School Colloquia Series - Dr Aravind Vijayaraghavan - Capacitive pressure and touch sensors with suspended graphene-polymer heterostructure membranes
Abstract: Single layer chemical vapor deposited (CVD) graphene has shown great promise in enabling Micro and Nano-electromechanical Systems (MEMS/NEMS) that can outperform current state of the art. However, existing methods in forming single layer graphene electromechanical devices result in low yields during the graphene device fabrication process. In addition, the suspended membranes that survive often suffer from a distorted topography due to transfer polymer residue, limiting the in-plane span, and poor device reproducibility.
I will present the fabrication and characterization of a suspended graphene/polymer heterostructure membrane that aims to tackle the prevailing challenge of constructing high yield, environmentally robust suspended graphene devices whilst preserving the mechanical and electronics properties.  The fabrication method enables the construction of suspended membrane structures that can be multiplexed over entire wafers with 100% yield.
Further, I will describe the fabrication and characterization of a capacitive pressure sensor formed by a graphene-polymer heterostructure membrane spanning a large array of micro-cavities each up to 30 µm in diameter with 100% yield. Sensors covering an area of just 1 mm2 show reproducible pressure transduction under static and dynamic loading up to pressures of 250 kPa. The measured capacitance change in response to pressure is in good agreement with calculations. Next, I will demonstrate a novel strained membrane transfer and optimizing the sensor architecture. This method enables suspended structures with less than 50 nm of air dielectric gap, giving a pressure sensitivity of 123 aF/Pa per mm2 over a pressure range of 0 to 100 kPa. 
Lastly, I will present a touch-mode capacitive pressure sensor (TMCPS) incorporating a SU-8 spacer grid structure.  This results in a partially suspended membrane configuration, which produces reproducible deflection, even after exposing the membrane to pressures over 10 times the operating range. The device shows a pressure sensitivity of 27.1 ± 0.5 fF/Pa over a pressure range of 0.5 kPa to 8.5 kPa.
I will present the operation of these devices as air pressure, water pressure and force-touch sensors.
 Berger, C.; Dirschka, M.; et al; Nanoscale, 2016, 8, 17928-17939.  Berger, C.; Phillips, R.; et al; Nanoscale, 2017, 9, 17439 - 17449.  Berger, C.; Phillips, R.; et al; 2D Materials, 2018, 5, 015025
Bio: Dr. Aravind Vijayaraghavan is a Reader in Nanomaterials in the School of Materials and the National Graphene Institute at The University of Manchester. He leads the Nano-functional Materials Group and his research involves the science and technology of graphene and 2-dimensional materials, particularly for applications in composites, sensors and biotechnology. He was previously a post-doctoral research fellow at Massachusetts Institute of Technology, USA and an Alexander von Humboldt Fellow at Karlsruhe Institute of Technology, Germany. He obtained his MEng (2002) and PhD (2006) from Rensselaer Polytechnic Institute, USA and his BTech (2000) from the Indian Institute of Technology - Madras, India. He has published over 70 papers in international peer reviewed journals and delivered over 60 presentations at international conferences. Dr. Vijayaraghavan is also a leader in public engagement and science communication and won the 2013 Joshua Phillips Award for Innovation in Science Engagement and was Science Communicator in Residence at the 2013 Manchester Science Festival. He has also been awarded a Royal Society Pairing Scheme Award (2013) and a British Science Association Media Fellowship (2017). He has delivered over 40 public lectures.
Group website: www.nanofunc.com. Twitter: @v_aravind