337 research outputs found
Graphene-Based Electromechanical Thermal Switches
Thermal management is an important challenge in modern electronics, avionics,
automotive, and energy storage systems. While passive thermal solutions (like
heat sinks or heat spreaders) are often used, actively modulating heat flow
(e.g. via thermal switches or diodes) would offer additional degrees of control
over the management of thermal transients and system reliability. Here we
report the first thermal switch based on a flexible, collapsible graphene
membrane, with low operating voltage, < 2 V. We also employ active-mode
scanning thermal microscopy (SThM) to measure the device behavior and switching
in real time. A compact analytical thermal model is developed for the general
case of a thermal switch based on a double-clamped suspended membrane,
highlighting the thermal and electrical design challenges. System-level
modeling demonstrates the thermal trade-offs between modulating temperature
swing and average temperature as a function of switching ratio. These
graphene-based thermal switches present new opportunities for active control of
fast (even nanosecond) thermal transients in densely integrated systems
Nonlinear Modeling of MEMS Fixed-Fixed beams
This dissertation studies critical topics associated with MEMS fixed-fixed beams. One of the typical devices of fixed-fixed beams is radio frequency microelectromechanical system (MEMS) capacitive switches. The interesting topic for this device includes the instability at the pull-in voltage; the switchesâ deformation characteristics when subject to an electrostatic force; nonlinear stretching effects, and the capacitance calculation in small scale. Specifically, the accuracy of parallel-plate theory in calculating the pull-in voltage and capacitance is investigated. The study shows that applying average displacement rather than maximum displacement into parallel-plate theory demonstrates better accuracy. The improvement increases with the bottom stationary electrode to moveable electrode ratio and it reaches 50% when the ratio is equal to 1. Besides average displacement, the nonlinear stretching effect and empirical linear correction coefficients are also added to the parallel-plate model to extend model\u27s validity range. In order to improve the lifetime of RF MEMS capacitive switch, a relationship between switches\u27 geometry and membrane strain is derived, which helps avoid switches operating beyond the elastic region.Furthermore, this dissertation presents a new coupled hyperbolic electro-mechanical model that is an improvement on the classical parallel-plate approximation. The model employs a hyperbolic function to account for the beamâs deformed shape and electrostatic field. Based on this, the model accurately calculates the deflection of a fixed-fixed beam subjected to an applied voltage and the switchâs capacitance-voltage characteristics without using parallel-plate assumption. For model validation, the model solutions are compared with ANSYS finite element results and experimental data. It is found that the model works especially well in residual stress dominant and stretching dominant cases. The model shows that the nonlinear stretching significantly increases the pull-in voltage and extend the beamâs maximum travel range. Based on the model, a graphene nanoelectromechanical systems (NEMS) resonator is designed and the performance agrees very well with the experimental data. The proposed coupled hyperbolic model demonstrates its capacity to guide the design and optimization of both RF MEMS capacitive switches and NEMS devices
MEMS tunable infrared metamaterial and mechanical sensors
Sub-wavelength resonant structures open the path for fine controlling the near-field at the nanoscale dimension. They constitute into macroscopic âmetamaterialsâ with macroscale properties such as transmission, reflection, and absorption being tailored to exhibit a particular electromagnetic response. The properties of the resonators are often fixed at the time of fabrication wherein the tunability is demanding to overcome fabrication tolerances and afford fast signal processing. Hybridizing dynamic components such as optically active medium into the device makes tunable devices. Microelectromechanical systems (MEMS) compatible integrated circuit fabrication process is a promising platform that can be merged with photonics or novel 2D materials. The prospect of enormous freedom in integrating nanophotonics, MEMS actuators and sensors, and microelectronics into a single platform has driven the rapid development of MEMS-based sensing devices. This thesis describes the design and development of four tunable plasmonic structures based on active media or MEMS, two graphene-based MEMS sensors and a novel tape-based cost-effective nanotransfer printing techniques.
First of all, we present two tunable plasmonic devices with the use of two active medium, which are electrically controlled liquid crystals and temperature-responsive hydrogels, respectively. By incorporating a nematic liquid crystal layer into quasi-3D mushroom plasmonic nanostructures and thanks to the unique coupling between surface plasmon polariton and Rayleigh anomaly, we have achieved the electrical tuning of the properties of plasmonic crystal at a low operating electric field. We also present another tunable plasmonic device with the capability to sense environmental temperature variations. The device is bowtie nanoantenna arrays coated with a submicron-thick, thermos-responsive hydrogel. The favorable scaling of plasmonic dimers at the nanometer scale and ionic diffusion at the submicron scale is leveraged to achieve strong optical resonance and rapid hydrogel response, respectively.
Secondly, we present two MEMS -based tunable near-to-mid infrared metamaterials on a silicon-on-insulator wafer via electrically and thermally actuating the freestanding nanocantilevers. The two devices are developed on the basis of the same fabrication process and are easy-to-implement. The electrostatically driven metamaterial affords ultrahigh mechanical modulation (several tens of MHz) of an optical signal while the thermo-mechanically tunable metamaterial provides up to 90% optical signal modulation at a wavelength of 3.6 ĂĂ”m.
Next, we present MEMS graphene-based pressure and gas flow sensors realized by transferring a large area and few-layered graphene onto a suspended silicon nitride thin membrane perforated with micro-through-holes. Due to the increased strain in the through-holes, the pressure sensor exhibits a very high sensitivty outperformed than most existing MEMS-based pressure sensors using graphene, silicon, and carbon nanotubes. An air flow sensor is also demonstrated via patterning graphene sheets with flow-through microholes. The flow rate of the air is measured by converting the mechanically deflection of the membrane into the electrical readout due to the graphene piezeroresistors.
Finally, we present a tape-based multifunctional nanotransfer printing process based on a simple stick-and-peel procedure. It affords fast production of large-area metallic and dielectric nanophotonic sensing devices and metamaterials using Scotch tape
Ultra-tuning of nonlinear drumhead MEMS resonators by electro-thermoelastic buckling
Nonlinear micro-electro-mechanical systems (MEMS) resonators open new
opportunities in sensing and signal manipulation compared to their linear
counterparts by enabling frequency tuning and increased bandwidth. Here, we
design, fabricate and study drumhead resonators exhibiting strongly nonlinear
dynamics and develop a reduced order model (ROM) to capture their response
accurately. The resonators undergo electrostatically-mediated thermoelastic
buckling which tunes their natural frequency from 4.7 to 11.3 MHz, a factor of
2.4x tunability. Moreover, the imposed buckling switches the nonlinearity of
the resonators between purely stiffening, purely softening, and even
softening-to-stiffening. Accessing these exotic dynamics requires precise
control of the temperature and the DC electrostatic forces near the resonator's
critical-buckling point. To explain the observed tunability, we develop a
one-dimensional physics-based ROM that predicts the linear and nonlinear
response of the fundamental bending mode of these drumhead resonators. The ROM
captures the dynamic effects of the internal stresses resulting from three
sources: The residual stresses from the fabrication process, the mismatch in
thermal expansion between the constituent layers, and lastly, the applied
electrostatic forces. The ROM replicates the observed tunability of linear
(within 5.5% error) and nonlinear responses even near the states of critical
buckling. These remarkable nonlinear and large tunability of the natural
frequency are valuable features for on-chip acoustic devices in broad
applications such as signal manipulation, filtering, and MEMS waveguides
Gradient Field Transduction of Nanomechanical Resonators
Das Forschungsgebiet nanomechanischer Systeme betrachtet die Bewegung von Strukturen, deren LĂ€nge in mindestens einer Richtung deutlich unter einem Mikrometer liegt. Meist werden dabei Auslenkungen untersucht, die in der NĂ€he einer mechanischen Resonanz angetrieben werden. Das wissenschaftliche Interesse an solchen Strukturen hat mehrere GrĂŒnde: aufgrund der kleinen Masse und oftmals geringen DĂ€mpfung (d.h. hohe GĂŒte) reagieren solche nanomechanischen Systeme sehr empfindlich auf Ănderungen ihrer Umgebung oder ihrer eigenen Eigenschaften wie etwa ihrer Masse. Die groĂe Vielfalt der nanomechanischen Systeme erlaubt die Kopplung an verschiedenste physikalische GröĂen wie (Umgebungs-)Druck, Licht, elektrische/magnitische Felder. Dies ermöglicht, die Wechselwirkung selbst zu untersuchen oder entsprechende Ănderungen empfindlich zu detektieren.
Im Rahmen der vorliegenden Arbeit wurde die Resonator Bewegung von doppelseitig eingespannten Balken untersucht; diese wurden mit konventioneller Mikrofabrikation aus verspanntem Silizium-Nitrid gefertigt. Die groĂe Zugspannung in den Balken fĂŒhrt zu einer hohen mechanischen StabilitĂ€t und ebenso zu hohen mechanischen GĂŒten.
Ein Teil der Arbeit befasste sich mit der Entwicklung neuer Detektions- und Antriebsmechanismen. Unter Ausnutzung der Polarisierbarkeit des Resonators wurde ein lokaler Antrieb realisiert, der sich durch besondere Einfachkeit auszeichnet. Ebenso wurden Fortschritte in der optischen Detektion erzielt. Ein Photodetektor konnte innerhalb einer optischen WellenlÀnge Abstand zum Resonator plaziert werden; dies ermöglicht die lokale Detektion seiner Bewegung.
Hochempfindliche Messungen nutzen oft optische Resonanzen; bisherige Umsetzungen basieren auf Reflexionen und sind daher auf Objekte beschrĂ€nkt, die gröĂer als die verwendete WellenlĂ€nge sind. In einer Zusammenarbeit mit Prof. Kippenberge konnte diese BeschrĂ€nkung umgangen werden, indem gefĂŒhrtes Licht in einem Mikro-Toroiden verwendet wurde.
Weiter wurde in der Arbeit die resonante Bewegung selbst untersucht. Im Bereich hoher Amplituden zeigt die rĂŒcktreibende Kraft nichtlineares Verhalten. Das sich dadurch ergebende bistabile Verhalten des Resonators wurde mit Hilfe von kurzen, resonanten Pulsen untersucht; schnelles Schalten wurde erreicht.
Die mechanische DĂ€mpfung der Siliziumnitrid Resonatoren wurde untersucht. Die hohen GĂŒten von Systemen unter Zugspannung konnte erklĂ€rt werden durch die sich ergebende erhöhte gespeicherte elastische Energie; im Gegensatz zu einem verĂ€nderten DĂ€mpfungsverhalten
Classical and fluctuation-induced electromagnetic interactions in micronscale systems: designer bonding, antibonding, and Casimir forces
Whether intentionally introduced to exert control over particles and
macroscopic objects, such as for trapping or cooling, or whether arising from
the quantum and thermal fluctuations of charges in otherwise neutral bodies,
leading to unwanted stiction between nearby mechanical parts, electromagnetic
interactions play a fundamental role in many naturally occurring processes and
technologies. In this review, we survey recent progress in the understanding
and experimental observation of optomechanical and quantum-fluctuation forces.
Although both of these effects arise from exchange of electromagnetic momentum,
their dramatically different origins, involving either real or virtual photons,
lead to different physical manifestations and design principles. Specifically,
we describe recent predictions and measurements of attractive and repulsive
optomechanical forces, based on the bonding and antibonding interactions of
evanescent waves, as well as predictions of modified and even repulsive Casimir
forces between nanostructured bodies. Finally, we discuss the potential impact
and interplay of these forces in emerging experimental regimes of
micromechanical devices.Comment: Review to appear on the topical issue "Quantum and Hybrid Mechanical
Systems" in Annalen der Physi
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