A Crane for Very Small Construction Sites

Hank Hogan

It is always best to have the right tool for the job at hand. Unfortunately, researchers investigating the high-resolution positioning of single nanoparticles have lacked the proper equipment. As a result, they have not been able to controllably integrate a definite number of nanoparticles in circuits and quantum dots.

Using an experimentally discovered technique, researchers pick up 20-nm-diameter gold nanoparticles on the tip of an atomic force microscope system and place them where desired (left). This approach avoids issues involved with pushing or pulling a particle across a surface, enabling deliberate arrangements of particles (A and B). Scan sizes are 10 μm (A) and 2 μm (B). Courtesy of Jun Hu and Yi Zhang, Shanghai Institute of Applied Physics.

Now scientists from Shanghai Institute of Applied Physics in China have demonstrated single-particle dip-pen nanolithography (SP-DPN), a technique that provides the precise particle placement that researchers have been missing.

“SP-DPN can deposit different kinds of functional individual nanoparticles onto a desired position to form an accurate nanofeature,” said professor Jun Hu.

Moving nanoparticles

Previously, investigators have manipulated an atomic force microscope (AFM) to laboriously push or pull nanoparticles across a flat surface, taking several hours to move a particle a distance of ∼100 μm. In addition, they have not been able to transfer nanoparticles easily from one surface to another or to deal with the vertical dimension.

Suggested solutions have included the use of a charged AFM tip or substrate, but this scheme will not work if the nanoparticles are nonconductive. Light-based manipulation — such as optical tweezers — will not work for particles measuring only a few nanometers across. In dip-pen nanolithography, a coated AFM tip writes a pattern of molecules on a surface. Because of the tip inking, however, this approach is not suitable for single-particle work.

SP-DPN, on the other hand, is. Normally, an AFM tip is scanned across a surface in tapping mode, with the cantilever that bears the tip vibrating up and down at a resonant frequency. As the tip interacts with the surface, frequency changes are detected, typically optically.

Using an AFM system made by Veeco Instruments Inc. of Woodbury, N.Y., and commercial tips made of silicon or platinum-titanium-coated silicon from MikroMasch Inc. of Wilsonville, Ore., the investigators found that they could pick up a single 20-nm-diameter gold particle. After relocating the particle on a mica substrate, they set the tip scan 10 nm below the normal scan height and lowered the tip in nanometer steps. After a few attempts, it picked up the particle, which remained while tapping resumed, until the tip was again forced below the normal scan height by 12 to 15 nm.

Experiments showed that the technique could easily rearrange gold nanoparticles into a parallelogram, taking 10 to 20 minutes per particle. In general, metal-coated tips did better.

“The most important thing is the operation mode we employed,” said team member and associate professor Yi Zhang. “It was found by chance in experiments, and the detailed mechanism is still unknown.”

Research into the exact mechanism by which particles are picked up is ongoing. These investigations could lead to more efficient manipulation and might explain why certain types of tips perform better than others. Applications of the technique, according to Zhang and Hu, include the fabrication of single quantum dots for single-photon sources and other nanodevices.

Applied Physics Letters, March 26, 2007, Vol. 90, 133102.