SciShow
Using Quantum Physics to Trace Atoms
02:06 - 07:06
Olivia discusses the story behind the physicists who invented a new type of microscope that could look at a single atom in sheet metal.
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Video Transcript
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they had passed through.
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Which is amazing!
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But these microscopes still didn’t have high enough resolution
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to capture every atom.
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They relied a lot on computers to fill in the blanks.
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So Binnig and Rohrer wanted to invent a new kind of microscope
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that could do even better.
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They came up with a design that could potentially zoom in on things
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10 times smaller than the best existing microscopes.
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According to their plan, it would work kind of like a needle
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hovering over a record.
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The resolution would come from the sharpness of their needle —
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because the more detail a needle can trace,
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the more detail it can reveal.
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In this case, Binnig and Rohrer wanted to be able to detect
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each and every atom on a surface, so their needle needed
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to be really sharp.
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In fact, its tip had to be on the order of one atom thick.
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That was the first big challenge.
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The two researchers used a technique called electrochemical etching
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to make a super-sharp metal tip.
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To make a needle this way, you start with a regular piece of wire.
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Binnig and Rohrer went with one made of tungsten.
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You connect that wire to another piece of metal —
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something like stainless steel.
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Then, you dunk the whole thing in a hydroxide solution
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and leave part of the tungsten wire poking out.
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Since that tip is exposed, the liquid forms what’s called
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a meniscus around it, meaning the liquid gets slightly drawn upward.
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Next, if you apply a voltage between the two metals, charge will
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start moving between them.
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And that will set off a chemical reaction at the meniscus.
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The submerged tungsten will react with hydroxide in the solution,
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producing something called tungstate.
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This tungstate dissolves away, leaving the wire to get thinner
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and thinner at the meniscus.
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Essentially, the metal gets chemically eroded away.
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Eventually, the wire becomes so thin that it breaks!
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And it leaves behind an extremely sharp tip — ideally one-atom thick.
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But this process isn’t perfect, so the tip normally still needs
3:53
to be sharpened a little.
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Fortunately, even on their first attempt,
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Binning and Rohrer were prepared.
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They were able to sharpen the tip by exposing it
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to very high electric fields.
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And I mean very high — like, high enough to make the molecules
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restructure themselves, which created a sharper point.
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But making the tip was only half the battle.
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Next, Binnig and Rohrer had to lower it into the surface
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they wanted to study.
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Except first, since they were dealing with such fine detail,
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they needed to completely control any vibrations —
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otherwise the tip or the sample could move in unpredictable ways.
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And that wasn’t easy.
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Because all sorts of things create vibrations — people talking,
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cars driving, the wind blowing.
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At the atomic level, even a footstep can seem like an earthquake.
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So the two researchers decided to levitate the entire apparatus
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using magnets.
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Which is super practical and as a bonus, gives your experiment
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a nice sci-fi vibe.
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Once their contraption was finally in place, it was time
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to actually trace the atoms in the silicon and get a reading.
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But to do that, they needed a way of determining when the needle
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was directly over an atom.
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Because, again, the needle itself was around the size of an atom,
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so to it, the metal didn’t look like a smooth sheet —
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it looked like a bunch of atoms bound together
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in some complex structure.
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So, they called on one of the quantum mechanics’ best party tricks,
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which just had been discovered a few decades before: quantum tunneling.
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Quantum tunneling is a phenomenon that happens because atoms
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are super strange.
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They don’t look or behave like anything we’re familiar
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with in the everyday world.
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And they don’t look anything like that classic model that was
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probably on the cover of at least half your science textbooks.
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In fact, they’re not even solid particles at all.
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They’re little nuclei surrounded by electrons.
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The thing is, those electrons don’t follow nice neat orbits.
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And—stick with me here—they truly don’t exist
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in a physical place at all.
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The most we can say is that an electron has a certain probability
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of being somewhere at a given time.
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And that’s not because we can’t see it or because we don’t have
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the precision to measure it or something.
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They actually don’t have a specific position.
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In fact, there is even some probability of electrons jumping
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from one location to another.
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And that jump is called quantum tunneling.
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Binnig and Rohrer encouraged the electrons to jump
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by giving the needle and the sample each
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a different electric potential.
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To try to even things out, electrons would jump between
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the two and create what’s called a tunneling current.
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And the strength of the tunneling current would depend
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a lot on how close the needle was to a given atom.
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This was the key that made the rest of the experiment fall into place.
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If there was a lot of current, that would basically mean
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the needle was hovering right on top of an atom.
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If the tunneling current was very weak, then the needle
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was probably far from an individual atom.
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This understanding was the breakthrough that made
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the whole technique possible.
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It sounds like the kind of mission that could take a lifetime
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to make into reality.
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You’re combining chemistry, electromagnetism, materials science,
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and quantum physics.
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Making this microscope was a tall order.
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But just three years later, in 1981, Binnig and Rohrer
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had a needle scanning the surface of a sample of silicon.
