There’s a wonderful video (see above) that has been circulating of Au nanoparticles merging under an electron transmission microscope (TEM). It’s not a new sight – TEM, scanning tunneling electron microscopes (STM), and others have been producing incredible images for some time. One of the most famous – certainly in terms of popular culture – is the one produced by IBM in the 1990s, showing individual atoms arranged in such a way as to spell the company’s name. Scanning electron micrographs have also captured in the past stunning images of various microbes and things likes like the human cochlea. But it is always nice to see such images emerge, from time to time, in popular media. And, really, it doesn’t matter how many times such images do emerge, the sight of whatever specimen being studied never feels any less remarkable. Electron microscopy offers a glimpse into a previously hidden world, and whether we’re talking physics or microbiology, the details of nature that we can now capture is nothing short of exciting.
In the case of the video above, I’ve seen discussion at a number of venues, including where it was originally posted, and there seems to be some confusion.
What we see is, how under the right conditions, the particles, made of gold atoms, can fuse or merge forming a larger cluster of atoms. So what is going to enable this behaviour? In short, what you are observing are the two nanoparticles made of gold atoms as they move in high temperature on top of a surface of FeO (Iron(II) oxide). It is the combination of the high temperature in addition to the extra energy introduced by the electron beam (transmitted by the TEM) that provides that triggers the event. Extra energy is introduced to the specimen, and the two particles become excited in such a way that they begin moving. This is key. The atoms occupy a higher energy level, rearranging their configuration, thanks to the extra energy introduced to the specimen. That is why, toward the end of the video, as the attractive intermolecular forces between particles draw them together, the particles rearrange in the correct orientation of the lattices. This reconfiguration, if you will, allows for the smaller particle to amalgamate with the larger particle. In other words, once appropriately rearranged, the even more rapid process initiates where the smaller particle merges with the larger particle, and a very short time later, we see the single larger particle take on a rather pleasant symmetrical shape (crystalline lattice) due it now rearranging itself to a lower energy (stable) state.
It’s a great presentation of some of the basic physics of chemistry. Notice, for example, the lines on each particle, which represents their lattice structure on an atomic level.
Some very general and introductory explanation can be found in this video by Sixty Symbols. For the curious. A similar phenomenon can also be observed in a different video, this one produced by FELMI ZFE (see the top of the page). In this case, you’re again observing Au nanoparticles. At 600°C, you see them diffuse on an amorphous carbon support and exhibit Ostwald ripening. It’s a remarkable sight. Take note, again, of some of the finer details, such as the texture on the particles – their atomic structure.