Humidity Sensors Based On Black Phosphorus

Humidity sensor is one of the most commonly used sensors,which is widely used in meteorological research,environmental monitoring,industrial process control,agricultural production,health care and many other fields.With the progress of science and technology,especially the rapid development of the Internet of things in the recent years,the growing demand of environmental information has put forward great requirements of humidity sensors with better performance.Therefore,because of the large specific surface area and excellent physical characteristics,it is now an important method to improve the performance by developing new humidity sensors with two-dimensional(2D)materials.Among the all kinds of new sensing materials,black phosphorus(BP),which is a p-type 2D semiconductor composed of phosphorus atoms,is considered as a kind of functional material with great potential because of its exceptional mechanical and electrical properties.In this dissertation,the humidity sensing characteristics of BP are studied and BP humidity sensors based on rigid and flexible substrates are fabricated.And the performance of these sensors is studied and analyzed.(1)The water adsorption characteristics of BP are analyzed with the assistance of molecular dynamics.The results show that the BP layers interact with the water molecules only through the Van der Waals force.The water molecules can easily escape from the surface of the BP layers when the humidity level varies,which result in the rapid adsorption and desorption processes.With the temperature increasing,the adsorption capacity of water molecules on the surface of BP layers is found increasing approximately linearly.Besides,it is found that the water molecules could not enter into the interlayers of the BP structure,but could be adsorbed between the stacked BP nanosheets.As a result,the BP films which consist of stacked nanosheets,can also absorb enough water molecules to realize the high sensitivity.(2)A BP humidity sensor based on rigid substrate is presented.The BP material used in experiments is prepared by liquid exfoliation methods,and then deposited on the interdigited electrodes by spin-coating methods to form a humidity sensing film with certain thickness and morphology.The surface morphology and stability of the BP films fabricated with different process parameters is studied to optimize the fabrication process.High sensitivity,fast response and good repeatability of the sensor are found in experiments.The response time of the BP sensor is 3.9 s while the recovery time is 0.9 s.According to the experiment results,an equivalent circuit model of the sensor is proposed.The sensing mechanism of the BP humidity sensor is analyzed,which is mainly dominated by the electrostatic interaction between the BP films and the adsorbed water molecules.(3)A BP humidity sensor based on flexible substrate is developed for the application of wearable sensors and bended surface.With the porous structure of the poly tetra fluoroethylene(PTFE)film,water molecules are allowed to pass through the micro-pore networks in the substrate,so that the BP layers can adsorb water from the double sides to improve the response speed of the sensor.It is found that the larger the pore size is,the better the film permeability is,and the faster response of the flexible humidity sensor is.Experiment results show that the flexible sensor remained a fast response to humidity(with the response time of 2 s and the recovery time of 1.7 s)though the thickness of BP films increases significantly compared to the sensors with glass substrates.On the other hand,the PTFE film is proved hydrophobic,so that water is not adsorbed on the substrate when the humidity sensor works.As a result,its influence on sensitivity,hysteresis and response speed is avoided so as to maintain the high performance of the humidity sensor.According to the experiment results,the flexible humidity sensor with BP sensing materials shows high sensitivity,fast response and good temperature characteristics,which can meet the requirements of various application areas.Finally,the impedance spectrum characteristics of the sensor is analyzed and an equivalent circuit is proposed.It is pointed out that the humidity sensing mechanism of the BP flexible sensor is quite different from that based on rigid substrate.In this dissertation,humidity sensors based on BP materials with high performance are proposed for different application requirements,which realize high sensitivity and fast response.Besides,the humidity sensing characteristics and mechanisms of BP humidity sensors are explored,providing some useful experience for the application of BP and the design of new humidity sensors.