Condenser microphones, which were discovered at Bell Labs a century ago, make use of a thin vibrating membrane as one plate of a capacitor charged by a polarized voltage. Incoming sound waves move the membrane, and this movement is then detected by an external electronic circuit.

Most condenser microphones are currently made from nickel, but graphene, which is a sheet of carbon just one atom thick, could be ideal for fabricating smaller devices thanks to its exceptional mechanical properties. The material is extremely lightweight but has a breaking strength of 42 N/m, for example, which is more than 200 times higher than a hypothetical steel film of the same thickness. Indeed, researchers have already succeeded in making good electrostatic loudspeakers and earphones out of it.

Vibrating membrane made from 60 layers of graphene

Taking things further now, a team led by Marko Spasenovic from the Institute of Physics at the University of Belgrade has created a vibrating membrane from 60 layers of graphene. This structure is more sensitive to sound (by 12 dB at low frequencies of up to 12 kHz) than a professional nickel-based microphone, but it is only 25 nm thick. What is more, theory calculations by the researchers reveal that a membrane made from 300-layer thick graphene might even work at frequencies of up to 1 MHz, that is, in the ultrasonic part of the spectrum.

Spasenovic and colleagues began by growing their multilayer graphene on nickel foil using a technique called chemical vapour deposition, or CVD, which produces uniform samples. Next, they etched away the nickel in an iron chloride/water solution to produce a floating multilayer graphene film. They then scooped the film out of the solution and onto a supporting polyethylene terephthalate (PET) frame, which has a round hole in the centre (see "Making the graphene membrane" figure). The graphene covers the hole to form the membrane.

Better than professional nickel-based devices

The team varied the diameter of the hole and the membrane to produce films that were between 5 and 12 mm across. They then dried the membranes for four hours before mounting them into conventional microphone casings.

The researchers, who report their work in 2D Materials, measured their microphones’ performance and found that the structures were more sensitive than professional nickel-based devices at all frequencies below 12 kHZ, regardless of their diameter.

“Our experiments show that graphene is a realistic microphone membrane material that, at least in theory, extends the range of microphones to the important ultrasound part of the spectrum, which is currently unreachable by conventional state-of-the-art devices,” Spasenovic tells nanotechweb.org. “We now need to be able to better control their fabrication and durability, and are busy redesigning the devices to this end.”