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NCN > TEM > Flashcards

TEM Flashcards

1
Q

TEM

A

TEM is an analytical tool that allows detailed investigation of the morphology,structure, and local chemistry of metals, ceramics, polymers, biological materials and minerals. It also enables the investigation of crystal structures,
crystallographic orientations through electron diffraction, as well as second phase, precipitates and contaminants distribution by x-ray and electron-energy analysis

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2
Q

TEM overview

A

-Gun emits electrons
-Magnetic (and electric) field control path of electrons
-Electromagnetic lens which focuses the electrons into a very fine beam
-Electrons scatter when they pass through thin sections of a specimen
-Transmitted electrons (those that do not scatter) are used to produce image
-Denser regions in specimen, scatter more electrons and appear darker
-Spatial resolution normally achievable @ 200KeV: ~1Å
-The higher the accelerating voltage, the smaller the wavelength of the
electrons and the higher the possible achievable resolution.

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3
Q

Source Types

A
  • Tungsten
    – Wire filament
  • Lab6
    – Lanthanum hexaboride
    crystal
  • Field Emission Gun
    – Tungsten tip
    – Thermionic (Schotkky) and Cold
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4
Q

Field emission Guns

A

Allows for emission of electrons
from fine tip via charge differential
of tip and anodes:
* Brighter
* More coherent source
* High signal-to-noise
ratio and spatial resolution
* Increased emitter life
* Atomic diameter point source is
future of FE guns

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5
Q

Accelerating voltage

A
  • accelerating voltages range from 60 kV to 200 kV.
  • Higher accelerating voltages give higher resolution, but less
    contrast. High accelerating voltages can also result in greater
    specimen damage.
  • For these reasons, studies of biological samples tend to
    employ low accelerating voltages (60 kV to 100 kV), while
    studies of inorganic materials, which often require higher
    resolution, usually employ an accelerating voltage of 200 kV.
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6
Q

LENSES:

A

-Electromagnetic lenses
-Condenser lens
-Objective lens
-Intermediate lens
-Projector lens

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7
Q

Electromagnetic lenses

A

-All modern TEMs use electromagnetic lenses. These consist of a coil of copper wires inside iron pole pieces.
-A current through the coils creates a magnetic field, symbolized by red lines in the diagram on the left. This field acts as a convex lens, bringing off axis rays back to focus. Focal
length can be changed by changing the strength of the current.

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8
Q

Electron optics elements

A

–Lense: Focus (or defocus) the beam on the specimen and
change the magnification.
–Deflection coil: Shift or tilt the beam.
–Stigmators: Correct the lenses. Ideally lenses are round and
symmetrical but in practice there are small deviation which is
corrected by the stigmators.

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9
Q

Condenser lens

A

–Illuminates the specimen.
–Controls the diameter of the beam.
–Relatively weak lens.
–Longer focal length than objective or protector lens.
–May bring electron beam into focus directly upon specimen, above the specimen (over focusing) or below the specimen (under focusing).

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10
Q

Objective lens

A

–Strong lens.
–Most important lens
–Forms initial image further magnified by other lenses
–Responsible for focus
–Highly concentrated magnetic field and short focal length. Causes electron beam, which has passed through specimen, to focus at a point a few nanometers below
specimen.
–The larger the aperture used the more phase contrast
–The smaller the aperture the more
aperture contrast

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11
Q

Intermediate lens

A

–The first intermediate lens magnifies the initial image that is
formed by the objective lens.
–The lens can be focused on:
Initial image formed by the objective lens, or Diffraction pattern formed in the back focal plane of the objective lens.
–This determines whether the viewing screen of the microscope
shows a diffraction pattern or an image. You may switch focus between the image and the back focal plane

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12
Q

Projector lens

A

-The projector lens magnifies the
image magnified by the intermediate
lens, and then forms the final image
on the fluorescent screen or the
detector.
-increase current = spreads beam =
higher magnification)

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13
Q

Apertures

A

–The condenser aperture controls the fraction of the beam which is allowed to hit the specimen. It therefore helps to control the intensity of illumination.
–The objective aperture selects which beams in the diffraction pattern contribute to the image, thus producing diffraction contrast.
–The selected area aperture is used to selected a region of the specimen from which a diffraction pattern is obtained.

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14
Q

Bright field

A

–An aperture only allows unscattered electrons to proceed and form the image,the scattered ones are blocked. Dark regions are strongly diffracting or dispersing the light. This is the most common imaging
technique.
–Contrast in an image is created by differences in diffraction. By inserting an aperture in
the back focal plane, an image can be produced with these transmitted electrons.

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15
Q

Dark field

A

In dark field (DF) images, the
direct beam is blocked by the
aperture while one or more
diffracted beams are allowed to
pass the objective aperture. Since
diffracted beams have strongly
interacted with the specimen, very
useful information is present in DF
images, e.g., about planar defects,
stacking faults or particle size.

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16
Q

TEM – Diffraction techniques

A

-Spot Patterns
-Ring patterns

17
Q

Spot Patterns

A

–Are created when electrons are diffracted in a single crystal
region of a given specimen. The center spot corresponds to
the transmitted electron beam. Other spots are diffracted
portions of the initial electron beam.
–can be used for unknown phase identification and identification of
crystal structure and orientation.

18
Q

Ring Patterns

A

–created when electron diffraction occurs simultaneously from many grains with different orientations relative to the incident electron beam.
–used to identify unknown phases or characterize the crystallography of a material.

19
Q

TEM imaging – phase or mass contrast

A

—Diffraction contrast/Phase - some grains diffract more strongly than others; defects may affect diffraction
—Mass-thickness contrast - absorption/scattering. Thicker areas or materials with higher Z are darker

20
Q

Bright field (stained polymers)

A

-In the case of polymer and biological samples, due low atomic number and similar electron densities, staining helps to increase the imaging contrast and improves the radiation damage. Staining agents work by selective absorption in one of the phases and tends to stain unsaturated C-C bonds.

21
Q

TEM – Sample requisites

A

–Thin enough for electrons to penetrate without excessive energy loss.
–Be representative of the structure and composition.
–For penetration of a 200 kV electron beam, a typical metal, ceramic, or semiconductor specimen must be less than 100 nm thick.
–The specimen is required to represent the unaltered bulk material in terms
of structure, chemistry, and content of defects.
–The specimen to have large amounts of electron-transparent thin area, be flat and unbent, and be strong enough to be easily handled, the task of making such a specimen from an arbitrary material is very difficult.
–In the case of biological samples, the preservation of the fine structures without induced artifacts and the contrast generation are important issues.
–Uniform thickness over the area of
interest
–Any deformation from thinning process
or previous processing must be removed
–Small area, < 3 mm or less, to fit
various TEM grids and holders
–Clean samples free from contamination

22
Q

TEM samples can be as:

A

-Bulk Sample
-thin film
-Powders

23
Q

Mechanical preparation

A

-Sample cutting
-Mechanical grinding plus polishing
-Dimple grinding
-ion Milling

24
Q

Mechanical preparation Advantages

A

– No foreign material implanted
– With experience it is repeatable
and fast
– Lower cost of required equipment
– Decrease milling time and costs

25
Q

Mechanical preparation disadvantages

A

– Requires careful handling throughout process
– Greater learning curve than other
techniques
– Time consuming process

26
Q

TEM lamella – focused ion beam

A

FIB is also a powerful tool for reaching the nanoscale
Mainly applied at the semiconductor industry
Ions are significantly larger/heavier than electrons and a sputtering phenomenon occurs. The atoms are forced out Ion beam interaction with the surface = produces ions,
neutrons and electrons

27
Q

Electropolishing

A
  • Sample is immersed in an electrolyte and then subjected to a direct electrical current.
  • The sample is maintained anodic, with the cathodic connection being made to a nearby metal conductor.
  • Anodic dissolution of sample creates polished surface.
  • Relatively quick and can produce
    samples with no mechanical damage, however can only be used for electrically conducting metals and alloys.
28
Q

TEM – Nanoparticles

A

-Suspend nanoparticles within a solution, typically isopropanol (IPA)
* Use sonication (vibration) to disperse the particles throughout the
suspension
* Drop cast them onto a TEM grid
Drop casting can result in excessive carbon contamination
Baking TEM samples at 60 ºc is effective to remove the contamination

29
Q

Transmission electron microscopy can:

A

–Image morphology of samples, e.g. view sections of material, fine powders, thin films, small whole organisms such as viruses or bacteria, nanodevices, among others.
–Analyze the composition and some bonding differences (through contrast and by using spectroscopy techniques: microanalysis.
–Perform several in-situ measurements.
–View frozen material (in a TEM with a cryostage).
–Acquire electron diffraction patterns

30
Q

TEM can’t do:

A

▪ TEM cannot take colour images. Colour is sometimes added artificially to TEM
images.
▪ TEM cannot image through thick samples
▪ A standard TEM cannot image surface information.

31
Q

Advantages TEM

A

▪ Highest spatial resolution (atomic scale resolution (~1 Å)
▪ Local crystallographic and chemical analysis at very high resolution
▪ Quantitative identification of structural defects

32
Q

Disadvantages of TEM

A

▪ TEM is an expensive instrument
▪ Destructive technique (during sample preparation)
▪ Sample preparation is time consuming
▪ Some materials are sensitive to electron beam radiation, resulting in a loss of
crystallinity and mass
▪ Sample dimension is small

33
Q

Scanning transmission electron microscopy - STEM

A
  • The basic principle of image formation fundamentally different from static beam TEM
  • small spot size is formed on the sample surface with the condenser lenses
  • This probe is scanned on the sample surface
  • the signal is detected by an electron detector, amplified and synchronously displayed on screen with the scan coils
34
Q

Electron backscatter diffraction

A

Conventional Measures of Microstructure
* Grain Size
* Grain Shape
* Optical Metallography
* Chemistry
* EDS
* Phases
* Phase Contrast

35
Q

FIB – 3D

A

FIB is used to remove slices of material from the sample. After each slice has been removed EBSD data is collected from the fresh surface. This cycle is repeated and
EBSD maps are collected from the volume of interest.
* A 3D data cube is generated and can be reassembled to see the microstructure, measure grain size and phase distribution in
volumes, using the 3D viewer software.

NCN (6 decks)

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