Looking at the small print

Looking at the small print

35 xenon atoms arranged by microscope • Image courtesy IBM

Originally published 4 June 1990

We don’t know who first imag­ined that the world was made of atoms.

An ancient Greek tra­di­tion attrib­uted the idea to a Phoeni­cian named Mochus, or Moschus. Mod­ern schol­ars are more like­ly to give the hon­or to Leu­cip­pus, who lived on the shores of the Aegean Sea in the 5th cen­tu­ry BC.

The name most of us asso­ciate with the inven­tion of atom­ism is Dem­ocri­tus, who was a pupil of Leucippus.

Who­ev­er first thought of it, the idea exer­cised a mighty hold on the human imag­i­na­tion for 2,500 years. Mind you, nobody ever saw an atom; they were much too small for that. Even the whisker of a flea was pre­sum­ably many atoms wide. But as Socrates urged upon Craty­lus, there must be some­thing that does­n’t change or the world would­n’t exist at all.

There is a love­ly pas­sage in De rerum natu­ra where the Roman writer Lucretius tries to con­vince his read­ers by anal­o­gy that just because you can’t see atoms does­n’t mean they don’t exist: “For often on a hill, crop­ping the rich pas­ture, wool­ly sheep go creep­ing whith­er the herbage all gemmed with fresh dew tempts and invites each, and full-fed the lambs play and butt heads in fun; all which things are seen by us blurred togeth­er in the dis­tance, as a kind of white­ness at rest on a green hill.”

Seeing what the ancients imagined

Look­ing back, we can only admire the clev­er­ness of those ancient thinkers who guessed the exis­tence of atoms. Their achieve­ment illus­trates the mys­te­ri­ous pow­er of the human mind to dis­cov­er the truth of things, some­times even with­out the guide of direct observation.

With­in recent cen­turies, dis­cov­er­ies in exper­i­men­tal chem­istry and physics made Greek atom­istic phi­los­o­phy seem almost cer­tain­ly cor­rect, but it’s only in our own time that we have seen what Lucretius could only imag­ine — visu­al images of indi­vid­ual atoms.

In the April 5th issue of Nature we are offered a pho­to­graph that would con­vince even the most obdu­rate skep­tic of the exis­tence of atoms: 35 fat xenon atoms lined up on a smooth nick­el sur­face to spell IBM.

The instru­ment which makes pos­si­ble the manip­u­la­tion of indi­vid­ual atoms is the scan­ning tun­nel­ing micro­scope, invent­ed in 1981 by Gerd Bin­nig and Hein­rich Rohrer at the IBM Zurich Research Lab­o­ra­to­ry in Switzer­land. The device is based on a law of quan­tum physics called the Uncer­tain­ty Principle.

To under­stand how it works, first think of a bunch of ping-pong balls rat­tling around in a card­board box, with anoth­er emp­ty box not far away. If the walls of the box­es are high enough, you will always find the balls inside box No. 1, nev­er box No. 2. Ping-pong balls can’t pass through the walls of the box­es, only up and over — and we have assumed they don’t have enough ener­gy for that.

Now think of the elec­trons in an atom as the ping-pong balls, and the force that holds the elec­trons in the atom as the walls of a box. But with this very big dif­fer­ence: The Uncer­tain­ty Prin­ci­ple says that we can nev­er know the posi­tion of an elec­tron exact­ly, only the prob­a­bil­i­ty that it will be found in a cer­tain place. We must think of the posi­tion of an elec­tron as a bit of a blur.

In the scan­ning tun­nel­ing micro­scope, the tip of a nee­dle is brought very close to a mate­r­i­al sur­face. If the posi­tion­al blur of an elec­tron in the tip of the nee­dle extends across the gap, there is a chance that the elec­tron will be found on the sur­face, like a ping-pong ball sud­den­ly appear­ing in box No. 2 with­out going over the walls. The elec­tron is said to have “tun­neled” the gap.

If a volt­age is applied between the tip of the nee­dle and the sur­face, the trans­fer of elec­trons shows up as an elec­tric cur­rent, and this cur­rent is exceed­ing­ly sen­si­tive to the dis­tance between tip and sur­face. As the nee­dle is moved lat­er­al­ly across the sur­face (scan­ning), vari­a­tions in cur­rent can be used to dis­play an image of the sur­face on a video screen. Atoms show up as bumps, and the spaces between the atoms as valleys.

Lining up atoms

In the work that led to the atom­ic-scale IBM logo, researchers at the IBM Almaden Research Cen­ter in Cal­i­for­nia used a scan­ning tun­nel­ing micro­scope to pick up and move around xenon atoms on a rel­a­tive­ly smooth nick­el sur­face, exploit­ing the tiny attrac­tive force that exists between the tip of the nee­dle and the atoms of the sur­face. Then, with the micro­scope in its imag­ing mode, the results were observed.

One-by-one, 35 xenon atoms were plucked out of a ran­dom scat­ter and lined up to form let­ters. It is not the first time atoms have been moved around with a tun­nel­ing micro­scope, but it is the first time that atoms have been indi­vid­u­al­ly arranged in a rec­og­niz­able human design.

You could line up 60,000 of these logos on the head of a pin. Call it, if you want, the small­est bit of self-pro­mo­tion in the his­to­ry of adver­tis­ing, but the atom­ic-scale IBM has its place in the his­to­ry of ideas. With this exper­i­ment, the atom­ism of Leu­cip­pus com­pletes a 2,500 year jour­ney from inspired guess to phi­los­o­phy to chem­istry to physics to plain old engineering.

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