Self Trapped & Charge Transfer Excitons

  1. I'm trying to understand the concept of "self-trapped" and "charge-transfer" excitons, and I'm hoping someone can break these concepts down for me. I've studied Wannier and Frenkel excitons a bit.

    I'm reasonably unfamiliar with the concept of "self-trapping". I only know that it involves some deformation of the lattice, which isn't a very complete or understandable description. I don't really understand the idea of self-trapped electrons or holes, much less excitons.

    I also need to understand "charge-transfer" excitons in the context of inorganic semiconductors. I'm guessing this is more applicable with semiconductors with at least some degree of ionic character. This is particularly confusing to me, as I understood excitons to transfer energy with no net charge.
  2. jcsd
  3. DrDu

    DrDu 4,092
    Science Advisor

    Where did you encounter these terms?
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  5. DrDu

    DrDu 4,092
    Science Advisor

    First, an exciton is almost any kind of electronic excitation, this can also go in hand with some charge separation. As nicely explained in the article, there are p-d and d-d type charge transfer excitons, where in the first case an electron is transferred from an oxygen p-orbital to a Ni-d-orbital correspondint to O2- + Ni2+ -> O- + Ni+, and in the second case a d-electron is transferred from one Ni atom to an adjacent one corresponding to Ni2+ + Ni2+ -> Ni+ + Ni3+.
    The self-trapping seems to be due mainly to Jahn Teller distortion in the Ni+ ion, i.e. a prolongation of the octahedra formed by the oxygen ions along one of the four-fold axes, as is well known from Cu2+ complexes (which are square-planar as you certainly remember from your chemistry classes).
  6. OK, aside from the charge-transfer explanation, what exactly is meant by self-trapping. I've read about charge carriers (electrons and holes) becoming "self-trapped", but "self" throws me off a bit. The extension to excitons is clearly lost on me, as I don't particularly grasp the concept on a single carrier basis, much less for coupled carriers.
  7. DrDu

    DrDu 4,092
    Science Advisor

    I think it refers to a strong increase of effective mass (equivalently a flattening of the exciton band) due to the coupling of the electronic excitation to deformations of the lattice (here Jahn-Teller distortions).
  8. Oh that's a really interesting idea. The effective mass point sounds very reasonable, considering a "trapped" carrier would obviously have very low mobility. Although, coupling the lattice distortion with the exitation makes me think of something similar to a polaron.
  9. DrDu

    DrDu 4,092
    Science Advisor

    Yes, I also learned about this mechanism in the context of polarons. I am also not an expert on this topic, so take my statement at best as an educated guess.
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