At some point in my life I spent several years trying to understand surfaces. It was a shock to me to learn that surfaces rule the world. It was even more of a shock when I discovered how difficult it was to understand or even measure what is happening on a surface. That makes a light switch made of a single hydrogen molecule sitting on a surface very interesting for what it can tell us about surfaces.
Before we get to the good stuff, we need to take a journey through (my) memory. What makes surfaces so important? What makes them so difficult to understand?
An inscrutable dictator
For starters, we’re not talking about surfaces like the one your kitchen table provides for your plates. Instead, the surfaces we’re talking about are the ones you’d see if you could zoom in on your kitchen table to the point where individual atoms were visible, and some molecules would bounce off the air in the room or stop occasionally to sit on the surface until vibrations set them off again.
My explanation in one word for the importance of these types of surfaces: catalytic converters. Almost all of modern life depends on catalytic converters. In very simple terms, catalysts are surfaces that initiate reactions. There’s more than that, but without the strange behavior we see on these surfaces, life would be very dull.
However, surfaces do not make this easy to understand. The atoms that make up a surface are in an unusual situation. In the material, all atoms are surrounded by their partners in a mutually satisfactory manner. In the interior, material properties can be understood by examining the structures that satisfactorily hold the atoms and the symmetries the structure possesses. On the surface, the symmetry is broken and, like Mick Jagger, he can’t get any satisfaction. The atoms’ attempts to find satisfaction make the surface both interesting and frustrating.
That lack of satisfaction affects everything on the surface. Because surface atoms don’t know what to do with their electrons, molecules like water or nitrogen will temporarily stick to a surface. The surface can even tear these adsorbed molecules apart. For the most part, however, the bonds between the surface atoms and the adsorbed molecule are often weak; they are constantly breaking and reforming.
This makes the surface an active site, as molecules attach, perhaps break and reform. The molecules move, they come loose and are replaced by new copies. All of this activity takes place in just a single layer of material, so any reading of it is often very weak and takes time to collect. As a result, our understanding of them is incomplete. Imagine you have to rate a movie, but you can only choose between viewing a single frame or the average of all the frames in the movie.
Watch hydrogen wave
That’s what makes this last piece of research interesting. This gives us an impression of how a molecule moves on a surface, albeit to a limited extent.
The researchers started with a gold surface. They then examined the surface with a very sharp point of gold – so sharp that it ends in a single atom. The tip is placed less than a nanometer from the surface and is moved around until it finds a hydrogen atom.
The tip also glows. The sharp tip shape constrains the local electrons, so as some electrons jump across the gap, others begin to slosh up and down in the tip. This sloshing pushes electrons in the surface around and also sets them in motion. If the incoming electrons have enough energy, the sloshing is powerful enough to generate visible light. That emission process is very sensitive to everything that stands between the gold surface and the tip.
When the tip comes over the top of a hydrogen molecule, the molecule creates a much smoother path for the current (something called resonant tunneling). As a result, the electrons in the tip and the surface cannot really push each other. End result: the hydrogen switches off the glow.
The hydrogen molecule also gets energy from the electrons passing through it. It begins to sway back and forth, like a branch in the wind. And a moment later it pops out from under the point. This causes the tip to glow. But the surface doesn’t like the hydrogen molecule’s new location, so after a while the molecule jumps back under the tip and the glow goes out. For example, a single hydrogen molecule can switch the glow of the tip on and off.
More than a flashing light?
That’s nice, but there’s another important point. The optical response of the tip is almost instantaneous when the hydrogen molecule moves. That means we get an accurate picture of how long the hydrogen molecule stays, and, once it’s gone, how long it will take to return.
This view of hydrogen diffusion is one of the most important parts of surface chemistry. Knowing how mobile a molecule is and how quickly diffusion occurs is knowing how quickly a chemical reaction can take place (molecules cannot react if they are not moving).
Unfortunately, this particular example of gold and hydrogen isn’t hugely important, and I’m not sure how well the technique will transfer to other surfaces and other molecules. Both gold and hydrogen are very special beasts that just happen to be just right for each other. I’ll be happier seeing this happen on a surface that isn’t gold (say platinum) and with molecules like water or a simple organic molecule.
Nano letters2018: DOI 10.1021/acs.nanolet.8b03753 (About DOIs)