Date: Wed, 26 Apr 2000 13:23:55 -0400

Author: Gerald Zani

Subject: Re: LED and Liquid Nitrogen

Post:

Dale,

Good question. Wish I knew. If you figure it out, fill me in. Below,
here is
some summer beach reading to get you started. Mostly these are articles about
using colored LED's to measure Planck's Constant. I have not yet found an
article that talks about the color change when the LED is cooled. I am
searching.

Phys. Teach., Vol. 12, No. 7, October 1974 Pages 423 - 425
Measuring Planck's constant using a light emitting diode
Patrick J. O'Connor
Leah R. O'Connor
DeVry Institute of Technology, Chicago, Ill. 60618

Phys. Teach., Vol. 35, No. 5, May 1997 Pages 261 - 261
Apparatus for LED measurement of Planck's constant
D.F. Holcomb
Department of Physics, Cornell University, Ithaca, NY 14853- 2051;
dfh1@Cornell.edu

Am. J. Phys. Vol. 64, No. 12, Dec 1996, Page 1448
Question #53. Measuring Planck's constant by means of an LED
F. Herrmann and D. Schatzle
Abteilung Didaktik der Physik, Universitat Karlsruhe, 76128 Karlsruhe, Germany

Phys. Teach., Vol. 35, No. 4, April 1997 Pages 198 - 198
LED's: their charm and pitfalls
D.F. Holcomb
Department of Physics, Cornell University, Ithaca, NY 14853-2501


Phys. Teach., Vol. 31, No. 5, May 1993 Pages 262 - 263
Light on LED's
W. N. Hubin
Smith Laboratory of Physics, Kent State University, P.O. Box 5190, Kent, Ohio
44242-0001

Phys. Teach., Vol. 34, No. 3, March 1996 Pages 144 - 146
LED's in physics demos: a handful of examples
Dan Lottis
Herbert Jaeger
Department of Physics, Miami University, Oxford, OH 45056

Phys. Teach., Vol. 35, No. 2, February 1997 Pages 108 - 109
Measuring the Planck Constant with LED's

Hi Jerry :
It was nice talking with you again. I looked up some
literature about the changes in LEDs when immersed in LN2. What I read
agrees with the second message down there. Yes, immersing the material in
LN2 will cause the atoms to move closer together (contraction) causing
greater interaction between individual atoms and hence a greater band gap.
Hence the light should shift to higher color (i.e higher energy) -- blue.
The shift to yellow contradicts this hypothesis.

The increase in brightness can be explained by the freezing out of
the phonon vibrational modes at very low temps. So that in an indirect
band gap semiconductor where lots of the energy is lost to phonons to
conserve momentum, at low temps. this loss does not occur and most of the
energy lost by the electron making the transition from conduction to
valence band is converted to light.

As regards an alternative explanation for the change in color (Green to
Yellow) of the LED, I will hold off on the one I gave you earlier
concerning impurity levels. As I told you over the phone, the energy
differnce between a shallow donor which is ionized at room temp. and the
conduction band edge should be of the order of a few tens of meV. Hence
unavailability of energy to ionize these donors should at most cause a
shift in energy by only about 20meV at best from room to LN2 temp. This
shift is insufficient to explain the observed shift from green to yellow
which would be at least an order of magnitude higher.

> ---------------------------------------------------------------------------
> Date sent: Mon, 18 Nov 1996 14:24:02 -0200
> From: Marcelo Magalhaes Fares Saba
>
> While showing my pupils some good demos with Liquid Nitrogen, it ocurred
> me to put a lit green LED into the LN2. Its colour changed to yellow!
> Has anyone done this? Has the semiconductor gap changed?
>
> The LED didn't simply get brighter but yellow. The red ones don't
> change colour.
>
> Date sent: Mon, 18 Nov 1996
> From: "Dave Baum - Drew University Physics Dept."

> Subject: Re: LED in LN2
>
> Generally, the change in band gap energy goes the other way:
> colling the material WIDENS the gap, shifting the colour more towards the
blue.
>
> db
_________________________________________________________>

Hey...Heat the thing with a heat gun...guess what...it is yellow...

Phys. Teach., Vol. 35, No. 2, February 1997 Pages 108 - 109
Measuring the Planck Constant with LED's
L. Nieves
G. Spavieri
B. Fernandez
R.A. Guevara
Departamento de Fisica and Centro de Astrofisica Teorica, Universidad de los
Andes, Merida 5101, Venezuela

Phys. Teach., Vol. 35, No. 5, May 1997 Pages 261 - 261
Apparatus for LED measurement of Planck's constant
D.F. Holcomb


At 11:44 AM 4/26/00 -0500, you wrote:
>
>Karl and JZ,
>
>You once showed me the demo where you took the large red-orange LED and
>cooled it in liquid nitrogen which changed the emitted color to yellow.
>I have used this several times but am not sure that I completely
>understand what is causing the color to change. Any detailed
>explainations available on this?? Any info at all?
>
>Thanks,
>Stille
>U of Iowa
>

********************************************************
Gerald Zani
Lecture Demonstration Technician
Brown University
Dept. of Physics
Box #1843
Providence, RI 02912

Gerald_Zani@brown.edu
Phone: (401) 863-3964
Fax: (401) 863-2024

http://www.physics.brown.edu/users/staff/zani/index.html
http://www.physics.brown.edu/Studies/Demo/

Do a little more of that work which you have confessed to be good, which you
feel that society and your most Just Judge rightly demand of you.
Cultivate the tree which you have found to bear fruit in the soil of your
soul. If you have any experiments you would like to try, try them.
Now's your chance.
H.D.T. Journal. 1850






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