Smashing the Microscope: Tiny crashes harnessed for nanoconstruction

Nanotechnologists have transformed what used to be a technical flaw in one of their premier atom-moving tools into a benefit that’s making nanoscale construction easier. Instead of lamenting how readily the tips of their scanning tunneling microscopes (STMs) crash into and damage underlying samples, a group of researchers has figured out how to exploit those crashes to build precise, atom-scale structures, including fences made of straight lines of silver atoms.

CRASHING IN. The tip of a scanning tunneling microscope makes craters (above) when it collides with an underlying surface, such as this one made of silver. American Chemical Society

The resulting debris of loose atoms provides a ready supply of building blocks for nanoscale constructions, such as the tiny fences that appear above as straight lines of dots. American Chemical Society

In the October Nano Letters, Saw-Wai Hla of Ohio University in Athens and his colleagues show how to use tip crashes to create a supply of loose atoms for performing fundamental studies of quantum phenomena, “hand making” specific molecules, or building nanogadgetry atom by atom.

In the past, the raw atoms for such STM studies and constructions have been supplied via a finicky and time-consuming process. Scientists would make atoms of a solid substance evaporate and then rain down on a surface of silicon or copper. The atoms that make up these surfaces usually are bound together in a crystal, so an STM can’t move them individually.

This is where a tip crash can be handy. It gouges out a crater in a surface and sprays loose atoms over it. If researchers are using the STM for the tool’s original purpose of nanoscale imaging, such crashes can wreck the sample. These bang ups also leave behind atomic detritus from both the sample and the tip, a messy situation that can hamper atom-scale construction projects.

Hla’s team now has developed a two-step method that uses these crashes to supply high-purity raw ingredients for nanoscale builders. The scientists first navigate a tungsten tip into a controlled crash, which coats the tip with atoms from the crystalline silver substrate. A second tip crash then gouges out silver atoms from the surface, without mixing them with tungsten from the tip.

“It’s a quick and easy technique to get some atoms on your surface,” says Michael F. Crommie of the University of California, Berkeley.

Andreas Heinrich at IBM’s Almaden Research Center in San Jose, Calif., who employs STMs to build nanoscale structures, says, “I’m going to use [this technique] next week.”

The new procedure “allows us to perform experiments at the nanoscale without moving the STM tip away from the surface region of investigation,” says Hla. That’s an advantage because it can be difficult to find the construction site again after the tip is moved.

To demonstrate the utility of the technique, Hla’s team used silver atoms freed up during tip crashes to fabricate linear structures, such as fences, and curved ones, including a happy face.

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