The unique properties and atomic thickness of two-dimensional (2D) materials enable smaller and better nanoelectromechanical sensors with novel functionalities. During the last decade, many studies have successfully shown the feasibility of using suspended membranes of 2D materials in pressure sensors, microphones, accelerometers, and mass and gas sensors. In this review, we explain the different sensing concepts and give an overview of the relevant material properties, fabrication routes, and device operation principles. Finally, we discuss sensor readout and integration methods and provide comparisons against the state of the art to show both the challenges and promises of 2D material-based nanoelectromechanical sensing.
Nanoelectromechanical system (NEMS) sensors and actuators could be of use in the development of next generation mobile, wearable, and implantable devices. However, these NEMS devices require transducers that are ultra-small, sensitive and can be fabricated at low cost. Here, we show that suspended double-layer graphene ribbons with attached silicon proof masses can be used as combined spring-mass and piezoresistive transducers. The transducers, which are realized using processes that are compatible with large-scale semiconductor manufacturing technologies, can yield NEMS accelerometers that occupy at least two orders of magnitude smaller die area than conventional state-of-the-art silicon accelerometers. With our devices, we also extract the Young's modulus values of double-layer graphene and show that the graphene ribbons have significant built-in stresses.
Graphene's unparalleled strength, chemical stability, ultimate surface-to-volume ratio and excellent electronic properties make it an ideal candidate as a material for membranes in microand nanoelectromechanical systems (MEMS and NEMS). However, the integration of graphene into MEMS or NEMS devices and suspended structures such as proof masses on graphene membranes raises several technological challenges, including collapse and rupture of the graphene.We have developed a robust route for realizing membranes made of double-layer CVD graphene and suspending large silicon proof masses on membranes with high yields. We have demonstrated the manufacture of square graphene membranes with side lengths from 7 µm to 110 µm and suspended proof masses consisting of solid silicon cubes that are from 5 µm × 5 µm × 16.4 µm to 100 µm × 100 µm × 16.4 µm in size. Our approach is compatible with wafer-scale MEMS and semiconductor manufacturing technologies, and the manufacturing yields of the graphene membranes with suspended proof masses were greater than 90%, with more than 70% of the graphene membranes having more than 90% graphene area without visible defects. The measured resonance frequencies of the realized structures ranged from tens to hundreds of kHz, with quality factors ranging from 63 to 148. The graphene membranes with suspended proof masses were extremely robust and were able to withstand indentation forces from an atomic force microscope (AFM) tip of up to ~7000 nN. The proposed approach for the reliable and large-scale manufacture of graphene membranes with suspended proof masses will enable the development and study of innovative NEMS devices with new functionalities and improved performances.
Graphene is an atomically thin material
that features unique electrical
and mechanical properties, which makes it an extremely promising material
for future nanoelectromechanical systems (NEMS). Recently, basic NEMS
accelerometer functionality has been demonstrated by utilizing piezoresistive
graphene ribbons with suspended silicon proof masses. However, the
proposed graphene ribbons have limitations regarding mechanical robustness,
manufacturing yield, and the maximum measurement current that can
be applied across the ribbons. Here, we report on suspended graphene
membranes that are fully clamped at their circumference and have attached
silicon proof masses. We demonstrate their utility as piezoresistive
NEMS accelerometers, and they are found to be more robust, have longer
life span and higher manufacturing yield, can withstand higher measurement
currents, and are able to suspend larger silicon proof masses, as
compared to the previous graphene ribbon devices. These findings are
an important step toward bringing ultraminiaturized piezoresistive
graphene NEMS closer toward deployment in emerging applications such
as in wearable electronics, biomedical implants, and internet of things
(IoT) devices.
The
electrical contact resistance at metal–graphene interfaces
can significantly degrade the properties of graphene devices and is
currently hindering the full exploitation of graphene’s potential.
Therefore, the influence of environmental factors, such as humidity,
on the metal–graphene contact resistance is of interest for
all graphene devices that operate without hermetic packaging. We experimentally
studied the influence of humidity on bottom-contacted chemical-vapor-deposited
(CVD) graphene–gold contacts, by extracting the contact
resistance from transmission line model (TLM) test structures.
Our results indicate that the contact resistance is not significantly
affected by changes in relative humidity (RH). This behavior is in
contrast to the measured humidity sensitivity of graphene’s
sheet resistance.
In addition, we employ density functional theory (DFT) simulations
to support our experimental observations. Our DFT simulation results
demonstrate that the electronic structure of the graphene sheet on
top of silica is much more sensitive to adsorbed water molecules than
the charge density at the interface between gold and graphene. Thus,
we predict no degradation of device performance by alterations in
contact resistance when such contacts are exposed to humidity. This
knowledge underlines that bottom-contacting of graphene is a viable
approach for a variety of graphene devices and the back end of the
line integration on top of conventional integrated circuits.
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