Unlocking the Secret of Manganese’s Hidden Light: Tracing the Origin of Mn²⁺ Near-Infrared Emission

Hey There, Fellow Light Enthusiasts!

So, you know how light comes in all sorts of colours? Well, there’s this whole other world just beyond the red end of the rainbow called near-infrared (NIR). It’s super useful for things like seeing in the dark, checking out plants, or even peeking inside your body without cutting you open. And for ages, folks have been trying to make tiny, efficient sources of this NIR light, especially ones that work with those cool blue LEDs we have everywhere now.

Getting these miniature NIR sources right means finding materials that can soak up the blue light and then ping out NIR light. We call these materials ‘phosphors’. Lots of different ingredients can do this trick – fancy rare earth ions, other transition metals, even Bismuth. But each has its quirks, like only emitting a narrow band of light or not being super bright.

One ingredient that’s been a bit of a puzzle is Manganese (II), or Mn²⁺. This little ion is a chameleon! Depending on where you stick it in a crystal structure, it can give you green light, red light, and sometimes… this mysterious NIR light. The green and red parts? We’ve got a pretty good handle on those – they usually depend on whether the Mn²⁺ is sitting in a spot surrounded by four oxygens (tetrahedral, often green) or six oxygens (octahedral, often red).

The Enduring Mn²⁺ Mystery

But that NIR emission from Mn²⁺? That’s been the head-scratcher. There have been all sorts of ideas flying around:

• Maybe it’s when two Mn²⁺ ions get cozy next to each other (Mn²⁺–Mn²⁺ pairs)?
• Perhaps it’s only when Mn²⁺ is in a weird spot surrounded by eight oxygens (cubically coordinated)?
• Could it be that some of the Mn²⁺ gets oxidized to Mn³⁺, and that’s the culprit?
• Or maybe it’s just random defects in the crystal structure causing it?

Honestly, none of these ideas have really stuck. It’s been a bit of an enigma, sparking intense debates in the academic world. We really needed to get to the bottom of this to unlock the full potential of Mn²⁺ for next-gen NIR tech.

Putting on Our Detective Hats with Garnet

Luckily, we found a fantastic playground to investigate this mystery: the garnet structure. You might know YAG (Yttrium Aluminum Garnet) – it’s the basis for the yellow phosphor in many white LEDs. The cool thing about garnet is that it has different spots (or ‘sites’) where ions can sit: distorted dodecahedral sites (surrounded by 8 oxygens), octahedral sites (6 oxygens), and tetrahedral sites (4 oxygens). And get this – Mn²⁺ can show *both* red and NIR emission in the garnet structure! This gave us a unique opportunity to compare the two and figure out what’s what.

We cooked up some YAG samples, adding a bit of Cerium (Ce³⁺) and varying amounts of Mn²⁺. The Ce³⁺ is a bit of a wingman here – it’s great at absorbing blue light from an LED and can then pass that energy over to the Mn²⁺, which is super helpful because Mn²⁺ itself isn’t the best blue light absorber. We checked our samples with all sorts of fancy tools – X-ray diffraction (XRD) to see if the structure was right, electron microscopy (STEM, HRTEM) to look at the atoms, and energy-dispersive X-ray spectroscopy (EDS) to make sure the Ce³⁺ and Mn²⁺ were spread out nicely. Everything looked solid – the ions were in the right places without messing up the structure or causing defects.

Confirming Mn²⁺’s Identity

Before we could figure out where the NIR light was coming from, we had to be absolutely sure the Manganese was indeed in the +2 state (Mn²⁺). We used techniques like X-ray absorption near edge structure (XANES), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR). All the evidence pointed to Mn²⁺ being the star of the show in our YAG samples. The EPR results were particularly interesting – they showed a pattern that strongly suggested the Mn²⁺ ions were sitting in sites with octahedral symmetry.

The Plot Thickens: Ce³⁺, Energy Transfer, and Shifting Colours

As we expected, the Ce³⁺ in YAG gave off its usual yellow light. But when we added Mn²⁺ and shone blue light on the samples, we saw something cool. The yellow emission from Ce³⁺ got weaker as we added more Mn²⁺, while new emissions from Mn²⁺ popped up – one in the red (~600 nm) and one in the NIR (~750 nm). This confirmed that energy was indeed transferring from Ce³⁺ to Mn²⁺. Pretty neat, right?

We also noticed something crucial: at low Mn²⁺ concentrations (like 2%), we *still* saw the NIR emission. This immediately made us raise an eyebrow at the idea that Mn²⁺–Mn²⁺ pairs were the main cause, because you’d typically need a higher concentration for ions to be close enough to pair up significantly. We could also rule out Mn³⁺ – we looked specifically for its characteristic emission band in the 1100-1200 nm range, and it just wasn’t there.

Different Lights, Different Homes?

One of the most telling clues came from looking at how long the light stuck around after we pulsed the blue excitation light. The red emission (~600 nm) faded relatively quickly, in about 1 millisecond. But the NIR emission (~750 nm)? That hung around for a whopping ~10 milliseconds! This huge difference in ‘fluorescence lifetime’ is a big deal. It strongly suggests that the red and NIR emissions are coming from Mn²⁺ ions sitting in *different* kinds of spots within the garnet structure. The long lifetime of the NIR emission points towards Mn²⁺ being in high-symmetry sites.

Given the different sites available in YAG (dodecahedral, octahedral, tetrahedral) and the evidence from our tests, we started forming a hypothesis. We figured the NIR emission, with its long lifetime and hint of octahedral symmetry from EPR, must be coming from Mn²⁺ ions sitting in the 6-coordinate octahedral sites (the AlO₆ spots). Why? Because these sites have a strong ‘crystal field’ – basically, the surrounding oxygen ions push and pull on the Mn²⁺ electrons in a way that splits their energy levels significantly. This strong splitting, we believe, is what shifts the emission way down into the NIR region for the specific electron transition (⁴T₁(⁴G) → ⁶A₁(⁶S)) of an *isolated* Mn²⁺ ion.

And the red emission? We think that’s coming from Mn²⁺ in the 8-coordinate distorted dodecahedral sites (the YO₈ spots). Even though the bonds here might be a bit longer than in the octahedra, the distortion of the site still creates a strong enough crystal field to make Mn²⁺ emit red light, similar to how Ce³⁺ in the same type of site gives yellow light in YAG.

Crunching the Numbers and Looking Closer

To really test our hypothesis, we turned to first-principles calculations – essentially, using powerful computers to simulate how Mn²⁺ behaves in different sites within the YAG structure. And guess what? The calculations strongly supported our idea! They showed that Mn²⁺ in an isolated octahedral site would emit light at an energy corresponding to NIR (~1.48 eV), while Mn²⁺ in an isolated dodecahedral site would emit at an energy corresponding to red (~1.87 eV). The calculations also showed that Mn²⁺–Mn²⁺ pairs, whether in octahedral or dodecahedral sites, *didn’t* significantly shift the emission energy compared to isolated ions, further challenging that old hypothesis.

We also used another powerful X-ray technique called Extended X-ray Absorption Fine Structure (EXAFS). This lets us figure out the average number of oxygen neighbours around the Mn²⁺ ions (the coordination number). Our previous work showed Mn²⁺ in garnet has an average coordination number of seven, meaning it’s in both 6- and 8-coordinate sites. To nail down which site gives which colour, we made a special garnet phosphor where Mn²⁺ *only* emitted red light. EXAFS analysis on *that* material showed the Mn²⁺ was primarily in 8-coordinate sites. This was the final piece of the puzzle! It confirmed that in the dual red/NIR emitting YAG:Ce,Mn, the red comes from Mn²⁺ in 8-coordinate sites, and the NIR must be coming from Mn²⁺ in 6-coordinate octahedral sites.

This finding isn’t just for YAG, either. We looked at other Mn²⁺-activated NIR phosphors, and many of them also have octahedral sites. This suggests our explanation – that Mn²⁺ in strong crystal field octahedral sites is the source of NIR emission – likely applies more broadly, helping to finally clear up this long-standing mystery.

Shining a Light on Applications

Of course, the real test is putting this knowledge to use! We took our YAG:Ce,Mn phosphor and slapped it onto a blue LED chip to make a prototype NIR pc-LED device. It worked! The device emitted strong NIR light, and its output increased nicely with more current. It also stayed pretty stable even when things heated up, retaining about 66% of its brightness at 100°C compared to room temperature. We even did some quick tests showing its potential for real-world uses, like seeing things a normal camera can’t in dim light, spotting foreign objects in fruit, and even getting clear images of blood vessels in a hand – pretty cool stuff for night vision, detection, and bioimaging!

While our YAG version had a little bit of red/far-red light mixed in, we saw that using a slightly different garnet (LuAG instead of YAG) shifted the NIR emission further towards the red end of the NIR spectrum. This shows we have room to tune the exact colour of the NIR light by tweaking the local environment around the Mn²⁺, which is exciting for developing even better NIR materials in the future.

Wrapping It Up

So, there you have it! We synthesized a YAG:Ce,Mn phosphor that primarily emits in the NIR range (700-900 nm) and showed it works in a prototype LED device. More importantly, we dug deep and found solid evidence that the NIR emission from Mn²⁺ in garnet structures comes from the Mn²⁺ ions sitting specifically in the 6-coordinate octahedral AlO₆ sites. This happens because the strong crystal field in these spots splits the Mn²⁺ energy levels just right, pushing the ⁴T₁(⁴G) → ⁶A₁(⁶S) transition into the NIR region. This finding challenges some of the older ideas about Mn²⁺ NIR and gives us a clear path forward for designing and developing new, high-performance Mn²⁺-activated NIR phosphors. The mystery, it seems, is finally starting to fade, replaced by a clear understanding of where that hidden light comes from!

Source: Springer