Nanoscale Accelerometer Enables Ultra-Small Wearable Graphene Sensors

A team of scientists at Graphene Flagship partners KTH Royal Institute of Technology, Sweden, RWTH Aachen University and AMO GmbH, Germany, have exploited the ultra-thin and highly conductive properties of graphene to develop an extremely small accelerometer with exceptional sensitivity.

Accelerometers are devices that enable smartphones and other mobile technology to sense their own orientation through axis-based motion sensing. They are commonly used in navigation systems, motion-sensitive video game controllers and smartphones, earthquake detectors and bionic limbs. Newer developments allow them to be applied in monitoring systems for cardiovascular diseases and ultra-sensitive wearable and portable motion-capture technologies.

Microelectromechanical systems (MEMS), which are made up of components between 1 and 100 micrometres in size, have been the basis for new innovations in these fields. The unique properties of graphene have enabled the Graphene Flagship to develop a new generation of ultra-small nanoelectromechanical systems, or NEMS, which can be used to develop new axis-based motion sensors on an extremely small scale. 

Xuge Fan, researcher at Graphene Flagship partner KTH Royal Institute of Technology, says that he has applied this new technology to create the smallest ever reported. "Based on the surveys and comparisons we have made, we can say that this is the smallest electromechanical accelerometer in the world," he says.

The device consists of a silicon mass suspended in space by a graphene ribbon, through which electrical conductivity measurements are taken. When the device is rotated, it experiences acceleration around one or more of its axes, which causes the suspended silicon mass to stretch the ribbon – resulting in an instantaneous change in electrical conductivity passing through it. These changes in conductivity measurements are then used to determine rotational acceleration, and therefore changes in orientation, with an unprecedented level of sensitivity. 

Peter Steeneken, the Graphene Flagship's Sensors Work Package Leader, is optimistic about this development. "Sensitive accelerometers require large masses and highly flexible springs, which usually leads to devices larger than 1 mm2. Graphene is an ideal material for this purpose: it is only one atom thick, and is therefore extremely flexible," he comments. "The authors have for the first time realized a graphene accelerometer, with the proof mass being more than 100,000 times heavier than the graphene springs by which it is suspended, while utilizing the excellent electrical properties of graphene for detection. Thus, they have realized a true breakthrough in size, with a graphene accelerometer in which the springs and mass have an area of less than 0.001 mm2"

The future for such small accelerometers is promising, continues Fan. "This could eventually benefit navigation apps and pedometers in mobile phones, as well as monitoring systems for heart disease and motion-capture wearables that can monitor even the slightest movements of the human body," he says. Other potential uses for these NEMS transducers include ultra-miniaturized NEMS sensors and actuators such as resonators, gyroscopes and microphones. In addition, NEMS transducers can also be used as a system to characterize the mechanical and electromechanical properties of graphene.

Max Lemme, lead author of the paper from Graphene Flagship partner RWTH Aachen University, is also excited about the results. "Our collaboration with KTH over the years has already shown the potential of graphene membranes for pressure and Hall sensors and microphones. Now, we have added accelerometers to the mix. This makes me hopeful to see the material on the market in a few years. For this, we are working on industry-compatible manufacturing and integration techniques."

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "this work showcases yet another possible application of graphene, whereby the thinness, low weight, electrical conductivity and mechanical properties all converge to produce a prototype device with superior properties with potential applications in nanoelectromechanical systems"

Reference

Graphene ribbons with suspended masses as transducers in ultra-small nanoelectromechanical accelerometers, Nature Electronics, 2019, 2, 394-404. Xuge Fan, Fredrik Forsberg, Anderson D. Smith, Stephan Schröder, Stefan Wagner, Henrik Rödjegård, Andreas C. Fischer, Mikael Östling, Max C. Lemme, Frank Niklaus.