The many uses of graphene membranes
Freestanding membranes of suspended graphene have unique applications in low-power displays, gas impermeable membranes, MEMS and NEMS pressure sensors, water filtration, high sensitivity ultrasonic speakers and microphones, quantum memories, and for fundamental scientific studies of opto- and nano-mechanics. All of these applications, for which graphene as a material stands unrivalled, put stringent demands on the mechanical integrity and quality of graphene. As such, graphene that is free of grain boundaries and wrinkles, the weakest point of graphene sheets, is in demand.
Arrays of suspended graphene “drums” have been used in novel low-power, high-resolution interferometric displays (GIMODs). The low power consumption and exceptional performance in bright environments are well-matched to the demands and trends in modern mobile device technology. Compared to other interferometric displays, graphene devices operate at much higher frame rates (up to 400 Hz) and with a full color gamut, all at a resolution of 12K (2500 pixels per inch). The first GIMOD prototype was displayed as a working exhibit at the Mobile World Congress in 2017.
Graphene membranes with molecular-sized pores could be an ultimate separator of mixed gases into individual components. Such a process is essential for multiple industrial applications, including biogas production, carbon capture, air enrichment in metal working, removal of toxic gases from natural gas, and hydrogen recovery from ammonia plants and oil refineries, but also for purification and desalination of water. The ultimate atomic thickness of graphene results in very efficient membranes, because there is no scattering of molecules from pore walls during transport. Such membranes need to have dimensions on the order of square millimeters or centimeters, while retaining structural integrity even when decorated with nanopores.
The properties of graphene make it ideal for use in NEMS sensors. Image: Photos.com.
Pressure sensors also very often rely on measuring deflection of freestanding membranes. Again, due to the very thin nature of graphene, demonstrated pressure sensors are 2-10 times more sensitive per unit area compared to standard silicon or carbon nanotube based sensors. The working sensor prototypes were made using MEMS and NEMS technology, yielding arrays of micrometer-sized membranes that react together to applied pressure. Given a set value for sensitivity, graphene sensors can have a much smaller footprint compared to traditional sensors, which is advantageous for applications in the aerospace sector. Larger area freestanding membrane sensors from graphene were made by supporting the graphene on a polymer or as part of other, more complex layered structures. These large-area sensors have their use in wearable devices for health monitoring, for example.
Due to the small mass and large ultimate tensile strength, graphene is of interest for acoustics. Graphene has been used as a vibrating membrane in active acoustic elements such as loudspeakers and microphones. These elements cover a broader frequency range and have higher sensitivity to sound than their high-end commercial counterparts, due to the mechanical superiority of graphene membranes. Graphene-based speakers and microphones demonstrated to date range in size and technology, from macroscopic headphone-sized speakers, to micro- and nanoscopic microphones integrated on MEMS microchips.
Graphene drum quantum memory. Credit: Delft University of Technology
Graphene drums have become an interesting tool for scientists to explore opto- and nanomechanics, by utilizing interactions between external light stimuli and motion of the membrane. Ultimately, these interactions can be used for quantum information storage, i.e. as memory devices in quantum optical computers. Light signals can be stored in mechanical vibrations of graphene membranes for up to 10 milliseconds, hence the drum acts as RAM memory of a quantum computer.
It is apparent that freestanding membranes made of graphene enable a host of novel technologies that range from visual displays to physical sensors to quantum computing. Many of these are now at the prototype stage of development, seeking their paths to the market. Therefore the availability of a graphene product with high mechanical integrity that is aimed at these applications would be very timely.