Science of Glow by Paige Whitehead

Nyoka Design Labs founder and LÜMI inventor shares her insights on the science of glow: forbidden transitions, dopant induced metastable states, and something called an electron hole.  Get ready for your deep dive with Paige!

By now you have probably heard of the quirky-quarky field of quantum and particle physics. Even the top scientists in the field will tell you, things can get rather ‘strange’. I dove into the mechanisms behind photoluminescent glow powder’s incredible shine along with Dr. Justin Albert, UVic Physics & Astronomy Professor and Harvard Graduate. Dr. Albert works  with the world-renowned ATLAS Experiment at the CERN Large Hadron Collider (LHC)”, which seeks to better understand why mass exists at all (apparently quite a conundrum!) and spends time designing high-tech balloons which launch into space in order to take precise measurements of the expansion rate of our universe (which is perplexingly accelerating, a state of affairs quite uncalled for). I picked his brain for a while about the mechanisms that occur in the photoluminescent compound during its activation and prolonged glow.  We I am here to tell you – it is quite complex and even stranger than you think. Stay with me here – it starts out a little dry, but the deeper we dive the more mind-boggling it gets. But before we get into the deep and dangerous world of physics, I’ll tell you a bit more about things that glow-in-the-dark, or to use the scientific term – luminesce.

Luminescence:

There are many types of non-electric light (aka cold light) that fall under the category of ‘luminescence‘; fluorescence, chemiluminescence, photoluminescence, bioluminescence, and more. These are the most common forms of luminescence you will see.

Fluorescence = road signs.

Chemiluminescence = glow sticks.

Photoluminescence = long-lasting glow powder and other commercial goods.

Bioluminescence = enzymatically produced light, most commonly found in the deep ocean and if you’re lucky, on a beach near you!  Swing over to our page on luminesce for more info!

This article is about phosphorescence, a class of photoluminescence where photons are absorbed by a material creating an ‘excited state’, and then re-emitted in a lower energy state. This is the material science you find our first market-ready product, LÜMI, but this isn't the first time I've seen this #sciencemagic in action; I worked with a company called Core Glow and they sold glow stones, crystals, and marble chips meant to be embedded in pathways. The result was a glow-in-the-dark pathway that was charged with light during the day (solar powered!) and glowed all night long. All of these materials are able to glow due to a very special compound called Strontium Aluminate, or simply glow powder (or magic dust, your pick.) There are many varieties of glow powder available, with the highest grade (ie. brightest and longest lasting glow) being of the Strontium Aluminate variety (and don’t worry, this is the safe isotope of Strontium!) It’s definitely not edible (strontium can out-compete calcium if ingested and cause bone fragility in growing children), but it is 100% non-radioactive and pretty benign overall as elements go.

 

The "Basics"

The SrAl2O4:Eu/Dy (Strontium Aluminate doped with Europium and Dysprosium) is an inorganic phosphorescent powder.  The powder itself is classified as a monocrystalline solid. The different colours available are due to slight differences in the chemical composition. The following chart shows the different colours available and the chemical formula for the colour variety. Currently blue, aqua, and green are available commercially. 

strontium aluminate

 

Don't you love a good visual aid!

How it Works

Just warning you – if you didn’t take physics the rest of this article may be a bit confusing. But if you want to sound impressive at the next party you go to.. I highly recommend trying to read through and looking up some terms on wikipedia.  You can get a pretty decent understanding online, and there’s something so charming about opening a conversation by casually dropping your favourite pi orbital. Well ok…. you might sound like an overeducated pretentious twat but it’s a good way to weed out people who don’t get excited about HOW THE WORLD WORKS and that is worth its weight in glow powder amirite? #geekout 

science of pi orbital

My favourite orbital is cherry pi harharhar

 

Light It Up

The phosphorescent properties of the compound are activated by light (ie. photons smashing into the crystal field), which cause non-ionizing excitation of the outermost 5d orbitals of the SrAl4O2 molecules electron shell, and form particles dubbed ‘excitons’ (more about these later). Any other time this would do nothing (photons hit us all the time and we don’t glow… so unfair!), but the chemical properties here are something special. Not only does this photon bombardment cause an emission of light, but the energy is ‘harvested’ and then slowly released over the span of 6-8 hours.
The excitation-time is lengthened due to our wonderful ‘dopant’ elements; Eu and Dy. The properties of these ‘rare earth elements’ cause metastable states, ie. accumulation of excited particles/energy. THIS IS RARE! Normally an excited state immediately breaks down (this is what happens in fluorescence), but in this case, the excited state is held for much longer. The slow re-emission of this energy causes the glow-in-the-dark phenomena aka. phosphorescence.

 

  Strontium Aluminate

3D structure of the Strontium Aluminate crystal field. Teeny ninja stars, tents, or 6 petaled flower?

 

Only Forbidden Transitions Allowed!

Makes sense if you don’t think about it too hard, but let’s go deeper. The splitting of the 5d orbitals is what initially causes the glow, but the excited state, split from the ground state, is extremely unstable. Again, normally the excited state would quickly return to ground state – there would be a bright flash, but no prolonged glow. This is where the forbidden transitions come in. These ‘forbidden transitions’ are not ‘allowed’ in ‘normal’ scenarios (pshh), but the properties of the dopants within the crystal field prevent de-excitation, and enable the irregularly long and stable excited-state that creates that hypnotizing glow. I thought it could be due to the higher mass of the Europium and Dysprosium atoms – their ‘pull’ could be what’s holding these excited states in place for so long. Turns out that’s partially true, but in the words of Dr. Albert, “the effects on the field are due to more than just the atomic weight of the Eu and/or Dy – the effects are really caused by how the Eu and/or Dy dopant molecules affect the whole crystal structure and the fields within it – those effects are very complicated.” And that is from the perspective of a world-class physicist!

Bring In The Electron Hole

Now you’re probably thinking, ok, that’s pretty neat. Well good, cause there’s more! This is the part where I started to go a little mental because we are describing a thing that does not really exist. In fact, the only reason it’s important in this case is precisely that is doesn’t exist. It’s called an electron hole. It’s not a thing, its a place where a thing used to be. In this case, an electron-hole is the absence of an electron at a point in the crystal field or ‘crystal-lattice’ inside the SrAl2O4:Eu/Dy crystals.

When an electron is excited, as we talked about earlier, it leaves its original ground state leaving an ‘electron hole’ behind. These holes flow through the crystal field, swept in the electro-chemical potential of the field like a bubble of air flows down a river (a cumulative effect of ‘exciton drift’, ‘phonon winds’, ‘diffusion’, etc). They exist as pseudo-positive charges that miss their electron buddies, and would rather stick together. YES THATS RIGHT – NOT ONLY DOES AN ELECTRON HOLE NOT EXIST AT ALL, BUT IT ALSO IS CONSIDERED TO HAVE A “PSEUDO-POSITIVE CHARGE’. double you tee eff physics. 

 

These holes are implicated in the glowing phenomena – the ‘exciton particles’ are actually best described as a bound state between an electron and an electron hole (trying to talk about particles in physics is like cracking open a Russian Doll – there is always more to talk about!) Excited electrons out of their ground state are attracted to their ‘theoretically positively charged’ electron holes, and combine in a process called ‘exciton-decay’ which releases light back into their safe home happy ground state. And then, we begin the process all over again. That’s the neat part about phosphorescence – unless the crystal is damaged, the light-producing effects last for ages (some say over twenty years!)

Strontium Aluminate

#sciencemagic in the palm of my hand 

 

The Learning Never Stops

If you made it this far I applaud you! As you can see, the closer we look the weirder it gets, and the weirder it gets the more we realise there is to learn. This little description is only one mechanism of action – we didn’t even talk about the vibrational energy of the crystal-field, excitation via non-visible light, or discuss why the act of producing ‘cold light’ is such an incredible feat. Nor did we discuss the fact that although we can talk for quite a while about the production of light, much of it is described by equations, and there is much more that we simply do not yet understand. That said, the longer we look into the quantum phenomena of light, there are times we attain moments of illumination. This is my attempt to share some of that with you.

Until next time!

Paige