Building structures molecule by molecule.

Workers constructing a building rely on heavy equipment to push, pull, and lift stone blocks into precise locations without damaging them. On a nanoscale level, materials scientists want to accomplish the same task.

To construct the tiniest objects, they are aiming for the ultimate molecular control-a mastery achieved by placing each molecule into a new substance according to an exact design.

The trouble with attempting such precise control is that scientists often end up damaging the very molecular building blocks they are trying to assemble.

Now, T.A. Jung, a materials scientist at IBM's Zurich Research Laboratory, and his colleagues have found a way to position the individual molecules into stable, predefined patterns without disrupting the bonds that hold the molecules together.

In the Jan. 12 Science, the researchers explain how they use the tip of a scanning tunneling microscope to push around molecules on a surface at room temperature, creating a two-dimensional array of intact molecules.

In one case, the researchers hooked four bulky porphyrins to a rigid copper-containing unit. These molecular blocks held strongly enough to the work surface to prevent thermal agitation from causing them to hop around-a substantial problem in manipulating and placing single molecules.

Next, the scientists pushed these blocks, one by one, into stable hexagonal rings. Such structures "do not naturally form" when these particular molecules come in contact with each other or a surface, they explain.

Nanofabrication, a group of techniques that have been developed during the last decade, combines physical and chemical methods to achieve precise molecular assembly, Jung says.

In the past, scientists used voltage pulses, electric fields, and mechanical contact to break atomic bonds. They succeeded in repositioning molecules only at temperatures low enough to quell thermal agitation.

The new molecular design technique works at room temperature and is, according to Jung's team, "a step that goes beyond current approaches of engineering on the molecular scale."

The success of this new technique depends largely on the shape and stability of the molecule to be positioned, Jung says.

"Their work is a major contribution," says Calvin F. Quate, a materials scientist at Stanford University. "They've had some spectacular results."

"This represents an advance in our ability to control, image, and move molecules," Quate says. "It means that our techniques are advancing to the point that we can handle molecules as individual entities. As the techniques mature, information obtained from these experiments may enable us to do interesting things with these molecular entities."