Electron Transport Characteristics In The Complex Conductive Networks Of Densely Packed Nanoclusters And Their Flexible Sensing Device Applications

The conductivity of the percolative networks in the densely packed nanoclusters is much sensitive to the change of the mean distance between clusters,which could be used to construct many sensing devices.If a flexible material is applied as the substrate,the densely packed nanocluster arrays could be fabricated as a novel flexible mechanical sensor.In the percolatvie networks of nanocluster arrays,the electronic transport depends on the electronic internal energy,the dielectric constrant of the tunneling junction,and other parameters,so that the conductance could respond to the change of the temperature and humidity.Therefore,based on percolative networks of the densely packed nanoclusters,we could obtain some multi-functional flexible sensors.With the popularity of the intelligent terminals,a huge market prospect has been shown in thewearable electronic devices.The flexible sensors,as the key components,will effect the designing functions and future development of the wearable devices.Through the multi-functional sensing devices,people could harvest various datum as many as possible for the analysis and evaluation of the information including the surrounding environment,the health state and the medicical treatments etc.And the mechanical sensors,especially the pressure or strain sensors,play a significant role in the process of these datum acquisition.Due to the special situations of application,the flexible mechanical sensors are demanded to not only respond to the basic perceptions of human skin,including touching,temperature,etc.,but also expand to more functions such as barometric pressure,humidity,vibration and so on.In recent years,some important progress has been made in the implantable and wearable mechanical sensors.Based on the novel nanotechnology,the flexible mechanical sensors with the characteristics of tiny size,light weight and portability,excellent electrical performace,high integration,low cost and reduced energy consumption,have gained the most attentions.There is a tremendous interest to develop a wearable mechanical sensor with high resolution,high sensitivity,fast response,lower cost and complex signal detections.Flexible sensing devices were generally constructed by compositing the low-dimensional nanomaterials with the elastic substrates.In this thesis,to construct the mechanical sensors,the nanoclusters were arranged on a flexible substrate in a densely packed manner to form a complex percolative conductive network.We fabricated a series of conductive networks in cluster arrays and the structure of nanocluster arrays compositing with the flexible substrate.The properties of electronic transport in the percolative networks of cluster arrays were investigated.The sensing elements responding to the pressure and temperature were fabricated,and applied to the precise barometers and multi-function electronic skins.The main results of this thesis are as follows:?1?The size-controlled metallic nanoclusters generated by the cluster beam system were deposited on the interdigital electrodes?IDEs?.The conductance evolution of the nanocluster films would be real-time monitored when the IDEs were connected to the micro-current monitoring circuit.Thereby,the deposition volume of the clusters on the substrate could be controlled,and the percolative networks of the densely packed nanoclusters with controllable conductance could be prepared.The cluster beam diameter could be modulated to micrometer level.Through the program-controlled scanning deposition with such cluster beam,a percolative network of non-uniform nanocluster arrays with directional gradient coverage distribution between parallel electrodes could be fabricated.?2?The relationship between the conductance of the nanoluster networks with temperature was analyzed.The conductance of the nanocluster networks increased with the raise of temperature,which indicated the quantum transport behavior of electrons in the cluster networks.The development of current-voltage curves accorded with the scale law and showed an obvious nonlinearity.Because of the coulomb blockade,there was a threshold voltage in the curves at lower temperature.The temperature dependence of the nanocluster networks conductivity proved that,in the low temperature range,the variable ranging hopping?VRH?played a major role in the electronic transport,while in the high temperature range,the temperature dependence of the conductivity was more consistent with the thermally activated tunneling model.When measuring the current-voltage curves of the gradient nanocluster networks,we found the asymmetry of electronic transport.It suggested that the conductance of electrons moving from the lower nanocluster coverage area to the higher coverage area was much higher than the one with the opposite direction.?3?The assemble nanoclusters networks from the deposition could compound with different polymer substrates to fabricate the sensing elements.These sensing elements could respond to the stimuli of the tactile pressure and temperature.And with different mechanical properties of the selected substrates,the pressure sensing capability of these elements were different.Using the PDMS substrate with a lower Young's modulus,the sensing elements had a sensitivity as high as 21.3 k Pa^-1,and could distinguish the tiny pressure of 0.5 Pa,with a response time of about 33 ms.The sensitivity of a sensing element with a PET layer was only 0.25 k Pa^-1,but it still kept a stable response capability in more than ten thousands of cycle tests.The sensitivity of the sensing element to the temperature is about 0.00126K^-1,helpping it can easily distinguish the temperature change of about 1 K.The coupling stimuli from temperature and pressure will induce the different conductive changing tendency of the sensing elements on the lower and upper surface of the substrate,so that the varying temperature and pressure could be decoupled,and the multi-function of the sensors could be realized.When the flexible sensors were worn over the body,the human behavior could be detected and distinguished,and the measurement distortion from the tactile stimulation would be reduced effectively.?4?A sensor enabling responding to the change of barometric pressure could be fabricated by sealing the sensing elements on a reference cavity.The sensing properties could be controlled by modifying the thickness of the flexible films.When a PET layer with a 0.05 mm thickness was applied as the substrate,the sensitivity of this pressure sensor was about 0.13 k Pa^-1,with a resolution of about 0.5 Pa in a limited measurement range of about 0-1 k Pa.However,the sensitivity and resolution of the sensor with a thicker PET layer were reduced correspondingly while the measurement range was expanded.This pressure sensor with such high resolution can distinguish the change of ambient barometric pressure between different floor heights.Therefore,the barometric pressure sensor can be applied as an altimeter,having a sensitivity of about 0.00026 m^-1and a resolution of about 1 m.