Quantum dots:emission versus absorption spectrum

[fisico30]

Hello Forum,

how is is possible for quantum dots to offer a wide, broadband absorption spectrum but still have a single distinct emission wavelength?

How is that asymmetry possible?
For example, I would think that if we can absorb, say, 4 wavelengths, we can also emit those say 4 wavelengths...

thanks
fisico30

 

[Cthugha]

What makes you think QDs have broad absorption spectra? The absorption spectra are pretty narrow and consist of several narrow lines corresponding to the different "shells", see e.g. Phys. Rev. Lett. 85, 389–392 (2000) by Pawel Hawrylak et al.

There are two possibilities to create a broad absorption spectrum:

1) You have an ensemble of QDs. Especially for self-assembled QDs, each individual QD is a bit different and as a consequence has a slightly different spectrum. In summary this gives a broad absorption spectrum (but also a broad fluorescence spectrum).

2) You pump at very high pump energies. This way you do not create the carriers directly in the QD, but in the surrounding wetting layer, which behaves like bulk material. There unbound electrons and holes are formed, which can move towards the QD and form excitons there, losing energy along their way. So if you pump above the confinement energy of the QD, the absorption of the whole system (QD+wetting layer) becomes broad.

 

[Darwin123]

Hello Forum,

how is is possible for quantum dots to offer a wide, broadband absorption spectrum but still have a single distinct emission wavelength?

How is that asymmetry possible?
For example, I would think that if we can absorb, say, 4 wavelengths, we can also emit those say 4 wavelengths...

thanks
fisico30

Many, if not most, materials in condensed matter have a wide, broadband absorption spectrum and an emission spectrum consisting of narrow bands.

There are two reasons:
1) Unbound states tend to be distributed continuously in energy while bound states have discrete energy states.
-Very high energy photons can cause photodisassociation of energy, which produces a wide absorption spectrum.
-Disassociated molecules can't emit photons because they are disassociated.
-Emission of narrow lines usually occurs between bound states of the electron.2) In a condensed matter system, every electronic level is split up by vibrational and rotational interactions. Each electronic state has a wide range of vibrational and rotational states associated with it.

-When light is absorbed, the energy doesn't have to match a single energy difference precisely. The electron can go into a higher vibrational state. The excess in energy goes into vibrational energy.

-Once excited, vibrational energy is lost to nearby atoms. The electron rapidly settles to the bottom most state of any electronic manifold. Thus, the range of vibrational states associated with that transition has narrowed. These rules apply to any condensed matter system, which includes all solids and liquids. Narrow band spectra only occur for dilute gases.

These rules apply as much to bulk semiconductor (large crystals) as quantum dot semiconductors (small crystals). However, there is a difference. Bulk semiconductors have very broad states associated with free carriers. Free carriers are electron states not bound to individual atoms.

Electrons in quantum dots are automatically bound states. They are bound by the boundaries of the quantum dot. So the bound states in a quantum dot are even more discrete than the bound states in a bulk semiconductor. Even the free carrier states become discrete in a quantum dot. Therefore, the emission bands of a quantum dot tend to be narrower then the emission bands of a bulk semiconductor.

 

[fisico30]

Hello Darwin123,
What is a bound and an unbound state? their difference?

best
fisico30