Chemists synthesize molecular p-n junction

Chemist Luping Yu, Professor in Chemistry and the College, led the research for the molecular diode. Yu was assisted by Man-Kit Ng, 2002 Ph.D. in Chemistry.

University chemists have successfully synthesized an electronic component the size of a single molecule that could prove crucial in the continuing push to miniaturize electronic devices.

The component, called a molecular diode, restricts current flow to one direction between electronic devices. In the semiconductor industry, these components, called p-n junctions, form half of a transistor. Man-Kit Ng, a 2002 Ph.D. in Chemistry, and Luping Yu, Professor in Chemistry, describe their diode in the Wednesday, Oct. 2 issue of the journal Angewandte Chemie, and an article appeared Thursday, Sept. 12, in the online publication of the Journal of the American Chemical Society.

Other researchers have synthesized other types of molecular components, but the Chicago chemists are believed to be the first to synthesize a p-n junction.

“There has been a tremendous amount of hyperbole surrounding this area of ‘molecular electronics,’ but Professor Yu’s advance is nothing less than a quantum leap forward in molecular electronics,” said Reginald Penner, professor of chemistry at the University of California, Irvine.

“The synthesis took us a long time, actually, but we made it,” Yu said. “Man-Kit Ng is terrific. He’s a superb organic chemist. He deserves the most credit.”

The Chicago molecular diode measures 2.5 nanometers in diameter, or approximately the width of a dozen atoms sitting side-by-side. Synthesizing it required a multistep process that involved creating two different compounds that display opposite electronic properties, then chemically bonding them together to form the diblock copolymer. The compounds, which are made mostly of hydrogen and carbon, are embedded in a monolayer, a sheet measuring only one molecule thick. The sheets are then transferred to a gold platform, where a scanning tunneling microscope measures the properties of the diodes.

It took Ng and Yu more than six months to develop the synthesis process, but now they can mass-produce molecular diodes with relative ease. Yu is confi-dent he can now synthesize a molecular transistor, but a more difficult hurdle remains: how to connect molecular components to make a working computer. “If you can solve that issue, that’s the ultimate computer you can have as far as component size is concerned,” Yu said.

Ng said the most challenging aspect of the project was translating the concept into an experiment involving synthetic chemistry, surface chemistry, film fabrication and scanning tunneling spectroscopy to measure electrical properties. “It took us quite awhile to understand how to prepare monolayer films on appropriate solid supports before we could start investigating the electronic properties of our new materials,” Ng said.

Yu has spent most of his career experimenting with a chemically versatile class of molecules called polymers. Only recently did he bring his polymer expertise to bear on the field of molecular electronics.

“I did not step in for a long time because if I do something, it has to be unique,” Yu said.

He appears to have succeeded.

“It is difficult to overstate the importance of this discovery,” Penner said. “Other types of molecular devices should be accessible using Professor Yu’s approach.” These would include light-emitting diodes, or LEDs, which are widely used in consumer electronics and transistors.

The National Science Foundation, the University’s Materials Research Science and Engineering Center, and the Air Force Office of Scientific Research funded the project.