Flexible Strain Sensor Based On Langmuir-Blodgett Self-assembly Graphene

Recently,portable and wearable flexible electronics develop rapidly.As one critical part of the flexible electronics,flexible strain sensors are paid tremendous attention all over the world.2D material graphene possesses high mobility and high tensile strength and is one of the promising candidates for strain sensors.Langmuir-Blodgett technology is one kind of self-assembly technologies.Langmuir-Blodgett technology can control the surface morphology of film and is a critical technology of film fabrication methods.Here,graphene flexible strain sensors are fabricated utilizing Langmuir-Blodgett assembly technology.The result are as follow:(1)Utilizing Langmuir-Blodgett technology to fabricated the graphene strain sensor,the result shows that the fabricated strain sensor possess satisfying performance.Firstly,diverse characterization methods are used to characterize the graphene material.The result shows that oxygen-containing group are functioned to improve the graphene dispersive capacity in solution.Using Langmuir-Blodgett technology,strain sensors on PDMS and PET substrates are fabricated.During the fabrication process,with the assistance of electron scanning microscope surface,it can be observed that surface pressure can manipulate the surface morphology of graphene thin film.The mechanical and electrical tests are carried out.The result show that the maximum stretchability is near 400%,the maximum detectable strain range is 93%,the maximum relative resistance change is 3400,revealing its high sensitivity and the response time is 50 ms,demonstrating its fast response to strain.The cyclic performance is good and the performance stays stable after over 3000 cycles of stretching and releasing.Meanwhile,the strain sensor can work under diverse frequencies.Taking advantage of controlling the surface morphology of film by surface pressure,the performance of strain sensor can be manipulated efficiently and remaining the stable and the high stability from device to device can be achieved.(2)Different ways to manipulate the performance of strain sensors.Firstly,taking the advantage of manipulation of surface pressure for surface morphology,the performance of strain sensors can be manipulated.The result show that with increasing surface pressure,the maximum strain increases from 45% to 93%.On the contrary,the relative resistance has a decreasing trend.The aspect ratio also can manipulate the performance of strain sensors and the result shows that,the strain sensors with bigger aspect ratio possess higher sensitivity under the constant strain range,larger detectable strain ranges and larger relative resistance changes.Further,manipulation of the inclined angle between directions of applying strain and testing is also studied.The result shows that with the increasing degree of angle,the sensitivity gets higher.(3)Explanation of the mechanism of strain sensors and verification.Here,the paper explains the mechanism of the strain sensors with scale-like graphene flakes fabricated by Langmuir-Blodgett technology.The percolation model is proposed and scanning electron microscope,simulation and transmission electron microscope are used to verify.The result show that the resistance changes depend on the changes of network of conductive graphene flakes.Without applied strain,the scale-like graphene flakes of sensor fabricated by Langmuir-Blodgett technology overlap with each other,forming the conductive network.When the strain sensor is stretched,the flakes slide due to the friction force with the flexible substrate.The overlapped area of graphene flakes decreases,leading to decrease of conductive paths,thus,resistance increasing.When the applied strain keeps increasing,the conductive paths break and resistance increases until it reaches the maximum value and stays stable,thus,relative resistance change becoming zero.(4)Application of strain sensors.Utilizing the sensor on PET substrate,resistance changes by compressing and transition between resistance and voltage,the orders of robot movements can respond to different voltages,realizing the interface of robot and human.The strain sensor on PDMS are in series with LED.By changing the resistance by stretching and releasing strain sensor,the manipulation of LED luminance can be achieved.For reading fatigue monitoring,strain sensor is places onto the spines of books.The resistance changes are transited into voltage changes according to the principle of voltage distribution.By monitoring the changes of input voltage in constant time,it can judge whether the reading fatigue happens.For health monitor of human cervical vertebra,the array of strain sensors on PDMS substrate are placed to the cervical vertebra.By collecting data of every strain sensor,the distribution diagrams of relative resistance can be achieved,offering the data for health monitor.In summary,a flexible graphene strain sensor is fabricated utilized LangmuirBlodgett assembly method.The performance of the sensor is fine.Besides,it is discovered that the surface morphology can be controlled by surface pressure,which is a critical parameter during fabrication process.It solves the uncontrollable problem of reported fabrication process.Controllable surface morphology make performance of sensors controllable.Apart from it,this paper also presents other methods of performance control using sample shape and test angle.As for the strain mechanism of graphene,this paper proves that it conforms to the percolation model by means of simulation.Finally,the applications of graphene strain sensor in human-computer interaction and health monitoring are also provided.