Zurich scientists position individual molecules at room temperature

Zurich, Switzerland, 12 Jan 1996—For the first time, scientists at IBM Research - Zurich have succeeded in moving and precisely positioning individual molecules at room temperature, using the extremely fine tip of a scanning tunneling microscope (STM). This is another important step towards being able to do a wide range of "engineering" on the nanometer scale (one millionth of a millimeter). It could help lead to the ultimate limits of miniaturization and open the way to fabricating molecules with specific properties and functions, constructing computers of ultimately small size, and even to building minute molecular machines capable of cleaning or repairing nano-scale electronic circuits, for example.

One key to this "nanocosmos" is the STM, which was invented at IBM Research - Zurich and for which its creators were awarded the Nobel prize in Physics in 1986. The STM can be used not only for imaging surfaces with atomic resolution but also for positioning individual atoms and molecules. However, as STM co-inventor and Nobel laureate Heinrich Rohrer explains, "Most atoms and molecules tend to stick quite strongly to the surface and to the STM tip, making it difficult to pick them up and release them in a precisely controlled way." On the other hand, the less "sticky" ones jitter and jump around at room temperature. Scientists at IBM's Almaden Research Center in San Jose, California, overcame the jitter problem by cooling the sample to –270 °C, which is nearly absolute zero. In 1989, they were first to position individual atoms when they wrote the letters "IBM" with 35 xenon atoms.

However, room-temperature positioning is required for broad practical uses, such as creating chemical reactions that build functional units from individual atoms and molecules. The first successful room-temperature manipulation of atoms was performed in 1991 at IBM's T.J. Watson Research Center in Yorktown Heights, New York, using electrical pulses to pick up and release individual silicon atoms. Most molecules, however, would be torn apart by the strong electrical pulses required by this technique.

Zurich scientists have now succeeded in positioning individual molecules at room temperature by purely mechanical means. The nature of the molecules and their interaction with the surface plays an important role: they have to stick tightly enough to remain at their position but not so tightly that they cannot be moved. The chemical bonds within the molecule, on the other hand, must resist being changed or broken when the molecule is pushed by the STM tip. Zurich scientists evaluated a wide range of molecules as possible candidates in experiments and performed elaborate molecular mechanical simulations in collaboration with colleagues at the French National Center for Scientific Research (CNRS) in Toulouse. They selected an organic molecule having a total of 173 atoms. Its core consists of a stable ring of atoms known as a porphyrin. Porphyrins are found widely in nature, for example as the basis of red blood cells (heme group of hemoglobin). Four strongly but flexibly bonded hydrocarbon groups attached vertically to the ring make the molecule, which has a diameter of approximately 1.5 nanometer, ideal for displacement experiments: its position and structure are easily identified by STM imaging, and the four hydrocarbon groups act as "legs" that lift the "body" of the molecule from the atomically flat copper surface. Computer simulation revealed that when pushed by the STM tip the molecule "walks" in uncorrelated steps and exhibits exactly the desired degree of stickiness.

IBM scientists, and colleagues at the University of Cambridge, UK, developed software that moves and positions the STM tip with extreme precision. The same STM can also be switched to the imaging mode by slightly increasing the distance between the tip and the surface.

This research 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) of the European Union. The long-term goal is to create new and complex molecular structures and to customize their specific properties and functions. The porphyrin-based molecule selected for these manipulation experiments has a number of potential technological uses. For example, the single copper atom at its center can be replaced by another metal atom having different electronic properties. This could be exploited in principle to construct data storage devices with densities 100,000 times higher than today's most advanced disk drives. Another technological vision involves wires only one molecule wide that could be used to build ultra-small electronic components.

The work at IBM Research - Zurich was performed by James Gimzewski, Thomas Jung, and Reto Schlittler; their colleagues are Christian Joachim and Hao Tang of CNRS, and Mark Welland, Martin Murrell, and Timothy Wong of the University of Cambridge. A scientific report was published in the January 12, 1996, issue of Science (Vol. 271).