TECHNIQUE CONTRAST - electron microscopy

TEM works by

shining large beam of electrons over sample

Resolution of TEM vs. SEM

TEM: Atomic level (nm)

SEM: (1-2 nm)

What does TEM look at?

Internal structure, including defects, crystallography, nanoparticles, dislocations, and interfaces

To look at TEM results, this screen is needed

Fluorescent 

Does TEM or SEM have intensive sample prep?

TEM

Involves grinding, thinning

Mass Density TEM

Thick regions scatter more e-, darker on screen

Diffraction contrast TEM

Strong diffraction- dark, weak - light

Selected Area Diffraction TEM

Select region can be scanned for orientation information

Phase contrast TEM

Takes pure phase —> pure amplitude signal

In-situ TEM

Real-life watch sample response to stimuli

Pros of TEM

• atomic level resolution

• can see image/diffraction

Cons of TEM

• Long sample process

• Thin samples only

• Small amount of sample

STEM

Focusing beam of electrons on one spot

How does picture on screen work for STEM?

Measurement of transmitted electron charge determines pixel brightness

Thermal diffuse scattering STEM

Large angle

Elastic scattering detection STEM

Small angle 

SEM focuses beam on sample _________

surface

Three signals from SEM

1. SE

2. BSE

3. EDS (X-Ray)

SEM samples must be conductive/nonconductive

Conductive (or coated to become conductive)

What controls resolution for SEM?

Working distance, apertures 

SE scan

Topography, leak from edges of topographical features

BSE Scan

Composition

BSE Scans depend on this principle of atoms

Z (atomic number)

Everhard-Thornley detector

SE - SEM

EDS detection

X-ray, hits inner electron with light, pushes out electron, outer electron sent in

This detection for SEM can look at elemental details

EDS

EBSD-SEM

Uses BSE to look at Kikuchi patterns formed when electrons hit lattice of crystal - angle of diffraction

ESEM

Low-vacuum, nonconductive/wet samples

Pros of SEM

• short sample prep

• surface composition info

• any level of sample thickness

• large amount of sample

Cons of SEM 

• worse resolution

To ionize electrons, use

GFIS (ionizes noble gases)

Characteristics of ion microscopy

0.5 nm achievable

Looks at top atomic layers

No need for conductive coatings

If you’re looking at a sample with : what would you use?

Morphology, grain size, coatings, fracture surfaces, microstructure

SEM

If you’re looking at a sample with : what would you use? 

Lattice imaging, nanoparticle analysis, crystal defects, interfaces, and phase identification

TEM

If you’re looking at a sample with : what would you use?

Atomic-resolution imaging, chemical and electronic structure mapping, studying interfaces or dopant distributions

STEM

If you’re looking at a sample with : what would you use?

Polymers, ceramics, nonconductive samples

Ion Microscopy

If you’re looking at a sample with : what type of TEM would you use?

Amorphous

Mass-Density TEM

If you’re looking at a sample with : what type of TEM would you use?

Noting defects, faults, crystalline materials

Diffraction contrast

If you’re looking at a sample with : what type of TEM would you use?

Atomic-level, thin-crystalline lattices

Phase contrast TEM

If you’re looking at a sample with : what type of TEM would you use?

Phase ID, orientation, lattice spacing

Selected Area Diffraction (SAD) TEM

Which is higher resolution, SEM or TEM

TEM

Which can produce 3D images, SEM or TEM?

SEM

Why does SEM need conductive samples

precent charge build-up, with non-conductive samples they would be hit with electron beams and build up a negative charge