TEM AES XPS, ESCA

Transmission Electron Microscopy

an electron beam is used to irradiate a small area on the sample

the sample is thin (10-100 nm) so electrons can be transmitted through

transmitted electrons are collected for image forming

Analytical TEM

no atomic resolution

crystallographic analysis possible (contrast due to diffraction on crystalline matter)

High Resolution TEM (HRTEM)

atomic resolution

contrast due to high or low angle atomic scattering

Scanning TEM (STEM)

image formation by scanning a fine probe

For chemical analysis

energy filtered TEM (EFTEM) or EDX

TEM sample preparation of inorganic samples

cutting of 2 mm slices (usually by wire erosion)

sample thinning down to 100 micrometers

electrolytic etching

ion milling or focused ion beam or suspension on a-C grid

TEM sample preparation of organic samples

fixation (chemical crosslinking of proteins with aldehydes or cryofixation with liquid ethane or helium)

dehydration

microtoming

(optional) staining with heavy metal for contrast improvement

Auger Electron Spectroscopy

surface sensitive analytical technique

able to determine elemental composition of materials

possible to determine chemical states of surface atoms

AES principle

primary electron beam (3-10 keV)

all elements with Z greater than 3 (+Li) emit Auger electrons

a spectrometer analyses the energy distribution of the electrons emitted from the sample

Auger electrons are used to

identify elements and to a lower extent chemical states

AES facts

depth 0.5-5 nm from the sample surface

the detection limit is about 1 percent

lateral resolution less than 1 micrometer

Auger Effect

radiationless internal rearrangement of electrons

X-ray or high energy electron bombardment of an atom can create vacant spaces in the inner shell that can be filled by

photon emission (X-ray fluorescence)

Auger process

Auger process steps

an inner shell electron is removed by an incident particle creating a hole by ionization

an electron from a higher orbital falls down to fill the hole (radiationless transition)

excess energy of the excited state is removed by the ejection of an Auger electron

Photon emission (X-ray fluorescence) is more likely to occur for

deep core holes (high binding energy)

high Z elements

Auger process is more likely to occur for

shallow core holes

low Z elements

Probability of relaxation by Auger emission varies with

core location and Z

Auger spectra contain

closely spaced groups of multiple peaks

Auger process steps

focused electrons are incident on the sample

emitted electrons are deflected into a cylindrical mirror analyzer

Auger electrons are multiplied in the electron detector

the signal is sent to data processing electronics

an ion gun can be added to sputter the sample’s surface (cleaning, depth profiling)

Auger process steps

emitted electrons have energies 50 eV to 3000 eV

short mean free path of electrons in solid

escape depth is few nm of target surface (extreme sensitivity to surface species)

most AES systems work in ultrahigh vacuum due to the low energy of Auger electrons (to prevent electron scattering or gas adsorption on the surface)

A typical Auger spectrum has

a large background due to inelastically sputtered secondary electrons which are superimposed on the characteristic Auger peaks of the elements present