Neopixels? Try Liquid Nitrogen To Color Shift Your LEDs Instead

If you’re like us, you’ve never spent a second thinking about what happens when you dunk an ordinary LED into liquid nitrogen. That’s too bad because as it turns out, the results are pretty interesting and actually give us a little bit of a look at the quantum world.

The LED fun that [Sebastian] over at Baltic Lab demonstrates in the video below starts with a bright yellow LED and a beaker full of liquid nitrogen. Lowering the powered LED into the nitrogen changes the color of the light from yellow to green, an effect that reverses as the LED is withdrawn and starts to warm up again. There’s no apparent damage to the LED either, although we suppose that repeated thermal cycles might be detrimental at some point. The color change is quite rapid, and seems to also result in a general increase in the LED’s intensity, although that could be an optical illusion; our eyes are most sensitive in the greenish wavelengths, after all.

So why does this happen? [Sebastian] goes into some detail about that, and this is where quantum physics comes into it. The color of an LED is a property of the bandgap of the semiconductor material. Bandgap is just the difference in energy between electrons in the valence band (the energy levels electrons end up at when excited) and the conduction band (the energy levels they start at.) There’s no bandgap in conductive materials — the two bands overlap — while insulators have a huge bandgap and semiconductors have a narrow gap. Bandgap is also dependent on temperature; it increases with decreasing temperature, with different amounts for different semiconductors, but not observably so over normal temperature ranges. But liquid nitrogen is cold enough for the shift to be dramatically visible.

We’d love to see the color shift associated with other cryogens, or see what happens with a blue LED. Want to try this but don’t have any liquid nitrogen? Make some yourself!

23 thoughts on “Neopixels? Try Liquid Nitrogen To Color Shift Your LEDs Instead

  1. Decades ago I read an article (by Lancaster? Mims?) Where they dunked red LEDs in LN2. They had poured the LN2 into a foam cup. The brightness of the LED increased dramatically while submerged.

        1. In bipolar transistors several things vary with temperature. Current gain usually falls at low temperatures, so the effectiveness of the transistor may decline even as the materials within it conduct better.

          Texas Instruments made positive temperature coefficient silicon resistors.

    1. I remember trying this in the late 90’s and none of the LEDs I had showed any noticable color change. One very old red LED got brighter, but stayed deep red. I don’t know what chemestry they were–there’ve been a half dozen different types of ‘red’ LEDs (or more).

      I wish there was a cheap place to get LN2. In college, it was $0.40/L–cheaper than milk. A lot of semi-dangerous fun was had over a few weekends!

      1. If you look at the color-spectrum in the video, you’ll notice that the “red”-range is extremely wide. Yellow was deliberately picked for my video because the yellow-range is extremely narrow. So even a tiny change is bandgap enery is immediately visible to the human eye. If you get a red LED at the higher photon energy end of red, you will observe a shift to orange.

      2. Me too! In fact, I likely still have a mini DV tape somewhere of it. I did get a green shift from a yellow LED, I think. I had one Cree SiC blue LED at the time (really more “cyan”, but I don’t think I brought it along.

        1. I would love to find some SiC blue LEDs. I bought two back in the day and I’ve lost them. I remember them having a very nice shade of blue-green. Seemed to have a very wide frequency spectrum.

    1. You’re absolutely right. What I didn’t mention in the video is how many different LEDs I fried until I found a batch that worked extremely well. My red and green ones die before any visible color-change becomes visible.

    2. What about -40f

      And no not unless it’s from the 70’s… heck it’s a basic requirement to spend days at -40 and thermal shock 105c to -40 in even basic automotive applications

      That’s just badicly normal stock with a paper qualification

  2. Can this be used to push a UV led further into the UV range?

    The shortest wavelength LED listed on DigiKey is 235 nm. The distance between yellow and green is hard to judge, because (the English terms for) green is 75 nm wide and yellow is 20 nm wide, but taking 1/4 the distance we can estimate the shift to be about 25nm.

    Can a 235 nm LED be shifted to 200 nm using this method? That might admit some interesting hacker opportunities.

    Additionally, could you bring the temperature down on an LED and simultaneously measure the temperature and wavelength, giving you an infinitely selectable wavelength within that range?

    This might lead to an interesting reflection/transmission spectrometer with tremendous resolution.

    (The 235 nm LED is pricey at $345, but there’s a $7 one at 255 nm, which is far enough in the UV range to be interesting.)

    1. LEDs are not as monochromatic as lasers. Take a look at a few datasheets to get an idea of the typical spread. This can be a disappointment if you’re looking for a very cheap and convenient pure color.

  3. I suspect that the yellow led is the best color to run this experiment with. Yellow is the narrowest color in the visible spectrum, a small shift in wavelength is thus easily visible.


  4. I’ve seen this effect in the opposite direction when soldering some SMT LEDs while they were powered on. The LED would color shift as I heated up the solder, I’d remove the iron after the solder flowed, and in a few seconds time I’d see it change back to its intended color.

  5. When I was doing research at an university my morning routine was to go get a thermos full of liquid nitrogen to cool down the IR thermal camera. Dunked a few LEDs in that thermos just for fun, the trick never gets old and impressed fellow researchers that were unaware of it.

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