The demonstration that individual molecules can be moved and precisely positioned on a surface by means of a scanning tunneling microscope (STM), without lowering the temperature to near absolute zero, represents a key step on the road to creating new atomic and molecular structures .
In Brief:
A team at the IBM Zurich Research Laboratory has
managed to move synthetic, organic molecules, equipped with "legs," along a metal surface. A scanning tunneling microscope (STM) nudges each molecule along, and the legs grip the surface, providing stability. The research is aimed at creating new, complex atomic and molecular structures.
The dreams of nanoscience have moved a step nearer to realization. In a significant recent advance, a team headed by Jim Gimzewski, of the IBM Zurich Research Laboratory, has, for the first time, moved molecules on a metal surface in a controlled and repeatable fashion. The team, which includes Thomas Jung and Reto Schlittler, reported its advance in Science (January 12, 1996).
The research enhances the prospect of developing the capability to create new, complex nanostructures; that is, structures on the scale of a few tens or hundreds of molecules. These structures might take the form of molecules with specially tailored properties; devices such as biosensors or electronic components. To reach that stage of molecular fabrication, scientists will need to master new techniques of moving and manipulating molecules, particularly at room temperature (see "Getting a Grip on Atoms").
Click and nudge
In their experiments, the researchers used an organic molecule called a "porphyrin." While porphyrins appear widely in nature, the Zurich group used "designer" molecules made in Japan. Generic porphyrin molecules consist of a ring of atoms about 1.5 nanometers in diameter, with a metal atom in the center. The molecules prepared for the Zurich team have, in addition, four "legs," each consisting of a hydrocarbon group.
The legs allow the molecule to grip the surface, an ability that counteracts the molecules' tendency to drift about randomly as a result of their own thermal energy. Yet the interaction between the legs and the surface does not prevent all motion. "When we nudge a molecule with the tip of the STM, it scoots along, almost as though it were on rollers," says Gimzewski.
Special software, written with collaborators at the University of Cambridge, helps control the motion of the tip, and hence of the molecules. "The software allows us to make many images - about 10 per second - with a small field of view, which means we can work interactively in real time," explains Gimzewski. "First, we image the molecule, then we move the cursor to where we want to position the tip and press the mouse button to automatically move the
tip next to the molecule. We can then move the tip again and push the molecule along."
The ability of the molecules to grip the surface results, in part, from the tendency of the legs to conform to the corrugations in the surface. By comparing a ball-and-stick model of the 173-atom molecule to the STM images, one can infer just how the legs bend in the conformation process. "We are actually able to study the mechanics of the molecule," says Gimzewski. Computer simulations at the National Center of Scientific Research in Toulouse, France, support his interpretation.
In addition to imaging and moving the molecules, the Zurich researchers have performed surgery on them. They can, for example, remove part of a leg or the entire limb, and, in principle, replace it with other groups of atoms to create more complex structures.
The molecule-moving work is part of the PRONANO project, sponsored by the Swiss Federal Office of Education and Science within the European Strategic Program for Research in Information Technology (ESPRIT) and includes collaborators at several European universities. One collaborator, former IBM Fellow Alec Broers, at Cambridge, is making nanometer-size (10 billionths of a meter or 390 billionths of an inch in diameter) wires using electron-beam lithography. He leaves gaps along the wires, and Gimzewski's group is examining the possibility of using the STM to move molecules into the gaps. If developed, that ability could prove useful in fabricating molecular electronic devices.
Getting a Grip on Atoms
IBM Fellow Don Eigler and his coworkers at the Almaden Research Center have pioneered low-temperature manipulation of atoms. In an early feat, they spelled out the letters "IBM" with xenon atoms and, more recently, created so-called quantum corrals by positioning iron atoms in circular patterns. They make these atomic constructions by bringing the STM tip so close to the atoms that the weak interatomic and chemical forces between the tip and an individual atom move the atom to the desired position.
Such feats are far more difficult to achieve at room temperature. Either the atoms and molecules jiggle around on their own (because of their intrinsic thermal energy), which makes it essentially impossible to position them, or they are so tightly bound to the surface that they can be moved only by breaking the chemical bonds, which limits the predictability of the result.
This bond-breaking technique has been pioneered by Phaedon Avouris and colleagues at the Thomas J. Watson Research Center, who move atoms at room temperature by applying a high electric field between tip and sample. In response to the field, a surface atom is evaporated onto the tip. The tip can then be moved elsewhere and the atom redeposited, by reversing the field.
