Portland, Ore. Carbon nanotubes harbor highly energetic properties, such as ballistic electron transport and electroluminescence. And the list just seems to keep growing. Now optical-to-mechanical transducers have been demonstrated based on a nanotube thin film, enabling light to directly control mechanical motion.
"In our experiments, we discovered independently that if we shone light on nanotubes, they would move," said Balaji Panchapakesan, a University of Delaware EE professor. "We are the first group to show you can make micro-optical-mechanical systems of actuators based on them."
Micro-opto-mechanical systems (MOMS) use lasers to actuate tiny mirror-tipped cantilevers instead of the electrical current needed for pneumatic, piezoelectric or electrostatic actuators. An optically active nanotube film enables MOMS to be actuated by an ultralow-power laser instead of a power-draining electrical current. Recently Panchapakesan's group demonstrated that Digital Light Processors (DLPs) could be catapulted into space if they switched from MEMS to MOMS.
Today, DLPs use microelectromechanical systems (MEMS) to tilt the angle of arrays of micromirrors, thereby projecting reflected images onto larger screens. For power-conservative applications, such as space exploration, even the microamps required to tilt the micromirrors could make DLPs prohibitive. Now Panchapakesan's group claims to have solved the MEMS-DLP problem by turning to MOMS-DLP instead. The researchers say that MOMS has low enough power consumption for space exploration and could enable new applications for field-emission displays and biomedicine.
"We have made many tiny robotic devices, such as a microgripper, with optical actuation, and we think MOMS could also be used to actuate microsurgical tools," said Panchapakesan.
The technique patterns a carbon nanotube thin film using standard CMOS processing steps, resulting in arrays of optically actuated cantilevers measuring 300 microns long x 30 µm wide x 7 µm thick.
"We can use our nanotube films on normal CMOS chips; they can be transparent or opaque and can go on silicon, silicon dioxide or silicon nitride wafers," said Panchapakesan. "Then, we use lithography and etching to define the actuators. [This is] the same way that MEMS works, but our material is nanotube films."
First, a 130-µm-thick layer of nanotubes is applied to an oxide-covered silicon wafer by vacuum filtration. After annealing, standard lithography and etching steps pattern the nanotube thin film into arrays of lightweight, optically activated cantilevers. The cantilevers deflected 23 µm when their base was illuminated with an 808-nanometer-wavelength, 170-milliwatt semiconductor laser, the team reported. The precise mechanism by which the nanotubes act as actuators is not known, but Panchapakesan is working to confirm a hypothesis. "We believe the primary mechanism is charge separation induced by the laser," he said. It "travels down the one-dimensional nanotube, inducing electrostatic forces that stretch the carbon-to-carbon bonds, causing actuation." Thermal expansion is a contributing factor as well, he added.
Annealing is best done at 75°C, he said. Moreover, lithography can succeed only if there is zero stress in the nanotube film. The other details of his process are described in the current issue of Applied Physical Letters (http://apl.aip.org).
Next, Panchapakesan's research group plans to try making the cantilevers wavelength-selective. His team also plans to build demonstration applications of MOMS showing how semiconductor laser actuators enable many types of nanorobotic mechanisms, such as a "cancer bomb" activated only inside tumors. The University of Delaware has received a provisional patent for its technique.