STM Key to Positioning Individual Molecules at Room Temperature
Scientists at IBM's Zurich Research Laboratory in Switzerland have used a scanning tunneling microscope (STM) to move and precisely position specially designed individual molecules on a copper surface at room temperatures for the first time. In addition to developing software that moves and positions the STM tip with extreme precision, the team was able to switch the same STM to the imaging mode by slightly increasing the distance between the tip and the surface.
The achievement is an important step towards developing the ability to perform a wide range of nanometer-scale engineering, according to Thomas Jung, who headed the Zurich effort. "Eventually, we hope to learn how to build molecules with specific properties and functions, construct computers of very small size, and even build minute molecular machines capable of cleaning or repairing nano-scale electronic circuits, for example," he said.
The STM, which earned its inventors at IBM/Zurich the 1986 Nobel Prize in physics, can image surfaces with atomic resolution and has been used to position individual atoms since late 1989, when scientists at IBM's Almaden Research Center wrote the letters "IBM" with 35 xenon atoms. However, most atoms and molecules tend to stick strongly to the surface, making it difficult to pick them up and release them in a precisely controlled way. Those that are less "sticky," on the other hand, tend to jitter too easily at room temperatures to make sustainable structures.
The Almaden team overcame the jitter problem by cooling the sample to nearly absolute zero. 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 by researchers at IBM's T.J. Watson Research Center, using electrical pulses to pick up and release individual silicon atoms. Most molecules would be torn apart by the pulses used in this technique, however.
To solve this problem, the Zurich scientists evaluated a wide range of molecules as possible candidates for the experiments, performing elaborate molecular mechanical simulations. "The molecules have to stick tightly enough to remain at their position, but not so tightly that they cannot be moved," said Jung of the role of the molecule's nature and its interaction with the surface. "On the other hand, the chemical bonds within the molecule must resist being changed or broken when the molecule is pushed by the STM tip."
In an invited paper at the March Meeting, Jung described how they manipulated an organic molecule consisting of 173 atoms, with a stable ring of atoms at its core known as porphyrins. Widely found in nature — they are the basis of red blood cells, for example — the position and structure of porphyrins are easily identified by STM imaging. The molecule also has four strongly but flexibly bonded hydrocarbon groups attached vertically to the ring, which act as "legs" that lift the "body" of the molecule from the atomically flat copper surface.
According to Jung, the porphyrin-based molecule has a number of potential technological uses. For example, the single copper atom at its center can be replaced by another metal atom with different electronic properties, which could be exploited to construct data storage devices with densities 100,000 times higher than today's most advanced disk drives. Another technological possibility involves wires only one molecule wide that could be used to build ultrasmall electronic components.
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