Electrical Transport And Gas Sensing Of Two-dimensional Materials And Their Heterostructures

Two-dimensional?2D?materials have attracted considerable interest in the past few years owing to their excellent physical and chemical properties.With their unique properties,such as ultra-large surface-to-volume ratio,excellent electrical properties and high surface activities,2D materials hold the promise for the construction of a generation of gas sensor with unprecedented performance.However,these gas sensors based on 2D materials still exhibited several issues such as low response,incomplete recovery at room temperature and poor selectivity.To improve these issues,new strategies such as developing novel nanomaterials or device structures are worthy of attention.In the thesis,we systematically investigated the sensing properties of molybdenum ditelluride?MoTe₂?,a new addition to the class of 2D materials.In addition,we demonstrated the potential of van der Waals heterostructure as a new platform for sensing application,and provide more insight into the interaction between gaseous molecules and 2D heterostructures.The main achievements of the study are as follows:1.The electrical and sensing properties of MoTe₂ field-effect-transistor?FET?have been systematically studied.Firstly,we demonstrated that the hysteresis characteristic of the MoTe₂ FET can be significantly influenced by gas molecules adsorption,quality of substrates,gate bias sweep rates and different gate sweep ranges.Then we demonstrated that both the sensitivity and recovery rate of the MoTe₂ sensor can be effectively adjusted by gate bias.By applying a proper gate bias,the MoTe₂sensor shows excellent reversibility at room temperature.Using gate bias to enhance the recovery kinetics of the MoTe₂ sensor takes advantage of the FET device structure and substantially simplifies the structure of chemical sensors.2.We reported the improvement of gas-sensing properties of MoTe₂ chemical sensor through light illumination.The dependence of sensing performance on the energy of photons and light intensity was systematically studied.With 254 nm ultraviolet light illumination,the responses to nitrogen dioxide and ammonia gas were dramatically enhanced by more than 7 and 25 times,respectively,and the recovery rate was also improved when detecting nitrogen dioxide.The results demonstrate that light illumination is a promising method to improve the sensing performance of 2D materials gas sensors.3.We demonstrated a van der Waals heterostructure chemical sensor based on few-layered black phosphorus and molybdenum diselenide flakes.The carrier transport and band alignment inside the heterostructure can be effectively modulated by gate bias,which provided a theoretical guidance for the following sensing application.Compared to sensors made of homogeneous materials,the hereojunction demonstrates considerably lower detection limit and higher sensitivity toward nitrogen dioxide.Kelvin probe force microscopy and finite element simulations have provided experimental and theoretical explanations for the enhanced performance,proving that chemical adsorption can induce significant changes in band alignment and carrier transport behaviors.