Modern techniques of materials characterization Universit Siegen by liaoqinmei


									Modern techniques of materials
                         Basic concept

•   Source – What kind of „probe“ is used?
•   How does the probe reach the sample?
•   Interaction between probe and sample
•   How does the signal of interest reach the analyzer?
•   Characteristics of the analyzer


                               Interaction        Analyzer

         What kind of probes are available?

• Each and every analysis technique is based on the interaction
  between a probe and a sample. The following probes are generally

       • Electrons            -       Hot cathode, field emission
       • Ions                 -       Plasma, liquid metal tips
       • Neutrons*            -       Nuclear reactions (e.g.
       • Photons              -       Laser
                                      Synchrotron radiation
       • Heat*                -       …
       • A field*             -       electric, magnetic fields
                    Analysis Techniques (principle)

                    Electrons   Ions   Neutrons   Photons   Heat   A field






        A field
Energy of a particle → Wavelength
                  Analysis of the structure

• Usually one starts with the direct physical imaging of a sample

   –   Optical microscope
   –   SEM/Auger (scanning electron microscopy)
   –   TEM (transmission electron microscopy)
   –   STM/AFM (scanning tunneling microscopy / atomic force microscopy)
   –   LEERM* (low energy electron reflection microscopy)
            Indirect analysis of the structure

• Diffraction of electrons, atoms or ions is used to gain insight to the
  atomic structure of the sample surface

    – XRD (x-ray diffraction) – surface analysis by crazing incidence X-ray
    – LEED (low electron energy diffraction) - MEED
    – ABS (atomic beam scattering)
    – LEIS (low energy ion scattering) – MEIS, HEIS
    – RBS (Rutherford back scattering)
    – RHEED (reflection high energy electron diffraction)

    – SEXAFS (surface enhanced X-ray absorption fine structure)
    – XANES (X-ray absorption near edge structure)
    – SEELFS (surface extended energy loss fine structure)
            Chemical analysis of the surface

• Basic determination of elements present at the surface
• Determination of chemical bonding and atomic or molecular states in
  the surface region
• Lateral and depth profiling of elemental distribution

   –   XPS (X-ray photoelectron spectroscopy)
   –   UPS (ultraviolet photoelectron spectroscopy)
   –   AES (Auger electron spectroscopy)
   –   SIMS (secondary ion mass spectrometry)
   –   FTIR (Fourier transform infrared spectroscopy), ATR (attenuated total
       reflectance spectroscopy), Raman
Scanning Probe Microscopy
    A plethora of possibilities
Basic idea
The SPM family
                   The scanning part of SPMs

•   Based on the piezoelectric
     – Piezo Tri-Pods
     – Piezo-Tube-Scanners
•   Problems of these
    scanners are:
     – Hysteresis, creep
     – Aging
     – Cross-correlations
       between the individual
•   These are addressed by
    extensive calibration-
    functions or closed-loop-
    systems utilizing laser-             Piezo-tube scanner and
    interferrometry                      sketch of a piezo tripod
                       AFM - interaction

• Lennard-Jones
  potential is often cited

• Consisting of a van-
  der-Waals and a
• Distance-dependence
  of interaction is
  changed in case of
  nanoscale objects
• Basic behavior,
  however, is
Various AFM modi
                        Non-contact mode

• Idea here is to
  sense the sample
  without touching it
  → essential in the
  context of most
  polymer and
• Cantilever is
  operated close to
  its resonance
  frequency via a
  piezo actuator
Electron microscope techniques
   Scanning Electron Microscope (SEM)
 Transmission Electron Microscope (TEM)
                        Electron sources

Electron guns:

•   Various examples
    of gun design
     – Thermionic
     – Schottky
     – Field emission
•   Cathode material
     – Tungsten
     – Lanthanum
     – Others…
•   Cathode material
    emission current
                         Energy scheme of various gun types
      What kind of species are generated?

Probe-sample interaction results in the „generation“ of

•   Secondary electrons
•   Backscattered electrons
•   X-rays
•   Auger electrons
•   Plasmons
                   Secondary electrons (SE)

•   SE (exit energies < 50 eV) are generated if the energy gain of these species
    is large enough to overcome the work function
•   This process needs to be treated quantum mechanically as the scattering of
    an electron wave at a potential barrier

•   SE are only able to escape from a small surface range (probability of
    escaping is based on their inelastic mean free path)
•   Backscattered electrons contribute to the SE yield 
              Backscattered electrons (BSE)

•   BSE are present in the whole energy range from 50 eV (definition) to the
    maximum acceleration energy of the primary electrons (PE)
•   Their spectrum shows a broad peak overlapped by SE and Auger peaks as
    well as plasmon loss

•   BSE and SE are the most important signals for imaging. Knowledge about
    the dependence of the backscattering coefficient and the SE yield on surface
    tilt, material and electron energy is essential for any interpretation.

•   Acceleration of a charged particle (electron) in the screened Coulomb
    potential of the nucleus leads – with a low probability – to an emission of a
    X-ray quantum (usually elastic scattering is observed)
•   Electron is decelerated by h (energy of the X-ray quantum) → continuous
    X-ray spectrum

•   This continuous spectrum is superposed on the characteristic X-ray
    spectrum generated by filling of inner shell vacancies
 Diffraction based techniques
X-ray, neutron, and electron based methods
                     Basic definition of diffraction

•    Diffraction is the bending, spreading and
     interference of waves when they pass by an
     obstruction or through a gap. It occurs with any
     type of wave, including sound waves, water
     waves, electromagnetic waves such as light and
     radio waves, and matter displaying wave-like
     properties according to the wave–particle duality.

                                                              Thomas Young (1773-1829),
                                                              ophthalmologist and physicist

    Thomas Young's sketch of two-slit diffraction, which he
    presented to the Royal Society in 1803
                             X-ray sources

Energy regime of Gamma- und X-ray radiation overlap – naming criteria is the
heritage: X-ray is created by electron processes whereas Gamma radiation is
a nuclear reaction product

Typically X-ray radiation is generated by deceleration of electrons
                 X-ray sources (Synchrotron)

Synchrotron radiation is emitted by
charged relativistic particles deflected by
a magnetic field tangentially to their path
of motion

In order to generate synchrotron radiation
so called storage rings are used that
keep the kinetic energy of the charged
particles constant in order to conserve a
constant energy spectrum of the radiation

Worldwide, about 30 laboratories are able to generate synchrotron radiation. In
Germany there are, among others, BESSY in Berlin, HASYLAB in Hamburg,
DELTA at Universität Dortmund and ANKA in Karlsruhe

A known natural source of synchrotron radiation is for example Jupiter which
bombards its moons with synchrotron radiation
                         Neutron sources

Nuclear reactor
     • Usually fission reactors are used to
        generate kinetic neutrons to serve in
        diffraction experiments

Spallation source
      • Nuclear spallation is one of the processes
         by which a particle accelerator may be
         used to produce a beam of neutrons. A
         mercury, tantalum or other heavy metal
         target is used, and 20 to 30 neutrons are
         expelled after each impact of a high energy
         proton. Although this is a far more
         expensive way of producing neutron beams
         than by a chain reaction of nuclear fission
         in a nuclear reactor, it has the advantage
         that the beam can be pulsed with relative
                               Bragg relation

•   The diffraction equation
    postulated by Bragg and
    his son in 1914 (Nobel
    laureate in 1915)

    Waves that satisfy this condition interfere
    constructively and result in a reflected wave of
    significant intensity
            X-ray diffraction – phase analysis

Rietveld method (Hugo Rietveld (1932-) allows a quantitative
   phase analysis in the context of X-ray and neutron

•   Analysis of the whole diffractogramm
•   Refinement of structure- as well as real-structure-
    parameters                                                 Hugo Rietveld
     – Quantitative phase analysis
     – Lattice parameters and temperature effects
     – Grain size and micro strain
•   Its not a structure analysis!
     – Basic lattice parameters,
     – phase composition, and
     – Space group needs to be known
Photon-based Techniques
                       Raman spectroscopy

•   The phenomenon behind this technique was first
    reported by Sir Chandrasekhara Venkata Raman (1888-
    1970) in 1928 – in 1930 he was awarded the Nobel
    Prize in physics for his findings

•   A small percentage of light scattered at a molecule is
    inelastically scattered (1 in 107 photons)

                                                             Sir C.V. Raman
               Raman spectroscopy - basics

•   At room temperature majority of molecules in initial (ground) state  anti-
    Stokes signal will be less pronounced: Ratio of anti-Stokes to Stokes can be
    used for temperature measurement
•   The energy of a vibrational mode depends on molecular structure and
    environment. Atomic mass, bond order, molecular substituents, molecular
    geometry and hydrogen bonding all effect the vibrational force constant
    which, in turn dictates the vibrational energy
•   Vibrational Raman spectroscopy is not limited to intramolecular vibrations.
    Crystal lattice vibrations and other motions of extended solids are Raman-
•   Raman scattering occurs when it features a change in polarizability during
    the vibration
•   This rule is analogous to the rule for an infrared-active vibration (that there
    must be a net change in permanent dipole moment during the vibration) -
    from group theory it is possible to show that if a molecule has a center of
    symmetry, vibrations which are Raman-active will be silent in the infrared,
    and vice versa
  Raman spectroscopy vs. IR

 IR = Change in dipole of molecule

Raman = Polarizability of Molecules
    Extended   Equilibrium   Compressed
             Raman spectroscopy - examples

•   The frequency of the
    RBS mode is inversely
    proportional to the
    diameter of the
•   RBS mode and double
    peaked high energy
    modes are prove of the
    existence of single-wall
    nanotubes in a sample
•   In metallic carbon
    nanotubes the lower
    high-energy mode is
    strongly broadened and
    shifted to smaller
    energies (1540 cm-1)

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