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Is my material Nanocrystalline or Amorphous? (XRD and Raman)

  1. Jun 13, 2013 #1
    Hi!
    How can I tell, based on my Raman and X-Ray Diffraction studies, if a thin film is nanocrystalline or amorphous?
    The maximum thickness of my films is about 200nm, the XRay diffractograms showed nothing but the substrate (is it because of the thickness?), and my Raman spectra showed some broad (~70 cm-1), low intensity peaks (not as intense as the ones of crystalline samples).
    I'm so confused, please help me!

    Have a nice day
    :smile:
     
  2. jcsd
  3. Jun 14, 2013 #2

    Claude Bile

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    Tough to say, crystalline media typically have stronger, sharper peaks, but lots of nanocrystallites might give you a broad distribution merged into a single peak which may resemble an amorphous-like Raman peak.

    Claude.
     
  4. Jun 14, 2013 #3

    ZapperZ

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    I don't think you'll be able to tell the difference using Raman. As for XRD, instead of clear diffraction pattern that one sees for single crystals, shouldn't you get something resembling what you would get for powder diffraction?

    Zz.
     
  5. Jun 15, 2013 #4

    cgk

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    This is just a guess, but if the material is really *nano*crystalline, could the pattern be quite different from a normal powder diffraction? I've seen some scanning electron microscopy images of materials which looked like they consisted of >20% grain boundaries, and I am not sure if the grains even all had the same crystal structure...

    To OP: I think you might want to ask if someone has a SEM around to take a look at your sample. Material-science SEMs often also have both back-scatter electron differaction detectors and local x-ray-detectors, both of which could tell you exactly what you are looking at (if it is not already obvious by looking at it, I mean).
     
  6. Jun 15, 2013 #5

    ZapperZ

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    That is certainly a strong possibility, especially if, as you said, a substantial portion of it is grain boundaries. I have no info if these nanocrystal are of the same structure or not.

    But aren't the EDX/EDS from SEM typically tells you the specie of the material, rather than being able to distinguish the crystal structure?

    Zz.
     
  7. Jun 17, 2013 #6

    cgk

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    Right, SEM EDX is only good for identifying the elemental composition of the grains. However, electron back-scatter diffraction (EBSD) provides a diffraction pattern which can be used to determine the orientation, lattice type, and lattice spacing of the grains. Together with the elemental composition (might be important if there is phase separation) this should allow for a reasonably good guess of what the grains are.

    For these techniques, spatial resolution might be a problem for really small grains, though. While with secondary electrons imaging about 1--2nm resolution is possible on a good day with a modern (expensive) SEM, I guess EBSD and EDX might be limited to 5-20nm resolution (but I am not an expert on this).
     
  8. Jun 27, 2013 #7
    It will be difficult to determine the structure in your thin film using the standard Bragg-Brentano geometry for the following reasons:
    1) This scattering geometry will only probe the electron density normal to the surface, i.e., you will only see diffraction from lattice planes (roughly) parallel to the substrate. The wavevector transfer, Q, extends normal to the surface as you increase θ-2θ.
    2) The penetration depth of the X-rays. With an incident angle much greater than say θ = 1deg (dependent on sample, X-ray λ etc.) the penetration of the X-rays into the substrate becomes extremely high so your diffractogram will be dominated by scattering/diffraction from the substrate.

    The solution to this is to use grazing-incidence X-ray diffraction. In this geometry you have the incident X-rays at a fixed incident angle θ, which is normally very small (θ < 1deg). This allows the scattering to arise only from your thin film and not from the substrate. In combination with a 2D detector (or by scanning a point detector vertically and horizontally) you are able to observe the in-plane and out-of-plane Bragg reflections. This technique is more complicated than standard XRD as the small incident angle gives rise to complicated refraction and reflection effects but it is the best way to get information on thin films.

    It also usually requires a synchrotron source or a very good specialized lab source so sorry if you don't have access to these!

    Hope that helps.
     
  9. Jul 8, 2013 #8
    You can use the Scherrer equation to estimate the grain size.

    http://en.wikipedia.org/wiki/Scherrer_equation

    Tom is right that signal/noise is going to be a problem, and that a grazing incidence geometry will maximize the
    signal from your film compared to the background from the bulk substrate. Another way of optimizing the signal
    is to use low-energy x-rays that penetrate less deep into the substrate.
     
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