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Feb 6th, 2014
 
Graphene outperforms silicon for NEMS sensors
 
Graphene can increase the sensitivity of NanoElectro-Mechanical System (NEMS) sensors by up to 100 times while the sensor size can be reduced at the same time, shows new research by the groups of Prof. Max Lemme at the University of Siegen in Germany, Prof. Mikael Östling and Prof. Frank Niklaus at KTH Royal Institute of Technology in Sweden.
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The researchers have also successfully demonstrated that mono-layer graphene membranes can be efficiently implemented as piezoresistive transducer elements for pressure sensors and other emerging NEMS sensors. ,  Because graphene is only one atomic layer thin, the sensors can be made smaller, with lateral dimensions of below 10 µm, and thus also be produced at a much lower cost, while at the same time providing improved sensitivities compared to existing state-of-the-art Micro-ElectroMechanical System (MEMS) sensors that are currently used e.g. in smartphones. These findings open the door for a whole range of new and innovative MEMS and NEMS sensor devices and they make graphene a very promising material for electromechanical transduction in emerging NEMS sensors. The resulting size reduction together with performance improvements and the possibility to integrate the NEMS structures with ICs are features that are beneficial for many sensor products in the future.

Figure 1: Schematic illustration of a piezoresistive graphene-membrane-based pressure sensor (left).
SEM image of a pressure sensor device (middle) and packaged graphene NEMS sensor (right).

In 2004, some of the unique characteristics of graphene were experimentally discovered. The record-breaking properties of graphene have caught the attention of the micro and nanotechnology research community and a huge number of innovative graphene-based devices and applications have been demonstrated.1,2,3, 4   Monolayer graphene consists of sp2-bonded carbon atoms arranged in a dense honeycomb crystal structure. It exhibits exceptional electronic and mechanical properties, including high carrier mobility, a high Young’s modulus of about 1 TPa, stretchability of up to approximately 20 % and near impermeability for gases.3,4,5  These properties make graphene a very promising material for different types of electronic and sensor applications. A large variety of graphene-based sensors have already been demonstrated, including chemical and biochemical sensors, gas sensors, magnetic and electric field sensors, optical sensors, strain sensors and mass sensors6.  Graphene exhibits a piezoresistive effect, which is induced by mechanical strain that changes the electronic band structure of the material.1,2,7, 8,  A large variety of conventional silicon-based MEMS sensors makes use of the piezoresistive effect in silicon. The piezoresistive effect in graphene can be utilized to implement NEMS sensors with dramatically improved sensitivity and reduced size.


Figure 2: SEM image of wire bonded graphene sensor with resistance model (left) and simulated graphene membrane deflection due to differential pressure (right).

The groups from KTH in Sweden and the University of Siegen in Germany were the first to demonstrate the piezoresistive effect in suspended mono-layer graphene membranes for electromechanical sensing.1,2 The experiments showed that the use of the piezoresistive effect in ultrathin graphene membranes for NEMS sensors enables unprecedented sensitivity per unit area and thus, is an enabling approach for novel graphene-based NEMS sensors with substantially improved performance.  Figure 1 depicts the NEMS sensor design that is based on a graphene membrane suspended over a sealed cavity. The graphene hangs freely like a drum membrane over the trench, which has a cross-sectional area of only 65 µm by 6 µm. The graphene patch is electrically contacted on both ends to measure resistance changes. The graphene membrane deflects due to a differential pressure and thus, the resistance of the graphene patch changes accordingly as depicted in Figure 2. The graphene-based NEMS pressure sensors showed very high sensitivity per unit area despite the moderate piezoresistive gauge factor of 3 to 4 observed in graphene1. The extraordinarily high sensitivity can be explained because the sensitivity of a membrane-based piezoresistive sensor is strongly dependent on the membrane thickness. Suspended graphene membranes are resilient and only one atom layer thick (~0.35 nm), which is several orders of magnitude thinner than that of typical silicon-based MEMS sensors today (~300-3000 nm).

The open-access publication can be downloaded: http://pubs.acs.org/doi/pdf/10.1021/nl401352k
Latest results have also been presented at the IEEE MEMS 2014 conference in San Francisco, USA.

Acknowledgments
The research was supported through three ERC Grants (OSIRIS, No. 228229, M&M’s, No. 277879 and InteGraDe ( www.integrade.org ), No. 307311) as well as the German Research Foundation (DFG, LE 2440/1-1) and the Italian MIUR through the Cooperlink project (CII11AVUBF).

About University of Siegen
Prof. Max Lemme holds the Chair for Graphene-based Nanotechnology at the University of Siegen. He is a recognized world leading expert for graphene-based micro and nano-electronic devices. His research interest is on integrated devices made from graphene and related 2-dimensional crystals and their co-integration with silicon technology. (http://www.eti.uni-siegen.de/nano/)

About Department of Integrated Devices and Circuits at KTH Royal Institute of Technology
Prof. Mikael Östling is heading the Department of Integrated Devices and Circuits at KTH (KTH-EKT). He conducts research on integrated devices and circuits based on silicon, SiGe and SiC material systems for both electronic and opto-electronic applications. (http://www.kth.se/en/ict/forskning/ickretsar)

About Department of Micro- and Nanosystems at KTH Royal Institute of Technology
Prof. Frank Niklaus is heading the Micro- and Nanomanufacturing Group at the Department of Micro and Nanosystems at KTH. The research focus of the group is on is manufacturing, integration and packaging technologies for emerging MEMS and NEMS components. (www.ee.kth.se/mst)

1- A.D. Smith, F. Niklaus, A. Paussa, S. Vaziri, A.C. Fischer, M. Sterner, F. Forsberg, A. Delin, D Esseni, P Palestri,
M. Östling, M.C. Lemme, “Graphene-based Piezoresistive Sensing”, Nano Letters, Vol.13, No.7, pp.3237-3242, 2013.
2- A.D. Smith, S. Vaziri, F. Niklaus, A.C. Fischer, M. Sterner, A. Delin, M. Östling, M.C. Lemme, ”Pressure Sensors based on Suspended Graphene Membranes”, Solid State Electronics, Vol.88, pp.89-94, 2013.
3- K.S. Novoselov, A.K Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, “Electric Field Effect in Atomically Thin Carbon Films”, Science, Vol.306, No.5696, pp.666-669, 2004.
4- A.K. Geim, “Graphene: Status and Prospects”, Science, Vol.324, No.5934, pp.1530-1534, 2009.
5- J.S. Bunch, S.S. Verbridge, J.S. Alden, A.M. Van Der Zande, J.M. Parpia, H.G. Craighead, P.L. McEuen, “Impermeable Atomic Membranes from Graphene Sheets”, Nano Letters,  Vol.8, No.8, pp.2458-2462, 2008.
6- E.W. Hill, A. Vijayaragahvan, K.S. Novoselov, “Graphene Sensors”, IEEE Sensors Journal, Vol.11, pp.3161-3170, 2011.
7- M. Huang, T.A. Pascal, H. Kim, W.A. Goddard, J.R. Greer, “Electronic−Mechanical Coupling in Graphene from in situ Nanoindentation Experiments and Multiscale Atomistic Simulations”, Nano Letters, Vol.11, pp.1241-1246, 2011.
8- X. Chen, X. Zheng, J.-K. Kim, X. Li, D.-W. Lee, “Investigation of Graphene Piezoresistors for Use as Strain Gauge Sensors”, Journal of Vacuum Science and Technology B, Vol.29, pp.06FE01-5, 2011
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