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In my world, extremely small LEDs are vital.

My question is, with today's technology—or any reasonable improvement of, i.e., “we don't have the machine to build it but we have the sciences and theories”—how tiny a LED can we make given the following constraints?

Despite the title explicitly stating LEDs, please assume "LED" in this question refers to any light-emiting device powered electrically. The components by which it electrically produces light is free game; gas/plasma bulbs, liquids, solids, etc., are all acceptable components/properties of the device. Preferably, these constituents would be materials common in the universe, nothing too crazily inabundant. Take that however you like, it is less a constraint than a sidenote.

The LED may have any lifespan, milliseconds or years, doesn't matter—this may be to an answer's advantage, as it suggests that the device may be spontaneous and self-destructive to accomplish the goal.

The LED needs to be bright, preferably bright enough to see an individual LED (at least above the range of human minimum visibility in total darkness).

The LED needs to fare well around others like it. It shouldn't cause others in its immediate vicinity to change state or become damaged in some way. Making a display panel with the devices should be conceivable.

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    $\begingroup$ en.wikipedia.org/wiki/Luciferase <-- is still small enough? $\endgroup$ – NofP Dec 18 '18 at 15:36
  • $\begingroup$ @NofP That's indeed small, but does it fit the constraints? Or rather, can it be made to fit the constraints? $\endgroup$ – B.fox Dec 18 '18 at 15:38
  • $\begingroup$ Atom thin graphene. $\endgroup$ – SZCZERZO KŁY Dec 18 '18 at 15:42
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    $\begingroup$ Are the LED's in my smart phone small enough? There are 1920 * 1280 *3 (7372800) packed into 4.5 * 2.5 inch panel. $\endgroup$ – pojo-guy Dec 18 '18 at 20:55
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    $\begingroup$ @B.fox Then tell us what pixel size you want. Note that the diffraction limit means there's no point in making a pixel smaller than 1.2-1.5x the wavelength of light, so that's about 1.2 um for visible light. Modern OLED displays are around 6x this limit and LCoS is about 3x. Camera sensors hit this limit. $\endgroup$ – user71659 Dec 18 '18 at 21:18
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Let's say LED stands for light emitting device.

I'd go for phosphorescence. Basically, a photon is "captured" by an electron in the form of energy to be released later, as a photon with a frequency that is specific to the element.

  • It occurs at subatomic level: you only need one atom for one frequency. If you want to have different colors and intensities, then you need more atoms.
  • Their lifespan ranges from nanoseconds to hours.
  • It seems that humans can sense single photons. To make sure you can pack a few.
  • To avoid one interfering with the next, they could go inside a multi-walled carbon nanotube. You'd probably need a number of layers to make the walls opaque (somewhat like graphite but made of concentric tubes instead of successive planes of graphene).

Not sure about the fine-tunning, but I don't think they would need to be more than one or two dozen atoms in radius (i.e., in the order of nanometers or even ångströms). Regarding the technology level needed, it is currently possible to build multi-walled carbon nanotubes as well as nanotubes of different radii, but I think you would still need a scientific breakthrough or two to manufacture them in the exact shape and arrangement needed.

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  • $\begingroup$ This is the nice, creative approach I was hoping for. The humans-seeing-single-photons thing is somewhat surprising. I thought only certain kinds of frogs could do that. Graphene and carbon nanotubes are good constituents for my world setting—not giving you ups for that because it wasn't mentioned in the question, just my appreciation. I suppose I'll need to find out which elements would suit best, though, colors aren't much of an issue. Oh, not to be nitpicky, but I think you mean Ångströms. Good answer. $\endgroup$ – B.fox Dec 18 '18 at 18:32
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Let me be blunt

A microscopic LED is entirely believable. I'd ignore all the explanations of why or why not and roll with the idea.

Now the answer

LEDs are diodes that emit light when they're in their operational state. Fundamentally, when you ignore the mechanics of miniturization (fabrication), your limitations are three-fold.

  • First, even at magical 100% efficiencies, it takes electrons to create light. The more light you want, the more electrons you need. Simplifying a bit, the quantity of electrons moving along a wire is amperage. And what happens when you push more electrons through a wire than it can handle? It gets hot and melts because no non-superconducting wire has zero reistance. The problem is, for a semiconductor object (diodes and transistors) to operate, you need resistance. Therefore, a diode will always be a fancy and expensive fuse. Theoretically you can improve the design of the diode — but it's unreasonable to believe that can go on forever.

  • Second, diodes kinda act like the flapper on the exit of your clothes dryer vent. When the pressure (voltage potential) is in the correct direction, the flapper opens and the air (electrons) exits. When the pressure is in the other direction, the flapper closes to keep the cold air out of your house. Not unlike those flappers, you need a minimum amount of pressure just to open the flapper (big diodes frequently need 0.3-0.7 volts to begin passing current, but this can change based on the manufacture of the diode). However, the system, like the flapper, isn't perfect. Enough reverse pressure will force the flapper to open inward (for diodes, this is called "breakdown voltage"). The problem with LEDs is that the breakdown voltage is low because of the physics of creating the photons. That was a very long and fancy way of saying they're intolerant. And the smaller you get, the lower the tolerance because you have less diode to work with to keep everything working. The compromise is lower light (e.g., the smaller the diode, the less light you can produce with it). Modern LED panels can be thought of as a huge number of small diodes all packed together to get lots of light. But the smaller those LEDs are, the bigger the panel must be to get the same amount of light. OK, that was really lengthy, but I can't stand not putting this in terms of Calculus. As the size of your LED approaches zero, the amount of light it can produce also approaches zero (which, if you think about it, is obvious).

  • Third, and the most dramatic problem, is that as you shrink a semiconductor, you eventually get to the point where you're using molecular thicknesses — and then atomic distances. Really weird things happen when you start goofing around with atomic distances. Back in the 80s when we were using micrometer (10-6) semiconductor design, it was frankly unbelievable to us that nanometer (10-9) design was even possible, because you hit molecular distances at angstrom (10-10) designs and nobody even knew if a semiconductor could operate that small. Then somebody made it work. Today the same arguments are occuring about picometer (10-12) and femtometer (10-15) designs because at that point your approaching atomic distances. From a realistic standpoint, once your semiconductor gate is a single atom wide, it stops working because you can't create the necessary resistance that makes a semiconductor work (remember, "semi-conductor" or "kinda-conducts" because there's resistance involved). So, the smaller you make your LEDs, the less believable they'll be to physicists and electrical engineers — but we're only a part of your audience, so forget about us and do it anyway.

But, to reiterate...

Sometimes good fiction is about believable science. Sometimes it's not. We literally wouldn't have most of the science fiction we enjoy today if we only stuck to what we knew could be done. And since I've lived through it, I know that even when everybody believes it can't be done — sometimes it can. So, if anybody here is telling you it can't be done. Ignore them and write a great story. Just because we can't do it today and don't think we can do it tomorrow doesn't mean at all that we won't do it the day after that. Humans are cool.

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    $\begingroup$ Wow. Your bluntness is absolutely excellent. Although I haven't limited it to just LEDs (I've rather crudely used it as a blanket term for anything that emits light), you really give insight to the difficulties such a technology would face. Given your training in electrical engineering, you probably don't think so, but to someone who rather isn't familiar with the minutiae, it's enlightening. Of course, go small enough, and effects like quantum tunneling start screwing up your nanomachines. Regarding your first note, yes, I like it when authors just create plausible ideas and roll with them. $\endgroup$ – B.fox Dec 18 '18 at 19:13
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    $\begingroup$ No mucking about in the boring specifics of it. Conversely, however, and regarding your last note, I also like it when authors subtly delve into the mechanics under the hood. It certainly adds credibility to a world if nothing else. Personally, and for my specific purposes, I like to adhere to the constraints of the technology I'm working with. Keeping with the continuity. $\endgroup$ – B.fox Dec 18 '18 at 19:14
  • $\begingroup$ Although you didn't posit a solution, your answer is penetrating into the real constraints, and is appreciable. Thank you. $\endgroup$ – B.fox Dec 18 '18 at 19:16
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    $\begingroup$ :-) I didn't posit a solution because, though I believe it will come to be, I don't know how to manipulate today's tech to make it happen. I can only tell you what the consequences of that manipulation are: dense doping on small substrates with remarkably precise control, leading to well defined transition areas between doped regions. But, frankly, that's boring for a book. I'm far more interested about how you're going to use the lights. $\endgroup$ – JBH Dec 18 '18 at 20:58
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    $\begingroup$ The real limiting factor isn't semiconductor physics, it's simply the wavelength of light. There's no point in making a pixellated source much smaller than a wavelength because diffraction means the individual sources cannot be resolved. Given that diffraction in the visible will start to become an issue at around 1 um, this limit can be effectively reached today. $\endgroup$ – user71659 Dec 18 '18 at 21:27
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Yes, it's possible to have LED displays with microscopic pixels. 3.74 μm qualifies as microscopic, I think.

The JD4704 is currently the world’s first 0.7”, color sequential, 4K2K LCoS microdisplay comprised of over 10 million of the world’s smallest all-digital pixels, with a 3.74 x 3.74 μm pixel size.

Order them from this company: https://www.jasperdisplay.com/tw/index.php/2014/07/04/20140704001/ Datasheet: https://www.jasperdisplay.com/products/lcos-panel/jd4704-4k2k/

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  • $\begingroup$ Wow. That's actually very impressive. I didn't know such a thing existed. $\endgroup$ – B.fox Dec 18 '18 at 19:20
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    $\begingroup$ @B.fox Impressive, but definitely not surprising. LEDs are just electronic devices at the core, and we've shrunk electronic devices to sizes of tens of nanometers already, creating chips with billions of transistors. Sure, it's expensive to scale down, but definitely doable. And there is no fundamental reason why an LED should be any bigger than a transistor. $\endgroup$ – cmaster Dec 18 '18 at 21:04
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    $\begingroup$ LCoS stands for Liquid Crystal on Silicon and is an LCD display, it doesn't emit light but is a controllable absorber. You need a separate light source which may or may not fit in the size constraints of the OP. $\endgroup$ – user71659 Dec 18 '18 at 21:17
  • $\begingroup$ @user71659 Nice observation, I do believe that puts a damper on things. $\endgroup$ – B.fox Dec 18 '18 at 21:21
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How about MicroLEDs? They're brighter, faster and cheaper than OLEDs.

by Rich Shibley/Digital Trends

Comparison LED vs. MicroLED, from digitaltrends.com

I can't find any precise measurements though.

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    $\begingroup$ Or at least potentially cheaper. The current challenges with microLEDs are mainly in how you move around thousands of μm-sized LEDs and assemble them into a panel. The "raw material" for LEDs are crystal layers grown on a wafer, so you can theoretically make LEDs as small as you can manage to dice it up and connect up the bits. I think this fits well with the criterion "we don't have the machine to build it but we have the science and theories." $\endgroup$ – IceGlasses Dec 18 '18 at 23:50
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The constrain on LED size is not given by technology.

State of the art technology allows us to realize features as small as 7 nm for industrial production, with feature of 3 nm being already realized in research.

The limit is given by the eye resolving limit (0.008 degrees).

Their minimum size therefore strongly depends on the distance at which they are supposed to be viewed.

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    $\begingroup$ You can see a light emitting point source much smaller than the eye's resolution. Don't believe it? Go outside on a clear night and look up. An "average" star seen from Earth has a disk in the micro arc second size range, yet we can clearly see them. And they're a VERY long way away -- the nearest ones are trillions of miles, and we commonly see stars (with the naked eye) that are up to tens of thousands of times that far. $\endgroup$ – Zeiss Ikon Dec 18 '18 at 18:54
  • $\begingroup$ @ZeissIkon, the OP stating "adjacent LED" excludes isolated point sources. $\endgroup$ – L.Dutch Dec 18 '18 at 19:04
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    $\begingroup$ All the OP says about adjacent LEDs is that they shouldn't damage adjacent ones, i.e. ought to be capable of assembling into a display. No reason nanoscale LEDs with several-atom construction couldn't do that. The limit to being able to see them is probably light emission power, which is limited by available electrical power and cooling. Nothing to do with the eye's resolution. $\endgroup$ – Zeiss Ikon Dec 18 '18 at 19:07
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Wikipedia notes "Self-assembled quantum dots are typically between 5 and 50 nm in size", and quantum dot displays are currently being manufactured by many of the various large display companies. If a display with very small luminescent pixels is your goal - as opposed to strictly a miniature LED display, this would seem like a promising approach.

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  • $\begingroup$ Definitely something I will look into, thank you for the suggestion! $\endgroup$ – B.fox Dec 18 '18 at 23:26
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As other answers suggest, the limit is a few atoms (if we're talking about actual LEDs), or potentially a single atom if we expand the definition to include light emitted by phosphorescence or fluorescence.

If you're talking about a large 2D matrix of these components, then you can't neglect the space needed for supporting machinery.

  • LEDs need wires to supply them with current, and some means of switching the current to address individual pixels. These wires can potentially be as narrow as a single atom, and since each pixel has an area of multiple atoms (see below), I think it is reasonable to assume that you can fit whatever circuitry you need for each pixel behind the display.
  • Fluorescent pixels need to be excited with (probably ultraviolet) light. This could come from behind the display, but that would mean that the fluorescent pixels have to sit on top of an LED display, so there's no space saving. Alternatively, you could have a plane of fluorescent atoms and illuminate them with an external source, like a laser. The problem then, though, is that if the pixels are smaller than the wavelength of the external light source, you will not be able to activate just one pixel at a time. This limits the pixel size to perhaps 350nm. It's also worth noting that when a fluorescent atom or molecule is excited, it will sometimes (often) return to the relaxed state without emitting a photon, so even if you could have single-atom fluorescent pixels, each one would only work some of the time.
  • Phosphorescent atoms can be excited in other ways, like a chemical reaction or a free electron (as in a CRT). In theory the wavelength of an electron is much shorter than that of UV light, so you could illuminate a single-atom pixel with a very precise electron beam. But there are the same yield issues as with fluorescent pixels, plus phosphorescent emission works over longer time scales, so I don't think there is any advantage here.
  • Quantum dots, for this purpose, function as giant fluorescent molecules. They are typically made of hundreds of atoms.
  • For completeness, I'll rule out incandescent lights. A single-atom light bulb filament would need a lot of space to isolate it from neighboring pixels, and would almost certainly burn out in a tiny fraction of a microsecond.

So, I think the most plausible candidate is LEDs. At least, that doesn't have any obvious roadblocks. I can imagine the front layer of a very hand-waving RGB nano-display looking like this:

LED nanopixel

I am guessing that you'd need more than one semiconductor atom in the front layer in order for the light to be emitted at the surface, rather than inside the display where you wouldn't see it. And the LEDs need to be electrically isolated in order to operate independently. So, pretending that atoms are all the same size (which they aren't), an RGB pixel has an area of 24 "atoms".

The important thing here is that each sub-pixel can easily be smaller than the wavelength of light it emits. In the way we normally think about resolution, the resolution couldn't be any higher than this anyway.

However, at this scale, it becomes theoretically possible for the display to work as a phased array. If you can get the LED elements to emit light with a single, controlled phase (which seems plausible(?) since we're emitting single photons to order), then you can basically make the whole display work as a programmable, full-color true hologram, so you can display 3D objects just as if the display were a glass window. Of course this would require enormous bandwidth and processing power.

Heat would definitely be an issue – you're doing a lot of switching in a very small volume – but I don't know of any fundamental reason why such a display couldn't work if it were built on, say, a liquid-cooled copper plate. So I'd accept that if I read it in a book.

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