79 research outputs found
Amplitude calibration of 2D mechanical resonators by nonlinear optical transduction
Contactless characterization of mechanical resonances using Fabry-Perot
interferometry is a powerful tool to study the mechanical and dynamical
properties of atomically thin membranes. However, amplitude calibration is
often not performed, or only possible by making assumptions on the device
parameters such as its mass or the temperature. In this work, we demonstrate a
calibration technique that directly measures the oscillation amplitude by
detecting higher harmonics that arise from nonlinearities in the optical
transduction. Employing this technique, we calibrate the resonance amplitude of
two-dimensional nanomechanical resonators, without requiring knowledge of their
mechanical properties, actuation force, geometric distances or the laser
intensity
Graphene Squeeze-Film Pressure Sensors
The operating principle of squeeze-film pressure sensors is based on the
pressure dependence of a membrane's resonance frequency, caused by the
compression of the surrounding gas which changes the resonator stiffness. To
realize such sensors, not only strong and flexible membranes are required, but
also minimization of the membrane's mass is essential to maximize responsivity.
Here, we demonstrate the use of a few-layer graphene membrane as a squeeze-film
pressure sensor. A clear pressure dependence of the membrane's resonant
frequency is observed, with a frequency shift of 4 MHz between 8 and 1000 mbar.
The sensor shows a reproducible response and no hysteresis. The measured
responsivity of the device is 9000 Hz/mbar, which is a factor 45 higher than
state-of-the-art MEMS-based squeeze-film pressure sensors while using a 25
times smaller membrane area
High-frequency stochastic switching of graphene resonators near room temperature
Stochastic switching between the two bistable states of a strongly driven
mechanical resonator enables detection of weak signals based on probability
distributions, in a manner that mimics biological systems. However,
conventional silicon resonators at the microscale require a large amount of
fluctuation power to achieve a switching rate in the order of a few Hertz.
Here, we employ graphene membrane resonators of atomic thickness to achieve a
stochastic switching rate of 7.8 kHz, which is 200 times faster than current
state-of-the-art. The (effective) temperature of the fluctuations is
approximately 400 K, which is 3000 times lower than the state-of-the-art. This
shows that these membranes are potentially useful to transduce weak signals in
the audible frequency domain. Furthermore, we perform numerical simulations to
understand the transition dynamics of the resonator and derive simple
analytical expressions to investigate the relevant scaling parameters that
allow high-frequency, low-temperature stochastic switching to be achieved in
mechanical resonators
Nonequilibrium Thermodynamics of Acoustic Phonons in Suspended Graphene
Recent theory has predicted large temperature differences between the
in-plane (LA and TA) and out-of-plane (ZA) acoustic phonon baths in
locally-heated suspended graphene. To verify these predictions, and their
implications for understanding the nonequilibrium thermodynamics of 2D
materials, experimental techniques are needed. Here, we present a method to
determine the acoustic phonon bath temperatures from the frequency-dependent
mechanical response of suspended graphene to a power modulated laser. The
mechanical motion reveals two counteracting contributions to the thermal
expansion force, that are attributed to fast positive thermal expansion by the
in-plane phonons and slower negative thermal expansion by the out-of-plane
phonons. The magnitude of the two forces reveals that the in-plane and flexural
acoustic phonons are at very different temperatures in the steady-state, with
typically observed values of the ratio between 0.2 and 3.7. These deviations from the generally used
local thermal equilibrium assumption () can affect the experimental analysis of thermal properties of
2D materials
Mass measurement of graphene using quartz crystal microbalances
Current wafer-scale fabrication methods for graphene-based electronics and
sensors involve the transfer of single-layer graphene by a support polymer.
This often leaves some polymer residue on the graphene, which can strongly
impact its electronic, thermal, and mechanical resonance properties. To assess
the cleanliness of graphene fabrication methods, it is thus of considerable
interest to quantify the amount of contamination on top of the graphene. Here,
we present a methodology for direct measurement of the mass of the graphene
sheet using quartz crystal microbalances (QCM). By monitoring the QCM resonance
frequency during removal of graphene in an oxygen plasma, the total mass of the
graphene and contamination is determined with sub-graphene-monolayer accuracy.
Since the etch-rate of the contamination is higher than that of graphene,
quantitative measurements of the mass of contaminants below, on top, and
between graphene layers are obtained. We find that polymer-based dry transfer
methods can increase the mass of a graphene sheet by a factor of 10. The
presented mass measurement method is conceptually straightforward to interpret
and can be used for standardized testing of graphene transfer procedures in
order to improve the quality of graphene devices in future applications
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