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Green LED in liquid nitrogen turns yellow

  1. Apr 7, 2014 #1
    Hi,

    While experimenting with various LED's in LN2, I noticed some strange behaviour from my green LED's.
    As expected all LED's (except the green one's) emitted light with a shorter wavelenth (for instance, my yellow led turned to red etc). However, my 2 (different manufacturer type LED) green LED's turned yellow'ish in liquid nitrogen. At first I thought this had to do with some property of the LED, but the second green LED showed the same shift (to a higher wavelength).

    I've thought about this a lot, but I can't think of any reason why the green LED's don't turn blue'ish. Any suggestions how to tackle this (for me still unsolved) mistery?

    Thanks in advance!
     
  2. jcsd
  3. Apr 8, 2014 #2

    Cthugha

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    Is that a typo? The red part of the em spectrum is at longer wavelengths than the yellowish part.

    Besides that: Nitrogen should not absorb much in the visible, so I guess it is indeed a property of the LED. Do you know some details about the LED? Is there some kind of cavity involved? Are there any localization effects which may play a role at low temperature?

    And have you actually measured the intensities or do you judge by eye? The eye is very sensitive in the green, but not really in the UV.
     
  4. Apr 8, 2014 #3
    What you describe (orange to red and green to yellow) is a shortening of wavelength, in both cases.
    This means an increase of energy. It show that the band gap increases when you decrease the temperature.

    However it may go the other way, too, in some types of LEDs, I believe.
     
  5. Apr 9, 2014 #4

    Andy Resnick

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    2016 Award

    Both the peak frequency and spectral width of (solid state) electroluminescence vary with temperature. Under the conditions of weak injection (weak biasing), the peak emission frequency hω= E + kT/2, where E is the bandgap energy, k is Boltzman's constant, h is Planck's constant, and T the temperature. The spectral width Δω≈ 1.8kT/h.

    Note, the peak emission is limited by the gap energy.
     
  6. Apr 9, 2014 #5

    Borek

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    Staff: Mentor

    I thought red is the longest of the visible, so orange to red should mean lengthening, or decreasing the energy. Say, 590 nm to 620 nm.

    What am I missing?
     
  7. Apr 9, 2014 #6
    Yes, you are right. It's my mistake completely. Thank you for pointing this out.
    I cannot edit my earlier message now.:cry:
     
    Last edited: Apr 9, 2014
  8. Jul 11, 2014 #7
    I wondered about similar thing, if it's a good idea to raise this old thread.

    Putting red, UV or blue LED in LN does not change their colour visibly.
    White LED stops working.

    But green LED turns orange.

    Videos i seen of that show an orange LED turn green - an opposite direction to my green one turning orange.
    The videos explain it as band gap changing, resulting in higher-energy light, i.e. here - http://www.ap.smu.ca/demos/index.php?option=com_content&view=article&id=181&Itemid=78

    However, that is not the case with a green LED.
    What is going on there?
     
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