Highly-sensitive wafer-scale transfer-free graphene MEMS condenser microphones

Since the performance of micro-electro-mechanical system(MEMS)-based microphones is approaching fundamental physical,design,and material limits,it has become challenging to improve them.Several works have demonstrated graphene’s suitability as a microphone diaphragm.The potential for achieving smaller,more sensitive,and scalable onchip MEMS microphones is yet to be determined.To address large graphene sizes,graphene-polymer heterostructures have been proposed,but they compromise performance due to added polymer mass and stiffness.This work demonstrates the first wafer-scale integrated MEMS condenser microphones with diameters of 2R=220-320μm,thickness of 7 nm multi-layer graphene,that is suspended over a back-plate with a residual gap of 5μm.The microphones are manufactured with MEMS compatible wafer-scale technologies without any transfer steps or polymer layers that are more prone to contaminate and wrinkle the graphene.Different designs,all electrically integrated are fabricated and characterized allowing us to study the effects of the introduction of a back-plate for capacitive read-out.The devices show high mechanical compliances C_(m)=0.081-1.07μmPa^(−1)(10-100×higher than the silicon reported in the state-of-the-art diaphragms)and pull-in voltages in the range of 2-9.5 V.In addition,to validate the proof of concept,we have electrically characterized the graphene microphone when subjected to sound actuation.An estimated sensitivity of S_(1kHz)=24.3-321 mV Pa^(−1)for a V_(bias)=1.5 V was determined,which is 1.9-25.5×higher than of state-of-the-art microphone devices while having a~9×smaller area.

Ultrasound Pulse Emission Spectroscopy Method to Characterize Xylem Conduits in Plant Stems

Although it is well known that plants emit acoustic pulses under drought stress,the exact origin of the waveform of these ultrasound pulses has remained elusive.Here,we present evidence for a correlation between the characteristics of the waveform of these pulses and the dimensions of xylem conduits in plants.Using a model that relates the resonant vibrations of a vessel to its dimension and viscoelasticity,we extract the xylem radi from the waveforms of ultrasound pulses and show that these are correlated and in good agreement with optical microscopy.We demonstrate the versatility of the method by applying it to shoots of ten different vascular plant species.In particular,for Hydrangea quercifolia,we further extract vessel element lengths with our model and compare them with scanning electron cryomicroscopy.The ultrasonic,noninvasive characterization of internal conduit dimensions enables a breakthrough in speed and accuracy in plant phenotyping and stress detection.

Sensitive capacitive pressure sensors based on graphene membrane arrays

The high flexibility,impermeability and strength of graphene membranes are key properties that can enable the next generation of nanomechanical sensors.However,for capacitive pressure sensors,the sensitivity offered by a single suspended graphene membrane is too small to compete with commercial sensors.Here,we realize highly sensitive capacitive pressure sensors consisting of arrays of nearly ten thousand small,freestanding double-layer graphene membranes.We fabricate large arrays of small-diameter membranes using a procedure that maintains the superior material and mechanical properties of graphene,even after high-temperature annealing.These sensors are readout using a low-cost battery-powered circuit board,with a responsivity of up to 47:8aF Pa^(−1) mm^(−2),thereby outperforming the commercial sensors.

Nanoelectromechanical Sensors Based on Suspended 2D Materials

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.