Research On Flexible Surface Acoustic Wave Humidity Sensor Based On Zinc Oxide Nanowires/Quantum Dots

With the aging of population and the increasing prevalence of chronic diseases,wearable devices that can detect human health in real time have shown a huge market prospect.Owing to the advantages of miniaturization,low power consumption,quasi-digital output and wireless passive control,the flexible surface acoustic wave humidity sensor can be embedded in a wearable device to detect human breathing and air humidity,which is of great significance for monitoring human apnea disease and improving human comfort.However,traditional surface acoustic wave sensors are based on hard substrates,which cannot be flexible and applied to curved surfaces.In this thesis,the preparation method of flexible surface acoustic wave device based on flexible glass and zinc oxide nanowire/graphene quantum dot humidity composite sensitive film is proposed,which realizes the significant improvement of device performance and sensor's sensitivity,and explores its wearable applications.This work first studies the preparation methods of ZnO piezoelectric films and surface acoustic wave devices on flexible glass substrates,and characterizes the properties of the prepared films and devices respectively,and compares them with related works.On this basis,a humidity sensing test platform was built to test the humidity response characteristics of the zinc oxide piezoelectric film as a sensitive film,and further analyze the mechanism of the semiconductor metal oxide's sensitivity to humidity;Then,the enhancement effect of graphene quantum dot concentration on the humidity sensitivity of the sensor is studied,and the sensor characteristics such as stability,repeatability and resolution are tested,the sensor sensitivity mechanism is analyzed,and the practical application scenarios of humidity sensing are explored.The signal amplitude of the device prepared on the flexible glass substrate reaches 45 d B,and the electromechanical coupling coefficient reaches 3.5 %,which is many times higher than that of the acoustic device based on polymer substrate(20 d B,1.21 %).It provides a good foundation for high-performance flexible sensors and flexible on-chip laboratories.By increasing the concentration of added quantum dots(1.81 mg/ml),the humidity sensitivity of the sensor is increased to 40.16 k Hz/%RH,and the response recovery time is shortened to 27s/12 s.When the relative humidity level is greater than 40 %,the frequency offset of the sensor changes linearly with the relative humidity levelThe increase in humidity sensitivity is attributed to the large specific surface area of the zinc oxide nanowires,the large number of hydrophilic functional groups of the graphene quantum dots,and the heterojunction formed between the graphene quantum dots and the zinc oxide nanowires,which also provides a reference for the design of other humidity sensitive membranes based on acoustics.The improvement of humidity sensitivity is attributed to the large specific surface area of ZnO nanowires,the large number of hydrophilic functional groups of graphene quantum dots,and the heterojunction interaction between graphene quantum dots and ZnO nanowires,which also provides a reference for the design of other humidity sensitive membranes based on acoustics.Finally,the typical applications of the flexible surface acoustic wave humidity sensor in curved surfaces,environmental humidity level detection,human respiration monitoring and other wearable fields are explored,and the development potential of the flexible surface acoustic wave humidity sensor in human-computer interaction(HMI),spatial localization and personal healthcare is revealed.