Researchers at Case Western
Reserve University
are developing atomically thin "drumheads" able to receive and
transmit signals across a radio frequency range far greater than what we can
hear with the human ear.
But the drumhead is tens of trillions
times (10 followed by 13 zeros) smaller in volume and 100,000 times thinner
than the human eardrum.
The advances will likely contribute
to making the next generation of ultralow-power communications and sensory
devices smaller and with greater detection and tuning ranges.
"Sensing and communication are
key to a connected world," said Philip Feng, an associate professor of
electrical engineering and computer science and corresponding author on a paper
about the work published March 30 in the journal Science Advances. "In
recent decades, we have been connected with highly miniaturized devices and
systems, and we have been pursuing ever-shrinking sizes for those
devices."
The challenge with miniaturization:
Also achieving a broader dynamic range of detection, for small signals, such
as sound, vibration, and radio waves.
"In the end, we need transducers
that can handle signals without losing or compromising information at both the
'signal ceiling' (the highest level of an undistorted signal) and the 'noise
floor' (the lowest detectable level)," Feng said.
While this work was not geared toward
specific devices currently on the market, researchers said, it was focused on
measurements, limits and scaling which would be important for essentially all
transducers.
Those transducers may be developed
over the next decade, but for now, Feng and his team have already demonstrated
the capability of their key components-the atomic layer drumheads or
resonators-at the smallest scale yet.
The work represents the highest
reported dynamic range for vibrating transducers of their type. To date, that
range had only been attained by much larger transducers operating at much lower
frequencies-like the human eardrum, for example.
"What we've done here is to show
that some ultimately miniaturized, atomically thin electromechanical drumhead
resonators can offer remarkably broad dynamic range, up to ~110dB, at radio
frequencies (RF) up to over 120MHz," Feng said. "These dynamic ranges
at RF are comparable to the broad dynamic range of human hearing capability in
the audio bands."
New dynamic standard
Feng said the key to all sensory
systems-from naturally occurring sensory functions in animals to sophisticated
devices in engineering-is that desired dynamic range.
Dynamic range is the ratio between
the signal ceiling over the noise floor and is usually measured in decibels
(dB).
Human eardrums normally have dynamicrange of about 60 to 100dB in the range of 10Hz to 10kHz, and our hearing
quickly decreases outside this frequency range. Other animals, such as the
common house cat or beluga whale (see illustration), can have comparable or
even wider dynamic ranges in higher frequency bands.
The vibrating nanoscale drumheads
developed by Feng and his team are made of atomic layers of semiconductor
crystals (single-, bi-, tri-, and four-layer MoS2 flakes, with thickness of
0.7, 1.4, 2.1, and 2.8 nanometers), with diameters only about 1 micron.
They construct them by exfoliating
individual atomic layers from the bulk semiconductor crystal and using a
combination of nanofabrication and micromanipulation techniques to suspend the atomic layers over micro-cavities
pre-defined on a silicon wafer, and then making electrical contacts to the
devices.
Further, these atomically thin RF
resonators being tested at Case Western Reserve show excellent frequency
"tunability," meaning their tones can be manipulated by stretching the
drumhead membranes using electrostatic forces, similar to the sound tuning in
much larger musical instruments in an orchestra, Feng said.
The study also reveals that these
incredibly small drumheads only need picoWatt (pW, 10^-12 Watt) up to nanoWatt
(nW, 10^-9 Watt) level of RF power to sustain their high frequencyoscillations.
"Not only having surprisingly
large dynamic range with such tiny volume and mass, they are also
energy-efficient and very 'quiet' devices", Feng said, "We 'listen'
to them very carefully and 'talk' to them very gently."
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