Self Trapped & Charge Transfer Excitons

[citw]

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.

 

[DrDu]

Where did you encounter these terms?

 

[citw]

Where did you encounter these terms?

Attached is one example of a self-trapped charge-transfer exciton in NiO. Here's the link:

http://istina.imec.msu.ru/media/publications/articles/cd6/ae7/545563/NiO-JETPLett.pdf

 

[DrDu]

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).

 

[citw]

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).

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.

 

[DrDu]

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.

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).

 

[citw]

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).

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.

 

[DrDu]

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.

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.