transmission electron microscopy (tem) - .:: … · mms 8110803- karakterisasi material + lab...

MMS 8110803 - KARAKTERISASI MATERIAL + LAB

Dr. Ir. A. Herman Yuwono, M. Phil. Eng.

Departemen Metalurgi dan Material Fakultas Teknik Universitas Indonesia

Tel: +(62 21) 7863510 Fax : +(62 21) 7872350 Email: [email protected]

TRANSMISSION ELECTRON MICROSCOPY

(TEM)

MMS 8110803- KARAKTERISASI MATERIAL + LAB [email protected]

DEPARTEMEN METALURGI DAN MATERIAL FAKULTAS TEKNIK UNIVERSITAS INDONESIA

What is Microscopy?

Microscopy: A technique for making visible images of structures or

details too small to otherwise be seen by the human eye.

How is this done?

• diffraction, reflection, or refraction of radiation incident upon the subject

of study.

• diffraction, reflection, or refraction of secondary radiation generated

within the subject of study

A microscopist must learn how are images are:

• formed

• collected

• interpreted



Why Use Electrons?

The primary answer is ‘resolution’

Resolution: The limit of resolution is the smallest separation at which two

points can be seen as distinct entities.

Higher magnification will not necessarily give higher resolution.

Unless a microscope is equipped to deliver higher resolution images, higher

magnification will only achieve ‘empty’ images.



What is TEM?

Transmission Electron Microscopy (TEM) is a method of producing

‘images’ from a sample by illuminating the sample with electrons (in a

vacuum), and detecting the electrons that are transmitted through the

sample.

Pros:

• Provides information on crystalline,

amorphous, incommensurate,

and periodic structures at fine scales

• Can gather crystallographic and

chemical data at unit cell or near

unit cell scale

• Subatomic resolution is coming

Cons:

• Requires that the sample be cut

into thin slices and placed in a

vacuum, possibly resulting in

artifacts

• High resolution TEM is

expensive, requiring high electron

voltages

• Data interpretation is time-

consuming



Basic Configuration



What Knowledge is Needed to Conduct TEM?

Diffraction Physics: TEM is a diffraction method, not a ‘magnifying’

method. An understanding of basic optics is essential.

Crystallography: Because TEM relies upon diffraction a grasp of

crystallography cannot be avoided if the instrument-specimen interaction

is to be meaningful and lead to interpretable data.

Patience: TEM is a difficult technique in terms of both data gathering and

data interpretation. It will normally take a graduate student 9-12 months

to obtain their first good results assuming weekly access to the

microscope.



Capability

Resolution:

TEM Mode < 2 Å

STEM Mode < 5 Å

Crystallographic Information:

Electron diffraction - Selected Area Electron Diffraction (SAD)

- Convergent Beam Electron Diffraction (CBED)

Imaging - Lattice Imaging

- High Resolution Electron Microscopy (HREM)

Chemical Information:

X-rays - Energy Dispersive X-ray Spectroscopy (EDS)

- Atomic Location by Channeling Enhanced

Microanalysis (ALCHEMI)

Electrons - Electron Energy Loss Spectroscopy (EELS)



Signals



Instrumentation

Basic Requirement • Intermediate voltage (200, 300, 400 keV)

• High brightness electron source

LaB6, FEG, hairpin W (possibly)

Desirable Extras • Scanning TEM

• Energy Dispersive X-ray Detector Si(Li) or Ge(Li)

• Electron Energy Loss Spectrometer (EELS)

• X-ray or EELS mapping software

• TV and CCD image capture and image

processing

• Cold stage, hot stage, tensile stage etc.

• SE & BSE Detectors

Essential Extras • Diffraction and image simulation software

• Image processing software



Principles of image formation:

Typically, a TEM consists of an

electron gun, a condenser lens system,

a specimen chamber, objective and

intermediate lenses, projector system

for producing images and diffraction

patterns, vacuum and computer

systems

With an electron gun, an electron

beam is formed, which is accelerated

by an electric field formed by a

voltage difference of typically

100−200 kV. By condenser lenses,

the electron beam is focused on the

sample to be investigated, where the

electrons are scattered.



Electron scattered in the same

direction are focused in the back focal

plane, giving rise to a diffraction

pattern, while electrons coming from

the same point of the object are

focused in the image plane. The first

image, which is formed by the

objective lens, is magnified typically

by 25 times, and the following

intermediate and projective lenses

give a final magnification of the image

in the viewing screen of more than

106 times.

Principles of image formation

(cont):



Image observation

A TEM image can be formed by either: the central spot of un-

scattered beam, or some or all of the scattered electrons.

Depending on beam selection, there are four common types of TEM

image:

a. Bright Field (BF) image;

b. Dark Field (DF) image;

c. Selected Area Diffraction (SAD); and

d. Lattice or High-Resolution TEM (HRTEM) image.



Bright Field (BF) image involves

the insertion of an aperture on

the back focal plane of the

objective lens so that only the

un-scattered electrons can pass

through and contribute to the

formation of image. As a

consequence, thick regions of

the specimen in which heavy

atoms are enriched or higher in

density such as crystalline

phase(s) will scatter more

strongly and appear as darker

areas in the resultant image. In

the absence of a specimen, a

bright background is observed.

a. Bright Field (BF) image



Dark Field (DF) image involves the

displacement of aperture which

allows some of the scattered

electrons to pass through the

objective aperture, while the un-

scattered electrons are blocked.

This is termed as dark field

imaging because the background

appears dark, in the absence of a

specimen, providing a reverse

contrast to the bright field image.

By this technique, the diffracted

beam can have strong interaction

with the sample; enabling some

useful information be obtained,

such as planar defects, stacking

faults or crystallite size

b. Dark Field (DF) image



Comparison of BF and DF Images

30-fold quasi crystalline Al-Mn film grown by sequential deposition of Mn on a (111)-

oriented Al film at 260oC.



c. Selected area diffraction (SAD)

Selected Area Diffraction (SAD) is

obtained when a special aperture namely

SAD aperture is inserted. It encloses a

small area through which both un-

scattered and diffracted electrons

contribute to the image formation. The

diffraction pattern obtained on the back

focal plane of the objective lens

represents an image of reciprocal lattice

and therefore contains information about

crystal structure.



For the purpose of analysis, the Bragg’s law can be applied with only the

first order diffraction (i.e. n =1) is considered. This is assumed so since

the Bragg’s angle is very small due to the very short wavelength of the

electron beam used in TEM. Therefore the simplified Bragg’s law can be

written as:

l = 2dq

For electrons which are diffracted through an angle q by the crystal

planes of spacing d and hit the screen or film which is a distance L from

the specimen, the resulting distance r from the un-diffracted spot at the

center can related by a simple geometry:

q2L

r



Combining the previous two equations results in:

or

Ll is called camera constant since the camera length L and the electron

wavelength l are independent of the specimen, and thus a constant.

The distance of a diffraction spot from the un-diffracted spot r, is therefore

inversely proportional to the d-spacing of the diffracting planes.

Given the camera constant, d can be determined simply by measuring r on

the pattern accurately.



Types of SAD Patterns

Diffraction type depends on the size of

the electron probe, as well as the crystallite size.



TEM image and corresponding SAD pattern of amorphous TiO2



TEM image and corresponding SAD pattern of crystalline TiO2



Crystalline diffraction pattern

from a twinned grain of FCC

Austenitic steel

Transmission Electron Micrograph of

Dislocations, which are faults in the

structure of the crystal lattice at the

atomic scale



d. Lattice or high-resolution TEM (HRTEM) image.

A lattice or High-Resolution TEM

(HRTEM) image is formed by the

interference between the diffraction

beam and un-scattered beam. In

practice, a larger objective aperture

is selected to allow both types of

beam to pass.

The incident parallel electrons

interact elastically when passing

through the sample, and the

resulting modulation of phase and

amplitude are present in the

electron wave leaving the sample.

This type of wave thus contains the

information about the object

structure.



d. Lattice or high-resolution TEM (HRTEM) image (cont)

Furthermore, all the diffracted beam and un-scattered beam are brought together

again in the objective lens and the Fourier transform (analysis) creates a

diffraction pattern of the object in the back focal plane.

The inverse Fourier transform (synthesis) is performed subsequently, making the

interference of diffracted beams back to a real space image in the image plane as

a lattice image.



Preparation of TEM

Specimen

In a TEM, the specimen you want to

look at must be of such a low density

that it allows electrons to travel

through the tissue.

There are different ways to prepare

your material for that purpose. You

can cut very thin slices of your

specimen from a piece of tissue

either by:

fixing it in plastic or working with it as

frozen material.



Negative Staining of Isolated

Material

The isolated material (can be a

solution with bacteria or a solution with

isolated molecules) is spread on a

support grid coated with plastic. A

solution of heavy metal salt is added.

The metal salt solution does not bind

to the material but forms a "shadow"

around it on the grid. The specimen

will appear as a negative picture when

viewing it in the TEM.



How do they look under microscope?

Hexagonal

Grid

Square Grid

•200 mesh

•55% open area,

•28 µm bar width

•Thick:17.8 µm

3 mm



TEM Grid Preparation

There are two sides in carbon coated ultra-thin grid:

1. Formvar side - lighter side (less hydrophobic than

the carbon film)

2. Carbon film is on the darker side.



TEM Grid Preparation

• Hold the grid with tweezers.

• Do not touch/contaminate the grid.

• Do not bend the grid.



Tem Grid Preparation

• Take a micropipette and put a droplet on top the grid. Let the

solution sit on the grid.

• After 10-12 min, wick away the rest of the solution with filter paper.

Do not touch the grid!

• Finally let it sit until the grid dries off.



TEM is a diffraction technique requiring a knowledge of optics and

crystallography to operate in a meaningful way.

TEM can collect signals from elastic and inelastic electron-matter

interactions to derive crystallographic and chemical information at atomic

or near-atomic scales.

The capability of TEM can be expanded by using hot stages, cold stages

and so on.

Image and/or spectral simulation are usually required for serious data

interpretation.

Summary

transmission electron microscopy (tem) - .:: … · mms 8110803- karakterisasi material + lab...

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