Researchers H.J. Cho of the University of Central Florida, J.H.Ahn of Sungkyunkwan University, and their colleagues have developed a method to fabricate graphene-on-organic film. This method stems out of the significant challenges in developing a compatible fabrication method of actuator materials for a large displacement and a rapid response at low voltages. Most work on actuators has focused on shape memory alloys, piezoelectric ceramics, and polymer-based materials; however, these materials require postprocessing steps that are not compatible with conventional batch microfabrication steps. The graphene-based organic film is compatible with microfabrication processes and used for electromechanically driven micro-actuators.
As reported in the January 31st online edition of Nano Letters (DOI: 10.1021/nl103618e), the researchers used a hybrid material of graphene/epoxy to fabricate the actuator using a series of steps as shown in the Figure. In the first steps (a, b), graphene films are grown by chemical vapor deposition (CVD) on a 4 in. SiO2/Si wafer coated with a Ni catalyst. The photolithography process is used to fabricate gold contact pads; additionally the process is coupled with O2 plasma reactive ion etching (RIE) to produce a serpentine micro heater pattern. A sequential photolithography two-step process is used to fabricate a thin graphene/epoxy cantilever beam and support body. The researchers used a buffer echant oxide and Fe3Cl to remove the nickel and SiO2 layer and used deionized water to clean the cantilever to be used for testing.
After the fabrication process, the researchers used four arrays of the graphene-epoxy hybrid and a field emission scanning electron microscope (FE-SEM) to confirm the structure of the graphene seropentine pattern. For biomorph actuation, graphene films were directly heated, and the organic epoxy film was warmed up by the diffused heat upon applying the electric power. The researchers report a resistance of graphene ranging 50–60 kΩ and a transfer yield approaching 99%.
Schematic diagram of the fabrication process for graphene and epoxy hybrid cantilever system. Reproduced with permission from Nano Lett. (DOI: 10.1021/nl103618e). © 2011 American Chemical Society.
The team also constructed a finite element theoretical model of the graphene actuator and calculated a coefficient of thermal expansion of (−6.9 ± 0.6) × 10−6 per °C.
To compare both theoretical and experimental results, the researchers investigated the effect of temperature as a function of input power. The team determined that the temperature of the cantilever changed linearly to 36°C with a supplied power up to 1.26 mW. The deflection of the cantilever increased linearly with temperature or the electrical input power resulting to conversion factors of 0.17 μm/°C and 2.58 μm/mW. These values are in agreement with theoretical values. The oscillation of the beam with a frequency of 0.91 Hz are observed.
