Triplet states in quantum dots

[McKendrigo]

Hi there,

I have a passing interaction with quantum dots and light emitting polymers as part of my research. I've been reading up on the physics of qdots and polymers recently. One thing that strikes me is that the existence of triplet states in polymers is mentioned, but not in quantum dots (at least, not as far as I'm aware).

Do triplet states exist in quantum dots? If not, why not? If so, why have I not seen anything about them yet? (are they only apparent under special circumstances, for example?)

I'm wondering if the carriers in the relatively small polymer molecules are more closely bound, whereas the carriers in the qdots are more free, which somehow mitigates carriers from falling into triplet states?

 

[Cthugha]

That depends on the kind of QD you are interested in. Usually the carriers are excitons, bound electron-hole pairs. In semiconductor QDs you usually have them composed mostly out of electrons ($$S_z=\pm \frac{1}{2}$$) and heavy holes ($$J_z=\pm\frac{3}{2}$$) (although they are never pure heavy holes in self-assembled QDs), so you get 4 possible exciton levels with angular momentum projections of $$M=\pm 1$$ and $$M=\pm 2$$, the so-called bright and dark excitons. Due to conservation rules, only the bright ones with $$M=\pm 1$$ couple to the em field.

If you consider doped QDs, you instead get negatively charged excitons or trions formed by 2 electrons and 1 hole for example. Here you get 8 states. Two of them are singlets with $$M=\pm \frac{3}{2}$$. The other six are triplet states. But as they require require an electron in a higher energy spatially confined state in the QD, they usually lie at much higher energies compared to the singlets and do not affect them. Going to more complicated exciton complexes, you will of course also get more complicated states.

For colloidal QDs the situation is, however, a bit different.