Electromechanical Piezoresistive Sensing in Suspended Graphene Membranes A. D. Smith, † F. Niklaus, † A. Paussa , ‡ S. Vaziri, † A. C. Fischer, † M. Sterner, † F. Forsberg, † A. Delin, † D. Esseni, ‡ P. Palestri, ‡ M. O stling, † and M. C. Lemme * ,† ,§ † KTH Royal Institute of Technology, Isafjordsgatan 22, 16440 Kista, Sweden ‡ DIEGM, University of Udine, Via delle Scienze 206, 33100 Udine, Italy § University of Siegen, Ho lderlinstrasse 3, 57076 Siegen, Germany * S Supporting Information ABSTRACT: Monolayer graphene exhibits exceptional elec- tronic and mechanical properties, making it a very promising material for nanoelectromec hani cal devic es. Here, we con- clusively demonstrate the piezoresistive eff ect in graphene in a nanoelectromechanical membrane conguration that provides direct electrical readout of pressure to strain transduction. This mak es it hig hly releva nt for an important cla ss of nan o- electromechanical system (NEMS) transducers. This demon- str ati on is consis ten t with our sim ula tio ns and pre vio usl y reported gauge factors and simulation values. The membrane in our exper iment acts as a str ain gauge indep endent of crysta llog raphic orientatio n and allo ws for aggre ssive size scalabili ty. Whe n compar ed wit h con vent ional pressure sensors, the sensors have order s of magnitud e high er sensi tivit y per unit area. KEYWORDS: Graphene, pressure sensor, piezoresistive e ff ect, nanoelectromechanical systems (NEMS), MEMS G raph ene is an intere sting material for nanoe lectr ome- chani ca l sys tems (NEMS) due to it s ex traordinary thinness (one atom thick), high carrier mobility, 1,2 and a high Young s modulus of about 1 TPa for both prist ine (exfoliated) and chemical vapor deposited (CVD) graphen e. 3 ,4 Graphene is further stretchable up to approximately 20%. 5 In addition, it shows str ong adh esi on to SiO 2 substrates 6 and is near ly impermeable for gases, including helium. 7 In this article, we dem ons tra te pie zor esisti ve pressure sensors based on sus- pended graphene membranes with direct electrical signal read- out. We utilize a piezoresistive eff ect induced by mechanical strain in the gra ph ene , which cha nge s the ele ctr onic ban d structure 8 and exploi ts the fac t that the sen si tiv ity of membrane-based electromechanical transducers strongly corre- lates with membrane thickness. 9 While graphene has been used as a piezoresistive strain gauge on silicon nitride 10 and polymer membranes, 11 we extend the use of the graphene to both membrane and electromechanical transduction simultaneously with an average gauge factor of 2.92. The sensitivity per unit area of our graphene sensor is about 20 to 100s of times higher than that of conventional piezoresistive pressure sensors. The piezoresistive eff ect is nearly independent of crystallographic orientation. In our experiments, graphene membranes made from CVD graphene are suspended over cavities etched into a SiO 2 lm on a silicon substrate. The graphene is electrically contacted and the dev ices are wir e-b onded int o a chi p pac kag e. Process sch ema tics are shown in Fi gur e 1a−c, while details of the fab ric ati on proces s are des cribed in Methods. A scanning ele ctr on microscope image of a wir e-b ond ed dev ice and a photograph of a packaged device are shown in Figure 1d,e, respectively. If a pressure di ff erence is present between the inside and the outside of the cavity (compare Figure 1 c), the graphene membrane that is sealing the cavity is deected and thus strained. This leads to a change of device resistivity due to the piezoresistive eff ect in the graphene. Measurements were performed in an argon environment in order to reduce the eff ects of adsorbates. If air is used instead of argon for the experiments, adsorption of noninert gases and/or molecules on the graphene will aff ect the resistivity (see details in Supporting Information ). In the experiments, the packaged devices are placed inside a vacuum chamber. The chamber is then evacuated from atmosp her ic pre ssure down to 200 mba r and then vented back to 1000 mbar. Thus the air sealed inside the cavity presses against the graphene membrane with a force proportional to the chamber pressure. The resistance of the graphene sensor is measured in a Wheatstone bridge (see Supporting Informa- Received: April 16, 20 13 Revised: June 17, 2013 Published: June 20, 2013 Letter pubs.acs.org/NanoLett © 2013 American Chemical Society 3237 dx.doi.org/10.1021/nl401352k | Nano Lett. 2013, 13, 3237−3242 Terms of Use
