Quantum Dot LEDs: Making Custom Color LEDs

Let me preface this by saying that this is NOT a cheap experiment by any means. At this time quantum dots are VERY pricey, with typical prices ranging from around $150 to $500 for 10mg. That being said, I used very little per LED. I estimate used about a tenth of a mg or less per LED. So assuming the high end. That is $5/LED and the low end $1.50/LED. Like I said, not cheap but it allows you to literally make ANY color you desire. Also, keep in mind the higher the cost the better quality materials. Don't pay attention to just quantum yield, also keep an eye on accepted variation in emission (this is REALLY important in the green-yellow-orange range) and FWHM. All of that being said, this is something that ANYONE can do. It's as simple as dipping them and evenly coating them in a mixture of QDots and glue.

I am in a unique position in that I research quantum dot synthesis (though for biotech use NOT for electronics use). So I have access to the QDots from failed reactions. All of the LEDs I am making are using failed materials (no I can't sell you any, I'd get fired quicked than you can say QDot :P) which didn't meet our rigorous specs. These are materials which would have been disposed of and are being repurposed just for fun.

I apologize now for the photos. They are grainy because they are all cell-phone pics. My stupid camera up and refused to turn on. So cell-phone pics it is.

Step 1: SCIENCE!

Now on with the article! Have you ever been designing a project and wishing that you had a very specific color of LED? Well thats what this article is going to discuss. But first, some science. If you don't care for the science and theory then skip on to step 3.

Step 2: Science

Quantum dots are tiny particles of semiconducting materials which emit light on a size dependent basis. Basically, the larger the QDot the more longer the wavelength. The other factor that determines emission color is the band-gap of the semiconductor. Now I won't go into discussing it here because honestly its kinda boring but the higher energy the band gap the bluer the emission. Commonly used materials in quantum dots are Cadmium Selenide, Cadmium Sulfide, Cadmium Zinc Sulfide, Indium Phosphide, Indium Gallium Arsenide, etc.

Structurally, these are typically encapsulated in a high band-gap material such as Zinc Sulfide forming a Core/Shell type structure. This outer ZnS shell makes them dramatically brighter and much much much more stable. The most common format is Cadmium Selenide/Zinc Sulfide (CdSe/ZnS) core-shell dots. These typically have emission wavelengths in the 500-650nm range (so from greenish-blue to deep red).

Now there are two methods of making a QDot based LED. The first is to isolate the QDots in between hole transport and charge transport layers a la OLEDs. This style is beyond the capabilities of most people. The other way is to cover the outside of a deep blue or UV LED with the QDots suspended in a solid matrix. This later method is the one which we will be using.

Now I know someone will want to ask can this project can be accomplished with other fluorescent materials as well, not just QDots. The answer is kind of. With careful planning, similar results can be achieved but QDots have some unique properties which make them very useful for this project. The first of which is they are VERY stable. Organic fluorophors and phosphors suffer from something called photobleaching where long term exposure to UV light causes a complete loss of emission. To be honest, over the long scale pretty much all phosphors and fluorophors do but the time scale for organic compounds (such as florescein) is incredibly short (hours or days). QDots likely can be measured in months or years. I have bottles sitting on a black lamp in my hood for 9 mo now and they still glow brightly, we put an organic dye on there and it bleached overnight. The second advantage is their absorbance is VERY VERY VERY high in the blue-UV region and it is continuous so any LED in the UV range will work well. The third advantage is that their emission wavelength can be very easily tailored. It isn't difficult to hit pretty much every pure color of the rainbow QDots, though blue and purple are harder/impossble with CdSe based dots. The fourth advantage is that their emission can be VERY narrow. Some organic and even many inorganic phosphors have a FWHM on the order of 50nm or greater. QDots can have under 30nm pretty easily. Combine that with the previous advantage and you get true and pure colors. Allowing for easier mixing and custom tailoring of colors.

Now the warnings:

• Cadmium is toxic. Please dispose of the material properly. If you don't have access to a proper waste disposal like me a better option is to dry it out and then dispose of it in a battery disposal area. They are used to dealing with the Cadmium from NiCad batteries. Please do not just throw it in the trash or worse wash it down the drain.
• Wear gloves when doing this. Not only will it keep you from gluing your fingers together it will protect you from the cadmium.
• UV light can blind you. Most true UV LEDs are intense enough to rapidly cause eye damage. They warn you of this and the risk is real.
• Don't bother trying to make money from this by selling custom color LEDs. The quantum dot IP landscape is a veritable minefield and likely someone has a bajillion patents to stop you from doing it. This instructable is just for experimenting and fun!

Step 3: Getting Started

Materials needed:

1. Cyanoacrylate Glue (aka superglue, the gel type works great)
2. True UV leds, ideally with a peak emission in the 350-400nm range. Keep in mind that the UV LEDs found on ebay are often not. THey are usually purple LEDs (which can still cause fluorescence) and this can give a purple hue to the LEDs.
3. Gloves - protect your hands from the glue AND the QDots
4. Some sort of non-polar solvent like Oops..! or Goo-B-Gone. Look for names like Toluene or Xylene on the ingredients. This isn't 100% necessary but is likely helpful for cleanup. Usually, you can find this at a hardware store. Another possible source is paint thinner. Some are pure toluene.
5. Something to transfer small amounts of a liquid such as a medicine dropper or pipette.
6. Quantum Dots

The last item of course is the most expensive part of the whole thing. You will be looking for organic-soluble nanocrystals or any that list a solvent such as decane, toluene, chloroform, etc.. You are not interested in the water soluble ones (they DO NOT work well with cyanoacrylate glue).

Some sources:

• Life Technologies, an Invitrogen Subsidiary
• eBioscience
• Crystalplex
• NN-Labs
• Voxtel
• Ocean Nanotech

Step 4: Lets Do This!

Before starting this. I did some test runs using nanocrystals in toluen and water. The ones in toluene tended to work the best (except for the yellow which have purity issues and did not respond well in any form).

OK, you have all of your stuff laid out and ready to go. The procedure honestly is quite easy if the nanocrystals are pure.

• Lay out some of the cyanoacrylate glue on a piece of uncolored material. I say uncolored because most dyes will absorb into toluene, coloring your mixture. My first trial was on one of those red plastic party cups and it ate the red right off. In my case I just used the plastic that the super glue came in. It seemed unaffected by both the glue and the toluene.
• Add a small amount of the nanocrystal solution into your glue and mix it up. The glue should get thinner and more liquid when the toluene hits it. Unfortunately, some impure nanocrystals will cause the cyanoacrylate glue to thicken up. I encountered this with one of my batches. It was one I could never get pure and sure enough it turned the glue to rubber.
• Now use this mixture to coat your LED any way you can. I found sort of rolling it around in the mixture generated the smoothest coat
• Let it dry for a bit
• Repeat the above for a total of about 3 coats (that tended to be the amount needed to get a nice uniform even coverage).

As you can see in my first test one. THe water soluble NCs result in

Step 5: Power Up!

Now its time to power up and enjoy your new color of LED!

Some improvements to try:

• Quite possibly a much better method would be suspending these in a clear coat paint and an air brush to apply a much more even coating. This would allow for controlled application and allow the end user to not worry so much about wasted NCs due to drying between multiple layers.
• Nanocrystal Powders
• Other formulations of nanocrystals such as CdZnSe and InP.