Whatever happened to the quantum dots.

[Clive Mitchell]

What's happening on the quantum dot front?

 

[Victor Roberts]

Quantum dots that produce white light could be the light bulb's
successor
http://www.eurekalert.org/pub_releases/2005-10/vu-qdt102005.php

Existing phosphors used with LEDs already have a quantum
efficiency of about 90%, and no one has explained how a
different type of phosphor, which is the function that
quantum dots provide when used with an LED, can improve
energy efficiency.

--
Vic Roberts
http://www.RobertsResearchInc.com
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[Clive Mitchell]

By the way, I love the job you have. I showed your various "at work"
photos to my fellow employees and told them that's what I'd rather be
doing. For several decades, I hung out with musicians and theater
folks. I devised a few lighting devices and musical effects boxes, but
never sought full time occupation in the field. Only one of my
effects, an envelope follower, got used in a recording studio.

It's easy to get a job like mine. You just throw financial security in
the air and bum around doing work you like. Fortunately I got a
trade first (25 years as an electrician this year) so I can turn my hand
to metal munching if money runs low.

It looks like quantum dot devices would probably end up as useful tools
in your arsenal of effects lighting.

Yeah they look interesting if anything comes of them.

I reckon I managed to get one of the first (incredibly expensive Nichia)
blue LEDs used on a BBC prop. Maybe I should try the same with a
quantum dot device if they don't turn out to be a flash in the pan.

 

[Victor Roberts]

Victor,
That particular science article has the statement "The full spectrum
emission is an intrinsic effect. ". Could I assume this is probably
the result of the various "broadening" effects I just learned about?

Ron,

Normal phosphors can have broad spectral output. I believe
that the tri-phosphor system users narrow band phosphors
because that is the best way to create high efficacy and
high CRI at the same time using our current system of CRI
measurement. (Note that there is a lot of discussion about
CRI measurements and whether or not the system should be
changed.) But I may be wrong. Perhaps the rare earth system
is naturally narrow band and to get wide band using
phosphors we would have to move back to the older
halophosphate system which is has lower efficacy, especially
when designed to have high CRI, and also shorter life when
bombarded with UV.

The fact that the article treats quantum dots as if they
provide some fundamental feature that was not available
before is why I classify it as a PR article instead a
serious attempt to introduce a new technology. There are
non-quantum dot phosphors that will produce a warm white
light when used with a blue LED. Perhaps the SPD produced by
the quantum dots is better than that produced by a phosphor,
but we will never find out from an article that just about
ignores the use of phosphor with LEDs. Perhaps the quantum
dots will produce a more efficient white light source than
phosphor, but since the quantum efficiency (the number of
visible photons produced for each exciting photon) is over
90% for current generation phosphors, it is hard to see how
quantum dots can provide higher efficacy unless they have a
QE greater than 1, which is possible, even for phosphors,
but very hard to achieve when the energy of the excitation
photon is so close to that of the emission photons - and
there is no mention of QE greater than 1 in the article -
just an implication that this is all new.

--
Vic Roberts
http://www.RobertsResearchInc.com
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[Victor Roberts]

On 25 Feb 2006 20:22:45 -0800, "Zink"

[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."

That was what I wondered about - the broadening effects. It is too bad
that it isn't the technological breakthrough they thought it might be.
I got used to noticing some "not so valid" aspects of some of the
science releases. In this case, whether or not the researchers were
even considering the efficiencies, they simply wanted to broadcast
their discoveries. Well hey, it's a university lab, not the real world
yet.
[snip]

they could have injected one group with THC and antibiotics. Then they
could have acknowleged the happy rats and declared antibiotics go
better with pot. Maybe in Amsterdam.

I always have to discern between "possible" discoveries, and
discoveries based on solid research principles. I still enjoy the
reading. No reply necessary.

Well - a reply you shall have

I also re-read the article, this time skipping over the
advertisements for LEDs, but reading the rest more
carefully. I also find the emission characteristics of
quantum dots quite interesting.

I had not thought much before about their size, but they are
smaller than the wavelength of the light they are producing,
which changes the game completely. This group has
previously discussed the beneficial characteristics of
emitters that are about the same size as the wavelength of
the light being emitted. For example, it has been proposed,
and even demonstrated in a simple way, that patterning the
surface of tungsten or carbon with a lattice of "cells" that
are about 1 micron in size can reduce 1 micron and longer
radiation from incandescent emitters, thereby increasing the
percentage of light that is emitted in the visible region.

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.

--
Vic Roberts
http://www.RobertsResearchInc.com
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[George Pontis]

On 25 Feb 2006 20:22:45 -0800, "Zink"

[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

--

 

[Victor Roberts]

Victor Roberts wrote:
[snip]

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!

Interesting! Is there a published reference or just private
discussions about this matter?

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.

That seems logical.

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.

Also logical.

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.

And again.

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.

But, in defense of quantum dots - this is a new area and we
cannot expect them to compete yet with a mature technology
like phosphor. The real question is do they have the
potential for improving the performance of white light LEDs
or will they just be only "as good as" phosphors when fully
developed? Based on your new information about the claimed
intrinsic broadening, it seems that the latter may turn out
to be true, but I believe that we still do not know.

--
Vic Roberts
http://www.RobertsResearchInc.com
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