Mark Hersam

Graduate student Nathan Yoder and Mark Hersam

A shrinking field

Nanotachnology promises electronic devices the size of a molecule

The increasingly fine focus of nanotechnology is enabling a world of scientific advances. But as the subjects under study shrink, nonscientists may find it harder and harder to picture what is happening at the atomic level. Coming to the rescue with a metaphorical pair of high-tech bifocals is Mark Hersam, assistant professor of materials science and engineering. Not only has Hersam built the devices that allow researchers to “see” matter on the atomic level, but he is also very good at explaining things.

For example, consider Hersam's explanation of how he and his colleagues are able to measure the chemical identity of individual molecules by gauging vibrational energies: “It's analogous to a mass on a spring — pull it, and it will vibrate. The frequency of that vibration is a function of the mass of the object. On the molecular level, the atoms in the molecule are held together by chemical bonds that act like springs. By measuring the frequency of those vibrations we can determine the identities of the atoms pulling on the springs.”

Hersam makes it sound simple — or at least understandable. In fact, measuring those vibrational energies involves ultrasophisticated equipment as well as expertise outside of his field, which is why he is teaming with Richard Van Duyne, Morrison Professor of Chemistry in Northwestern's Judd A. and Marjorie Weinberg College of Arts and Sciences, on a project to study single-molecule sensing, characterization, and actuation. Van Duyne is an expert on laser spectroscopy, a technique that offers high spectral resolution but poor spatial resolution. Hersam's group has built several scanning tunneling microscopes (STMs), devices that offer high spatial resolution. “Combining the very high special resolution of STMs with the advantages of lasers allows us to measure the vibrational energies and identify the atomic makeup of molecules,” explains Hersam.

To fabricate the silver tip of the microscope's probe, Hersam called on Teri Odom, assistant professor of chemistry in Weinberg College. “She's an expert in fabricating nanowires,” says Hersam, “and she's developed unique strategies for our project.”

That interdisciplinary approach marks much of Hersam's research. In addition to his collaboration with Van Duyne and Odom, Hersam is working closely on several other projects with faculty members from Weinberg College and the McCormick School. “That's the beauty of Northwestern,” says Hersam. “You hear about the work your colleagues are doing, you get together with them, you toss around crazy ideas — and often you're able to turn those ideas into reality.”

A diverse bunch

For research on silicon-based molecular electronics, Hersam is teaming with Mark Ratner, Charles E. and Emma H. Morrison Professor of Chemistry. For a project on nanoscale organic light-emitting diodes, he is tapping the expertise of Tobin Marks, professor of materials science and engineering and Vladimir Ipatieff Research Professor in Organic Chemistry. With Michael Bedzyk, professor of materials science and engineering and of physics and astronomy in Weinberg College, Hersam is exploring conductive scanning probe microscopy nanopatterning.

Indeed, all seven areas of Hersam's ongoing research involve collaborators from Northwestern and universities around the world as well as from industry. Working alongside Hersam are five postdoctoral fellows — “a diverse bunch,” says Hersam, with degrees in chemistry, materials science, and electrical engineering — plus a dozen graduate students and nine undergraduates.

The interactions work both ways. When Hersam collaborates with chemists, for example, not only does he take advantage of their specialized knowledge, but he may also be contributing to a revolution in the way chemistry is studied. “Instead of mixing substance A and B together and guessing about what's happening at the molecular level,” says Hersam, “we will actually be able to see it. STMs will become tools for doing fundamental science.”

One area of Hersam's ongoing work that has generated special interest of late is molecular electronics, which offers the potential of using individual molecules as tiny electronic devices. “A single-molecule device likely represents the ultimate scalability of electronic technology,” says Hersam, who is collaborating with Ratner in this research.

Sharing the credit

Although most researchers in the field of molecular electronics study the electronic properties of individual molecules with metallic contacts — such as thiol molecules on gold — the approach of Hersam's group has been to replace one of the metallic contacts with a semiconductor, silicon. The semiconductor contact has several significant advantages, says Hersam:

  • It builds on existing covalent binding chemistry for organic molecules on silicon.
  • Semiconductors possess an energy band gap that enables novel modes of charge transport.
  • The electronic properties of the semiconductor contact can be tailored easily via doping.
  • Since silicon is the dominant material in the microelectronics industry, molecular-scale devices built on silicon substrates can be directly integrated with preexisting technology.

The interface between organic and inorganic molecules that is at the heart of Hersam's approach to molecular electronics increases the functionality of potential devices and multiplies possible applications. “Organic molecules have selective reactivity, whereas silicon reacts with many things,” explains Hersam. “The idea isn't for organic molecules to replace silicon but to add value to it, to complement the silicon.” Attaching organic molecules to silicon substrates promises to capitalize on the advantages of both materials. One application that may result from this research is the development of tiny sensors sensitive to subtle shifts in the environment, a potential application that has attracted interest and funding from the U.S. government.

To study the structure and properties of individual organic molecules on silicon surfaces, Hersam and his team have custom-built three ultra-high vacuum STMs. The team uses atomically precise feedback-controlled lithography to pattern the assembly of organic molecules on the silicon surfaces. Unlike other fabrication processes, which are performed at cryogenic, or very low, temperatures and are impractical for consumer goods, this process works at room temperature, paving the way for eventual integration with conventional silicon microelectronics.

The richness of Hersam's research — along with his knack for explaining it — has garnered the 30-year-old professor a spate of prestigious awards and grant money of late: an Alfred P. Sloan Foundation Research Fellowship; an Army Research Office Young Investigator Award; an Office of Naval Research Young Investigator Award; and the 2006 Robert Lansing Hardy Award from the Minerals, Metals, and Materials Society (TMS).

“The fact that I'm winning these awards reflects upon the high quality of the students and postdocs who are in this group,” says Hersam. Sharing the credit comes naturally to Hersam, perhaps because he finds research rewards him amply all by itself. Research, says Hersam, offers “the excitement of generating new knowledge.”

Having worked closely with Hersam, Ratner believes the recognition is well deserved. “Mark Hersam has championed the use of scanning probe techniques to answer important questions in materials science, chemistry, physics, and electrical engineering,” says Ratner. “I think his work is the best of its kind in the world. He's a tremendous resource and an exciting presence at Northwestern.”

—Leanne Star

Northwestern University