Researchers at Penn State have developed a new technique using organic molecules as molecular rulers to mass fabricate ultra-tiny metal nanostructures.

The results were written in a report written by Penn State scientists Paul Weiss and Anat Hatzor and published by Science magazine earlier this month.

The research is a step in understanding more about the growing field of molecular electronics. It could also make an impact in higher-density data storage disks and circuitry.

Weiss, associate professor of chemistry and director of the Center for Molecular Nanofabrication and Devices, has been doing research in nanofabrication for two years. The end goal of all the research is to be able to make extremely small structures and precisely place them close together, he said.

As it stands now, techniques that make extremely small structures are hard to accurately position close together.

Weiss and Hatzor's molecular rulers act by helping make smaller copies of parent nanostructures.

"The first step was to use conventional nanofabrication," Weiss said, adding they used a technique called electron-beam lithography to synthesize 21 patterned groups of 10 gold parent structures on a glass-like oxidized silicon wafer.

The parent structures were shaped like rectangular blocks. They were about 25 nanometers high, 200 nanometers wide, and one micrometer long, said Hatzor, post-doctoral fellow and member of the Penn State National Nanofabrication Users Network Facility. Between two parent structures was a gap of about 40 to 100 nanometers.

A nanometer is one-billionth of a meter and a micrometer is one-millionth of a meter.

After creating the patterned parent structures, Weiss and Hatzor broke the wafer into its 21 parts and performed different experiments on the groups of the 10 gold blocks. "We took those structures and attached the molecules to them," Weiss said.

The organic molecules they used are called mercaptoalkanoic acid. On one of end of the molecule, which is shaped like a straight, long chain, is a sulfur atom, which likes to bind to copper ions and gold.

One the other end of the molecular chain is a carboxylic acid group that prefers to bind with copper ions.

The researchers covered the gold structures with one layer of the mercaptoalkanoic acid, with the sulfur atom bonded to the gold parent structures and the carboxylic acid sticking out from the other end of the rigid chain.

Then they added a layer of copper ions, which stuck to the exposed carboxylic acid. After that came another layer of the mercaptoalkanoic acid, with the sulfur bonding to the copper ions and the carboxylic acid sticking straight out again.

In adding layer over layer of alternating mercaptoalkanoic acid and copper ions, the gap between the two gold parent structures narrowed.

"We know how thick each layer is very accurately, so when we're done, we've chosen the thickness of that film," Weiss said.

Hence, the gap length in between two parent structures is directly related to the organic molecules that the researches put down, thus showing how the organic molecules can be used as molecular rulers.

"We can change that gap by changing the molecule we use, and/or changing the number of layers we put down," Weiss said.

After precisely reducing the gap length between two parent structures, the researchers covered everything with a metal; in their case, they used gold.

"Once we're done with that layer-by-layer growth of film, then we evaporate a metal onto that," Weiss said. He added that in future research, they will probably experiment with evaporating other types of metal onto the wafer.

The gold that they evaporate onto everything attaches not only to the top of the multi-layered film of mercaptoalkanoic acid, but also to the wafer itself in the reduced gap.

The gold that attaches to the wafer becomes the daughter structure. It is smaller than the parent structure and as a result of the meticulous layering of the molecular rulers, it has been precisely placed between the parent structures.

Because there are more than just two parent structures, the daughter structures are mass-produced on the silicon wafer.

The daughter rectangular blocks they have fabricated have the same length as the parent structure, one micrometer, but they are only about 10-nanometers high and their widths range from 15 to 70 nanometers.

"It's about one-thousandths smaller than bacteria," Hatzor said of the daughter structures.

The researchers have also experimented with other parent structures more complex than rectangular blocks, like rings.

"The shapes are the demonstration that we can make more complex structures," Weiss said. "We can design something in the parent structure so that the feature that we produce has some complexity to it."

The significance of the newly developed nanofabrication technique can be far-reaching, Weiss said.

"There are two areas of impact. One is more short-term. In the semiconductor world, there's an international roadmap for what scales we need to reach," he said, referring to the ever-increasing density of data storage. "This may be a way to move ahead in a steady way, or even leapfrog a number of years," he said.

"The other area of impact is hooking up molecules, which ultimately may lead to smaller scale devices," he said, talking about the idea of using molecules as on/off switches in data storage, which has not been completely actualized as of yet.

Hatzor also said the technique could help in understanding molecular electronics. "In basic research, it will help find the basic properties of a single molecules."

One of their goals now is to make the width of the daughter structure less than 10 nanometers. "There starts to be interesting physical and electronic properties at those small-scale structures," Weiss said.

Weiss described the research like a bridge that starts to cross the gap between the chemistry of atoms and fabrication of nanostructures. "Those two areas really haven't met yet," he said, adding their technique "doesn't solve all the problems."

But it is a step in the right direction, he said, as the science of electronics and world atoms collide.