Making Metals (More) Interesting

Metals are boring. Sure, they’re nice and shiny. And they’re definitely useful. But could you honestly, hand on your heart, say that the last time you saw a bit of metal you jumped for joy? Probably not.

Metals: a bit boring?

The trouble is, we’ve been playing around with chunks of metal for so long that the novelty has worn off. You can melt down metal and create swords, axes and other cool things. That’s pretty neat, but it’s not exactly cutting-edge technology (see what I did there?). You can melt metals and mix then up to get alloys, which are awesome for jazz bands (brass instruments) and eating (stainless steel) among many other uses. Don’t get me wrong, I’m a great stickler for jazz and a big fan of the classic knife and fork combo, but neither of those things are particularly new.

Worse still, as a physicist working in the realm of optics whose job is to find things, shine light on them and see what happens, I find chunks of metal truly boring.

Fortunately for us optical physicists, lots of researchers in other fields were having a similar problem and suggested a solution: make things really really small and try measuring them again. The beauty of nanoscience is that conventionally boring materials can be made interesting again by studying them on the nanoscale (billionths of a metre). A recent example of this approach can be found in the world of carbon. Graphite is a pretty boring form of layered carbon atoms (as found in pencils), but reducing a piece of graphite down to a single layer (graphene) makes weird stuff happen.

Metal nanoparticles in stained glass windows: quite a bit more interesting.

What happens, then, when we shine light on tiny pieces of metal? It turns out we’ve had the answer right before our eyes for centuries without even knowing it. The brilliant colours we see in stained glass windows are caused by pieces of gold or silver a few tens of nanometres across. These pieces of metal absorb extra wavelengths of light which they wouldn’t absorb if they were of a more “conventional” size. Change the size of these metal nanoparticle by a few dozen nanometres (say, from 80 nm down to 50 nm) and the wavelengths of light they absorbed (and so the colour of the glass they are dispersed in) changes. This is all a bit weird, isn’t it? Compare a gold ring with a bar of gold, or a gold statue, and they all look to be the same colour – even though we’ve gone from something a few millimetres in length to something on the order of a metre. Yet change the size of a nanoparticle by a mere 10 or 20% and we get an enormous change.

So, what’s going on?

It all comes down to everybody’s favourite subatomic particle, the electron. Metals are made of regularly arranged atoms, this arrangement leaves one of the electrons (or more, depending on the metal) from each atom free to wander around the atomic lattice more or less as they feel. These delocalised electrons can be made to move in an ordered way by giving them energy. For example, by applying a voltage across a long, thin piece of metal (commonly known as a “wire”) the delocalised electrons will all flow along the metal in the same direction – this is electricity. But attaching crocodile clips to metallic nanoparticles is, as you can probably imagine, rather tricky. Instead we can induce collective motion in a nanoparticle’s delocalised electrons by shining light at it. In a nanoparticle, the delocalised electrons are confined to a small region, and they will strongly absorb certain wavelengths of light and convert this energy into collective oscillations. These light-induced electron oscillations are known as plasmons. The wavelength of light which excites these plasmons depends on the size of the particle but also on their shape and electron density (which in turn depends on the metal used).

The principle of plasmon resonances in metallic nanoparticles.

The fun doesn’t stop with stained glass windows, either. Tuning the properties of metal nanoparticles, placing them in arrays and combining them with other materials allows for all kinds of weird and wonderful properties with potential applications in biosensing, optoelectronics, cancer therapy, and possibly even invisibility cloaks (each of which will be discussed in subsequent blog posts). Best of all for a generation of bored metal-optics researchers, this cocktail of weird results means that metals have once again become interesting and will likely remain interesting for some time.

Post by: Philip Thomas