Is a dye an indirect semiconductor if it looks black under monochromatic light that is of different colour?

Question

I have noticed that most objects such as the red dye in a plastic stool look black under blue or green light.
From what I understand about direct band gap semiconductors, the dominant recombination mechanism is radiative recombination. Therefore If the dye was a direct band gap semiconductor I should expect it to still appear red under blue or green light as the electron in the red dye should absorb the light, thermalize and radiatively recombine releasing red light that's equal to the band gap in its energy. Therefore if it was a direct band gap I would expect it to fluoresce red in blue or green light.

The fact that it appears black implies there is another recombination mechanism such as indirect recombination that would cause the blue or green light that is absorbed to be entirely lost due to heat rather than by radiative emission.

Based on this I would assume that all indirect semiconductors would appear black, but for direct semiconductors, they should fluoresce.

I also understand that materials can have both indirect and direct band gaps. But for wide direct band gap semiconductors that have an indirect band gap that far offsets the direct band gap in energy should fluoresce under blue or green light.

Is my understanding correct or is there something I'm missing? Many thanks.

Answer 1

Maybe it is better to think through your observations it in terms of energy levels and molecules. You want to pick the right set of tools and nomenclature to solve your problem.

One way to get to band theory is to start with an atom, then multiple atoms and chains of atoms, then atoms periodically spaced in crystals.

Along the way, if you’re an atomic physics person you might think carefully about energy splitting of a certain energy level as two atoms are bought together. if a chemist maybe more in terms of molecular orbitals and for conductive polymers consider HOMO and LUMO levels and how electrons might move along some chain of atoms. If a Condensed matter physicist or electrical engineer start thinking conduction and valence bands or even band structures and with that direct and indirect bandgaps. But in all these different approaches you have you quantum, Pauli exclusion principle, molecular vibrations, phonons, excitons etc. The way you problem solve and describe the problem depends on what community you are in.

The point is that depending you seems to be trying to force fit a some concepts suitable for one case to a different case.

Dye molecules are typically very good absorbers, incidentally this implies they tend to be good emitters. But the emission peak will tend to be lower in energy than the absorption peak.

But not just that, you might notice that there could be different lifetimes for the emitters light.

So for a dye molecule, since you don’t really have a semiconductor in the sense of Bloch waves periodicity etc. talking about being direct or indirect for the Dye molecule isn’t really a correct approach. However you do still have a separation of energy levels you still have phonons etc and you look at what transitions are allowed.