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Shaping the Future

01.10.1998

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Frydman, Maoz, Sagiv and Cohen. Nano-building techniques

History repeats itself. However, can the history of "big things" be deliberately duplicated when developing "the small stuff"? Can massive be made into diminutive? Most importantly, can the process be revealed ­ and then restructured? And what does all of this have to do with cavemen?

Using the history of man-made architecture as an example, is it replicable when designing, manufacturing, and building electronic apparatus ­ when its size is only one-millionth of a millimeter?

Sounding the philosopher-architect in addition to the materials scientist, Prof. Jacob Sagiv of the Materials and Interfaces Department at the Institute is asking these questions, and he believes this is exactly the direction tomorrow's technology will take.

"Our ancestors," says Prof. Sagiv, "lived in the caves that nature provided. Later, they learned how to carve out the rock and to extend their caves. In this way, for example, ancient water cisterns were crafted. Thus, the city of Petra was carved out of the red rock in the southern part of the Kingdom of Jordan.

"Early man showed that with rock carving, one can produce some very impressive achievements. But at a certain stage this technology reaches the limit of its possibilities; those with the determination to progress must abandon it and find a more advanced replacement. This is exactly what the ancients did. Instead of continuing to carve out living accommodation from the existing natural rock, they began to chisel away at smaller stones, so that they could be used as building blocks to fabricate various structures with them: water cisterns, walls, and housing.

"Using the technique of building blocks, it was possible to reach a level of sophistication and complexity that surpasses previous, ancient methods. It cannot be compared with the products that were made using the technique of carving out natural rock. For example, today's building methods, to which various strengthening techniques were added, allow for the construction of the most complicated skyscrapers built by man. And he's still testing the limits.

"A similar development is also likely to take place in the field of constructing small electronic devices. Presently, integrated electronic circuits are being manufactured using methods based on 'digging,' by etching the crystals of semiconductors. The trend toward miniaturization of integrated circuits, to whatever extent possible, demands chemical 'carving' to a very delicate degree. But what level of delicacy can be reached? What are the boundaries of complexity of the structures that we can create in this way?

"As it's clear that we're approaching these limits, if we're going to continue to advance, we have to imitate the ancient architects. We've got to abandon the technique of carving out existing materials and start building structures and apparatus from the tiniest of building blocks available: molecules and atoms."

Prof. Sagiv, Dr. Rivka Maoz and research student Eli Frydman of the Institute's Materials and Interfaces Department, together with Dr. Sidney Cohen of the Institute's Chemical Services Unit, have recently taken some first steps in this direction; they've succeeded in building planned, simple three-dimensional structures out of molecules.

In the first stage, the researchers manufactured a base layer, one molecule thick, which was attached in a well-defined manner to the outer surface of a selected solid material, in this case, silicon. To do this, they used molecules that on one side are hydrophilic (i.e., water "loving") and on the other side, hydrophobic (i.e., water "hating"). When a plate of glass or a silicon chip is immersed in a solution containing such molecules, their hydrophilic sides attach themselves to the plate (as its properties are similar to those of water), while the hydrophobic sides face outwards.

Then they were ready for the building stage. This is how it works.

To mark the foundations of the molecular building, the researchers used an atomic force microscope. This is a lensless instrument that works by blindly groping about the face of the surface of the material using a needle with a fine tip which "feels out" the atoms and the molecules protruding from the top of the surface. The needle sends "sensations" to the computerized system which translates the "feelings" into pictures on a computer screen.

The distance between the sensing tip and the material's surface must be less than one nanometer (one-millionth of a millimeter). Following an idea suggested by Dr. Maoz, the Weizmann Institute team discovered that when a small electric current flows down the fine sensing tip, it can induce a localized chemical reaction which changes the properties of the molecules located directly beneath it, on the surface of the material being treated. In this way, the researchers were able to change the properties of the surface molecules in a specified area. Only "the chosen" will be adapted for additional connections to other selected molecules.

Using the needle of the atomic force microscope, the scientists could deliberately change the chemical properties of the hydrophobic, outwards-facing side of the molecules attached to the silicon chip, so that at any point where the tip "draws" lines of such chemical changes, new molecules can attach themselves to those first attached to the surface. In this way the "walls" of the molecular building are constructed.

In one experiment, for example, the members of the research team made a two-story molecular building in the shape of a tiny Star of David whose sides had a width of one-thousandth the width of a human hair.

These first steps in the architecture of molecular buildings are likely to bring about the development of tiny devices and other products which cannot be constructed using existing miniaturization technologies. The devices will be designed for high efficiency and, by their nature, will offer maximum space saving.

New and unconventional properties are likely to be derived both from their minuscule dimensions andfrom the purpose-designed spatial organization of their molecular components.

Prof. Sagiv and Dr. Maoz dream of giving new meaning to "Small is beautiful." While most of us are still struggling with "Think big," it's apparent that creative living solutions are literally right under our noses, in miniature.

From big to small

 

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