Nanotechnology involves working with materials on the scale of atoms. And for years, nanotechnology researchers have used what’s called a “scanning tunneling microscope” to “see” atoms.
Now they’ve converted this tool into an atom bulldozer that can move atoms from place to place. Bob Celotta is a physicist at the National Institute of Standards and Technology in Maryland. He and Joe Stroscio are taking the atom bulldozer further. They’re teaching a computer to assemble just a few atoms into “nanostructures” such as circles or triangles.
Bob Celotta: We just feed it a drawing of what we’d like and it figures out how to move the atoms and in what order, and in what speeds, and in which ways … to create the structures we want.
But the computer needs rules. For example, if two atoms get too close, they might stick together. So the computer has to plan each atom’s route. It also looks for the most efficient path — in the same way mapping software can tell you the most direct way to get from Chicago to New York.
Bob Celotta: Our vision is that we leave it with homework overnight to create a whole bunch of structures — that the next day we can come in and try to understand the physics of these nanostructures…
Celotta hopes that some of them will someday be useful as nano-electronic parts. One application would be faster computer chips.
The scanning tunneling microscope uses a probe that hovers over surfaces and senses atoms. It takes advantage of a weird feature of quantum physics known as “tunneling. In the new device, the probe is brought even closer to an atom — close enough to form a temporary chemical bond. Then, the probe can be manually moved around, dragging the atom to any desired spot. Celotta’s innovation is to automate the process — making the atom bulldozer into an “autonomous atom assembler.”
According to Clayton Teague, Director of the National Nanotechnology Coordination Office in Arlington, Virginia, “all STM atom assemblers today operate in an iterative fashion: (1) put the STM into an imaging mode of operation and obtain an image of a random array of atoms that have been deposited on a surface; (2) put the STM into an atom-moving mode and move the atoms around converging toward the desired ordered structure based on an image of the initial atom locations; (3) take another image of the resulting atom arrangement determining if the desired change in atom locations occurred; (4) iterate until the desired ordered atomic structure is obtained. Celotta and his team’s significant advancement is that they have developed methods for automating the processes necessary for transitioning between the two modes of operation and for iterating the processes.”
The autonomous atom assembler’s operation is like dragging atoms around with a bulldozer right now. It would be nice instead to pick them up one by one like a crane and set them down. Celotta says, “And this has been done by others, but it’s not been done reliably yet. You can’t be sure you pick it up. You can’t be sure you put it down. And the last thing you want to do is remove atoms from the surface and have them disappear on the tip somewhere and not be sure what atoms are on your tip and what atoms are on your surface. So it’s something that really has to be worked out.”
Celotta’s autonomous atom assembler does it’s work in a high vacuum, at low temperatures. This insures that there are no stray atoms from the surrounding air dropping onto the surface. It also means that the atoms on the surface don’t have enough energy to move around. Celotta says, “So once we achieve this, we can study the same atom for many months. I mean, everything is very quiet, everything is very stable. One can move atoms around, and study patterns and build things up over a long period of time. So once you’ve achieved this experiment running, you have quite a long time to make your measurements and try different things.”
This work is still in its infancy. Eventually, Celotta would like to create 3-dimensional structures and — instead of making structures out of just one kind of atom — use a variety of different atoms.
Excerpts from an interview with Bob Celotta:
-Other scientists have moved atoms using a modified scanning tunneling microscope. What’s innovative about your work?
[2:30]: But now we go one step further — as others have also — by moving the atom tip even closer so that the atom on the tip and the atom on the surface can form a temporary, sort of tunable, temporary chemical bond and then using the forces of that chemical bond, we move atoms around on the surface. Now that’s been done before, by hand to make very beautiful structures on the surface. And what we’re in the middle of is teaching a computer how to do this. So this can autonomously assemble whatever structures we want starting from a random collection of atoms. We just feed it a drawing of what we’d like and it figures out how to move the atoms and in what order and at what speeds and in which ways to create the structures we want. And we call this an autonomous atom assembler.
[3:30]: It’s just beginning to work and we’re making simple structures on the surface of circles, triangles and squares and once we learn the kind of rules it needs to have in order to make these structures — let me give you an example of a rule — if you’re going to move an atom past another atom, you don’t want to get too close because there’ll be a chemical attraction between the two and they’ll form a dimer — a two atom pair. So you have to give it a certain wide berth when you put them together and we also have to make the quickest routes possible and we have to worry about other defects in the surface. So we’re teaching it a whole bunch of rules to follow and then trying to see how well we can construct a surface where it just does all the thinking. And of course, when you try to teach a computer to do something, you have to know all the rules. So it forces it to understand all the kinds of interaction that can happen on the surface and be able to convey that to the computer so it can work correctly.
-What do you enjoy most about your work?
[18:20]: The best times are when everything works just right — just the way you imagined it — and you can move atoms from one place to another and see what nature is going to do in response to your stimulus. You can just put two atoms in a particular place and say what would happen if I put a third atom here — would they move — would they move closer — would they move further apart? What happens to the electronic structure — what happens to the electronic properties of this system if there were another atom here? Is this going to be something that conducts electricity or is this going to be an insulator depending on how these atoms are arranged? So that’s the part we work for — trying to be able to play with the individual atoms and see what the results are going to be.
-What’s the software that you use called?
[21:55]: It doesn’t really have a name. The type of software is called “path planning software.” And it’s not unlike the software that you would find in MapQuest, you know if you were trying to find your way from one part of the country to another. There’s a bunch of routes and you want to find the quickest way to get there. Well, this is a little like that. You can find which routes you can take by moving the atom around on the surface to get where you want to put it. There are other atoms on the surface. So if you’re going from New York to Los Angeles and there’s an atom in Chicago, you’d have to route yourself around Chicago to get there and basically, you have a lot of atoms located in various places that all have to be assembled without bumping into each other.
When it works correctly, everything happens pretty quickly — to move one atom from a place to another — and we haven’t really tried to speed this up yet. It usually takes about a second — on the surface that we’re working in now. So the whole plan doesn’t take very long to move 20 or 30 atoms around. But we’re just at the first stages. We’re just beginning to start here. So we’re moving atoms of all the same kind on a flat surface in two dimensions and we’re not moving them terribly close to each other. We’re just making large geometries to confine electrons.
-So what’s next?
[23:35]: Another step would be to have different kinds of atoms and keep them straight, so that you can assemble things out of multiple species of atoms on the surface. Another step would be to do things in three dimensions to bring atoms up so you assemble a small cluster of atoms, say a square that was all filled in on one layer and then build something on top of that square. That’s still another step to this. So there are many, many different steps to this.
Another major step would be to actually understand the interaction between the atom on the tip and the atom on the surface — that is actually understand the forces … and it’s really chemistry on a very fine level. I mean you’ve got an atom sitting there nestled in among a few other atoms on the surface and you have another atom approaching it from above … it has a certain voltage on it and it’s a certain current passing between them and you can explore hopefully the forces between those two atoms — one interesting thing about that is if you could reliably figure out how to pick an atom up on the surface and put it on the end of the tip and then also reliably put it back exactly onto the surface where you want it. It’s kind of like a crane on a construction site. So we’re actually now moving the atoms around more like bulldozers where we’re pushing them around on the surface or dragging them around on the surface might be more correct. A future enhancement might be to pick each one up selectively and put it down.
And this has been done by others, but it’s not been done reliably yet. You can’t be sure you pick it up. You can’t be sure you put it down. And the last thing you want to do is remove atoms from the surface and have them disappear on the tip somewhere and not be sure what atoms are on your tip and what atoms are on your surface. So it’s something that really has to be worked out.
-What impact do you think your work will have?
[27:25]: And if we can really understand what happens in these nanometer dimensions and how we can modify the structure over nanometer scales to make better devices, better materials, we can generally improve the technological infrastructure of the country — either by having more durable materials, having more effective pharmaceuticals, having more efficient or higher speed computers, having better information storage, to have more information at your fingertips, using battery operated devices that are attached to your belt or cell phones that have a miraculous amount of information in them. So it’s a new venture into a new regime that offers many, many opportunities. And we are trying to understand how to measure these devices — how to measure their properties on the nanoscale to allow us to help fuel this revolution toward nanoscale technology and the devices it’s going to foster.