Research On Micro/Nano Structures Design For Electronic Skin Devices

Tactile sensing via the skin is one of the basic means for human beings to interact with the outside world.Over the past decades,researchers have been pursuing a kind of flexible thin-film electronic devices called electronic skin to mimic the haptic functions of human skin.And electronic skin devices have been demonstrated the application prospects in intelligent robots,bionic prostheses,health monitoring and human-machine interface and other fields.The pressure sensation is the core of haptics.Unlike the human skin pressure sensation,the "pressure sensation" of the electronic skin is a quantitative measurement,which is achieved by flexible pressure sensors,and evaluated by performances of sensitivity,hysteresis,response time,linearity,repeatability and stability.Among them,the sensitivity is most concerned,and the sensitivity of the flexible pressure sensor is usually improved by using soft materials or microstructures that can achieve lowapparent Young's modulus.However,such methods often fail to balance other performances such as hysteresis,response time and linearity,etc.,which severely limit the applicability of electronic skin.Therefore,on the basis of achieving high sensitivity,how to improve other performances is still an important challenge in the research field of electronic skin devices.In this thesis,the micro-and nanostructures and their surface/interface effects in the pressure sensor are utilized to enhance the performances of hysteresis and response time of the flexible pressure sensors.The following research results have been achieved:First,a flexible capacitive pressure sensor with high sensitivity and low hysteresis performance is designed and implemented based on a hierarchical microstructured electrode.The sensitivity and hysteresis of a flexible pressure sensor are offten trade off between each other in most devices.In recent years,researchers are seeking methods to develop flexible pressure sensors with sensitively surpassing human skin,most of them tried to decrease the apparent Youngs' Modulus by using softer materials and microstructures.With lower apparent Youngs' Modulus,these materials gain larger deformation hence higher sensitivity,however,the tradeoff is a high hysteresis(>20%)resulted from internal and interfacial energy dissipation accompanying with flexible materials.The hysteresis of the pressure dependent electric properties leads to a large difference in the reading outputs between loading and unloading for the same pressure,especially in a dynamic loading-unloading process.In this thesis,we report the design of flexible capacitive pressure sensors with both high sensitivity and low hysteresis by adopting a hierachical pyramid microstructure design for the electrode.By testing and analyzing the pressure sensors with electrodes of different micropyramid density,the interfacial hysteresis is identified as an important factor for the large hysteresis of a flexible pressure sensor,which was rarely noticed in previous strudies.By inserting small pyramid arrays into large pyramid arrays can siginficantly reduce the interfacial adhesion,and decreasing the area density of large pyramids is favorable for higher sensitivity.As a result of adjustiing the size,density,and number ration of micropyramids,the sensor finnally achieves sensitivity as high 3.73 kPa-1 and hysteresis as low as 4.42%(the lowest value when reported),extremely low pressure detection limit(0.1 Pa),a short response time(21 ms),and excellent repeatability and stability,which allows precise measurement in the situation involveing dynamic pressure,such as wrist pulse.Our pulse sensing comparison showed that both high sensitivity and low hysteresis of the hierarchically microstructured device are essential to achieve accurete pulse information,which shows great potential for advanced electronic skin devices.Secondly,a flexible capacitive pressure sensor with low hysteresis,fast-response and high sensitivity is designed and implemented based on silicon nanowires.In this thesis,we report a silicon nanowires based pressure sensor for the first time.The silicon nanowires are grown by vapor-liquid-solid(VLS)process,and are well crystallized,needle-like,1?2 ?m long with a root diameter below or around 100 nm,standing obliquely and non-orientationally on the substrate.Due to the nano-scale effect,silicon nanowires with good crystallinity transform from macroscopic brittle materials into fine Hooke's elastomers that can be bent at large angles,and their bending deformations are non-viscoelastic,repeatable,and time-independent.In this thesis,silicon nanowires arrays are adopted as a functional layer for the pressure sensor,and the morphology of the VLS-grown silicon nanowires are favorable for ultralow apparent Youngs' Modulus at the vertical direction hence high sensitivity.On the other hand,the needle-like standing silicon nanowires also in favor of negligible interfacial adhesive hysteresis.As a result,the silicon nanowires based pressure sensor achieves a new record of low hysteresis(?2.26%),an extremely short response time(?3 ms at large base pressure range of 0-50 kPa)and ultrahigh sensitivity(8.21 kPa-1,?236.0 pF/kPa)among flexible capacitive pressure sensors.And also,we designed a real-time sensing of a bouncing water droplet dropped onto the superhydrophobic surface of the silicon nano wires based pressure sensor.The sensor was capable to clearly record the 11 times bouncing induced pressure changes in a short period of-420 ms,indicating its advanced applicability for high-speed and accurate pressure sensing.In the following chapaters,we will first introduce the development history and current research status of electronic skin in the first chapter.The second chapter and the third chapter will introduce the detailed two main work of this paper,and we will draw a brief summary and propose some prospects for electronic skin in the last chapater.