Graphene, with its enormous stiffness and low density, is an excellent material for nanoelectromechanical system (NEMS) applications. Such applications include various types of physical, biological, and chemical sensors, all expected to have superb sensitivities, exceeding those of competing technologies. Graphene sensors and NEMS are also attractive due to their small size.
First studies of graphene membranes for NEMS appeared only about five years ago, with some of the first papers published by the group of Paul McEuen at Cornell University. Those first sensors detected minute changes in electronic charge, with sub-millielectron sensitivity. The sensor, consisting of graphene membranes suspended above trenches in the underlying substrate, also represented the world's thinnest mechanical resonators, only one atom thick. With frequencies in the megahertz range and the ability to actuate the devices either electronically or optically, the sensors were also highly versatile, promising a bright future for the field.
A couple of years later, teams around the world, including those at Columbia University, extended that work to include graphene mass and temperature sensors. Fabrication methods at that time reached a level that allowed for high sensor quality. Now, in 2013, Max Lemme and coworkers at KTH in Sweden are using graphene to build the world's most sensitive – and smallest - pressure sensors.
In two recent papers, the group of Lemme demonstrated superior pressure-sensing performance of graphene NEMS sensors compared to competing technologies. The graphene sensor is constructed by first cutting open a trench in a silicon dioxide substrate and then depositing a graphene sheet over the trench. The graphene hangs freely like a drum membrane over the trench, which has a cross-sectional area only 65 by 6 micrometers.
To test the sensor, the researchers change the pressure of the surrounding atmosphere. The graphene sheet is impermeable to air, meaning that the pressure in the drum remains constant, while the outside pressure varies. The pressure gradient between the drum and its surroundings cause a force proportional to the pressure of the ambient, making the sheet to bulge out by up to 200 nanometers. The bulging of the sheet is detected through graphene's piezoelectric property, which makes the electrical resistance change under strain. As the pressure goes up, the resistance goes down, causing an increase in current.
The pressure sensitivity of this novel sensor is about 4 microvolts per volt per mmHg. Compared with standard silicon- or carbon nanotube-based microelectromechanical sensors (MEMS), the graphene sensor packs a 2 to 10 times better sensitivity per unit area. Given a certain requirement on sensitivity, the graphene sensor can thus be scaled down to much smaller dimensions than conventional sensors.
This promising technique uses conventional CVD graphene and the graphene transfer process that is commonly used here at Graphenea. With these recent results, the field of graphene NEMS sensors has shown to be moving forward and to be a serious competitor to present-day technology. Pressure sensors are used for control and monitoring in thousands of everyday applications, for example for monitoring weather, and for various functions in aircraft and cars. A graphene-based pressure sensor would be particularly attractive for the aircraft industry, given the sensor's small footprint and weight. As we anxiously anticipate further developments in this field, stay tuned to find out Graphenea's role in the future of graphene NEMS.
Figure: Graphene NEMS pressure sensor, consisting of a single graphene layer suspended above a trench in the substrate.