ABSTRACT
Limited Z-axis bandwidth of piezotube scanners employed in conventional Scanning Tunneling Microscopes (STMs) has been a major limiting factor in achieving high scan speeds in STM applications. Slow Z-axis dynamics of typical piezotube scanners combined with the weight of the STM tip/tip holder assembly, that the scanner has to carry, substantially limit the achievable Z-axis bandwidth in both imaging and lithography modes. To tackle this issue, we propose a high bandwidth microelectromechanical-system-based nanopositioner to be integrated into an existing STM scanner. The device is designed to replace the STM tip and fine Z-positioning mechanisms in the conventional STM setup, while providing an order of magnitude higher bandwidth in Z axis. The device is microfabricated using double silicon-on-isolator technology, and standard cleanroom processes. Experiments show that tunneling current between the device tip and a highly ordered pyrolytic graphite sample can be successfully established and maintained in air using the proposed device in a feedback loop. Results indicate that the proposed device uniquely combines a very high resolution and a large stroke with a substantially larger Z-axis bandwidth compared to that of conventional STM piezotube scanners, enabling higher scanning speeds in STM operations.
ABSTRACT
Ultralight graphene elastomer-based flexible sensors are developed to detect subtle vibrations within a broad frequency range. The same device can be employed as an accelerometer, tested within the experimental bandwidth of 20-300 Hz as well as a microphone, monitoring sound pressures from 300 to 20â¯000 Hz. The sensing element does not contain any metal parts, making them undetectable by external sources and can provide an acceleration sensitivity of 2.6 mV/g, which is higher than or comparable to those of rigid Si-based piezoresistive microelectromechanical systems (MEMS).
ABSTRACT
Ultrasensitive flexible strain sensors were developed through the combination of shear alignment of a high concentration graphene oxide (GO) dispersion with fast and precise patterning of multiple rectangular features on a flexible substrate. Resistive changes in the reduced GO films were investigated under various uniaxial strain cycles ranging from 0.025 to 2%, controlled with a motorized nanopositioning stage. The devices uniquely combine a very small detection limit (0.025%) and a high gauge factor with a rapid fabrication process conducive to batch production.
ABSTRACT
This study presents a novel acoustic mixer comprising of a microfabricated silicon nitride membrane with a hole etched through it. We show that the introduction of the through hole leads to extremely fast and homogeneous mixing. When the membrane is immersed in fluid and subjected to acoustic excitation, a strong streaming field in the form of vortices is generated. The vortices are always observed to centre at the hole, pointing to the critical role it has on the streaming field. We hypothesise that the hole introduces a discontinuity to the boundary conditions of the membrane, leading to strong streaming vortices. With numerical simulations, we show that the hole's presence can increase the volume force responsible for driving the streaming field by 2 orders of magnitude, thus supporting our hypothesis. We investigate the mixing performance at different Peclet numbers by varying the flow rates for various devices containing circular, square and rectangular shaped holes of different dimensions. We demonstrate rapid mixing within 3 ms mixing time (90% mixing efficiency at 60 µl min(-1) total flow rate, Peclet number equals 8333 ± 3.5%) is possible with the current designs. Finally, we examine the membrane with two circular holes which are covered by air bubbles and compare it to when the membrane is fully immersed. We find that coupling between the holes' vortices occurs only when membrane is immersed; while with the bubble membrane, the upstream hole's vortices can act as a blockage to fluid flow passing it.
