Determining the most efficient geometry through simulation study of ZnO nanorods for the development of high-performance tactile sensors and energy harvesting devices

The piezoelectric nanomaterial ZnO exhibits an excellent piezoelectric response that can transduce mechanical energy into electrical signals by applying pressure. The piezoelectric behavior of ZnO nanostructures (especially nanorods or microrods) is getting considerable attention in the fabrications of piezo tactile sensors, energy harvesting devices, and other self-powering implantable devices. Especially vertically aligned ZnO nanorods are of high interest due to their higher value of piezoelectric coefficient along the z-direction. In this report, various geometries and alignments of ZnO nanorods are explored and their effect on strength of piezoelectric output potential has been simulated by COMSOL Multiphysics software. Best suited geometry and inclination are explored in this simulation to achieve high piezoelectric output in haptic and energy harvester devices. The simulation results show out of many geometries and inclinations the highest piezoelectric output is demonstrated by the inclined ZnO nanorods due to the application of higher torque force or shear stress in similar applied force. The high torque force or shear stress at 60 degree orientation and optimized contributions from all the piezoelectric coefficients resulted in a high piezoelectric output potential close to 215 mV which is much higher than the vertically aligned ZnO nanorod which is approximately 25 mV. The results are contrary to the accepted understanding that the vertical ZnO nanorods should produce the highest output voltage due to the high piezoelectric coefficient along the z-axis.Comment: 22 pages, 8 figure

A Micro-Fabricated Force Sensor Using an All Thin Film Piezoelectric Active Sensor

The ability to measure pressure and force is essential in biomedical applications such as minimally invasive surgery (MIS) and palpation for detecting cancer cysts. Here, we report a force sensor for measuring a shear and normal force by combining an arrayed piezoelectric sensors layer with a precut glass top plate connected by four stress concentrating legs. We designed and fabricated a thin film piezoelectric force sensor and proposed an enhanced sensing tool to be used for analyzing gentle touches without the external voltage source used in FET sensors. Both the linear sensor response from 3 kPa to 30 kPa and the exact signal responses from the moving direction illustrate the strong feasibility of the described thin film miniaturized piezoelectric force sensor

SYSTEM-LEVEL APPROACHES FOR IMPROVING PERFORMANCE OF CANTILEVER-BASED CHEMICAL SENSORS

This work presents the development of different technologies and techniques for enhancing the performance of cantilever-based MEMS chemical sensors. The developed methods address specifically the sensor metrics of sensitivity, selectivity, and stability. Different techniques for improving the quality and uniformity of deposited sorbent polymer films onto MEMS-based micro-cantilever chemical sensors are presented. A novel integrated recess structure for constraining the sorbent polymer layer to a fixed volume with uniform thickness was developed. The recess structure is used in conjunction with localized polymer deposition techniques, such as inkjet printing and spray coating using shadow masking, to deposit controlled, uniform sorbent layers onto specific regions of chemical sensors, enhancing device performance. The integrated recess structure enhances the stability of a cantilever-based sensor by constraining the deposited polymer layers away from high-strain regions of the device, reducing Q-factor degradation. Additionally, the integrated recess structure enhances the sensitivity of the sensor by replacing chemically-inert silicon mass with ‘active’ sorbent polymer mass. Finally, implementation of localized polymer deposition enables the use of sensor arrays, where each sensor in the array is coated with a different sorbent, leading to improved selectivity. In addition, transient signal generation and analysis for mass-sensitive chemical sensing of volatile organic compounds (VOCs) in the gas phase is investigated. It is demonstrated that transient signal analysis can be employed to enhance the selectivity of individual sensors leading to improved analyte discrimination. As an example, elements of a simple alcohol series and elements of a simple aromatic ring series are distinguished with a single sensor (i.e. without an array) based solely on sorption transients. Transient signals are generated by the rapid switching of mechanical valves, and also by thermal methods. Thermally-generated transients utilize a novel sensor design which incorporates integrated heating units onto the cantilever and enables transient signal generation without the need for an external fluidic system. It is expected that the thermal generation of transient signals will allow for future operation in a pulsed mode configuration, leading to reduced drift and enhanced stability without the need for a reference device. Finally, A MEMS-based micro thermal pre-concentration (µTPC) system for improving sensor sensitivity and selectivity is presented. The µTPC enhances sensor sensitivity by amplifying low-level chemical concentrations, and is designed to enable coarse pre-filtering (e.g. for injection into a GC system) by means of arrayed and individually-addressable µTPC devices. The system implements a suspended membrane geometry, enhancing thermal isolation and enabling high temperature elevations even for low levels of heating power. The membranes have a large surface area-to-volume ratio but low thermal mass (and therefore, low thermal time constant), with arrays of 3-D high aspect-ratio features formed via DRIE of silicon. Integrated onto the membrane are sets of diffused resistors designed for performing thermal desorption (via joule heating) and for measuring the temperature elevation of the device due to the temperature-dependent resistivity of doped silicon. The novel system features integrated real-time chemical sensing technology, which allows for reduced sampling time and a reduced total system dead volume of approximately 10 µL. The system is capable of operating in both a traditional flow-through configuration and also a diffusion-based quasi-static configuration, which requires no external fluidic flow system, thereby enabling novel measurement methods and applications. The ability to operate without a forced-flow fluidic system is a distinct advantage and can considerably enhance the portability of a sensing system, facilitating deployment on mobile airborne platforms as well as long-term monitoring stations in remote locations. Initial tests of the system have demonstrated a pre-concentration factor of 50% for toluene.Ph.D