Self Trapped & Charge Transfer Excitons

In summary, the conversation revolves around the concepts of "self-trapped" and "charge-transfer" excitons, with a focus on understanding their definitions and applications in inorganic semiconductors. The discussion also touches on the idea of self-trapping in relation to effective mass and lattice distortions, and the possible similarity to polaron behavior.
  • #1
citw
72
0
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.
 
Physics news on Phys.org
  • #2
Where did you encounter these terms?
 
  • #4
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).
 
  • #5
DrDu said:
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.
 
  • #6
citw said:
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).
 
  • #7
DrDu said:
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.
 
  • #8
citw said:
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.
 

1. What is a self-trapped exciton?

A self-trapped exciton is an electron-hole pair that becomes trapped in a localized region of a material due to the interaction with its surrounding atoms or molecules. This can occur when the electron and hole have opposite charges, causing them to be attracted to each other, or when the material has a low dielectric constant.

2. How do self-trapped excitons form?

Self-trapped excitons can form through various mechanisms, such as through photoexcitation, where an electron is excited to a higher energy state and leaves a hole behind, or through thermal excitation, where the thermal energy of the material causes electrons and holes to become separated and form a self-trapped exciton.

3. What is the role of charge transfer in exciton formation?

Charge transfer refers to the movement of electrons or holes from one atom or molecule to another. In the context of self-trapped excitons, charge transfer plays a crucial role in the formation of these localized electron-hole pairs. This can occur through the transfer of an electron or hole to a neighboring atom or molecule, leading to the trapping of the opposite charge.

4. How do self-trapped excitons affect the properties of a material?

Self-trapped excitons can significantly influence the optical and electrical properties of a material. They can lead to the creation of new energy states within the material's bandgap, resulting in changes in its absorption and emission spectra. They can also affect the material's charge transport properties, such as its conductivity and mobility.

5. What are some potential applications of self-trapped and charge transfer excitons?

Self-trapped and charge transfer excitons have applications in various fields, including optoelectronics, photonics, and energy storage. They can be used to create efficient light-emitting devices, such as LEDs, as well as solar cells and sensors. They also play a crucial role in the coloration of gemstones, such as rubies and sapphires.

Similar threads

  • Atomic and Condensed Matter
Replies
1
Views
1K
  • Atomic and Condensed Matter
Replies
4
Views
4K
  • Atomic and Condensed Matter
Replies
2
Views
2K
  • Other Physics Topics
Replies
22
Views
3K
  • Atomic and Condensed Matter
Replies
5
Views
2K
  • Biology and Chemistry Homework Help
Replies
1
Views
565
  • Introductory Physics Homework Help
Replies
11
Views
623
  • Introductory Physics Homework Help
Replies
4
Views
1K
Replies
2
Views
2K
Replies
2
Views
2K
Back
Top