Smartphones are increasingly multi-purpose electronic computers that allow a number of functions beyond a mere telephone. These include GPS navigation, video communications, mobile games, smart shopping, environmental monitoring, metal detection, and even health monitoring or seismic monitoring. But do you know how many sensors in your smartphone? It's half a dozen or more gadgets packed into a single slab, with a wide range of sensors. Microelectromechanical systems (MEMS) accelerometers are a type of such sensors in smartphones, which are used for detecting motion, such as counting your steps. MEMS accelerometers have been widely used in our daily life, such as in vehicles airbag deployment systems, in navigation systems and in cameras. To meet the growing demand for miniaturization of accelerometers in emerging applications such as smartphones, consumer electronics, wearable devices, IoT devices, nano-scale robotics, and micro/nanoscale biomedical implants where more traditional sensors cannot easily operate, development of next-generation of micro-accelerometers with ultra-small dimensions and high sensitivity is required. In addition, continuous miniaturization of accelerometers results in smaller packages, and ultimately in reduced costs.

Graphene is an extremely promising two dimensional material for nanoelectromechanical systems (NEMS) with great potential for substantial device scaling due to its atomic-scale thickness, excellent electrical and mechanical properties. In a recently published work, we successfully fabricated ultra-small piezoresistive NEMS accelerometers by suspending heavy proof masses on graphene ribbons (Figure 1). We demonstrate that the graphene ribbons with a suspended silicon proof mass are robust and graphene NEMS accelerometers occupy orders of magnitude smaller die area than conventional state-of-the-art silicon MEMS accelerometers while retaining competitive sensitivities. In addition to applications in miniaturized NEMS accelerometers, our graphene ribbons with an attached proof mass can be used as an excellent mechanical system to analyze the mechanical properties of graphene such as stiffness. We extracted Young's modulus of 0.22 TPa and built-in stresses of hundreds of MPa in the suspended double-layer graphene ribbons, both of which have a tangible influence on the transducer properties.

Fig. 1 Ultra-small Graphene NEMS accelerometers. a, SEM image of an ultra-miniaturized NEMS accelerometer. b, SEM image of an ultra-miniaturized NEMS accelerometer with bond wires. c, Die containing a single NEMS accelerometer with bond pads, placed on a coin. d, Packaged and wire-bonded die containing 64 NEMS accelerometers.

Our ultra-small graphene NEMS accelerometers can be fabricated by using conventional large-scale semiconductor and MEMS fabrication technologies. Our findings set the stage for a new class of ultra-small and highly sensitive graphene NEMS devices and are very encouraging for the practical applications of graphene, which is a step closer to real-world and commercial use. Beyond the ultra-small NEMS accelerometers, our suspended graphene ribbons with an attached proof mass have the potential for use in other types of ultra-miniaturized NEMS devices such as resonators, gyroscopes, and microphones. It is very exciting to envision that this research might promote the transformation of graphene NEMS and our daily lives.

This blog-post was written by Xuge Fan.

Original journal article: Xuge Fan, et al. Graphene ribbons with suspended masses as transducers in ultra-small nanoelectromechanical accelerometers. Nature Electronics.
https://www.nature.com/articles/s41928-019-0287-1

Xuge Fan is a Researcher in the Department of Micro and Nanosystems of the School of Electrical Engineering and Computer Science at KTH Royal Institute of Technology. He received his M.S. in Electronics and Communication Engineering under supervision of Prof. Wendong Zhang from Taiyuan University of Technology, Taiyuan, China, in 2013, and his Ph.D. in Microsystem Technology under supervision of Prof. Frank Niklaus and Prof. Max C. Lemme from KTH Royal Institute of Technology, Stockholm, Sweden, in 2018. Since 2018, he has been a researcher at KTH Royal Institute of Technology, Stockholm, Sweden. His research interest includes graphene-based two-dimensional materials and their heterostructures, MEMS/NEMS, integration of graphene-based two-dimensional materials into NEMS for material studies and device applications, observation and characterization of grain boundary-based defects of graphene-based two-dimensional materials, humidity and gas sensing properties of graphene-based two-dimensional materials, flexible and wearable devices based on graphene-based two-dimensional materials, silicon nanowires, ZnO nanowires, and carbon nanotubes.