What Are Quantum Dots and How Do They Work?

[ThomasEdison]

Can someone either explain Quantum dots for me or point me in a good direction?
I didn't have much luck with the wiki's I've seen.

I would like an intuitive grasp of: what they are, how they are made, and their basic functions.

 

[MATLABdude]

Quantum dots (and quantum wells) fluoresce--you apply energy to them, and they emit the energy as light. Light with a very narrow spread of frequencies, and converted from the applied energy (broad spectrum light, voltage, etc.) with very high efficiency.

That's very simplified, but a very brief Reader's Digest description of it.

EDIT: There's a nice picture on this site (first one down) that shows quantum dots being illuminated using a UV light source:
http://www.viewsfromscience.com/documents/webpages/nanocrystals.html

 

[ThomasEdison]

Quantum dots (and quantum wells) fluoresce--you apply energy to them, and they emit the energy as light. Light with a very narrow spread of frequencies, and converted from the applied energy (broad spectrum light, voltage, etc.) with very high efficiency.

That's very simplified, but a very brief Reader's Digest description of it.

EDIT: There's a nice picture on this site (first one down) that shows quantum dots being illuminated using a UV light source:
http://www.viewsfromscience.com/documents/webpages/nanocrystals.html

Thank you for the link although I am still a bit confused.

There are a few things which confuse me.

1. I can accept that electrons can be confined into a cage ( a small crystal in this case) but I get confused at the idea that the absence of electrons can also be confined. I think the concept of exciton has me a bit stumped. Escpecially where you separate an exciton from atoms altogether. Is there anyway that in relation to Quantum Dots this process is explained in detail? How this is all formed?


2. Also what is stopping the electrons from leaving the trap or tunneling out?

3. Are Quantum dots permanent objects or do they decay very quickly?

4. How are these used to form qubits?

5. Or an accumulator to store data?

6. I've read about them called "artificial atoms" like an atom without a nucleus but still having electrons that form valence all of which is trapped. In that case the size of the "trap" is considerably larger than an atom since it is a molecule right? So wouldn't that artificial valence be huge because there is so much more space? I'm just trying to get an image of what this all looks like as clear as possible.

7. Why aren't Fullerenes used to create quantum dots?

8. Is an artificial ring of electrons that form a sort fo valence without a nucleus.. isn't that a monopole?

 

[Sybren]

1) Electrons and holes appear when looking at the difference between an excited and an unexcited quantum dot. This difference of excited minus ground state is what is measured in pump-probe experiments.

In a semiconductor, an electron can be excited into the conduction band. By subtracting this excited conduction band from the groundstate conduction band, it is found that the difference equals one negatively charged electron of charge -e (-e-0=-e). By subtracting the excited valence band from the groundstate valence band, it is found that the difference equals one positively charged 'hole' of charge +e (0--e=+e).

These two charges are real in the sense that pump-probe experiments see them.

2) electrons can tunnel out, but to stay outside of the quantum dot there has to be an energetically favourable place for them, such as an electron acceptor.

3) they are permanent objects: nanometer-sized crystals.

6) the discrete energy levels are what makes a quantum dot an 'artificial atom'.

 

[ThomasEdison]

1) Electrons and holes appear when looking at the difference between an excited and an unexcited quantum dot. This difference of excited minus ground state is what is measured in pump-probe experiments.

In a semiconductor, an electron can be excited into the conduction band. By subtracting this excited conduction band from the groundstate conduction band, it is found that the difference equals one negatively charged electron of charge -e (-e-0=-e). By subtracting the excited valence band from the groundstate valence band, it is found that the difference equals one positively charged 'hole' of charge +e (0--e=+e).

These two charges are real in the sense that pump-probe experiments see them.

2) electrons can tunnel out, but to stay outside of the quantum dot there has to be an energetically favourable place for them, such as an electron acceptor.

3) they are permanent objects: nanometer-sized crystals.

6) the discrete energy levels are what makes a quantum dot an 'artificial atom'.

Thank you.
Something clicked and I can accept electrons forming valence shells without an atom's nucleus because they are trapped and "fixed" in place by their own repulsion to each other and band gap. That makes sense to me. I can picture it.

To get more info I picked up a book which covers them (for a layman) recently and started it. I have no science background; I'm just an electrician who must understand intuitively how every electronic solid state device functions (as a hobby) and I was doing fairly well understanding transistors (since I did learn those in school) and some of the more advanced logic gates; but I'm no scientist or grad student or even an engineer. Quantum dots threw me for a loop because I found out they are fast becoming commonplace and have been around decades yet they were not even mentioned in school.
They appear like game changers to me even more so than I thought CNT and graphene would be to industry!

I was a bit humbled (but mostly excited) to find out that valence shells could be independant of an atomic nucleus because no where in high school physics or chemistry was this mentioned to me. I didn't even hear about it at all in my 1990's transistor theory/lab classes and in all the technology articles I read daily I must have glazed over exactly what a quantum dot is which I regret. Thank you for the help.

I found this at Borders.
http://en.wikipedia.org/wiki/Hacking_Matter


I still have a few questions.
I'm a bit more interested in their properties as matter and for computing instead of the fancy light properties at present.

In the case of treating them as artificial matter/programmable matter(?):
1.If two quantum dots or more are each within their own crystal trap in order to keep the electrons from escaping then how can those two separate quantum dots interact with eacthother to form an artificial molecule? If they are not separate then wouldn't they just form one large artificial atom?


2. How are they adjusted? By what mechanism is an artificial atom made to mimic an entirely different one by adding or subtracting electrons? How is this achieved? I am not talking about the creation of a Quantum Dot( or sheet of them) but the adjustment of existing one's and groups of them on the fly to make their properties programmable.

 

[Sybren]

Well, to correct you a little: quantum dots are nano-sized crystals, which are built up from atoms arranged in a crystal lattice, there are thus nucelei present. It is really the same as an ordinary crystal, for example NaCl (salt), which is built up from Na and Cl atoms. The difference for nanocrystals is that the size of the crystal is not a few millimeters as in salt grains, but is a few nanometers.
This small size confines the electronic wavefunctions, and because of this size-dependent properties are observed. Smaller crystals (less nanometers wide) confine the electronic wavefunction more, and the spacing in the allowed energy levels increases.

The analogy to a quantum dot is really a particle in an infinite potential well, from quantum mechanics. The name artificial atom only applies because of the larger spacing between energy levels, the other quantum dot properties are not atom-like.

I am currently working with quantum dots for my PhD, and I have some experience with them (making them, measuring their properties).