Chem 177 Slide 1

• Material Characterization


  The study of the composition, structure, and properties of materials
• Metrology

  The study of measurements, broadly known as metrology, is also of much industrial relevance with many companies having separate metrology departments to monitor the production and characterization of materials, devices, and components
• Probes for Characterization

  • Electromagnetic radiation
  • Electrons
  • Protons
  • Neutrons
  • Ions
  • Scanning probe methods

  
• Methods for Characterization

  • Spectroscopy
  • Scattering and/or diffraction
  • Imaging

  
• Materials Used for Characterization

  • Bulk
  • Composite
  • Coating
  • Thin-film
  • Nanoparticle
  • Patterned surface
• The core activities of a materials scientist/engineer can be represented by a tetrahedron

  
• Only some characterization techniques will be covered in this course, failure analysis will not be covered
• Elastic interactions


  No difference in the energy of the probe and signal, applies predominantly to scattering or diffraction methods
• Inelastic interactions


  There is a change in energy and such dispersions of signal intensity, as a function of energy, form the basis of spectroscopy techniques

  
• Signals from both elastic and inelastic interactions can be spatially resolved or mapped, and such spatial intensity distributions form the basis of imaging methods

  
• The probe and signal may or may not be the same (e.g., XPS or electron microscopy)

  
• Energy is not expressed in the standard SI or MKS unit of joule (J), but in unit of electron volts (eV), which is the kinetic energy gained by an electron accelerated from rest through a potential difference of 1 V

  
• The photoelectric effect demonstrates the particle nature of light (photons)

  
• Electron diffraction demonstrates the wave-like properties of electrons, which is a particle

  
• Diffraction studies of surfaces require electrons with energies of the order of 100 eV, and such surface techniques are termed low energy electron diffraction (LEED) or microscopy (LEEM)

  
• To probe the internal crystal structure of materials, electrons with substantially higher energy (100–200 keV), e.g. in TEMs, with wavelengths in the range (0.037–0.0251 Å) are used

  
• High-energy electrons can also probe the structure of surfaces in reflection mode (RHEED)

  
• Penetration depth


  A measure of how deep the electromagnetic radiation can penetrate a material; often, it is defined as the depth at which the intensity falls to 1/e (37%) of its original value
• Mean free path length

  The average distance traveled by a moving particle in a material between successive impacts/collisions that modify its direction or energy

  
• For any technique, if the probe and signal radiations are not the same and have different mean free path lengths in the material, the volume analyzed (or the sampling depth) will be determined by the radiation—either probe or signal—with the smaller mean free path length

  
• UV, visible and IR radiation can only examine the surface of the sample

  
• Higher-energy probes, particularly X-ray radiation, have a uniform and predictable behavior in all materials. The absorption of X-rays, is defined by the attenuation coefficient, μ, which increases with the average atomic number of the material and determines the depth of penetration

  
• The intensities of γ-rays show the same exponential dependence on thickness as X-rays, but with their much higher energies (∼50 keV – 50 MeV) they penetrate much larger distances

  
• For low energy (∼0–2000 eV) electrons, the mean free path length is of the order of a few Å, and curiously, for all materials, satisfy a universal curve as a function of energy

  
• For high-energy (E ≥ 5 keV) electrons, the penetration depth, dP, behaves as dP ∝ E1.7/Z, where Z is the average atomic number of the material

  
• Neutrons have ∼1,000 times the mass of electrons but do not have an electric charge. As a result, neutrons penetrate much greater distances than electrons and X-ray photons

  
• The interaction of high-energy ions with materials is also complex. At low (∼eV) energy they are reflected from the surface, following simple rules of conservation of energy and momentum. At higher energies, they interact with the material, causing atomic displacements, formation of clusters and sputtering (the removal of atoms, ions, and electrons from the specimen)

  
• Depth resolution


  The resolution is equivalent to the penetration depth already described
• Spatial resolution


  The resolution in a direction normal to the direction of incidence, depends on the diameter of the incident beam, its wavelength, and its mean free path length in the material
• Temporal resolution


  The precision of a measurement with respect to time, is a very important criterion for designing in situ and dynamic experiments for the study of growth, morphological evolution, and response of materials to various applied stimulus
• Damage to the specimen can be caused by the transfer of energy and momentum from the probe
• For photons, the transfer of energy in the form of heat largely causes the damage. The degree and spatial extent of the damage will be determined by the penetration of the radiation in the material, and the energy and flux of the incident photon

  
• Electrons can cause significant damage by breaking interatomic bonds, particularly in polymeric materials. However, if the electrons are accelerated through higher voltages (∼1 MeV), such as in high-voltage electron microscopes (HVEMs), the momentum transferred to the atomic nuclei by elastic large-angle scattering is sufficient to cause significant atomic displacements even in inorganic materials and alloys

  
• Similar atomic displacements are also caused by ions, and the extent of the damage is determined by the incident ion flux
• Specimen preparation or requirements

  • Clean surface (dust-free, no adsorbed substances)
  • Vacuum compatibility
  • Conducting surface for SEM and STM
  • Electron transparent thin foil for TEM
  • Somewhat rigid surface for AFM
• Three basic processes underpin the foundations of most characterization methods

  • Spectroscopy
  • Scattering and/or diffraction
  • Imaging

  
• Spectroscopy: Absorption, Emission, and Transition Processes


  1. Primary electron (probe) with sufficient energy, EP, to overcome the binding energy, EB, of the inner-shell electron, ejects the core electron

  2. The primary electron is scattered in some new direction with reduced energy, EP'

  3. The core electron that is removed is now free and is referred to as the secondary electron

  4. The energy lost by the primary electron, EP - EP′ = EB + ES, is sensitive to the binding energy of the core electron, and by measuring this loss of energy accurately, the electronic structure of the atom/material can be probed

  5. It is also possible to map the distribution of such secondary electrons as the primary electron beam is rastered along the specimen surface, and this technique forms the basis of SEM

  6. Alternatively, instead of the primary electron if a photon of sufficient energy, EP = hf , is incident, the related process of photoionization occurs

  
• Mean free path length

  The average distance that the primary electron travels between such collisions in the material

  
• Secondary electron


  The core electron that is removed and is now free
• Energy loss (EP - EP')


  EB + ES, sensitive to the binding energy of the core electron, can probe the electronic structure of the atom/material