Device Physics And Applications Of Memristors Based On Two-Dimensional Materials

Memristor is a resistive switch which can keep the internal resistance based on the applied voltage and current status.Having great application prospects,it also faces many challenges in the key stages from scientific research to applications.By utilizing the atomic thickness of two-dimensional crystalline materials and their excellent performance in electrical,optical,thermal,mechanical and other aspects,as well as rich physical connotation and regulation of related heterostructures,we can provide an important opportunity to achieve a breakthrough for the memristor in the applications of new materials,improvements of the structure and performance optimization.In this article,we investigated the applications of graphene,molybdenum disulfide(MoS2),tungsten disulfide(WS2)and tungsten diselenide(WSe2)in memristors and related electronic devices,the working mechanism of these devices was also studied through transmission electron microscopy characterization and theoretical analysis.At the beginning,we gave a brief introduction about the research history,switching mechanism,advantages and challenges of the memristor.We also introduced the two-dimensional materials and its heterostructures.Then,we discussed the application prospects and research status of two-dimensional materials in memristors.In Chapter Two,we fabricated a memristor with ultra-low switching power based on graphene bottom electrode.By comparing the device with a Pt-electrode memristor,which has a similar structure,we found that the graphene electrode could reduce the switching power by about 600 times,retaining the excellent switching performances of TiOx-based memristors.The experimental results and theoretical simulations suggest that the interface between graphene and oxide is mainly responsible for the ultra-low switching power of the memristor.Due to the tunnel barrier at the interface,the ?-?curve of the device exhibits nonlinear characteristics,which could potentially solve the leakage current problem.We also demonstrated that the performance of the device could be tuned by the quality of graphene electrode.In Chapter Three,we utilized the tunnel barrier of the interface between graphene and Titanium oxide to fabricate a novel selector.The ?-? curve of the device exhibits both high nonlinearity and symmetry characteristics,which are highly desirable for selector devices.Our control experiments,together with temperature-dependent transport studies and theoretical simulations,suggest that the nonlinearity arises from the graphene/TiOx interface tunneling barriers.By connecting this selector with a TaOx based memristor,we demonstrated that the selector can effectively reduce the leakage current in the cross-bar array without sacrificing the switching performance of the memristor.In Chapter Four,we fabricated a memristor based on MoS2 switching layer and investigated its switching mechanism.The conduction channels in the device were successfully positioned through pressure-modulated conductance microscopy.We found that the formation of conduction channels is related to device damage.We utilized transmission electron microscopy to investigate the conduction channels in the memristor and found that there are a large amount of oxygen atoms in the channel,indicating the important role of oxygen atoms in the switching process of the device.In Chapter Five,we selected MoS2-xOx and graphene as the switching layer and electrode,respectively,and fabricated a memristor with high thermal stability based on heterostructure of two-dimensional materials.The results showed that the memristor could achieve very stable switching:endurance over 107,switching in less than 100 ns,and non-volatile memory.We found that the device could work stably at the temperature up to 340 ? and maintain excellent switching performance,which is record-high for memristors.MoS2-xOx layer was found to induce the observed high thermal stability after performing high temperature in situ high-resolution transmission electron microscopy studies.Further in situ scanning transmission electron microscopy investigations revealed a switching mechanism based on the migration of oxygen ions.We found that the conduction channel in the memristor is well protected by the single crystal graphene and MoS2-xOx during the switching process,which ensures the stability of the conductive channel at elevated temperatures.The mechanical flexibility of such structured devices was demonstrated by showing a good endurance against mechanical bending of 1,200 times on polyimide substrate(corresponding to a strain of?0.6%),suggesting possible flexible electronic applications.In Chapter Six,we developed vertical structured devices with negative differential resistance based on two-dimensional transition metal chalcogenides.We found that Au/MoS2/Au,Au/WS2/Au,Au/WSe2/Au,and graphene/MoS2/graphene all exhibit similar negative differential resistance phenomenon.By performing temperature-dependent ?-? characterization,we concluded that the negative differential resistance in these devices is caused by the decrease of the resistance of transition metal chalcogenides after self-heating.With some basic circuit components and the negative differential resistance devices,we built the circuit that could implement the reverse,amplification,and frequency multiplication of AC signal.We also successfully generated neuron spikes with the principle that a negative differential resistance device can generate self-oscillation in RC circuits.In the last chapter,we summarized all the researches in this article,discussed and analyzed the progress we made and the problems we faced,and listed some plans for future investigation.