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Well, as the article points out, the things being imaged are most definitely actual atoms. The only thing TFA is nitpicking about is that light isn't used to image those atoms.

This is about as interesting as pointing out the fact that an ultrasound picture of a baby isn't an "actual picture", since we use sound instead of light to make the image.

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The concept of the scanning tunneling microscope is so simple and ridiculous that it was probably thought of and dismissed long before someone built one. "You're going to image atoms by dragging a tiny needle across them"? Yet that's pretty much how it works.

The first one was built in 1981, but one could have been built in the 1950s. Piezoelectric crystals were known. Raster scanning circuits were known. Feedback circuits for controlling the height were known. The tricks for making one-atom sharp points were not known, but the earliest STMs just stretched out a tiny platinum wire until it broke, and sometimes you got a one-atom point.

Hobbyists have built STMs.[1] It's simpler than building a 3D printer.

[1] https://dberard.com/home-built-stm/

I know, I did a benchtop STM lab for my Nuclear Physics course some years ago. Most of the day was spent trying to make single atom needles by a wrist-flick technique for cutting platinum wire with some sharp pliers.

I worked in the lab at the Nation Physical Lab in the UK that had the low pressure + low temperature STM, we used to make the tips by dipping the ends in a very strong solution of NaOH. You relied on the surface tension as it dissolved to produce you a very sharp tip.

Surprisingly easy.

Cool. I know people also buy tips, but they're expensive.

For really cool stuff, check out the CO functionalized tip AFM coming out of IBM in Zürich: https://www.zurich.ibm.com/st/atomic_manipulation/pentacene....

The AFM I used was mainly for teaching the concepts, so they wanted simple, cheap, low risk => strong NaOH was out.

The technique back then was, somehow, cutting at about 45° angle and twisting the pliers to make this angle go to zero while cutting/pulling. Then you had to put the tip in the machine and do test images to see whether the tip was good. Usually it wasn't, so you tried again. Finally you got a perfect tip, and maybe got two or three good images before you messed up the tip (from being a noob).

TBH, more people were using AFMs than STMs, because they're cheaper (I think) and the fact that the STM is based on conductivity limits what can be imaged.

Getting pictures of atoms is nice, but sometimes you want to image molecules, especially organic ones (tricky to do in a STM).

How do you know if your tip is one atom wide or not?

Also, what would the shape of the tip look like if you drew it? I'm wondering what kind of general angles the surface has. Is it like a cone with a single atom at the tip? What kind of slope?

The only real way to know if you had a single atom tip was to image a known surface. If the image was junk you probably had some funky tip states going on. HOPG (graphite) was the standard we typically used.

A good tip could be just about anything, from a nice cone to really jagged. One problem was any of the methods one has to view the tip can't actually resolve the single atom that is doing the imaging.

Man .. I know what my next hobby project is going to be :)

Dumb question .. I get that the needle scans a surface and you get the quantum tunneling effect between the atom you are "looking at" and the tip of the needle. What I don't get is how one figures out depth. For each X,Y position, do you just keep going down until you touch something, and then move up, and go to the next position? If so, apart from the issue with 1 atom tip, I imagine the next problem would how to increment X and Y by 1 atom.

P.S. I think some of the marketing put out on these things really confuses the issue. Sure .. it gets people excited about science but it gives people the wrong intuition. As a non-physics person, I got a lot out of this article.

In the simplest configuration have a X-Y stage that you raster, along with Z axis control - generally these are all piezo controlled. Your signal is then acquired by maintaining a constant tunneling current while you raster, so you track the Z axis position as you raster.

This means that you're getting a "pseudo-height" map - if you had a surface with 2 types of atoms, both the same size, but with different tunneling barriers, you would see them appear to be different sizes.

Yeah, we used to use graphite (as flat as we could get it, tried to ensure we had a single surface) to see how good it was. You could tell how good it was from how much noise you got and therefore how good your image was.

Take a picture of it ?

Could you post a quick summary describing the technique? Sounds like my definition of esoteric fun, haha.

I think actually the hard part in the 50s might be the current amplifier but I'm not au fait enough with the tech of the time to know if nanoamp amplifiers were easy. I think it's not so different from radio but would be interested in an educated opinion.

That's a good point. A very high gain DC amplifier with low noise would have been hard then. A STM needs to sense about 1na at 0.1V, which means a series resistance of about 100 megohms. Vacuum tube voltmeters of the 1940s and 1950s era were getting close, with about 10 megohms input resistance.[1] Sensing 0.1V through 100 megohms with tubes doesn't seem out of reach.

High RF gains are easier to get than DC gains; you can filter out everything but the frequency of interest and reduce noise. That's basically how radios amplify weak signals. But I don't think you can run an STM on RF.

[1] http://www.tubebooks.org/Books/rider_VTVM.pdf

How do you verify that the tip is indeed one atom sharp without already having something like an STM?

dan's work is awesome I wish teensy was around in 1996 http://www.phys.ttu.edu/~tlmde/thesis/INTRODUCTION.html

It is the distinction between pictures and photographs: they definitively are pictures of atoms, but they aren't nor could ever be photographs.

Ultrasounds also are pictures, but not photographs.

No I disagree. An ultrasound picture of a baby accurately reflects the actual form of the baby. A Scanning Tunneling Electron Microscope does not accurately reflect the form of an atom because it simply uses an artificial dot to represent the presence or absence of an atom. In short, the "map" that STM's give us is abstracted from reality and does not accurately portray an atom (in large part because STM's still simply due not measure at the scale of an atom).

Well, we can image molecular orbitals using STM [1], which I'd say is one of the best ways to represent the shapes of atoms.

[1] https://physics.aps.org/featured-article-pdf/10.1103/PhysRev... (open access)

I found the article really interesting and learned a few things from it.

I do know how an ultrasound image works, and I had some basic knowledge about scanning tunneling microscopes, but this article - and the many links in it - were definitely worth my time reading.

Isn't it possible to make an image using light, but by computing the "real" image from the interference pattern (of a single non-repeating atom/structure)? And if there is more than one solution, then perhaps by using images from multiple angles, or with different wavelengths?

Isn't that basically what X-ray crystallography is (taking the liberty to include X-rays as "light")? Though with the (significant) restriction that it only works for repeating crystal structures, not unique individual targets...

Xrays are human-invisible light, part of the EM spectrum. No liberty needed.

Yes, so my question is if we can do it also on individual targets.

Ah, yes, sorry, I missed your "of a single non-repeating atom" :-)

I'm also curious -- but I wonder, would this allow for imaging anything that a scanning electron microscope can't?

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