Atomic Force Microscopes Used As Molecular Milling Machines - New Technology

Duke University’s, Pratt School of Engineering researchers are at the vanguard of efforts to remake the "atomic force microscope" (AFM), an instrument typically used to obtain molecular scale images, into a tool to build precisely aligned structures at those tiny dimensions.

"I think this will be a very good tool for research in the laboratory because we should have very good control and get results relatively easily," said Stefan Zauscher, an assistant professor of mechanical engineering and materials science who brought his expertise in designing and using atomic force microscopes to Duke.

Conventional AFMs allow scientists to image surfaces at resolutions approaching the atomic scale by recording the motions of a sharp pivoting tip while the tip moves over a surface in a way resembling a phonograph needle's interactions with a record groove.

AFMs can also measure forces between molecules. And in recent years, researchers have discovered that, besides imaging surfaces and sensing forces, AFM's can be used like medicine droppers to deposit "nanoscale" amounts of materials where desired.

The term nanoscale means billionths-of-a-meter dimensions, which can translate into structures just 10 hydrogen atoms wide, noted Robert Clark, who is the Thomas Lord professor of engineering and senior associate dean at Pratt.

Trying to use a conventional AFM for nanoscale construction projects introduces problems of repeatability, reproducibility, accuracy and fidelity, said Ashutosh Chilkoti, an associate professor of biomedical engineering and materials science and engineering.

These issues, "really come from taking existing atomic force microscopes that were actually designed to do something quite different," said Chilkoti, who has worked with Zauscher to manipulate proteins with an AFM.

In response to these technical shortcomings, Clark and his laboratory group are collaborating with Zauscher's team to modify AFMs so they can lay down materials with more precision.

"We bring to bear control strategies that are beyond the typical instruments," said Clark. "I think some of the advances we'll make, both software and hardware, have potentials for commercialization."

Duke researchers have already used conventional AFMs as molecular construction tools through a procedure known as "dip pen nanolithography"(DPN) -- so-called because the process works something like a fountain pen laying down ink.

Originally developed by Northwestern University professor Chad Mirkin, DPN allows an AFM tip to pick up molecules and deposit them on another surface. The molecular transfer is greatly aided by the presence of a "nanomeniscus," a miniscule layer of water that condenses on the AFM tip in a moist atmosphere.

Jie Liu, a Duke assistant professor of chemistry, has used a DPN nanomeniscus as a "chemical reactor" in which to synthesize electrically conducting organic polymers out of molecular building blocks dissolved in the water. Liu then uses the AFM tip to deposit the finished products onto the surfaces of other molecules.

The Pratt School's Chilkoti has also experimented with DPN as an alternative way to do other kinds of microscopic chemical engineering within mere dots of materials. In one experiment, he deposits tiny samples of protein mixtures on a surface and then adds water and applies heat to separate one protein molecule from another.

While Chilkoti has previously used flexible "microstamps" to lay out tiny and precise patterns of chemicals, DPN allows experimentation with much smaller-scale features, Zauscher said.

Chilkoti said that an atomic force microscope "allows you to build a structure very quickly without having to go through expensive fabrication facilities." For years, those facilities have used masks, chemicals and beams of energy to etch and layer elaborate microstructures such as those engraved on computer chips.

Using the latest energy sources, such as X-ray lithography, a centralized fabrication facility can now turn out circuitry of nanoscale size for those willing to pay and to wait. In contrast, the AFM approach "empowers people to do nanoscience research in their own labs," Chilkoti said. "It's a relatively low tech-way of building structures quickly. In principle, you can go in and build something in an afternoon."

The problem with using a conventional AFM for such fabrication is that its scanning system typically lacks the precise control and repeatability of standard fabrication techniques, Zauscher said. For example, he said, in attempting to place dots of material on a surface, the dots might not be evenly spaced.

Such relatively poor spacing control is why Zauscher is collaborating with Clark's Adaptive Systems and Structures Laboratory to build more-precise AFMs for nanolithography.

Clark's group is designing and building an AFM with a "multi-axis" guidance system to better control the manipulation of AFM tips. Ultimately, he envisions a more advanced version that could create nanostructures on surfaces much like a sophisticated milling machine creates patterns.

"We want to create a simple interface so that you don't have to be an expert in programming the hardware to actually articulate the instrument," said Clark, who is a mechanical engineer and materials scientist. Such a simple interface would enable use of standard computer aided design (CAD) software packages to draw patterns that the AFM would then automatically and precisely execute.

Another feature might be "adaptive controls," a specialty of Clark's lab, which provides a way to adjust a controller according to varying conditions. Examples might be responding to probe tips with different stiffnesses or surface materials with changing properties.

Posted 19th May 2003

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