Applying tiny forces
Fields and applications as varied as biomaterials,1,2 artificial muscles,3 nanorobots,4 and nanoantennas5 could benefit from the ability to convert other forms of energy into motion. For instance, it might be possible to inhibit the growth of malignant cells (such as tumor cells) by applying a properly controlled force. For such research to be undertaken, however, we first need to develop nanoactuators that can generate and control a force on a sufficiently small scale.
Microscale actuators have been reported for mirrors,6–8 conveyer systems,9,10 tweezers,11–14 ciliary systems,10,15 and fixtures.16 Further miniaturization may enable more interesting applications by making it possible to apply lower and controllable forces to smaller objects and create higher-frequency (up to terahertz) nanodevices. However, difficulties in fabricating nanoactuators and measuring nano-Newton (nN) forces have hindered development of nanomechanical actuators.
We have developed a simple way to fabricate nanoactuators along with a method of measuring nN forces in a repeatable and controllable fashion. We used this force-measurement technique to obtain Young's modulus and the coefficient of thermal expansion (CTE) of the polymer polypyrrole (Ppy). A material's CTE describes the amount it expands or contracts with temperature and knowing Young's modulus enables us to predict the force a material exerts when it expands.
At an early stage, we used individual multiwall nanotubes (MWNTs) to create bimorph nanoactuators that generate forces on the micro-Newton level based on the high Young's moduli (~1TPa) and small CTEs (3×10−6/K) associated with MWNTs.17 To create such nanoscale bimorph structures, we used a directional deposition method (pulsed-laser deposition: PLD) to precisely deposit a metal film on only one side of a MWNT, leaving the other side bare. We measured the force generated from the nanoactuator by sweeping an atomic-force-microscope tip laterally across the middle of the bimorph.
We also demonstrated nanoactuators based on Ppy nanowires with a low Young's modulus.18 We synthesized the nanowires using an anodized alumina oxide membrane as the template in an electrolyte bath. We subsequently dissolved the membrane to leave it suspended in solution. The nanowires were deposited on the wafer edge as cantilevers. We then deposited a thin metallic (copper) film on one side of the cantilevered nanowires using PLD (see Figure 1).
We observed thermal deflections over multiple identical temperature cycles inside a scanning-electron microscope using a thermal stage prepared in the microscope. We used lateral-force microscopy to measure the thermally generated force due to CTE mismatch. The force generated by the bimorph applied a torque at the tip of an atomic-force microscope (AFM)'s cantilever. We used the AFM's pre-calibrated photodiode signal to record the torsion at the cantilever and calculated the applied force. At 100K, a 13μm-long bimorph produced a force of 1nN. As expected, considering the typical polymer Young's modulus of ~2GPa, this is 1000 times smaller than that of aluminum carbon-nanotube (Al-MWNT) actuators.
Using our force-measurement technique, we investigated the mechanical properties and behavior of nanoactuators composed of several materials. Further, we are developing a nickel aluminum (Ni-Al) bimorph nanoactuator for applications toward a nanoscale antenna.19
In summary, we are pursuing the fabrication, assembly, and manipulation of nanoactuators for cross-disciplinary applications. A large number of such actuators with repeatable and controllable nanoactuator properties and performance will be required for practical applications. We are focusing on fabricating various materials of nanowires in different dimensions to optimize performance. We are also developing a technique to produce large numbers of nanoactuators in a controlled way, which is necessary for cell biology and high-frequency nanoantenna applications.
The experimental part of this work was performed by Onejae Sul and Seongjin Jang under supervision of the author. This work has been partially supported by the National Science Foundation's Major Research Instrumentation Program (DMI-0619762) and the Air Force Office for Scientific Research (FA9550-08-1-0134).
Eui-Hyeok (‘EH’) Yang's research group is investigating engineered carbon-nanotube and graphene nanostructures and devices for nanosensors/actuators and nanoelectronics applications. Current research projects include a novel nanolithography technique associated with carbon-nanotube and graphene nanostructures, graphene-based nanoelectronics, and nanoactuator-based active nanostructures. EH Yang is the recipient of a number of awards, including NASA Inventions and Contributions Board Space Act Awards, Level B and C Awards, and Class 1 NASA Tech Brief Awards. In recognition of his excellence in advancing the use of microelectromechanical-systems-based actuators for NASA's space applications, he received the prestigious Lew Allen Award for Excellence at NASA's Jet Propulsion Laboratory. He is currently principal or co-principal investigator on grants and contracts from the Air Force Office of Scientific Research, the National Science Foundation, NASA, NASA Small Business Innovation Research, and the US Army Armament Research, Development, and Engineering Center. He holds over 10 patents issued or pending. He is associate editor of the IEEE Sensors Journal and a member of the editorial boards of Nanoscience and Nanotechnology Letters, Science of Advanced Materials, and IEEE Sensors Journal. He is the director of the Micro Device Laboratory, a multi-user facility at the Stevens Institute of Technology.
