Re: Whatever happened to the quantum dots.

Victor Roberts wrote:

On 25 Feb 2006 20:22:45 -0800, "Zink"
<rons@xxxxxxxxxxxxxxxxxx> wrote:

[snip]

I re-read the article and found that the answer to my question
already there. They stated:
"In the larger nanocrystals, which produce light in narrow spectral
bands, the light originates in the center of the crystal. But, as
the size of the crystal shrinks down to the magic size, the light
emission region appears to move to the surface of the crystal and
broadens out into a full spectrum."

...
...

I don't fully understand the emission mechanism of something
smaller than the wavelength of something smaller than the
wavelength of the emitted energy. The few articles I have
read speak of quantum dots as "artificial atoms" since their
small size creates quantized energy levels, and these levels
change with the size. This can apparently explain why larger
quantum dots emit reddish light while smaller ones emit
light closer to the blue end of the spectrum, but none of
these explanations explain why dots of a certain size should
emit light of many wavelengths, as would be required for
white light.

So the broadening mechanism, your original question, remains
unanswered. Unless, perhaps, the "dots" created in this
experiment were not semi-spherical at all and had such a
complex shape that they supported many different sets of
quantized energy levels, each corresponding to one of the
many dimensions of their complex share, and these many sets
of quantized energy levels then lead to the emission of many
different wavelengths of light. Just a theory from someone
who knows nothing about the subject.

I have just a little knowledge of this area from having working
alongside chemists that were making quantum dots like this. I am surely
not a chemist or an authority on the subject, but from what I learned
this broadening effect did not happen!

The whole thing that was interesting about dots is that one could
control the wavelength by choosing the material (CdSe being popular for
visible wavelengths), and the size of the dot. One measure of how well
the dots were grown was how narrow the spectral distribution was. In
process that was not well controlled, dots might be produced with a
range of sizes and thus the spectrum would spread as a consequence.

There is a finite range of dot sizes that is convenient to grow yet
still provide quantum confinement, so to cover a wider range of
wavelengths you can use different materials to shift the entire
spectral range up and down.

We are talking about dots that have 80-100 atoms in them. This is a
hard thing to control, since if you lose a few atoms then the
wavelength wanders off. Usually some kind of protection is needed to
help keep the dot intact. When the whole thing is put together, the QD
may not be as rugged as one would hope. I doubt that anyone has dots
that can compare with the mature and high performance phosphors that
are already in common use in lamps. If they did, you can imagine some
high value applications that would justify a lot more money than would
any lighting application.

George

--

.

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