Well, there you are. You’re looking at individual atoms within a crystal at the highest resolution that is physically possible. Researchers at Cornell University just published the methodology they used to get here, called multislice electron ptychography, on May 20 in Science.
It not only broke their own previous resolution record by a factor of 2, but it also allowed them to use thicker samples, thick enough to determine three-dimensional structure at this resolution, even including single-atom defects that can be so critical in semiconductors and superconductors.
When you look at a photo of someone you know who is represented in a new way, like when they’re standing on top of a mountain or dressed in a crazy costume, the photo means a lot more to you because you recognize who you’re looking at. You probably don’t feel that so much with the image above, so I’m going to try to show exactly what it is we’re looking at so you’ll appreciate it more.
First let me say just a couple other things about the image:
The magnification here is ONE HUNDRED MILLION. That’s like making a golf ball the same width as the United States.
Also, this image is not just a simple snapshot, but a reconstruction. That doesn’t make it inferior to other images. Far from it. it just means that a lot of data was needed to piece it together. The first image of a black hole in 2019 was also a reconstruction, using data from multiple instruments around the world. Every image is a mere representation of reality; some are just a lot harder to put together than others.
So what are we viewing here?
It’s a crystal of praseodymium orthoscandate, or PrScO3. We have three different types of atoms: praseodymium (Pr), scandium (Sc), and oxygen (O).
A 3-D representation of a chunk of this kind of crystal looks like this:
This lets you see how the atoms are linked to each other: each oxygen is bonded to 5 other atoms, each scandium 6, and each praseodymium 8. It doesn’t look too much like the image above, but we’re getting there.
Let’s take away the bonds, so we’re just seeing the atoms:
One thing you might notice about the Pr atoms is those little pairs. If you look at the three pairs of them that go across the center of the cartoon structure, you notice they alternate in how far they’re tipped to the side, just like they do in the main image.
Also notice that the Sc atoms show up between rows of Pr pairs, in rows of their own, but as singles. Keep in mind that this is only one angle for viewing the crystal; go HERE if you want to rotate the structure around to your heart’s content.
One other little adjustment we need to make has to do with atomic radius. Oxygen atoms are actually pretty tiny compared to Pr and Sc atoms, as you see here:
The image was generated by firing a beam of electrons through the crystal and noting how those electrons got deflected. Bigger atoms like Pr and Sc will deflect electrons a lot more than tiny atoms like O. The nucleus of any atom in the crystal has a positive charge, and the electron has a negative charge. Opposites attract, after all, so an electron gets tugged at by the nucleus of any atom it gets close to, and the bigger the atom, the greater the effect. If an electron zips by a nucleus really closely, it could even be thrown backward:
So keep in mind that oxygen atoms aren’t going to show up as big dots as they do in the crystal cartoon diagram; in the reconstructed image, they’ll be the dimmer dots that didn’t deflect electrons all that well.
And now, without further ado, I’m going to line up the crystal cartoon diagram with the actual image, and now you will recognize exactly what you are seeing:
Holy cow, exactly as predicted, right down to the individual atom. I have to tell you, I actually got a bit teary-eyed when I lined these two up for the first time. There is that pang of recognition.
The authors did a nice thing for us to explain why this resolution is the best that can possibly be achieved. Let’s check out the four images below:
Image (A) is what we would see by their method if the atoms were at absolute zero, the coldest temperature physically possible, where atoms have very little motion. Then image (B) adjusts for the fact that this was done at room temperature, and so the thermal movement of the atoms makes them vibrate around and thus deflect electrons over a bigger effective radius. Image (C) adjusts even further to reflect the fact that we cannot collect every single electron, because some are deflected too far afield, and this makes the calculation slightly less precise. Finally, image (D) is a reconstruction from real data, and it is not substantially different from what they predicted in (C).
In other words, unless something about physics changes, we are not going to do any better than this.
We arrived here with multislice electron ptychography, a way to send an electron beam through a sample, move the electron beam around, and watch how the deflection pattern of the electrons in the beam changes, then use some really sophisticated math to reconstruct where the atoms in the sample must be. I should leave it to the experts to describe what ptychography does and the possibilities it opens up:
Team leader David Muller said:
“This doesn’t just set a new record. It’s reached a regime which is effectively going to be an ultimate limit for resolution. We basically can now figure out where the atoms are in a very easy way. This opens up a whole lot of new measurement possibilities of things we’ve wanted to do for a very long time. It also solves a long-standing problem – undoing the multiple scattering of the beam in the sample, which Hans Bethe laid out in 1928 – that has blocked us from doing this in the past.”
“We’re chasing speckle patterns that look a lot like those laser-pointer patterns that cats are equally fascinated by. By seeing how the pattern changes, we are able to compute the shape of the object that caused the pattern.”
Muller and his team, including lead author Zhen Chen, made it magnificent! Debbie Harry could only have dreamed as much back in 1979…...