Technology: Atomic Force Microscopy Finds Numerous Applications

David L. Shenkenberg, david.shenkenberg@laurin.com

In the March 3, 1986, publication of Physical Review Letters, Gerd Binnig, Calvin Quate and Christoph Gerber announced that they had developed the first atomic force microscope, or AFM. Quate is a professor at Stanford University, Binnig was at Stanford on sabbatical from IBM in Switzerland, and Gerber was working mostly at IBM in San Jose, Calif.

On the same floor as Calvin Quate’s at Stanford is Olav Solgaard’s lab, in which Ali Fatih Sarioglu is a PhD student. Sarioglu and Solgaard reported their development of cantilevers with flexible ends in the July 16, 2008, issue of Applied Physics Letters.

This scanning electron microscope image shows the flexible tip developed by A.F. Sarioglu et al measuring force interactions in tapping mode. Reprinted with permission of Applied Physics Letters.

The flexible ends can respond to subtle changes in force interactions that a standard cantilever cannot. The flexible ends resemble a comb because they have linear holes in them. Using the laser and detector that came with the AFM, the researchers focus a laser spot on the comb, and the spot is divided into many spots. By measuring the power from one of the spots, they can determine the force; by measuring the positions of all spots simultaneously, they can map the topography.

“I’m interested in DNA, RNA proteins [and] their interaction forces,” Sarioglu said. AFMs can enable scientists to visualize the surface of cells and biological molecules with greater magnification than with optical microscopes, without modifying the sample and with less damage to the sample than electron microscopy causes.

AFM also has been increasingly used for analysis of cracks in materials or for characterizing carbon nanotubes and other very small materials. To examine those properties on sample surfaces, some researchers have used cantilever tips with special electrical properties or with special chemical coatings.

At the University of Michigan, Ann Arbor, researchers in the labs of Kevin P. Pipe and Max Shtein have been developing AFM cantilevers in combination with an organic light-emitting diode (OLED) that provide topographic and optical information.

The Michigan team has developed three variations. In the first, the investigators mill a hole in the cantilever tip with a beam of focused ions and deposit an electron transport layer and organic light emitter and coat it with a cathode. In the second, they deposit an insulator and then lop off the top of the tip. In the third iteration, they deposit the layers without milling. If they apply a forward electrical bias, the OLED is a light emitter; in reverse bias, it’s a light detector.

“We’re starting to work with people … to get topographical information about living cells,” Shtein said. “There’s an interesting microcavity effect of these probes that we think we could use to measure flaws in materials, negative-index materials, that have interesting properties like plasmonic materials.”

IBM researchers have been developing AFMs for data storage in cell phones, and Swiss scientists developed the first AFM to go to Mars.

The biggest inherent drawback of AFM is that mechanical motion of the cantilever is slow. Researchers have increased the speed by using piezoelectric materials and by increasing the number of probes. In fact, video-rate AFM has been achieved. Even though the AFM is a relatively new invention, innovations already have enabled it to find numerous applications.