The Humidity Sensing Propterties Of Graphene Oxide And Its Application In Micro/Nano Humidity Sensors

As an important physical quantity, relative humidity (RH) plays an important role in human daily life, industry and agriculture. With the continuous development of science and technology, the increasing demands for high performances humidity sensor bring out unprecedented accident and challenge for humidity sensor industry. In recent years, novel nanostructure materials have frequently been adopted in the application of humidity sensor, and gradually become the development trends and the hot spots of the humidity sensing materials. This dissertation centres on and spreads out from the humidity sensing properties of graphene oxide (GO), which is a novel two dimension carbon nanomaterial. We present several types of humidity sensor based on GO thin films, and investigate the influence of ambient humidity on the electrical properties of GO films. The main content summarized as follows:Firstly, the effects and mechanism of ambient humidity on the electrical properties of GO films is studied by using alternating current complex impedance technique and direct current I-V method.(1) Utilizing alternating current complex impedance technique, the humidity sensing mechanism of GO films is studied, and the correspoding equivalent circuits are proposed. By analyzing the characteristics of the Cole-Cole diagram and Bode diagram of GO films, we find that the conductive mechanisms of GO films at low RH are intrinsic electron conduction and electronic polarization of GO films; Because of the intrinsic electron conduction of GO films is weak, GO films exhibit large impedance and the polarization is dominant in the impedance of GO films. At high RH, however, the conductive mechanisms of GO films are ion conduction and polarization of water, GO films exhibit poor conductivity. We further dicuss the application of GO films in impedance humidity sensor. The results suggest that GO films based impedance humidity sensor show good humidity sensing property at low frequency exciting point (50Hz). In addition, the sensor exhibit low humidity hysteresis, fast response and recovery.(2) The I-V characteristics of GO films at various humidity levels are investigated. The results indicate that the I-V characteristic of GO films under a lower loading voltage (-2V～2V) is different from that of a higher loading voltage (-4V～-2V and2V～4V). In the case of a lower loading voltage, the I-V curves of GO films are linear at various RHs. The resistance of GO films decreases with inceasing RH at a constant loading voltage. The resistance of GO films is independent on the loading voltage. However, in the case of a higher loading voltage, the I-V characteristics of GO films are not only dependent on humidity, but also on the loading voltage, the resistance of GO films decreases with increasing loading voltage. It is infered that the reduction of GO films occurs in the presence of strong electric fields, a part of cabon atoms of GO films transform from sp3hybridization state to sp2hybridization state; it results in the enhancement of the conduction of GO films. The increase in loading voltage and humidity level can also strengthen the reduction of GO films, leading to the large enhancement of the conduction of GO films. The above-mentioned studies indicate that the graphene electrical device arising from solution processed GO shoud consider the effect of humidity and loading voltage.Second, due to these excellent properties of GO films, such as strong hydrophilic, high specific surface area, high mechanical modulus, GO modified quartz crystal microbalance (QCM) humidity sensor with both large humidity sensitivity and high frequency stability is presented. The sensor utilizes QCM as transducer element. GO films as humidity sensing layer is deposited on the electrode of the QCM. At first, we study the humidity sensing properties of GO films modified QCM humidity sensor by oscillation circuit method. The results show that GO films modified QCM exhibit high humidity response sensitivity, linear humidity-frequency response in the wide detection range of6.4-93.5%RH, fast response and recovery, low humidity hysteresis, good long-term stability. In addition, the influence of the thickness of GO films on the humidity sensing characteristics of QCM humidity sensor is discussed; it is found that QCM humidity sensor with the thicker GO films show higher sensitivity response, but worse linearity. Next, the resonant behaviors of GO films modified QCM humidity sensor in various RHs are investigated through impedance circuit method, and are compared with conventional polymer modified QCM. The results indicated that the quality factor (Q) of GO films modified QCM is much higher that of polymer modified QCM in high RH environment, as a result, GO as humidity sensing material is very suitable to realize high stability humidity sensor by combining with QCM transducer.Thirdly, a novel MEMS humidity sensor is presented based on the swelling property of GO films due to water adsorption. The sensor utilizes silicon microbridge as transducer element. GO films as humidity sensing layer is deposited on the silicon microbridge, which forms GO-silicon bilayer structure. The transducer principle of GO films based MEMS humidity sensor is as follow: the adsorption/desorption of water molecule from GO films produce volume swelling/shrinking. This stain applies on silicon microbridge, resulting in the bending of silicon microbridge. The full piezoresistive Wheatstonebridge integrated in silicon microbridge transforms this deformation into a measurable output electrical signal. The experiment results show that GO films based MEMS humidity sensor exhibited excellent linear humidity response in a wide humidity range of10-98%RH, fast response and recovery, good repeatable property and low humidity hysteresis, and so on. Moreover, the dependence of the thickness of GO thin films on response sensitivity and linearity is investigated, and the effect of temperature on the sensor output is also dicussed. Since the fabrication process of GO films based MEMS humidity sensor is compatible with classic silicon integrated circuit process, this approach can easily realize singlechip humidity sensors by integrating signal process circuit on MEMS chip, and form multi-informations and smart sensors by integrating other types of sensor on one chip.