to reduce the thickness in the central are of specimen.

Ion milling

➢ An ion beam with energy of 1–10 keV bombards the specimen. The specimen is placed in the center at an angle of about 5–30◦ to the ion beam in order have a high yield of sputtering.

Laser autoterminator

Ar ion

stage

Laser light

Specimen



Ion milling machine



▪ Ultramicrotomy: Methods to section a specimen to the 100 nm scale.


➢ A specimen is mounted in a holder against the cutting tool (glass knife or diamond knife).

➢ The specimen should be trimmed to have a tip held against the knife. The cross-section of the trimmed tip usually is only about 1 mm2

for diamond knife cutting.

Ultramicrotomy

➢ The holder gradually moves toward the knife while it repeatedly moves up and down. The firmly mounted specimen is sectioned as it passes the edge of the knife blade.

Arm

Back-and-forth motion

Up-and-down motion

Specimen

Diamond knife

Sections

Water



Ultramicrotomy machine

Segment arc to adjust sample to the knife edge

Specimen arm

Knife carrierGlass knife with water through

Pump to adjust water level

Specimen

Specimen arm movement

F. Shaapur, “An Introduction to Basic Specimen Preparation Techniques for Electron Microscopy of Materials”, Arizona State University, (1997) http://www.asu.edu.class/csss

TEM modes


• A TEM can be operated in two modes including image mode and diffraction mode.

➢ In the image mode, the image of the specimen is focused and projected onto a fluorescent screen.

➢ In the diffraction mode, the diffraction pattern formed on the back-focal plane is projected onto a fluorescent screen


TEM imaging


▪ The contrast in TEM images is determined by the geometry of the electron illumination, the electron scattering within the specimen, and the path of the transmitted electrons through the post specimen lenses, apertures, and detection system

➢ Bright-field (BF) imaging➢ Electron diffraction➢ High-resolution TEM➢ Scanning TEM (STEM)➢ High angle annular dark field (HAADF)

TEM imaging


❑ Image mode: Bright-field (BF) imaging

➢ Scattered electrons can be blocked by using an aperture (objective aperture) positioned in the back focal plane of the objective lens. A BF image is formed from only unscattered electrons.

Condenser aperture

Sample

Objective aperture


Objective lens

TEM imaging


❑ Image mode: Bright-field (BF) imaging

➢ Scattered electrons can be blocked by using an aperture (objective aperture) positioned in the back focal plane of the objective lens. A BF image is formed from only unscattered electrons.

➢ In a BF image, areas of the specimen which are actively scattering have fewer electrons, and therefore darker contrast.

➢ These include regions of increased thickness, increased mass (high Z-number/density), grain boundaries, and dislocations.

http://www.microscopy.ethz.ch/TEM_BF.htm

Optical axis

Bright-field image

Diffraction pattern

TEM imaging


❑ Image mode: Dark-field (DF) imaging

➢ The objective aperture can also be placed in the back focal plane to block unscattered electrons and choose a selection of scattered electrons to form a dark-field (DF) image.

➢ DF images are used to map regions of sample that generate specific electron scattering (those that pass through the aperture), which appear bright in the DF image.


Optical axis

Bright-field image

Diffraction pattern

TEM imaging


❑ Diffraction mode: Electron diffraction

➢ Constructive and destructive interference of coherent elastically scattered electrons can generate strong beams of transmitted electrons at specific angles to the incoming electrons determined by the crystal structure and orientation of the material. This phenomenon is called electron diffraction or Bragg scattering.

➢ When the orientation of a crystal is in a favorable orientation with respect to the incoming electron beam, elastically scattered electrons can be focused by the post specimen lenses to form an electron diffraction pattern.

TEM imaging



➢ The diffraction contrast can generate bright-field and dark-field TEM images.

➢ A bright-field image is obtained by allowing only the transmitted beam to pass the objective aperture. A dark-field image, however, is obtained by allowing one diffraction beam to pass the objective aperture.


Diffraction pattern

Bright-field Dark-field

Transmitted

DiffractedObjective aperture

TEM imaging



➢ The TEM diffraction pattern exhibits a plane of the reciprocal lattice of crystal.➢ Such a pattern is obtained by diffraction from a selected area in the specimen.


Polycrystalline

Bright-fieldDiffraction pattern

Single crystal

Bright-fieldDiffraction pattern

TEM imaging


▪ Selected Area Diffraction (SAD)

➢ Electron diffraction is not only useful to generate images of diffraction contrast, but also for crystal structure analysis.


➢ Single-Crystal Diffraction

• Index the pattern in order to know the crystal structure or crystal orientation that the pattern represents.

• Assign a set of ℎ, 𝑘 and 𝑙 to each diffraction spot in the pattern.

Incident

(ℎ𝑘𝑙)

1

𝜆

Specimen

Camera length (L)

R

g

Ewald shape1

𝜆≫ 𝑔

2𝜃

TEM imaging

2189341 Material Characterisation 32https://charuslukv.wordpress.comLeng, Yang. Materials characterization: introduction to microscopic and spectroscopic methods. John Wiley & Sons, 2009.

• In the pattern, the central spot of the transmitted beam is brightest and should be indexed as (0 0 0) of the reciprocal lattice plane𝑅𝑚 𝑅𝑛

• Then, we can select two spots, for example 𝑚 and 𝑛 in the pattern to measure their 𝑅 lengths and angle between them.

𝑅𝑚, 𝑅𝑛, 𝜃𝑚𝑛

• The indices of the spots can be determined by the following relationship for cubic crystals such as NaCl.

𝑅(ℎ𝑘𝑙) =𝜆𝐿 ℎ2 + 𝑘2 + 𝑙2 1/2

𝑎

TEM imaging


• The wavelength of electrons (𝜆) for 200 kV, which is used to obtain the pattern, should be 0.00251 nm.

𝑅𝑚 𝑅𝑛• The lattice parameter of NaCl (𝑎) is 0.563 nm.

• The diffraction photograph is taken at the camera length (𝐿) of 1.0 m.

𝑅(ℎ𝑘𝑙) =𝜆𝐿 ℎ2 + 𝑘2 + 𝑙2 1/2

𝑎

𝑅𝑚 = 8.92 𝑚𝑚𝑅𝑛 = 12.6 𝑚𝑚𝜃𝑚𝑛 = 45°

NaCl.

TEM imaging


• NaCl exhibits a face-centered cubic (FCC) structure for which the diffraction pattern should not include (ℎ𝑘𝑙) indexes of mixed odd and even numbers.

Lattice type Diffraction present

Simple all

Body-centered (ℎ + 𝑘 + 𝑙) even

Face-centered ℎ, 𝑘 and 𝑙 unmixed

• Thus, there are no diffraction spots for 100, 110. . .

• We can use the structure extinction laws to determine whether a cubic crystal is FCC or body-centered cubic (BCC).

structure extinction rules

Real lattice Real lattice parameter

Simple cubic 𝑎

FCC 𝑎

BCC 𝑎

Reciprocal lattice Reciprocal lattice parameter

Simple cubic 𝑎∗ = Τ1 𝑎

BCC 𝑎∗ = Τ2 𝑎

FCC 𝑎∗ = Τ2 𝑎

• The reciprocal lattice of an FCC structure is a BCC type or vice-versa.

TEM imaging


• The wavelength of electrons (𝜆) for 200 kV, which is used to obtain the pattern, should be 0.00251 nm.

𝑅𝑚 𝑅𝑛• The lattice parameter of NaCl (𝑎) is 0.563 nm.

• The diffraction photograph is taken at the camera length (𝐿) of 1.0 m.

𝑅(ℎ𝑘𝑙) =𝜆𝐿 ℎ2 + 𝑘2 + 𝑙2 1/2

𝑎

𝑅𝑚 = 8.92 𝑚𝑚𝑅𝑛 = 12.6 𝑚𝑚𝜃𝑚𝑛 = 45°

NaCl.

TEM imaging




➢ Identification of Crystal Phases

• Three crystal structures may exist for calcium phosphates precipitated from supersaturated aqueous solutions:i. hydroxyapatite (HA)ii. octacalcium phosphate (OCP)iii. dicalcium phosphate (DCPD).

calcium phosphates precipitates

Image Diffraction pattern

TEM imaging


• Both patterns exhibit a d-spacing of about 0.68 nm, which is calculated from the 𝑅1length.

calcium phosphates precipitates

Diffraction pattern

𝑅2

𝑅1

Thus, the precipitates should only be either OCP or HA, not DCPD.

• The crystallographic data indicate that the (001) d-spacings of OCP and HA are about 0.68 nm, while DCPD does not have any d-spacing close to 0.68 nm.

𝒂

𝒃

𝑑1 = 0.68 𝑛𝑚𝑑2 = 0.94 𝑛𝑚𝜃12 = 90.3°

𝑑1 = 0.68 𝑛𝑚𝑑2 = 0.82 𝑛𝑚𝜃12 = 90°

TEM imaging


• The calculated d-spacing is 0.94 in Figure 𝑎and 0.82 nm in Figure 𝑏.

Diffraction pattern

𝑅2

𝑅1

Thus, Figure 𝑎 is likely to be OCP.

• The 𝑹2 length on Figure 𝑎 is smaller than that on Figure 𝑏.

𝑑1 = 0.68 𝑛𝑚𝑑2 = 0.94 𝑛𝑚𝜃12 = 90.3°

𝑑1 = 0.68 𝑛𝑚𝑑2 = 0.82 𝑛𝑚𝜃12 = 90°

𝑅(ℎ𝑘𝑙) =𝜆𝐿 ℎ2 + 𝑘2 + 𝑙2 1/2

𝑎

𝒂

𝒃• The crystallographic data show that only OCP exhibits d-spacing of 0.94 nm in the (ത110) plane.

Then, Figure 𝑏 is HA.

TEM imaging




➢ Multi-Crystal Diffraction

• If an area selected for diffraction contains a large number of single crystals that are randomly oriented, a ring pattern composed of tiny grains will be obtained in SAD.

polycrystalline specimen

BF ImageDiffraction pattern

• The ring formation in the multi-crystalline specimen is equivalent to the rotation of the SAD diffraction of a single crystal around the axis of the transmitted electron beam, because the grains in a polycrystalline specimen may show all orientations similar to the pattern produced by single crystal rotation.

TEM imaging



https://www.physicsforums.com/threads/kikuchi-lines-vs-kikuchi-bands-ebsd-in-sem.776916/

➢ Kikuchi Lines

• Pairs of parallel lines consisting of one bright and one dark line in the diffraction mode

• Kikuchi lines appear when the selected area for diffraction is moved to a thicker section in the specimen where the diffraction spots become weaker, or even disappear.

• Kikuchi lines result from inelastic scattering of electrons in specimens.

https://www.physicsforums.com/threads/kikuchi-lines-vs-kikuchi-bands-ebsd-in-sem.776916/

TEM imaging


❑ Phase contrast: High-resolution TEM (HRTEM)

➢ The TEM phase contrast produces the highest resolution of lattice and structure images for crystalline materials.

➢ If specimens are ultrathin, of the order of 100 nm

thickness or less, the elastic scattering dominates over inelastic scattering.

➢ It involves at least two electron waves (the transmitted beam and a diffraction beam) that are different in wave phase to interact with the crystal lattice diffract and form complex interference patterns .

HRTEM Image

TEM imaging


❑ Scanning TEM (STEM)

➢ Some TEM instruments are fitted with scan coils, which can scan a focused electron beam across the specimen.

➢ This STEM mode is extremely useful for carrying out sequential chemical analysis across areas of the specimen.

TEM imaging


❑ High angle annular dark field (HAADF)

➢ In STEM mode, unscattered electrons are collected to form BF images, and scattered electrons are collected to form ADF images.

➢ Electrons incoherently scattered through very high angles can be collected using a high angle annular dark field (HAADF) detector.

➢ HAADF images show very strong contrast changes due to local changes in atomic number of the specimen (Z-contrast), and can be used to analyze chemistry at the atomic scale.

TEM imaging


▪ Images of Crystal Defects

• One of the great advantages of a TEM is its ability to reveal dislocations, which are crystal line defects.

➢ DislocationsRandomly distributed

Lined up

Tangled cell walls

• The dislocations appear as fine, dark lines in the bright-field images and as bright lines in the dark-field images.

• The configurations of dislocations vary with specimens.

BF Image

TEM imaging



• An edge dislocation in a crystal generates local lattice distortion.

➢ Dislocations

• A portion of distorted lattice planes is severely bent near the dislocation core. A significant proportion of electrons in the transmitted beam will be deflected into the diffraction direction and the diffraction intensity immediately increases at such a portion of the lattice plane.

Dislocation core

• The formation of dislocation images can be graphically explained using an example of edge dislocations.

TEM imaging



• In the bright-field image, a dark image forms at point a near the dislocation core.

➢ Dislocations

Dislocation core

BF

inte

nsi

ty

a b

Yu, Qian, et al. "Origin of dramatic oxygen solute strengthening effect in titanium." Science 347.6222 (2015): 635-639.

Edge dislocation core in Ti. Oxygen atoms can be seen at the interstitial positions

TEM spectroscopy


❑ Energy-Dispersive X-ray Spectrometry (EDS)

➢ The spatial resolution of X-ray analysis in TEM is much higher than in SEM, because the X-rays can only originate from the very restricted thickness of the electron transparent TEM specimen (usually less than 200 nm thickness) rather than the large interaction volume in SEM.

➢ The local chemical composition of a material can be determined to an accuracy of approximately 0.1% weight, 10 times better than SEM.

TEM spectroscopy


❑ Electron Energy Loss Spectroscopy (EELS)

➢ Inelastic scattering inside the specimen causes the primary electrons to lose energy. This can be detected after the specimen by passing the transmitted electrons through energy filters to separate the electrons according to their energy loss.

➢ The electrons can then be counted according to their energy to form an EELS spectrum, with typical energy resolution of 0.5-1 eV.


Electron Diffraction(ED)

High-Resolution Transmission Electron Microscopy

(HR-TEM)

Bright- and Dark-Field Imaging(BF/DF imaging)

• Crystallographic Info• Internal ultrastructure• Nanostructure dispersion• Defect identification

• Interface structure• Defect structure

Energy-Dispersive X-ray Spectrometry (EDS)

• Elemental composition, mapping and line scans

• Chemical composition• Other Bonding info

Electron Energy Loss Spectroscopy (EELS)TEM

Advantages vs Disadvantages


Pros Cons

Very powerful magnification and resolution.

large and very expensive

A wide-range of applications and can be utilized in a variety of different scientific, educational and industrial fields

Laborious sample preparation

Provide information on element and compound structure.

Operation and analysis requires special training

Images are high-quality and detailed. Require special housing and maintenance

Images are black and white

Documents

2189341 Material Characterisation - WordPress.com · 2020. 8. 30. · Materials characterization using nondestructive evaluation (NDE) methods. Woodhead Publishing, 2016. Consists