Graphene Electrostatic Microphone and Ultrasonic Radio
Qin Zhou, Jinglin Zheng, Seita Onishi, M.F. Crommie, and A. Zettl
Modern wireless communication is based on generating and receiving electromagnetic (EM) waves that span a wide frequency range, from Hz to THz, providing abundant band resources and high data transfer rates. However, there are draw-backs to EM communication, including high extinction coefficient for electrically conductive materials and antenna size. On the other hand, animals have effectively used acoustic waves for short-range communication for millions of years. Acoustic wave based communication, while embodying reduced band resources, can overcome some of the EM difficulties and complement existing wireless technologies. For example, acoustic waves propagate well in conductive materials, and have thus been explored for underwater communication by submarines. Marine mammals such as whales and dolphins are known to communicate effectively via acoustic waves. In land-based acoustic wave communication, the audible band is often occupied by human conversations, while the subsonic band can be disturbed by moving vehicles and building construction. The ultrasonic band, while having a wide frequency span and often free of disturbance, is rarely exploited for high data rate communication purposes. One possible reason for this is the lack of wide bandwidth ultrasonic generators and receivers. Conventional piezoelectric-based transducers only operate near their resonance frequencies, preventing use in communications where wider bandwidth is essential for embedding information streams.
In a conventional acoustic transducer such as microphone, air pressure variations from a sound wave induce motion of a suspended diaphragm; this motion is in turn converted to an electrical signal via Faraday induction (using a magnet and coil) or capacitively. The areal mass density of the diaphragm sets an upper limit on the frequency response (FR) of the microphone. In the human auditory system, the diaphragm (eardrum) is relatively thick (~100 µm), limiting flat FR to ~2 kHz and ultimate detection to ~20 kHz. In bats the eardrums are thinner, allowing them to hear reflected echolocation calls up to ~200 kHz. Diaphragms in high-end commercial microphones can be engineered to provide flat FR from the audible region to ~140 kHz. Thinner and lighter diaphragms allow for more faithful tracking of sound vibration at high frequencies.
The ultra-low mass and high mechanical strength of graphene makes it extremely attractive for sound transduction applications. We have previously demonstrated an electrostatically-driven graphene diaphragm loudspeaker with a equalized frequency response (FR) across the whole human audible region (20 Hz - 20 kHz). The ultimate high frequency cut-off of the speaker was not determined, the measurement being limited to 20 kHz by available detection equipment (indeed, as shown below, the graphene loudspeaker operates to at least 0.5 MHz). Graphene allows air damping to dominate over the diaphragm's own mass and spring constant over a wide frequency range. In principle, graphene’s exceptional mechanical properties and favorable coupling to air and other media could enable wideband transducers for both sound generation andreception, core requirements for ultrasonic radio.
We here describe the successful design, construction, and operation of a wideband ultrasonic radio. A key ingredient of the radio system is an electrostatically-coupled, mechanically vibrating graphene diaphragm based receiver that can be paired with the graphene-based acoustic transmitter. We find that the graphene microphone has an outstanding equalized frequency response (within 10 dB variation of perfect flat-band response) covering at least 20 Hz to 0.5 MHz (limited by characterization instrumentation), and a sensitivity sufficient to record bats echo-locating in the wild. The highly efficient graphene ultrasonic transmitter/receiver radio system successfully codes, propagates, and decodes radio signals. The same radio system can be used to accurately measure distances using interference between ultrasonic and electromagnetic waves.
Bats often use echolocation to navigate and forage in total darkness. Bat call frequencies range from as low as 11 kHz to as high as 212 kHz, depending on the species. We acquired ultrasonic bat sound signals (bat calls) in the field using the graphene electrostatic microphone, at Del Valle Regional Park, Livermore, California where the bat species Western Pipistrelle (parastrellus hesperus) is prevalent. The spectrogram shows that these bat calls consist of periodic chirps during which the emitted frequency consistently ramps down in frequency from ~100 kHz to ~50 kHz. The duration of each chirp is around 4 ms, and the repeating period is around 50 ms. It is believed that bats utilize the frequency sweeping technique to distinguish multiple targets, improve measurement accuracy, and avoid interference from each other.
A direct recording (amplitude vs time) of the bat calls (slowed by a factor of 8 to bring the signal into the human hearing range) can be found here:
Introduction
Humans and other animals effectively use acoustic waves to communicate with each other. Ultrasonic acoustic waves are intriguing because they do not interfere with normal voice communication and can be highly directional with long range. Therefore, wireless ultrasonic radio is a useful communications method. Here we find that graphene has mechanical properties that make it ideally suited for wide-band ultrasonic transduction. Using simple and low-cost fabrication methods we have produced an ultrasonic microphone and ultrasonic radio prototypes. When acting as loudspeaker/microphone alone, the graphene based acoustic devices also show ideal flat-band frequency response spanning the whole audible region as well as ultrasonic region to at least 0.5MHz; such flat frequency response has significant acoustic applications implications.Modern wireless communication is based on generating and receiving electromagnetic (EM) waves that span a wide frequency range, from Hz to THz, providing abundant band resources and high data transfer rates. However, there are draw-backs to EM communication, including high extinction coefficient for electrically conductive materials and antenna size. On the other hand, animals have effectively used acoustic waves for short-range communication for millions of years. Acoustic wave based communication, while embodying reduced band resources, can overcome some of the EM difficulties and complement existing wireless technologies. For example, acoustic waves propagate well in conductive materials, and have thus been explored for underwater communication by submarines. Marine mammals such as whales and dolphins are known to communicate effectively via acoustic waves. In land-based acoustic wave communication, the audible band is often occupied by human conversations, while the subsonic band can be disturbed by moving vehicles and building construction. The ultrasonic band, while having a wide frequency span and often free of disturbance, is rarely exploited for high data rate communication purposes. One possible reason for this is the lack of wide bandwidth ultrasonic generators and receivers. Conventional piezoelectric-based transducers only operate near their resonance frequencies, preventing use in communications where wider bandwidth is essential for embedding information streams.
In a conventional acoustic transducer such as microphone, air pressure variations from a sound wave induce motion of a suspended diaphragm; this motion is in turn converted to an electrical signal via Faraday induction (using a magnet and coil) or capacitively. The areal mass density of the diaphragm sets an upper limit on the frequency response (FR) of the microphone. In the human auditory system, the diaphragm (eardrum) is relatively thick (~100 µm), limiting flat FR to ~2 kHz and ultimate detection to ~20 kHz. In bats the eardrums are thinner, allowing them to hear reflected echolocation calls up to ~200 kHz. Diaphragms in high-end commercial microphones can be engineered to provide flat FR from the audible region to ~140 kHz. Thinner and lighter diaphragms allow for more faithful tracking of sound vibration at high frequencies.
The ultra-low mass and high mechanical strength of graphene makes it extremely attractive for sound transduction applications. We have previously demonstrated an electrostatically-driven graphene diaphragm loudspeaker with a equalized frequency response (FR) across the whole human audible region (20 Hz - 20 kHz). The ultimate high frequency cut-off of the speaker was not determined, the measurement being limited to 20 kHz by available detection equipment (indeed, as shown below, the graphene loudspeaker operates to at least 0.5 MHz). Graphene allows air damping to dominate over the diaphragm's own mass and spring constant over a wide frequency range. In principle, graphene’s exceptional mechanical properties and favorable coupling to air and other media could enable wideband transducers for both sound generation andreception, core requirements for ultrasonic radio.
We here describe the successful design, construction, and operation of a wideband ultrasonic radio. A key ingredient of the radio system is an electrostatically-coupled, mechanically vibrating graphene diaphragm based receiver that can be paired with the graphene-based acoustic transmitter. We find that the graphene microphone has an outstanding equalized frequency response (within 10 dB variation of perfect flat-band response) covering at least 20 Hz to 0.5 MHz (limited by characterization instrumentation), and a sensitivity sufficient to record bats echo-locating in the wild. The highly efficient graphene ultrasonic transmitter/receiver radio system successfully codes, propagates, and decodes radio signals. The same radio system can be used to accurately measure distances using interference between ultrasonic and electromagnetic waves.
Illustrations
If you use any of the following images, please include the credit "Courtesy Zettl Research Group, Lawrence Berkeley National Laboratory and University of California at Berkeley."
 Construction of graphene electrostatic wideband receiver (microphone). The graphene membrane is suspended across the supporting frame(a). The membrane is electrically contacted with gold wires, and spacers are added (b) to control the distance from the membrane to the gold-coated stationary electrodes (c). Operation principles: the microphone can be modeled as a current source imic. The conventional circuit (d) is not suitable for high frequency operation because parasitic capacitance Cp is in parallel with the current-voltage conversion resistor R. The fast-photodiode-detector-like circuit (e) avoids charging Cp and maintains a consistent gain at higher frequencies.  Measured frequency response of graphene acoustic transmitter and receiver. (a) and (b) are measured in reference to SONY® ICD-SX700 in audible region. 0dB corresponds to a response of 3.3 nA/Pa. (a): Without an acoustic cavity, the frequency response suffers from interference at lower frequencies when measuring far-field sound waves; (b): Graphene microphone exhibits rather equalized frequency response with near-field coupling and proper acoustic design. (c): Response including ultrasonic region, measured with identical pair technique (see main text and SI). The response fall-off beyond 0.5MHz is not intrinsic to the acoustic device, but rather reflects limitations of the operational amplifier used in the detection circuit.  Applications of wideband ultrasonic graphene acoustic transducers. (a): Spectrogram of bat calls (parastrellus hesperus) recorded in the field at a local park. During each 4ms wide emission chirp, the frequency ramps down from ~100 kHz to ~50 kHz. The time between chirps ranges from 30ms to 50ms. (b): The transmission and reception of amplitude modulated (AM) acoustic signals. The wideband acoustic radio well-preserves the sharp edges of the sawtooth envelope. (c): A novel way of measuring distance by frequency sweeping. The oscillation comes from the interference between the signals picked up from acoustic waves and electromagnetic waves, and the distance between the speaker and microphone is derived to be equal to the speed of sound divided by the pitch between the peaks.  Construction of graphene microphone. A 1 cm2 piece of 25-µm-thick nickel foil is first electrochemically polished, cleaned by DI water, and loaded into a 25 mm diameter quartz tube furnace (Fig. a). After hydrogen annealing, the graphene layers are grown by chemical vapor deposition process at 1050 ˚C with 50 sccm methane and 50 sccm hydrogen co-flow. The growth chamber pressure is controlled at 1 Torr. The growth lasts 15 minutes and the methane flow rate is increased to 200 sccm for the last 2 minutes to improve the stitching between graphene grains. The foil is then quickly cooled down to quench the graphene growth(Fig. b). After unloading, a layer of poly methyl methacrylate (PMMA) is spin-coated on top of the nickel foil (Fig. c), and the graphene on the other side of the foil is etched away using an oxygen plasma (1 min @ 100W)(Fig. d). A circular aperture of 8 mm diameter is created with a disc cutter on a sticky Kapton® tape serving as a supporting frame. The supporting frame is then attached to the PMMA layer on the nickel foil (Fig. e). The nickel foil is subsequently etched away in 0.1 g/mL sodium persulfate solution (Fig. f). Compared to the iron chloride solution used previously, here the etch rate is much lower (typically overnight etching is required to remove the 25-µm-thick nickel), but the resulting graphene diaphragm is very clean and free of amorphous carbon. The exposed (not covered by the supporting frame) area of the PMMA layer is then dissolved in acetone, and the graphene layer supported by the frame is cleaned twice with isopropanol and dried in air (Fig. g). The PMMA between the supporting frame and graphene serves as a buffer material and improves the yield to ~100% (the PMMA-free process has a typical yield of ~30%). The membrane is measured by light transmission to be approximately 20 nm thick, or 60 monolayers of graphene. A 25µm diameter gold wire is attached to the edge of the graphene membrane for electrical contact (Fig. h). Finally, spacers of approximately 150 µm thick are attached to both sides of the frame, followed by perforated electrodes made from silicon wafers using deep reactive ion etch (DRIE). The rigid electrodes are also wired with gold wires attached by silver paste (Fig. i). The surfaces of the electrodes facing the graphene membrane are coated with conductive metal layers (20 nm sputtered gold) to allow ohmic contact between the gold wire and the electrodes. This gold coating is essential for eliminating any contact barrier that could block the current flow during microphone operation, since the voltage variation on the membrane is very small. We note that for loudspeaker applications, this metal coating is not necessary since large voltages are there applied. Optionally, a waveguide or a Helmholtz acoustic cavity can be attached to the microphone assembly, modifying the FR of the microphone in the low frequency region by altering the damping or creating/eliminating interference. Without these modifications, the sound pressure forces at the front and the backside of the diaphragm tend to cancel at low frequencies, resulting in diminished response.  Photos of microphone without acoustic cavity and with acoustic cavity. |
Recordings of Bat Call
If you use any of the following media or images, please include the credit "Courtesy Zettl Research Group, Lawrence Berkeley National Laboratory and University of California at Berkeley."Bats often use echolocation to navigate and forage in total darkness. Bat call frequencies range from as low as 11 kHz to as high as 212 kHz, depending on the species. We acquired ultrasonic bat sound signals (bat calls) in the field using the graphene electrostatic microphone, at Del Valle Regional Park, Livermore, California where the bat species Western Pipistrelle (parastrellus hesperus) is prevalent. The spectrogram shows that these bat calls consist of periodic chirps during which the emitted frequency consistently ramps down in frequency from ~100 kHz to ~50 kHz. The duration of each chirp is around 4 ms, and the repeating period is around 50 ms. It is believed that bats utilize the frequency sweeping technique to distinguish multiple targets, improve measurement accuracy, and avoid interference from each other.
A direct recording (amplitude vs time) of the bat calls (slowed by a factor of 8 to bring the signal into the human hearing range) can be found here:
- Bat Calls Direct Recording (3.93 MB)