auger electron spectroscopy is a surface sensitive analytical technique used mainly to determine...

Auger electron spectroscopy is a surface sensitive analytical technique used mainly to determine elemental compositions of material and, in certain cases to identify the chemical states of surface atom.

1.Etching the sample with X-rays or electron bombardment

2.A vacancy is created in an atomic core level

3.An electron from an upper level fill the hole

4.The energy difference liberated, and it can form of a photon (XRF) or eject an Auger electron.

Auger emissions are described in term of the type of orbital transition involved in the production of an electron.

KLL: 1.removes a K electron

2.transition of an L electron to the K orbital

3.ejection of second L electronLMM & MNN are also common transitions.Auger electron emission and X-ray fluorescence are

competitive processes:Lower atomic number auger electron emissionHigher atomic number X-ray fluorescence

Measurement of kinetic energies of the emitted electrons is performed.

Spectra typically displayed as a derivative dN(E)/dE

SP UTTER T IM E (M IN .)

PE

AK

-TO

-PE

AK

Fracture surface of Carbon fibers in BN matrix - analysis area outlined in black

Depth profile on fiber to determ ine point of fracture. Variations in fracture surfaceinterface for d ifferent sam ple treatments will be reflected in depth profile.

From research by C . Cofer/J. Econom y, M aterials Science D ept.

Sensitivity to atoms of low atomic numberMinimal matrix effect High spatial resolutionDetailed examination of solid surfaceNon-destructiveRapid analysis

Classical methods

Optical microscope

Resolution is limited by diffractioneffects to about wavelength of light

classical methods

modern microscopic methods

spectroscopic methods

Modern microscopic methods (higher resolution)Scanning electron microscopy (SEM)

Scanning probe microscopy (SPM)Scanning tunneling microscopy (STM)Atomic force microscopy (AFM)

classical and modern microscopic methods provide information about the

physical nature or morphology of surfaces but less about their chemical

nature.

Scanning Electron MicroscopySEM

They take advantage of the short wavelength of electrons. As the wavelength is shorter, higher resolution is possible than with regular light microscopes

• Electron microscopes were developed around the same time as the development of television sets (1960s) and utilize much of the same technology

Electron Interactions: All Types

Sample

Cathodaluminescence

Secondary e–

Backscattered e–

Incident e–

Auger e–

X-rays

ElasticallyScattered e–

InelasticallyScattered e–

Unscattered e–

Electron Beam-Specimen interactions

SEM is very good for looking at surfaces

SEM cannot show colorWith the right detectors SEM can be

used to determine the elements in a sample

SEM can go to high magnifications, but cannot achieve magnifications as high as those achieved with Transmission Electron Microscopy

Principles and Construction of the SEM

+

-Magnet lenses

Beam deflector

Filament

FocussedElectron beam

Electrons

Stub

Sample

…..

-

+

Magnet lenses

Beam deflector

Filament

FocussedElectron beam

Electrons

Stub

Sample

Reflected electrons

Electron detector

Fly Head

Bee Eye

Leaf With Bacteria

This is an image of an aluminum copper alloy formed using backscattered electron imaging. The light area is mostly copper and the dark area is mostly aluminum.

Intel’s transistors – current and near-future

M. Deal Stanford, 6/30/2005

SEM showing the vertical growth of nanowires for electronic devices (Stanford) Human hair

Single-walled nanotubes (SWNTs)

Multi-walled nanotubes (MWNTs)

Bi/opal Pb/opal

NaNO3/opal

2 µm 500 nm

500 nm 20 µm

After annealing at Т > Tm

NaNO3/opal

300nm

Latex opal 300nm on Latex opal 240nm

240nm

Substrate

5μmSEM image

SEM Images: Improved Depth of Focus

• Secondary electrons of SEM provide higher depth of focus compared to optical microscopy.

Optical Image SEM Image

screw

cells

From Brundle

From Flegler

1. Relatively easy to use

2. Large depth of focus

3. Large magnification range (> 80,000X)

4. 3D images

6. High resolution 1-5 nm

5. Can produce images whose contrast is based on composition variations

7. Characteristic x-rays used to identify and image elemental distributions

Inability to produce colorSpecimen must be stable under vacuum.Specimen at least must exhibit slight

electrical conductivity.

Scanning Tunneling Microscope

IntroductionSTM has unique capability to resolve

topological and electronic structures at atomic level.

Ability to operate in a variety of different environments such as air, reactive gases, liquids, electrolytes and biological fluids.

Application: electrochemistry, biology, tribology, medicine . . .

Basic principlesPhenomenon of tunneling

According to quantum mechanics, a subatomic particle can pass through a spatial region in which the particle’s kinetic energy is less than its potential energy.

The tunnel current will change with the tip location on the surface (eq. 1).

The tunneling current is held constant by a feedback mechanism that moves the tip up and down so that s remains constant.

Tip movement is controlled by the voltage at z-piezo. This voltage yields a topographic profile of the surface.

e-e-

e-

e-e-

e-

e-

e-

e-

< 1nm

Resolution in z is 0.01 nm and in x/y is ~0.2 nm, resulting in atomic resolution images.

It=Vexp(-Cd)

Constant current mode

Structure of crystal surfaceMetals, Semiconductors, Superconductors

and OxidesAdsorption, surface Diffusion and Surface

ChemistryThin film morphology, Nucleation, Growth,

etcSurface electronic structureOrganic molecules, Biological molecules Atom manipulation and Nanolithography

Atomic Force Microscope

STM-single atom interactionSTM-single atom interaction

AFM-several atoms on tip interact with several atoms on surfaceAFM-several atoms on tip interact with several atoms on surface

AFM

STM

InIn contactcontact AFM electrostatic and/or AFM electrostatic and/or surface tension forces from the surface tension forces from the adsorbed gas layer pull the scanning tip adsorbed gas layer pull the scanning tip toward the surface. It can damage toward the surface. It can damage samples and distort image data. samples and distort image data. Therefore, contact mode imaging is Therefore, contact mode imaging is heavily influenced by frictional and heavily influenced by frictional and adhesive forces compared to non-adhesive forces compared to non-contact or tapping mode.contact or tapping mode.

Non-contactNon-contact imaging generally imaging generally provides low resolution and can also be provides low resolution and can also be hampered by the contaminant layer hampered by the contaminant layer which can interfere with oscillation.which can interfere with oscillation.

TappingModeTappingMode AFM was developed as a AFM was developed as a method to achieve high resolution method to achieve high resolution without inducing destructive frictional without inducing destructive frictional forces both in air and fluid. With the forces both in air and fluid. With the TappingMode technique, the very soft TappingMode technique, the very soft and fragile samples can be imaged and fragile samples can be imaged successfully. successfully.

Polymer latex particle on mica

Contact Noncontact Vibrating (tapping)

Cantilever soft hard hardForce 1-10nN 0.1-0.01nNFriction large small smallDistance <0.2nm ~ 1nm >10nmDamage large small small

Contact Mode

Advantages:

• High scan speeds

• The only mode that can obtain “atomic resolution” images

• Rough samples with extreme changes in topography can sometimes be scanned more easily

Disadvantages:

•The combination of lateral forces and high normal forces can result in reduced spatial resolution and may damage soft samples (i.e. biological samples, polymers, silicon) due to scraping

Advantages and Disadvantages Advantages and Disadvantages of the 3 main Types of AFMof the 3 main Types of AFM

Tapping Mode AFM

Advantages:

• Higher lateral resolution on most samples (1 to 5nm)

• Lower forces and less damage to soft samples imaged in air

Disadvantages:

• Slightly lower scan speed than contact mode AFM

Advantages and Disadvantages Advantages and Disadvantages of the 3 main Types of AFMof the 3 main Types of AFM