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Microscope techniques (TEM (Essential Components (Electron Gun, Condenser…

- - - - Diffraction photons can be added and subtracted to give different areas of photon intensities - Destructive Interference
        When there is a collision of photons their intensities may combine - Constructive Interference
        When the wavelengths of photons are only slightly unaligned a mixture of constructive and destructive interference occurs - Partial Interference
  - - - Minimal sample preparation is required
      - Powerful and rapid (< 20 min) technique for identification of an unknown mineral
      - In most cases, it provides an unambiguous mineral determination
      - XRD units are widely available
      - Data interpretation is relatively straight forward
    - - Homogeneous and single phase material is best for identification of an unknown
      - Must have access to a standard reference file of inorganic compounds (d-spacings, hkls)
      - Requires tenths of a gram of material which must be ground into a powder
      - For mixed materials, detection limit is ~ 2% of sample
      - For unit cell determinations, indexing of patterns for non-isometric crystal systems is complicated
      - Peak overlay may occur and worsens for high angle 'reflections'
- - - - Discrimination of phases based on mean atomic number (related to relative density)
      - Aquiring elemental maps or spot chemical analyses using EDS
      - Compositional maps based on differences in trace element 'activators'
      - Identify phases based on qualitative chemical analysis and/or crystalline structure
      - Precise measurement of very small features and objects down to 50nm in size
  - - - Electron source 'Gun'
      - Electron lenses
      - Sample stage
      - detectors for all signals of interest
      - Display/data output devices
      - Infrastructure requirements:
        
        Power supply
        
        vacuum system
        
        cooling system
        
        vibration free floor
        
        room free of ambient magnetic and electric fields
  - - - No other instrument with the breadth of applications in the study of solid materials
      - Easy to operate, user friendly interfaces, minimal sample preparation
      - Critical in all fields that require characterisation of solid materials
      - Data is generated in digital formats, highly portable
    - - Samples must be solid and fit in the mircoscope chamber
      - Samples are likely to outgas at low pressures are unsuitable for examination
      - Most machines are unable to detect elements with an atomic number less than 11
      - Xray detectors have relatively poor energy resolution and sensitivity to elements present in low abundances
- - - - Scattered beam electrons Beam electrons may be elastically scattered by the nuclei of sample atoms. In a bulk specimen in a SEM, elastically scattered beam electrons that have been directed back out of the sample constitute the backscattered electron (BSE) signal. In STEM, transmitted beam electrons have been scattered through a relatively large angle are detected using a high angle annular dark field reactor (HAADF) detector.
      - X-ray microanalysis. Electrons bombarding the specimen cause it to emit X-rays whose energy is characteristic of the elemental composition of the sample. X-ray microanalysis uses an energy dispersive X-ray (EDX) spectrometer to count and sort characteristic X-rays according to their energy.
      - Wavelength dispersive X-ray (WDX) spectrometry measures and counts X-rays by their wavelength (a correlate of energy). A wavelength spectrometer uses a crystal or grating with known spacing to diffract characteristic X-rays.
      - Electron energy loss spectrometry (EELS) analyzes transmitted electrons to determine the amount of energy they have lost in interactions with the sample. It provides information about the interacting atoms, including elemental identity, chemical bonding, valence and conduction band electronic properties, surface properties, and element-specific pair distance distribution functions.
      - Principal advantage over TEM is enabling the use of other signals that cannot be spatially correlated in TEM, including secondary electrons, scattered beam electrons, characteristic X-rays, and electron energy loss.
      - Disadvantages
        
        Samples need to be electrically conductive
        
        Specimen damaged by fast electron beam
        
        Sensitive to certain environments
        
        maintenance on the equipment is difficult
- - - - reduce interference with the sample
    - - elemental maps
      - phases on qualitative chemical analysis
    - - scanning electron microscope
  - - - X-ray diffraction can be used to discover what elements are in a compound, how accurate is this method? Can only discover trace amounts
  - - - Name 1 component of a TEM (electron gun)
    - - Which has the higher resolution (light, TEM) = TEM
    - - one million times or more