Engineering News - Spring 2006

The cause for excitement in Mechanical Engineering these days is often traced to infinitesimally small structures that are the subject of research by many of SEAS’s newest faculty members. While still maintaining its classic areas of inquiry, mechanical engineering has included in its scope an evolving emphasis on nanoelectromechanical systems, or NEMS, the smaller version of MEMS, microelectromechanical systems.

Assistant Professors James Hone and Chee Wei Wong are representative of this new direction of inquiry taken by the department. How small is nano? In a chip the size of this box , there could be 1,000 transistors squeezed onto it in the horizontal direction and another 1,000 vertically.

Mechanical engineering professors James Hone, left, and Chee Wei Wong discuss Dr. Wong’s work on optical lasers in his lab.

Dr. Wong and his team do mainly experimental work, supported by numerical work, in optical nanostructures. “If we can control the fabrication of structures smaller than the wavelength of light, we can use physical geometry to control light from first principles,” says Prof. Wong. “In subwavelength optical nanostructures, light behaves differently; we can slow light down by controlling geometry with rods and cylinders; we can control the interaction of light with matter, and we can put light on a silicon chip.”

One area of their work has immediate applications for high-speed semiconductor chips. His team is hoping to replace the electrical interconnects on computer chips with optical nanowires. “Once you have multicore chips or chips with very high speed processing, it becomes necessary for these chips to talk to each other and the traveling time of electrons actually starts to become a consideration. We are using light, which is the fastest thing we know,” he says.

Moreover, optical nanowires allow multiple colors of light to pass through at the same time and potentially dissipate little power per tens of gigabits of signal modulation. Optical NEMS control the flow of light on silicon chips, with information bits remaining solely in the photonic state.

In addition, his team is working with negative refraction, a principle new to science that allows light to be bent “the wrong way.” This strange behavior allows focusing beyond the diffraction limit, an application potentially helpful in semiconductor manufacturing and optical imaging tools, in photonic crystal nanostructures. While this behavior was predicted by leading theoretical groups, Dr. Wong, together with researchers in applied physics, electrical engineering and chemistry, have very recently experimentally observed this phenomena in the near-infrared wavelengths. “This is an example where our interdisciplinary team can control the behavior of light by being able to fabricate structures, smaller than the wavelength of light,” he says.

“Carbon nanotubes are like rolled chicken wire, but they are only a single atomic layer thick and have a few tens of atoms around the circumference.”

The behavior of photons is also fundamentally different when a quantum dot is placed into a small confined cavity. Investigation of quantum dots and photonic crystals holds additional promise for enhancing light emission and non-classical light sources. With the scientific community’s ability to fabricate quantum dots and nanostructures, Dr. Wong and his group are excited to have the ability to explore the quantum mechanical behavior of photons. Dr. Wong’s team also is exploring nonlinearities in optical nanostructures, with applications toward optical memories and novel light generation in photonic crystals. By being able to numerically design photonic band gap nanostructures from first principles and fabricate them with careful precision, he says, “we are now able to explore nonlinearities at lower thresholds and channel them into useful and creative applications.”

While Prof. Wong is making chips with optical nanostructures, Prof. Hone is producing carbon nanotubes that are only one nanometer thick but can be centimeters long, an aspect ratio of over ten million to one. He is one of many SEAS researchers working on nanotubes, including faculty from disciplines that include electrical engineering, materials science, chemical engineering, applied physics and physics.

“Carbon nanotubes are like rolled chicken wire,” says Prof. Hone, “but they are only a single atomic layer thick and have a few tens of atoms around the circumference. We want to make engineered nanotube devices, in which we have full control over the placement and properties of the constituent nanotubes. Carbon nanotube properties depend on how they are wrapped; the wrapping pattern changes behavior,” he says.

When a nanotube can be stamped onto a silicon chip and the crystal structure is known, the device can be built in a controlled way. “We can test the properties by seeing what properties change when the structure changes,” he says. Dr. Hone is also combining carbon nanotubes with carbon fibers for fuel cells, using them as electrical leads to molecules—by cutting a nanotube and, with the right chemistry, insert a single molecule—and creating devices for signal processing that use mechanical rather than electrical signals.

Dr. Hone and his group are also using nano-tools for biological studies. “We have an opportunity to do really interesting stuff driven by real biological questions.” He noted that “top-down” fabrication can be pushed down to a size scale that is similar to that of individual protein molecules. In “nano-bio,” he says, “we can have controlled fabrication using electron beam lithography similar to the way computer chips are made. We can have features as small as five nanometers, which is at the scale of DNA binding and antigen binding. We can study a binding site and see how it works, and we can make an artificial organization, control the chemistry and see how it all fits together.”

“All of this makes what we’re doing really fascinating,” Prof. Hone says, “and it is exciting to be able to create such potentially big advances at the nano-level.”

MechE Faculty Have Big Hopes for Nanoscale Work

MechE Faculty Have Big
Hopes for Nanoscale Work