Date: Tue Oct 7 12:09:49 2008

Author: Adam Beehler

Subject: Re: LED's in LN2

Post:

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> Hi Adam
> I saw a good version of this demo at a physics teachers conference in
> the Czech Republic.
> They had an unsilvered dewar they lowered in the green LED and as it
> cooled it turned yellow.
> The lecturer solicited opinions about how it worked.
> Then after many suggestions he smiled and said now let's apply your
> hypotheses to this experiment.
> He lowered a yellow LED (at room temperature) into the liquid
> nitrogen and it turned green.
> oops.
> Paul D

This describes my problem now. If any of you have watched the video I
posted, then you might have noticed that each of the LED's I put into
the liquid nitrogen have had their spectrum shift to LOWER energies,
which is contrary to most of the explanations I have read or received.
And thank you everyone for your responses! Now I am confused though.
The explanations make "more" sense to me now, but my results conflict
with the explanation, which says that the shift should be to HIGHER
energies. Any suggestions to what I might be doing wrong in carrying
out the experiment? I have tried other LED's and I have always seen a
spectrum shift to lower energy wavelengths.

I have temporarily posted a link to a video I took here:
http://www.physics.utah.edu/~beehler/uofu/pnJxnLEDLN2.mpg

Here are some of the explanations I have found:
- "As the temperature of the diode is lowered, the band gap and the
donor-acceptor gap both increase together."
- "Let me ask you to predict what will happen when these blue LEDs are
dunked in liquid nitrogen. ... You can see that the color shifts toward
the violet, consistent with the idea that the reduction in internuclear
distances at 77 K causes the electrons to be held more tightly,
increasing the band gap energy and causing the emitted light to shift to
higher energy."
- "The band gap of a LED varies with temperature because the crystalline
lattice of the semiconductor contracts at low temperatures. This causes
the atom to become closer together and the increased atom-atom overlap
widens the band gap."
- "But the LED is the opposite of a black body; As the temperature is
made lower the wavelength gets shorter. ... When an LED is cooled the
density of the states of the atoms are made greater and the band gap
energy increases."

Adam Beehler



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Hi
Adam

I saw a good version of this demo at a physics teachers conference in
the Czech Republic.

They had an unsilvered dewar they lowered in the green LED and as it
cooled it turned yellow.

The lecturer solicited opinions about how it worked.

Then after many suggestions he smiled and said now let's apply your
hypotheses to this experiment.

He lowered a yellow LED (at room temperature) into the liquid nitrogen
and it turned green.

oops.

Paul D



This describes my problem now. If any of you have watched the video I
posted, then you might have noticed that each of the LED's I put into
the liquid nitrogen have had their spectrum shift to LOWER energies,
which is contrary to most of the explanations I have read or received.
And thank you everyone for your responses! Now I am confused though.
The explanations make "more" sense to me now, but my results conflict
with the explanation, which says that the shift should be to HIGHER
energies. Any suggestions to what I might be doing wrong in carrying
out the experiment? I have tried other LED's and I have always seen a
spectrum shift to lower energy wavelengths.

I have temporarily posted a link to a video I took here:
http://www.physics.utah.edu/~beehler/uofu/pnJxnLEDLN2.mpg


Here are some of the explanations I have found:
- "As the temperature of the diode is lowered, the band gap and the
donor-acceptor gap both increase together."
- "Let me ask you to predict what will happen when these blue LEDs are
dunked in liquid nitrogen. ... You can see that the color shifts toward
the violet, consistent with the idea that the reduction in internuclear
distances at 77 K causes the electrons to be held more tightly,
increasing the band gap energy and causing the emitted light to shift
to higher energy."
- "The band gap of a LED varies with temperature because the
crystalline lattice of the semiconductor contracts at low
temperatures. This causes the atom to become closer together and the
increased atom-atom overlap widens the band gap."
- "But the LED is the opposite of a black body; As the temperature is
made lower the wavelength gets shorter. ... When an LED is cooled the
density of the states of the atoms are made greater and the band gap energy
increases."

Adam Beehler





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