NISE NETWORK: SMALL IS DIFFERENT: THINK SMALL

The concept of the atom has been around in one form or another since the ancient Greeks. Yet at the beginning of the twentieth century, there were still some scientists who doubted the atom’s existence. To them, an atom was simply a useful fiction. It took Albert Einstein, in a 1905 paper, to explain the indirect evidence for the existence of atoms and to show that the sizes of atoms and molecules could be determined. French scientist Jean Baptiste Perrin did the experimental work that confirmed Einstein’s theory.

Nonetheless, scientists doubted that we’d ever be able to observe an atom, or anything else smaller than a few hundred nanometers, because of the limitations of the optical microscope. This microscope uses light to make an image, so its performance is constrained by the nature of light. Light behaves as a wave, and for an optical microscope to form a clear image of a sample, the sample has to be larger than a wavelength of light (the distance between successive wave crests or other identical parts of a wave.) The wavelengths of visible light range from about 700 nanometers to about 400 nanometers. But atoms are only 0.1 nanometers to 0.5 nanometers in diameter.

Then scientists discovered that electrons have wave properties, and that the wavelength of an electron is determined by its speed. It was a short step to recognize that high-speed, short-wavelength electrons could be used instead of light to look at tiny samples. In 1933, the first electron microscope was designed by Ernst Ruska at the Fritz Haber Institute of the Max Planck Society in Berlin. Just as glass lenses are used to focus light in an optical microscope, electromagnets are used to focus electrons in an electron microscope. The image is formed on a photographic plate. With this tool, scientists could see structures as tiny as viruses. But the smallest viruses are about 200 times the size of an atom.

A field ion microscope image of a tungsten surface taken by Erwin Müller, the inventor of the field ion microscope.

Ions—atoms that have a positive or negative charge because they have lost or gained electrons—have wavelengths shorter than those of electrons, and, in 1951, this led Erwin Müller at Pennsylvania State University to develop the field-ion microscope: A metal sample formed into a sharp needle and a florescent detector screen were enclosed in a vacuum chamber along with a small amount of an inert gas such as helium. The sample was cooled to less than 100 kelvin (-170 degrees Celsius) to reduce the vibrations of its atoms. Then the sample was positively charged and the screen was negatively charged. Gas atoms that came in contact with the sample became ionized—positively charged—and were repelled by it. These ions zoomed toward the screen in straight lines perpendicular to the curved surface of the sample, causing the screen to glow where they hit it. This produced an image that showed the arrangement of the atoms on the surface of the sample.

The field-ion microscope provided the first real “look” at individual atoms, but it could only be used with a limited number of materials and didn’t show the atoms in great detail. As the century progressed, electron microscopes were improved to the point that they, too, were able to produce images of single atoms, although rather fuzzy ones.

In 1981, Gerd Binnig and Heinrich Rohrer at the IBM Zurich Research Laboratory in Switzerland developed a significantly superior tool for observing surfaces atom by atom: the scanning tunneling microscope (STM). (Binnig and Rohrer would share the 1986 Nobel prize with Ernst Ruska, designer of the electron microscope.)

Here’s the basic concept: The STM has a metal needle that scans a sample by moving back and forth over it, gathering information about the curvature of the surface. Imagine closing your eyes and running your finger along the top of a row of books—you could easily identify the changes in height. Now imagine replacing your finger with a needle that has a tip tapering down to a single atom, and you can understand how the tip can follow the smallest changes in the contours of a sample.

The needle doesn’t touch the sample, however, but stays about the width of two atoms above it. The STM takes advantage of what’s called the tunnel effect: If a voltage is applied to the tiny distance between the needle and the sample, electrons are able to tunnel, or jump, between the needle and the sample, creating an electric current. A computer receives the electrical signal and directs the needle to move up or down to keep the current constant—which keeps the distance between needle and sample constant. The path of the needle is recorded, and the computer can display that information as a grayscale image or topographical map. Scientists can add color to make the image easier to interpret. (See the Quantum Corral for more about creating images.)

In 1989, Eigler and Schweizer spelled “IBM” by positioning thirty-five xenon atoms on a nickel surface.(Image reproduced by permission of IBM Research, Almaden Research Center. Unauthorized use not permitted.)

The result is a visual way to learn about the sample—but it’s not a picture of the atoms on the surface of the sample. For example, the atoms appear to have solid surfaces in STM images, but in reality they don’t. The nucleus of an atom is surrounded by electrons that are in constant motion. What appears to be a solid surface is actually a haze of electrons. The STM shows the positions of atoms—or more precisely, the positions of some of the electrons. It doesn’t show the atoms themselves.

Just eight years after Binnig and Rohrer used the STM to observe surfaces at the atomic scale, Donald Eigler and Erhard Schweizer at IBM’s Almaden Research Center first used the STM to manipulate individual atoms. They discovered that they could bring the tip of the probe just close enough to an atom for the atom to stick to it. Then if they moved the tip horizontally, it dragged the atom along with it.

This additional ability of the STM opens up an entirely new range of possibilities. If atoms can be moved around, then molecules can be constructed or altered. And materials for products as diverse as integrated circuits and biomedical devices could be created specifically for the function they need to perform.

This intriguing nanoscale image, called the Quantum Corral, was created by Donald Eigler, Michael Crommie, and Christopher Lutz in 1993 at IBM’s Almaden Research Center.

The STM may be conceptually simple, but there are complexities in its use. For instance, a small vibration, even a sound, could smash the tip and the sample together. The STM needs to be in a vacuum chamber, which isolates it from vibrations. The vacuum chamber also protects against contamination. A single dust particle, for example, could damage the needle.

Atomic Force Microscope
Like the scanning tunneling microscope, the atomic force microscope uses a probe to scan back and forth over the surface of a sample. But instead of using an electrical signal, the AFM relies on forces between the atoms in the tip and in the sample.

The probe of the AFM is a flexible cantilever—think of a diminutive diving board—with a tip attached to its underside. As the tip scans the sample, the force between them is monitored. To keep the force constant, the cantilever is moved up and down. A detection device, usually a reflected laser beam, measures the vertical movement of the cantilever, which corresponds to the hills and valleys of the sample’s surface. A computer translates this vertical movement into an image.

In addition to gathering information about the topography of a sample, the AFM can measure the friction between the tip and the sample, and it can also measure the elasticity, or softness, of a sample.

The AFM can operate in three modes: The tip can be in constant contact with the sample, it can be slightly above the sample, or it can be in “tapping” mode, intermittently tapping gently on the sample. This latter approach works well with soft samples that might be harmed if the tip stayed in contact.

While the STM is generally used with samples that conduct electricity, the AFM can be used with almost any type of material, including biological samples. It’s been used to image DNA, individual proteins, and even living cells.

The AFM was developed in 1985 by Gerd Binnig, IBM scientist Christoph Gerber, and Stanford University professor Calvin Quate.

From Vision to Action
In 1959, Nobel laureate Richard Feynman delivered a talk to the American Physical Society titled “There’s Plenty of Room at the Bottom—An Invitation to Enter a New Field of Physics” (available at http://www.its.caltech.edu/~feynman/plenty.html). In this talk, he discussed the opportunities and promises of manipulating and controlling things on a very small scale, outlining what was to become the field of nanotechnology. And he talked about how interesting it would be to explore this nanoscale world.

This striking image taken by Ming Lee Tang at Stanford University shows a very thin crystalline film composed of stacked “terraces” (a 45-nanometer-thick layer of an organic semiconducting compound, a fluorenethiophene-oligomer).

In the course of his lecture, Feynman made this prediction: “In the year 2000, when they look back at this age, they will wonder why it was not until the year 1960 that anybody began seriously to move in this direction.”

But there’s a simple reason people didn’t immediately begin working at the nanoscale. They didn’t have the tools.

The scanning tunneling microscope and the atomic force microscope were the two most important tools at the beginning of the nanoscale revolution, but now there’s an expanding toolkit of devices used to observe, measure, and manipulate nanoscale structures. And as we learn more about the nanoscale world, we’ll be able to make even better tools.